Nickel based metal oxide electrocatalysts: From model to operando conditions studied by XPS and vibrational spectroscopy

Electrocatalysts play an important role in a future renewable energy economy. Therefore, a thorough understanding of their functionality can help to improve and replace existing rare and expensive Pt-group based metal and metal oxide materials for fuel cell applications and also materials used for alkaline electrolysis. Here, we aim to synthesize thin film model systems to study and understand the critical processes taking place at the electrode/electrocatalyst/ electrolyte interface under operating conditions by a surface science approach combined with electrochemical processing and treatment. NiOx(OH)y electrocatalysts are prepared on different substrates by electrochemical and by magnetron sputtering methodologies. The so prepared electrodes are characterized with regard to their activity for the hydrogen evolution reaction (HER) as well as for the oxygen evolution reaction (OER) by standard electrochemical procedures. In our approach we characterize the samples before and after electrochemical treatment mainly by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy with regard to their chemical surface composition. To obtain a deeper insight into the changes taking place under working settings, we also apply Raman spectroscopy during operando conditions for selected electrodes. From the experimental data we conclude, that a mixture of metallic and different oxidic phases leads to the highest overall HER activity. The pure phases on its own show only a strongly reduced activity. In general, the preparation of the samples plays the most important role for the final activity. This is especially observed for the OER, where an easy transformation of the starting phases is required for improved electrical conductivity and electrocatalytic activity. A large number of defects favor the formation of the Ni3+ oxyhydroxide phase, which can then form the O2 evolving site under operating conditions. Even if our model systems are still too complicated to deliver a thorough understanding of all underlying processes, we can show the validity of our approach. Furthermore, we obtained a rich spectral and electrochemical database for NiOx(OH)y based systems as catalysts for electrochemical water splitting.