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ACS Catal ; 12(6): 3256-3268, 2022 Mar 18.
Artigo em Inglês | MEDLINE | ID: mdl-35359579


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.

Angew Chem Int Ed Engl ; 59(49): 21904-21908, 2020 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-32729209


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.

J Am Chem Soc ; 139(12): 4532-4539, 2017 03 29.
Artigo em Inglês | MEDLINE | ID: mdl-28252295


The surface restructuring of Pt(111) electrodes upon electrochemical oxidation/reduction in 0.1 M HClO4 was studied by in situ grazing-incidence small-angle X-ray scattering and complementary scanning tunneling microscopy measurements. These methods allow quantitative determination of the formation and structural evolution of nanoscale Pt islands during potential cycles into the oxidation region. A characteristic ripening behavior is observed, where these islands become more prominent and homogeneous in size with increasing number of cycles. Their characteristic lateral dimensions primarily depend on the upper potential limit of the cycle and only slightly increase with cycle number. The structural evolution of the Pt surface morphology strongly resembles that found in studies of Pt(111) homoepitaxial growth and ion erosion in ultrahigh vacuum. It can be fully explained by a microscopic model based on the known surface dynamic behavior under vacuum conditions, indicating that the same dynamics also describe the structural evolution of Pt in the electrochemical environment.

J Phys Chem Lett ; 8(5): 1067-1071, 2017 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-28201875


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.