2D MXene-Based Materials for Electrocatalysis

Abstract

MXenes, as an emerging 2D material, are expected to exert a great influence on future energy storage and conversion technologies. In this review, we systematically summarize recent advances in MXene-based materials in electrocatalysis, particularly in the hydrogen evolution, oxygen evolution, oxygen reduction, nitrogen reduction, and CO₂ reduction reactions. Crucial factors influencing the properties of these materials, such as functional groups, conductivity, and interface, are discussed, and challenges to the future development of MXene-based electrocatalysts are presented.

Introduction

Since graphene was first prepared by mechanical exfoliation in 2004 [1], various two-dimensional (2D) materials have attracted extensive attention on account of their unique physical and chemical properties [2]. These materials consist of atomically thin sheets with inherently large surface areas; they can be used extensively in various areas, such as electrocatalysis [3], photocatalysis [4], energy storage [5], membrane separation [6, 7], and biotherapy [8]. Besides graphene, a wide range of atomically thin 2D materials have also been successfully prepared, including transition metal dichalcogenides [9,10,11], phosphorenes [12,13,14], silicenes [15, 16], germanene antimonenes [17, 18], boron nitrides [19,20,21], and layered double hydroxides [22].

Transition metal carbides, carbonitrides, and nitrides (MXenes) are a new addition to the family of 2D materials [23]. The common form of MXene is M_n+1X_nT_x (n = 1, 2, 3), where M represents an early transition metal, X represents carbon and/or nitrogen, and T_x denotes surface functional groups, such as −O, −OH, or −F [24]. MXenes have representative structures of M₂XT_x, M₃X₂T_x, and M₄X₃T_x with n layers of X elements covered by n + 1 layers of M (Fig. 1) [25]. Since the first MXene, Ti₃C₂T_x, was synthesized in 2011, over 19 types of MXenes have been subsequently prepared, and more MXenes are predicted to exist. As-synthesized MXene-based materials are widely applied in various fields, such as in alkali metal batteries [26,27,28,29], photothermal conversion [30,31,32], photocatalysis [33,34,35], supercapacitors [36,37,38], and membrane separation [39,40,41], where they consistently show excellent performance. The outstanding electrical conductivity and hydrophilic surfaces of MXenes also ensure their stable performance as electrocatalysts in the hydrogen evolution reaction (HER) [42,43,44], oxygen evolution reaction (OER) [45, 46], oxygen reduction reaction (ORR) [47, 48], nitrogen reduction reaction (NRR) [49, 50], and CO₂ reduction reaction (CO₂RR) [51, 52].

Fig. 1

Three different formulas (M₂XT_x, M₃X₂T_x, and M₄X₃T_x) and compositions (mono-M MXenes and double-M MXenes) of MXenes. Reproduced with permission [25]. Copyright 2019, Wiley-VCH

Although the preparation, properties, and applications of MXenes in energy storage and conversion have been summarized [23, 25, 53,54,55], explorations of MXene-based materials for electrocatalysis have not been developed as quickly. Moreover, reviews focusing on MXene-based materials for electrocatalysts are scarce [56]. Therefore, new reviews covering research from the early stages of MXene-based materials to their present use in electrocatalysis, including their preparation, properties and the latest advances, are essential. In this review, we aim to describe recent progress on MXene-based materials for electrocatalysis.

Synthesis of MXenes

MXenes are usually obtained by selective etching of specific atomic layers from their layered precursors, such as MAX phases. To date, over 70 types of MAX phases have been reported.

Since the first MXene was synthesized by etching with HF at room temperature (Fig. 2a) [57], other types of MXenes, such as TiC₂T_x [58], Ti₃CN_xT_x [59], TiNbCT_x [60], Mo₂CT_x [61], Mo₂TiC₂T_x [62], Mo₂Ti₂C₃T_x [62], V₂CT_x [61], Ta₄C₃T_x [60], Nb₂CT_x [26], Nb₄C₃T_x [61], Zr₃C₂T_x [63], Hf₃C₂T_x [64], (Nb_0.8Ti_0.2)₄C₃T_x [65], and (Nb_0.8Zr_0.2)₄C₃T_x [65], have been successfully prepared by this method. However, the aqueous HF etching method requires the handling of high-concentration HF and a strenuous multi-step procedure. Ghidiu et al. [38] proposed a safer and easier synthetic route to MXene synthesis by in situ formation of HF via the reaction of HCl and LiF. Subsequently, other fluorides, such as NaF [66], KF [66], CsF [38], CaF [38], FeF₃ [67], and tetra-n-butylammonium fluoride [38], have been used to synthesize MXenes. NH₄F [68] and NH₄HF₂ [69] have also been employed to synthesize MXenes (e.g., Ti₃C₂T_x). Some MXenes, such as Ti₄N₃, have been obtained through etching with molten fluoride salt mixtures at high temperature [70].

Fig. 2

Schematic of the exfoliation process of Ti₃AlC₂. A HF etching method. Reproduced with permission [57]. Copyright 2011, Wiley-VCH. B LiF + HCl etching method. Reproduced with permission [71]. Copyright 2016, Wiley-VCH. C Organic base (TMAOH) method. Reproduced with permission [72]. Copyright 2016, Wiley-VCH. D KOH method. Reproduced with permission [73]. Copyright 2017, American Chemical Society. E Proposed electrochemical etching mechanism of Ti₂AlC in HCl electrolyte. Reproduced with permission [74]. Copyright 2019, American Chemical Society. F Schematic of the exfoliation process of Ti₃AlC₂ by molten ZnCl₂. Reproduced with permission [75]. Copyright 2019, American Chemical Society

Although various MXenes have been achieved by etching with HF or in situ formation of HF (Fig. 2b) [71], these methods limit the large-scale preparation and application of the catalysts owing to the acute toxicity of HF. Therefore, the development of novel HF-free methods is necessary. Xuan et al. [72], for instance, presented a strategy involving the organic base-driven intercalation and delamination of TiC (Fig. 2c). In another work, Li and co-workers [73] reported that KOH in the presence of a small amount of water can serve as an etchant to prepare MXenes (Fig. 2d). Pang et al. [74] reported a HF-free facile and rapid MXene synthesis method via thermal-assisted electrochemical etching (Fig. 2e). Li et al. [75] proposed an element-replacement approach by reaction with Lewis-acidic molten salts (Fig. 2f). Thus, future developing trends may focus on safe and efficient preparation methods for MXenes.

Structural and Electronic Properties

Structural Properties

The overall crystal geometry of MXene presents a hexagonal close-packed structure, which is analogous to its MAX-phase precursor. Here, M atoms are arranged in a close-packed structure, and octahedral sites are occupied by X atoms. The adjacent layered units are connected via van der Waals forces, similar to other 2D materials [53].

