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Electrochemical Energy Reviews

, Volume 1, Issue 3, pp 324–387 | Cite as

Core–Shell-Structured Low-Platinum Electrocatalysts for Fuel Cell Applications

  • Rongfang Wang
  • Hui Wang
  • Fan Luo
  • Shijun LiaoEmail author
Review article

Abstract

Pt-based catalysts are the most efficient catalysts for low-temperature fuel cells. However, commercialization is impeded by prohibitively high costs and scarcity. One of the most effective strategies to reduce Pt loading is to deposit a monolayer or a few layers of Pt over other metal cores to form core–shell-structured electrocatalysts. In core–shell-structured electrocatalysts, the compositions of the core can be divided into five classes: single-precious metallic cores represented by Pd, Ru, and Au; single-non-precious metallic cores represented by Cu, Ni, Co, and Fe; alloy cores containing 3d, 4d or 5d metals; and carbide and nitride cores. Of these, researchers have found that carbide and nitride cores can yield tremendous advantages over alloy cores in terms of cost and promotional activities of Pt shells. In addition, desirable shells with reasonable thicknesses and compositions have been recognized to play a dominant role in electrocatalytic performances. And recently, researchers have also found that the catalytic activity of core–shell-structured catalysts is dependent on the binding energy of the adsorbents, which is determined by the d-band center of Pt. The shifting of this d-band center in turn is mainly affected by strain and electronic effects, which can be adjusted by adjusting core compositions and shell thicknesses of catalysts. In the development of these core–shell structures, optimal synthesis methods are of primary concern because they directly determine the practical application potential of the resulting electrocatalysts. And in this article, the principles behind core–shell-structured low-Pt electrocatalysts and the developmental progresses of various synthesis methods along with the traits of each type of core and its effects on Pt shell catalytic activities are discussed. In addition, perspectives on this type of catalyst are discussed and future research directions are proposed.

Graphical Abstract

Keywords

Core–shell structure Shell formation Shell thickness Core composition Core–shell interaction 

PACS

82.45.Jn surface structure, reactivity and catalysis 

1 Introduction

1.1 Low-Temperature Fuel Cells and Pt-Based Catalysts

Energy is an essential component of modern civilizations and is of great concern both nationally and globally. Currently, much of the world’s energy production comes from coal, natural gas, oil, and other similar carbonaceous sources such as fossil fuels. However, these sources are rapidly dwindling and fast becoming less readily available for the production of cheap energy with worldwide demands for fossil fuels continuously outpacing supply. In addition, the demand for energy is expected to increase by as much as 36% from 2014 to 2035 and global CO2 production from fossil fuel usage may become 20% higher in 2035 than it was in 2014 [1]. Therefore, to mitigate the effects of dwindling non-renewable energy resources and the detrimental climate challenges originating from CO2 emission, significant efforts are required to develop alternative clean energy sources and new energy utilization patterns. Here, fuel cell technologies, which directly convert chemical energy into electric energy through electrochemical reactions, are some of the most promising technologies to tackle these challenges because of high efficiencies and eco-friendly operations [2, 3]. And among the various fuel cell technologies, low-temperature fuel cells (LTFCs) represented by proton exchange membrane fuel cells (PEMFCs) are attracting special attention because of their high energy density and relatively low working temperatures, making them suitable for mobile and stationary applications [2]. The automotive industry has also been researching fuel cell-powered vehicles for commercialization to replace conventional internal combustion vehicles [4, 5, 6]. However, the commercialization of fuel cell vehicles is severely impeded by several issues with the major issue being the prohibitive cost, with the current cost of FCVs being approximately $350 kW net −1 (Fig. 1 shows the cost of fuel cell-powered vehicles, as represented by the Toyota Mirai, which was launched in 2015). This cost is significantly higher than that of Li-ion battery-powered electric vehicles ($260 kW net −1 ) and of conventional internal combustion vehicles ($100 kW net −1 ), and is far higher than the target price of FCVs issued by the United States Department of Energy in 2016 ($53 kW net 1 at the production rate of 500,000 units/year) [7]. Therefore, to obtain a competitive edge in the automotive industry, fuel cell power systems need to achieve cost, performance, and durability criteria as defined by the techno-economic analysis of the envisioned application. Unlike Li-ion battery-powered electric vehicles and conventional internal combustion vehicles, the fuel cell power system in FCVs accounts for approximately 71% of the total cost with the hydrogen storage unit accounting for approximately 8% and the fuel cell system itself accounting for approximately 63% (as shown in Fig. 1). And of the 63% total cost of the FCV, catalysts can account for up to 45% of the total cost. The reason catalysts cost so much in FCVs is because in current LTFC technologies, platinum-based nanomaterials are necessary components of the main electrocatalysts used at both the anode and cathode of LTFC systems. This is because platinum (Pt) exhibits the highest catalytic activity for both H2 oxidation and oxygen reduction among other pure metals, particularly in acidic media [8, 9, 10]. Therefore, the reduction of Pt loading in LTFC electrocatalysts is the most efficient approach to reduce FCV costs.
Fig. 1

The cost of various types of vehicles: Pt loading via its applications and the cost breakdown of fuel cell vehicles. FCEV, fuel cell-powered electric vehicle; LBEV, lithium-ion battery-powered electric vehicle; CICV, conventional internal combustion vehicle; TPS, total platinum supply

1.2 Importance of Low-Platinum Catalysts

Pt is one of the rarer elements in the earth’s crust with the average abundance of approximately 5 μg kg−1. In addition, its reserves are decreasing with continuous mining driven by a huge boost in demand, especially in usage as jewelry or as investments. And because of this, the price of Pt will continue to increase as demand continues to rise, with supplies being extremely tight and production deficits mounting [11]. To add to this increased demand, Pt also plays an increasingly important role in a variety of industrial processes, including chemical catalysis, particularly in oil refineries, catalytic converters of car exhaust systems, fuel cells, and electronic components [12]. Therefore, this increasing demand for Pt coupled with the expanding application of Pt in other areas will require sustained growth of global Pt productions in the long term. However, with current mining technologies, total average annual Pt productions from 2006 to 2016 were only 240 tons (As shown in Fig. 1, total Pt supply (TPS) in 2016 was 243 tons), and of this supply, 59% were used for jewelry, investments and industrial catalytic processes, and the other 41%, approximately 99.6 tons, were used for automotive applications.

In an example of automotive application, Toyota unveiled its first mass-produced FCV, the Mirai, in 2015 and the catalyst used in the Mirai fuel cell required approximately 30 g Pt. And according to the Hydrogen and Fuel Cells Strategic Roadmap issued by the Strategic Consultation of the Ministry of Economy, Trade and Industry in Japan, penetration of household fuel cell appliances will reach 1.4 × 106 units by 2020, and 5.3 × 106 units by 2030. Based on current Pt loadings for electrocatalysts, the Pt requirement for this forecast will be approximately 160 tons per year (Fig. 1), meaning that based on Pt supplies in the last 10 years, it will be difficult for current mining technologies to meet this demand. In addition, if all global light vehicles (about 89 million units in 2015 [13]) were powered by LTFCs, the total amount of Pt required in these vehicles will reach approximately 2.67 × 106 kg, which is about 15 times higher than the current yield. Evidently, the production of Pt will be unable to meet the requirements of the future, and with the commercialization of fuel cell vehicles, Pt demand will rapidly outpace supply and drive up prices, further impeding fuel cell applications. Therefore, methods to reduce Pt loading in electrocatalysts must be developed for the commercial application of fuel cell technologies.

1.3 Core–Shell-Structured Pt-Based Catalysts

Electrocatalysis is a chemical process that occurs at the solid–liquid interface of the catalyst, and evidence provided by surface science techniques, analytical instrumentations, and first-principles calculations has shown that the catalytic activity of heterogeneous catalysts is mainly determined by the available catalytic sites on the surface and the electronic structures of the catalytic sites [14, 15, 16, 17, 18, 19, 20]. Studies on Pt-based alloy electrocatalysts have also demonstrated that catalytic sites on Pt-based alloy electrocatalysts originate from Pt atoms located on the surface and that the introduction of other metals into the catalyst alloy can change the availability of catalytic sites or binding strengths of species during reactions [21, 22, 23, 24, 25, 26, 27]. These results suggest that high catalytic performances can be achieved by arranging Pt atoms onto the surface of electrocatalysts and by modifying Pt atom electronic structures through the addition of other metallic elements as cores. In this arrangement, referred to as a core–shell structure, Pt atoms can directly act as surface catalytically active sites and added metal alloys can act as the cores, which can act as a basal support and change the electronic structure of surface Pt atoms. In these core–shell structures, the thickness of Pt shells has been reported to be in the range of a few nanometers to monolayers of Pt atoms and if the core sizes remain the same, the cost of the resulting Pt-based catalysts can be reduced by forming thin layers of Pt atoms on the surface only, in which only a monolayer of Pt atoms is ideally formed on the core surface with 100% Pt utilization. And in this ideal situation, Pt loading can be reduced to only 20% pure Pt nanoparticles and the cost of the resulting catalyst can be reduced by 80% (Fig. 2).
Fig. 2

Relationship between shell structures, Pt utilization and cost reduction; red sphere, Pt atoms; silver sphere, core atoms; ML, monolayer

Based on these promising cost reduction figures, the development of core–shell-structured electrocatalysts has become an active research topic, and great efforts have been dedicated to the preparation and improvement of core–shell-structured Pt-based electrocatalysts, such as tuning the composition of both shells and cores, and controlling Pt shell thicknesses, particle sizes, and shapes. And although Minoru and Hideo reviewed the development of Pt core–shell catalysts for PEMFCs recently and other review papers have studied core–shell-structured electrocatalysts, none has provided detailed discussions on core–shell structures [10, 28, 29, 30, 31]. Therefore, it is of great significance to summarize the development of core–shell-structured Pt-based electrocatalysts and provide a detailed discussion. Based on this, this review will include the following three main sections: (1) principles and development of synthesis methods, comparing core–shell-structured materials synthesized using different methods with the advantages and disadvantages of each method being discussed based on their structural parameters (e.g., composition, shape, thickness, and feasibility for practical application); (2) classification of the core compositions for core–shell Pt-based catalysts, providing discussions and specifications on the features of each type of core and its effects on the catalytic activity of Pt shells; and finally (3) summarization of the interaction effects between cores and shells on electrocatalytic performances, systematically discussing and explaining the interactions between the core and shell, such as strain effects, electronic effects and ensemble effects, as well as the influence of core composition and shell thickness on core and shell interactions, which will provide a theoretical basis for analyzing and adjusting electrocatalytic performances.

2 Core–Shell-Structured Catalysts: Preparation

For commercial catalysts, prices are based on the cost of raw materials and the synthesis technology used because tedious synthesis processes increase production costs and decrease raw material utilization. Therefore, facile, large-scale, and green synthetic methods are vital for the synthesis of core–shell-structured catalysts. And because of this, there have been significant efforts and studies to develop optimal synthetic methods to fabricate core–shell structures with low-Pt loading, with many reviews being also conducted to evaluate and summarize performances and synthesis methods [30, 31, 32, 33, 34, 35]. For example, Inaba and Daimon [30] studied and modified the electrochemical strategy of utilizing underpotential deposition (UPD) of Cu atoms to prepare Pd/Au core Pt shell structures, and Oezaslan et al. [33, 36] evaluated core–shell nanoparticle electrocatalysts for oxygen reduction reactions (ORRs) prepared through the colloidal method, dealloying/leaching, Cu-UPD, and surface segregation [33, 36]. Xu et al. [34] also reviewed surface segregation in bimetallic nanoparticles. And because of all of the reviews and studies conducted on individual core–shell-structured catalysts being reported in the literature, a comprehensive summary of the synthesis methods for core–shell-structured Pt-based electrocatalysts is necessary.

According to the principles of core–shell-structured formation, strategies to prepare core–shell-structured electrocatalysts can be divided into six categories, including chemical deposition, dealloying, electrodeposition, surface segregation, atomic layer deposition, and physical deposition (Fig. 3).
Fig. 3

General classification of synthesis approaches for the formation of core–shell-structured electrocatalysts based on the formation process of the shell

2.1 Wet Chemical Deposition

2.1.1 Principles and Development of Wet Chemical Deposition

Wet chemical deposition is a two-step process that has been widely applied in the preparation of core–shell-structured electrocatalysts by researchers. The first step of wet chemical deposition is the synthesis of a core using a reduction reaction in which suitable capping ligands such as polyvinylpyrrolidone (PVP) or oleyamine (OAm) are introduced into the reaction to passivate the core surface and impede agglomeration. In the second step of wet chemical deposition, successive reduction occurs with the addition of shell precursors, and a shell is formed through epitaxial growth over the core. This wet chemical deposition method is also referred to as the two-step seed-mediated growth method and has been used as early as 1996 to prepare bimetallic core–shell nanoparticles of Au@Pd, Au@Pt, and Pt@Pd using ethanol as a reducing agent in the presence of PVP [38]. Many other core–shell-structured electrocatalysts have also been prepared by using this method [37, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]. For example, Wang et al. [37] prepared a core–shell-structured Pd@Pt concave decahedra (Fig. 4) through Pt epitaxial growth covered on Pd decahedral seeds with the presence of PVP ligands, ascorbic acid (AA) reducer, and KBr supplement. Other core–shell-structured electrocatalysts prepared by using this two-step wet chemical deposition method are listed in Table 1, in which precursors, ligands, and reducers are displayed to compare roles. From this, it can be seen that the various morphologies and structures of the core and shell can be adjusted through the control of growth kinetics, including precursors, ligands, reducers, and reaction conditions. For example, if EG and DEG reducers were replaced by AA, the morphology of the resulting Pd@Pt changes from spheres to dendrites, and particle sizes increase from approximately 7.7 nm to 23.5 nm, with shell thicknesses increasing from 0.5~1.2 nm to 14.4 nm [45, 46]. In addition to these studies, many studies have also investigated core–shell electrocatalysts prepared through the two-step seed-mediated growth method, studying different synthetic conditions.
Fig. 4

a Schematic of the two-step seed-mediated method for the formation of core–shell structures; b HAADF–STEM image of the concave decahedra and corresponding EDX mappings of elemental Pd and Pt.

Reprinted with permission from Ref. [37]. Copyright 2015 American Chemical Society

Table 1

Collection of various core–shell-structured electrocatalysts prepared by using the two-step seed-mediated growth method

Core@shell electrocatalyst

Precursor of core/shell

Ligands

Reducing agent

Shape/size/thickness of shell

References

Pd@Pt

Na2PdCl4/Na2PtCl4

PVP + KBr

AA

Decahedron/20 nm/5 atomic layers

[37]

Pd@Pt

Na2PdCl4/K2PtCl4

PVP + KBr

CA

Octahedra/23 nm/1~4 monolayers

[50]

Pd@Pt

Na2PdCl4/K2PtCl4

PVP

EG and DEG

Sphere/7.7 nm/0.5~1.2 nm

[45]

Pd@Pt

Na2PdCl4/K2PtCl4

PVP

AA

Dendrites/23.5 nm/14.4 nm

[46]

Pd@Pt

Na2PdCl4/H2PtCl6

PVP

CA

Plate/50~100 nm/none

[51]

Pd@Pt

Pd(AcO)2/K2PtCl4

PVP

EG

Sphere/1.5~5.5 nm/none

[52]

Pd@Pt

Pd(NO3)2/K2PtCl4

PDDA

FA

Hydrangea/48~60 nm/none

[53]

Pd@Pt

K2PdCl(Br)4/K2PtCl4

CTAC

AA

Hollow octahedron@dendrities/25 nm/8.0 nm

[54]

Pd@Pt

Pd(acac)2/

TOP/OAm

OAm

Sphere/9 nm/3 nm

[55]

Pd@Pt

Na2PdCl4/H2PtCl6

PVP

EG

Sphere/7 nm/0.6 nm

[56]

Pd@Pt

H2PdCl4/H2PtCl6

CTAB

AA

Cube@multi-arm/10, 50 nm/2 nm

[57, 58]

Pd@Pt

Pd(acac)2/Pt(acac)2

PVP

KI

Cube/14.6 nm/0.2~2.2 atomic layer

[59]

Pd@Pt

Na2PdCl4/H2PtCl6

PVP

AA

Cube/20 nm/none

[60]

Pd@Pt

Na2PdCl4/Pt(acac)2

PVP

AA

Cube/18 nm/none

[61]

Pd@Pt

Na2PdCl4/Na2PtCl4

PVP

AA

Octahedra/2.2~4.3 monolayer

[62, 63]

Pd@Pt

Na2PdCl4/H2PtCl6

None

N2H4

Sphere/5 nm/none

[64]

Pd@Pt

PdCl2/Pt(acac)2 + Ni(acac)2

CTAB

AA

Cube and porous sphere/25~30 or 30~40 nm/3~4 nm

[65]

Pd@Pt

PdCl2/K2PtCl4

PVP

HCHO/AA

Tetrapod/150 nm/4 nm

[66]

Pd@Pt

PdCl2/K2PtCl4

None

AA

Star/26 nm/4 nm

[67]

Pd@Pt

PdCl2/H2PtCl6

None

AA

Flower/6.3 nm/3 nm

[68]

Pd@PtNi

Na2PdCl4/K2PtCl4 + NiCl2

PVP

AA

Octahedra/10 nm/none

[69]

Pd@PtNi

Na2PdCl4/K2PtCl4 + NiCl2

PVP + C6H8O7

NH2OH

Octahedra/28 nm/4 monolayers

[70]

Pd@Pt

K2PdCl4/H2PtCl6

PVP

AA/methylamine

Cube/12 nm/2 nm

[71]

Pd@Pt

Na2PdCl4/K2PtCl4

Citric acid

CA

Cuboctahedron, octahedron, plate, cube/9 or 20 nm/1~2 nm

[72]

Pd@Pt

Na2PdCl4/H2PtCl6

PVP

EG

Sphere/7.2~11.2 nm/none

[73]

PdAu@Pt

Pd(C5H7O2)2 + HAuCl4/H2PtCl6

OAm

borane tert-butylamine

Sphere/4~6 nm/1~3monolayers

[74]

Pd@PtFe

Pd(acac)2/Pt(acac)2 + Fe(CO)5

OAm + borane t-butylamine

OAm + borane t-butylamine

Sphere/6~8 nm/1~3 nm

[75]

Pd3Co5@Pt

Na6Pd(SO3)4 + Co(NO3)3/Na6Pt(SO3)4

None

H2O2

Sphere/6.2 nm/none

[76]

Pd3Cu1@Pt

Pd(acac)2 + Cu(acac)2/H2PtCl6

None

Borane t-butylamine/Hantzsch ester

Sphere/5 nm/1 nm(2~3monolayers)

[77]

PdAg@Pt

AgNO3/PdCl2 + Pt(acac)2

OAm

OAm

Twinned icosahedron/15 nm/none

[78]

Pt3Pb@Pt

Pb(acac)2 + Pt(acac)2

OAm + oleic acid

Borane-triethylamine

Particle/4.0,4.9 nm/0.3,1.2 nm

[79]

PtIn@Pt

In(acac)2 + Pt(acac)2

OAm

1,2-Hexadecanediol

Sphere/3 nm/none

[80]

Au@Pt

HAuCl4/K2PtCl4

CTAB

NaBH4/AA

Cube, rod, octahedron/80 nm/7~9 nm

[81]

Au@Pt

HAuCl4/H2PtCl6

C76H52O46

C76H52O46/N2H4

Sphere/7 nm/1monolayer

[82]

Au@Pt

HAuCl4/H2PtCl6

C6H5Na3O7

C6H5Na3O7/AA

Sphere/15~30 nm/none

[83]

Au@Pt

HAuCl4/H2PtCl6

PVP

AA/HCOOH

Sphere@nanorod/250~300 nm/90~120 nm

[84]

Au@Pt

HAuCl4/K2PtCl4

Trp

Trp

Truncated polygonal@sponge-like shape/100~200 nm/2~8 nm

[85]

Au@Pt

HAuCl4/K2PtCl4

HEPES

HEPES

Popcorn/15 nm/1.3 monolayer

[86]

Au@Pt

HAuCl4/H2PtCl6

Trition-114

KBH4

Wire/none/0.125~1 monolayer

[87]

Au@Pt

HAuCl4/H2PtCl6

C6H5Na3O7/PVP

KBH4/AA

Dendrities/1~4.5 nm/none

[88]

Au@Pt

HAuCl4/K2PtCl4

C6H5Na3O7

AA

Dendrities/14 nm/none

[89]

Au@Pt

HAuCl4/H2PtCl6

PVP

NaBH4/FeSO4

Sphere/4 nm/none

[90]

Au@NimPt2

HAuCl4/Pt(acac)2 + Ni(acac)2

PVP

NaBH4/AA

Sphere/5.0~6.5 nm/none

[91]

