Modeling the Influence of Electrolyte Distribution in Silver Gas Diffusion Electrodes for CO2 Electroreduction

Abstract

The chemical industry has been increasingly focusing on reducing CO2 emissions in recent years. One promising way to do this is via CO2 electroreduction, a technology that does not only reduce CO2 emissions but also allows CO2 to be used as a source of carbon for making commodity chemicals. A key to efficient CO2 electroreduction is the electrolyte management inside the porous system of gas diffusion electrodes (GDEs). Flooding of electrodes can severely hinder mass transfer and thereby prevent CO2 reduction, giving rise to the parasitic hydrogen evolution reaction [1]. Although the measurement of electrolyte saturation is possible through methods like synchrotron radiography, these measurements are highly expensive, and thus not always readily available [2]. Modeling can help here to better understand the influence of electrolyte saturation, and predict possible measures for improving the process.In most CO2 electrolysis models in literature, the electrolyte saturation is a fixed parameter [3,4]. This assumption does not always hold true, as effects like electro-wetting change the wetting behavior of the GDE for different operating conditions. In this new modeling approach, correlations between capillary pressure and saturation, as well as between contact angle and potential are introduced, making the description of these changes possible. These correlations are gained from pore network simulations of the electrode material reconstructed from FIB-SEM measurements. The correlations are implemented in a one dimensional continuum model that includes discretized balance spaces for liquid and gas phase inside the porous system of the GDE, ideally mixed gas and electrolyte bulk spaces on the respective sides of the electrode, as well as boundary layers between bulk and electrode. Homogeneous reactions of the carbonate buffer system in the liquid phase and electrochemical reactions inside the GDE are considered. Transport of species is calculated using Maxwell-Stefan diffusion, migration in the potential field resulting from the Nernst-Planck equation, and flow through the porous electrode according to Darcy’s law. Additionally phase transfer processes like gas dissolution and evaporation of water are considered. The GDE model is validated with experimental data for CO2 electrolysis, enabling insights into the processes in the GDE and elucidating the influence of electrolyte saturation on the CO2 mass transfer.[1] Bienen, F., Paulisch, M. C., Mager, T., Osiewacz, J., Nazari, M., Osenberg, M., Ellendorff, B., Turek, T., Nieken, U., Manke, I., & Friedrich, K. A., Investigating the electrowetting of silver‐based gas‐diffusion electrodes during oxygen reduction reaction with electrochemical and optical methods. Electrochemical Science Advances (2022) e2100158.[2] Hoffmann, H., Paulisch, M. C., Gebhard, M., Osiewacz, J., Kutter, M., Hilger, A., Arlt, T., Kardjilov, N., Ellendorff, B., Beckmann, F., Markötter, H., Luik, M., Turek, T., Manke, I., & Roth, C., Development of a Modular Operando Cell for X-ray Imaging of Strongly Absorbing Silver-Based Gas Diffusion Electrodes. Journal of The Electrochemical Society 169 (2022) 044508.[3] Löffelholz, M., Osiewacz, J., Lüken, A., Perrey, K., Bulan, A., & Turek, T., Modeling electrochemical CO2 reduction at silver gas diffusion electrodes using a TFFA approach. Chemical Engineering Journal 435 (2022) 134920.[4] Weng, L.-C., Bell, A. T., & Weber, A. Z., Modeling gas-diffusion electrodes for CO2 reduction. Physical Chemistry Chemical Physics 25 (2018) 16973–16984. Figure 1