Electroreduction of CO₂ in a Non-aqueous Electrolyte-The Generic Role of Acetonitrile

Transition metal carbides, especially Mo₂C, are praised to be efficient electrocatalysts to reduce CO₂ to valuable hydrocarbons. However, on Mo₂C in an aqueous electrolyte, exclusively the competing hydrogen evolution reaction takes place, and this discrepancy to theory was traced back to the formation of a thin oxide layer at the electrode surface. Here, we study the CO₂ reduction activity at Mo₂C in a non-aqueous electrolyte to avoid such passivation and to determine products and the CO₂ reduction reaction pathway. We find a tendency of CO₂ to reduce to carbon monoxide. This process is inevitably coupled with the decomposition of acetonitrile to a 3-aminocrotonitrile anion. Furthermore, a unique behavior of the non-aqueous acetonitrile electrolyte is found, where the electrolyte, instead of the electrocatalyst, governs the catalytic selectivity of the CO₂ reduction. This is evidenced by in situ electrochemical infrared spectroscopy on different electrocatalysts as well as by density functional theory calculations.