MXenes are usually prepared in aqueous solutions, including acidic fluorides. Therefore, the surface of MXenes is occupied by a mixture of −OH, −O, and −F terminations. For brevity, these molecules are denoted M_n+1X_nT_x, where T represents the surface termination. Non-terminated MXenes have never been obtained [23, 76]. Recent computational studies demonstrate that the surface termination exerts significant impacts on the properties of MXenes. For example, Hu et al. [77] systematically studied the chemical origin of termination-functionalized MXenes by Bader charge analysis and thermodynamic calculations; the materials revealed stability in the order of Ti₃C₂O₂ > Ti₃C₂F₂ > Ti₃C₂(OH)₂ > Ti₃C₂H₂ > Ti₃C₂, which was attributed to the splitting of the highly degenerated 3d orbitals of surface Ti. In another study, Fu and co-workers [78] systematically explored the effects of several functional groups (i.e., −Cl, −F, −H, −O, and −OH) on the stabilization, mechanical properties, and electronic structures of a representative MXene (Ti₃C₂); the authors found that oxygen-functionalized Ti₃C₂ shows better thermodynamic stabilization and strength than their other counterparts due to significant charge transfers from inner bonds to the outer surface of the material. While MXenes with specific terminations may be gained by a post-synthesis method, very few studies on this topic have been reported. For example, Meng et al. [79] predicted that S-functionalized Ti₃C₂ displays metallic behavior, a stable structure, a low diffusion barrier, and outstanding storage capacity for Na-ion batteries.

Besides theoretical explorations, surface termination of MXenes such as Ti₃C₂T_x and V₂CT_x has also been investigated by using experimental methods. For instance, Wang et al. [80] revealed the surface atomic scale of Ti₃C₂T_x through aberration-corrected scanning transmission electron microscopy (STEM); the group found that surface functional groups (e.g., −OH, −F, and −O) are randomly distributed on the MXene surfaces and prefer to occupy the top sites of the central Ti atom. Karlsson’s group [81] observed individual and double sheets of Ti₃C₂ by aberration-corrected STEM-EELS and revealed sheet coverage and intrinsic defects and TiO_x adatom complexes. In another study, Sang and co-workers [24] observed the different point defects in monolayer Ti₃C₂ nanosheets via STEM through the minimally intensive layer delamination method. Hope et al. [82] quantified the surface functional groups of Ti₃C₂T_x by ¹H and ¹⁹F nuclear magnetic resonance (NMR) experiments and found that the proportions of different surface terminations highly rely on the preparation method of the material. Harris et al. [83] directly measured the surface termination groups of V₂CT_x MXenes via solid-state NMR.

Electronic Properties

The applications of MXenes in electrochemistry energy storage and electrocatalysis largely rely on the inherent excellent electronic properties of the catalyst materials. Recent theoretical computational studies have been carried out to explore the effect of different M, X, and surface functional groups on the electronic properties of most MXenes. Since MXenes include various transition metals, the electronic properties of MXenes may be expected to range from metallic to semiconducting [55]. However, some MXenes have been predicted to be topological insulators because they contain heavy transition metals, such as Mo, W, and Cr [23, 25]. Additionally, surface termination could change the electronic properties of bare MXenes. For example, Fredrickson et al. [84] investigated the structural and electronic properties of layered bulk Ti₂C and Mo₂C with multiple functional groups in aqueous media by density functional theory (DFT) calculations. The out-of-plane lattice parameter of bulk MXenes is obviously affected by surface functional groups and intercalation of water. At zero applied potential, bulk MXenes (Ti₂C and Mo₂C) were functionalized by one monolayer of O. However, bare MXenes were unstable, regardless of the applied potential. In addition, changes in the surface functional groups of Ti₂C from O-covered to H-covered could promote metal–insulator transition under an applied potential. In another study, Tang et al. [28] reported that bare MXenes (e.g., Ti₃C₂) show metallic properties; however, functionalizing Ti₃C₂ with different groups (e.g., –OH, –F, and –I) yields semiconductor properties with narrow band gaps. This study also demonstrated that the M layer could obviously affect the electronic properties of the resulting material. Interestingly, whereas Ti₃C₂T_x is metallic, MXenes containing Mo display semiconductor properties. Wang and Liao [85] reported that Ni₂N MXenes show intrinsic half-metallicity using DFT calculations. The electronic properties of MXene have been related to their nanostructures. Enyashin and Ivanovskii [86], for instance, predicted that hydroxylated Ti₃C₂ nanotubes have metallic-like characteristics. Zhao et al. [87] predicted that Ti₃C₂ nanoribbons have distinct electronic properties different from those of MXenes nanosheets.

Several experiments have been performed to study the electronic properties of MXenes. However, only the electronic properties of some MXenes, such as Ti₂CT_x, Ti₃C₂T_x, and Mo₂CT_x, have been experimentally evaluated thus far [55]. For example, Halim et al. [88] evaluated the electronic conduction of Ti₃C₂T_x and Mo₂CT_x films. Lipatov and co-workers [71] measured the electronic properties of monolayer Ti₃C₂T_x flakes. Lai et al. [89] revealed the excellent electronic properties of 2D Ti₂CT_x. Computational and experimental results demonstrate that MXenes have excellent electronic properties and are promising candidate materials for electrochemistry, energy storage, and electrocatalysis.

Applications in Electrocatalysis

In the following section, we summarize current progress on the applications of MXene-based materials in electrocatalysis, including HER, OER, ORR, NRR, and CO₂RR, as presented in Table 1.

Table 1 Summary of the current progress on MXene-based materials as electrocatalysts

Hydrogen Evolution Reaction

Hydrogen is a promising energy carrier that may be harnessed to solve energy and environmental problems due to its high energy density and environmental friendliness. Electrochemical water splitting via HER offers the possibility of obtaining hydrogen through a clean and sustainable strategy. Pt-based catalysts display excellent performance for HER, but their high cost and scarcity seriously hinder their practical applications [121, 122]. Therefore, exploring earth-abundant electrocatalysts that can potentially replace Pt is of paramount importance. As described earlier, MXenes exhibit outstanding electronic properties due to their inherent metallic character. Thus, the development of MXene-based HER electrocatalysts has attracted extensive attention.

Several theoretical calculations and experiments on MXene-based catalysts have been carried out to explore their applications in HER. Seh et al. [90] first performed a combined theoretical calculation and experimental study on pristine MXenes as electrocatalysts for HER. DFT calculations revealed that Mo₂CT_x is a promising candidate HER catalyst, as shown in Fig. 3a, b. Experiments indicated that Mo₂CT_x requires an overpotential of 189 mV to reach a current density of 10 mA/cm² (Fig. 3c) and, thus, is superior to Ti₂CT_x. In addition, theoretical calculations indicate that the basal planes of Mo₂CT_x could act as active sites for HER, which is clearly different from the mechanism of the widely studied 2H phase MoS₂.