Ru@Pt

RuCl3/H2PtCl6

PVP

EG

Sphere/10 nm/1.5~3.6 monolayers

[92]

Ru@Pt

RuCl3/H2PtCl6

PVP

EG

Sphere/2.6~6.6 nm/none

[93]

Ru@Pt

RuCl3/H2PtCl6

None

ethanol

Sphere/5 nm/0.8 nm

[94]

Ru@Pt

RuCl3/H2PtCl6

None

CH3COONa/NaBH4

Sphere/1.9, 2.5 and 3.1 nm/0.2~0.5 nm

[95]

Au@PtCu

HAuCl4/H2PtCl6 + Cu(acac)2

OAm

OAm

Sphere/8 nm/2 nm

[96]

Au@Pt

HAuCl4/H2PtCl6

PVP/CTAB

AA

Sphere/30 nm/none

[97]

Ag@Pt

AgTFA/Pt(acac)2

OAm

OAm

Sphere/13 nm/none

[98]

Au@Pt

HAuCl4/H2PtCl6

None

NH2OH

Sphere/90 nm/none

[99]

Ag@Pt

AgNO3/H2PtCl6

None

NaBH4

Sphere/7 nm/none

[100]

Rh@Pt

Pt(acac)2/Rh(acac)2

OAm

EG

Twinned nanodumbbell/

[101]

Pd@Pt

Na3RhCl6/K2PtCl4

PVP

AA

Cube-like cage/20 nm/none

[102]

Ag@Pt

AgNO3/K2PtCl4

P123

NaBH4/AA

Sphere@dendrities/40, 60 nm/10, 15 nm

[103]

Ag@Pt

AgNO3/K2PtCl4

C6H5Na3O7/PVP

NaBH4/AA

Hexagon/15.4 nm/1.3~1.6 nm

[104]

Ag@Pt

AgNO3/Na2PtCl4

None

HCHO + EG

Sphere/3 nm/none

[105]

Ag@PtPd

AgNO3/Pd(NO3)2 + H2PtCl6

C6H5Na3O7

NaBH4

Sphere/9.1 nm/none

[106]

AgNi@AgNiPt

AgNO3 + Ni(acac)2/Pt(acac)2

OAm

OAm

Sphere/15.6 nm/4 nm

[107]

Ru@Pt

RuCl3/H2PtCl6

None

ethanol

Sphere/3.7,4.2 and 4.7 nm/0.6 nm

[108]

Ru@Pt

RuCl3/H2PtCl6

None

ethanol

Sphere/none/1~2 monolayer

[109]

Au@Pt

HAuCl4/Pt(acac)2

C10H18/OAm

OAm

sphere@tendril/6~18 nm@7 nm/7 nm

[110]

Ru@Pt

Ru(acac)3/PtCl2

PVP

glycol

Sphere/3.0 nm/1~2 monolayers

[111]

Rh@Pt and Sn@Pt

SnCl2 + RhCl3/H2PtCl6

None

NaBH4

Sphere/4~8 nm/none

[112]

Ru@Pt

RuCl3/H2PtCl6

C6H5Na3O7

EG

Sphere/3 nm/none

[113]

TOP, trioctylphosphine; EG, ethylene glycol; C10H18, decahydronaphthalene; TTAB, tetradecyltrimethylammonium bromide; PDDA, poly(diallyldimethylammonium chloride); FA, formic acid; AA, ascorbic acid; CAc, citric acid; C6H5Na3O7, sodium citrate; TOABr, tetraoctylammonium; OAm, oleylamine; OAc, oleyl acid; Na-OAc, sodium oleate; C76H52O46, tannic acid Trp, l-tryptophan; HEPES, 4-(2-hydroxyethyl)-1-piperazinee-thanesulfonic acid; CTAB, cetyltrimethylammonium bromide; BP7A, Ac-Thr-Lue-His-Val-Ser-Ser-Tyr-CONH2; CTAC, cetyltrimethylammonium chloride; AgTFA, silver trifluoroacetate; P123, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (average molecular weight: B5800)

To simplify the two-step seed-mediated growth method, galvanic replacement reactions have been exploited by researchers to replace the second step [133, 134, 135]. Here, galvanic displacement reactions occur between the core and the shell precursor in an aqueous solution and are achieved by the discrepancies in standard reduction potentials which act as the driving force for the galvanic displacement reaction between M core x+ /Mcore and M shell y+ /Mshell. And because of the absence of an extraneous reducer, the deposition of the shell occurs exclusively on the core surface and no monometallic nanoparticles of the shell metal are produced as by-products [127]. In addition, the thickness of the shell can be tuned through varying the amount of shell precursors [121, 129]. Therefore, from an electrochemical point of view, the galvanic displacement method is also an electrochemical method (reviewed by Petrii et al. [136]) which can be regarded as a chemical procedure for the synthesis of core–shell-structured electrocatalysts. For example, in a study conducted by Ryu et al. [114], a Pt shell was deposited onto a PdCu core (Fig. 5) as a result of the difference in standard reduction potentials between Cu2+/Cu (E0 = 0.3 V) and Pt4+/Pt (E0 = 1.44 V). A series of electrocatalysts possessing Cu or Cu alloy cores and Pt shells have also been prepared in various experiments by using this simplified two-step seed-mediated growth method consisting of the galvanic replacement reaction [114, 124, 125, 137]. Other core–shell-structured electrocatalysts such as Au@Pt and Ag@Pt have also been synthesized by using this principle as well [117, 130]. Various examples are presented in Table 2.
Fig. 5

a Schematic illustration of the synthetic procedure of core–shell PdCu@Pt catalysts; b, f TEM images; c, g STEM images; d, h EDX mappings [red for Cu and green for Pd in (d); red for Pt, blue for Pd, and green for Cu in (h)]; and e, i EDX line profiling data of PdCu/C and PdCu@Pt catalysts.

Modified with permission from Ref. [114]. Copyright 2015 Elsevier

Table 2

Collection of various core–shell-structured electrocatalysts prepared by using the two-step seed-mediated growth method containing the galvanic replacement reaction in the second step

Core@shell electrocatalyst

Precursor of core/shell

Ligands

Reducing agent

Shape/size/thickness of shell

References

PdCu5@Pt

Na2PdCl4 + CuSO4/K2PtCl4

None

EG/Cu

Sphere/3.9 nm/none

[115]

PdCu@Pt

Pd(NO3)3 + Cu(NO3)3/H2PtCl6

None

NaBH4/Cu

Sphere/9.2 nm/none

[114]

Au@Pt

HAuCl4/K2PtCl4

CTAC/CTAB + PVP

AA/Ag

Concave cube/13 nm/none

[116]

Au@Pt

HAuCl4/H2PtCl6

C6H5Na3O7

NaBH4/Au

Sphere/3.8 nm/none

[117]

Au@Pt

HAuCl4/K2PtCl4

PVP

NaBH4/Ag

Sphere/55 nm/25 nm

[118]

PdNi@Pt

K2PdCl4 + K2Ni(CN)4/K2PtCl4

None

NaBH4/Ni + Pd

Corallite-like shape/none/none

[119]

AuAg@Pt

HAuCl4 + AgNO3/H2PtCl4

CA

NaBH4/Ag

Sphere/5 nm/none

[120]

AuCu@Pt

HAuCl4 + CuCl2/K2PtCl4

None

NaBH4/Cu

Sphere/16 nm/1.5monolayers

[121]

Ag@Pt

AgNO3/H2PtCl6

None

Ag

Sphere/none/none

[122]

Cu@PtPd

CuCl/H2PtCl6

hexadecylamine

Glucose/Cu

Wire/10 nm/none

[123]

Cu@Pt

CuSO4/H2PtCl6

CA + sodium

NaBH4/Cu

Sphere/1.9~7.3 nm/none

[124]

Cu@Pt

CuSO4/H2PtCl6

None

NaBH4/Cu

Sphere/3 nm/none

[125]

Pd@Pt

Pd(acac)2(RuCl3)/PtCl4

OAm

NaBH4/Pd(Ru)

Sphere/4.7(6.4)nm/none

[126]

PdFe@PdPt

FeCl3/K2PtCl4

None

Fe

Sphere/8.1 nm/none

[127]

PdCu@Pt

CuSO4 + K2PdCl4/H2PtCl6

None

EG/Cu

Sphere/6.7 nm/none

[128]

Ag@Pt

AgNO3/K2PtCl4

C6H5Na3O7

NaBH4/Ag

Sphere/9.6 nm/none

[129]

Ag@Pt

AgNO3/K2PtCl4

PVP

EG/Ag

Box, popcorn-like shape/50,90 nm/1.7 nm

[130]

Au@Pt

HAuCl4/K2PtCl4

C6H5Na3O7

Ag

Sphere/4~6 nm/2~3 nm

[131]

Ru@Pt

RuCl3/H2PtCl6

None

Ru

Sphere/2.5 nm/none

[132]

Recently, the two-step method was further simplified to a one-step method referred to as the one-pot synthesis, which is a more facile and economical method of preparing core–shell-structured catalysts [44, 165, 166, 167]. In this one-step method, core–shell structures result from different reduction sequences of metal, which are affected by two factors: the reduction potential difference between the soluble metal salts of the core and the shell, and the reactant concentrations [168, 169, 170, 171, 172]. For example, in a system consisting of two soluble precursors of H2PtCl6 and HAuCl4, AuCl4−, and [PtCl6]2−, the metals can be reduced according to the following reactions:
$$ {\text{AuCl}}^{4 - } + 3{\text{e}}^{ - } \to {\text{Au}} + 4{\text{Cl}}^{ - } + \, 1.00{\text{ eV versus SHE}} $$
(1)
$$ \left[ {{\text{PtCl}}_{6} } \right]^{2 - } + 2{\text{e}}^{ - } \to \left[ {{\text{PtCl}}_{4} } \right]^{2 - } + 2{\text{Cl}}^{ - } + 0.68{\text{ eV versus SHE}} $$
(2)
$$ \left[ {{\text{PtCl}}_{4} } \right]^{2 - } + 2{\text{e}}^{ - } \to {\text{Pt}} + 4{\text{Cl}}^{ - } + 0.76{\text{ eV versus SHE}} . $$
(3)
And based on the driving force created by the large difference in reduction potentials, the reduction of AuCl4− with lower concentrations preferentially occurs in a shorter time to form Au cores (Fig. 6) and Pt dendritic nanowires will form through reducing higher-concentration [PtCl6]2− over Au seeds [138, 139, 147]. By using this method, a considerable number of core–shell-structured electrocatalysts (Table 3) have been prepared through altering synthesis conditions, such as precursors, concentrations and reaction times. Moreover, the morphology and structure of the obtained particles can be further modified through tuning the ligands, which can also act as structure-directing agents.
Fig. 6

a Schematic of the one-step method for forming core–shell structures; bd TEM images of samples taken at different reaction times of 1, 6, and 24 h during the synthesis process of Au@Pt. Reprinted with permission from Ref. [138]. Copyright 2010 American Chemical Society; eg HAADF–STEM–EDS mapping images of dendritic Au@Pt with ring regions showing different contents of Pt and Au.

Reprinted with permission from Ref. [139]. Copyright 2016 Elsevier

Table 3

A collection of various core–shell-structured electrocatalysts prepared by using the one-step method

Core@shell electrocatalyst

Precursor of core/shell

Ligands

Reducing agent

Shape/size/thickness of shell

References

Pd@Pt

Na2PdCl4/K2PtCl4

CTAB

AA

Cube and sphere/25 and 30 nm/3~4 nm

[65, 140]

Pd@Pt

Na2PdCl4/H2PtCl6

Pluronic F127

AA

Cube and sphere/50 nm/none

[141]

Pd@Pt

K2PdCl4/K2PtCl4

PVP + CTAB + Pluronic F123 + Pluronic F127

AA

Dendrites/150 nm/3 nm

[142]

Pd@Pt

PdCl2/K2PtCl4

Pluronic F123

AA

Dendrites/8~15 nm/3 nm

[143]

Pd@Pt

Na2PdCl4/K2PtCl4

Pluronic F127

AA

Concave sphere/45 nm/47 nm

[144]

Pd@Pt

Pd(acac)2 + Pt(acac)2

PVP + Ni(acac)2 + FeCl3

AA

Concave polyhedron/25~33 nm/1.4 nm

[145]

Pd@Pt

H2PdCl4/H2PtCl6

PEO

AA

Dendrites/37.7 nm/none

[146]

Pd@Pt

Pd(NO3)2/Pt(NH3)4(NO3)2

None

NaBH4

Sphere/5 nm/none

[147]

PdPt@Pt

Pd(acac)2/Pt(acac)2

TOP + OAm

SA

Sphere/11.5 nm/< 2.5 nm

[148]

Pt@PtPb

Pb(acac)2/Pt(acac)2

CTAB + glucose + OAm

Glucose + OAm

Wire/18~21 nm/none

[149]

PdPt@Pt

Na2PdCl4/H2PtCl6

hexadecylpyridinium

Formic acid

Nanoring/110~130 nm/none

[150]

PdPt@Pt

H2PdCl4/H2PtCl6

None

Formic acid

Particle/24.5 nm/none

[151]

FePt@Pt

C12H22O14Fe + H2PtCl6

None

NaBH4

Nanodendrites/22 nm/none

[152]

Fe@PtRu

Fe(acac)2/Pu(acac)2 + Pt(acac)2

None

EG

Sphere/2.3 nm/< 1 monolayer

[153]

PtNi@Pt

Ni(acac)2 + Pt(acac)2

CTAC + OAm + OAc

OAm

Sphere/40~50 nm/none

[154]

Au@Pt

HAuCl4/H2PtCl6

AOT/isooctane

N2H5OH

Sphere/3~4.5 nm/none

[155]

Au@Pt

HAuCl4/H2PtCl6

PVP

EG

Sphere/3~8 nm/none

[156]

Au@Pt

HAuCl4/H2PtCl6

Pluronic F127

AA

Dendrites/27 nm/0.3~3.0 nm

[138]

Au@Pt

HAuCl4/H2PtCl6

OAm + CTAB + TOPO

OAm

Star/16 nm/none

[157]

Au@Pt

HAuCl4/H2PtCl6

PIA

AA

Dendrites/69 nm/none

[139]

AuPt@Pt

HAuCl4/H2PtCl6

OAm

OAm

Sphere/7 nm/0.33~1.16 nm

[158]

Au@PtNi

HAuCl4/K2PtCl4 + NiCl2

Brij58

AA

Sphere/30 nm/12 nm

[159]

PtNi@Pt

Pt(acac)2 + Ni(acac)2

PVP

None

Truncated octahedron/7.5 nm/none

[160]

Pt2Ni@Pt

H2PtCl6 + NiCl2

OAm

OAm

Concave tetrahedron@dendrities/40 nm/20 nm

[161]

Pt3Co@Pt

Pt(acac)2 + Co(acac)2

None

EG

Sphere/2.3 nm/none

[162]

Au@Pd@Pt

HAuCl4/Na2PdCl4/K2PtCl4

Pluronic F127 or PVP

AA

Sphere@sphere@porous dendrities/35 nm/Pt 3 nm

[163, 164]

AOT, sodium di-2- ethylhexylsulfosuccinae; CTAB, cetyltrimethylammonium bromide; TOP, trioctylphosphine; C12H22O14Fe, ferrous gluconate; SA, stearic acid; CTAC, cetyltrimethylammonium chloride; TOPO, hexadecyltrimethylammonium; PEO, poly(ethylene oxide); PIA, polyinosinic acid; Brij58, polyoxyethylene 20 cetyl ether

In addition to the two-step seed-mediated growth method and the one-step method, several other methods, including three or more steps chemical deposition in solution methods, were also developed to prepare core–shell-structured electrocatalysts. For example, Lai et al. [173] prepare a core–shell-structured electrocatalyst by using a three-step process in which Co spheres were first prepared through a reduction reaction. The resulting Co spheres were subsequently used as a hard template for the deposition of Pd atoms by using the galvanic replacement reaction between [PdCl4]2− and Co to form hollow spheres. In the final step, Pt atoms were deposited onto the hollow Pd spheres through the reduction of [PtCl4]2− with AA to form a core–shell structure comprised of a hollow sphere-like core and a nanorod-like shell. Other pre-synthesized materials, such as Te wires and TiO2, have also been used as hard templates to synthesize core–shell-structured electrocatalysts, such as Au@Pt, Pd@Pt and Ru@Pt [174, 175, 176]. Compared with the two-step seed-mediated growth method and the one-step method; however, there has been little research into other methods of core–shell-structured electrocatalyst synthesis as a result of their complex processes [177, 178].

2.1.2 Merits of Wet Chemical Deposition

Based on the information listed in Tables 1 and 3, merits of chemical deposition can be summarized as follows:
  1. 1.

    The operation is simple and rarely requires special equipment.

     
  2. 2.

    The precursors are mostly inorganic salts, which are inexpensive compared with organic salts.

     
  3. 3.

    The morphology, size, and composition of cores and shells prepared by using wet chemical deposition are tunable through the use of different ligands and reducing agents, or through the control of precursor amounts and reaction times.

     
  4. 4.

    The multi-step wet chemical deposition method can be used to synthesize more complex core–shell structures, such as core–shell-structured Cu@Pt/Cu@Pd porous-skeletons and core–double-shell catalysts [48, 179, 180, 181].

     

2.1.3 Demerits of Wet Chemical Deposition

Based on the information listed in Tables 1 and 3, demerits of chemical deposition can be summarized as follows:
  1. 1.

    The use of capping agents and organic solvents leads to complex reaction microenvironments and tedious cleaning processes after synthesis. And if these clean processes are not carried out, serious impairments to electrocatalytic activities can occur as a result of a “dirty” surface [82, 142, 182, 183, 184, 185, 186, 187, 188].

     
  2. 2.

    The core materials used in wet chemical deposition require precious metals such as Au, Pd, and their alloys because these precious metal-based cores are stable in physicochemical fuel cell operating conditions, and because there are relatively small crystal lattice mismatches with Pt. The incorporation of non-precious metals such as Ni, Co, Fe and their alloys as cores in electrocatalysts for low-temperature fuel cells is desirable, because liberal usage of non-precious metals can significantly decrease electrocatalyst costs. Here, Ni, Co, Fe have all been shown to be highly effective in balancing Pt surface energetics and improving electrocatalytic activities. [93, 189, 190, 191, 192, 193, 194, 195].

     
  3. 3.

    Chemical deposition is usually incapable of fully controlling shell thicknesses and epitaxial growth of shells over cores. Here, ideal core–shell-structured catalysts should possess a Pt monolayer shell on the core of the catalyst, which can produce maximum catalytic performances with minimal Pt usage. However, as seen from Tables 1, 2 and 3, the shell thicknesses of most core–shell-structured electrocatalysts prepared through wet chemical deposition are larger than 1 nm, which is equivalent 2–3 atomic layers. Alternatively, although the formation of dendrite-like shells is another indication of the imprecise control of epitaxial growth of shells over cores, the presence of dendrite-like Pt shells can actually decrease Pt usage.

     

2.2 Dealloying

2.2.1 Principles and Development of Dealloying

Dealloying is an ordinary corrosion behavior that involves the selective dissolution of one or more active components from an alloy precursor, producing nanoporous metal alloys [209, 210, 211, 212]. This method is also referred to as the depletion gilding and has been used to prepare highly porous metal catalysts such as RANEY-nickel and Au island-rich surfaces over Ag particles [213, 214]. In the electrocatalytic field, the dealloying method was first applied by Mukerjee et al. [215] for the propose of pre-leaching base-metals from Pt alloys to minimize membrane electrode assemble (MEA) contamination during fuel cell operations. And with more in-depth research, this process has recently been developed as an effective method to prepare core–shell-structured electrocatalysts from non-noble-metal-rich Pt alloy precursors, such as the preparation of CuPt@Pt through the selective dissolution of Cu atoms from PtCu alloy precursors (Fig. 7) [196]. The dealloying method can be conducted through two different approaches with one being chemical dealloying, which uses chemical reactants to dissolve one or more metals out of the alloy (Table 4). For example, porous and core–shell-structured PtCu@Pt particles with an average size of 3~4.5 nm can be obtained by dealloying PtCu alloy particles under 80 °C for 36 h by using a H2SO4 solution as an etchant [155]. In another example, porous and core–shell-structured PtCu@Pt can be obtained by dealloying PtCuMn alloy precursors with (NH4)2SO4 as the etchant [200]. Overall, for chemical dealloying processes, acids are usually selected as etchants.
Fig. 7

Illustration of the stepwise in situ preparation of dealloyed Pt–Cu electrocatalysts through the selective dissolution of Cu from a Pt25Cu75 precursor; a The Pt25Cu75 precursor; b Dissolution of Cu atoms from the precursor; c Formation of a core–shell structure.

