Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations

To slow down the growth of the steadily increasing carbon footprint, a transition to renewable energy is imperative. Electrochemical water splitting (2 H₂O → 2 H₂ + O₂) offers a zero-carbon route to hydrogen (H₂) from water, where the main cause of energy loss and cost in this process is the anodic oxygen evolution reaction (OER). To make water a viable source for H₂, efforts therefore need to be focused on more efficient OER electrocatalysts. Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Here, we investigate the OER activity and redox-activity using X-ray absorption spectroscopy (XAS) of a Ni–Fe oxyhydroxide (OOH) electrocatalyst in the presence of alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), which is one of the best performing catalysts in alkaline media.

In conclusion, we demonstrate that the effect of alkali metal cations on the OER activity of the Ni₆₅Fe₃₅(OOH) catalyst can be explained by differenes in the electrolyte pH, supported by the directional shift of the Ni²⁺→Ni^3+/4+ redox-peak and the OER activity. Since the basicity increases in the order LiOH <  NaOH < KOH < RbOH < CsOH following the cation size (explained by their pK_b values), the electrolyte pH increases accordingly from small to large alkali metal cations due to an increase in the amount of dissociated OH⁻. Since the OER activity is sensitive to pH, this is able to explain previously puzzling discrepancies between the trend in cation size and the OER activity, since any factor introducing uncertainties in the OH⁻ concentration (or electrolyte concentration) can affect the OER activity. Our DFT-derived reactivity descriptors further support that alkali metal cations do not significantly alter the intrinsic reactivity properties of either Ni, Fe, or O lattice in contrast to Fe substitution, which instead increases the Lewis acidity of neighboring Ni-sites (although Fe remains the most Lewis acidic site). The alkali cations are therefore unlikely to account for the differences in the OER activity. Thus, our DFT results are in line with the experimental findings that the electrolyte pH constitutes the strongest activity descriptor. Future studies therefore need to explore possible pH-effects in oxide-derived catalysts in striving for understanding the role of alkali metal cations in OER electrocatalysis.