Watermelon-like Rh x S y @C nanospheres: phase evolution and its influence on the electrocatalytic performance for oxygen reduction reaction

Abstract

Design and synthesis of oxygen reduction reaction (ORR) electrocatalysts with desired constituent based on understanding and exploitation of electrocatalytic mechanism is of great significance for applications of hydrochloride acid electrolysis. Herein, strongly coupled hybrid of watermelon-like Rh x S y @C nanospheres has been rationally designed and synthesized via a one-step solvothermal method. X-ray photoelectron spectroscopy results reveal that the rhodium element presents in a mixture phase of metallic Rh and three sulfides, i.e., Rh17Sl5, Rh3S4 and Rh2S3. The ratio of the constituent phases, Rh/Rh17Sl5/Rh3S4/Rh2S3, is closely related to the synthesis parameter, and the average S/Rh molar ratio increases along with the increasing solvothermal temperature and duration. When assessed as electrocatalysts for ORR in hydrochloric acid, the Rh x S y /C composite with a constituent of 29.1% Rh17S15, 45.8% Rh3S4 and 25.1% Rh2S3, where the ORR-active Rh17S15 and Rh3S4 phases are up to 75%, exhibits the best catalytic activity and stability, even outperforms Pt/C. In contrast, the metallic Rh-involved counterpart suffers from severe degradation in the highly corrosive environment, and the inactive Rh2S3 phase-dominant composite is less active toward ORR. Our results may give a new impetus to the rational design of rhodium sulfides with improved performance for hydrochloric acid electrolysis.