Water Dissociation on NiOOH in Alkaline Water Electrolysis Improves with Increasing Alkali Metal Cation Size

The activity of nickel-based electrocatalysts toward the oxygen evolution reaction (OER) is influenced by the presence of alkali metal cations in the electrolyte. Since the underlying mechanism is not fully resolved yet, a study combining Raman, Fourier-transform infrared, and photoelectron spectroscopies is conducted. It is found that an improved OER activity correlates with structural changes of the catalyst. The cations are adsorbed in increasing amounts in the order Li⁺ < Na⁺ < K⁺ < Cs⁺, opening the layers of the NiOOH layered double-hydroxide structure and promoting a transition from a more β-like to a more γ-like NiOOH phase. In addition, the NiOOH surface gets increasingly deprotonated with increasing alkali cation size. The activated catalyst materials are stabilized in ultra-high vacuum and exposed to controlled doses of H₂O to analyze the catalyst-electrolyte interface in a quasi in situ approach with photoelectron spectroscopy. Going from Li⁺ to Cs⁺, more OH groups are found on the surface after the exposure to H₂O, demonstrating that such structural changes are facilitating the dissociation of H₂O. As the dissociation of H₂O is a crucial step in many OER mechanisms, its modified efficiency can be correlated with the observed trends in OER activity in LiOH, NaOH, KOH, and CsOH.