ABSTRACT
Transition metal-based composites are one of the most important electrocatalysts because of their rich redox chemistry. The reaction kinetics of a redox couple is dependent on the chemical valence and is a key issue in electrocatalytic performance. In this study, a metallic Co catalyst was synthesized by pyrolyzing Co(OH)2. The effect of the chemical valence of Co on the oxygen reduction reaction (ORR) was investigated by comparing the electrocatalytic properties of three Co-based catalysts containing Co0, Co2+, and Co3+. The electrocatalytic properties were evaluated mainly by linear scan voltammetry (LSV) and a direct borohydride fuel cell (DBFC) where the Co-based catalysts were used as cathodes. The LSV results show that the ORR peak current density increases with a decrease in chemical valence. The DBFC with the Co0 cathode exhibits highest power density and good durability. In situ X-ray diffraction combined with in situ X-ray absorption fine structure tests was carried out to reveal the dynamic microstructure evolution of the Co0 cathode during ORR. The in situ results clearly demonstrate the evolution of metallic Co to Co(OH)2 and then to CoOOH during the ORR.
