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

Abstract Alkaline water electrolyzers promise very high purity hydrogen 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 (Li x Ni 1− x O, 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 LiNiO 2 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 LiNiO 2 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 Ni 3+ 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 Ni 3+ sites (probably due to the presence of chemically unstable LiNiO 2 ).