Fig. 3

A HER volcano curves with theoretical overpotentials for the studied MXenes. The stars represent two MXenes (Ti₂C and Mo₂C) in the experiment. B Zoomed-in portion of the top of the volcano in A. C Polarization curves of Ti₂CT_x and Mo₂CT_x nanosheets and Pt nanoparticles. Reproduced with permission [90]. Copyright 2016, American Chemical Society. D Free energy diagrams of HER processing on Ti₂CO₂, V₂CO₂, Nb₂CO₂, Ti₃C₂O₂, and Nb₄C₃O₂. Reproduced with permission [123]. Copyright 2016, American Chemical Society. E Polarization curves of glassy carbon, Ti₃C₂T_x (F:Ti = 0.28), Mo₂Ti₂C₃T_x (F:Mo = 0.05), Mo₂TiC₂T_x (F:Mo = 0.04), Mo₂CT_x (F:Mo = 0.02), and 20% Pt/C. Reproduced with permission [91]. Copyright 2018, American Chemical Society. F Polarization curves of E-Ti₃C₂T_x, E-Ti₃C₂(OH)_x, E-Ti₃C₂O_x, E-Ti₃C₂T_x-450, and Pt/C. Reproduced with permission [44]. Copyright 2011, Wiley-VCH. G Polarization curves of pristine Ti₂AlC, layered Ti₂CT_x nanosheets, and Pt/C. Reproduced with permission [124]. Copyright 2018, Elsevier Ltd

Gao et al. [123] studied the HER performance of various O-terminated 2D MXenes, such as Ti₂C, V₂C, Nb₂C, Ti₃C₂, and Nb₄C₃, by DFT calculations. As shown in Fig. 3d, different MXenes displayed different Gibbs free energies for the adsorption of atomic hydrogen (ΔG_H*0) under different coverages. Among the MXenes tested, Ti₃C₂O₂ notably showed the lowest ΔG_H*0 with a hydrogen coverage of 4/8. The ΔG_H*0 of Cr₂CO₂ MXene with different hydrogen coverages has also been obtained by Cheng et al. [92].

Theoretical calculations indicate that modifying the surface functional groups of MXenes is an effective route to boost their HER performance, and many of these results have been verified by experiments. For example, Handoko et al. [91] first investigated the effect of five MXenes with different F coverages on their basal plane for HER and found that the presence of F terminations deteriorates the HER performance of these materials. As-obtained Mo₂CT_x featuring low F coverage only required 189 mV to achieve a current density of 10 mA/cm² (Fig. 3e). Besides, oxygen groups on the basal planes of Mo₂CT_x proved to be catalytically active for HER. Jiang et al. [44] prepared oxygen-functionalized ultrathin Ti₃C₂T_x and achieved a HER performance (190 mV at 10 mA/cm²) higher than that of untreated Ti₃C₂T_x, as shown in Fig. 3f. Li et al. [124] synthesized rich F-terminated Ti₂CT_x and obtained excellent HER performance with a small overpotential of 170 mV at a current density of 10 mA/cm² (Fig. 3g). V₄C₃T_x MXene has also been synthesized and directly used as a HER catalyst [93].

Yoon et al. [94] synthesized nitrided-Ti₂CT_x (N-Ti₂CT_x) via the high-temperature nitridation of 2D Ti₂CT_x using NaNH₂ (Fig. 4a). As shown in Fig. 4b, the obtained N-Ti₂CT_x showed high HER catalytic performance with an overpotential of 215 mV at a current density of 10 mA/cm², which is over three times smaller than that of pristine-Ti₂CT_x (645 mV). The effects of nanostructures on the HER catalytic activity of MXenes have been explored. For instance, Yang et al. [125] constructed 12 types of MXenes nanoribbon models and evaluated the role of MXenes nanoribbon edges on catalyzing HER. Nanoribbons of Ti₃C₂ and solid solution (Ti, Nb)C showed outstanding performance for HER and revealed low adsorption free energies (close to 0 eV) and small Tafel barriers below 0.42 and 0.17 eV, respectively (Fig. 4c–e). Ti₃C₂T_x MXene nanofibers were successfully prepared by Yuan and co-workers [95], and the obtained nanofibers displayed enhanced HER activity with a small overpotential of 169 mV at 10 mA/cm² (Fig. 4f) and a Tafel slope of 97 mV/dec.

Fig. 4

a Preparations of the N-Ti₂CT_x nanosheets. b Polarization curves of Ti₂CT_x with different nitridation degrees, pristine-Ti₂CT_x, TiN, and Pt/C. Reproduced with permission [94]. Copyright 2018, Royal Society of Chemistry. c Free energy diagrams for hydrogen evolution on the edges of various MXene nanoribbons. Free energy profiles of the Tafel reaction for H₂ formation on d the edges of Ti₃C₂ and e (Ti, Nb)C MXene nanoribbons, respectively. Reproduced with permission [125]. Copyright 2018, Royal Society of Chemistry. f Polarization curves of Ti₃C₂ flakes, Ti₃C₂ nanofibers, and Pt/C. Reproduced with permission [95]. Copyright 2018, American Chemical Society

Hybridizing active components with MXene is yet another effective route to improve HER catalytic activity. Thus, the development of MXene-based nanohybrids has drawn wide attention. You et al. [126] predicted that Schottky barrier-free hole contacts could be formed at six MXenes (i.e., V₄C₃O₂, Mo₂CO₂, V₂NO₂, Cr₂NO₂ Cr₂CO₂, and V₂CO₂) and 2H-MoS₂ contacting interfaces. The formation of unique interfaces was attributed to the high work functions of the MXenes (Fig. 5a), which were larger than the ionization energy of monolayer 2H-MoS₂, and the absence of the formation of interfacial gap states that usually strongly pin the Fermi level in the midgap of the semiconductor. The authors also found that efficient charge injection into MoS₂ facilitated by the Schottky barrier-free contact could also increase the HER activity of the 2H-MoS₂ basal plane by improving its conductivity as well as its ability to adsorb hydrogen, being comparable to 1T-MoS₂ (Fig. 5b, c). In another study, Ling et al. [127] used DFT calculations to predict that a low S vacancy concentration (~ 2.5%) in MoS₂/MXenes-OH heterostructures could result in the ideal free energy needed to enhance hydrogen evolution. The simulation results indicated that the HER catalytic performance of MoS₂ could be remarkably improved by forming heterostructure with MXenes. These findings were also verified by recent experimental studies. For instance, our group developed a facile method to prepare hierarchical nanoroll-like MoS₂/Ti₃C₂T_x hybrids by combining liquid nitrogen-freezing and annealing (Fig. 5d) [42]. The resulting unique hierarchical MoS₂/Ti₃C₂T_x nanohybrid showed outstanding HER catalytic activity with a low onset overpotential of 30 mV (Fig. 5e) and an over-25-fold increase in exchange current density compared with MoS₂. Attanayake et al. [100] prepared vertically aligned interlayer expanded MoS₂ on a 2D Ti₃C₂ MXene by the microwave-assisted method. The resultant few-layered MoS₂ showed a uniform interlayer spacing of 9.4 Å and delivered a small onset potential of 95 mV (Fig. 5f) and Tafel slope (~ 40 mV/dec). Wu et al. [103] presented hierarchical MoS₂/Ti₃C₂-MXene@C nanohybrids by coupling MoS₂ nanosheets on carbon-stabilized Ti₃C₂ MXene. The obtained catalyst exhibited excellent performance with a low overpotential of 135 mV at 10 mA/cm² and a low Tafel slope of 45 mV/dec; these values are smaller than those of other counterpart catalysts (Fig. 5g). Indeed, our group presented Co-MoS₂/Mo₂CT_x nanohybrids by engineering Co-doped MoS₂ coupled with Mo₂CT_x MXene [98]. The resulting hybrids exhibited a low overpotential of 112 mV at 10 mA/cm² and good stability in 1 mol/L KOH aqueous solution.

