Optimizing the Ink Formulation for Preparation of Cu-Based Gas Diffusion Electrodes Yielding Ethylene in Electroreduction of CO₂

Cu-based gas diffusion electrodes (GDE) show excellent performance in the electrochemical reduction of CO₂ to ethylene. In the present work, we evaluate how the solvent of ink formulations containing Nafion-ionomer and unsupported Cu particles affects the polymerized Nafion-copper distribution in the as-prepared GDE and the obtained performance in the electrochemical reduction of CO₂. Isopropanol (IPA), dimethyl sulfoxide (DMSO), ethylene glycol (EG), or N-methyl- ₂-pyrrolidone (NMP)) were used. Microscopic analyses of the Cu-GDEs demonstrate that NMP and DMSO lead to exposed islands of copper, with Nafion acting predominantly as an interparticle binder. Such geometry is confirmed by the relatively high electrochemical surface area (ECSA) and the low charge-transfer resistance (R_ct). IPA or EG induce the formation of Cu-catalyst particles embedded into and covered by (polymerized) Nafion films, in agreement with the relatively low ECSA, and high R_ct, likely due to significant polymerization and agglomeration of Nafion in the ink formulation induced by the protic solvents prior to preparation of the catalyst layer. When evaluated in the electrochemical reduction of CO₂ at -1.1 V vs the reversible hydrogen electrode (RHE), the exposed particles prepared using NMP and DMSO lead to higher FE toward ethylene than EG or IPA-based GDEs (23.5% and 19.2% vs 10.2% and 13.4%, respectively). The lower ethylene FE for Nafion covered systems is tentatively attributed to the acidity and highly effective transport of protons by Nafion.