Improved Electrochemical Water Oxidation over Chromium-Substituted Cobalt Aluminate Spinels

Spinel-type Co-Al-Cr mixed oxides (CoAl2−xCrxO4) were developed as electrochemical water oxidation catalysts. CoAl2−xCrxO4 powders were synthesized by a sol-gel method, followed by calcination at elevated temperatures in air. Electrochemical water oxidation was conducted in a phosphate-buffered aqueous solution (pH 7.5) using CoAl2−xCrxO4-loaded fluorine-doped tin oxide (FTO) conductive glass support under potentiostatic control. The electrochemical water oxidation activity was enhanced by substitution of Cr3+ ions for B-site Al3+ in CoAl2O4 in all activity metrics (i.e., geometric, specific and mass activities). Among the Cr-substituted CoAl2O4 examined, the highest activity was obtained for CoAl1.6Cr0.4O4, which was stable at +1.80 V (vs. RHE) for the generation of O2 with a Faradaic efficiency of close to unity and with minimal impact of undesirable Cr3+ oxidation. Physicochemical analyses by means of transmission electron microscopy, X-ray absorption spectroscopy, and electrochemical impedance spectroscopy indicated that the key to achieve high geometric activity was a reduction in the resistance of the electrode by the use of high surface area CoAl2−xCrxO4, which was achieved by controlling the preparation conditions of CoAl2−xCrxO4, i.e., appropriate adjustment of the Cr concentration and the calcination temperature.