Fig. 5

a Work functions (denoted by dots) of different MXenes with O terminations compared with the ionization energies (denoted by dashed lines) of monolayer MoS₂, WS₂, MoSe₂, WSe₂, and MoTe₂. b Schematic of the MoS₂-catalyzed HER and metallic energy-band feature of MoS₂ induced by p-type Schottky barrier-free contact. c Hydrogen adsorption energies of 2H-MoS₂, 1T-MoS₂, and 2H-MoS₂/MXene heterostructures. The numbers in brackets represent the corresponding different H coverages. Reproduced with permission [126]. Copyright 2019, American Chemical Society. d Typical TEM and SEM images of a nanoroll-like MoS₂/Ti₃C₂T_x hybrid and schematic of the MoS₂/Ti₃C₂T_x-catalyzed HER process. e Polarization curves of a MoS₂/Ti₃C₂T_x hybrid, pure MoS₂, Ti₃C₂T_x nanosheets, and Pt/C. Reproduced with permission [42]. Copyright 2019, Elsevier Ltd. f Polarization curves of interlayer expanded-MoS₂/Ti₃C₂ at various temperatures. Reproduced with permission [100]. Copyright 2018, Royal Society of Chemistry. g Polarization curves of MoS₂/Ti₃C₂-MXene@C, MoS₂/oxidized MXene, MoS₂/rGO@C, Ti₃C₂ MXene, and Pt/C catalysts. Reproduced with permission [103]. Copyright 2011, Wiley-VCH. h Reaction free energy (ΔG_H*) of HER on the most active sites of different graphene/MXene heterostructures and on the Pt (111) surface. i Changes in ΔG_H* during HER on N-doped graphene over a V₂C MXene monolayer. Reproduced with permission [128]. Copyright 2018, Royal Society of Chemistry

Du et al. [102] reported the in situ growth of the Ni-based bimetal phosphorus trisulfide (Ni_1−xFe_xPS₃) on the surface of Ti₃C₂T_x MXene nanosheets by a simple self-assembly and subsequent solid-state reaction process. The optimized hybrids (Ni_0.7Fe_0.3PS₃@MXene) exhibited a low overpotential of 196 mV for HER in 1 mol/L KOH solution. Zhou et al. [128] theoretically designed several heterostructures of N-doped graphene/MXenes (Ti₂C, Nb₂C, V₂C, and Mo₂C) as catalysts for HER. DFT calculations suggested that N-doped graphene/heterostructures possess the lowest reaction free energies (close to 0 eV) and a low Tafel reaction barrier (1.3 eV) for HER (Fig. 5g, h) owing to the strong electronic coupling between the MXene and N-doped graphene.

Recent studies indicate that the HER performance of MXenes could be improved by doping with metal atoms. Li et al. [129] studied the HER properties of modified M₂XO₂-type MXenes bearing transition metal atoms by high-throughput computational methods. Addition of transition metal atoms to several combinations, such as Os-Ta₂CO₂, Ir-Sc₂CO₂, Ag-Nb₂NO₂, Re-Nb₂NO₂, and W-Nb₂NO₂, could change the relevant reaction mechanism (from Volmer–Heyrovsky to Volmer–Tafel), induce electron redistributions on the surface of the MXene, and, ultimately, result in distinct enhancements in HER activity. Du et al. [43] reported an MXene (Ti₃C₂T_x)-based hybrid with simultaneous Nb doping and surface Ni/Co alloy modification. DFT calculations indicated that Nb doping could shift the Fermi energy level toward the conduction band, leading to improved conductivity. Moreover, the surface M–H affinity was modified by the Ni/Co alloy, and the optimized catalyst showed the lowest Gibbs free energy for adsorbed H* (Fig. 6a, b). The resultant Ni_0.9Co_0.1@NTM (Nb-doped Ti₃C₂T_x) hybrids delivered excellent HER performance, only requiring a small overpotential of 43.4 mV to deliver a current density of 10 mA/cm² in 1 mol/L KOH solution (Fig. 6c, d), and exhibited long-term stability. Using in situ co-reduction, Li et al. [96] prepared Pt/Ti₃C₂T_x via alloying Pt with Ti from the surface of Ti₃C₂T_x. In situ X-ray absorption spectroscopy revealed that Pt transforms from a single atom into intermetallic compounds with increasing temperature (Fig. 6e). The as-prepared Pt/Ti₃C₂T_x-550 showed outstanding HER performance and only needed a low overpotential of 32.7 mV at 10 mA/cm² (Fig. 6f); it also demonstrated a small Tafel slope of 32.3 mV/dec. HER current normalization processing revealed that the respective mass activity and specific activity of Pt/Ti₃C₂T_x-550 are 4.4 and 13 times higher than those of Pt/Vulcan at an overpotential of 70 mV (Fig. 6g, h). As shown in Fig. 6i, DFT calculations demonstrated that (100)- and (111)-terminated Pt₃Ti nanoparticles show H* binding comparable with Pt (111). However, (110)-termination showed that H* adsorption was excessively exergonic, leading to poisoning of the relative overpotential.

Fig. 6

a Atomistic configuration of pristine monolayer Ti₃C₂O₂ with H* adsorption, Nb doped on pristine monolayer Ti₃C₂O₂ with H* adsorption, Co/Ni replacement of Ti atoms on Nb-doped pristine monolayer Ti₃C₂O₂, and the three different H* adsorption O sites. b Gibbs free energies for H* adsorbed on active sites shown in a M-doped Ti₃C₂O₂. c Polarization curves of a series of NiCo@Nb-doped Ti₃C₂T_x MXene nanohybrids, Ni@Nb-doped Ti₃C₂T_x MXene nanohybrid, Nb-doped Ti₃C₂T_x MXene, and Pt/C in 1 mol/L KOH. d Corresponding Tafel plots of a series of NiCo@Nb-doped Ti₃C₂T_x MXene nanohybrids, Ni@Nb-doped Ti₃C₂T_x MXene nanohybrid, Nb-doped Ti₃C₂T_x MXene, and Pt/C in 1 mol/L KOH. Reproduced with permission [43]. Copyright 2019, Wiley-VCH. e Magnitude of the Fourier transform of the k² weighted Pt L_III edge in situ EXAFS of Pt/Ti₃C₂T_x reduced at different temperatures compared with that of Pt/SiO₂. f Polarization curves of Pt/Vulcan, Pt/Ti₃C₂T_x at different temperatures, and Ti₃C₂T_x. g Mass activity of Pt/Vulcan and Pt/Ti₃C₂T_x catalysts with different treatments. h Specific activity of Pt/Vulcan and Pt/Ti₃C₂T_x catalysts. i DFT-calculated free energy diagrams of hydrogen evolution at the Pt (111), Pt₃Ti (111), Pt (100), Pt₃Ti (100), Pt (110), and Pt₃Ti (110) surfaces. Reproduced with permission [96]. Copyright 2019, American Chemical Society

