Monitoring electrochemical and electrocatalytic interface processes on the atomic and nanometer scale by operando surface X-ray scattering

The need for sustainable energy, reduction of pollutants, and the environmental benign processing of chemicals has spurred worldwide scientific activities in electrochemical energy science and electrocatalysis. These processes occur at the interfaces of solid electrodes in contact with complex liquid environments under conditions, which are difficult to access by most surface analytic techniques. For a better understanding of structure-property relationships and reaction-induced morphological changes, experimental approaches are required that provide direct insight into the atomic and nanoscale interface structure.
Modern synchrotron-based X-ray scattering methods provide unique opportunities for such operando studies. They allow monitoring structural changes at electrode surfaces with high structure sensitivity on (sub-) second time scales. The talk will give an overview of the current state of this field. It will specifically highlight the advantages of using very high photon energies (70 keV) in combination with 2D X-ray detectors. This new approach enables e.g. detailed determination of complex, dynamically changing interface structures and microdiffraction studies of spatially heterogeneous materials. The application of these methods will be demonstrated by studies of Pt oxidation and transition metal oxide catalysts for water splitting. In the first case, a strong dependence of the oxidation mechanism on the Pt surface structure was observed, resulting in pronouncedly different Pt dissolution and nanoscale restructuring. The second example will focus on epitaxial Co oxide thin films, which were monitored in the oxygen evolution regime up to current densities as high as 150 mA cm^-2. Whereas CoOOH(001) films were perfectly stable under these conditions, oxygen evolution on Co₃O₄(111) films occurs on a disordered nm-thick skin layer, which forms highly reversibly at potentials 300 mV negative of the onset of this electrocatalytic reaction.