Reprinted with permission from Ref. [196]. Copyright 2008 American Chemical Society

Table 4

Collection of core–shell-structured electrocatalysts prepared from their alloy precursors by using chemical leaching

Core@shell electrocatalyst

Solvent/conditions

Shape/size/thickness of shell

References

PtCu@Pt

1 M H2SO4 80 °C 36 h

Sphere/3~4.5 nm/none

[155]

Pd@Pt

10 mL Concentrated HNO3 for 24 h

Icosahedron/17.7 nm/1 nm

[197]

PtCu@Pt

1.5 M H2SO4 3 h at ambient temperature

Sphere/10~15 nm/none

[198]

PtCu@Pt

Different concentrations of HNO3 for different times at ambient temperature

Wire/none/none

[199]

PtCu@Pt

1 M (NH4)2SO4 3d at ambient temperature (PtCuMn)

Ribbon/none/none

[200]

PtCu@Pt

HNO3(v/v 1:1) for different times

Three dimension/none/none

[201]

PtNi@Pt

1 M H2SO4 + 1 M HNO3 20 °C 10 h

Sphere/5~7 nm/none

[202]

PtNi@Pt

H2SO4 or HNO3

Sphere/6 nm/0.8~1.6 nm

[203]

PtNi@Pt

0.5 M H2SO4 20 °C 24 h

Sphere/6 nm/0.6 nm (2~4 atomic layers)

[204]

PtCo@Pt

1 M H2SO4 60 °C 24 h

Sphere/4.0 nm/1~4 monolayer

[205]

AgPt@Pt

1 M HClO4 90 °C 1 h

Sphere/2.5 nm/none

[206]

FeNiPt@Pt

25 mL AA 70 °C, N2, 24 h

Wire/none/none

[183]

PtRuNi@PtRu

50 mL HNO3(v/v 1:1) for 5 min

Sphere/7.4, 10.5 nm/none

[207]

PtRuCu@PtRu

4.0 M H2SO4 + O2, 5d at ambient temperature

Irregular Sphere/8.5 nm/none

[208]

The other dealloying approach is the electrochemical dealloying method, in which electrons produced from dissolution (e.g., the oxidation of active metal components) transfer to an external circuit. This procedure can spontaneously occur on alloy electrodes in suitable electrolytes at open circuit potentials and is also known as the anodic corrosion. For example, electrochemical dealloying occurs spontaneously if alloys of AgAu are immersed into NaCl solution or if MnCu alloy systems are immersed into an HNO3 solution [214, 239, 240, 241, 242]. Electrochemical dealloying can also be carried out through applying a potential by using an external power source, and this is the preferred synthesis method for core–shell-structured electrocatalysts in recent years [243, 244, 245, 246]. For example, core–shell-structured PtCu@Pt sphere electrocatalysts (Fig. 8) can be produced through dealloying PtCu alloys by using cyclic voltammetry (CV) or the application of a constant potential [216, 219, 220, 223, 224]. Additional core–shell-structured electrocatalysts synthesized by using electrochemical dealloying, including PtFe@Pt, PtNi@Pt, and PtCo@Pt are listed in Table 5. To further improve the electrochemical performance of chemically and electrochemically dealloyed nanoparticles, alloy precursors can be pre-treated by using thermal annealing, which can provide high degrees of ordered metal alloys, allowing for the improvement of dealloyed nanoparticle stability and the simplification of the dealloying process [23, 196, 198, 247, 248, 249, 250].
Fig. 8

a Cyclic voltammograms of Cu3Pt/C ordered intermetallic nanoparticles during electrochemical dealloying in 0.1 M HClO4 at a scan rate of 50 mV s−1. Color-coded legends indicate the cycle number; b, c TEM overview images of Cu3Pt/C intermetallic nanoparticles after electrochemical dealloying. EELS mappings of d Cu; and e Pt analysis; f The combination of (d) and (e), showing the Pt shell.

(modified from Ref. [216]). Copyright 2012 American Chemical Society

Table 5

Collection of core–shell-structured electrocatalysts prepared by using electrochemical dealloying

Core@shell electrocatalyst

Electrochemical method

Shape/thickness of shell

References

Pt3M(Fe,Co,Ni)@Pt

A constant potential control of 0.05 V versus Ag/AgCl in 0.1 M HClO4

None

[217]

PtNi@Pt

A constant potential control of 0.06,0.4,0.6 and 0.8 V RHE in 0.1 M HClO4

Sphere/none

[218]

PtCu@Pt

A constant potential control of + 1.2 V versus RHE in 0.1 M HClO4 for 5 h

Sphere/0.5 nm

[219]

PtCu@Pt

A constant potential control of +  0.6 V versus RHE 0.1 M HClO4 until the current decreased below a threshold value of 5 μA

Sphere/0.6 nm

[220]

PtCr@Pt

A constant potential control of + 1.25 V versus RHE in 0.1 M H2SO4

Sphere/2~3 monolayers

[221]

AuAgPt@AuPt

A constant potential control of 0.8 V versus Hg/Hg2SO4 until dealloying stops (the material turns black, and the dissolution current drops to zero)

Leaf/none

[222]

PtCu@Pt

200 CV cycles between 0.06 and 1.2 V versus RHE at 1000 mV s−1 in 0.1 M HClO4

Sphere/1~2 nm

[223]

PtCu@Pt

200 CV cycles between 0.05 and 1.0 V versus RHE at 200 mV s−1 in 1 M H2SO4

Sphere/0.6~1 nm

[224]

PtCu@Pt/

100 CV cycles between -0.2~1.2 V versus RHE at 50 mV s−1 in 0.5 M H2SO4

Sphere/none

[225]

PtCu@Pt

1600 CV cycles between 0.05 and 1.2 V versus RHE at 1000 mV s−1 in 0.1 M HClO4

Sphere/none

[226]

PtCo@Pt

3 CV cycles at 100 mV s−1 + 200 CV at 200 mV s−1 + 3CV at 100 mV s−1 cycles between 0.05 and 1.0 V versus RHE at 200 mV s−1 in 1 M HClO4

Sphere/none

[227]

PtCo@Pt and PtCu@Pt

3 CV cycles at 100 mV s−1 + 200 CV cycles at 500 mV s−1 between 0.06 and 1.0 V versus RHE in 1 M HClO4

Sphere/0.8~1.2 nm

[228]

PtNi@Pt

200 CV cycles at 500 mV s−1 between 0.06 and 1.0 V versus RHE in 1 M HClO4

Sphere/none

[218, 229]

PtNi@Pt

200 CV cycles between 0.05 and 1.2 V versus RHE at 500 mV s−1 in 0.1 M HClO4

Sphere/none

[230]

PtNi@Pt

50 + n CV cycles between 0.05 and 1.2 V versus RHE at 250 mV s−1 in 0.1 M HClO4 until CV reaches a steady state

Sphere/none

[231]

PtNi@Pt

400 CV cycles between 0.05 and 0.95 V versus RHE at 1 V/s in 0.1 M HClO4

Cube and cube-octahedra/none

[232]

PtNi@Pt

100 CV cycles between 0.05 and 1.0 V versus RHE at 1 V/s in 0.1 M HClO4

Sphere/2~3 atomic layers

[233]

PtxNi1−x@Pt

200 CV cycles between 0.06 and 1.0 V versus RHE at 100 mV s−1 in 0.1 M HClO4

Sphere/2~3 atomic layers

[234]

PtNi6 or PtNi3@Pt

200 CV cycles between 0.06 and 1.0 V versus RHE at 100 mV s−1 in 0.1 M HClO4

Sphere/0.2,0.4 nm

[235]

Pt1.5Ni, PtNi or PtNi1.5@Pt

28 CV cycles between 0.06 and 1.0 V versus RHE at 100 mV s−1 + in 0.1 M HClO4

Octahedra/5~12,1~4 atomic layers

[236]

PtCuCo@Pt

250 CV cycle between 0.05 and 1.2 V versus RHE in 1 M H2SO4

None

[237]

PtCuAu@PtAu

n CV cycles between − 0.25 and 1.5 V versus RHE at 50 mV s−1 in 0.5 M H2SO4 until CV reaches a steady state

Sphere/none

[238]

In addition, comparisons between the electrochemical dealloying procedure and the chemical dealloying procedure (acid washes) have been conducted in literature [196, 216] and the consensus is that (1) both chemical and electrochemical Cu dealloying procedures do not result in thermodynamically and compositionally stable active catalyst phases; (2) electrochemical dealloying methods can lead to the formation of thin Pt skins of approximately 1 nm in thickness with an ordered Cu3Pt core structure, whereas chemical leaching gives rise to “spongy” porous structures without ordered core structures. However, if the size of the alloy precursor is large enough, porous structures can also be formed through electrochemical dealloying, as shown in Fig. 9 [218, 228, 231, 251]; and (3) electrochemical dealloying (cyclic voltammetric) results in the complete Cu dissolution of the obtained core–shell structure and electrochemical dealloying can, to a large extent, combine potential-induced surface atomic rearrangements to expose subsurface atomic layers, leading to more complete Cu dissolution [214, 239, 252, 253, 254].
Fig. 9

Transmission electron micrographs (TEMs) of carbon-supported Ni75Pt25 obtained before and after dealloying (a, b 8 nm; c, d 15 nm).

Reprinted with permission from Ref. [231]. Copyright 2012 American Chemical Society

2.2.2 Merits of Dealloying

Based on the information listed in Tables 4 and 5, merits of dealloying can be summarized as follows:
  1. 1.

    The dealloying method is a facile and reproducible method to synthesize core–shell structures. Moreover, nanoporous electrocatalysts with core–shell structures prepared by using chemical dealloying can be produced in large scale to meet electrocatalyst commercialization requirements.

     
  2. 2.

    The dealloying method allows the surfaces of prepared electrocatalysts to be extremely clean [255, 256]. In addition, dealloying can roughen electrochemical interfaces, which provides surfaces with high densities of defects at the nanoscale level, resulting in increased surface areas and improved electrocatalytic activities [209, 221, 237, 250, 257].

     
  3. 3.

    The dealloying method allows surface metal atoms to evolve into bi-continuous structures of metal-and-void extending in three dimensions, which can provide excellent electronic conductivity and improved mass transfer channels, thus facilitating electrochemical reaction kinetics on electrode surfaces [222, 258, 259].

     
  4. 4.

    Various multiple-component alloys can be designed and synthesized through combining mechanical alloying [260, 261].

     

2.2.3 Demerits of Dealloying

Based on the information listed in Tables 4 and 5, demerits of dealloying can be summarized as follows:
  1. 1.

    The dealloying of active metals will increase the cost of obtained core–shell-structured electrocatalysts.

     
  2. 2.

    Nanoporous and core–shell-structured electrocatalysts prepared by using electrochemical dealloying cannot be produced in large scale.

     
  3. 3.

    The tunability of alloy compositions is limited and until now, the performance optimization of dealloyed electrocatalysts is achieved through the adjustment of precursor compositions or precursor treatment temperatures rather than the control of dealloying procedures [224, 243].

     
  4. 4.

    The dealloying process is generally not good at controlling shell thicknesses, and the resulting Pt shells are usually thicker than monolayer shells. Moreover, considerable amounts of Pt atoms are still distributed in the core in addition to the Pt atoms exposed on the surface of electrocatalysts during the dealloying process, decreasing Pt utilization.

     

2.3 Electrodeposition

2.3.1 Principles and Development of Electrodeposition

In principle, electrodeposition; also referred to as electroplating, can be thought of as a special class of liquid-phase chemical deposition involving electron transfer between chemical reactions and external circuits [136]. Metal electrodeposition is the reduction of metal ions in electrolytes and the generation of electrons on cathode electrodes as supplied by an external circuit. And dating back to the early 19th century, precious metal particles such as Pt and Au have been synthesized by using this method [136, 262, 263]. To date, electrodeposition methods have been developed to synthesize various metals and corresponding alloys with multi-scale syntheses and morphologies, along with core–shell-structured electrocatalysts, which have been discussed in several reviews [136, 264, 265, 266, 267]. Electrodeposition is also controllable through the adjustment of either potential or current, which can be presented in various ways such as the potentiostatic mode, potential sweep mode, galvanostatic mode, alternating-current mode, and pulse mode [268], with underpotential deposition and pulse deposition being the most effective methods to control electrodeposition of core–shell-structured electrocatalysts. Below, these two methods are discussed in detail.

Underpotential deposition (UPD) refers to the electrodeposition of a metal monolayer(s) over a metal substrate at potentials more positive than the equilibrium potential of the metal/metal ion system. Early UPD methods, which have been reviewed by several studies, were used to prepare and maintain single-crystal electrodes with well-defined surface structures and cleanliness [266, 271, 272, 273, 274]. And based on the deliberate and precise control of surface layers facilitated by the UPD method, associated UPD and galvanic displacement were introduced as a combined process to fabricate core–shell-structured electrocatalysts by Adzic and Weaver [275, 276, 277, 278, 279] (Fig. 10) in which sacrificial Cu metal is first applied as a UPD monolayer onto a Pd substrate and subsequently displaced spontaneously by a Pt monolayer in a PtCl42− solution [270]. This procedure has also been used by other researchers to deposit Pt monolayers onto Au colloids [280, 281]. Other sacrificial metal layers such as Pb and Ag can also be used for depositing M2 monolayers [274] and more specific examples of the UPD are collected in Table 6.
Fig. 10

a Schematic of the two major steps involved in the synthesis of a Pt monolayer on a Pd core through a combination of electrochemical deposition and galvanic replacement; b HAADF–STEM image and c EDX line-scanning profile showing the formation of a Pt monolayer on a Pd nanoparticle that was modified [269] based on the work of Adzic et al. [270].

Reprinted with permission from Ref. [269]. Copyright 2005 American Chemical Society

Table 6

Core–shell-structured electrocatalysts prepared through combining UPD and galvanic displacement

Core@shell electrocatalyst

Shell precursors

UPD conditions

Shape/size/thickness of shell

References

Pd@Pt

1 mM K2PtCl4

0.38~0.005 V versus Cu/0.1 M Cu2+ at 5 mV s−1

Nanowire/r = 1.7~2.7 nm/monolayer

[282]

Au@Pt

5 mM H2PtCl6

0.18 V versus Ag/AgCl for 5 min in 0.1 M H2SO4 + 1 mMCuCl2

sphere/15 nm/1~8 monolayers

[283]

Au@Pt

5 mM K2PtCl4

0.5~0.01 V versus Ag/AgCl at 10 mV s−1 in 1 mM CuSO4 + 0.1 M H2SO4

sphere/5 nm/1~5 monolayers

[284]

Au@Pt

1 mM H2PtCl6

0.5~0.27(0.23) V versus Ag/AgCl at 5 mV s−1 in 10 mM CuSO4 + 0.05 M H2SO4

Urchin-like clusters/none/0.67~2 monolayers

[285]

Au@Pt

1 mM K2PtCl4

− 0.85 V versus Hg/Hg2SO4 for 3 s in 1.0 mM Pb(NO3)2 + 0.1 M HClO4

Dendrimer/1.4 nm/monolayer

[286]

Au@Pt

0.5 mM K2PtCl4

For Pb UPD on Au, 0.05 to − 0.85 V versus Hg/Hg2SO4 at 20 mV s−1 in 0.1 M NaClO4 + 0.01 M HClO4 + 0.003 M Pb(ClO4)2

Cluster/3~5 nm/monolayer

[287]

Au@Pt

0.5 mM K2PtCl4

0.05 to − 0.38 V versus Hg/Hg2SO4 at 20 mV s−1 in 0.1 M HClO4 + 0.001 M Cu(ClO4)2

Cluster/3~5 nm/monolayer

[287]

PdAu@Pt

1 mM K2PtCl4

0.38~0.005 V versus Cu/0.1 M Cu2+ at 5 mV s−1

Irregular sphere/5 nm/monolayer

[270, 288]

Pd@PdAu

@Pt

1 mM HAuCl4, PdCl2/K2PtCl4

0.05~0.5 V versus Ag/AgCl at 5 mV s−1 in 0.05 M CuSO4/0.05 M H2SO4 electrolyte

Irregular shape/5 nm/monolayer

[289]

Pd9Ru@Pt

1 mM K2PtCl4

0.764~0.314 V versus RHE at 20 mV s−1 in 50 mM CuSO4 + 50 mM H2SO4

Irregular particle/3 nm/monolayer

[290]

PtPb@Pt

PdPb@Pt

PdFe@Pt

1 mM K2PtCl4

0.05~0.5 V versus Ag/AgCl at 5 mV s−1 in 0.05 M CuSO4/0.05 M H2SO4 electrolyte

Irregular shape/none/monolayer

[291]

Ru@Pt

Ru/C/K2PtCl4

0.38~0.005 V versus Cu/0.1 M Cu2+ at 5 mV s−1

Sphere/3.8 nm/1~3 monolayer

[292]

Au@Pt

2.4 mM PtCl42−

− 0.02 V versus Ag/AgCl for 120 s in 1 mM CuSO4

Sphere/19 nm/monolayer

[293]

Pd1Ru1Ni2@Pt

1 mM K2PtCl4

0.38 V versus Ag/AgCl for 600 s in 50 mM CuSO4 + 50 mM H2SO4

Sphere/2.87 nm/monolayer

[294]

Os@Pt

OsCl3/K2PtCl4

A potential control of 0.35 V versus RHE in 50 mM CuSO4 + 50 mM H2SO4

Sphere/none/monolayer

[295]

Au@Pt

1 mM K2PtCl4

Few CV 0.9~0.275 V versus RHE in 50 mM CuSO4 + 0.5 M H2SO4 at 10 mV s−1, subsequently, a potential control of 0.275 V for 15 min in flowing electrolyte of 0.5 M H2SO4

Octahedrons/10 nm/monolayer

[296]

Pd@Pt

PdCo@Pt

0.1 mM K2PtCl4

0.38~0.98 V versus RHE in 50 mM H2SO4 and 50 mM

CuSO4

Sphere/4 nm/monolayer

Sphere/4.6 nm/monolayer

[297]

Pd2Co(111)@Pt

K2PtCl4

A potential control of 0.4 V versus Ag/AgCl in 50 mM CuSO4 until the current becomes a steady value near zero

Sphere/5.1 nm/monolayer

[298]

Pd@Pt

K2PtCl4

A potential control of 0.4 V versus Ag/AgCl in 50 mM CuSO4 until the current becomes a steady value near zero

Sphere/5 nm/monolayer

[299]

Au@Pt

K2PtCl4

A potential control of 0 V versus Ag/AgCl in 1 mM CuSO4 +  0.1 M H2SO4 until the current becomes a steady value near zero

Sphere/3, 10 nm/monolayer

[300]

Pd@Pt,Ni@Pt Co@Pt,

Ru@Pt

1.0 mM K2PtCl4

A potential control of 0.7~0.3 V versus RHE at 20 mV s−1 in 0.05 M H2SO4 + 50 mM CuSO4

Sphere/10 nm/monolayer

[35, 301]

Au@Pt

5 mM K2PtCl4

A potential control of 0.27 to − 0.002 V versus Ag/AgCl at 400 mV s−1 for 180 s in

0.1 M H2SO4 + 1 mM CuSO4

Sphere/3.7 nm/monolayer

[302, 303]

Pd@Pt

Pd(acac)2

Defect-mediated thin-film growth (DMTFG)

15 cycles CV 0.4~0.67 V followed by LSV 0.4~1 V for stripping Cu at 10 mV s−1 in 1 mM Cu(NO3)2

Sheet/none/monolayer

[304]

Ru@Pt

0.5 mM K2PtCl4

A potential control of 0.3 V versus RHE in 2 mM CuSO4 + 0.5 M H2SO4

Sheet/a few hundred nanometers/1.5 monolayer

[305]

From Table 6, it can be seen that the UPD processes are influenced by deposition potentials, times and sweep rates; and that the thickness of Pt shells can be well controlled through coulometric control [303]. Besides Pt monolayers, other metal monolayers, such as Pd, Au, Ag and Cu, can also be deposited on metal substrates by using the UPD [306, 307, 308], and researchers are constantly improving the coverage and smoothness of these monolayers through modifying the UPD procedure [296, 297, 309, 310, 311]. For example, based on kinetic limitation effects, smooth and near perfect Pt monolayers can be grown in the absence of UPD-type energetic stabilization by using a flow cell setup [296, 309]. In another example, Tao et al. [312] modified a Cu-UPD process by carefully preparing a UPD solution containing Cu, CuSO4 and AA to allow the process to be free of potential control, resulting in 1~2 nm Pt nanoclusters being deposited onto Pd nanorods. Adzic et al. [289] also performed the Cu-UPD twice to deposit double shells consisting of an outmost monolayer shell of Pt and a monolayer subshell of PdAu onto a Pd core.