In another work, Zhang et al. [104] reported a novel electrochemical exfoliation method to prepare Mo₂TiC₂T_x MXene nanosheets for HER. The obtained nanosheets possessed an abundance of exposed basal planes and Mo vacancies providing numerous active sites on which to immobilize single atoms and improve the HER catalytic property of the MXenes (Fig. 7a). Pt atoms anchored onto the Mo₂TiC₂T_x nanosheets showed excellent catalytic performance. The obtained Mo₂TiC₂T_x–Pt_SA catalysts only needed low overpotentials of 30 and 77 mV to deliver current densities of 10 and 100 mA/cm², respectively. The as-prepared catalyst showed an outstanding mass activity of 8.3 A/mg, which is around 40 times greater than that of commercial Pt/C (0.21 A/mg; Fig. 7b, c). Strong covalent bonding between Mo₂TiC₂T_x and positively charged Pt atoms endowed the Mo₂TiC₂T_x–Pt_SA catalyst with outstanding long-term stability. DFT calculations suggested that single-atom Pt could lead to the redistribution of the electronic structure of Mo₂TiC₂T_x and move up the d orbitals-electron domination close to the Fermi level (Fig. 7d, e), resulting in improved catalytic activity. As presented in Fig. 7f, the obtained Mo₂TiC₂T_x–Pt_SA catalyst showed a low adsorption energy of − 0.08 eV, which is significantly lower than those of Mo₂TiC₂O₂ (− 0.19 eV) and Pt/C (− 0.10 eV). Other Pt-modified MXenes nanohybrids have been achieved by different methods, such as photo-induced reduction [97], NaBH₄ reduction [99], and solution plasma modification [101], and the resulting hybrids generally showed remarkably improved performance for HER compared with pristine MXenes. Xiu et al. [105] synthesized CoP-3D MXene hybrids and studied their catalytic performance for HER. The obtained nanohybrids showed a low overpotential of 168 mV at a current density of 10 mA/cm² in 1 mol/L KOH solution. This superior electrocatalytic activity could be attributed to the hierarchical 3D architecture of the catalysts, which greatly boosts their active surface area, promotes higher charge transfer kinetics, and increases their mass diffusion rate.

Fig. 7

a Schematic of the electrochemical exfoliation process of MXene with immobilized single Pt atoms. b Polarization curves of carbon paper, Mo₂TiC₂T_x, Mo₂TiC₂T_x–V_Mo, Mo₂TiC₂T_x–Pt_SA, and Pt/C. c Mass activity of state-of-the-art Pt/C and Mo₂TiC₂T_x–Pt_SA. d Top view of the slab models used to describe Mo₂TiC₂O₂ and Mo₂TiC₂O₂–Pt_SA. Circles in blue, purple, green, brown, and red represent Ti, Mo, Pt, C, and O atoms, respectively. Charge density distribution differences between Mo₂TiC₂O₂ and Mo₂TiC₂O₂–Pt_SA are also shown. e Calculated PDOS of Mo₂TiC₂O₂ and Mo₂TiC₂O₂–Pt_SA with aligned Fermi levels. f Free energy profiles of HER on Mo₂TiC₂O₂, Mo₂TiC₂O₂–Pt_SA and Pt/C. Reproduced with permission [104]. Copyright 2018, Nature Publishing Group

Oxygen Evolution Reactions

OER plays a crucial role in many important renewable energy conversion and storage methods, such as electrochemical water splitting and metal–air batteries. Nevertheless, the sluggish kinetics and high overpotential of OER make it imperative to search for high-performance catalysts. Noble-metal-based oxides (e.g., RuO₂ and IrO₂) are high-performance catalysts for OER, but their high cost and scarcity seriously hamper their broader applications [130]. Hence, great efforts have been devoted to develop earth-abundant and high-activity catalysts that can replace precious metal-based materials.

MXene-based materials have received extensive attention for their potential applications in OER. For instance, Yu et al. [45] prepared hierarchical FeNi-LDH/Ti₃C₂T_x nanohybrids by coprecipitation of Ni²⁺ and Fe³⁺ in the presence of Ti₃C₂T_x and urea (Fig. 8a). The as-synthesized hybrids showed superior OER activity and only needed a low overpotential of 298 mV to deliver a current density of 10 mA/cm²; they also demonstrated a low Tafel slope of 43 mV/dec (Fig. 8b–e). This achievement could be attributed to strong interfacial interactions and electronic coupling with prominent charge transfers between Ti₃C₂T_x and FeNi-LDH. Such interaction and coupling not only enhance the conductivity and stability but also obviously facilitate the redox process of FeNi-LDH for OER. Ma et al. [108] prepared free-standing flexible films via the layer-by-layer self-assembly of graphitic carbon nitride (g-C₃N₄) and titanium carbide (Ti₃C₂), as shown in Fig. 8f. The obtained hierarchically porous films featured a highly hydrophilic surface and showed good OER activity; indeed, the catalysts only needed a small overpotential of 420 mV to achieve a current density of 10 mA/cm² and revealed a small Tafel slope of 74.6 mV/dec in 0.1 mol/L KOH solution (Fig. 8i, j). Such excellent performance was attributed to the Ti–N_x motifs of the catalyst, which act as electroactive sites. This hypothesis was verified by XPS and near-edge X-ray absorption fine structure spectroscopy (Fig. 8g, h). Tang and co-workers [109] constructed S-NiFe₂O₄@Ti₃C₂@NF hybrids and studied their OER performance. The obtained hybrids displayed a low overpotential of 270 mA at a current density of 20 mA/cm² and a small Tafel slope of 46.8 mV/dec in 1 mol/L KOH solution.

