Activation of bimetallic AgCu foam electrocatalysts for ethanol formation from CO2 by selective Cu oxidation/reduction

Bimetallic AgCu metal foams (15 at% Ag, 85 at% Cu) have been synthesized by means of an additive-assisted electrodeposition process using the dynamic hydrogen bubble template approach. Ag and Cu remain fully phase-segregated in the as deposited bimetallic foam exhibiting a high degree of dispersion of pure nm-sized Ag domains embedded in the Cu matrix. An activation of this bimetallic material towards ethanol formation is achieved by thermal annealing of the as deposited foam under mild conditions (200 degrees C for 12 h). Such annealing quantitatively transforms the Cu in the bimetallic system into a mixture of crystalline Cu2O and amorphous CuO whereas the Ag remains in its metallic state due to the thermal instability of Ag2O above temperatures of 180 degrees C. The selective oxidation of Cu in the bimetallic Ag15Cu85 catalyst goes along with an enrichment of Cu oxides on the surface of the formed mixed AgCuxO foam. Both operando X-ray diffraction and operando Raman spectroscopy demonstrate, however, that the oxide reduction is completed before the electrochemical CO2 reduction sets in. The thus formed oxide-derived (OD) bimetallic Ag15Cu85 foam catalyst shows high selectivity towards alcohol formation with Faradaic efficiencies of FEEtOH = 33.7% and FEn-PrOH = 6.9% at -1.0 V and -0.9 V vs RHE, respectively. Extended electrolysis experiments (100 h) indicate a superior degradation resistance of the oxide-derived bimetallic catalyst which is ascribed to the effective suppression of the C1 hydrocarbon reaction pathway thus avoiding irreversible carbon contaminations appearing in particular during methane production.