Investigation of gas-diffusion electrodes applied in alkaline CO2 electrolysis by electrochemical impedance spectroscopy (EIS)

Kurzfassung

Closing the anthropogenic carbon cycle is one of the major challenges for the present mankind. Low temperature electrochemical reduction of carbon dioxide to useful chemicals can contribute to the solution of this problem by changing the role of CO2 from harmful waste into a valuable feedstock. The electrolyzer can be operated with renewable energy and CO2 taken from concentrated off-gas of industrial plants. One of the main challenges of this process is to bring gaseous CO2 in contact with the catalyst and the aqueous phase to maintain high space-time-yields. By applying porous gas-diffusion electrodes (GDEs) instead of plane electrodes the triple-phase-boundary - reaction zone – and gas transport are significantly enhanced. Electrochemical impedance spectroscopy is a powerful tool to analyze the nature and velocity of single processes occurring at the microscopic level in electrochemical devices. These insights provide valuable information for a specific optimization of electrodes regarding electrolysis performance and long-term stability. However, there is a lack of knowledge which processes are displayed in the impedance spectrum during alkaline CO2 electrolysis on GDEs. This work focusses on the identification of these processes displayed in the impedance spectrum by changing experimental parameters e.g. temperature, CO2 partial pressure & current density and analyzing the obtained response in the spectrum. The shape of the impedance spectrum dramatically changes depending on the supplied gas mixture at the GDE. Compared to the absence of CO2 where hydrogen evolution prevails, two additional processes become visible for a pure CO2 gas feed which can be associated to the CO2 reduction reaction.