Hierarchical N-Doped Porous Carbons for Zn–Air Batteries and Supercapacitors
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Hierarchical N-doped porous carbons (NPCs) with large surface area and controllable N-doping are synthesized by ball milling, followed by pyrolysis.
As a Zn–air battery cathode, NPCs have comparable discharge performance to precious metal catalysts and more stability.
NPCs also exhibit an excellent specific capacity and cycling stability when used as supercapacitor electrodes.
KeywordsPorous carbon Ball milling Nitrogen doping Oxygen reduction reaction Zn–air battery Supercapacitor
Traditional fossil resources are progressively being replaced by sustainable and clean energy sources. However, the movement toward a greener economy requires energy storage. Electrochemical energy devices, such as fuel cells, metal–air batteries, and supercapacitors, are expected to play a crucial role in the transition to a sustainable future . The oxygen reduction reaction (ORR) is an important cathodic reaction in fuel cells and metal–air batteries. The development of highly efficient and stable ORR catalysts could resolve the bottleneck of fuel cells and metal–air batteries [2, 3]. Noble metal Pt and Pt-based alloys are the most efficient ORR electrode materials. However, although Pt-based catalysts have high ORR activity, there are several associated hindrances, such as CO poisoning, metal dissolution, poor stability, scanty supply of noble metals, and high costs . To resolve these issues, extensive efforts have been devoted to designing high-activity and more stable non-precious metal-based ORR catalysts [5, 6]. In 2009, Gong prepared a metal-free catalyst (nitrogen-doped VA-CNT) as an ORR electrocatalyst, and its performance exceeded that of commercial Pt/C . Subsequently, carbon-based electrocatalysts with other heteroatoms (such as B, P, and S) have also been developed, which exhibited excellent ORR catalytic activity [8, 9, 10]. An excellent ORR electrocatalyst typically exhibits the following characteristics: (1) a large number of active sites, (2) high specific surface area, which increases the three-phase boundary and active surface area , (3) outstanding conductivity, which accelerates electron transfer during electrocatalysis, and (4) excellent stability, which must be considered in practical applications. The high specific surface area and high conductivity considered desirable here are also required in supercapacitors .
Among the many metal-free catalysts, N-doped porous carbon (NPC) could be used in precious metal catalysts due to its high catalytic activity and specific surface area, good electrical and thermal conductivity, and easily adjustable microstructure [2, 9, 13, 14]. Furthermore, NPCs could improve the capacitance of the catalyst via surface Faradaic reactions . Micropores play an important role in the adsorption of ions [16, 17], while mesopores accelerate the transport of ions to the bulk of the material [18, 19]. Hierarchical micro-mesoporous structures can provide numerous accessible active sites and accelerate the mass transport of ORR-related species (O2, protons, and H2O) [20, 21, 22]. Li et al. demonstrated that N-doped hierarchical porous carbon (N-HPC) has a larger specific surface area, faster charge transferability, and better surface wettability than undoped HPC [23, 24].
The pyrolysis of N-containing organic polymers is a very simple and efficient method of preparing NPCs and allows a certain degree of morphological control [25, 26, 27]. The porosity and catalytic activity of the NPC can be optimized by adjusting the carbonization temperature . However, the synthesis of N-containing polymers often requires the use of catalysts or large amounts of organic solvents to promote the reaction. Thus, the synthesis process is relatively complicated [25, 26, 27]. In addition, the mass use of organic solvents may also cause harm to both humans and the environment; therefore, the process requires further improvement. Compared to the various strategies for preparing NPCs, ball milling induces chemical polymerization reactions by mechanical force, which is a scalable, solvent-free, low-cost, environmentally and economically sustainable approach [28, 29, 30]. Additionally, ball mills are common in modern industrial production; therefore, preparing NPCs by a mechanochemical method is more likely to result in a technology that can be translated.
