Full Cell Modeling of Flooding Effects on Ionomer-Based CO2 Electrolyzers

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Electrochemical reduction of carbon dioxide (eCO2R) is a promising route to enable a circular economy for production of carbon-based materials using non-fossil-based feed stocks. Copper catalysts have been shown to be proficient at converting CO2 to ethylene as a precursor to polymer-based material synthesis. Production of ethylene at high faradaic efficiency and low voltage is necessary for commercial-scale viability of eCO2R as a technology, but the current state of the art does not meet these key performance indicators. We have synthesized and characterized quaternary ammonia-based polyphenylene oxide (QPPO) ionomers as alternatives to commercially available ionomers, and the QPPO ionomers showed increased faradaic efficiency towards ethylene while also decreasing full cell voltage. In this presentation, we demonstrate how continuum modeling was used to identify that underlying cause for achieving higher ethylene faradaic efficiency and lower cell voltage was the prevention of flooding of the gas diffusion layer in a zero-gap CO2 electrolyzer. We simulated two cell assemblies to elucidate the phenomena governing performance differences: one with a bare copper catalyst between the gas diffusion medium and membrane, and the other with an ionomer layer between the catalyst layer and the membrane. To validate our full cell model against experimental results for each cell assembly, we introduced a flooded region of the cathode diffusion media. By fitting the thickness of the flooded region, we were able to capture all trends for cell voltage and faradaic efficiency toward four products over three applied current densities. Voltage breakdown analysis showed that the ~150 mV lower cell voltage with the addition of the ionomer layer resulted from decreased charge and mass transfer overpotentials at the cathode due to the high aqueous CO2 concentration in the cathode catalyst layer and decrease in CO2 consumption to bicarbonate via the presence of liquid water in the gas diffusion layer. Overall, these results show the importance of incorporating liquid water into accurate modeling of CO2 electrolyzers and water management in experimental work to achieve commercial-scale electrolyzer performance. Figure 1