Nonprecious metal single‐atom materials have attracted extensive attention in the field of electrocatalysis due to their low cost, high reactivity, high selectivity, and high atomic utilization. However, the high surface energy of a single atom causes agglomeration during preparation and catalytic measurement, resulting in damage to the catalytic sites. The strong interaction between substrate and monoatoms is the key factor to prevent the aggregation of individual metal atoms, and the geometry and electronic structure of the catalysts can be adjusted to optimize the catalytic activity. Due to the hierarchically pores, high specific surface area, and defect effect, nitrogen‐doped porous carbon (NPC) has been widely studied as an ideal nonprecious metal single‐atom support, which synergistically enhance the electrocatalytic performance toward oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction with non‐noble metal single atoms. This review summarizes the controllable synthesis, characterization, theoretical calculation, and application of M (M = Co, Fe, Ni, Cu, Zn, Mo, etc.) single atoms on nitrogen‐doped porous carbon. Finally, the future development and challenges of nitrogen‐doped porous carbon supported nonprecious metal single‐atom electrocatalysts for practical commercialization are concluded.

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Molybdenum carbide (MoxC)‐based nanomaterials have shown competitive performances for energy conversion applications based on their unique physicochemical properties. A large surface area and proper surface atomic configuration are essential to explore potentiality of MoxC in electrochemical applications. Although considerable efforts are made on the development of advanced MoxC‐based catalysts for energy conversion with high efficiency and stability, some urgent issues, such as low electronic conductivity, low catalytic efficiency, and structural instability, have to be resolved in accordance with their application environments. Surface and interface engineering have shown bright prospects to construct highly efficient MoxC‐based electrocatalysts for energy conversion including the hydrogen evolution reaction, oxygen evolution reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. In this Review, the recent progresses in terms of surface and interface engineering of MoxC‐based electrocatalytic materials are summarized, including the increased number of active sites by decreasing the particle size or introducing porous or hierarchical structures and surface modification by introducing heteroatom(s), defects, carbon materials, and others electronic conductive species. Finally, the challenges and prospects for energy conversion on MoxC‐based nanomaterials are discussed in terms of key performance parameters for the catalytic performance.

Figure 6. a,b) Structural simulation and HAADF-STEM of Ru-N/G SACs. c) Fourier transform magnitudes of the experimental Ru K-edge extended X-ray absorption fine structure (EXAFS) spectra of the Ru-N/G and other comparative samples.Reproduced with permission. [28] Copyright 2017, American Chemical Society. d) Structural simulation of Pt-Ru dimer dispersed on NCNT. e) ΔG H* plots for different H coverages. Reproduced with permission. [95] Copyright 2019, Nature Publishing Group. f) Schematic illustration of the synthesis of NiRu 0.13 -BDC by ion-exchange method. g) The simulated charge difference between NiRu 0.13 -BDC and Ni-BDC. Yellow represents charge accumulation and blue represents charge depletion. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. h) In situ X-ray absorption near edge structure (XANES) of Ru K-edge under OER electrochemical conditions. i) Gibbs free energy diagram during OER process on Ru/CoFe-LDHs. Reproduced with permission.

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Electrochemical storage and conversion systems, such as water electrolyzers, proton exchange membrane (PEM) fuel cells and metal-air batteries, have drawn extensive attention in the last decades. Hydrogen and oxygen conversion reactions, including hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), are important reactions in the above-mentioned systems. Specifically, water electrolyzer with HER and OER at cathode and anode, respectively, can transform electricity into clean energy (H 2 ); [4] the fuel cells can convert chemical energy into electricity with HOR at anode and ORR at the cathode; [5] the electrochemical ORR/OER is the pair of inverse reactions during the discharge/charge processes in rechargeable metal-air batteries. [6] Currently, precious metal-based electrocatalysts are still required to drive these electrochemical reactions for high performance. However, the high price and low distribution of precious metals block further commercial applications. [7] Therefore, the exploration of precious metals with low loading and/or nonprecious metal-based catalysts without the compromise of their catalytic activity has been the core of developing renewable energy.In recent years, nanosized catalysts, that is, nanoparticles, quantum dots, and clusters, have been developed rapidly for various electrochemical conversion reactions, such as HER, Single-atom catalysts (SACs) have attracted tremendous research interest due to their unique atomic structure, maximized atom utilization, and remarkable catalytic performance. Among the SACs, the carbon-supported SACs have been widely investigated due to their easily controlled properties of the carbon substrates, such as the tunable morphologies, ordered porosity, and abundant anchoring sites. The electrochemical performance of carbonsupported SACs is highly related to the morphological structure of carbon substrates (macro-environment) and the local coordination environments of center metals (micro-environment). This review aims to provide a comprehensive summary on the macro/micro-environment regulating carbonsupported SACs for highly efficient hydrogen/oxygen conversion reactions. The authors first summarize the macro-environment engineering strategies of carbon-supported SACs with altered specific surface areas and porous properties of the carbon substrates, facilitating the mass diffusion kinetics and structural stability. Then the micro-environment engineering strategies of carbon-supported SACs are discussed with the regulated atomic structure and electronic structure of metal centers, boosting the catalytic performance. Insights into the correlation between the co-boosted effect from the macro/ micro-environments and catalytic activity for hydrogen/oxygen conversion reactions are summarized and discussed. Finally, the challenges and perspectives are addressed in building highly efficient carbon-supported SACs for practical applications.

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