Structure activity stability and in situ Raman spectroscopic studies of oxygen evolution catalysts in acidic environment
Water electrolysis emerges as key technology for the long term storage of electricity from renewable sources which is essential to overcome their intermittent availability. In this context, proton exchange membrane (PEM) electrolyzers constitute the most promising technology. Considering PEM electrolyzers, the main catalytic efficiency losses (overpotentials) and stability problems are related to the noble metal based anode catalyst, were the oxygen evolution reaction (OER) proceeds. This work aims to establish a more profound understanding of the electrocatalytic OER and the related catalysts under acidic conditions to facilitate the knowledge based improvement of the catalyst's activity and stability and, additionally, to lower its noble metal content. Initially, an appropriate model catalyst for the fundamental investigation of PEM OER catalysts was contrived, which allows for microkinetic studies and the investigation of catalyst substrate interactions. In this context, thermally prepared thin homogenous oxide films were found to constitute an adequate model system. Based on this model system, first Ir oxide was studied. Thereby, two chemically distinct Ir oxides with distinctly different catalytic properties were identified as a function of calcination temperature. The amorphous low temperature Ir oxide provided a considerably higher OER activity but, unfortunately, a lower Ir stability than the crystalline Ir oxide formed at higher calcination temperatures. Catalyst substrate interactions between the Ir oxide layer and the Ti substrate were solely identified at the highest investigated calcination temperature of 550degC and decreased the OER activity of the catalyst. Although indications for the OER activity and stability determining material properties were obtained in the study of pure Ir oxides, a more detailed investigation of this aspect was performed based on Ir Ni mixed oxides. Considering Ir Ni mixed oxides, the OER activity and the Ir stability can be varied continuously, which allows the identification of the material properties relevant for the OER activity and stability as well as their optimization. Based on Ir Ni mixed oxides the Ir mass based OER activity could be increased by a factor of ~20 compared to a similarly prepared pure Ir oxide reference sample. Furthermore, this study revealed that the oxide surface termination critically determines the OER activity of Ir Ni oxides. Hereby, the fraction of surface hydroxyl groups was found to be correlated to the surface specific OER activity of the oxide. The surface processes occurring during the OER were studied by in situ surface enhanced Raman spectroscopy (SERS) on a Ru oxide catalyst. In order to provide the required surface enhancement a recent approach called shell isolated nanoparticleenhanced Raman spectroscopy was adapted to PEM OER catalysts and its applicability for SERS studies of OER catalysts was demonstrated. In this study an OHx species was identified which appeared exclusively during the OER.