Intensified Electrocatalytic CO₂ Conversion in Pressure-Tunable CO₂ -Expanded Electrolytes

Multimolar CO₂ concentrations are achieved in acetonitrile solutions containing supporting electrolyte at relatively mild CO₂ pressures (<5 MPa) and ambient temperature. Such CO₂ -rich, electrolyte-containing solutions are termed as CO₂ -eXpanded Electrolytes (CXEs) because significant volumetric expansion of the liquid phase accompanies CO₂ dissolution. Cathodic polarization of a model polycrystalline gold electrode-catalyst in CXE media enhances CO₂ to CO conversion rates by up to an order of magnitude compared with those attainable at near-ambient pressures, without loss of selectivity. The observed catalytic process intensification stems primarily from markedly increased CO₂ availability. However, a non-monotonic correlation between the dissolved CO₂ concentration and catalytic activity is observed, with an optimum occurring at approximately 5 m CO₂ concentration. At the highest applied CO₂ pressures, catalysis is significantly attenuated despite higher CO₂ concentrations and improved mass-transport characteristics, attributed in part to increased solution resistance. These results reveal that pressure-tunable CXE media can significantly intensify CO₂ reduction rates over known electrocatalysts by alleviating substrate starvation, with CO₂ pressure as a crucial variable for optimizing the efficiency of electrocatalytic CO₂ conversion.