Herein, we explore a mechanochemical strategy of synthesizing N-containing polymer precursors using planetary ball mills without solvents and catalysts with isophthalaldehyde and p-phenylenediamine as the carbon sources. During ball milling, the amine and aldehyde groups condense, leading to the formation of polyamine-based polymers . The pyrolysis of such polymer precursors would produce NPCs with large numbers of micropores [27, 31]. To optimize the pore structure, we introduced nanosized SiO2 spheres as a template to produce mesoporous structures during ball milling. The hierarchical NPC is obtained by the pyrolysis of the polymer precursor containing SiO2, followed by the removal of the SiO2 template. The resultant NPC has a large specific surface area (641–1013 m2 g−1), the abundant micropores and mesopores which are beneficial for the adsorption and transport of ions and reactants. NPC-1000, which was prepared at 1000 °C, exhibits excellent ORR activity in KOH solutions. The onset potential (Eonset, defined as the potential when the current density reaches 0.1 mA cm−2) and half-wave potential (E1/2) of NPC-1000 are only 30 mV lower than those of commercial Pt/C. However, the stability and methanol tolerance of NPC-1000 are much better than that of Pt/C. Zn–air batteries (ZABs) with an NPC-1000 cathode achieve a high open-circuit voltage of 1.43 V, and comparable discharge performance and energy density to those of Pt/C. Its cycling stability is also better than that of Pt/C. Furthermore, owing to the higher nitrogen content and hierarchical pore structure, NPC-800 exhibits outstanding capacitive behavior as a supercapacitor electrode (256 F g−1 at 0.5 A g−1 and 431 F g−1 at 10 mV s−1) and excellent cycling stability (98.7% retention after 10,000 cycles at 10 A g−1) in an aqueous 6-M KOH electrolyte.
2.1 Preparation of Nitrogen-Doped Porous Carbon
All chemical reagents are of analytical-grade and used without further purification. To prepare the NPC, isophthalaldehyde (1.13 g, 8.5 mmol), p-phenylenediamine (0.91 g, 8.5 mmol), and 2.04 g of 12-nm silica spheres (mass ratio of SiO2 to the carbon sources is 1:1) are placed in a zirconium oxide grinding jar with twenty-four 5-mm zirconium oxide grinding balls. The mixture is milled at 500 rpm for 5 h using an XGB2 planetary ball mill to form a yellow precursor. The precursor is subsequently dried at 80 °C overnight and pyrolyzed in a tube furnace at 700–1100 °C for 2 h at a heating rate of 5 °C min−1 under an argon atmosphere (denoted as NPC-T, where T is the pyrolysis temperature). The silica template is removed by leaching with 2 M NaOH at 80 °C for 4 h. The above leaching process is repeated twice to ensure that the silica is completely removed. The final product is washed with water and ethanol until the pH of the filtrate is approximately 7 and then dried at 80 °C.
NPC-1000-0 and NPC-1000-2 are synthesized following the approach described above, where 1000 indicates that the pyrolysis temperature is 1000 °C, and 0 and 2 indicate the mass of SiO2, which is zero and two times that of the total carbon sources (isophthalaldehyde and p-phenylenediamine).
Fe-doped NPC (Fe-NPC) is also synthesized following the same procedure as NPC-1000, with the addition of 0.081 g of FeCl3 during ball milling.
2.2 Material Characterization
Powder X-ray diffraction (XRD) analysis is conducted using a D8 ADVANCE instrument with Cu Kα radiation (40 kV, 60 mA). The morphologies are characterized from scanning electron microscopy (SEM) images obtained using a field emission scanning electron micro-analyzer (FEI Magellan 400), and transmission electron microscopy (TEM; JEM-2100F). Raman spectra are obtained with a DXR Raman Microscope (Thermal Scientific Co., USA) with an excitation length of 532 nm. Two spectra are obtained for each sample to ensure accuracy. Nitrogen adsorption–desorption isotherms are measured at − 196 °C using an ASAP 2010 accelerated surface area and pore size analyzer system (Micrometitics, Norcross, GA). The specific surface areas are obtained following the multipoint Brunauer–Emmett–Teller (BET) method. The pore-size distribution curves, pore volume, and pore diameter are calculated using the adsorption branch of the isotherms following the Barrett–Joyner–Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) measurements are used to analyze the surface of the samples with an ESCALAB 250 X-ray photoelectron spectrometer and Al Kα (hν = 1486.6 eV) radiation.
2.3 Electrochemical Measurements
The catalyst ink is prepared by blending the catalyst powder (5 mg) with 20 μL of a Nafion solution (5 wt%), 500 μL of ethanol, and 500 μL of deionized water in an ultrasonic bath, and 20 μL of the catalyst ink is pipetted and spread onto the glassy carbon (GC) electrode (catalyst loading amount of 0.5 mg cm−2). For comparison, commercial 20 wt% platinum on Vulcan carbon black (Pt/C) with the same loading amount is analyzed under the same conditions.
All electrochemical measurements are conducted in a conventional three-electrode cell using a CHI760E electrochemical workstation (CH Instrument) at room temperature. A GC electrode (5.0 mm diameter) is used as the working electrode, and a saturated calomel electrode (SCE) and graphite rod are used as the reference and counter electrodes, respectively. The ORR electrochemical experiments are conducted in an O2-saturated 0.1 M KOH electrolyte. To remove the capacitive current of the working electrode, the background current is measured by running the above electrodes in N2-saturated 0.1 M KOH. In the reported ORR polarization curves, the background current is subtracted from the capacitive current. All measured potentials are converted to the potentials versus the reversible hydrogen electrode (RHE) based on ERHE = ESCE + 0.2415 + 0.059 × pH.