Fig. 8

a Schematic of the preparation of 2D hierarchical FeNi-LDH/Ti₃C₂-MXene nanohybrids. b Polarization curves of FeNi-LDH/Ti₃C₂-MXene with different FeNi-LDH contents, pure FeNi-LDH, Ni(OH)₂, and pristine Ti₃C₂ MXene. c Polarization curves of FeNi-LDH/Ti₃C₂-MXene with 80 wt% FeNi-LDH, FeNi-LDH + Ti₃C₂-MXene, FeNi-LDH/rGO, and RuO₂ catalysts. d Comparison of the catalysts in b in terms of onset overpotential and overpotential required to achieve a current density of 10 mA/cm². e Tafel plots of FeNi-LDH/Ti₃C₂-MXene, FeNi-LDH + Ti₃C₂-MXene, FeNi-LDH/rGO, FeNi-LDH, Ni(OH)₂, and RuO₂ catalysts. Reproduced with permission [45]. Copyright 2018, Elsevier Ltd. f Preparation of porous Ti₃C₂/g-C₃N₄ hybrid films. g High-resolution XPS spectra of Ti 2p in the Ti₃C₂/g-C₃N₄ hybrid film in (f). h N K-edge NEXAFS of TCCN and g-C₃N₄; the inset displays the relevant N sites. i Polarization curves of the Ti₃C₂/g-C₃N₄ hybrid film, g-C₃N₄, Ti₃C₂, and IrO₂/C. j Tafel plots of the Ti₃C₂/g-C₃N₄ hybrid film, g-C₃N₄, Ti₃C₂, and IrO₂/C. Reproduced with permission [108]. Copyright 2016, Wiley-VCH

Interestingly, metal organic framework (MOF) and MOF derivatives have also been successfully hybridized with MXene nanosheets to achieve improved OER performance. For example, Zhao et al. [107] synthesized an MXene/MOF hybrid (Ti₃C₂T_x–CoBDC) via an interdiffusion reaction-assisted method (Fig. 9a). The resultant hybrids needed a low overpotential of 410 mV to deliver a current density of 10 mA/cm² and showed a low Tafel slope of 48.2 mV/dec in 0.1 mol/L KOH solution (Fig. 9b, c). This superior OER performance could be attributed to the well-defined interface between the CoBDC layer and Ti₃C₂T_x nanosheets, which allows fast charge and ion transfer. The presence of metallic Ti₃C₂T_x nanosheets not only prevents the porous CoBDC layers from aggregating but also improves charge and ion transfers. In another study, CoNi-ZIF-67@Ti₃C₂T_x was prepared via a simple coprecipitation reaction [106]. Owing to the presence of Ti₃C₂T_x, the CoNi-ZIF-67 particles became smaller in size, and the average oxidation of Co/Ni elements increased, thus endowing the catalyst with excellent OER performance. The CoNi-ZIF-67@Ti₃C₂T_x hybrids showed a low onset potential of 275 mA versus RHE and a Tafel slope of 65.1 mV/dec. Zou et al. [110] prepared a novel NiCoS/Ti₃C₂T_x hybrid using an MOF-based method (Fig. 9d). The hybrids showed a small overpotential of 365 mV at 10 mA/cm², a small Tafel slope of 58.2 mV/dec (Fig. 9e, f), and excellent stability.

Fig. 9

a Preparation procedures of Ti₃C₂T_x–CoBDC hybrids for OER. b Polarization curves and c the corresponding Tafel plots of various electrodes modified by Ti₃C₂T_x, CoBDC, IrO₂, and a Ti₃C₂T_x–CoBDC hybrid in 0.1 mol/L KOH solution. Reproduced with permission [107]. Copyright 2017, American Chemical Society. d Synthesis of NiCoS/Ti₃C₂T_x hybrids. e Polarization curves and f the corresponding Tafel plots of NiCoS/Ti₃C₂T_x, NiCoS, NiCo-LDH/Ti₃C₂T_x, NiCo-LDH, and RuO₂. Reproduced with permission [110]. Copyright 2018, American Chemical Society. g Preparation of hierarchical Co-B_i/Ti₃C₂T_x hybrids at room temperature. h Polarization curves and i the corresponding Tafel plots of Co-B_i nanosheets, a Co-B_i/Ti₃C₂T_x hybrid, and Ti₃C₂T_x nanosheets. Reproduced with permission [46]. Copyright 2018, Wiley-VCH

Many new MXene-based hybrids also show promising applications in OER. For example, we synthesized a unique hierarchical cobalt borate/Ti₃C₂T_x MXene (Co-B_i/Ti₃C₂T_x) by a rapid chemical reaction at room temperature (Fig. 9g) [46]. The metallic Ti₃C₂T_x nanosheets not only improved the electron transfer capacity of the material but also hindered the aggregation of Co-B_i nanosheets. The strong interaction between Ti₃C₂T_x and Co-B_i nanosheets ensured strong charge transfer abilities and enhanced the electrostatic attraction of more anionic intermediates to achieve fast redox processes. Thus, the as-synthesized hybrids exhibited outstanding OER performance with a low overpotential of 250 mV at 10 mA/cm² and a small Tafel slope of 53 mV/dec (Fig. 9h, i).

Oxygen Reduction Reaction

ORR is the key half-reaction in renewable energy conversion devices; it is characterized by inherent environmental friendliness and low cost and has been applied to fuel cells and rechargeable metal–air batteries. However, ORR often suffers from sluggish kinetics, which seriously hinders the overall power performance of these devices. Today, Pd-based catalysts are regarded as the optimal ORR catalyst [131]. However, developing low-cost and high-efficiency catalysts for ORR remains a crucial endeavor.

Liu and Li [132] simulated a series of Pt/v-Ti_n+1C_nT_x (n = 1–3, T = O and/or F) heterostructures by DFT calculations. As displayed in Fig. 10a, F-terminated MXenes were predicted to display better performance in ORR than their O-terminated counterparts; however, F-terminated MXenes may demonstrate lower stability on account of their weaker chemical bonding. A variety of MXene-based materials have been explored to enhance ORR performance. For instance, Li et al. [48] prepared FePc/Ti₃C₂T_x hybrids by a facile self-assembly method in dimethylformamide solution. Owing to the presence of Ti₃C₂T_x, obvious Fe 3d electron delocalization and spin-state transition of Fe(II) ions were confirmed by a series of characterization analysis, such as ESR and Mössbauer spectroscopy, as presented in Fig. 10b–d. More importantly, changes in electron configuration led to lower local electron densities and higher spin states in the Fe(II) centers, which promoted oxygen adsorption and reduction in active FeN₄ sites. As shown in Fig. 10e, f, the optimized hybrids showed lower half-wave potentials (− 0.886 vs. RHE) compared with pure FePc (− 0.886 vs. RHE) and commercial Pt/C (− 0.84 V vs. RHE). The catalysts also, respectively, showed two- and fivefold higher specific ORR activity than pure FePc and commercial Pt/C in 0.1 mol/L KOH solution.

