Quantitative Evaluation of the Activity of Low-Spin Tetravalent Nickel Ion Sites for the Oxygen Evolution Reaction

Oxide catalysts containing tetravalent nickel exhibit high oxygen-active catalytic activity. We investigated the effects of lithium content (x) on the electrocatalytic activity of LixNi0.5Mn1.5O4, a spinel that provides only divalent and tetravalent nickel ions in a KOH aqueous electrolyte for the oxygen evolution reaction (OER). The lithium content was controlled by the chemical delithiation of Li0.96Ni0.49Mn1.51O4 using NO2BF4. Upon employing this approach, nickel changed from the divalent to tetravalent state. The OER activity increased with decreasing x, and a higher activity than that of standard LaNiO3 was observed at x = 0.00. Comprehensive Tafel analyses and X-ray spectroscopic investigations revealed a downshift in the Ni conduction band, indicating that high OER activity correlated with strong hybridization of the Ni 3d and O 2p orbitals. These insights into the role of Ni4+ in high OER activity are expected to facilitate the development of other highly active Ni-based electrocatalysts.