Highlights
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General principles for designing atomically dispersed metal-nitrogen-carbon (M–N-C) are briefly reviewed.
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Strategies to enhance the bifunctional catalytic performance of atomically dispersed M–N-C are summarized.
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Challenges and perspectives of M–N-C bifunctional oxygen catalysts for Rechargeable zinc-air batteries are discussed.
Abstract
Rechargeable zinc-air batteries (ZABs) are currently receiving extensive attention because of their extremely high theoretical specific energy density, low manufacturing costs, and environmental friendliness. Exploring bifunctional catalysts with high activity and stability to overcome sluggish kinetics of oxygen reduction reaction and oxygen evolution reaction is critical for the development of rechargeable ZABs. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts possessing prominent advantages of high metal atom utilization and electrocatalytic activity are promising candidates to promote oxygen electrocatalysis. In this work, general principles for designing atomically dispersed M-N-C are reviewed. Then, strategies aiming at enhancing the bifunctional catalytic activity and stability are presented. Finally, the challenges and perspectives of M-N-C bifunctional oxygen catalysts for ZABs are outlined. It is expected that this review will provide insights into the targeted optimization of atomically dispersed M-N-C catalysts in rechargeable ZABs.
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1 Introduction
To alleviate the dependence on traditional fossil fuels and reduce environmental degradation, significant efforts have been focused on the development of harvesting, conversion, and storage of low-cost and environmentally friendly renewable energy [1,2,3,4]. As one of the most promising next-generation energy storage devices, rechargeable zinc-air batteries (ZABs) are considered to be potentially used in electric vehicles, flexible wearable electronic devices, etc., due to their high theoretical energy density (1086 Wh kg−1), high cell voltage (1.66 V), good safety, low cost, and environmental friendliness [5,6,7,8,9]. While there have been considerable advances in recent years, ZABs still face some critical challenges. The absence of a reliable and effective bifunctional air electrode has been the most significant impediment to its practical application [10,11,31, 39,40,41,42,47]. When carbon materials are doped with TM atoms, they possess both high electrical conductivity of carbon materials and high intrinsic activity of metal-based active sites [48,49,50]. Furthermore, the TMs in such catalysts can also significantly improve the graphitization of carbon materials, providing them with excellent protection from corrosion and accumulation during electrochemical reactions [ Schematic diagram of general design principles and strategies aimed at enhancing the bifunctional catalytic activity of atomically dispersed M-N-C electrocatalysts in this review
2 General Principles for Designing Atomically Dispersed M-N-C Catalysts
The interaction between metal atoms and the carbon matrix is weak, which makes individual atoms stabilize on the carbon matrix quite difficult. The metal center usually needs to cooperate with other atoms on the carrier to maintain stability. N-doped porous carbon as a carrier has a high specific surface area, abundant pores, and high N content, which is considered one of the most widely used substrates for stabilizing single metal atoms [74,75,76]. On the one hand, N is more electronegative and reactive than carbon, which makes the interactions between the metal and N atoms stronger [75]. The introduction of N into the carbon lattice by chemically bonding with adjacent carbon atoms plays a crucial role in anchoring isolated metal atoms [5, 31]. On the other hand, N atoms in the carbon network cannot only adjust the electronic structure of adjacent carbon atoms to make charge redistribution but also reduce the adsorption energy of oxygen-containing substances. It was identified that pyridinic N, graphitic N, and regulated carbon atoms are important sources of catalytic activity [5, 16, 31]. In general, the metal atom forms M-N4 by coordinating with four N atoms such as pyridine and pyrrole N, which can influence the adsorption efficiency of the reactant on the surface of the catalyst and ultimately affect the catalytic activity, selectivity, and stability toward ORR and OER [59, 77].
