Simulation-Driven Enhancement of an Alkaline CO2 Capture Electrochemical Cell

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Electrochemical cells using anion exchange membranes (AEM) have been previously demonstrated to regenerate aqueous K2CO3 solutions obtained via CO2 absorption from air with aqueous KOH [1]. Preliminary results suggest this technique might significantly outperform currently employed methods in terms of specific energy expenditure (MJ/kgCO2). In order to accomplish this, we propose a computational study of an electrochemical cell designed and operated at the Paul Scherrer Institut. A multi-phase multi-component model has been developed and implemented via appropriate customization of the commercial CFD software Ansys Fluent. The model accounts for all the physical and chemical phenomena relevant to the electrochemical process, i.e. fluid flow mechanics, heat and mass transfer, electrical potential propagation and chemical and electrochemical reactions. The results of the simulations suggest two possible causes of cell overpotential. Firstly, the formation of hydrogen bubbles by the hydrogen evolution reaction in the cathode electrode, which partially cover the liquid-solid interface where the electrochemical reaction occurs. This is clearly shown by direct comparison with idealized single-phase simulations. Secondly, the flow structure of the catholyte is not optimized to favor the diffusion/migration of carbonate rather than hydroxide anions towards the membrane. This requires increasing the gap between the electrode and the membrane in order to maximize the faradaic efficiency, but it also increases the cell specific resistance. Although we show that there exists an optimal ratio of electrode-membrane gap to cell height, our simulations indicate that a topological optimization of the electrode fine structure might be beneficial to eliminate the undesired effects mentioned above. [1] Muroyama, A. P., & Gubler, L. (2022). Carbonate Regeneration Using a Membrane Electrochemical Cell for Efficient CO2 Capture. ACS Sustainable Chemistry & Engineering.