How the Choice of Electrolyte Affects the Carbon Efficiency in Electrochemical CO2 Reduction at Silver Gas Diffusion Electrodes

Abstract

Reducing CO2 emissions by replacing fossil fuels with renewable energy sources is an important step in the context of climate change. Electrochemical CO2 reduction is a promising technology for supporting this shift, since it provides the possibility to store electrical energy in form of high value chemicals while consuming CO2. It is now generally accepted that the use of gas diffusion electrodes (GDEs) is beneficial due to the limited solubility of CO2 in water [1].To achieve a better understanding of the complex processes taking place in the electrode, the description by a mathematical model is desirable. We present a TFFA (thin-film flooded agglomerate) model for silver GDEs based on the approach presented by Franzen et al. [2] for the oxygen reduction reaction. The model describes the processes in the GDE, including reaction, diffusion, and migration. Therefore, the model provides an in-depth view on the local reaction environment in the electrode, which becomes highly alkaline at elevated current densities due to the formation of hydroxide ions by the electrochemical reactions [3]. This causes the rate of bicarbonate/carbonate formation to exceed the rate of electrochemical CO2 reduction, resulting in a limited carbon efficiency [4]. Using acidic electrolytes might significantly reduce this limitation by decreasing the local alkalinity. This has already been experimentally demonstrated for gold electrodes using electrolytes with pH values in the range of 2-4 [5]. In this study, we present experimental results for silver GDEs using acidic electrolytes. These results show that the carbon efficiency can be increased while maintaining a similar Faradaic efficiency as obtained for KHCO3 electrolytes at industrially relevant current densities. Our model calculations support these findings and give insights about the development of the local pH in dependence of the applied current density for different pH values of the bulk electrolyte.[1] T. Burdyny, W. A. Smith, CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions, Energy & Environmental Science 12 (5) (2019), 1442-1453.[2] D. Franzen, M. C. Paulisch, B. Ellendorff, I. Manke, T. Turek, Spatially resolved model of oxygen reduction reaction in silver-based porous gas-diffusion electrodes based on operando measurements, Electrochimica Acta 375 (2021), 137976.[3] M. Löffelholz, J. Osiewacz, A. Lüken, K. Perrey, A. Bulan, T. Turek, Modeling electrochemical CO2 reduction at silver gas diffusion electrodes using a TFFA approach. Chemical Engineering Journal 435(2) (2022), 134920.[4] J. A. Rabinowitz, M.W. Kanan, The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem. Nature Communications 11 (2020), 5231.[5] M.C.O. Monteiro, M.F. Philips, K.J.P. Schouten et al., Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media. Nature Communication 12 (2021), 4943. Figure 1