Effect of different alkali metal cations on the oxygen evolution activity and battery capacity of nickel electrodes in concentrated hydroxide electrolytes

Abstract

The effect of different alkali metal cations on the oxygen evolution activity and battery capacity of nickel electrodes has recently been studied in low concentration alkali hydroxide electrolytes. As high concentration hydroxide electrolytes are favored in applications due to their high conductivity, we investigate if the cation effects observed in low concentration electrolytes translate to more industrially relevant conditions for both alkaline water electrolysis and nickel iron batteries. We investigate the alkali metal cation effect on the electrochemical properties of nickel electrodes in highly concentrated hydroxide electrolytes by adding Li⁺, Na⁺, Cs⁺ and Rb⁺ cations to a 6.5 M KOH electrolyte, while keeping the hydroxide concentration constant. For OER we find a trend in activity similar to that at low concentrations Rb⁺>Cs⁺>K⁺>Na⁺>Li⁺, where especially larger additions of Rb⁺ and Cs⁺ (1 M or 0.5 M) cause a significant decrease in OER potential. Smaller cations interact with the layered hydroxide structures in NiOOH to stabilize the α/γ phases and increase the potential for OER. The intercalation of small cations also causes an increase in battery electrode capacity because of the higher average valence of the Ni(OH)2/NiOOH α/γ pair. Small concentrations of Li⁺ added to a concentrated KOH electrolyte can therefore be beneficial for the nickel electrode battery functionality and for an integrated battery and electrolyser system, where it increases the battery capacity without a significant increase in OER onset potential.