The Effect of the Tetraalkylammonium Cation in the\nElectrochemical CO<sub>2</sub> Reduction Reaction on Copper Electrode

Aprotic organic solvents\nsuch as acetonitrile offer a potential\nsolution to promote electrochemical CO<sub>2</sub> reduction over\nthe competing hydrogen evolution reaction. Tetraalkylammonium cations\n(TAA<sup>+</sup>) are widely used as supporting electrolytes in organic\nmedia due to their high solubility and conductivity. The alkyl chain\nlength of TAA<sup>+</sup> cations is known to influence electron transfer\nprocesses in electrochemical systems by the adsorption of TAA<sup>+</sup>, causing modifications of the double layer. In this work,\nwe elucidate the influence of the cation chain length on the mechanism\nand selectivity of the CO<sub>2</sub>RR reaction under controlled\ndry and wet acetonitrile conditions on copper cathodes. We find that\nthe hydrophobic hydration character of the cation, which can be tuned\nby the chain length, has an effect on product distribution, altering\nthe reaction pathway. Under dry conditions, smaller cations (TEA<sup>+</sup>) preferentially promote oxalate production via dimerization\nof the CO<sub>2</sub><sup>·–</sup> intermediate, whereas\nformate is favored in the presence of water via protonation reaction.\nLarger cations (TBA<sup>+</sup> > TPA<sup>+</sup> > TEA<sup>+</sup>) favor the generation of CO regardless of water content.\nIn situ\nFTIR analysis showed that TBA<sup>+</sup> cations are able to stabilize\nadsorbed CO more effectively than TEA<sup>+</sup>, explaining why\nlarger cations generate a higher proportion of CO. Our findings also\nsuggest that higher cation concentrations suppress hydrogen evolution,\nparticularly with larger cations, highlighting the role of cation\nchain length size and hydrophobic hydration shell.