Intrinsically stable in situ generated electrocatalyst for long-term oxidation of acidic water at up to 80 °C

Intrinsically stable in situ generated electrocatalyst for long-term oxidation of acidic water at up to 80 °C
Professor Leone Spiccia

Deep studies of water oxidation electrocatalysts is an area that was introduced into our Australian Centre for Electromaterials Science many years ago by the late Professor Leone Spiccia. The significance of this field has blossomed in recent years in Australia as a result of the growing recognition of the enormous renewable energy resources that are available here if a carrier such as hydrogen can be efficiently produced. Inspired by Leone, our goal is to contribute to the technologies that will make this wonderful resource available to the world. To allow this vision to come true, we aim to develop new catalysts for water splitting, focusing simultaneously on three key metrics – activity, stability and price.

In the most successful scenario, the catalytic material would be highly active, robust on a time-scale of decades under relevant operating conditions, and cheap. A much more frequently encountered outcome is that only two of these requirements are met, for example when high activity and stability initially require very expensive catalysts. The challenge that we faced in embarking on this work was “What to do when even noble metals are unstable under the conditions of the reaction of interest, and cheaper analogues lag far behind in performance?”

This is precisely the scenario that emerges in the electrolysis of water at low pH and high temperature, conditions that are ideal for the water reduction to hydrogen gas, but are seriously challenging in regard to catalysis of water oxidation to oxygen gas. Publications in this field very rarely mention that the oxygen evolution reaction at low pH has a significantly longer history of research. Indeed, electrooxidation of highly acidic water was commercialised long time ago as part of electrowinning and electroplating processes – a technology that has a much higher industrial significance than water electrolysis at present. At the start of our research into the low-pH oxygen evolution reaction to support renewable fuel synthesis we delved deeply into the immense literature on water oxidation catalysts for electrowinning. This provided us with a much clearer understanding of the directions that are worth pursuing in our endeavours. As usual, a few days of thorough reading of the literature saved months of work in the laboratory.

As a result it was not a big surprise for us to confirm that the currently known materials that exhibit very high activity for the electrooxidation of hot, acidic water are all inherently unstable under the actual operation conditions and/or during shut down periods, whether they are noble-metal-based or not. This is a familiar problem to us that arose frequently in our previous investigations of in situ electrogenerated water oxidation catalysts for neutral/alkaline conditions. Searching for a strategy here, the self-healing concept, popularised more than a decade ago by Daniel Nocera and his colleagues, appeared to us as one of the most feasible solution to the acidic water oxidation conundrum. Combining this idea with the immense knowledge of the electrowinning anode field formed the basis of our latest developments of the intrinsically stable catalytic systems for the electrooxidation of water at low pH. Our first attempt was published a while ago in a short investigation on the electrolysis of weakly acidic solutions contaminated with metal cations. A more thorough study, focusing on catalysts operating robustly at both very low pH and high temperature has been now published in Nature Catalysis. All members of our multifaceted team have made invaluable contributions to this project, but most of the hard work was completed by Manjunath and Jamie with a continuous support from Max.

Maxime Fournier, Manjunath Chatti and James Gardiner

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