Ultrafast Electronic and Structural Dynamics in Oxygen Evolution Reaction Catalysts

We investigate the ultrafast electronic and thermal properties of the bulk amorphous cobalt oxide water oxidation catalysts cobalt-phosphate (CoPi) and cobalt borate (CoBi) using optical pump/X-ray probe correlated with in situ electrochemical transient absorption spectroscopy. The electronic signature of a light-generated intermediate species is compared to steady-state in situ heating X-ray absorption spectroscopy (XAS), suggestive of non-thermal contributions to charge transfer formation on ultrafast timescales. With Co K-edge transient absorption spectroscopy, we observe a net photoreduction following 400 nm excitation that also initiates a potential-dependent increase in the excited-state fraction of trapped charge carriers that persists on nanosecond to microsecond timescales and is identified as a potential precursor state to oxygen evolving reactions (OER). Distinctly, the formation of excited-state species cannot be fully explained by photothermal reaction dynamics alone, with potential contributions from electronic motion acting in concert with macroscopic redox activity. The results can influence the design of water-splitting catalytic materials for tuning dimensionality, confinement, and charge delocalization across structures.