Pulse electrochemical deposition (PED) (as shown in Fig. 11) is an electroplating procedure in which the polarity of the applied current is periodically reversed. Here, external pulse power sources can supply various current waveforms such as sinusoidal, triangular, rectangular, and saw-toothed, in which rectangular current waveforms are always used for PED [313, 314, 315]. PED processes also possess three wide-ranging independent variables: the pulse current (ip), the ON time (Ton), and the OFF time (Toff) [313, 316]. The PED was initially developed by researchers to deposit electrocatalysts onto Nafion-coated electrode surfaces in order to decrease Pt catalyst particle sizes and reduce Pt loading [317, 318, 319], and by using this method, different-shaped Pt, PtRu, and PtAg alloy electrocatalysts have been synthesized [320, 321, 322, 323, 324, 325, 326, 327]. In the case of PtAg alloy electrocatalysts, the increased ORR activity of the PtAg alloy deposited on a glass carbon plate can be attributed to the formation of Pt shells through the absence of Ag oxidation/reduction features in its CVs [326, 327], implying that core–shell-structured electrocatalysts can be fabricated through the PED. Based on this, Kulp et al. [328] demonstrated that Pt shells can be directly electrodeposited onto seed surfaces to form core–shell structures by using the PED, in which carbon-supported Au or Ru were used as the seed, and the thickness of the Pt shell can in principle be tuned by the cycles of deposition pulses applied [328]. In addition, and of greater importance, researchers have also found that Pt shells can be controllably electrodeposited over noble metal-free TiNiN cores by using the PED (Fig. 12) [329]. Various core–shell-structured electrocatalysts developed through the PED are listed in Table 7.
Fig. 11

General concept of the pulse-current technique.

Reprinted with permission from Ref. [313]. Copyright 1998 Elsevier Science

Fig. 12

a Schematic of the synthesis of a Pt monolayer on a TiNiN core through pulse deposition; b TEM; and c, d HAADF/STEM images of a single TiNiN@Pt nanoparticle, and EDS elemental mappings of e N (green); f Ni (yellow); g Pt (blue); and h Ti (red) in the same area

(modified from Ref. [329]). Copyright 2016 American Chemical Society

Table 7

Core–shell-structured electrocatalysts prepared by using pulse electrochemical deposition

Core@shell electrocatalyst

PED procedure

Shape/size/thickness of shell

References

Au@Pt

Ru@Pt

− 0.61 V versus Ag/AgCl for Au/C, − 0.55 V versus Ag/AgCl for Ru/C, Ton: 100 ms Toff: 100 ms in 0.1 mm Pt(NO3)2 + 1 mM NaNO3

Sphere/~ 6.4 nm/~ 0.5 nm

[328]

Au@Pt

0.4 VSCE for 2 s, − 0.8 VSCE for 15 s, 0.4 VSCE for 30 s in 0.5 M NaCl + 3 mM K2PtCl4

None/90 nm/1~5 monolayer

[330]

Ru@Pt

A pulsed current of 3 mA cm−2, Ton: 0.0003 s Toff: 0.00015 s in 0.1 M Na2SO4 + 1.25 M C6H5Na3O7 + 0.5 M Pt(NH3)4Cl2

Sphere/5 nm/1 nm

[331]

Ir@Pt

A pulsed current of 2.5 mA cm−2, Ton: 0.03 s Toff: 0.015 s in 0.1 M Na2SO4 + 1.25 M C6H5Na3O7 + 0.5 M Pt(NH3)4Cl2

Sphere/2.3 nm/0.6 nm

[332]

AuPt@Pd

A pulsed current of 2.5 mA cm−2, Ton: 0.03 s Toff: 0.015 s in 0.1 M Na2SO4 + 1.25 M C6H5Na3O7 + 0.5 M PdCl2

Sphere/4.9 nm/1 nm

[315]

TiNiN@Pt

A pulsed current of 2.5 mA cm−2, Ton: 0.03 s Toff: 0.015 s in 0.1 M Na2SO4 + 1.25 M C6H5Na3O7 + 0.5 M Pt(NH3)4Cl2

Sphere + cube/9.2 nm/0.9 nm(2~3 atomic layer)

[329]

Cu@Pt

0.008 V versus RHE for 120 s in 10 mM Pt(NO3)2 + 100 mM NaNO3

Sphere/200 nm/none

[333]

Pd@Pt

A pulsed current of 50 mA cm−2, Ton: 0.3 ms Toff: 0.15 ms in 0.1 M Na2SO4 + 1.25 M C6H5Na3O7 + 0.5 M Pt(NH3)4Cl2

Sphere/6~8 nm/1~2 nm

[334]

Pt@Pt PtPdRu@Pt

A pulsed current of 400 mA cm−2, 400 rpm, Ton: 25 ms, Toff: 50 ms in 2.5 mM H2PtCl6 + 0.5 M H2SO4 25 °C

Sphere/20 nm/none

[335]

Aside from the two electrodeposition procedures discussed so far, other electrodeposition procedures have also been developed to fabricate core–shell-structured electrocatalysts. For example, porous Ni@Pt core–shell nanotube arrays were synthesized by using ZnO as a template through a sequential deposition method with a control current of 0.25 mA cm−2 for 20 min in a solution of 0.01 M Ni (Ac)2 + 0.05 M H3BO3 + 0.05 M NH4Cl and 20 mM H2PtCl6 + 0.5 M NaCl + 2.5 mM C6H5Na3O7. Electrodeposition methods can also be adapted to form different structures such as Pt@Ni@Pt, Ni@Pt@Ni@Pt and Pt@Ni@Pt@Ni@Pt [336, 337], and CVs can be used to synthesize Pt@Pd and Pt@Pd@Pt core–shell-structured electrocatalysts for methanol oxidation [338, 339]. These other electrodeposition methods can be referred to in the review article conducted by Petrii [136].

2.3.2 Merits of Electrodeposition

Merits of electrodeposition can be summarized as follows:
  1. 1.

    Catalyst surfaces prepared by electrodeposition are clean because there are no surfactants or reductants required in this method, and products are easily collected from precipitates.

     
  2. 2.

    Catalyst particles can be grown directly onto current collectors with excellent contact by using electrochemical deposition.

     
  3. 3.

    Electrodeposition, particularly the PED, allows for the precise and reproducible control of shell thicknesses from monolayer to several atomic layers, maximizing Pt utilization.

     
  4. 4.

    The PED allows Pt shells to be grown directly onto core surfaces, providing a facile procedure to obtain high Pt utilization for practical applications.

     

2.3.3 Demerits of Electrodeposition

Demerits of electrodeposition can be summarized as follows:
  1. 1.

    Although Adzic’s group synthesized gram-quantities of Pt monolayer electrocatalysts in one experiment using a new electrochemical cell [298, 299], product yields of electrodeposition are limited as compared with chemical deposition, hindering large-scale applications.

     
  2. 2.

    From the above-listed samples, shells obtained by using electrodeposition are always comprised of a single metal, implying that shell composition tunability is limited. In addition, there is a lack of control for core compositions in electrodeposition because the electrodeposition of shells is often carried out over cores which were synthesized by using chemical methods.

     
  3. 3.

    In terms of the UPD process, the process is complex and requires a large amount of inert gas to protect the catalysts. In addition, for the galvanic replacement reaction, the two metals involved need to possess large differences in electrochemical redox potentials, which restricts possible combinations of metals.

     

2.4 Surface Segregation

2.4.1 Principles and Development of Surface Segregation

Surface segregation is a phenomenon in which one metal component of a bulk alloy assembles spontaneously onto the surface, leading to surface concentration enrichment. Thermodynamically, this is a behavior in which alloys preferentially rearrange into a more steady state through the decrease in surface free energies in response to ambient environments [341, 342, 343]. This behavior also applies to cases involving alloy nanoparticles as well [344, 345, 346]. Therefore, surface segregation has become an effective method to control electrocatalyst surface compositions at the atomic level [340, 347, 348, 349, 350]. For example, the surface composition of Au0.5Pt0.5/C nanoparticles can be manipulated to form either Pt or Au enriched surfaces through controlling heat treatment conditions (Fig. 13), leading to enhanced intrinsic activities [340]. In addition, the high degree of segregation induced by properly applied conditions tends to form core–shell structures such as PtNi@Pt catalysts as illustrated in Fig. 14 [351, 352]. And through surface segregation, numerous core–shell-structured electrocatalysts, such as Pd@Pt, Ru@Pt, Au@Pt, Cu@Pt, Fe@Pt, Co@Pt, and Ni@Pt have been synthesized [353, 354, 355, 356, 357, 358, 359]. These samples can be referred to in Table 1 in the review written by Xu et al. [34]. Surface segregation of alloy nanoparticles can also be affected by many factors, such as NP sizes, temperatures, and adsorbates because these parameters can drastically alter surface energies. More detailed discussions into these induction factors are provided in the review written by Xu et al. [34].
Fig. 13

Schematic models of Au0.5Pt0.5 nanoparticles undergoing surface-energy-driven restructuring by using different thermal treatments. Differential surface energies of Au and Pt in different environments enable adsorbate-driven segregation, which leads to different surface compositions despite all having the same bulk composition.

Reprinted with permission from Ref. [340]. Copyright 2013 American Chemical Society

Fig. 14

Schematic illustration of the surface reconstruction of PtNi nanoparticles by using thermal annealing.

(modified from Ref. [351]). Copyright 2014 American Chemical Society

2.4.2 Merits of Surface Segregation

Merits of surface segregation can be summarized as follows:
  1. 1.

    Surface segregation is an easily controllable and repeatable procedure, making it suitable for the large-scale production of core–shell-structured electrocatalysts.

     
  2. 2.

    Catalyst surfaces formed by surface segregation are clean because no thermal processes are required and surface compositions can be adjusted through tuning synthetic conditions.

     
  3. 3.

    During surface segregation, the degree of order for the catalyst structure is improved, resulting in high catalytic performances [247, 248, 360].

     
  4. 4.

    Cheap transition metals can be easily used as cores in high quantities.

     

2.4.3 Demerits of Surface Segregation

Demerits of surface segregation can be summarized as follows:
  1. 1.

    Surface segregation can only be used for bimetallic systems.

     
  2. 2.

    Surface segregation does not allow for the control of shell shapes and thicknesses from the monolayer to several atomic layers.

     
  3. 3.

    The core formed by surface segregation always contains a certain amount of Pt. In addition, these Pt atoms cannot directly act as active sites for electrocatalysis, resulting in low-Pt utilization.

     
  4. 4.

    Large amounts of reducing gases, such as H2 and CO, are often used as adsorbates, which increase electrocatalyst costs. Furthermore, these reducing gasses are often toxic.

     

2.5 Atom Layer Deposition

2.5.1 Principles and Development of Atom Layer Deposition

Atomic layer deposition (ALD) is a chemical vapor deposition technique based on chemical reactions between precursors, which are isolated into successive self-limiting surface chemical reactions (sequential and self-limited surface reactions are displayed in Fig. 15). During the ALD, precursors are kept separated and sequentially reacted with surface species on substrates in a self-limiting process. A purging step is required following each surface reaction to remove unreacted precursors and by-products. Here, the sequence of self-limiting surface reactions and purging steps constitutes a cycle and through controlling the number of cycles, a highly uniform and conformal film coating can be deposited onto substrate surfaces. This feature is especially attractive for the synthesis of heterostructured materials used in catalysis and nanodevices [362, 363, 364, 365].
Fig. 15

Schematic of an ALD cycle. Step 1: The pulse of reactant A leading to its absorption onto the surface. Step 2: The purge of unreacted precursor A and of the by-products. Step 3: The pulse of reactant B, which reacts with surface species created by precursor A. Step 4: The purge of unreacted precursor B and of the by-products.

Reprinted with permission from Ref. [361]. Copyright 2005 the American Institute of Physics

The ALD has also been explored as an alternative method for the preparation of advanced heterostructured and core–shell-structured electrocatalysts, such as Pd@Pt and Ni@Pt [368, 374, 377, 378, 379, 380]. For example, Cao et al. [366] synthesized uniform Pd@Pt core–shell-structured electrocatalysts using the ALD technique with ODTS self-assembled monolayers as substrates (Fig. 16). Various core–shell-structured electrocatalysts prepared by using the ALD are listed in Table 8 and from these, it can be seen that Pt shells can be coated onto the surface of not only metal/alloy particles but also carbides by using the ALD, and that even multilayer structured TiO2@Al2O3@W@Pt can be fabricated by using the ALD [381]. It should be noted, however, that reports on the synthesis of core–shell-structured electrocatalysts through the ALD are limited, which may be because of the relatively complex processes involved, suggesting that there is much room for development for core–shell-structured electrocatalysts by using the ALD.
Fig. 16

a Schematic illustration of the fabrication of core–shell nanoparticles through AS-ALD on ODTS modified substrates; b HAADF–TEM image of the Pt/Pd core–shell NPs. The sample was oxidized through exposure to gentle N2/O2 plasma to increase Z-contrast and enable core–shell structure observations. The inset image presents the SAED pattern of the sample before oxidation; c TEM-EDX line scan of the Pt/Pd core–shell NP. The inset is the HAADF image.

(modified with permission from Ref. [366]). Copyright 2005 Science

Table 8

Collected samples of core–shell-structured electrocatalysts prepared through the ALD

Core@shell electrocatalyst

Core and shell precursors

Carrier gases

Shape/size/thickness of shell

References

Pd@Pt

Pd(hfac)2/MeCpPtMe3

O2, N2 purge

Sphere/4 nm/1.5 nm

[366, 367]

Pd@Pt

Pd(hfac)2/MeCpPtMe3

H2 plasma/O2, Ar

Sphere/controlled 2~10 nm/controlled thickness

[368]

Ru@Pt

Ru(Cp)2/MeCpPtMe3

O2, N2 purge

Cauliflower/controlled size/controlled thickness

[369]

PtCo@Pt

Pt(acac)2, Co(acac)3

N2

Sphere/1.68 nm/controlled thickness

[370, 371]

Ru@Pt

Ru(EtCp)2/MeCpPtMe3

O2

Sphere/2.7~4.3 nm/varied thickness

[372]

Pt@Ru

MeCpPtMe3/Ru(EtCp)2

O2

Sphere/1.5~6 nm/varied thickness

[373]

Ni@Pt

Me3(MeCp)Pt/Ni(NO3)2 6H2O

O2, H2

Particle/1~2.4 nm/varied thickness

[374]

Ru@Pt

Ru(EtCp)2/MeCpPtMe3

Ar/O2 plasma, N2 purge

Ribbon/2~3 nm in width/1.2 nm

[375]

WC@Pt

MeCpPtMe3

O2, N2

Particle/controlled size/controlled thickness

[376]

Pd(hfac)2, Pd(II)hexafluoroacetylacetonate; Ru(Cp)2, bis(cyclopentadienyl) ruthenium; Pd(hfac)2, palladium hexafluoroacetylacetonate; MeCpPtMe3, trimethyl(methylcyclopentadienyl) Pt(IV); Ru(EtCp)2, bis(ethylcyclopentadienyl) Ru(II)

2.5.2 Merits of Atom Layer Deposition

Merits of atom layer deposition can be summarized as follows:
  1. 1.

    The ALD process can allow for the precise control of thicknesses at the Ångstrom or monolayer level, and the self-limiting aspect of the ALD can lead to excellent step coverage and conformal deposition on high aspect ratio structures.

     
  2. 2.

    The ALD process can allow for the precise control of catalyst loading and composition through the control of cycle numbers involving self-limiting surface reactions and purging steps.

     
  3. 3.

    The ALD process can allow for the coating of Pt and other precious metals onto the surface of oxides and carbides, which can possibly lead to noble metal-free core components of electrocatalysts, further decreasing Pt loading.

     
  4. 4.

    The ALD process is also extendible to highly complex core–shell structures.

     

2.5.3 Demerits of Atom Layer Deposition

Demerits of atom layer deposition can be summarized as follows:
  1. 1.

    The ALD process depends on the special apparatus and is not generally designed for powder preparation. Furthermore, reaction chamber sizes limit ALD product output.

     
  2. 2.

    The ALD process requires organic metal compounds as precursors which are expensive.

     
  3. 3.

    The ALD process requires a large quantity of ultra-high purity gas, which further increases cost.

     
  4. 4.

    The ALD process in the fabrication of core–shell-structured electrocatalysts often involves relatively complex processes.

     

2.6 Physical Deposition

Sputtering deposition is a physical phenomenon in which target atoms are removed from a target plate (or cathode) after being bombarded by energetic ions generated by a glow discharge plasma situated in front of the target. Subsequently, these targeted atoms can condense onto a substrate to form a thin film [391]. The sputtering deposition process has been modified and developed into various types of sputtering techniques; such as ion-beam sputtering deposition, and has been proven to be the most important method in the preparation of various types of films [392, 393]. This technology has also been successfully applied to the preparation of one-layer or multilayer film electrocatalysts such as Pt, PtRu, PtFe, PtCo, and PtNi [394, 395, 396, 397]. Bimetallic films can also be prepared by using successive sputtering techniques, and core–shell-structured electrocatalysts can be formed in the case in which one layer of deposited metal is covered by a subsequent layer of deposited metal. Room temperature ionic liquids (RTIL) can also be introduced into sputtering deposition procedures to control metal nanoparticle sizes and morphologies, and this process is called the RTIL/metal sputtering technique [398, 399, 400]. And by using this technique with the ionic liquid of 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (HyEMI-BF4), uniform Au@Pt electrocatalysts for ORR can be prepared (Fig. 17) [382]. Many other core–shell-structured electrocatalysts such as Pd@Pt and CrN@Pt have also been prepared by using sputtering deposition and are listed in Table 9.
Fig. 17

(Left) Schematic illustration of Pt deposition on individual Au particles floating on an HyEMI-BF4 surface; (right) TEM images of the original Au particle monolayer film a and Au–Pt bimetallic particle films with Pt sputtering times of 5 b, 20 c, 40 d min.

(modified with permission from Ref. [382]). Copyright 2016 American Chemical Society

Table 9

Collected samples of core–shell-structured electrocatalysts prepared by using sputtering deposition

Core@shell electrocatalyst

Sputtering deposition method

Shape/size/thickness of shell

References

Pd@Pt

Magnetron-ion-beam sputtering deposition

Sphere/1~2/monolayer

[383]

Pd@Pt

Direct-current magnetron sputtering deposition

Particle/none/none

[384, 385]

Ru@Pt

Electron-beam sputtering deposition

Cluster/1.9 nm/0.2 nm

[386]

CrN@Pt

Glancing angle deposition in a load-locked magnetron sputter deposition system

Vertically oriented particle/100~200 nm/none

[387]

Ni@PtNi

Glancing angle deposition in a load-locked magnetron sputter deposition system

Nanorod/diameter: 20~50 nm/none

[388]

Au@Pt

RTIL/metal sputtering technique

Sphere/4.2~4.8 nm/monolayer

[382]

Ni@Pt

RTIL/metal sputtering technique

Sphere/2.0 nm/monolayer

[389]

Pt@Pd

Metal sputtering technique

Sphere/none/none

[390]

HOPG, highly ordered pyrolytic graphite

2.6.1 Merits of Physical Deposition

Merits of physical deposition can be summarized as follows:
  1. 1.

    Sputtering deposition is a controlled procedure, making it suitable for the large volume production of core–shell-structured electrocatalysts.

     
  2. 2.

    Sputtering deposition can precisely adjust shell thicknesses to form monolayers through the tuning of applied conditions.

     
  3. 3.

    Sputtering deposition can allow for the easy introduction and maintenance of cheap transition metals into the core of core–shell-structured electrocatalysts.

     

2.6.2 Demerits of Physical Deposition

  1. 1.

    Physical deposition is dependent on the special apparatus and is generally not designed for nanoparticle preparations.

     
  2. 2.

    Physical deposition can result in low precious metal utilization for the resulting electrocatalyst.

     
Based on the above discussions, it is obvious that there is no perfect preparation method for core–shell-structured catalysts. Some researchers have, however, combined two methods together to prepare core–shell-structured electrocatalysts, which offers inspirational strategies for the rational design of precise structures, including desirable morphologies, adjustable compositions, and controllable sizes. For example, Wang and Yamauchi [401] prepared core–shell Pd@Pt nanocages that allowed for the effective usage of interior catalytic sites in the hollow nanoparticles by using a combination of one-step wet chemical deposition followed by dealloying (Fig. 18). In another example, Shao et al. [402] synthesized a Pt monolayer on porous PdCu core catalyst for ORRs by using a combination of electrochemical dealloying followed by the displacement method. These examples prove that synthesis strategies that combine methods can diversify catalyst structures, favoring great improvements in catalytic activities.
Fig. 18

a Schematic illustration of the formation of core–shell-structured Pd@Pt nanocages. HAADF–STEM images, elemental mapping images and compositional line profiles for bd Dendritic Pt-on-Pd nanoparticles before chemical etching; and eg Dentridic nanocages after chemical etching.

