Degradation Processes in Bipolar Membrane CO2-Electrolysis Studied by Time-Resolved X-Ray Tomography

Abstract

Electrochemical reduction of carbon dioxide via CO2 co-electrolysis (CO2ELY) is an essential process to produce non-fossil chemical feedstock acting as CO2 sink. The required electrical energy can be harvested from temporal surplus of renewable energy sources like solar or wind. Employing a bipolar electrolyte membrane (BPM) in forward-bias is a promising approach to mitigate fundamental problems of the current technological fore-runner design that employs an anion exchange membrane (AEM). We employed synchrotron time-resolved X-ray tomographic microscopy (XTM) to study degradation processes in BPM CO2ELY.In forward-bias BPM co-electrolysis, the cation exchange layer (CEL) faces the anode to provide favorable acidic conditions for the oxygen evolution reaction. Such acidic anode allows using pure water as electrolyte eliminating the salt precipitation at the cathode observed for AEM CO2ELY. As the second major issue in AEM CO2ELY, (bi)carbonate ions formed at the cathode are transported to the anode presenting a major obstacle for high CO2 conversion efficiency. The then required effort to separate CO2 from the electrolyte further reduces the overall energy efficiency. Employing a BPM and introducing the CEL prevents (bi)carbonate ions reaching the anode. BPM CO2ELY retains at the same time the advantages of AEM CO2ELY of performing CO2 reduction under alkaline conditions with the anion exchange layer (AEL) of the BPM facing the cathode.However, BPM CO2ELY suffers from low current density and poor stability in the range of only a few hours. Current limiting factors and degradation processes as well as the affected components are rarely investigated and poorly understood. Processes that have been brought forward are cathode flooding similar to fuel cells or catalyst poisoning. We employ synchrotron XTM to investigate structural dynamics in CO2ELY during operation. The brilliant synchrotron source allows us to acquire one full high-quality scan with a voxel size of 2.75 um in 1 s. We acquired one scan every minute for one hour during electrochemical operation.The image data reveals the formation of gas bubbles at the AEL-CEL junction and ultimately delamination of the BPM. We explain the gas formation as the recombination of (bi)carbonate ions with hydrogen ions at the junction producing gaseous CO2. This CO2 additionally reaches the anode by diffusion causing partial delamination of the anode catalyst layer. CO2 escaping from the AEL surface at the anode forms high-pressure gas bubbles that break the water wet catalyst layer. A diffusive model considering the transport of ions and neutral CO2 is able to capture the mechanism.Our imaging experiments provide insights not attainable by other diagnostic methods and highlight the need for fundamental research on forward-bias BPM CO2ELY. Knowledge based on AEM CO2ELY and fuel cells is not necessarily directly applicable and specific studies on BPM CO2ELY are required to increase performance and stability.We gratefully acknowledge provision of beamtime at TOMCAT, SLS, Paul Scherrer Insititut, Villigen PSI, Switzerland.Figure 1: Tomographic slice of CO2-electrolyser (cathode to the left) a) before operation and running water through anode b) after 23 minutes operation at 3.5V constant potential showing liquid water dynamics at cathode, membrane swelling, gas formation within the membrane and anode catalyst layer disintegration. Figure 1