Decoupled electrochemical CO2 reduction using redox mediators

Potter, Mark and Toghill, Kathryn (2025) Decoupled electrochemical CO2 reduction using redox mediators. PhD thesis, Lancaster University.

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Abstract

Climate change, caused by the use of fossil fuels for both primary energy and as a source of commodity chemicals, is of growing concern. It is imperative that alternate technologies are developed quickly to meet the internationally agreed emission reduction targets to limit global warming to 1.5°C above pre-industrial levels. Electrolytic conversion of CO2 and water for energy storage and chemical synthesis offers a potential solution to this issue, whereby renewable energy is used to drive the otherwise unfavourable reactions. Presently, conventional electrolysers rely on intricate membrane electrode assemblies which pose many chemical and mechanical challenges, resulting in low lifetime and limited activity. As an alternative technique, redox mediators can be used to decouple the chemical reaction from the electrode surface. Through this, charge transfer at the electrode and the electrocatalytic reaction occur effectively independently and can be optimised separately. A range of redox active molecules were considered as possible mediators. Initially, a novel catalytic response to CO2 was observed using a dithiolene complex of iron. This was explored further, where it was found that high selectivity towards formate could be achieved using trifluoroethanol as a proton source. Focus was then shifted to aqueous conditions more favourable for CO2 reduction, where a complex of chromium with 1,3-propanediamine-N,N,N’,N’-tetraacetic acid was selected due to its highly negative reduction potential. It was found that the two-electron reduction products CO and formate were both accessible by using optimised catalysts based on gold and bismuth nanomaterials respectively. A brief exploration of homogeneous co-catalysis using the mediator alongside Ni cyclam also found CO and formate to be the main products, with selectivity influenced by the supporting electrolyte. Finally, the reaction was scaled up, where it was found that production rates comparable to state-of-the-art systems are achievable.