Operando Identification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution

Co oxides and oxyhydroxides have been studied extensively in the past as promising electrocatalysts for the oxygen evolution reaction (OER) in neutral to alkaline media. Earlier studies showed the formation of an ultrathin CoO_x(OH)_y skin layer on Co₃O₄ at potentials above 1.15 V vs reversible hydrogen electrode (RHE), but the precise influence of this skin layer on the OER reactivity is still under debate. We present here a systematic study of epitaxial spinel-type Co₃O₄ films with defined (111) orientation, prepared on different substrates by electrodeposition or physical vapor deposition. The OER overpotential of these samples may vary up to 120 mV, corresponding to two orders of magnitude differences in current density, which cannot be accounted for by differences in the electrochemically active surface area. We demonstrate by a careful analysis of operando surface X-ray diffraction measurements that these differences are clearly correlated with the average thickness of the skin layer. The OER reactivity increases with the amount of formed skin layer, indicating that the entire three-dimensional skin layer is an OER-active interphase. Furthermore, a scaling relationship between the reaction centers in the skin layer and the OER activity is established. It suggests that two lattice sites are involved in the OER mechanism.

Electrochemical Stability of the Reconstructed Fe3 O4 (001) Surface

Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between -0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm^-2 and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe₃ O₄ bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction.

Transmission Surface Diffraction for Operando Studies of Heterogeneous Interfaces

Processes at material interfaces to liquids or to high-pressure gases often involve structural changes that are heterogeneous on the micrometer scale. We present a novel in situ X-ray scattering technique that uses high-energy photons and a transmission geometry for atomic-scale studies under these conditions. Transmission surface diffraction gives access to a large fraction of reciprocal space in a single acquisition, allowing direct imaging of the in-plane atomic arrangement at the interface. Experiments with focused X-ray beams enable mapping of these structural properties with micrometer spatial resolution. The potential of this new technique is illustrated by in situ studies of electrochemical surface phase transitions and deposition processes.