Kinetic study of oxygen reduction reaction on tantalum oxide-based electrocatalysts produced from oxy-tantalum phthalocyanines in acidic media

In our previous study, we prepared tantalum oxide-based cathodes from oxy-tantalum phthalocyanines by heat treatment under low oxygen partial pressure for polymer electrolyte fuel cells. We revealed that the high onset potential for oxygen reduction reaction (ORR) indicated a low hydrogen peroxide formation rate due to the large extent of crystalline distortion of orthorhombic Ta2O5. However, the hydrogen peroxide formation rate was consistent with an apparent four-electron reaction rate because it included the effect of the hydrogen peroxide reduction reaction. Therefore, in this study, we calculated the individual rate constants of the ORR for tantalum oxide-based cathodes to obtain the four-electron reaction rate (four-electron reduction rate of O2/two-electron reduction rate of O2) and investigated the factors influencing the four-electron reaction rate. The values of the four-electron reaction rate for the tantalum oxide-based cathodes were larger than 1, indicating that the four-electron reaction mainly occurred on the oxide surface. A higher four-electron reaction rate was obtained in the case of severe crystalline distortion and shrinkage of orthorhombic Ta2O5 due to the large number of oxygen vacancies.