Effect of Se modification on RuSey/C electrocatalyst for oxygen reduction with phosphoric acid

Carbon-supported Se-modified-Ru catalysts (RuSey/C) were synthesized, and their phosphate adsorption characteristics were evaluated using electrochemical techniques and in-situ X-ray absorption spectroscopy (XAS). When phosphoric acid was added, the ORR activity of unmodified Ru/C decreased by 26.8% because the active sites were blocked by electrochemical adsorption, as confirmed by CV. However, for RuSey/C, the ORR activity was enhanced with phosphoric acid (RuSe1.56/C: 63.8%), which indicates that the kinetics at each site increased to compensate for the site blocking effect. The XAS results demonstrated that, for RuSey/C, phosphoric acid molecules or phosphate anions primarily interacted with Se atoms, and the oxidation state of Ru atoms decreased. Therefore, it was concluded that the enhanced ORR kinetics originated from the decreased oxygen binding energy with larger electrostatic repulsion.

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► RuSey/C showed enhanced ORR activity by 17.8–63.8% with an addition of 10 mM H3PO4.
► The addition of 10 mM H3PO4 led to the decrease of Ru oxidation state in RuSey/C.
► In-situ XAS and cyclic voltammetry suggested interaction between Se and H3PO4.
► Electrostatic repulsion between H3PO4 and Ru–Oad likely decreased Ru oxidation state.