Optimal pre-treatment of a Ni-11Fe-10Cu anode for efficient molten salt electrolysis of carbon dioxide: Toward net-zero emission manufacturing

Molten salt electrolysis offers an attractive pathway to net-zero emission manufacturing, including the electrolysis of carbon dioxide to generate high value carbon materials. This is only possible with the development of an anode material which is active towards oxygen evolution at elevated temperatures and stable within the melt. Here, it is shown that optimal pre-treatment of a Ni-11Fe-10Cu alloy includes formation of an active oxide layer which incorporates the molten salt. The molten ternary carbonate electrolyte (Li-Na-K CO3) is investigated at 600°C and pre-oxidation conditions considered include chemical oxidation of the alloy surface under air, and electrochemical oxidation of the surface below that of oxygen evolution, at the onset of metal oxide formation. It was seen that chemical oxidation forms oxides more likely to destabilise the electrode surface when subsequently exposed to the carbonate salt. The unmodified electrode was also significantly corroded due to the fast rate of surface oxidation when stepped to oxygen evolution potentials. Extended (5 h) electrochemical metal oxide formation also causes a build-up of a reactive sub-surface nickel oxide rich layer during pre-treatment, however short-term pre-treatment (1 h) was seen to have the best outcome in terms of both stability and oxygen evolution activity. This is attributed to the formation of a stable LiFeO2 film at the Ni-11Fe-10Cu alloy surface which is highly active towards oxygen evolution. Understanding the interaction between the electrode surface and molten salt chemically and electrochemically under varied polarisation, as carried out here, is crucial to allow progression of this technology for application to carbon capture and other low emission manufacturing approaches.