(modified with permission from Ref. [401]). Copyright 2013 American Chemical Society

3 Core–Shell-Structured Catalysts with Monometallic and Alloy Cores

In the previous sections, different synthesis methods for core–shell-structured electrocatalysts along with corresponding strengths and weaknesses were discussed. As presented in Tables 1, 2, 3, 4, 5, 6, 7, 8 and 9, these catalysts possess different compositions, sizes, and morphologies, and therefore, exhibit different catalytic activities for ORRs and fuel oxidation including hydrogen oxidation and organic small molecule oxidation. In the following sections, the catalytic performance of core–shell-structured Pt-based electrocatalysts will be discussed. Currently, the cores of most core–shell-structured electrocatalysts are mainly composed of 3d, 4d and 5d metals (Fig. 19) and noble metals around Pt, including Ru, Rh, Pd, Ag, Os, Ir, and Au have been composited with Pt to form core–shell structures to catalyze electrochemical reactions of fuel cells. Among these, Ru, Pd, and Au have been especially promising because of their special performances. Therefore, these three metals will be first reviewed as cores for Pt shells. Following this, non-noble metals such as Cu, Fe, Co and Ni will also be reviewed with discussions on their electrochemical performances.
Fig. 19

Examples of 3d, 4d and 5d metals and their electronegativities (top left corner) and atomic radii (lower right corner, in nanometers), plotted based on data presented in Refs. [403, 404]

3.1 Pd, Ru, and Au as Cores

3.1.1 Pd as Core

Pd is a desirable base core used to form core–shell-structured electrocatalysts because of the following features: (1) Pd is much cheaper than Pt (the current price of Pd is only 40% that of Pt), reducing the cost of Pt catalysts by using Pd cores; (2) Pd shares a similar face-centered cubic (fcc) structure and an almost identical lattice constant (with a mismatch of only 0.77%) with Pt, allowing for the epitaxial growth of Pt atoms over Pd cores, leading to core–shell structures; and (3) the catalytic activity of pure Pd is almost on a par with pure Pt for electrocatalysis. And because of these features, Pd cores have been extensively developed to form core–shell-structured electrocatalysts with Pt and in the last couple of decades, researchers have conducted the controlled synthesis of core–shell-structured Pd@Pt electrocatalysts with a full variety of compositions and shapes (Tables 1, 2, 3, 4, 5, 6, 7, 8, 9).

These Pd@Pt electrocatalysts also exhibit better catalytic activities for ORRs and small organic molecule oxidation, including methanol, ethanol and formic acid, than pure Pd or Pt electrocatalysts [41, 367, 405]. And in particular, Pd@Pt electrocatalysts possess the best ORR catalytic activity among M@Pt electrocatalysts (M = Pd, Ru, Ir, Rh and Au) according to calculations and experimental results (Fig. 20) [403, 406, 407, 408], and in the past few years, an enormous amount of studies related to Pd@Pt core–shell-structured catalysts for ORR have been conducted, and great efforts have been made to improve catalytic activities through controlling shell thicknesses, core sizes or morphologies. In terms of shell thickness, a monolayer is in theory ideal and has been elaborately achieved through the use of different methods including the UPD, the ALD and the sputtering deposition, exhibiting better catalytic activities for ORRs than pure Pt due to strain and ligand effects [45, 59, 65, 66, 299, 301, 304, 334]. However, experimental results have indicated that optimum Pt thicknesses involve 2~3 atomic layers, rather than the monolayer for catalytic performances [297, 299, 409, 410]. For example, a Pd@Pt catalyst with a two atomic layer shell was found to exhibit a higher mass activity normalized by the mass of Pt and Pt-group metals (MAP, MAPGM) and a higher specific activity (SA) than catalysts with a monolayer shell [297, 409]. And as for Pd@Pt icosahedra (Fig. 21), the highest activity and best durability for ORR were achieved with a Pt shell thickness of 2.7 atomic over-layers [411].
Fig. 20

Polarization curves for O2 reduction on Pt monolayers (PtML) on Ru (0001), Ir (111), Rh (111), Au (111), and Pd (111) in a 0.1 m HClO4 solution on a disk electrode. The curve for Pt (111) was obtained from Ref. [21] and included for comparison. The rotational rate was 1600 rpm, and the sweep rate was 20 mV s−1 (50 mV s−1 for Pt (111)); j represents the current density and RHE represents the reversible hydrogen electrode.

Reprinted with permission from Ref. [406]. Copyright 2005 Wiley

Fig. 21

a HAADF–STEM image. The Scale bar, 20 nm; b The energy-dispersive X-ray spectroscopy mapping of Pd and Pt of the two Pd@Pt2.7L icosahedra, confirming a core–shell structure; c Atomic-resolution HAADF–STEM image taken from the edges, revealing the detailed arrangement of Pd and Pt atoms (green dots: Pd atoms; red dots: Pt atoms). The Scale bar, 1 nm; d ORR polarization; e, f Specific and mass ORR activities given as kinetic current densities (jk) normalized to the ECSAs and Pt masses of the catalysts, respectively. The color scheme in (d) applies to all other panels; g The ORR mass activities at 0.9 V versus RHE for the Pd@Pt2.7L catalyst before and after accelerated tests.

(modified with permission from [366]). Copyright 2015 Nature

As for Pd@Pt particles, studies have found that particle sizes influence strain effects, which in turn affect oxygen binding energies (BE-O) on surface metals, altering intrinsic ORR activities [41, 270, 297, 412]. As illustrated in Fig. 22, an optimal BE-O of 3.89 eV on a Pt (111) surface can be achieved with Pd@Pt particles in the 3–6 nm size range [41]. In addition, based on Fig. 22, the large difference in N111/Ns between the two particle shapes suggests the potential of enhancing ORR activities through shape- and morphology-controlled synthesis. This is because low-coordination sites tend to oxidize and dissolve, assuring a smooth and (111)-orientation-rich Pt surface that can simultaneously enhance the activity and stability of the ORR catalyst. And based on this, a series of Pd@Pt core–shell-structured electrocatalysts possessing different morphologies such as Pd@Pt nanowires [282], concave Pd@Pt polyhedra [145], cube-like Pd@ multi-armed Pts [57] and concave Pd@Pt decahedra [37] have also been synthesized to improve ORR catalytic performances. In addition, cage-like Pd@Pt core–shell icosahedra with (111) facets were synthesized by using a two-step seed-growth method and demonstrated much better ORR activities that were 10 times higher for specific activity and 7 times higher for mass activity than commercial Pt/C [197]. However, there was no distinct morphology that was significantly better than the rest for ORRs in the comparisons of performance for different Pd@Pt core–shell-structured catalyst morphologies (Fig. 23). This is because multiple factors, including shape, size, and composition, exist in catalysts and work together to affect catalytic performances.
Fig. 22

ORR specific activities at 0.9 V measured at 10 mV s−1 (symbols, left axis) and surface fractions of atoms on the (111) facet (solid and dashed lines, right axis) as a function of particle size. The vertically dotted lines mark the size of the three nanoparticle models that have BE-O close to the optimal value of 3.89 eV. The blue, red, and green colors represent particles with 1-, 2-, and 3-Pt layers, respectively; (N111/Ns): the calculated surface fraction of atoms on the (111) facet of the icosahedral and cuboctahedral particles [297]

Fig. 23

Comparison of the catalytic activities of core–shell-structured Pd@Pt catalysts for ORR. This figure was plotted based on data presented in Refs. [37, 45, 55, 57, 62, 66, 197, 411] corresponding to the position from left to right in the figure

3.1.2 Ru as Core

Ru has also been extensively studied because PtRu alloys possess the best overall catalytic performance among bimetallic electrocatalysts for MORs through bi-functional mechanisms and ligand effects [20, 413, 414]. In addition, Ru is much cheaper than Pt, allowing the addition of Ru into Pt catalysts to reduce costs. However, because Ru possesses a lower oxidation potential (0.68 V vs. NHE) than Pt, Ru dissolution in PtRu alloys in fuel cell systems, especially at higher potentials experienced during startup and shutdown, is a serious issue. Ru@Pt core–shell structures, on the other hand, do not appear to have this issue, in which the advanced architecture of the Ru@Pt core–shell structure appears to not only protect Ru cores from dissolving, but also further decreases Pt loading compared with PtRu alloys. Recent experimental and DFT results have demonstrated that Pt monolayers coated on Ru possess substantially enhanced catalytic activities for CO oxidation through a novel pathway of weakly bonded Pt–(OH)ads on the Pt monolayer/Ru nanomaterial originating from compressive strains resulting from the apparent lattice mismatch (2.55%) between Pt and Ru [35, 111, 132, 177, 415, 416, 417]. And because of these reported performance enhancements, many types of Ru@Pt core–shell-structured electrocatalysts have been synthesized by using different procedures by researchers and have been reported to produce enhanced catalytic activities for MORs [93, 95, 113, 176, 305, 328, 418, 419, 420, 421, 422, 423, 424]. For example, a Ru@Pt core–shell catalyst possessing a 3 nm core and a 3~4 atom layer Pt shell was prepared by using the PED (Fig. 24) by Chen et al. [331] which exhibited a higher catalytic activity and stability for MORs than PtRu alloy catalysts and Pt catalysts.
Fig. 24

a HRTEM and b HAADF–STEM images of Ru@Pt/C nanoparticles; c Pt + Ru signal profiles obtained from (b); d original cyclic voltammograms in a solution of 0.1 M HClO4 and 1 M CH3OH, at room temperature with a scanning rate of 50 mV s−1; e mass activities of Ru@Pt/C, PtRu/C-D, Pt/C-P, and JM-Pt/C toward the anodic oxidation of methanol; the shaded bars are calculated based on the forward peak current density and the unshaded bars are based on the backward peak current density; f Relationship between the peak current of the anodic oxidation of methanol and the cycling numbers for Ru@Pt/C and JM-Pt/C. All data were recorded in a solution of 0.1 M HClO4 and 1 M CH3OH at room temperature with a scanning rate of 50 mV s−1.

(modified with permission from [331]). Copyright 2015 Science

Analogous to the particle-size-dependent electronic effects reported in previous papers [425, 426, 427], the size of Ru cores can induce electronic modifications of Pt shells, affecting the electrochemical CO desorption of Ru@Pt core–shell-structured electrocatalysts. For example, Goto et al. [95] reported that an Ru core 1.9 nm in size affected the lower-lying d states of weaker Pt–CO bonds on Pt shells, leading to CO desorption at less positive potentials compared with a 1.3 nm Ru core. This result suggests that the optimal size for Ru cores of core–shell Ru@Pt electrocatalysts for MORs is approximately 2.0 nm. The thickness of the Pt shell over Ru cores is also an important factor, in which Chen et al. [92] reported that the optimum thickness of Pt shells over a 1.6 nm Ru core is 1.5 atomic layers in Ru@Pt core–shell-structured electrocatalysts for MORs, and Xie et al. [108] reported that three atomic layers are the optimum thickness for Pt shells over a 3 nm Ru core. Here, the difference between the two reports is a result of the structure and size of the different Ru cores. This suggests that the structure of Ru cores can influence the catalytic performance of Ru@Pt core–shell-structured catalysts. Therefore, intrinsic effects of Ru core structures on Pt shells need to be systemically studied in the future.

3.1.3 Au as Core

Compared with Pd and Ru, the price of Au is much more expensive and is comparable to that of Pt, therefore the use of Au as a core will not significantly reduce Pt catalyst costs. In addition, the Au atomic radius is larger than that of Pt and if a Pt monolayer was deposited onto an Au core, tensile strains can arise and in the case of ORRs, the binding of oxygen atoms or oxygen-containing fragments on Au (111)@Pt surfaces becomes stronger than on pure Pt surfaces. This in turn can lead to the hindrance of oxygen atom or OH hydrogenation kinetics, in which slow O or OH hydrogenation rates can cause the build-up of surface O or OH and the blocking of O2 adsorption, dissociation, or hydrogenation sites, which is not favorable to ORR on Au (111)@Pt surfaces. And because of this, Au (111)@Pt should be less active than Pt (111) for ORRs (Fig. 20) [406]. However, in contrast to this, various reports have stated that Au@Pt core–shell-structured electrocatalysts can exhibit comparable or higher ORR catalytic activities than pure Pt, attributing the enhanced performance to ligand and ensemble effects [117, 284, 285, 287, 330, 429, 430]. In addition, it has also been reported the catalytic performance of core–shell Au@Pt was similar to pure Pt if Pt shells were Open image in new window 3 atomic layers thick due to the disappearance of ligand and ensemble effects. Au also possesses a higher oxidation potential (1.48 V vs. NHE) than Pt, making Au more stable in acidic media. And because of this, if Pt shells are attached to Au, the surface redox properties of the Pt shells are changed, improving stability [431]. For example, Zhang et al. [428] reported an Au/Pt/C electrode which displayed no apparent loss of ECSA and only 5 mV degradation of ORR half-wave potential (E1/2) after cycling between 0.6 and 1.1 V (vs. RHE) for 30,000 cycles in HClO4 (Fig. 25), whereas a Pt/C electrode displayed a 45% ECSA loss and a 39 mV negative shift of ORR E1/2. In this study, as evidenced by XANES, the greatly improved durability of the Au/Pt/C catalyst was attributed to the increased oxidation potential of Pt in the presence of Au clusters. Furthermore, from the reports [117, 284, 285, 287, 330, 429, 430], it can be seen that in cases of 1~2 atomic layer thick Pt shells, Au cores are a viable alternative to fabricate core–shell-structured ORR electrocatalysts with high stability in acidic media.
Fig. 25

Polarization curves for the O2 reduction reaction on a Au/Pt/C and c Pt/C catalysts on a rotating-disk electrode, before and after 30,000 potential cycles; the sweep rate was 10 mV s−1 and the rotation rate was 1600 rpm. Voltammetry curves for b Au/Pt/C; and d Pt/C catalysts before and after 30,000 cycles; the sweep rates were 50 and 20 mV s−1, respectively. The potential cycles ranged from 0.6 to 1.1 V in an O2-saturated 0.1 m HClO4 solution at room temperature. For all electrodes, the Pt loading was 1.95 mg (or 10 nmol) on a 0.164 cm2 glassy carbon rotating-disk electrode. The shaded area in (d) indicates the lost Pt area.

Reprinted with permission from Ref. [428]. Copyright 2007 Science

Another outstanding advantage of AuPt bimetallic nanoparticles with alloy or core–shell structures is the significantly higher catalytic activities for CO oxidation relative to pure Pt and Au nanoparticles, suggesting that AuPt bimetallic nanoparticles are an excellent candidate for low-temperature anode electrocatalysts [110, 283, 432]. As stated previously, Au possesses higher electronegativity than Pt and this contact between Pt and Au atoms results in subtle net charge transfers from Pt to Au [433, 434]. This subtle net charge transfer can increase the d-band vacancy of Pt sites, subsequently weakening the binding of CO on Pt sites, facilitating the removal of intermediate CO-like species and promote the anti-poisoning abilities of Pt catalysts in MORs [435, 436]. As a result of this, some Au@Pt core–shell-structured catalysts have been shown to demonstrate much higher catalytic activities for MORs than Pt catalysts [88, 340, 437, 438, 439, 440]. Overall, Au@Pt core–shell-structured catalysts can exhibit much higher catalytic activities for formic acidic oxidation as compared with Pt catalysts because of their high CO tolerance [286, 441, 442].

In order to take full advantage of each component, alloy nanoparticles composed of precious metal alloys discussed above have been used as cores as well. For example, Zhang et al. [443] reported that the ORR activities of PdAu@Pt can be linearly tuned through varying the alloy core composition and that optimal activities can be achieved with a 28% Au/72% Pd alloy core supporting a Pt shell. Alloy cores composed of other precious metals have also been reported, such as PdAu@Pt [288] and PdPt@Pt [148, 150, 151, 444].

In addition to the precious metals discussed here, other precious metals, such as Rh, Ag, Os, and Ir, have also been used as the cores to synthesize core–shell catalysts to catalyze electrochemical reactions in fuel cells [101, 179, 295, 445, 446, 447, 448, 449] and their promotion mechanisms toward Pt catalysts are similar to those of Pd, Ru, and Au.

3.1.4 Merits and Demerits of Pd, Ru, and Au Cores

Pd, Ru, and Au cores possess obvious merits and each corresponding core–shell-structured catalyst provides important properties, such as (1) Pd@Pt has the highest ORR catalytic activity among bimetallic systems; (2) Ru@Pt has the high CO tolerance; and (3) Au@Pt has the high CO tolerance and superior stability. Demerits for Pd, Ru, and Au cores include the fact that they are precious metals and are expensive with Au being sometimes more expensive than Pt. Furthermore, the reserves of these precious metals are limited and from a long-term perspective, the decrease in the usage of these precious metals is inevitable. And because of this, non-noble metal cores have attracted increasing attention from researchers and in the next section, non-noble metal cores of Cu, Fe, Co, and Ni will be discussed.

3.2 Non-noble Monometallic Cores

3.2.1 Cu as Core

Cu is a relatively stable non-noble metal in non-oxidizing acid solutions and various studies have stated that Cu@Pt core–shell-structured electrocatalysts can exhibit more favorable ORR reaction kinetics than Pt catalysts (Fig. 26) [125, 333, 450]. For example, in a study by Sarkar and Manthiram [124], a volcano-type plot for the ORR mass activity of Cu@Pt versus nominal Cu content was presented in which at the low initial nominal Cu content ( Open image in new window  60%), the Pt mass activity of Cu@Pt was lower than that of commercial Pt catalysts, and that the surface area specific activities of Cu@Pt demonstrated a linear increase with increasing nominal Cu content, which can be attributed to the electronic modification of the shell by the inner core. Here, the experimental results indicate that the optimized Cu@Pt core–shell electrocatalyst possessed a higher catalytic activity than Pt catalysts with DFT results indicating that Cu@Pt particles can display favorable properties for O2 dissociation comparable to pure Pt systems in the initial stages of O2 chemisorption due to the significant distortion of the (111) facet. However, in this study, there was little difference between pure Pt and Cu@Pt particles in terms of the Pt d-band center, implying that Cu@Pt particles are also likely to over-bind oxygen species, inhibiting later stages of the ORR [403]. And based on the above report, the increase in Pt catalysts by using pure Cu cores is uncertain and perhaps, this is the reason studies related to Cu@Pt electrocatalysts are limited. Therefore, the effects of Cu cores need to be further studied, because Cu is significantly cheaper than Pt, translating to formidable cost reductions through the usage of Cu@Pt electrocatalysts.
Fig. 26

a Schematic representation of the synthesis and dispersion method of novel core–shell nanoparticles; b TEM image of well dispersed Cu50–Pt18 particles on Vulcan XC-72; c Magnified display of the core–shell features of supported nanoparticles and the inset shows a magnified single core–shell nanoparticle with its core and shell dimensions; d Comparative cyclic voltammograms at 100 mV s−1 in a desecrated 0.5 M H2SO4 electrolyte with an Hg/Hg2SO4 reference electrode (MSE) and a Pt wire counter electrode; e Comparative LSVs at 900 rpm with O2 flow at a 10 mV s−1 scan rate.

(modified with permission from Ref. [450]). Copyright 2011 The Royal Society of Chemistry

3.2.2 Ni as Core

Theoretically, Froemming and Henkelman predicted in 2009 [451] by adopting a genetic algorithm with density functional theory that Ni@Pt core–shell nanoparticles can be effective ORR catalysts because of the reduction of oxygen binding energies through compressive strain and electron effects. Yang et al. [452] also suggested that icosahedron (Ih) Ni@Pt12 core–shell nanoparticles can serve as good ORR catalysts because of the enhancement of O2 adsorption, diffusion, and dissociation, as well as the adsorption and diffusion of atomic O as a result of the compressive strain and electron effects. These theoretical results were also verified by experimental results related to core–shell-structured Ni@Pt electrocatalysts for ORRs in various studies [190, 453, 454, 455, 456, 457, 458, 459] in which the maximum catalytic activity and stability of Ni@Pt for ORRs was reported by Ramos-Sanchez et al. [460]. Here, optimal performances were achieved by using a monolayer of Pt, whereas shell thicknesses of 2~3 atomic layers lead to behaviors which were almost identical to pure Pt nanoparticles. Chen et al. [194] also reported that optimal ORR activity and stability under continuous potential cycling of Ni@Pt core–shell electrocatalysts occurred with a Pt monolayer [194]. All of these reports suggest that a monolayer Pt shell is the optimal thickness for core–shell-structured Ni@Pt ORR electrocatalysts.

