Structure-activity correlations in a nickel-borate oxygen evolution catalyst

An oxygen evolution catalyst that forms as a thin film from Ni(aq)(2+) solutions containing borate electrolyte (Ni-B(i)) has been studied by in situ X-ray absorption spectroscopy. A dramatic increase in catalytic rate, induced by anodic activation of the electrodeposited films, is accompanied by structure and oxidation state changes. Coulometric measurements correlated with X-ray absorption near-edge structure spectra of the active catalyst show that the nickel centers in activated films possess an average oxidation state of +3.6, indicating that a substantial proportion of nickel centers exist in a formal oxidation state of Ni(IV). In contrast, nickel centers in nonactivated films exist predominantly as Ni(III). Extended X-ray absorption fine structure reveals that activated catalyst films comprise bis-oxo/hydroxo-bridged nickel centers organized into sheets of edge-sharing NiO(6) octahedra. Diminished long-range ordering in catalyst films is due to their ostensibly amorphous nature. Nonactivated films display a similar oxidic nature but exhibit a distortion in the local coordination geometry about nickel centers, characteristic of Jahn-Teller distorted Ni(III) centers. Our findings indicate that the increase in catalytic activity of films is accompanied by changes in oxidation state and structure that are reminiscent of those observed for conversion of β-NiOOH to γ-NiOOH and consequently challenge the long-held notion that the β-NiOOH phase is a more efficient oxygen-evolving catalyst.