Modular CO2-to-CO Electrolysis Short-Stack Design─Impact of Temperature Gradients and Insights into Position-Dependent Cell Behavior

Abstract

The performance of flow cells for aqueous CO2-to-CO electrolysis at ambient conditions is reportedly close to meeting industrially applicable rates when operating with membrane electrode assemblies (MEAs) based on anion exchange membranes. However, the challenges of the stacking of these cells are underrepresented in the literature, despite being a major milestone for scaling the technology. Therefore, we report a modular short-stack design for MEA cells and demonstrate its operation with three cells. The short stack replicates the performance of its respective single cell at current densities between 100 and 200 mA/cm2. At higher current densities (300-400 mA/cm2), the short stack surprisingly outperforms the single cell showing a lower stack voltage and varying cell voltage depending on the cell’s position in the short stack. The temperature distribution affected the membrane conductivity and activation energies for the reactions at the electrodes, which was verified by electrochemical impedance spectroscopy. It could be demonstrated that the temperature distribution is the leading cause of the observed position dependency of the individual cell voltages in the short stack. We show that the inherent asymmetry of the cells results in an asymmetric temperature distribution in the short stack. Taking advantage of the modular stack design, we designed a quasi-symmetric cell eliminating the problem. In this cell, we observed a much smaller voltage variation at 400 mA/cm2, caused by shunt current, which is well-known from alkaline water electrolysis. In the CO2 electrolysis short stack, however, their effect was found negligible.