Fig. 10

a Free energy diagram of ORR intermediates on Pt/v-Ti_n+1C_nT₂ (n = 1–3, T = O or F) surfaces. Reproduced with permission [132]. Copyright 2019, American Chemical Society. b Fe Mössbauer transmission spectra and c deconvolution of pristine FePc and FePc/Ti₃C₂T_x. d X-band ESR spectra of pristine Ti₃C₂T_x, FePc, and FePc/Ti₃C₂T_x. e Polarization curves and f the corresponding Tafel plots of pristine FePc, FePc/Ti₃C₂T_x, and Pt/C. Reproduced with permission [48]. Copyright 2018, Wiley-VCH. g Preparation of Co/N-CNTs@Ti₃C₂T_x composites. h C K-edge XANES spectra of Ti₃C₂T_x MXene, Co/N-CNTs@Ti₃C₂T_x, and Co/N-CNTs. i N K-edge XANES spectra of Co/N-CNTs and Co/N-CNTs@Ti₃C₂T_x. j Ti L-edge XANES spectra of Ti₃C₂T_x MXene and Co/N-CNTs@Ti₃C₂T_x. k Polarization curves of Ti₃C₂T_x, Co/N-CNTs, Co/N-CNTs@Ti₃C₂T_x, and Pt/C. Reproduced with permission [111]. Copyright 2018, Wiley-VCH

Zhang et al. [111] presented a new type of Co/N-CNTs@Ti₃C₂T_x hybrid synthesized by an in situ growth strategy (Fig. 10g). The resulting catalyst showed superior ORR catalytic performance with a low onset potential of 0.936 V versus RHE and a half-wave potential of 0.815 V versus RHE in 0.1 mol/L KOH aqueous solution (Fig. 10k); such performance was attributed to strong interfacial coupling and electron transfers in the composite, which were well verified by XANES (Fig. 10h–j). A series of nanohybrids, such as Mn₃O₄/Ti₃C₂T_x nanocomposites [114], C₃N₄/Ti₃C₂ heterostructures [47], FeNC/MXene nanohybrids [112], urchin-like MXene-Ag_0.9Ti_0.1 nanowire composites [113], and FeCo (3:1)-N-d-Ti₃C₂ MXene hybrids [115], have also been proven to display outstanding ORR performance.

Nitrogen Reduction Reaction

NH₃ is considered a promising alternative energy carrier on account of its high energy density. At present, large-scale NH₃ production is primarily conducted via the Haber–Bosch method at high-pressure and high-temperature conditions using H₂ and nitrogen N₂ as the virgin gas. However, this process consumes large amounts of energy and generates massive amounts of CO₂. Thus, developing sustainable and economical N₂-fixation methods is urgently needed. Electrocatalytic NRR has attracted much attention due to its innate advantages, including reaction under ambient conditions and water as the hydrogen source [133]. However, NRR processes remain at the infant stages of development, and designs of efficient and low-cost electrocatalysts continue to challenge researchers.

MXene-based materials have recently been studied as catalysts for NRR. For example, Azofra et al. [50] predicted the N₂-capture behaviors of M₃C₂ MXenes using DFT calculations and found that V₃C₂ and Nb₃C₂ are excellent candidates as NRR catalysts due to their low reaction energies of 0.32 and 0.39 eV (vs. a standard hydrogen electrode), respectively (Fig. 11a). V₃C₂ showed a low activation barrier of 0.64 eV, which is smaller than that of Nb₃C₂ (0.85 eV), for the first proton–electron transfer (rate-determining step). Gao et al. [134] predicted the catalytic activity of a series of single atoms (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Cd, and Au) anchored onto Ti₃C₂O₂ by calculating their Gibbs free energies. The authors suggested that end-on N₂ adsorption is energetically advantageous and that negative free energies represent outstanding N₂ activation properties. Hydrogenations of N₂ into *NNH and of *NH₂ into NH₃ were considered possible potential-limiting steps. In another study, Cheng et al. [135] carried out DFT calculations to investigate the catalytic activity of single transition metal atom (Mo, Mn, Fe, Co, Ni, or Cu)-decorated M₂NO₂-type MXenes (M = Ti, V, and Cr) for NRR. Mo/Ti₂NO₂ was screened as a very promising candidate catalyst with a low overpotential of 0.16 eV. This result could be ascribed to the strong bonding strength between Mo and Ti₂NO₂. Moreover, Mo/Ti₂NO₂ showed a low Gibbs free energy (0.12 eV) for NH₃ desorption, which promotes NH₃ release, and exhibited excellent metallic characteristics, which could effectively promote electron transfer between Mo and Ti₂NO₂. Zheng and co-workers [136] studied the NRR performance of single-atom B-decorated MXenes using DFT calculations. Here, B-doped Mo₂CO₂ and W₂CO₂ MXenes showed excellent catalytic activity and selectivity with limiting potentials of − 0.20 and − 0.24 V, respectively (Fig. 11b–d). Hydrogenation of *N₂ into *N₂H could be facilitated by the high tendency of B-to-adsorbate electron donation. However, conversion of *NH₂ into *NH₃ was seriously hindered by strong B–N bonding.

Fig. 11

a Minimum energy path for N₂ conversion into NH₃ catalyzed by V₃C₂ (top) and Nb₃C₂ (bottom) MXenes. Reproduced with permission [50]. Copyright 2016, Royal Society of Chemistry. Free energy profiles for the NRR catalyzed by group b IV (Ti, Zr, Hf), c V (V, Nb, Ta), and d VI (Cr, Mo, W) MXenes with B centers. Reproduced with permission [136]. Copyright 2019, American Chemical Society

Some experiments have been performed to investigate the NRR activity of MXene-based materials. For example, Luo et al. [49] first verified that the central Ti atom in the MXene Ti₃C₂T_x is the most active site for N₂ adsorption (1.34 eV) by comparison with C (− 0.16 eV), O (− 1.21 eV), and lateral Ti (− 0.95 eV) atoms. In addition, the basal plane of MXene is inert relative to edge planes owing to the former’s lower exposure of Ti sites, as shown in Fig. 12a, b. When smaller Ti₃C₂T_x MXenes were dispersed on vertically aligned metal FeOOH nanosheets, a faradaic efficiency of 5.78% under − 0.2 V versus RHE was obtained; this value is 1.25 times higher than the maximum value obtained from an MXene/stainless steel mesh (4.62%) under − 0.1 V versus RHE, as presented in Fig. 12c, d. Li and co-workers [118] directly applied small-sized (~ 50–100 nm) F-free Ti₃C₂T_x nanosheets for NRR. The obtained catalyst showed an NH₃ yield of 36.9 μg/(h mg_cat) and faradaic efficiency of 9.1% at − 0.3 V versus RHE in 0.1 mol/L HCl (Fig. 12e). These values are, once again, much larger than those of F-based MXenes due to the unique size effect and fluorine-free characteristics to the novel catalysts. In another study, Zhao et al. [119] reported that Ti₃C₂T_x MXene nanosheets could serve as catalysts for NRR. The catalysts achieved an NH₃ yield of 20.4 μg/(h mg_cat) and a faradaic efficiency of 9.3% at − 0.4 V versus RHE. DFT results demonstrated that the distal NRR mechanism was more favorable, and the related *NH₂/NH₃ reaction was the rate-determining step. Zhang et al. [117] prepared TiO₂/Ti₃C₂T_x hybrids by using a simple hydrothermal method and studied their catalytic activity for NRR. The obtained hybrids were tested in 0.1 mol/L HCl and showed good catalytic performance with an NH₃ yield of 26.32 μg/(h mg_cat) and faradaic efficiency of 8.42% at − 0.60 V versus RHE (Fig. 12f, g); these results are believed to originate from the synergistic effect between TiO₂ nanoparticles and Ti₃C₂T_x nanosheets. Kong and co-workers [120] reported that an MnO₂-decorated Ti₃C₂T_x MXene nanohybrid could serve as an electrocatalyst for NRR with excellent durability and outstanding selectivity. This nanohybrid showed a large NH₃ yield of 34.12 μg/(h mg_cat) and high faradaic efficiency of 11.39% under 0.55 V versus RHE in 0.1 mol/L HCl (Fig. 12h). As shown in Fig. 12i, DFT calculations indicated that unsaturated surface Mn atoms could serve as active sites for adsorption and activation of N₂. The first hydrogenation process in this strategy was identified as the rate-determining step.

