Unraveling the Cobalt Oxidation State at the Surface of Epitaxial Cobalt Oxide Films during the Oxygen Evolution Reaction by Operando X-ray Absorption Spectroscopy/Surface X-ray Diffraction

Abstract

Better understanding of the oxygen evolution reaction on cobalt oxides requires insights into the oxide−solution interface structure and composition under reaction conditions. We here present operando studies of electrodeposited epitaxial thin films with planar surface morphology that couple X-ray absorption spectroscopy, surface X-ray diffraction, and electrochemical measurements. This enabled us to disentangle bulk and surface contributions of the XAS signal and to correlate the cobalt oxidation state with the surface structure of cobalt oxide films. In the case of Co 3 O 4 (111) films, we show a one-to-one correlation between the Co oxidation state increase in the pre-OER potential range and the potential-dependent thickness of the reversibly formed amorphous layer on the oxide surface. From this correlation, we conclude that this amorphous layer is exclusively composed of Co 3+ . In the case of CoOOH(001) films, we show that no such surface amorphization takes place and that the small oxidation state change with potential may be attributed to the progressive deprotonation of the surface Co-OH groups. For both oxides, the amount of Co 4+ remains below the detection limit.

Nanoscale Surface Refinement of CoCrMo Alloy for Artificial Knee Joints via Chemical Mechanical Polishing

“…This hints that neither the oxidative action of the H 2 O 2 , the complexing function of the glycine, nor the inhibitory role of the BTA modifies the phase structure of the CoCrMo alloy itself; these chemical reactions are restricted to the surface. The XRD pattern manifests peaks associated with metallic Co, with the peak at 44.2° linked to the (111) crystal plane of Co, in alignment with the PDF card PDF#15-0806 [ 31 ]. Such an occurrence is due to the formation of a solid solution by the metals in the alloy, which are fully soluble in each other.…”