General approaches for preparing atomically dispersed M-N-C catalysts include top-down and bottom-up techniques [104,105,106,107]. Besides N-do**, other heteroatoms (e.g., S, P, B, and O) can also provide sites to anchor isolated metal atoms, improving the metal atom deposition in carbon materials [54, 59]. In addition, they can destroy the electric neutrality of the carbon matrix and refine the charge and spin distribution to effectively form additional active sites, which will increase the amount and the reactivity of active sites simultaneously [5, 61, 75]. Based on pristine M-N-C catalyst, inserting secondary heteroatoms with a discrepancy in electron spin density and electronegativity can modulate the coordination environment and correspondingly alter electronic configurations of the M-N-C sites. Heteroatoms can also influence the catalytic activity of the M-N-C sites through the long-range delocalization and charge transfer effect, even if they cannot bind directly to the M-N-C sites [18, 61, 66, 76, 104, 108]. Moreover, injecting heteroatoms into the electrocatalyst surface can significantly increase the density of active sites without severe structural collapse [5d). Yang et al. discovered that the increased S atoms incorporation is an efficient way to increase the density of atomically distributed active sites and boost bifunctional catalytic efficiency [61]. Guo et al. proposed a B-doped Co-N-C bifunctional oxygen electrocatalyst by a soft template self-assembly pyrolysis approach (Fig. 5e) [64]. According to experiments and DFT calculations, it is confirmed that the B atom do** can improve electronic conductivity and generate more active sites by activating electron transfer near the Co-N-C site. It is noteworthy that the B-doped catalysts possess a lower onset potential than RuO2 and Pt/C and demonstrate excellent bifunctional electrocatalytic activity (ΔΕ = 0.83 V) (Fig. 5f). The synergistic effect of the active CoNx species and the introduced phosphorus (P) atoms can also significantly increase the ORR and OER activities and durability [67].
Copyright 2017, Wiley–VCH. c Atomic structure model of Fe-NSDC. d Overall ORR and OER polarization curves of different catalysts in O2-saturated 0.1 M KOH [63]. Copyright 2019, Wiley–VCH. e Synthetic procedure of Co–N, B-CSs. f Overall polarization curves of different catalysts in 0.1 M KOH [64]. Copyright 2018, American Chemical Society
a The synthetic procedure for S, N-Fe/N/C-CNT. b LSV of all samples for ORR and OER in 0.1 M KOH [16, 10b) [133]. The study found that ORR activities have little correlation with BET and micropores surface area but are related to the conditions for the full utilization of the active sites, including the electrolyte wetting and mass transport kinetics (Fig. 10c, d). Macro- and mesoporous structures contribute to different stages of reaction kinetics in ORR catalysis, making the systematic control of pore size distribution crucial to catalyst activity.
Copyright 2019, Royal Society of Chemistry. b Three N-doped carbon model catalysts “standard”, “meso-free” and “macro-free”. c ORR activity, BET and micropore surface areas of three N-doped carbon model catalysts. d Schematic illustration of electrochemically wettable and kinetically accessible area for three model catalysts [133]. Copyright 2019, American Chemical Society
a Schematic diagram of hierarchically porous structures [132].
Hence, it is essential to regulate both the pore size and structure (e.g., pore continuity, length, and pore size distribution) of bifunctional catalysts to get more exposed active sites, faster electron/mass transportation pathways, and higher surface areas [137]. However, disordered structure and a large number of blocked pores attacked by caustic and oxidizing conditions will collapse the electrode structure and seriously impede the transport efficiency, resulting in a loss of a large number of gas transport channels and active sites [12]. It is, therefore, necessary for an interface to be designed rationally with good channel connectivity and organized structure. According to many reports, the structure of interconnected fractals and ordered arrays have been considered the most promising architectures. The ordered array configuration allows lower transfer resistance, while the interconnected fractal structure ensures homogeneous transfer and conduction properties [128]. For creating pores with consistent size and architecture, template methods are most popular and straightforward [82]. Using organometallic synthesis, Mu et al. fabricated an advanced Co-Nx/C nanorod array (Fig. 11a). Remarkably, as a bifunctional catalyst, the impressive catalytic performance (ΔE ≈ 0.65 V) is primarily attributed to the synergistic effect of the abundant Co–N active sites and the unique nanorod geometry with enriched porosity and high surface area (Fig. 11b, c) [71]. Li et al. gave an in-depth analysis of the influence of nanoarchitecture and microporosity in carbon materials on reversible oxygen catalytic activity and stability [70]. A facile method for synthesis of meso/micro-FeCo-Nx-CN as a high-performance bifunctional oxygen catalyst is using a salt template and silica template simultaneously (Fig. 11d). The results of analyzing the pore size distribution by the QSDFT model show that the addition of the silicon template increases the ratio of mesopores to micropores, even though the surface area is not significantly affected (Fig. 11e). The hierarchical structure and enriched reversible oxygen electrocatalytic sites attribute to the remarkable bifunctional activity (ΔE = 0.78 V) of meso/micro-FeCo-Nx-CN (Fig. 11f). Furthermore, a charge–discharge test to determine the stability of the ZAB equipped with meso/micro-FeCo-NxCN lasts for over 40 h and the voltage remains fairly constant.
