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

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.