What limits the rate of thermally activated self-discharge of nickel oxyhydroxide electrodes?

Abstract

Nickel (oxy)hydroxide electrodes are widely used in alkaline batteries and water electrolyzers. Their operation involves reversible phase transformations (Ni(OH)₂ + OH⁻ $\rightleftharpoons$ NiOOH + H₂O + e⁻) upon charge and discharge at potentials below the onset of oxygen evolution; electrocatalytic oxygen evolution reaction (OER, 4OH⁻ → O₂ + 2H₂O + 4e⁻) at higher potentials; and spontaneous chemical self-discharge (4NiOOH + 2H₂O → 4Ni(OH)₂ + O₂) that is accelerated at elevated temperatures. This work studied compositional and microstructural changes in nickel-boride-based electrodes during cold (room temperature) charge and hot (95 °C) self-discharge cycles that enable decoupled membraneless water electrolysis. Pristine electrodes comprised agglomerates of equiaxed nickel boride nanoparticles that transformed into boron-depleted Ni(OH)₂ platelets upon chemical aging and electrochemical activation treatments followed by operation in cold charge and hot self-discharge cycles. Cyclic operation in alternating cold and hot alkaline electrolytes resulted in crystallization and growth of mosaic hexagonal plates that seem to have been formed by detachment of small platelets from the agglomerates and oriented reattachment onto the edges of the growing plates. Concomitant with the plate growth, the self-discharge kinetics (at 95 °C) decreased from one cycle to another whilst the OER kinetics (at room temperature) slightly enhanced. This observation suggests that bulk processes involving solid-state diffusion and phase transformation limit the rate of thermally activated self-discharge rather than OER. This finding sheds new light on the mechanism of the self-discharge reaction that limits the performance of alkaline batteries and decoupled alkaline water electrolyzers.