Ag modified Cu hollow fiber for gas-phase CO<sub>2</sub> electrocatalytic conversion to oxygenates

The conversion of CO₂ to oxygenates driven by renewable electricity is of great importance, since not only the carbon emission would be reduced by utilizing CO₂ as feedstock, but also the renewable energy could be stored as high value-added chemicals. The development of gas-phase electrocatalytic system is significant for the practical application of CO₂ electroreduction, which could avoid the obstacles of liquid-phase CO₂ electroreduction system, such as the insufficient CO₂ solubility and separation dilemma of soluble products. In this work, Ag modified Cu hollow fibers via galvanic replacement are utilized as the electrode for gas-phase CO₂ electroreduction to produce multi-carbon oxygenates. Hollow fiber with its large surface area, adjustable pore structure, good chemical stability and conductivity shows unique advantage in electrocatalysis application. The morphology, surface and bulk structure characterizations of Ag-Cu hollow fiber electrodes indicate that the modification via galvanic replacement forms Ag nanodendrites in different scales on the surface of Cu hollow fiber electrodes. The relationship between the surface structures and electrocatalytic performances of the as-prepared Ag-Cu electrodes is also unveiled. Compared with Cu hollow fiber electrode, not only the current densities but also the faradaic efficiencies of C₃ products (propanal and acetone) for Ag-Cu hollow fiber electrodes are greatly improved. 0.1wt.% Ag-Cu hollow fiber electrode shows an elevated current density of −0.19, −0.35 and −0.72 mA cm⁻² at the potential of −1.4, −1.6 and −1.8 V vs. counter electrode respectively, compared with those of −0.11, −0.16 and −0.33 mA cm⁻² for Cu hollow fiber. And 0.1wt.% Ag-Cu hollow fiber electrode also gives promoted faradaic efficiencies of not only C₂ product ethanol (with a yield of 56 nmol cm⁻² h⁻¹), but also C₃ product acetone. Although the further increase of Ag loading narrows the current density difference comparing with Cu hollow fiber, the faradaic efficiencies and yields of C₃ products propanal and acetone are greatly improved for 0.3wt.% and 1wt.% Ag-Cu hollow fiber electrodes. 1wt.% Ag-Cu hollow fiber electrode provides faradaic efficiencies of 19% for propanal and 7% for acetone with a current density of 0.13 mA cm⁻² at the potential of −1.4 V vs. counter electrode. And its propanal yield reaches 57 nmol cm⁻² h⁻¹, even higher than the ethanol yield of other electrocatalysts in reported gas-phase CO₂ electroreduction works, indicating the better activity of Ag-Cu hollow fiber electrode on multi-carbon product formation. The modification of Ag dendrites on hollow fiber surface not only promotes the activity of gas-phase CO₂ electroreduction, but also facilitates the formation of multi-carbon oxygenates. The fractal dendrite structure of Ag forms abundant conductive networks on the surface of Cu hollow fiber electrode, which facilitates the electron transport and mass transfer processes, leading to superior kinetics and elevated current densities for gas-phase CO₂ electroreduction. Also, the high curvature surface of Ag dendrites induces high local electric field and high local concentration of CO₂ as well as active intermediates, which would be beneficial for the C-C coupling and the formation of multi-carbon products. The present study provides an inspiration and scientific instruction for designing and constructing gas-phase CO₂ electrocatalytic systems to produce multi-carbon oxygenates with high activity and high selectivity.