In addition, CH3OH chemisorption and CO intermediate adsorption are also predicted to be suppressed on platinized Ni catalysts due to strain effects [461, 462], implying that core–shell Ni@Pt electrocatalysts possess good catalytic activities for small organic molecules such as methanol, ethanol and formic acid [336, 463, 464, 465, 466, 467, 468]. For example, Yuan et al. [464] synthesized core–shell Ni@Pt nanoparticles through chemical reduction for use as an electrocatalyst for methanol oxidation which exhibited a 40 mV negative shift in onset potential, a 5.6 times better specific activity, and a higher forward anodic peak current density (If) to reverse anodic peak current density (Ib) (If/Ib, 1.57) than Pt/C catalysts. Moreover, studies have reported that further enhancements in MOR catalytic activities can be achieved if the shape of core–shell-structured Ni@Pt was modified into nanotube arrays [336, 337]. However, despite these promising findings, information related to long-term stability during MORs in the above reports is absent, indicating that more studies focused on this aspect is required in the future.

3.2.3 Co as Core

Similar to Ni, Co cores also appear to be promising due to desirable Co properties such as its low cost, its small atomic radius leading to compressive strain effects, its small electronegativity resulting in charge transfers from Co to Pt. For example, Lin et al. [469] synthesized Co@Pt electrocatalysts for ORRs and obtained a mass activity of 50.3 mA mg Pt −1 at 0.9 V versus RHE, which is 1.43 times that of commercial Pt/C (JM) catalysts. Other groups have also reported the good catalytic activity of core–shell-structured Co@Pt electrocatalysts for ORRs [455, 470] and Kettner et al. [471] attributed this activity to the decrease in oxygen binding energies mainly originating from electronic effects [471]. However, there is a gap for the catalytic activity of Co@Pt in these reports in which the size of Co cores influences the continuity of Pt shells. Here, “large” Co cores (> 10 nm) tend to cause aggregated Pt depositions (e.g., non-continuous 3D Pt structures), whereas “small” Co cores (< 6 nm) tend to provide layer-by-layer growth, implying that the catalytic activity of Co@Pt differs with differing core sizes. And because non-continuous Pt shells lead to the quick corrosion of Co in 0.3 M H2SO4 [472], the thickness of the shell greatly affects the catalytic activity of core–shell Co@Pt samples, with the monolayer Pt shell over Co core yielding the highest ORR activity [472].

Besides good ORR catalytic activities, core–shell-structured Co@Pt can also exhibit good catalytic activities for MORs [93, 461, 473, 474]. And to further improve Co@Pt catalytic activities for MORs, Li et al. [191] synthesized porous Pt shells over Co nanochains, and obtained higher activities than Pt catalysts, including a mass activity of 234 mA mg Pt −1 , high CO tolerance (1.36 of If/Ib) and only a 20.2% loss of Pt electrochemical surface area after 2000 cycles of CVs. In another study by Zhang et al. [189], the researchers optimized Pt shell thicknesses, and obtained catalytic activities for Co@Pt/C that outperformed PtRu/C catalysts. In addition, the high CO tolerance of core–shell-structured Co@Pt also makes it a good catalyst for ethanol oxidation [475].

3.2.4 Fe as Core

DFT theoretical calculations conducted by Yang et al. [476] demonstrated that the use Fe as a core to form D5h Fe@Pt12 core–shell nanoparticles is an attractive and viable option that can enhance both structural stability and catalytic activity. Jennings et al. [403] in their study also predicted that Fe@Pt38 core–shell particles were good catalysts for ORRs through low O2 dissociation barriers on its surface. However, there have been few studies into the synthesizing of core–shell-structured Fe@Pt catalysts in literature. In one example, Jang et al. [193] synthesized Fe@Pt nanoparticles with various shell thicknesses using one-step sonochemical reactions. Here, experimental and theoretical data showed that the resulting PtFe1.2/C with the highest Fe content and a monolayer Pt shell provided the highest ORR activity. In another example, Matin et al. [153] prepared a core–shell-structured Fe@PtRu catalyst using one-step sonochemical reactions that demonstrated enhanced MOR electrocatalytic activities by a factor of up to 2.5 based on If and improved stability as compared with a commercial PtRu-alloy (PtRu) catalyst, particularly in the complete suppression of CO-poisoning. As shown in Fig. 19, the electronegativity of Fe is smaller than Co and Ni, implying that Fe tends to be oxidized. Therefore, pure Fe cores are difficult to retain during the successive multi-step synthesis and in practice, the use of pure Fe as cores to synthesize core–shell-structured catalysts is rare.

3.2.5 Merits and Demerits of Non-noble Monometallic Cores

Cu, Ni, Co and Fe cores can effectively reduce the cost of Pt catalysts. However, there are some issues as follows:
  1. 1.

    Uniform Pt shells are difficult to form on the surface of these metals due to the large mismatch between Pt and these 3d metals, resulting in small clusters of Pt shells. And because of this, parts of the Pt atoms will exhibit similar characteristics to pure Pt, which limits Pt catalytic activity. In addition, non-continuous Pt shells will lead to the corrosion of the 3d metal cores [462, 464, 472].

     
  2. 2.

    Because of the small electronegativity of these metals, segregation on the surface becomes favorable if the catalysts are exposed to ambient conditions, affecting the stability of the Pt shell structures. For example, the calculated adsorption energies and relative stabilities predict a magnitude of up to 75% Cu migration to the topmost surface layer [477]. And in the case of Ni, if the concentration of Ni in core–shell Ni@Pt structures is increased, Ni tends to appear on the surface due to its oxyphilic nature [478].

     
  3. 3.

    The large mismatch between Pt and these 3d metals can result in strong compressive strain, which leads to serious distortions of Pt lattices, resulting in decreased stability of Pt skin structures [147, 194].

     

3.3 The Various Alloy Cores Comprised of 3d Metals with 4d or 5d Metals

The above discussions indicate that precious metal cores and non-precious metal cores both possess advantages and disadvantages. Therefore, combining precious metals with non-precious metals into a single core is a logical method to overcome individual disadvantages by using individual advantages. And based on this strategy, many types of alloy nanoparticles composed of 3d metals with 4d or 5d metals have been synthesized and used as cores to fabricate core–shell-structured electrocatalysts, and among these, Pt, or Pd-based cores have been intensely investigated.

3.3.1 PtNi Alloy as Core

Core–shell-structured PtNi@Pt catalysts have been extensively studied because of its predicted high ORR catalytic activity among Pt3M@Pt (M = Ni, Co, and Fe) catalysts. Here, di Paola et al. [478] and Stamentovic et al. [479] were the first to study core–shell-structured PtNi@Pt using DFT methods to set forth its development; and based on these two studies, various methods have been developed for the synthesize of PtNi@Pt core–shell structures [182, 202, 204, 218, 230, 231, 233, 234, 251, 480, 481]. In addition, various factors affecting catalytic performance such as composition, size, morphology, and shell thickness have also been thoroughly studied. For example, Gan et al. [229] prepared three PtNi@Pt catalysts through the electrochemical dealloying of PtNi, PtNi3, and PtNi5 precursors. And after dealloying, the resulting dealloyed PtNi catalyst possessed a simple core–shell structure with a 0.5 ± 0.2-nm-thick shell and a maximum Ni composition of 34 ± 7% at the particle center, whereas the dealloyed PtNi3 catalyst possessed a thicker 0.8 ± 0.2 nm shell and a higher maximum Ni composition of 47 ± 11% at the formed Ni-enriched inner shell, and the dealloyed PtNi5 catalyst possessed a 0.4 ± 0.2 nm-thick shell and a Ni composition of 40 ± 11% at the formed Ni-enriched inner shell (Fig. 27). In this study, physical characterizations indicated that initial Ni content can affect the amount of Ni dissolution during electrochemical dealloying in which strong tendencies of near-surface spherical enrichment of Ni in D-PtNi3 and D-PtNi5 catalysts can lead to the formation of Ni-enriched inner shells. In addition, electrochemical data indicated that dealloyed PtNi3 possessed the highest activity for ORRs among the three catalysts, strongly suggesting that compositional Ni distributions below Pt shells (up to ~ 10 atomic layers) play an important role in catalytic activities [229]. In another study, Carpenter et al. [232] studied core–shell-structured PtNi@Pt catalyst performances and indicated that PtNi@Pt with a 1.7:1 ratio of Pt–Ni displayed the maximum catalytic activity (0.81 A mg Pt −1 , 2.27 mA cm Pt −2 at 0.9 V vs. RHE) among five different Pt–Ni atomic ratios of 0.6:1, 0.9:1, 1.7:1, 2.1:1 and 3.2:1. Other optimal results such as Pt73Ni27 [233], Pt2Ni [161], Pt26Ni74 [234], Pt1Ni1 [202], have also been reported, but from these results, the most suitable composition appears to vary from study to study.
Fig. 27

a High-resolution HAADF–STEM images and EELS compositional line profiles of individual nanoparticles of D-PtNi(left), D-PtNi3 (middle), and D-PtNi5 (right); b Structural models of distinctly different compositional core–shell fine structures of dealloyed PtxNi1−x catalysts; c Polarization curves of ORRs by using linear scanning voltammetry from 0.06 to 1.0 V at 5 mV s−1 in O2-saturated 0.1 M HClO4 aqueous solution; d Comparison of mass activities and specific activities at 0.9 V.

(modified with permission from Ref. [229]). Copyright 2012 The American Society of Chemistry

For core–shell-structured PtNi@Pt catalysts, the thickness of the Pt shell is another major concern. In a study by Gan et al. [229], the researchers found that optimal catalytic activities can be obtained in dealloyed PtNi3 catalysts that possess 3~5 atomic layers of Pt shells. This was further verified by Wang et al. [233] who adjusted the optimal particle size (< 5 nm) of a series of dealloyed PtxNi1−x alloys for catalytic performances and found that dealloyed PtNi catalysts with Pt shells of ~ 0.5 nm in thickness (2~3 atomic layers) demonstrated the best catalytic activity for ORRs. In addition the same authors also demonstrated in a further study [482] that PtNi@Pt with 3 atomic layers of Pt shells produced higher specific activities than catalysts with 1, 5 and 7 atomic layers of Pt shells. And based on these results, it appears that there is fixed value for optimal Pt shell thicknesses and that this value is based on the different compositions of PtNi cores used in each study. Studies have also shown that the thickness of Pt-rich layers are dependent on initial particle compositions, in which Pt shell thicknesses increase with increasing Ni content, and that Ni content in PtNi cores follows a volcanic trend with increasing initial compositions [229, 233]. Therefore, Pt shell thickness and Ni content in catalyst cores can simultaneously change PtNi@Pt catalytic activities, and only through the balancing of the two factors can maximum catalytic activities be achieved for PtNi@Pt.

There have also been reports stating that the surface roughness of Pt shells can also affect the catalytic activity of core–shelled PtNi@Pt catalysts [218]. Here, Wang et al. [482] suggested that the vacancies formed by the dissolution of Ni can lead to lower average coordination of Pt surface atoms (Fig. 28) and stronger adsorption of intermediate oxygenated species, causing lower activities. This issue is a defect of the dealloying method for the fabrication of core–shell-structured PtM (M = 3d metals)@Pt catalysts and smooth Pt skins can be formed by using thermal annealing which can induce subsurface Pt to segregate to the alloy particle surface [22, 217, 483]. This information suggests that conducting a thermal process after dealloying can further enhance catalytic performances of electrocatalysts (Fig. 28). In addition to the various physical factors, porosity can also influence catalytic activities [204, 218, 230, 231]. Here, large alloy nanoparticle precursors can result in porous structures, and although these porous structures can enhance PtNi@Pt ORR catalytic activities, stability is also simultaneously decreased. Studies have also reported that core–shell-structured PtNi@Pt catalysts can exhibit good catalytic activities for MORs [461, 484, 485], but compared to its outstanding ORR catalytic activities, the activity for MORs is not very attractive.
Fig. 28

Electrochemical studies of Pt thin films deposited over PtNi substrates by using RDE: a Cyclic voltammograms; b Polarization curves, and c Summary of specific activities and corresponding improvement factors (vs. polycrystalline Pt surface) for Pt films of various thicknesses. Cyclic voltammograms were recorded in an Ar saturated 0.1 M HClO4 electrolyte with a sweeping rate of 50 mV s−1. Polarization curves were recorded in the same electrolyte under O2 saturation with a sweep rate of 20 mV s−1. Specific activities were presented as kinetic currents normalized by ECSAs obtained from integrated Hupd, except for the annealed 3 ML Pt/PtNi surface which was obtained based on COad stripping polarization curve.

Copied with permission from Ref. [482], Copyright 2012 The American Society of Chemistry

3.3.2 PtCo Alloy as Core

Experimental data has shown that core–shell-structured Pt3Co@Pt catalysts are more active than Pt3Ni@Pt and Pt3Fe@Pt catalysts for ORRs [479]. And as a result of this, a series of PtCo@Pt catalysts [162, 227, 245, 370, 487, 488, 489, 490] have been studied and have been found to exhibit better catalytic activities for ORR than PtCo alloys. Similar to PtNi@Pt catalysts, the performance of PtCo@Pt catalysts are affected by Pt–Co ratios, Pt shell thicknesses and physical factors such as porosity. For example, Wang et al. [489] reported that ordered Pt3Co@Pt catalysts with a core diameter of 5 nm and a Pt shell of 2~3 atomic layers can exhibit an over 200% increase in mass activity, an over 300% increase in specific activity and a minimal loss of activity after 5000 potential cycles as compared with disordered Pt3Co alloy nanoparticles and Pt/C catalysts. In this study, the authors attributed this high stability to the stable intermetallic Pt3Co core, which remained virtually intact through the testing process. In another study, Hu et al. [486] investigated a more refined, highly ordered homogeneous, cuboctahedral core–shell Pt3Co nanoparticle structure based on detailed experimental characterizations as well as extensive DFT calculations and found that in order to achieve optimal performances, the proposed cs-Pt3Co nanoparticles must comprise a Pt-rich skin, a Pt-rich shell configuration of 5~6 layers that also needs to contain Co atoms, and a core that is relatively Co-rich (Fig. 29). And if these conditions were met, the resulting PtCo@Pt catalyst can produce a high electrocatalytic activity that is 6 times higher than commercial Pt/C, a high activity that is 5 times more active than non-faceted alloy sp-Pt3Co nanoparticles and a high stability. These above results are helpful in the design and control of inner core structures during the synthesis of PtCo@Pt catalysts.
Fig. 29

Core–shell configurations of a cs-Pt–Co-x; and b cs-Pt3Co-x systems; here, the shell thicknesses vary from 1 to 6 metal layers; c Atomic-resolution HADDF–STEM image of a core–shell Pt3Co cuboctahedral nanocrystal; d The polarized curves of the catalyst electrode (Pt/Vulcan is shown as a black line, sp-Pt3Co/Vulcan as a red line, and cs-Pt3Co/Vulcan as a blue line) in an O2 saturated 0.1 M HClO4 solution at a scan rate of 0.01 V s−1 with a rotating speed of 1600 rpm; e Polarized curves of the cs-Pt3Co/Vulcan catalyst for ORRs before and after 5000 potential cycles at a rotating speed of 1600 rpm.

(modified with permission from Ref. [486]). Copyright 2016 The Royal Society of Chemistry

3.3.3 PtFe Alloy as Core

In the case of core–shell PtFe@Pt catalysts, theoretical and experimental results have shown that it is an active catalyst for ORRs and MORs [152, 353, 355, 396]. Studies have also reported that the effects of PtFe cores on Pt shells are similar to that of PtNi and PtCo catalysts due to their similar structure. For example, Liu et al. [353] reported that PtFe@Pt core–shell nanoparticles supported on carbon nanotubes with ordered L10 intermetallic PtFe cores and approximately three atomic layers of Pt shells can exhibit superior catalytic activities and significantly improved durability toward ORRs and the results here were similar to that of ordered intermetallic PtCo cores in Ref. [489].

3.3.4 PtCu Alloy as Core

Electrochemical experiments have also shown that PtCu@Pt catalysts possess good activity for ORRs and MORs [196, 198, 199, 200, 201, 220, 224, 225, 226, 491]. For example, Strasser et al. [224] presented a 3~6-nm-diameter PtCu@Pt catalyst with a 1:3 mol ratio of Pt–Cu and a 0.6-nm-thick Pt shell that produced a high catalytic activity of 0.72 mA cm Pt −2 and 0.56 A mg Pt −1 for ORR (Fig. 30). To further improve the catalytic activity of PtCu@Pt catalysts, compositions, shell thicknesses, and core structures have also been optimized by several groups. For example, Fu et al. [201] conducted a study to optimize PtCu@Pt core–shell nanofoams using Pt–Cu ratios ranging from 25:75 to 69:31. Here, the researchers found that a composition of Pt62Cu38 comprised of fused ~ 3-nm-diameter nanoparticles produced optimal activities, in which the resulting catalyst exhibited optimal catalytic activities that were sevenfold more active in terms of mass activity, 14 times more active in terms of specific activity, and significantly more durable for ORRs than commercial Pt/C catalysts. In another study, Sohn et al. [198] reported that based on their obtained experimental data, PtCu3@Pt/C catalysts were more active than PtCu1@Pt/C and PtCu5@Pt/C catalysts, and exhibited great improvements in ORRs with a mass activity of 0.501 A mg Pt −1 , which is 2.24 times greater than that of commercial Pt catalysts. Similar results were also reported by Wang et al. [216] and Liu et al. [354]. Despite these findings, however, DFT calculation results indicate that a 50:50 Pt–Cu atomic ratio is suitable for 3~7-nm-diameter PtCu@Pt particles [492], suggesting that the optimal composition of core–shell PtCu@Pt catalysts is not a fixed value, which is similar for PtNi@Pt and PtCo@Pt catalysts. Alternatively, optimal shell thickness values in PtCu@Pt catalysts are fixed. For example, Wang et al. [216] reported that the appropriate thickness of Pt shells for PtCu@Pt core–shell catalysts with an ordered PtCu3 core structure is approximately 1 nm. Moseley and Curtin [492] in their study used DFT calculations to predict that for 3~7 nm-diameter PtCu@Pt particles, 3~5 layer thick Pt shells can achieve peak activities for ORRs as a result of its approximately 2.5% surface strain corresponding to a shift in the EO of 0.2 eV relative to unstrained (111) Pt. These obtained results, such as compositions and shell thicknesses, vary as a result of varying physical characterizations of PtCu@Pt catalyst particles in each report.
Fig. 30

a High-resolution EDS (HR-EDS) elemental map of PtCu@Pt with a ratio of Pt25Cu75, Pt is represented in blue, Cu in red; b HR-EDS line profiles across an individual ~ 4-nm-diameter PtCu@Pt particle, c Sweep voltammetric measurements of the activity for the electroreduction of oxygen (ORR) of three dealloyed PtCu bimetallic catalysts from (a) as compared to pure Pt. Here, the catalysts were prepared and measured on a rotating-disk electrode with experimental conditions of: 0.1 M HClO4, Oxygen saturation, T = 25 °C, 1600 rpm, deaerated conditions, a scan rate of 5 mV s−1; d Pt surface and e Pt mass normalized ORR activities of PtCu@Pt catalysts with three Pt–Cu ratios compared with pure Pt evaluated at 0.9 V versus the reversible hydrogen electrode (RHE) potential. The dotted line represents the US Department of Energy activity goal for fuel cell cathodes.

(modified with permission from Ref. [224]). Copyright 2010 Nature

As for PdM (M = Ni, Co, Fe, and Cu) cores, these have also been extensively studied as catalysts for ORRs and organic small molecules [76, 114, 115, 119, 127, 128, 135, 298, 400, 493, 494, 495, 496, 497, 498, 499]. In these reports, there are many reported similarities between PdM@Pt with PtM@Pt catalysts. For example, various researchers have reported that ORR and MOR catalytic activities of Pt shells can be enhanced by using ordered Pd3Co alloys as the core due to strain effects [76, 297, 499]. However, the effects of PdM cores on Pt shells were not elaborated in these reports. Other precious metals such as Au and Ir have also been composited with 3d transition metals to form cores, and have been reported to exhibit promotional effects for the catalytic activity of Pt shells in ORRs and MORs. Here, the use of Au has been reported to particularly enhance catalytic stability [121, 500, 501, 502, 503]. Furthermore, cores composed of three metals such as AuNiFe [504], PdRuNi [294], PtCuCo [237], PtCuAu [238], PtRuCu [208], and PtFeNi [183], have also been synthesized and used to enhance the catalytic activity of Pt shells for ORRs and MORs.

Overall, cores composed of 3d transition metals with 4d or 5d precious metals are suitable for core–shell-structured electrocatalysts because of several advantages: (1) catalyst cost reductions as a result of the addition of non-noble metals; (2) increased core stability as a result of the presence of precious metals; and (3) increased tunability of alloy core compositions as compared with single metal cores, allowing for the adjustment of core effects on Pt shells to optimize catalytic performances. However, despite these advantages, Pt or Pd3M (M = Ni, Co and Fe) cores will always exhibit better enhancement effects than the cores discussed above, implying that the content of M in Pt or Pd3M (M = Ni, Co and Fe) cores will always be low, limiting the cost reduction of the resulting catalyst. Therefore, the search for alloy cores containing high non-precious metal content still requires significant efforts for the commercial application of core–shell-structured catalysts.

