Potassium ion modulation of the Cu electrode-electrolyte interface with ionomers enhances CO2 reduction to C2+ products

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31. Residual styrene starting materials were removed from the ionomers by precipitation of polymer from a concentrated MeCN solution with Et2O, washing with hot 1 M KHCO3(aq), and followed by reprecipitation with MeCN and Et2O. Due to the difference in solubility, a modified purification procedure was employed for the methoxystyrene homopolymer, with residual starting material removed by precipitation of a concentrated THF solution into MeOH.

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36. While ICP-MS measurements might not exactly match K+ content as measured by integration in 1H NMR of the material, ionomer 4 provides 57% and 79% greater local [K+] than 2 in the absence of applied potential and after electrolysis, respectively. Exact measurements of local [K+] are limited by the ability of K+ to exchange with Li+ in the mother liquor (see Supporting Information, Electrochemical Measurments in GDE. (Flow) Cell).

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54. We acknowledge the complicated relationship between gas solubility and diffusivity as models have previously highlighted an inverse relationship when comparing ionomers (Kim et al.16) while others have experimentally observed direct correlations (Sato et al.53) Other models have invoked both improved mass transport and greater local CO2 concentration as phenomena to explain greater FEC2+ and |jC2+| with Cu electrodes (Wang et al.21). In our system, minimization of Helmholtz free energy is attributed to increased CO2 entropy (SCO2, Table S9) which is a result of increased diffusion, as well as decreased internal energy due to enhanced K+-related Coulomb stabilization.

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