Trends in Monoclinic Metal Selenides (M 3 Se 4 , M = Fe, Co, Ni) for Selective Two-Electron Electrochemical Oxygen Reduction to H 2 O 2

Hydrogen peroxide (H₂O₂) is a widely used oxidant in wastewater treatment, disinfection, and industrial processes. Conventional anthraquinone production process is complex and energy intensive, and produces highly concentrated H₂O₂ that is unsafe and expensive to transport. As an alternative, distributed electrochemical synthesis of H₂O₂ via the two-electron oxygen reduction reaction (2e⁻ ORR) provides a more sustainable approach, enabling on-site production from air and electricity. In this study, we explore a series of monoclinic transition metal 3:4 selenides (M₃Se₄, M = Fe, Co, Ni), including Co₃Se₄, FeCo₂Se₄, Fe₂CoSe₄, and Fe₃Se₄, as potential electrocatalysts for 2e⁻ ORR. Systematic hydrothermal synthesis of this series of compounds and structural characterization are conducted. Then their activity and selectivity for 2e⁻ ORR are evaluated using rotating ring-disk electrode (RRDE) measurements. The Fe:Co ratio in (Fe x Co 1- x )₃Se₄ is further adjusted to investigate its effect on their electronic structure and catalytic behaviors. The results show that FeCo₂Se₄ exhibits the best activity and selectivity for 2e⁻ ORR across a broad potential range, demonstrating the potential of M₃Se₄ as electrocatalysts for electrochemical H₂O₂ production. This study provides insights into rational design of transition metal chalcogenides for electrocatalysis and contributes to the development of sustainable H₂O₂ electrosynthesis.