2.4 Zn–Air Battery Measurements
Liquid Zn–air battery (ZAB) tests are conducted using a homemade Zn–air cell. The air cathode consists of a hydrophobic carbon paper with a gas diffusion layer on the air-facing side and a catalyst layer on the water-facing side. The loading amount for all catalysts is 0.25 mg cm−2. A polished Zn plate with a thickness of 0.3 mm is used as the anode. The electrolyte used for ZAB is 6.0 M KOH containing 0.20 M Zn(Ac)2. To evaluate the potential of NPC-1000 in a real device, NPC-1000 and NiFe-LDH (NiFe-layered double hydroxide) with a mass ratio of 1:1 are used as the air cathode. NiFe-LDH achieves excellent OER activity, which reduces the charging voltage of the Zn–air battery. Cycling tests are conducted using a Land CT2001A system, and each discharge and charge period are set to be 30 min. The charge–discharge polarization curve is tested using a PINE electrochemical workstation (Pine Research Instrumentation, USA). For comparison, a mixture of noble metal Pt/C and RuO2 (mass ratio 1:1) with the same loading amount is also tested.
2.5 Supercapacitor Measurements
3 Results and Discussion
The surface chemical compositions and surroundings of the as-prepared samples are investigated by XPS analysis, and an XPS survey scan of the NPC-1000 is presented in Fig. 3d, which shows the three peaks attributable to C, N, and O. This suggests that N atoms are still present in the carbon subjected to high-temperature pyrolysis. The high-resolution N 1s spectra show three peaks for pyridinic N (~ 398.7 eV), pyrrolic N (~ 400.5 eV), and graphitic N (~ 401.2 eV) [21, 37]. Previous studies demonstrated that pyridinic N and graphitic N are beneficial for improving the electrochemical performance, as N-doping could modify the electronic structure and improve the surface hydrophilicity, conductivity, and adsorption performance, as well as the Fermi levels of the adjacent carbon atoms [38, 39, 40, 41]. The presence of C–N in the high-resolution C 1s spectra further confirms that N atoms are doped into the carbon skeleton. For comparison, the XPS of NPC-800 and NPC-1100 are also measured (Fig. S2, Table S2), and the results show that the proportions of pyridinic N, pyrrolic N, and graphitic N are almost the same after pyrolysis at different temperatures. NPC-800, NPC-1000, and NPC-1100 have nitrogen contents of 4.3, 2.2, and 1.1 at.%, respectively. That is, the higher the pyrolysis temperature, the lower the nitrogen content. Additionally, the oxygen-containing functional groups on the carbon surface can also affect the doping of N and promote the introduction of N to the highly active sites of the carbon lattice .
The effect of specific surface area and pore structure on the catalytic activity of ORR is also investigated by changing the amount of SiO2 template particles. As shown in Fig. S4, NPC-1000 performs better than NPC-1000-0; therefore, a high specific surface area and the presence of mesopores in the hierarchical pore structure are essential for improving ORR. The higher current density of NPC-1000 than that of NPC-1000-2 indicates that the pore structure impacts the diffusion process. The collapse of the NPC-1000-2 mesoporous structure hinders diffusion, resulting in a lower limiting current density than that of NPC-1000.
Our results are consistent with results that demonstrated that nitrogen doping (especially pyridinic and graphitic nitrogen) plays an important role in ORR electrocatalysis [38, 39, 40]. Furthermore, for NPCs, a large specific surface area is beneficial for more active sites and offering good electrical conductivity, which can accelerate electron transport during the reaction process. Considering the above, we speculate that the excellent ORR activity of NPC-1000 is mainly attributed to the high density of active sites, good conductivity, and hierarchical pore structure. First, although the nitrogen contents of NPC-700, NPC-800, and NPC-900 are higher than that of NPC-1000, their low specific surface areas resulted in the incomplete exposure of active sites. Second, although NPC-1100 has a higher specific surface area and conductivity, its nitrogen content is insufficient, resulting in an insufficient number of active sites. Finally, the hierarchically micro-mesoporous structure accelerates the transport of electrons and ions. Therefore, NPC-1000 offers the best ORR activity among all NPCs.
This mechanochemical method is also suitable for preparing porous carbons with metal doping. For example, when Fe ions are incorporated during ball milling followed by pyrolysis, the resulting Fe-NPC catalyst exhibits outstanding ORR activity that is comparable to that of commercial Pt/C (Fig. S5). This high ORR activity may be due to the formation of Fe–Nx sites, which is highly efficient for ORR in an alkaline solution [43, 44, 45].