4 Carbide and Nitride Cores

4.1 Carbides as Core

Transition metal carbides (TMCs) have drawn increasing interest from researchers ever since Levy and Boudart reported that tungsten carbide phases exhibit ‘‘Pt-like’’ catalytic features for several reactions which were previously only catalyzed by Pt-group metals [505]. And although review articles in the electrochemical field have focused on the potential of TMCs as replacements for precious metal catalysts through a more solid understanding of their catalytic activity and stability [506, 507, 508], the electrocatalytic activities of TMC catalysts are not high enough for practical applications as electrocatalysts [509, 510, 511]. In addition, most TMCs oxidize at low potentials in acidic media to produce TMC oxides that are inactive for catalytic reactions. Despite this, theoretical and experimental results have shown that they can be promising cores for core–shell-structured electrocatalysts based on the following factors: (1) TMCs possess similar sp electronic properties to that of Pt, implying good adhesion between Pt and TMCs, leading to the formation of core–shell structures without the formation of stable carbides [508, 512, 513]; (2) the presence of C atoms can lock transition metal atoms in their positions, preventing their segregation toward the surface and the dissolution of parent metals into the acidic electrolyte solution, leading to long-term stability [514]; and (3) the presence of C can reduce lattice mismatches between the parent metal and Pt elements leading to reduced strain effects introduced by the subsurface of TMCs as compared with that of corresponding parent metals. This lowered strain subsequently can lead to more moderate binding energies for adsorbates, resulting in improved electrocatalytic activities [514, 515]. Overall, these advantageous factors provide ample motivation toward the development of core–shell-structured TMCs@precious metal electrocatalysts [387, 516].

Among the various TMC cores, tungsten carbide cores (WC) have received extensive attention from researchers because these cores can support full Pt encapsulation [513] and possess relatively high electrochemical stability below potentials of 0.8 V versus NHE [517]. As for ORRs, however, Liu et al. [514] reported that core–shell-structured WC@Pt catalysts with monolayer Pt shells are ineffective ORR electrocatalysts because strongly oxidizing conditions can lead to significant instabilities. In addition, Liu and Mustain [517] demonstrated that at least two different WOx species can form on WC surfaces at elevated potentials during ORRs. Furthermore, oxygen binding energies calculated by DFT suggest that core–shell-structured WC@Pt catalysts with monolayer Pt shells can only provide ORR catalytic activities that are similar to bulk Pt [518]. And because of these detrimental results, a growing endeavor has been undertaken by researchers to identify potential applications of core–shell-structured WC@Pt catalysts with monolayer Pt shells in the fields of the hydrogen evolution reaction (HER), the hydrogen oxidation reaction (HOR) and MOR electrocatalysis [376, 510, 515, 519]. In terms of the catalytic activity of core–shell WC@Pt electrocatalysts for MORs, however, surface science experiments have revealed that the CO binding energy of the Pt monolayer on WC is weaker based on the lower surface desorption temperature of CO than Pt [520]. This weaker CO binding energy can subsequently facilitate CO oxidation by using surface –OH species to produce CO2, and thereby increase the availability of active sites on the Pt monolayer. This has been confirmed by both DFT calculations and experimental results [509, 510, 521, 522, 523]. However, nanosize core–shell-structured WC@Pt catalysts for MORs have only been reported by a few groups [516, 524], and in one example, Chen et al. [516] synthesized core–shell WC@meso-Pt nanocatalysts through the carburization of ammonium tungstate and copper nitrate using gas–solid reactions followed by a Pt replacement reaction (Fig. 31). Here, the resulting nanocatalyst exhibited a peak current density of 275 mA cm−2 for methanol oxidation in the positive CV; which was approximately twofold that of Pt/C, and revealed inherent catalytic stability as well.
Fig. 31

a SEM image (inset shows particle-size distribution); b TEM image; c EDS line-scan analysis; and d HRTEM image of the as-prepared nanosize WC@(Cu)Pt sample; e CVs of WC@m-Pt and Pt/C in 2 M CH3OH + 1 M H2SO4 solution at a scan rate of 50 mV s−1 at 25 °C; the inset shows CVs in 1 M H2SO4 supporting electrolyte; f Chronoamperometric data (current–time profiles) of the samples at 0.3 V in 2 M CH3OH + 1 M H2SO4 solution.

(modified with permission from Ref. [516]). Copyright 2013 The Royal Society of Chemistry

Titanium carbide cores (TiC) have also attracted interest because it is the most electrochemically stable carbide, and is more stable than carbon or graphite materials in various acidic media [514, 525]. Therefore, because of this stability, efforts have been undertaken to introduce TiC into electrocatalyst systems and a majority of studies have focused on the effects of TiC as a support with both positive and negative results being currently reported [526, 527, 528, 529, 530]. Recently, Xia et al. [531] carried out a DFT study on Pt layer deposition on TiO2, TiN and TiC and investigated the structural stability and wetting tendency of Pt over-layers using energetic descriptors. Here, the researchers concluded that at low-Pt loadings on the support, Pt atoms have a stronger tendency to go to subsurface sites on TiN and TiC surfaces. These results imply that the encapsulation of a full Pt monolayer over TiC surfaces is difficult, leading to a lack of reported nanosize TiC core–Pt shell electrocatalysts despite the fact that Pt monolayers can be deposited on TiC films [532]. Other carbides, such as vanadium carbides (VC), iron carbides (FexC), and molybdenum carbides (MoC) have also been studied as supports and have been proven to improve ORR catalytic activities of Pt [514]. However, once again, corresponding nanosize core–shell-structured electrocatalysts are seldom reported.

Hunt et al. [533] has, however, developed a high-temperature self-assembly process to synthesize nanosize electrocatalysts comprised of carbide cores and precious metal shells through the carburization of mixtures of noble metal salts and transition metal oxides encapsulated in removable silica templates. And by using this method, nanosize WC@Pt core–shell-structured spheres with controlled sizes and Pt shell thicknesses can be synthesized. And based on the superior electrochemical stability of TiC, the researchers in this study synthesized core–shell-structured W0.1Ti0.9C@Pt and W0.1Ti0.9C@PtRu (Fig. 32) and found that the two resulting catalysts exhibited superior catalytic activities for HORs and ORRs (Fig. 32). In addition, the methods used in this study can also be extended to the synthesis of TiWC core nanoparticles with controllable sizes (3~10 nm) and mono- and bimetallic shell compositions with controllable thicknesses by using mixtures of Ru, Rh, Ir, Pt, and Au. Other bimetallic semi-carbide cores such as (Cu0.2W0.8)2C, (Co0.2W0.8)2C, and (Ni0.3W0.7)2C were also used to fabricate core–shell nanoparticles in this study [533]. Overall, this high-temperature self-assembly method provides new and highly tunable methods for the formation of nanosize carbide@precious-metal core–shell nanoparticles with increased catalytic activities and long-term stability in electrocatalysis.
Fig. 32

a TEM image; b STEM image; and c EDX map after stability cycling of PtC-S; d TEM image; e STEM image; and f EDX map of PtRuC-S; g HOR/HER Tafel plots; h HOR LSVs with and without CO contamination; i CVs for MOR normalized by CO-ECSA roughness factors; j Steady-state specific activities at fixed potentials after stability cycling and regeneration in alkaline media.

(modified with permission from Ref. [533]). Copyright 2016 Science

The use of TMCs as promising cores can offer many benefits, including fuel cell catalyst cost reductions, improved metallic electrical conductivity, and increased corrosion resistance. In addition, similar sp electronic properties between TMCs and Pt can intuitively lead to good adhesion between TMC cores and Pt shells, inhibiting the tendency of Pt shells to assemble into bulk Pt particles that can result in decreased catalytic activities. Despite these benefits, however, there has been only limited research conducted into these materials and the research that is out there indicate challenges such as impure phases and too big sizes of obtained powders [534, 535, 536, 537, 538, 539], suggesting that significant amounts of further studies are required.

4.2 Nitrides as Core

Transition metal nitrides (TMNs) possess similar properties as transition metal carbides and as early as 1963, researchers have found that TiNs are electrochemically active toward oxygen evolution in acidic solutions [540]. And later, Xia et al. [541] and Zhong et al. [542] in separate studies introduced Mo2N as ORR catalysts, but found in their results that these TMNs lacked the expected catalytic activities required for ORR catalysts. However, the same authors also reported that these TMNs possessed good CO tolerance and electrochemical stability. Therefore, although Mo2N, even if alloyed with Pt, does not possess high enough catalytic activities to be used as electrocatalysts for ORRs or the oxidation of formic acid and methanol, its high electrochemical stability allows it to act as a core so that Pt-based TMNs become promising electrocatalysts [543]. However, methods to efficiently improve the catalytic activity of Pt-based TMN electrocatalysts will require further research.

The vigorous development of core–shell structures have greatly assisted researchers in the fabrication of TMN core–Pt shell catalysts for electrochemical reactions. For example, Adzic et al. [544] successfully synthesized sphere-like nanoparticles consisting of a Pt shell and a Ni nitride core with an average diameter of 3.5 nm (Fig. 33). Here, the Pt shell thickness of the resulting catalyst was around 0.5~1.0 nm, equivalent to 2~4 monolayers of Pt and for ORR, the PtNiN catalyst displayed a mass and specific activity of 0.86 A mg Pt −1 and 1.65 mA cm−2 at 0.9 V versus RHE, respectively. These results were around 4.5~6.5 times larger than that of Pt/C catalysts (0.20 A mg Pt −1 and 0.24 mA cm−2), and after 35,000 cycles, LSVs only showed a 11 mV loss in half-wave potential for the resulting catalyst, demonstrating extremely high stability for ORRs, which was further attested by the intact core–shell structure of the PtNiN nanoparticle. The researchers in this study also conducted DFT calculations and found that the nitriding of the Ni core can adjust the electronic structure of the outer Pt shell to exhibit higher ORR activities than commercial Pt/C (electronic effect); and simultaneously, interacted Pt atoms located at the inner shells can easily diffuse to the outer surface where vacancy sites can be filled due to the existence of N atoms in the inner core (geometric effect), leading to the augmented stability of the catalysts. In addition, the researchers here also predicted that higher N concentrations may facilitate further Pt diffusion and therefore increased durability [544]. Later, Adzic et al. [545] again reported that core–shell-structured FeN@Pt and CoN@Pt catalysts synthesized by using the same method also showed good ORR catalytic activities and high cycling stabilities. Moreover, using DFT calculations, these researchers predicted that NiN@Pt can produce the highest ORR activity among NiN@Pt, CoN@Pt, and FeN@Pt catalysts because FeN and CoN cores impart higher compressive strains to Pt shells than NiN cores, which only provide a slight but moderate compressive force to the Pt shell. In the case of FeN cores, Ding et al. [546] reported that the catalytic activity of FeN@Pt for ORRs presented a volcanic-like variation with increased Fe concentrations in the FeN core, in which PtFe3N/C possessed a mass activity of 369.32 mA mg Pt −1 in 0.90 V versus RHE for ORRs, as compared with 190.24 mA mg Pt −1 for PtFeN/C and 315.44 mA mg Pt −1 for PtFe5N/C (315.44 mA mg Pt −1 ), and was about 3 times larger than that of commercial Pt/C (129.15 mA mg Pt −1 ). Here, the experimental results of the FeN core appear to be inconsistent with those of NiN cores based on theoretical results. Therefore, to obtain scientific results, more system studies are required.
Fig. 33

a HAADF–STEM image of a PtNiN core–shell nanoparticle with its corresponding two-dimensional EELS mappings of Pt M and Ni L signals. (dotted lines for visualization purpose only); b STEM image of PtNiN core–shell nanoparticles; c EELS line scan profiles of Pt and Ni in a single nanoparticle along with schematic representations of a single PtNiN nanoparticle (blue represents Pt; gray represents Ni; purple represents N); d Synchrotron XRD pattern for a PtNiN catalyst showing Pt and Ni4N phases (blue line denotes Ni3N phase); e Polarization curves for ORR and mass and specific activities (the inset) for PtNiN/C and Pt/C catalysts on a RDE electrode. Pt loadings for PtNiN/C and Pt/C were 7.84 and 7.65 μg cm−2, respectively; f ORR polarization and voltammetry (the inset) curves of PtNiN core–shell nanoparticles before and after 35,000 cycle tests between 0.6 and 1.05 V in 0.1 M HClO4.

(modified with permission from Ref. [544]). Copyright 2012 American Chemical Society

To further optimize catalytic performances of TMN based catalysts, secondary metals have been introduced into TMN cores as well. For example, Tian et al. [329] introduced Ni into TiN cores to synthesize core–shell-structured TiNiN@Pt catalysts. Here, the half-wave potential for the ORR polarization curve of the resulting TiNiN@Pt was 893 mV, which was 16 and 44 mV higher than for TiN@Pt and Pt/C, respectively. In addition, this resulting TiNiN@Pt also exhibited a specific activity of 0.49 mA cm Pt −2 and a mass activity of 0.83 A mg Pt −1 , which was comparable to the activities reported for most core–shell catalysts with precious metal cores. Furthermore, the onset potential of TiNiN@Pt was positively shifted by 20 mV compared with that of Pt/C catalysts, even though the Pt loading of the TiNiN@Pt catalyst was only one-fourth that of the Pt/C catalyst. And after 10,000 potential cycles, the TiN@Pt and TiNiN@Pt catalysts only degraded by a negative shift of 17 and 10 mV in its half-wave potential, respectively, whereas Pt/C showed a degradation of more than 25 mV (E1/2), demonstrating the long-term stability of the resulting TiNiN@Pt catalyst. Other bimetallic nitride cores covered by Pt shells, such as PdNiN, TiWN, and TiCuN, have also been reported to display enhanced catalytic activities and stabilities for ORR [547, 548, 549] and these developments open up broad feasibilities for the design and synthesis of various TMN nanoparticle-based core–shell structures for applications in electrocatalysis. However, many problems still exist for TMN core applications. For example, according to previous reports [544, 545], in TMN cores comprised of two phases, more research is required to determine which phase contributes more to Pt shell performances, and which synthesis parameters need to be controlled to obtain TMNs with one pure phase. And because of this, significant efforts are still required before TMNs can be widely applied as cores for electrocatalysis (Fig. 34).
Fig. 34

a Illustration of the synthesis of PtRuCu6-A/C by using chemical corrosion; b STEM and EDX images of PtRuCu6-A/C; c CVs on different catalysts at 50 mV s−1 at 0.20 V; d Current versus time curves of different catalysts; e CO stripping CVs of different catalysts at 50 mV s−1.

(modified with permission from Ref. [208]). Copyright 2016 Elsevier

5 Core–Shell-Structured Catalysts with Alloy Shells

As discussed in Sects. 3 and 4, various cores have been thoroughly and carefully investigated based on different aspects including composition, size, and shape, in order to optimize the catalytic activity of core–shell-structured electrocatalysts. And aside from the manipulation of thicknesses to improve activities, modifications of shell compositions can also lead to optimizations as well.

For example, Ru has been proposed by researchers for shell composition modifications because it can provide well-known bifunctional mechanisms to provide remarkably higher CO tolerances in PtRu alloys. Another feature of Ru that distinguishes it from other metals is that in terms of bifunctional mechanisms, it can provide adsorbed hydroxyls (Ru–OHads) to catalyst surfaces which can help adjacent active Pt atoms (Pt–COads) to easily catalyze CO oxidation reactions [550]. And with this in mind, Ru has been introduced into Pt shells to form PtRu alloy shells by several groups [153, 207, 208, 494, 551, 552, 553, 554]. For example, Huang et al. [208] synthesized PtRu shells through the chemical corrosion of PtRuCu alloys (denoted as PtRuCu6-A/C) and the resulting catalyst exhibited a forward peak with a 1632.5 A g Pt −1 current density that was approximately 3.3 and 10.6 times more active than PtRu/C and Pt/C, respectively. In addition, the onset potential of MOR for the resulting PtRuCu6-A/C was − 173 mV, presenting a negative shift of 42 and 173 mV as compared with PtRu/C and Pt/C. Here, Matin et al. [153] suggested that the good CO tolerance and catalytic activity of PtRu shells originate from both bifunctional mechanisms with the presence of Ru and effects from the core such as compressive strains.

Pd is another precious metal that has been composited into Pt shells by many groups because PtPd bimetallic systems are the best among bimetallic catalysts for ORRs [106, 123, 150, 555, 556, 557]. Here, experiments carried out on mixed-PtPd-shell PtPdCu nanoparticle nanotubes have demonstrated that PtPd shells over Cu cores can produce not only better catalytic activities but also superior durability toward ORRs than Pt shells on Cu cores. Li et al. [123] attributed the enhanced activity of Cu@PtPd catalysts to the alloyed shell in which a negative shift in the d-band center of Pt occurs through compressive strains and ligand effects caused by alloying, resulting in a decrease in adsorption binding of hydroxyl intermediates.

PtAu shells have also been used to fabricate core–shell-structured electrocatalysts [222, 238, 558] and the choice of using Au is based on its ability to greatly enhance stabilities through the modification of Pt electronic structures. In addition, there have also been various reports related to the introduction of other precious metals such as Ag into the shells of electrocatalysts for ORRs [107]. But the performances reported in these studies have not surpassed PtPd shells in other reports.

Aside from precious metals, non-noble metals have also been added into the shells of core–shell-structured electrocatalysts. For example, Shi et al. [159] covered an Au core with a shell consisting of PtNi alloy layers and reported an onset potential and E1/2 of ORRs for the resulting Au@PtNi nanoparticles that were 7 and 20 mV higher than those of Au@Pt nanoparticles, respectively. In addition, the mass activity of the resulting Au@PtNi nanoparticles was 0.538 and 0.185 A mg Pt −1 at 0.80 and 0.85 V vs. RHE, respectively, and were 1.6 and 2 times that of Au@Pt nanoparticles at 0.80 V and 0.85 V vs. RHE, respectively. And after 10,000 cycling tests, the Au@PtNi nanoparticles exhibited a roughly 11.7 mV smaller negative potential shift than Au@Pt nanoparticles (12.7 mV). Various studies have also reported that if PtNi alloy shells are used with other cores such as Pd, Ni, and PtPb, good catalytic activities and stabilities for ORRs can also be obtained [160, 388, 559, 560, 561]. Wang et al. [562] also reported that the use of PtCo alloy shells on Pd cores can produce better ORR activities than other samples of Pt/C, Pd/C and Pd@Pt/C, and increased stability in which the maximum power density of Pd@Pt3Co/C showed almost no obvious decay in experimental testing. In addition, other experiments have demonstrated that Fe [75, 563], Cu [96, 175, 564, 565, 566] and Pb [149] can also be introduced into Pt shell layers to form core–shell-structured electrocatalysts that exhibit better catalytic activities in electrochemical reactions than Pt shells alone.

Recently, Shen et al. [92] synthesized nearly monodispersed core–shell-structured Au@NimPt2 nanoparticles 5.0~6.5 nm in diameter (with controlled shell compositions and a thickness of 0.75~1.5 nm) to be used as ORR catalysts (Fig. 35). And during the CV cycle process, changes in voltammetric characteristics resulting from electrochemical dealloying was observed, suggesting that Ni atoms in the shell were selectively dissolved during CV cycles. STEM–EDX elemental maps of the Au@Ni2Pt2 particles after electrochemical dealloying (Fig. 35) indicated that its structure possessed an Au core and a double shell consisting of a NiPt alloy sublayer and an outermost Pt layer. And in the performance testing conducted in this study, these Au@NimPt2 nanoparticles were immobilized on Vulcan XC-72 carbon and provided an up to twofold higher catalytic activity and better durability than commercial Pt/C catalysts for ORR in acidic electrolytes [91]. These results suggest that the introduction of non-noble metals into shells can result in core–double-shell structures for catalyst particles in working environments. These results also provide guidance for the optimization of core–shell structures for commercial catalysts in the future.
Fig. 35

a TEM image of carbon-supported Au@Ni2Pt2; b and c EDX images of Au@Ni2Pt2 before and after electrochemical dealloying; d Schematic showing the enrichment of Pt atoms at the outermost layers; e ORR polarization curves in an O2-saturated 0.1 M HClO4 solution; f IA and MSA data at 0.9 V; g IA; and h MSA changes during durability testing. Pt/C E-TEK, ■; Au@Ni1Pt2/C, ○; Au@Ni2Pt2/C, △; Au@Ni1Pt2–CO/C, ●; and Au@Ni2Pt2–CO/C, ▲.

