Temporal Ni K-Edge X-ray Absorption Spectroscopy Study Reveals the Kinetics of the Ni Redox Behavior of the Iron-Nickel Oxide Bimetallic OER Catalyst

Operando X-ray absorption spectroscopy (XAS) can be utilizedtoprobe the phase and structural changes of FeNiO (x) and similar transition metal oxide electrocatalysts duringelectrocatalytic reactions. However, capturing the temporal changesoccurring in the operando chemistry of electrocatalysts has been littlestudied. In this work, we successfully capture the time-resolved changesat the Ni K-edge for both the X-ray absorption near-edge structure(XANES) and extended X-ray absorption fine structure (EXAFS) regionsof an aqueous phase synthesized FeNiO (x) bimetallic nanoparticle electrocatalyst. During a stepped voltageexperiment, the temporal change in the Ni K-edge was observed as theapplied voltage was stepped from 0.7 to 0.8 V versus the silver/silverchloride reference electrode, and the change observed is associatedwith the Ni redox transition from the 2+ to 3+/4+ oxidation state.Individual XAS spectra were obtained in 90 s, and a total of 9 scanspost voltage step showed a unique transition from a hydroxide to anoxyhydroxide phase. The shift in absorption edge energy position andthe changes in spectral shape for individual scans explain the typicallyobserved broadening of the Ni K-edge XANES spectrum during time-averagedoperando XAS, and a kinetic analysis revealed a first-order observedrate constant, |k (obs)|, of 0.00426 s(-1) and a half-life (t (1/2)) of 163 s. Multivariate curve resolution-alternating least squares analysis followed by linear combination fitting analysis for time-averagedversus time-resolved Ni K-edge changes upon voltage step show distinctdifferences in estimated contributions from Ni2+ oxide/hydroxidephases. Detailed EXAFS modeling shows the phase transition from hydroxideto oxyhydroxide on top of the unchanged metallic core.