Element-Specific Restructuring of Anion- and Cation-Substituted Cobalt Phosphide Nanoparticles under Electrochemical Water-Splitting Conditions

Studying the restructuring behavior of doped catalyst materials under electrochemical reaction conditions is important for understanding the structure–property relationships and for developing design principles for better catalysts. As a well-known catalyst for both the cathode (H2 evolution reaction, HER) and anode (O2 evolution reaction, OER) reactions of water electrolysis, CoP can be made even more active by cationic or anionic substitution. However, the dependence of catalytic reactivity on substitutional doping has not been sufficiently understood in the context of restructuring under working conditions. In this work, cation (Fe)- and anion (S)-substituted CoP nanoparticles are synthesized, and their surface oxidation under ambient conditions and restructuring under HER and OER conditions are investigated. For Fe0.5Co0.5P, the Fe substituents are more easily oxidized than Co in the air; they are also more difficult to reduce under HER conditions in alkaline electrolyte, and the remaining Fe–OH species on the surface hampers the activity for HER. For CoP0.5S0.5, the S substituents are less oxidized than P in the air; they are also more difficult to oxidize under OER conditions in alkaline electrolyte, and the remaining sulfate-like species enhances the activity for OER.