Revealing Mechanistic Processes in Gas-Diffusion Electrodes During CO2 Reduction via Impedance Spectroscopy

The use of gas-diffusion electrodes (GDEs) as CO2 converting electrodes increases the achieved current densities for the CO2 reduction reaction (CO2RR) by a multiple (>100 mA cm(-2)) compared to planar electrodes (10 mA cm(-2)), whereas the long-time stability of the employed GDEs is a crucial factor for the industrial realization of this technology. We investigated carbon-supported tin-based GDEs by electrochemical impedance spectroscopy with the goal to identify the underlying physical processes by varying the applied current density, temperature, CO2 partial pressure, and electrolyte, an unexplored task so far. The spectrum displays four features which we interpreted as (i) ionic and electronic conductivity in the porous system, (ii) reaction of CO2 with OH- to form bicarbonate, (iii) charge transfer converting CO2 (aq) to CO2 center dot-, and (iv) liquid phase diffusion of CO2 (aq). Besides the assignment of the physical processes to the features obtained in the spectrum, we conclude that the observed spectrum shape is affected by CO2RR and hydrogen evolution reaction, whereby the shape dominating reaction switches with varying experimental conditions such as temperature, CO2 volume fraction, current density, and electrolyte.