Copyright 2018, Wiley–VCH. d The synthetic procedure of and e pore size distribution curves of meso/micro-FeCo-Nx-CN-30. f Overall polarization curves of different catalysts (ORR in 0.1 M KOH, OER in 0.1 M KOH) [70]. Copyright 2018, Wiley–VCH
a Synthetic procedure for the Co-functionalized carbon nanorod array. b LSV curves of Co-Nx/C NRA and contrastive catalysts for ORR in 0.1 M O2-saturated KOH. c LSV curves of Co-Nx/C NRA and IrO2 for OER before and after 10,000 potential sweeps [71].
4 Summary and Perspective
The past twenty years have witnessed the great progress of electrocatalysts for ZABs. The emergence of atomically dispersed catalysts with high atom utilization efficiency and tunable active sites has reinvigorated research targeting the discovery of low-cost electrocatalysts for oxygen catalysis. This review surveys the general principles for designing atomically dispersed M-N-C. The endeavors to enhance the intrinsic activity for ORR and OER are summarized including designing appropriate metal centers, introducing heteroatoms, manufacturing defects, and structuring bimetals active sites. Moreover, architecting favorable hierarchical-pores structures with a high specific surface area can facilitate the utilization efficiency of active sites and accelerate mass/electron transportation. Despite the achievements so far, there are still many challenges remaining to be addressed to develop highly efficient ZAB systems in the future. Thus, those challenges and perspectives are discussed as follows:
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(1)
An in-depth understanding of the bifunctional reaction process is required. The complicated mechanisms of OER and ORR are strongly determined by different catalytic environments, raising intricate debates on the understanding of the real active sites for the attribution of bifunctional catalysts. Considering that single metal atoms, defects, and heteroatoms can act as active sites independently or cooperatively, the superior catalytic performance of atomically dispersed M-N-C is always ambiguously defined as a “synergistic effect”. However, the potential “synergistic effect” or even “inhibited effect” between different active sites shall be carefully considered. Previous studies have shown that the coordination structure and electronic configuration of metal atoms change dynamically during the reaction, and even morphology/composition reconstruction. It is necessary to measure and track the catalyst surface reconstruction process in time to capture the dynamic evolution of the bifunctional catalyst, which provides new insights for accurately identifying the real active sites on the electrocatalyst surface and revealing the catalytic mechanism. Given this, greater efforts should go toward combining in-situ/operando characterization technologies (such as X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure spectroscopy (EXAFS), and surface-enhanced Raman spectroscopy (SERS)) and theoretical calculations to accurately capture the dynamic reconstruction of the electrocatalyst, identify active sites and understand the reaction mechanism. It will provide the guidelines to finely tune the properties and activities of M-N-C catalysts with optimal activity, selectivity, and stability, which is crucial for practical applications.
