Linking gas diffusion electrode composition to CO2 reduction in a flow cell

Gas diffusion electrodes (GDEs) mediate the transport of reagents, products, and electrons in electrochemical reactors designed to reduce CO2 into fuels or chemicals. While the ratio of ionomer to electrocatalyst in the precursor catalyst ink is typically assumed not to change after being deposited on the GDE, we show herein that this assumption is likely not valid. Moreover, we discovered that the faradaic efficiency for formate, which is considered to be inconsequential relative to CO when using Ag electrocatalysts, can be modulated by 20% by a mere 5 wt% change in GDE Nafion® content. We were able to resolve these small differences in GDE composition by developing an X-ray fluorescence (XRF) spectroscopic protocol that quantifies the sulfonate groups appended to the polytetrafluoroethylene (PTFE) backbone of Nafion®. Using this protocol, we were able to determine how to precisely control the relative amount of ionomer to electrocatalyst for each GDE. We also found that maintaining a uniform ionomer–catalyst composition across the entire GDE can likely be done more effectively with automated spray coating than with manual deposition methods. We recommend following these procedures in order to generate reproducible CO2RR performance parameters in flow cells.