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Abstract

A series of Co-based nanoparticles supported on activated carbon was synthesized by using waste tea leaves as a template as well as a sustainable carbon source. The crystal structure of the Co particles was adjusted by post-treatments with H₂O₂, ethanol vapor, and H₂, which result in Co₃O₄, CoO, and metallic Co phases, respectively. After these different treatments, the composite materials consist of small Co-based nanoparticles with an average particle size of 6-14 nm supported on activated carbon with specific surface areas up to 1065 m² g^-1. Correlations between the structure of the materials and their activity for the oxygen evolution reaction (OER) were established, whereby the post-treatment with ethanol vapor was found to yield the most effective electrocatalyst. The material shows good stability at 10 mA cm^-2 over 10 hours and reaches a mass activity of 2.9 A mg_Co^-1, which is even higher than pristine ordered mesoporous Co₃O₄. The superior electrocatalytic performance is ascribed to a high dispersion of Co-based nanoparticles and the conductivity of the activated carbon that facilitate the charge transport.