High oxygen evolution reaction activity on lithiated nickel oxides - Activity descriptors

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Alkaline water electrolyzers promise very high purity hydrogen gas production but suffer from large overpotential for anodic oxygen evolution reaction (OER). Here we describe the effect of lithium (Li+)-substitution into nickel oxide on the electrocatalytic activity towards OER in alkaline electrolyte. The X-ray diffraction patterns of lithiated nickel oxides (LixNi1−xO, x = 0.00–0.50) synthesized by the solution-combustion method suggest that pure phase of lithiated nickel oxide was formed until x = 0.30; thereafter, a secondary phase of LiNiO2 was observed. Rietveld analysis showed that Li+-substitution caused a contraction in the lattice structure as shown by the decrease in lattice parameters upon Li+-substitution. Further, the weight fraction of LiNiO2 was found to be dominant for x = 0.50. Deconvolution of the high resolution X-ray photoelectron spectroscopy for O 1s and Ni 2p spectra suggested that concentration of oxygen vacancies increased linearly, whereas that of Ni3+ increased till x = 0.30 and it decreased when Li+-substitution was further increased to x = 0.40 and 0.50. Although electrical conductivity increased upon Li+-substitution, no significant effect was observed for lithiated samples with varying Li+-content (x = 0.10–0.50). The activities for OER were measured using the rotating disk electrode in 0.5 M NaOH electrolyte, and the data suggest that lithiated nickel oxide synthesized with x = 0.30 shows the highest current density at 1.70 vs. RHE (V). The decrease in OER activity for x = 0.40 and 0.50 was attributed to the decline in OER active Ni3+ sites (probably due to the presence of chemically unstable LiNiO2).