In-Operando Changes of the Cation Exchange Membrane in CO2 electrolyzers

CO2 electrolysis provides a promising solution for reducing CO2 concentration levels in the atmosphere and for decarbonizing current CO2 sources. Through CO2 electrolysis, we can synthesize syngas, formic acid, and other useful hydrocarbon feedstocks [1] for carbon-neutral fuels, creating a cyclic consumption of hydrocarbons. A critical limitation of industrializing current CO2 electrolyzer designs is understanding the mass and ionic transport effects of the reactants and products across the membrane-electrode assembly [1,2]. These effects are directly dependent on the operating conditions and reduce cell performance [1]. Membrane hydration is an important parameter that significantly affects cell performance and optimum performance can be achieved by supplying humidified CO2 gas supply at the cathode, as evident in fuel cell technologies [3]. In this study, we investigated the impact of humidified CO2 gas supply on cell performance and correlated it with mass transport effects causing changes in the cation exchange membrane (CEM). We operated a CEM CO2 electrolyzer with varied cathode gas inlet relative humidity (RH) to study the performance change at various current densities while simultaneously using synchrotron X-ray radiography to quantify membrane hydration changes and water dynamics. These results were coupled with electrochemical impedance spectroscopy (EIS) data, which gave us the high frequency resistance (HFR) highlighting the ohmic losses present in the membrane at various RH conditions. Similar work on other electrochemical devices (fuel cells [4] and water electrolyzers [5]) has shown the relationship between cell performance and changes in membrane hydration and transport resistance. Significant improvement in cell performance was observed as the cathode inlet RH increased. This improvement is a direct result of increased membrane hydration. Furthermore, EIS data exhibited a significant decrease in HFR as RH increased, indicative of lower ionic transport resistance across the membrane. As the electrolyzer is operated at various current densities, the through plane visualizations of the CEM provide insight into the relative changes in the membrane. Mass transport phenomena have a significant effect on cell performance at elevated current densities, which is evident by in-operando membrane hydration changes and membrane ohmic losses. From our observations, the humidification of CO2 gas supply has a significant effect on cell performance. We believe this knowledge can be translated into substantially improving the performance of CO2 electrolyzers for industrial-scale implementation. References: [1]A. Martín, G. Larrazábal and J. Pérez-Ramírez, "Towards sustainable fuels and chemicals through the electrochemical reduction of CO2: lessons from water electrolysis", Green Chemistry, vol. 17, no. 12, pp. 5114-5130, 2015. [2]S. Narayanan, B. Haines, J. Soler and T. Valdez, "Electrochemical Conversion of Carbon Dioxide to Formate in Alkaline Polymer Electrolyte Membrane Cells", Journal of The Electrochemical Society, vol. 158, no. 2, p. A167, 2011. Available: 10.1149/1.3526312. [3]M. Saleh, T. Okajima, M. Hayase, F. Kitamura and T. Ohsaka, "Exploring the effects of symmetrical and asymmetrical relative humidity on the performance of H2/air PEM fuel cell at different temperatures", Journal of Power Sources, vol. 164, no. 2, pp. 503-509, 2007. Available: 10.1016/j.jpowsour.2006.11.065. [4]R. Banerjee et al., "Identifying in operando changes in membrane hydration in polymer electrolyte membrane fuel cells using synchrotron X-ray radiography", International Journal of Hydrogen Energy, vol. 43, no. 20, pp. 9757-9769, 2018. Available: 10.1016/j.ijhydene.2018.03.224. [5]C. Lee et al., "The effect of cathode nitrogen purging on cell performance and in operando neutron imaging of a polymer electrolyte membrane electrolyzer", Electrochimica Acta, vol. 279, pp. 91-98, 2018. Available: 10.1016/j.electacta.2018.05.066.