Enhancement of Oxygen Evolution Activity of Nickel Oxyhydroxide by Electrolyte Alkali Cations

Abstract

Herein, the effect of the alkali cation (Li⁺ , Na⁺ , K⁺ , and Cs⁺ ) in alkaline electrolytes with and without Fe impurities is investigated for enhancing the activity of nickel oxyhydroxide (NiOOH) for the oxygen evolution reaction (OER). Cyclic voltammograms show that Fe impurities have a significant catalytic effect on OER activity; however, both under purified and unpurified conditions, the trend in OER activity is Cs⁺ > Na⁺ > K⁺ > Li⁺ , suggesting an intrinsic cation effect of the OER activity on Fe-free Ni oxyhydroxide. In situ surface enhanced Raman spectroscopy (SERS), shows this cation dependence is related to the formation of superoxo OER intermediate (NiOO^- ). The electrochemically active surface area, evaluated by electrochemical impedance spectroscopy (EIS), is not influenced significantly by the cation. We postulate that the cations interact with the Ni-OO^- species leading to the formation of NiOO^- -M⁺ species that is stabilized better by bigger cations (Cs⁺ ). This species would then act as the precursor to O₂ evolution, explaining the higher activity.