Interplay between alkaline water oxidation temperature, composition and performance of electrospun high-entropy non-equimolar (Cr,Mn,Fe,Co,Ni) oxide electrocatalysts

High-entropy transition-metal oxides are a promising alternative to platinum group metal-based electrocatalysts for alkaline water oxidation. Their electrochemical performance can be improved by engineering surface defects and optimizing their composition. In this study, the composition of electrospun (Cr,Mn,Fe,Co,Ni) oxide nanofibers is varied by doubling the amount of each metal in turn. To obtain ultra-small (< 10 nm) oxide grains with high density of surface defects, calcination is carried at low temperature (400 °C). The evaluation of nanofibers as electrocatalysts for alkaline water oxidation at different temperatures (2060°C) evidences that the (Cr,Mn,Fe,Co,Ni) oxide with doubled Cr concentration exhibits the highest activity at any reaction temperature (overpotentials at 10 mA cm2: 326 and 255 mV at 20 and 60°C, respectively). The results obtained clearly prove the key role of surface defects. The oxygen-vacancy concentration is confirmed as a descriptor of the alkaline water oxidation process able to fully explain the behavior of the present electrocatalysts. The activity and the energy barrier under applied overpotential (11-33 kJ mol-1) are mainly influenced by the concentration of chromium hydroxide species on the catalyst precursor surface, while all surface hydroxide species contribute to enhance reaction kinetics and control the energy barrier at equilibrium (86-147 kJ mol-1).