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(2)
It is still challenging to fabricate single metal atoms dispersed on the porous N-doped carbon with a precise coordination structure and high metal loading. Despite the widespread availability of the pyrolysis process for the fabrication of the atomically dispersed M-N-C, the precise synthesis at the atomic level is a major challenge. This is because pyrolysis usually collapses the pre-designed precursor structure and forms the undefined structure of the M-N-C active sites. Based on this fact, it is critical to promote reliable strategies and synthetic methods that can precisely tune the atomically dispersed active sites. MOFs are considered the most promising precursors to develo** atomically dispersed M-N-Cs. However, MOFs are extremely expensive, complex to synthesize, and low yield, significantly limiting their application on a large scale. It is desirable to enhance the catalyst activities by combining MOFs with other low-cost precursors to enrich the single atomic site population within M-N-C catalysts, by increasing their metal loadings. However, obtaining high metal loading without aggregation for increasing the density of the active site is another challenge. It may be effective to solve this problem by rationally designing hierarchical porous structures with suitable pore-size distribution to increase metal do** content, which can also provide good mass transport/electron transfer. The synthesis of hierarchical porous structures usually requires complex procedures with high cost. Therefore, it is necessary and urgent to develop a low-cost and simple synthesis method to produce high-metal-loaded atom-dispersed catalysts on a large scale.
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(3)
The challenge of the insufficient stability of the bifunctional M-N-C catalysts for long-term operation remains, particularly in cases of the OER process. After repeated deep charging and discharging of the rechargeable ZABs, the conductivity and hydrophobicity of the air electrode are severely reduced. In this case, surface/interface nanoengineering should be used to maintain the stability of rechargeable ZABs. For the carbon material with a low graphitization degree during the charging process, a large number of defects are susceptible to corrosion at high overpotentials, which causes a serious performance decay of rechargeable ZABs. To address this problem, raising the graphitization degree of a stable carbon structure through heat treatment and stable heteroatoms do** can increase the stability of M-N-C catalysts.
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(4)
In addition to improving the activity and stability of bifunctional catalysts, upgrading the battery configuration of ZABs can also not be ignored. Depending on the state of the air electrode in ZABs, the mechanism of ORR and OER could be influenced by the different conditions, including the irreversible deposition and accumulation of ZnO, the formation of Zn dendrites, the high concentration of electrolyte, and the configuration of batteries. The deposition of ZnO can block the pores of the air electrode and cover the active sites, which will prevent the diffusion of reactive intermediates and finally cause premature battery death. Battery performance is also significantly affected by alkaline electrolyte loss and degradation. To guarantee the stable and durable charge–discharge operation of ZABs for practical application, there are still many important topics including composition modification of the Zn electrodes, constructing a protective layer to inhibit the excessive enrichment of ZnO on the air electrode, and adopting a flowing-electrolyte design. For all-solid-state ZABs, the backward development of high-performance electrolyte membranes has also severely restricted their applications. In addition to the formation of dendrites and surface passivation at the electrolysis-zinc electrode interface, the resistance at the air electrode–electrolyte interface is much higher than that of the water interface, which severely limits the performance of solid-state ZABs. Exploring solid electrolytes with high ionic conductivity and mechanical strength, better water absorption and retention, and good safety and stability and solving the problems of the electrolyte–electrode interface are necessary to meet the commercial needs of solid-state ZABs. For flexible ZABs, flexible collectors, flexible electrolytes, and appropriate packaging technologies should all be considered and fully optimized to maintain stable electrochemical performance for a long time under bending conditions.
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Acknowledgements
This work is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Centre Québéco is sur les Materiaux Fonctionnels (CQMF), Fonds de Recherche du Québec-Nature et Technologies (FRQNT), and Institut National de la Recherche Scientifique (INRS). This work is also supported by the National Natural Science Foundation of China (21972017) and the “Scientific and Technical Innovation Action Plan” Hong Kong, Macao and Taiwan Science & Technology Cooperation Project of Shanghai Science and Technology Committee (19160760600). F. Dong gratefully acknowledges scholarships from the China Scholarship Council (CSC).
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Dong, F., Wu, M., Chen, Z. et al. Atomically Dispersed Transition Metal-Nitrogen-Carbon Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries: Recent Advances and Future Perspectives. Nano-Micro Lett. 14, 36 (2022). https://doi.org/10.1007/s40820-021-00768-3
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DOI: https://doi.org/10.1007/s40820-021-00768-3