Self-Assembly of Spinel Nanocrystals into Mesoporous Spheres as Bifunctionally Active Oxygen Reduction and Evolution Electrocatalysts

Abstract

The present work introduces spinel oxide nanocrystals self-assembled into mesoporous spheres that are bifunctionally active towards catalyzing both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The electrochemical evaluation reveals that (Ni,Co)₃ O₄ demonstrates a significantly positive-shifted ORR onset and half-wave potentials [-0.127 and -0.292 V vs. saturated calomel electrode (SCE), respectively], whereas Co₃ O₄ results in a negative-shifted OER potential (0.65 V vs. SCE) measured at 10 mA cm^-2 . Based on the DFT analysis, the potential at which all oxygen intermediate reactions proceed spontaneously is the highest for (Ni,Co)₃ O₄ (U=0.66 eV) during ORR, whereas it is the lowest for Co₃ O₄ (U=2.09 eV) during OER. The high ORR activity of (Ni,Co)₃ O₄ is attributed to the enhanced electrical conductivity of the spinel lattice, and the high OER activity of Co₃ O₄ is attributed to relatively weak adsorption energy promoting rapid release of evolved oxygen.

Keywords: bifunctional catalysts; density functional theory; oxygen evolution; oxygen reduction; spinel oxides.