Impact of over compressing gas diffusion electrodes in alkaline zero-gap CO2 electrolyzers

The electrochemical reduction of CO2 offers a promising route to valuable fuels; however, stable performance is hindered by gas–liquid transport limitations in the cathode gas diffusion electrode (GDE), which are directly affected by GDE compression. Here, we showed that operating an alkaline zero-gap CO2 electrolyzer at a compression ratio (CR) of 10% achieved higher CO faradaic efficiency compared to CR = 20% and 30%. The improved performance is attributed to the fewer obstructions to gas transport in the 10% compressed cathode GDE, as evidenced by lower quantities of flooded anolyte and salt precipitates observed via synchrotron X-ray radiography. Less salt precipitate formation in the cathode at CR = 10% is further demonstrated by an 82% smaller mass transport resistance compared to CR = 30%, a result attributed to larger pores in the less compressed GDE that mitigate anolyte entrapment. Additionally, lower amounts of anolyte are observed at the cathode flow field–GDE interface at CR = 10%, whereas at CR = 30%, excess liquid at this interface led to CO2 gas inlet blockage and cell voltage instabilities. As such, operating the CO2 electrolyzer at lower CRs is recommended to facilitate gas–liquid transport within the cathode GDE and thereby improve overall cell performance.