Iridium-free catalysts for acid water electrolysis investigated
Scanning electron micrograph of a cobalt-based catalyst on a fibre substrate (micrograph was manually coloured) and schematic representation of a multi-technique operando material characterization indicated by artificially added light ray, bubbles and rising spectra. © Marc Tesch/MPI-CEC
Hydrogen will play an important role, both as a fuel and as a raw material for industry. However, in order to produce relevant quantities of hydrogen, water electrolysis must become feasible on a multi-gigawatt scale. One bottleneck is the catalysts required, with iridium in particular being an extremely rare element. An international collaboration has therefore investigated iridium-free catalysts for acidic water electrolysis based on the element cobalt. Through investigations with various methods, among them experiments at the LiXEdrom at the BESSY II X-ray source in Berlin, they were able to elucidate processes that take place during water electrolysis in a cobalt-iron-lead oxide material as the anode. The study is published in Nature Energy.
The oxygen evolution reaction (OER) in water electrolysis requires special catalytic support. However, iridium catalysts are probably not suitable for large-scale use due to their price and limited availability, so alternatives must be found. An international team led by Dr Alexandr N. Simonov from Monash University in Melbourne, Australia, has now investigated the acidic oxygen evolution reaction on cobalt-based catalysts and elucidated the changes at the active cobalt sites. The research teams used different methods and combined their findings to a new picture.
Processes during the Oxygen evolution reaction
The stabilisation of catalysts during OER involves the interaction of corrosion and oxidation processes and is considered key to catalyst development. ‘In this study, we have discovered that the corrosion and deposition processes are not directly linked to the catalytic process, but run in parallel,’ says Dr Marc Tesch from the Max Planck Institute for Chemical Energy Conversion, one of the authors of the study. The time-resolved measurements also show that the development of the catalyst to a stabilised active state is not a rapid process, but takes place on a time scale of minutes. X-ray spectroscopy shows that the catalytically active cobalt sites adopt an oxidation state higher than 3+ during the acidic OER and do not exhibit long-range order. This distinguishes them from previously described cobalt μ-(hydr)oxo structures, which are present in neutral and alkaline reaction environments.
International collaboration under Corona conditions
A significant part of the research was carried out at BESSY II during the coronavirus pandemic, when international travel and external access to the synchrotron facility were severely restricted. ‘The support provided by the local team at BESSY II was therefore particularly important,’ says Tesch.
The findings are helpful for developing cost-effective cobalt-based anode catalysts for use in proton exchange water electrolysers.
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