Positive and Negative Synergistic Effects of Fe-Co Mixing on the Oxygen and Hydrogen Evolution Reaction Activities of the Quadruple Perovskite CaCu3Fe4-xCoxO12

Highly active and earth-abundant catalysts for the oxygen and hydrogen evolution reactions (OER and HER) are crucial for attaining a sustainable society. Perovskite-related oxides containing Fe and Co ions, Ba0.5Sr0.5Co0.8Fe0.2O3-delta, Ca2FeCoO5, and CaFe0.5Co0.5O3, have been reported as potential OER catalysts, and the Fe-Co mixing in these oxides has been found to synergistically enhance OER activity. However, this effect has not been verified in quadruple perovskite oxides, including CaCu3Fe4O12, which exhibits the highest OER catalytic activity among perovskite oxide catalysts. We therefore synthesized CaCu3Fe4-xCoxO12 with 0 <= x <= 4 using a high-pressure/high-temperature method and investigated their catalytic properties for the OER and HER. Rietveld analyses based on synchrotron X-ray diffraction measurements revealed that all the CaCu3Fe4-xCoxO12 samples crystallized into almost single-phase solid solutions between x = 0 and 4 with a homogeneous distribution of Fe/Co cations over the entire x range. At x > 0.5, Fe-Co mixing improved the overpotentials and specific activity for both the OER and HER relative to those of the parent compounds, reaching broad maxima at 2 <= x <= 3, thus resulting in the first appearance of synergistic effects on the catalytic activities of quadruple perovskite catalysts. In contrast, at x <= 0.5, this positive synergistic effect disappeared for the OER and even became negative for the HER. Bond valence sum analyses and X-ray absorption spectroscopy clarified that the positive effects were caused by the isovalent substitution of Co similar to 3+ by Fe similar to 3+ on the CaCu3Co4O12 side at x > 0.5, whereas the negative effect was triggered by the aliovalent substitution of Fe similar to 4+ by Co similar to 3+ on the CaCu3Fe4O12 side at x <= 0.5. This information provides an effective strategy for further enhancing the catalytic activities of complex transition metal oxides in multiple valence-variable elements.