In summary, we prepared N-containing polymer precursors via mechanochemical polymerization without a solvent or catalyst, followed by pyrolysis and the removal of the SiO2 template to obtain hierarchical micro-mesoporous NPCs. Owing to the sufficient hierarchical pore structure, high specific surface area, and nitrogen doping, the NPC-1000 prepared by this method exhibited excellent ORR electrocatalytic activity, stability, and methanol tolerance. When used as a ZAB cathode, NPC-1000 exhibited excellent discharge performance comparable to that of Pt/C. Furthermore, its discharge stability is much better than that of Pt/C. The NPC-800 prepared by the same method also exhibited excellent supercapacitance performance due to its high specific capacity (256 F g−1 at 0.5 A g−1 and 431 F g−1 at 10 mV s−1). A high rate performance and excellent cycling stability (98.7% retention after 10,000 cycles at 10 A g−1) in an aqueous 6-M KOH solution were observed. This study confirmed the feasibility of preparing nitrogen-containing polymers by ball milling to prepare NPCs with high electrocatalytic activity, which could replace noble-metal electrocatalytic materials.
The authors are grateful to the financial support from NSFC (51602332), the National Key Research and Development Program of China (2016YFB0700204), Science and Technology Commission of Shanghai Municipality (15520720400, 16DZ2260603), and Equipment Research Program (6140721050215). M. Yang would like to thank the National 1000 Youth Talents program of China and financial support from Ningbo 3315 program. T.T. thanks DST Solar Energy Harnessing Centre (DST/TMD/SERI/HUB/1(C)), DST Materials for Energy Storage program, Ministry of Electronics and Information Technology (India) (Project ID: ELE1819353MEITNAK).
- 9.T. Zhou, R. Ma, Y. Zhou, R. Xing, Q. Liu, Y. Zhu, J. Wang, Efficient N-doping of hollow core-mesoporous shelled carbon spheres via hydrothermal treatment in ammonia solution for the electrocatalytic oxygen reduction reaction. Microporous Mesoporous Mater. 261, 88–97 (2018). https://doi.org/10.1016/j.micromeso.2017.10.050 CrossRefGoogle Scholar
- 21.R. Xing, T. Zhou, Y. Zhou, R. Ma, Q. Liu, J. Luo, J. Wang, Creation of triple hierarchical micro-meso-macroporous N-doped carbon shells with hollow cores toward the electrocatalytic oxygen reduction reaction. Nano-Micro Lett. 10, 3 (2018). https://doi.org/10.1007/s40820-017-0157-1 CrossRefGoogle Scholar
- 24.M. Demir, B. Ashourirad, J.H. Mugumya, S.K. Saraswat, H.M. El-Kaderi, R.B. Gupta, Nitrogen and oxygen dual-doped porous carbons prepared from pea protein as electrode materials for high performance supercapacitors. Int. J. Hydrogen Energy 43, 18549–18558 (2018). https://doi.org/10.1016/j.ijhydene.2018.03.220 CrossRefGoogle Scholar
- 25.M. Zhong, E.K. Kim, J.P. McGann, S.E. Chun, J.F. Whitacre, M. Jaroniec, K. Matyjaszewski, T. Kowalewski, Electrochemically active nitrogen-enriched nanocarbons with well-defined morphology synthesized by pyrolysis of self-assembled block copolymer. J. Am. Chem. Soc. 134, 14846–14857 (2012). https://doi.org/10.1021/ja304352n CrossRefGoogle Scholar
- 40.X. Bao, X. Nie, D. Deak, E.J. Biddinger, W. Luo, V. Asthagiri, U.S. Ozkan, C.M. Hadad, A first-principles study of the role of quaternary-N doping on the oxygen reduction reaction activity and selectivity of graphene edge sites. Top. Catal. 56, 1623–1633 (2013). https://doi.org/10.1007/s11244-013-0097-z CrossRefGoogle Scholar
- 51.H. Zhou, Y. Zhou, L. Li, Y. Li, X. Liu, P. Zhao, B. Gao, Amino acid protic ionic liquids: multifunctional carbon precursor for N/S codoped hierarchically porous carbon materials toward supercapacitive energy storage. ACS Sustain. Chem. Eng. 7, 9281–9290 (2019). https://doi.org/10.1021/acssuschemeng.9b00279 CrossRefGoogle Scholar
- 55.J. Shao, X. Zhou, Q. Liu, R. Zou, W. Li, J. Yang, J. Hu, Mechanism analysis of the capacitance contributions and ultralong cycling-stability of the isomorphous MnO2@MnO2 core/shell nanostructures for supercapacitors. J. Mater. Chem. A 3, 6168–6176 (2015). https://doi.org/10.1039/c4ta06793b CrossRefGoogle Scholar
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