Tracking Gas Diffusion Electrode Flooding in CO2 Electrolyzers Via Electrochemical Double Layer Capacitance

Abstract

As electrochemical technologies such as batteries, fuel cells, and water electrolyzers advance and transform the electric and transportation sectors, there is increasing interest around the role of electrochemistry in sustainable chemical manufacturing. As an example, blending electrochemically generated carbon monoxide (CO) and hydrogen, derived from carbon dioxide (CO2) and water electrolyses respectively, could constitute a renewable scheme to produce syngas for Fischer-Tropsch gas-to-liquids processes1. Decades of fundamental research into electrochemical CO2 reduction (CO2R) coupled with emerging engineering and economic incentives have shifted the field’s focus towards high-performance, gas-fed electrolyzers. Catalyst-coated gas diffusion electrodes facilitate such reactor configurations by providing physical separation between the gaseous reactants and the liquid electrolyte. While high geometric-area-specific electrochemical activity has been demonstrated with commercial gas diffusion electrode materials for a variety of both CO- and hydrocarbon-selective metal catalysts2,3, longevity remains a challenge and performance decay is often attributed to electrode deficiencies. To date, most gas diffusion layers reported in the CO2R literature have been repurposed from fuel cell applications, in which the transport of water to and from the catalyst layer is crucial to device operation. To this end, in fuel cells, densely-packed microporous layers serve both as catalyst-layer substrates and effective media for water management. However, the efficacy of this layer as a barrier to liquid electrolyte flooding in CO2R is limited by increasing hydrophilicity upon exposure to reducing potentials and high local pH. New operando diagnostic techniques are needed to probe the stability of the gas-liquid interface in gas-fed CO2 electrolyzers with flowing liquid electrolytes4. In this presentation, we propose a new experimental approach for determining the relationship between cell operating conditions and the eventual degradation of CO2-to-CO faradaic efficiency. Specifically, we propose combining periodic in-situ electrochemical-double-layer-capacitance-based electrolyte wetting predictors with in-line gas chromatography characterization of CO2R products. Voltammetric- or impedance-based methods are often used to estimate electrochemically active surface area of porous carbons in supercapacitor applications, but have yet to be used to probe electrolyte wetting in CO2R gas diffusion electrodes5. To demonstrate this technique, we evaluate the flooding tolerance of silver-coated gas diffusion electrodes under a range of operating conditions and using a number of commercial gas diffusion layers. Additionally, we discuss the impact of electrolyte properties (e.g., composition, pH), electrode properties (e.g., PTFE-content, macroporous layer, cracking), and operating conditions (e.g., pressure differential, current density, temperature) on flooding phenomena to identify key descriptors that can inform the design of more resilient electrode configurations. Funding Acknowledgement We gratefully acknowledge funding support from the US Department of Energy SBIR Program Grant # DE-SC0015173 and Royal Dutch Shell plc through the MIT Energy Initiative. References (1) Bushuyev, O. S.; De Luna, P.; Dinh, C. T.; Tao, L.; Saur, G.; van de Lagemaat, J.; Kelley, S. O.; Sargent, E. H. What Should We Make with CO2 and How Can We Make It? Joule 2018. https://doi.org/10.1016/j.joule.2017.09.003. (2) Ma, S.; Sadakiyo, M.; Luo, R.; Heima, M.; Yamauchi, M.; Kenis, P. J. A. One-Step Electrosynthesis of Ethylene and Ethanol from CO2 in an Alkaline Electrolyzer. J. Power Sources 2016, 301, 219–228. https://doi.org/10.1016/j.jpowsour.2015.09.124. (3) Verma, S.; Lu, X.; Ma, S.; Masel, R. I.; Kenis, P. J. A. The Effect of Electrolyte Composition on the Electroreduction of CO2 to CO on Ag Based Gas Diffusion Electrodes. Phys. Chem. Chem. Phys. 2016, 18 (10), 7075–7084. https://doi.org/10.1039/C5CP05665A. (4) Higgins, D. C.; Hahn, C.; Xiang, C.; Jaramillo, T. F.; Weber, A. Z. Gas-Diffusion Electrodes for Carbon-Dioxide Reduction: A New Paradigm. ACS Energy Lett. 2018. https://doi.org/10.1021/acsenergylett.8b02035. (5) Verbrugge, M. W.; Liu, P. Analytic Solutions and Experimental Data for Cyclic Voltammetry and Constant-Power Operation of Capacitors Consistent with HEV Applications. J. Electrochem. Soc. 2006, 153 (6), A1237–A1245. https://doi.org/10.1149/1.2194610.