Fig. 12

a Optimized structures of Ti₃C₂T_x MXenes and the corresponding adsorption energies for N₂ on various atomic sites and H₂O on the middle Ti atomic site. b–d Faradic efficiencies of a Ti₃C₂T_x MXene/stainless steel mesh and Ti₃C₂T_x MXene/FeOOH at different potentials, respectively. Reproduced with permission [49]. Copyright 2018, Elsevier Ltd. e NH₃ yields and faradaic efficiencies of F-free Ti₃C₂T_x nanosheets and Ti₃C₂T_x/carbon paper at various potentials. Reproduced with permission [118]. Copyright 2019, Royal Society of Chemistry. f NH₃ yields and faradaic efficiencies of TiO₂/Ti₃C₂T_x at various potentials. g Amounts of NH₃ obtained from carbon paper (CP), TiO₂/CP, Ti₃C₂T_x/CP, and TiO₂/Ti₃C₂T_x/CP at − 0.6 V after 2 h of electrolysis. Reproduced with permission [117]. Copyright 2019, American Chemical Society. h NH₃ yields and faradaic efficiencies of MnO₂–Ti₃C₂T_x at various potentials. i Gibbs free energy profiles for NRR over MnO₂ (110)–MXene surfaces through the traditional distal pathway. Reproduced with permission [120]. Copyright 2019, Royal Society of Chemistry

CO₂ Reduction Reaction

Large-scale anthropogenic CO₂ emissions cause serious environmental issues, including global warming and extinction of species, among others. Converting CO₂ by CO₂RR into value-added chemicals and fuels has attracted extensive research attention due to the environment-friendly characteristics of this technology [137, 138].

The electrocatalytic CO₂RR activity of MXenes has been explored by using theoretical DFT calculations. For example, Chen et al. [139] studied different −OH terminated MXenes for CO₂RR by theoretical calculation and found that Sc₂C(OH)₂ is a highly promising candidate for catalyzing the CO₂RR of CO₂ into CH₄ with a limiting potential of − 0.53 V. This excellent performance could be attributed to the high reactivity of H atoms in the −OH termination groups of the MXene, which is conducive to the formation of stable structures with intermediates and lowering of the necessary overpotential. MXene catalysts with low charge migration during the potential-limiting step have also been suggested to demonstrate good CO₂RR performance. Li et al. [140] predicted that IV–VI series MXenes show excellent performance for CO₂ capture. Cr₃C₂ and Mo₃C₂ MXenes have been considered highly promising candidates for the selective conversion of CO₂ into CH₄. The authors also found that the formation process of OCHO· and HOCO· radicals occurs as a spontaneous reaction in the early hydrogenation steps, which was the rate-determining step of CO₂ into CH₄ conversion process. According to the calculated minimum energy path results, the CO₂ → CH₄ conversion process over bare Cr₃C₂ and Mo₃C₂ required overpotentials of 1.05 and 1.31 eV, respectively (Fig. 13a, b). However, functional group (e.g., −O or −OH)-terminated MXenes (Mo₃C₂) required very low energy inputs (Fig. 13c, d). In another study, Handoko and co-workers [51] reported that W₂CO₂ and Ti₂CO₂ are highly promising M₂XO₂ MXene candidates for CO₂RR owing to their low overpotential and good selectivity. This excellent performance could be attributed to the accessibility of the *HCOOH pathway, which is energetically more favorable compared with *CO pathway. In addition, O termination groups on MXenes help stabilize the reaction intermediates. Thus far, however, no experimental study on MXene-based catalysts for CO₂RR has yet been reported.

Fig. 13

a Side view of the minimum energy path for CO₂ conversion into *CH₄ and **H₂O catalyzed by Mo₃C₂. b Minimum energy path for CO₂ conversion into *CH₄ and **H₂O catalyzed by Cr₃C₂. Minimum energy path for CO₂ conversion into CH₄ and H₂O over c Mo₃C₂(OH)₂ and d Mo₃C₂O₂. Reproduced with permission [140]. Copyright 2019, American Chemical Society

Summary and Outlook

As an emerging class of 2D materials, MXenes show tremendous potential in electrochemical energy conversion. In this review, we systematically summarized recent advances in MXenes-based materials in electrocatalysis, including HER, OER, ORR, NRR, and CO₂RR. Many high-performance MXenes-based catalysts featuring distinct inherent properties, such as excellent metallic conductivity, rich surface chemistry, and unique morphology, have been prepared. We outlined two common strategies for improving the electrocatalytic property of MXene-based catalysts. First, surface functional groups (e.g., −O, −OH, and –F) and exposed terminal metal sites (e.g., Ti, Mo, Nb, and V) can serve as catalytic activity sites, as verified by theoretical calculations and experiments. Thus, regulating the surface chemistry of these molecules is a promising strategy to enhance the electrocatalytic property of MXenes. Second, constructing nanohybrids with other active components (e.g., nanoparticles, monoatomics, and other 2D materials) is another effective strategy to improve the electrocatalytic performance of MXene-based materials. The surface functional groups of MXenes endow them with the ability to easily form strong interactions with different components. Many metallic MXenes show enhanced charge-carrier transfer properties, and their 2D structure can prevent the active materials from aggregating.

Despite the initial successes obtained from MXene-based electrocatalysts, however, many challenges remain to be solved. For example, more novel MXenes must have been predicted and synthesized by theoretical calculations and experimental methods. The electrocatalytic performance of these materials should also be systematically investigated. The electrocatalytic performance of MXenes-based materials for some applications (e.g., CO₂RR) remains mostly theoretical. Thus, experimental studies should be performed to verify the results of theoretical calculations. Moreover, great efforts have been exerted to develop MXene-based catalysts for electrocatalysis, but elucidating the relevant catalytic mechanism has proven to be difficult. Therefore, more advanced characterizations (e.g., in situ microscopy and spectroscopy) and theoretical calculations must be conducted to promote the rational design of MXene-based catalysts.