Title : In situ and Operando soft X ray absorption spectroscopy on water oxidation catalysts
Abstract:
Hydrogen gas (H2) is currently being pursued as an alternative energy carrier to replace fossil fuels. Solar-driven splitting of water into O2 and H2 is one of the intensively discussed approaches to obtain hydrogen. For a knowledge driven development and optimization of stable low-cost photocatalytic systems a detailed understanding of the structure-function correlation of these materials is required. Synchrotron-based operando soft X-ray spectroscopy can provide important insights into the electronic structure during the catalytic function. However, bridging the pressure gap between the experimentally necessary vacuum conditions and the operando conditions involving electrolyte solution, applied voltage or light irradiation is challenging. Here, a recently developed soft X-ray transmission cell for in-situ and operando X-ray absorption spectroscopy studies at the synchrotron facility BESSYII will be presented together with recent studies on catalytic systems. Using the example of a MnOx water oxidation catalyst the power of transition metal L-edge spectroscopy will be presented, allowing to reveal how contributions from different material structures and oxidation state species change during the activation process of the catalyst. The XAS results show that the freshly prepared film at OCP contains a dominant contribution of MnO2 (~75 %) and a contribution from a birnessite-like material (~25%). No or only neglectable percentage of MnO, Mn3O4 or Mn2O3-like Mn species were found in the freshly prepared sample. After 51 min of in situ activation at 1.2 V vs NHE at pH 7.0, the birnessite-contribution increased to 75% in the spectrum. As another example oxygen K-edge measurement will be shown, that were applied to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(OxHy) electrocatalyst. For the first time, we tracked here the spectral changes in a dynamic fashion in the soft X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. At operando conditions we observed the formation of a pre-peak feature at the O K-edge which we correlated to the formation of an electron deficient oxygen site.
