(Invited) Revealing Structural Evolution of Nickel-Based Nanocatalysts By X-Ray Absorption Spectroscopy during Oxygen Evolution Reaction in Alkaline Medium

Nickel-based materials are known to undergo structural changes during electrooxidation reactions such as the oxygen evolution reaction (OER). The structural evolution is relevant to the Ni oxidation transition from the 2+ to 3+/4+ oxidation state, converting from hydroxide to oxyhydroxide. In this work, we apply operando x-ray absorption spectroscopy (XAS) to probe the structural changes of Ni-based electrocatalysts during OER. The time-resolved changes at the Ni K-edge XAS has been successfully captured during a stepped voltage experiment on a FeNiOx bimetallic nanoparticle electrocatalyst. The change observed is associated with the Ni oxidation transition from hydroxide to oxyhydroxide. A kinetic analysis reveals a first order observed rate constant, |kobs|, of 0.00426 s-1 and a half-life (t1/2) of 163 s. Detailed extended x-ray absorption fine structure modeling shows the phase transition from hydroxide to oxyhydroxide on top of the unchanging metallic core. This time-resolved XAS method is extended to study a series of Ni-based OER-active catalysts to reveal the effects of composition on the kinetics of the Ni oxidation transition under OER conditions.