Atomic-Scale Insights into Morphological, Structural, and Compositional Evolution of CoOOH during Oxygen Evolution Reaction

Co-based electrocatalysts are an emerging class of materials for the oxygen evolution reaction (OER). These materials undergo dynamic surface changes, converting to an active Co oxyhydroxide (CoOOH) layer during OER. A better understanding of the structural, morphological, and elemental evolution of CoOOH formed during OER is, therefore, crucial for the optimization of Co-based electrocatalysts. Herein, we propose an innovative multimodal method, which combines X-ray photoelectron spectroscopy, electron backscatter diffraction, high-resolution transmission electron microscopy, and atom probe tomography with electrochemical testing to investigate the temporal evolution of the oxidation state, thickness, morphology, and elemental distribution of the CoOOH layer grown on Co(0001) during OER. We reveal that the oxyhydroxide layer with the highest OER activity is a 5−6 nm thick, strongly hydrated film resembling a β-CoOOH(0001) structure and consisting of stacks of nanocrystals; which explains the X-ray amorphous characteristics of active species formed on Co-based electrocatalysts observed by operando X-ray-based techniques. The large interfacial area at these hydrated β-CoOOH(0001) nanocrystals enables efficient mass transport of reacting hydroxyl ions to catalytically active sites, and thus, high OER rates. As OER proceeds, the well-hydrated β-CoOOH(0001) nanocrystals grow into a monolithic crystalline β-CoOOH(0001) film. This goes along with a decrease in water content and electrochemically accessible catalyst area, resulting in slightly decreased OER currents. Overall, our study unprecedentedly unveils that in situ generated thin β-CoOOH(0001) layer undergoes dynamic morphological and elemental changes along with (de)incorporation of water molecules and hydroxyl groups during OER, which in turn alters OER performance. We demonstrate the strength of our multimodal characterization approach when seeking mechanistic insights into the role of structural and compositional evolution of hydrous oxides in activity during electrocatalytic reactions.