(modified with permission from Ref. [91]). Copyright 2016 The American Chemical Society

6 Interactions Between the Shell and Core

Catalytic performances depend on metal surface characteristics. And according to Nørskov et al. [567, 568, 569, 570, 571, 572, 573, 574], the energy center of the surface metal valence d-band density (εd) plays a crucial role in surface catalytic reactivity. Here, as the d-band center lowers, a distinctive anti-bonding state below the Fermi level appears, filling electronic orbit states and weakening bonds, which cause adsorbates to bind less strongly. Alternatively, high d-band centers have the opposite effect, thereby altering catalytic activities. For example, two reactions (O2 → O + O and O + H → OH) can be chosen to represent the elementary reaction steps of ORRs on Pt sites regardless of detailed mechanisms, and the O–O bond- breaking step and O–H bond-forming step can be viewed as a representative O2 dissociation and O hydrogenation, respectively. Here, surfaces that can strongly bind adsorbates can enhance the kinetics of bond-breaking steps whereas surfaces that bind species weakly can enhance the kinetics of bond-making steps. And in cases in which d-band centers shift to a moderate location, the binding of adsorbates on Pt surfaces moderates, providing a balance between bond-breaking kinetics and bond-making kinetics, resulting in maximized ORR catalytic activities [223, 224, 330, 406, 483, 575]. As for core–shell-structured electrocatalysts, electrocatalytic activities depend on the characteristics of the shells, which is affected by the core. Here, many experimental and theoretical studies have been carried out to elucidate the effects affecting activity and durability and three major effects; strain effects, electron effects, and ensemble effects have been proposed in the literature to influence the position of εd for shells, Pt, and over-layers [330, 576, 577, 578].

6.1 Strain Effect

The strain effect, also called the geometric effect, originates from the different reactivities of surface atoms because of lattice mismatches between surface atoms and sublayer atoms [578]. In core–shell structures, lattice parameters between core and shell metals are different and lattice mismatches exist at the interface between two metals, inducing the shift of metal d-band centers. For example, the atomic distance of Pt is compressed if Pt atoms are deposited onto Ru or Pd substrates, whereas the atomic distance of Pt is expanded if Pt atoms are deposited onto Au substrates, resulting in the contraction or expansion of Pt layer lattice parameters. Here, a compressed lattice causes a downshift in Pt d-band centers, leading to the strong binding of adsorbates on its surface, whereas an expanded lattice causes the opposite effect [224, 574, 579]. And as discussed above, surfaces that strongly bind adsorbates are inclined to improve bond-breaking kinetics and surfaces that weakly bind adsorbates are inclined to improve bond-making kinetics.

Experimental data and theoretical calculations have demonstrated the effect of strain effects on ORRs [406, 572]. For example, in precious metal cores (Fig. 36), the lattice mismatch between Pd and Pt is smaller than that of other precious metals with Pt due to the smaller radii difference between the two metals. Therefore, compressive strains induced by Pd cores on Pt shells are smaller than others, resulting in the downshift of d-band centers to a moderate location and the reasonably weak binding of adsorbates onto the Pt shell. And as a result of this, the ORR activity of Pd@Pt is higher than both bulk Pt and Pt monolayers over Ir, Ru, Rh and Au substrates [406, 580, 581].
Fig. 36

Models of pseudo-morphic monolayers of Pt on three different substrates inducing compressive strain (Ru (0001) and Pd (111)) and expansive strain (Au (111)).

Reprinted with permission from Ref. [35]. Copyright 2007 Springer

In the case of non-noble metals presented in Fig. 19, their atomic radii are noticeably smaller than Pt and the resulting large lattice mismatch induces strong compressive strain effects, leading to the shifting of the Pt monolayer d-band center toward a relatively low position. This causes the relatively weak binding of adsorbates, enhancing the ORR activity of non-precious metal core Pt shell catalysts [189, 203, 224, 582]. However, these non-precious metal catalyst activities do not surpass Pd@Pt catalysts for ORRs, proving once again that compressive strains of suitable strength provide maximized ORR activities of catalysts. In addition, strong compressive strains can lead to serious lattice distortions for Pt, resulting in poor stability during catalysis, and there are major defects in non-precious metal cores.

Strain effects also influence the adsorption of carbonaceous and hydroxyl ligands at Pt sites, affecting CO oxidation on Pt surfaces and facilitating the oxidation of organic small molecules. For example, if Pt monolayers are coated onto Ru (001) thin-films, the bonding of adsorbed carbonaceous and hydroxyl ligands at Pt sites are weakened due to compressive strains, leading to substantially enhanced CO oxidation activities [574, 583, 584]. In addition, weakly bonded Pt–(OH)ads can serve as novel pathways for the superior activity of CO deportation and water dissociation on Pt monolayer/Ru nanomaterials [111], enhancing methanol oxidation on Pt monolayers over Ru substrates. Overall, compressive strain effects play a dominant role in the enhancement of catalytic activities. Therefore, the control of strain effects is an attractive strategy to tune catalytic activities.

As for the factors influencing strain effects, compressive strain is formed by a Pt shell surrounding a core with a smaller atomic radius, implying that compressive strains are affected by core components. Here, alloy cores composed of 3d and 4d or 5d metals are good examples. By alloying 3d metals into 4d or 5d cores, the lattice parameters of the 4d or 5d core becomes smaller than that of 4d or 5d metals, increasing lattice mismatches with the Pt shell. This in turn strengthens the compressive strain on Pt shells, leading to weakened binding toward adsorbates and improved surface activities on the Pt monolayer [119, 121, 229, 298, 502, 582, 585]. Based on this concept, researchers have also tuned the ratios of the components in cores to optimize catalytic activities [128, 586]. For example, Cochell et al. [128] adjusted the lattice parameter of a PdCu core by controlling the Pd–Cu ratio (Fig. 37), resulting in lattice parameter changes of the Pt monolayer due to compressive strains. And in their results, the behavioral trend of the catalytic activity was consistent with that of decreasing core lattice parameters, illustrating the connection between the two aspects [128]. Similar reports have also been provided by other groups [224, 229].
Fig. 37

a and b Variations of lattice parameters for the as-synthesized and heat-treated PdxCuy/C and Pt@PdxCuy/C as a function of bulk alloy composition, with expected lattice parameter values based on Vegard’s law indicated by the dashed line in their XRD patterns; c Hydrodynamic polarization curves of the Pt@PdxCuy/C and Pt/C samples in an O2-saturated 0.1 M HClO4 at 1600 rpm and a scan rate of 5 mV s−1; d Pt and noble metal mass activities obtained at 0.9 V.

(modified with permission from Ref. [128]). Copyright 2012 The American Chemical Society

Compressive strain effects are also sensitive to shell thicknesses, especially in small nanoparticles. Here, strain effects can be affected by metals from the upper monolayer to the several atomic layers around the interface with strain effects being strongest in Pt monolayer shells in which stability is still controversial. Optimum Pt shell thicknesses are often reported to be 2~3 atomic layers [62, 92, 121, 297, 299, 409] and the strain in thinner shells shows weaker dependence on nanoparticle diameters as compared with thicker shells [492]. Therefore, based on the above discussion, the modulation of compressive lattice strains can be controlled through adjusting core components and shell thicknesses to achieve optimal catalytic activities.

6.2 Electronic Effect

The electronic effect, also referred to as the ligand effect, is an effect in which the presence of core metals around a shell metal atom in core–shell structures changes the electronic environment of the shell metal atoms, giving rise to the modification of upper shell metal atom electronic structures, leading to the alteration of band-structures and chemical properties [217, 415, 587]. The electronic effect includes charge transfer resulting from different electronegativities, and orbital overlap originating from hybridization [477, 588].

And as discussed above, Pd substrates covered with a Pt monolayer possess higher ORR activities than bulk Pt surfaces and is thought to be a result of the small compressive strains. However, charge transfer effects can also play a role in this enhancement. Here, the electronegativity of Pt is larger than Pd, causing charge transfer from Pd to Pt (Fig. 38a). This charge transfer subsequently leads to the filling of higher d-bands of Pt, resulting in increased Fermi energy (EF) and a downshift of Pt εd (Fig. 38b). Alternatively, the enhanced ORR activity can also be attributed to orbital overlap effects. Here, the interactions between Pt and Pd result in orbital overlap, facilitating charge redistribution through the hybridization of the states from each atom, broadening the d-band, and causing a downshift in the d-band center [301]. This lowered d-band center subsequently weakens the binding of adsorbates and Pt–OH intermediates; an ORR blocking species is easily formed on Pt surfaces because of the higher εd of Pt and is easily removed from the surface of Pd@Pt. In comparison with pure Pt surfaces, this characteristic can help to improve kinetics and enhance ORR activities [589, 590].
Fig. 38

a The differential charge density based on first-principles simulation illustrating the alterations of electron distribution with Pt thickness. The olive and cyan areas represent the increase and decrease in electron density, respectively (copied with permission from Ref. [71]). Copyright 2014 Wiley–VCH Verlag GmbH & Co. KGaA; b Illustration of the electronic effect of Pd on Pt d-band centers

The higher CO tolerance of Pt-sub-monolayers on Ru electrocatalysts as compared with PtRu alloys is also likely to be a consequence of electronic effects, which can cause Pt d-bands to downshift to maintain pure d-band filling and charge neutrality. This lowering of the d-band center in turn weakens CO adsorption due to decreased back-donation from Pt to anti-bonding CO orbitals [35, 568, 591, 592]. Moreover, a situ FTIR study conducted by Brankovic et al. [593] revealed different electronic interactions between Pt-sub-monolayers and Ru, and between Pt and Ru in alloys.

Del Popolo et al. [594] also reported that Au cores can modify the electronic structure of Pt shells toward lower Pt surface energies, or lower-lying Pt d-band states [594]. In many of these reports, however, the authors usually use charge transfer between surface metals and substrates to provide simplistic explanations of electronic effects. But in many cases, the direction of charge transfer is different. For example, in PtPd systems, the electronic effect is usually stated as the charge transfer from Pd to Pt [589]; however, in core–shell-structured Pt@Pd catalysts for ethanol oxidation, the direction of the charge transfer is from Pt to Pd [595]. Here, it would appear that the direction of the charge transfer is not completely determined by the electronegativity difference of the two metals. Similar findings were also reported by other groups, such as charge transfer from Au to Pt [596, 597], and charge transfer from Co to Pt [598]. These results demonstrate the importance of ligand effects on both precious and non-precious metal cores in the chemical properties of the shell.

Similar to strain effects, electronic effects decrease with increasing shell thicknesses [295] and are actually more sensitive to shell thicknesses than strain effects. For example, Strasser et al. [224] studied a Pt/Cu system (Fig. 39) and reported the absence of detectable Cu signals in the photoelectron spectrum of Pt with five monolayers (5 ML), indicating no valence bands of Pt in the 5 ML Pt grown on Cu (111), representing a pure Pt state with no ligand effects from the underlying Cu substrate. The researchers in this study also reported that lattice parameters during the growth of 5 ML Pt on Cu (111) provided a 2.5 + 0.3% compressive strain based on the low-energy electron diffraction (LEED) (Fig. 39b). Xie et al. [62] also reported that Pt shells of more than three atomic layers deposited on Pd nanocubes would result in weakened ligand effects which are subsequently screened by Pt electrons. As for core–shell-structured Au@Pt catalysts, Deng et al. [330] reported that as Au moves deeper beneath the surface, ligand effects lessen and completely vanish at 3~4 layers. However, the authors here also reported that strain effects were still present in the layers where ligand effects vanished by comparing the binding energies of pure Pt with the 5 layers thick Pt skin on Au.
Fig. 39

a Photoelectron spectra of the valence-band region for Cu (111), Pt (111) and 5 ML of Pt on Cu (111) measured with a photon energy of 620 eV and a grazing electron-emission angle of 158°. The inelastic background was subtracted. The spectra of the 5 ML Pt on Cu (111) were completely dominated by emissions from Pt because no sharp Cu d-band emissions were seen at 2.3 eV from the underlying substrate (similar spectra were also found for 3 ML of Pt); b Strain in Pt layers deposited on Cu (111), defined as [(afilm − aPt)/aPt × 100%], deduced from LEED patterns as a function of film thickness.

(modified with permission from Ref. [224]). Copyright 2010 Nature

Both electronic and strain effects are important in the determination of the reactivity of monolayer metals over substrates. And in general, these two effects co-exist in core–shell structures [295]. For example, in cases in which electronic effects decay completely as Pt layers increase, the activity of core–shell-structured catalysts can be directly determined by strain effects. So in cases in which later d-metals possess less charge transfer effects, stain effects are likely to play a key role in determining changes in Pt d-centers [471, 579]. Alternatively, for early 3 d-metals such as Co, electronic effects are likely to dominate changes in Pt d-band characteristics, in which significant charge transfers are observed between M cores and Pt shells [471, 579].

6.3 Ensemble Effect

Ensemble effects refer to catalytic property changes of an ensemble of surface atoms which has undergone chemical composition changes [599]. Ensemble effects are governed by the adsorption of metals on surfaces of the shell due to the presence of distinct atomic groups and these effects are usually difficult to be distinguished from ligand effects. Therefore, to illustrate the difference between the two effects, Liu and Nørskov [415] developed a rational model for CO binding sites on AuPd/Pd (111) surfaces through relating the adsorption energies to two properties of the adsorption site. As seen in Fig. 40, there are distinct differences for CO adsorption in a threefold site with varying ratios of Au and Pd atoms. This effect, related to the nature of the ensemble of metal atoms to which adsorbates bind, can be referred to as the ensemble effect. As for adsorption sites with the same composition, adsorption energies can still vary (cf. site 1 and 4) because the atom(s) to which adsorbates bind possess different surroundings, and this effect related to the nature of the ligands of active sites, can be referred to as the ligand effect [415]. And based on previous descriptions, ensemble effects occur with the presence of hetero-atoms on shell surfaces, meaning ensemble effects are present in core–shell electrocatalysts with alloy shells. As for M@Pt core–shell-structured electrocatalysts, all M atoms are in the subsurface and have no direct contact with adspecies; therefore, the activity of Pt shells is mainly affected by stain and electronic effects [600]. System studies focused on ensemble effects in core–shell-structured electrocatalysts with alloy shells cannot be found in literature for the purpose of this review but similar studies on alloyed [601, 602] or decorated [603] electrocatalysts have been carried out by several groups, with the possibility that these investigated ensemble effects in alloy electrocatalysts can be used to explain the ensemble effects in core–shell-structured electrocatalysts.
Fig. 40

Correlations between adsorption sites and adsorption energies for CO adsorption on a Au/Pd (111); and b Au1Pd3/Pd (111). The numbers listed along the horizontal axis indicate the different adsorption sites, as shown in the inserted figures. Green balls represent Pd and yellow balls represent Au.

(modified with permission from Ref. [415]). Copyright 2001 The Royal Chemical Society

7 Perspectives on Core–Shell-Structured Catalysts

The demand for large-scale applications of fuel cell technologies in commercial, residential, and transportation applications is rapidly increasing with the depletion of fossil fuels and the increase in environmental pollution. However, in order to achieve this, anodes and cathodes in fuel cells require high-performing, low-cost electrocatalysts. And to date, Pt-based catalysts are still considered the most practical catalysts for both electrodes, particularly cathodes on which ORR is the rate-determining step of overall fuel cell reactions due to its sluggishness in comparison with anode reactions. However, because of the high cost and scarcity of Pt, the reduction of Pt loading in catalyst layers has become a key target to achieve widespread commercialization of fuel cells. Here, researchers have recognized that the most effective strategy for Pt loading reduction is the deposition of monolayers or a few layers of Pt over other metal cores to form Pt shell catalysts, in which the use of core–shell-structured electrocatalysts not only significantly reduces Pt loading, but also increases catalytic activities toward electrochemical reactions. And to date, significant progresses have been made in the preparation and evaluation of Pt shell core–shell-structured electrocatalysts, along with the improvement of their catalytic activities toward electrochemical reactions such as ORRs, MORs and HORs, which have been revealed to be strongly dependent on the composition and size of cores, as well as the thickness of shells.

In the development of core–shell-structured electrocatalysts, synthesis techniques must be first considered because it determines the feasibility of the commercial application of the resulting electrocatalyst. And in terms of the synthesis methods for the formation of core–shell-structured electrocatalysts, widely reported methods can be divided into six categories, including wet chemical deposition, dealloying, electrodeposition, surface segregation, atom layer deposition, and physical deposition. In this review, the principles and developments of these methods are introduced and through the comparison of the characterization of the obtained core–shell-structured materials prepared by using each method, the merits, and demerits of each method are summarized. And based on these, it can be seen that although each method possesses its own advantages and disadvantages, any single method cannot meet commercial requirements in which synthesis methods are required to provide the precise tunability of structural parameters (e.g., composition, size, shape, and shell thickness) and allow for the feasibility for practical mass production. However, based on the information provided in this review, two or more synthesis methods can be combined to fabricate high-quality, core–shell-structured electrocatalysts with optimized structural parameters. Significant challenges remain, however, in the exploitation of feasible and efficient methods for the formation of designed core–shell structures.

As for core–shell-structured electrocatalysts, the composition of the core significantly influences the surface activity of Pt shells through strain and electronic effects. Here, in an attempt to provide general observations on the effects of different cores on shell activities, different cores are divided into five classes depending on composition. These classes include single-precious metallic cores represented by Pd, Ru and Au, single-non-precious metallic cores represented by Cu, Ni, Co and Fe, alloy cores containing 3d metals and 4d or 5d metals, carbide cores, and nitride cores. In this review, the traits of each type of cores are specified based on reported studies and the effects of these cores on the catalytic activity of Pt shells are broadly examined through both theoretical and experimental studies reported in the literature. The merits and demerits of each type of cores are also provided based on systematic and comparative examinations. And from these, the conclusion drawn in this review is that cores synthesized through the alloying of 3d metals with 4d or 5d metals can efficiently yield electrocatalysts with enhanced catalytic activities for chemical reactions, whereas carbide and nitride cores can yield tremendous advantages over alloyed cores in terms of cost and promotional activities of Pt shells. In addition to the type and composition of cores in the design and preparation of optimal electrocatalysts, desirable shells with reasonable thicknesses and compositions have also been recognized to play a dominant role in the improvement of catalysis. With increasing discoveries in the development of advanced core–shell-structured electrocatalysts, catalytic activities are often found to be dependent on the binding energies of adsorbents that are determined by Pt d-band centers which are mainly affected by strain and electronic effects. And in these cases, these two effects can be adjusted through controlling the core composition and shell thickness of catalysts.

Based on the issues discussed in this review, several important future research directions are proposed to understand the relationship between core–shell structures and surface activities, and to design highly active, core–shell-structured electrocatalysts:
  1. 1.

    The development of efficient approaches to form core–shell structures in order to achieve high activity and durability. Here, three deposition strategies can be considered: the first is to precisely control the shell thickness in wet chemical deposition through tuning reaction parameters; the second is to design or improve the equipment used in electrodeposition, ALD and physical deposition methods to allow for the possibility of large-scale production and cost reduction; and the third is to combine two or more methods (i.e., dealloying followed by surface segregation) to ensure the uniform formation of Pt shells over alloy cores with controllable shell thicknesses.

     
  2. 2.

    The optimization of cores through the tuning of elements, compositions, sizes, and shapes to achieve optimal designs. Here, elements in cores can play important roles in balancing high surface activities and stabilities of Pt shells and three types of cores should be widely developed. The first are cores composed of alloyed 3d and 4d or 5d metals which need to be optimized in terms of composition, size, shape, and structure, to further decreases precious metal loading in cores while catalytic performances are maintained. The second are nanoscale carbide cores which need to be deeply developed in terms of synthesis methods that can successfully control pure phases, sizes, and shapes, inducing variation trends of Pt shells. The third are nanoscale nitride cores which need to be deeply investigated in terms of high throughput controlled synthesis, making these cores more applicable in commercial core–shell-structured electrocatalysts.

     
  3. 3.

    The development of alloy shells to achieve better surface activities. Here, Ru, Pd, and Au can be selected and introduced into Pt shells.

     
  4. 4.

    The deeper understanding of strain and electronic effects and the identification of correlations between the two effects and catalyst features such as composition, size, and shape. And with the elucidation of the correlations between catalytic activity and catalyst physical features, catalytic activities can be precisely tuned through the adjustment of catalytic features. Theoretical studies and modeling also play a guiding role in this effort.

     

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© Shanghai University and Periodicals Agency of Shanghai University 2018

Authors and Affiliations

  1. 1.College of Chemical EngineeringQingdao University of Science and TechnologyQingdaoChina
  2. 2.The Key Laboratory of Fuel Cell Technology of Guangdong Province and the Key Laboratory of New Energy Technology of Guangdong University, School of Chemistry and Chemical EngineeringSouth China University of TechnologyGuangzhouChina

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