Electrolyte Dependence of CO₂ Electroreduction: Tetraalkylammonium Ions Are Not Electrocatalysts

Abstract

The ability of tetraalkylammonium ions (NR4+) to facilitate CO2 electroreduction at various electrode surfaces has been investigated. Scan rate dependence shows this process to be diffusion-controlled and largely independent of working electrode material. Variation of the R groups on NR4+ is shown to have little effect on the reduction potential, indicating that catalysis via reduced NR4• species, as previously proposed in the literature, is not a viable mechanism for CO2 electroreduction. Rather, CO2 is reduced via outer-sphere electron transfer according to the mechanism put forth by Savéant and co-workers [Lamy, E.; Nadjo, L.; Savéant, J.-M. J. Electroanal. Chem. 1977, 78, 403−407]. This view is supported by a full analysis of the reaction products, which consist exclusively of CO and CO32–, as well as solvent decomposition products formed at the counter electrode. No degradation of NR4+ ions is observed, which rules out the transient formation of NR4• radical species during electroreduction. However, Li+ ions are definitively shown to inhibit CO2 reduction, even in the presence of NR4+ ions. This inhibition likely occurs via surface adsorption of the Li+ ions.