Developing highly-efficient bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts is crucial for the widespread application of rechargeable Zn–air batteries (ZABs). Herein, an efficiency electrodeposition and pyrolytic strategy to synthesize the three-dimensional (3D) N-doped carbon coating multiple valence Co and MnO heterostructures supported on carbon cloth substrate (Co-MnO@NC/CC). It contains Co–Co, Co–N, and Co–O bonds, which synergistically enhance the oxygen reaction activity with MnO. It exhibits a working potential of 1.473 V at 10 mA·cm⁻² for OER and onset potential of 0.97 V for ORR. Theory calculations demonstrate that the synergy between cobalt and manganese species could optimize the d-band center and reduce the energy barrier of Co-MnO@NC/CC for both OER and ORR processes. Besides, the MnO acts as the main OER active site could significantly optimize the energy barrier of O* → OOH*, thus further promoting the OER activity. It can be directly used as the air-cathode for both liquid-state and solid-state ZABs, which could afford a small voltage gap of 0.75 V at 10 mA·cm⁻², a high power density of 172.5 mW·cm⁻² and a long-term durability for 400 h, surpassing those of the Pt/C + RuO₂-based ZAB. Importantly, the assembled batteries show potential applications in portable devices.

Electrocatalytic water splitting is an essential and effective means to produce green hydrogen energy structures, so it is necessary to develop non-precious metal catalysts to replace precious metals. Cobalt-based catalysts present effective alternatives due to the diverse valence states, adjustable electronic structures, and plentiful components. In this review, the catalytic mechanisms of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for electrocatalytic water splitting are described. Then, the synthesis strategies of various cobalt-based catalysts are systematically summarized, followed by the relationships between the structure and performance clarified. Subsequently, the effects of d-band center and spin regulation for cobalt-based catalysts are also discussed. Furthermore, the dynamic electronic and structural devolution of cobalt-based catalysts are elucidated by combining a series of in-situ characterizations. Finally, we highlight the challenges and future developed directions of cobalt-based catalysts for electrocatalytic water splitting.

To achieve the goals of the peak carbon dioxide emissions and carbon neutral, the development and utilization of sustainable clean energy are extremely important. Hydrogen fuel cells are an important system for converting hydrogen energy into electrical energy. However, the slow hydrogen oxidation reaction (HOR) kinetics under alkaline conditions has limited its development. Therefore, elucidating the catalytic mechanism of HOR in acidic and alkaline media is of great significance for the construction of highly active and stable catalysts. In terms of practicality, Pt is still the primary choice for commercialization of fuel cells. On the above basis, we first introduced the hydrogen binding energy theory and bifunctional theory used to describe the HOR activity, as well as the pH dependence. After that, the rational design strategies of Pt-based HOR catalysts were systematically classified and summarized from the perspective of activity descriptors. In addition, we further emphasized the importance of theoretical simulations and in situ characterization in revealing the HOR mechanism, which is crucial for the rational design of catalysts. Moreover, the practical application of Pt-based HOR catalysts in fuel cells was also presented. In closing, the current challenges and future development directions of HOR catalysts were discussed. This review will provide a deep understanding for exploring the mechanism of highly efficient HOR catalysts and the development of fuel cells.