GONG Qing ^{1, 2}, CHEN Bohong ^{1, 2}, CHEN Gang ^{1, 2}, ZOU Junhua ^{1, 2}

(1. Jiangxi Provincial Key Laboratory of Greenhouse Gas Accounting and Carbon Reduction, Institute of Energy Research, Jiangxi Academy of Sciences, Nanchang 330096, Jiangxi, China; 2. Jiangxi Carbon Neutralization Research Center,Nanchang 330096, Jiangxi, China)

Extended Abstract:[Significance] Proton exchange membrane fuel cells (PEMFCs) have various advantages, such as high efficiency, fast start-up, and zero emissions, making them promising candidates to replace traditional power sources in the field of transportation. As the core component of PEMFCs, electrocatalysts primarily facilitate the hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode, which critically govern the performance, durability, and cost of PEMFCs. Notably, the sluggish kinetics of the ORR heavily depend on precious platinum (Pt) as cathode catalysts, hindering the large-scale application of PEMFC. Therefore, it is urgent to develop non-platinum catalysts with low cost, high activity, and high stability to promote the commercial application of PEMFCs. Ruthenium (Ru), the most cost-effective metal in the platinum group, with a price approximately one-third that of Pt, has relatively abundant reserves. When combined with chalcogenides (Ch=S, Se, Te), Ru forms ruthenium-based chalcogenides, which exhibit good ORR activity and methanol resistance, making them potential candidates for practical applications in PEMFCs or direct methanol fuel cells. In recent years, researchers have conducted extensive studies on Ru-based chalcogenide catalysts, including preparation methods, compositions, structures, electrocatalytic performances, enhancement mechanisms, and fuel cell performances, with significant progress achieved. This paper summarizes the preparation methods of ruthenium-based chalcogenides and comprehensively reviews their current research status as oxygen reduction reaction (ORR) catalysts, while addressing existing challenges and future development trends.[Progress] Based on composition and structure, ruthenium-based chalcogenides can be categorized into three types: (1) Nonstoichiometric Ru_xCh_y (y/x˂1.0), in which a small fraction of chalcogen elements modify the surface of Ru without forming stable compounds; (2) Stoichiometric RuCh₂ (e.g., RuS₂, RuSe₂, RuTe₂) with stable crystallographic structures; (3) Ternary Ru-based chalcogenides, in which a third element is introduced to modify Ru_xCh_y (y/x˂1.0) or RuCh₂ catalysts. It is generally believed that metallic Ru serves as the active site in the Ru_xCh_y (y/x˂1.0) catalyst, where a small content of chalcogen elements distributed on the surface to suppresses Ru oxidation, thereby weakening the oxygen adsorption and enhancing ORR activity. The type of chalcogen element has an important effect on the size of the Ru core clusters and the interaction strength between Ru and surface chalcogens, ultimately modulating the ORR activity. Among Ru_xCh_y (y/x˂1.0) catalysts, Ru_xSe_y (y/x˂1.0) exhibits the highest ORR activity. However, the main component of Ru_xSe_y catalyst is Ru, and Se and Ru have not formed stable chemical bonds. The surface Se used to suppress Ru oxidation can be easily oxidized, causing the exposed Ru surface to be oxidized and deactivated, weakening the electrochemical stability. In contrast, RuCh₂ compounds (e.g., RuS₂, RuSe₂, RuTe₂) have stable Ru-Ch chemical bonds and lower Ru loading, demonstrating promising fuel cell performance. RuTe₂ and RuSe₂ exhibit ORR activity comparable to Ru_xSe (y/x˂1.0) while demonstrating significantly enhanced stability. Notably, the orthorhombic-structured RuTe₂/C catalyst achieved a breakthrough maximum power density of 672 mW·cm⁻² in H₂/O₂ fuel cells. The cell performance remains unchanged after a 55-hour constant-current test, highlighting its promising performance and durability. To further reduce the Ru content and enhance the catalytic performance, researchers have attempted to modify Ru-based chalcogenides by introducing elements such as Cr, Mo, W, Fe, Co, and Ni, aiming to tailor their surface structures and electronic environments. However, ternary Ru-based chalcogenide catalysts have not demonstrated significant improvements in fuel cell performance. Currently, the maximum power density of ternary catalysts (e.g., RuxMoySez/C) in fuel cells remains limited to 247 mW·cm⁻².[Conclusions and Prospects] With the development of PEMFC towards commercialization, it is necessary and urgent to explore high-performance non-platinum catalysts. Ru-based chalcogenides, as promising candidate catalysts, have garnered significant attention. The preparation methods of Ru-based chalcogenide catalysts and their recent advances in structure/performance regulation and fuel cell applications are systematically summarized, aiming to provide insights for future research on Ru-based chalcogenide catalysts. In recent years, the synthesis of ruthenium-based chalcogenides has become more efficient and simplified, with steady improvements in catalytic activity and fuel cell performance. However, their development and practical application in PEMFCs still face critical challenges:(1) The primary component of Ru_xCh_y (y/x˂1.0) remains precious metal Ru, and its poor electrochemical stability results in unsatisfactory durability in practical fuel cell applications. Reducing Ru loading while simultaneously enhancing stability remains a critical unresolved challenge for Ru_xCh_y (y/x˂1.0) catalysts.(2) RuCh₂ catalysts demonstrate promising fuel cell performance yet still lag far behind commercial Pt/C catalysts, with their oxygen reduction activity requiring further enhancement. Current limitations include relatively large particle sizes (>5.0 nm) and ill-defined active sites, both of which hinder further improvement in ORR activity. Developing novel synthesis methods to reduce particle size and elucidating their oxygen reduction catalytic mechanisms represent critical future directions for RuCh₂ catalysts.(3) Research on ternary Ru-based chalcogenide catalysts has predominantly focused on modifying Ru-Se systems, while studies targeting Ru-Te systems remain scarce. Exploring the incorporation of elements such as Co, Ni, Fe, N, and P into RuTe₂ catalysts is considered a promising strategy to enhance their ORR activity and fuel cell performance in future studies.

Key words: Ru-based chalcogenide; proton exchange membrane fuel cell; oxygen reduction reaction; electrocatalyst