Understanding the Effects of Cations on the Selectivity Modulation of the Conversion of Electrochemical Acetylene to Ethylene

Unraveling the not-yet-definitive mechanism of the extensively studied cation effects in the electrocatalytic hydrogenation (ECH) process would provide methods to suppress competitive side reactions and address selectivity issues, which have impeded the development of the electrosynthesis strategy. Here, taking electrochemical acetylene (C 2 H 2 ) to ethylene (C 2 H 4 ) hydrogenation (EAH) as a model case, the C 2 H 4 selectivity over commercial polycrystalline copper (p-Cu) exhibits a volcano-type trend with increasing cation radius (Li + < Na + ≈ K + > Cs + ). The observed cation-dependent selectivity modulation results are attributed to the opposite trend in the hydrogen evolution reaction (HER) (inhibited) and C 2 H 2 dimerization (boosted) with increasing cation size. Further mechanistic studies reveal that the increased electronic interaction, asymmetric charge distribution, and increased discontinuity of the hydrogen bond network with increasing cation size result in easier activation and adsorption of C 2 H 2 feedstocks, accounting for the suppressed HER and enhanced coupling, respectively. Notably, the mechanism mentioned is also appropriate for explaining the highest C 2+ selectivity of CO 2 electroreduction with Cs + -containing electrolytes.