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Hydrogen (H2) is a new type of renewable energy that can meet people's growing energy needs and is environmentally friendly. In order to improve the industrial application prospects and electrochemical performance of hydrogen evolution catalysts, extensive research on transition metal materials has been carried out. Among the many catalytic materials, cobalt is an element with potential for the hydrogen evolution reaction (HER) due to its abundant reserves, low cost, and small energy barrier for H adsorption. This review classifies the latest research on cobalt-based catalysts according to the types of compound, including cobalt-based sulfides, phosphides, carbides, borides, oxides, etc., and summarizes the latest research progress of cobalt-based compound catalysts in acidic and alkaline media. Strategies to tune the properties of cobalt-based compound catalysts for high catalytic activity for HER are focused on, including structural engineering, defect engineering, and doping, etc. The advantages and limitations of each modified approach are reviewed. Not only that, but also the catalytic activity and advantages of the catalyst are evaluated by using density functional theory (DFT) calculation-related descriptors, activity evaluation parameters, etc. Finally, limitations and challenges of cobalt-based materials for HER are presented, as well as prospects for future research. This paper aims to understand the chemical and physical factors that affect cobalt-based catalysts, and to find directions for future research on cobalt-based catalysts. [ABSTRACT FROM AUTHOR]

Summary: The cost of platinum group metal (PGM) catalysts is one of the major obstacles in commercializing proton exchange membrane water electrolyzers (PEMWEs). The non‐PGM substituents are often more financially beneficial but low in activity and durability in the acidic environment. In this study, cobalt phosphide catalysts, which are promising non‐PGM alternatives for the hydrogen evolution reaction (HER) and have enhanced durability and single‐cell performance, were fabricated directly on carbon paper using the pulse electrodeposition method. As the dissolution potential (reported as ‐x vs saturated calomel electrode) of the pulse electrodeposition shifted in the positive direction, the P/Co ratio of the Co‐P‐x catalysts increased because of severe Co dissolution. Among the catalysts, Co‐P‐0.6, Co‐P‐0.5, and Co‐P‐0.4 (where the number indicates the negative dissolution potential) were rapidly degraded in acid, whereas Co‐P‐0.3, Co‐P‐0.2, and Co‐P‐0.1 showed high stability because of the relative amounts of CoP and Co2P phases. The acid‐dissolved Co‐P‐0.3 catalyst showed the best half‐cell performance (an overpotential of 143.85 mV at 10 mA/cm2) and durability, and the P‐Co and Coδ+ surface states are critical for its performance. Single‐cell tests using the Co‐P‐0.3 cathode revealed its remarkable performance of 1.89 A/cm2 at 2.0 Vcell, indicating its promise as a non‐PGM cathode material for PEMWEs. [ABSTRACT FROM AUTHOR]