Morphology Matters: Tuning the Product Distribution of CO2 Electroreduction on Oxide-Derived Cu Foam Catalysts

Mesoporous Cu foams formed by a template-assisted electrodeposition process have been identified as CO2 electrocatalysts that are highly selective toward C2 product formation (C2H4 and C2H6) with C2 efficiencies (FEC2) reaching 55%. The partial current of C2 product formation was found to be higher than that of the (parasitic) hydrogen evolution reaction (HER) at any potential studied (−0.4 to −1.0 vs the reversible hydrogen electrode). Moreover, formate production could largely be suppressed at any applied potential down to efficiencies (FEformate) of ≤6%. A key point of the Cu foam catalyst activation is the in operando reduction of a Cu2O phase, thereby creating a large abundance of surface sites active for C–C coupling. The cuprous oxide phase has been formed after the Cu electrodeposition step by exposing the large-surface area catalyst to air at room temperature. The superior selectivity of the Cu foam catalyst studied herein originates from a combination of two effects, the availability of specific surface sites for C–C coupling [dominant (100) surface texture] and the temporal trapping of gaseous intermediates (in particular CO and C2H4) inside the mesoporous catalyst material during CO2 electrolysis. A systematic CO2 electrolysis study reveals a strong dependence of the C2 efficiencies on the particular surface pore size of the mesoporous Cu catalysts with a maximal FEC2 between 50 and 100 μm pore diameters.