Beyond Copper in CO₂ Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam Catalysts

Abstract

Ag-foam catalysts have been developed for the electrochemical CO2 reduction reaction (ec-CO2RR) based on a concerted additive- and template-assisted metal-deposition process. In aqueous media (CO2-saturated 0.5 M KHCO3 electrolyte), these Ag foams show high activity and selectivity toward CO production at low and moderate over-potentials. Faradaic efficiencies for CO (FECO) never fell below 90% within an extremely broad potential window of ∼900 mV, starting at −0.3 V and reaching up to −1.2 V versus a reversible hydrogen electrode (RHE). An increased adsorption energy of CO on the Ag foam is discussed as the origin of the efficient suppression of the competing hydrogen-evolution reaction (HER) in this potential range. At potentials of <−1.1 V versus RHE, the FEH2 values significantly increase at the expense of FECO. Superimposed on this anti-correlated change in the CO and H2 efficiencies is the rise in the CH4 efficiency to the maximum of FECH4 = 51% at −1.5 V versus RHE. As a minor byproduct, even C–C-coupled ethylene could be detected reaching a maximum Faradaic efficiency of FEC2H4 = 8.6% at −1.5 V versus RHE. Extended ec-CO2RR reveals the extremely high long-term stability of the Ag foam catalysts, with CO efficiencies never falling below 90% for more than 70 h of electrolysis at −0.8 V versus RHE (potential regime of predominant CO production). However, a more-rapid degradation is observed for extended ec-CO2RR at −1.5 V versus RHE (potential regime of predominant CH4 production), in which the FECH4 values drop to 32% within 5 h of electrolysis. The degradation behavior of the Ag-foam catalyst is correlated to time-resolved identical-location scanning electron microscopy investigations that show severe morphological changes, particularly at higher applied over-potentials (current densities) at −1.5 V versus RHE. This study reports on the first ec-CO2RR catalyst beyond copper that demonstrates a remarkably high selectivity toward hydrocarbon formation, reaching a maximum of ∼60% at −1.5 V versus RHE. The experimental observations presented herein strongly suggest that this newly designed Ag-foam catalyst shares, in part, mechanistic features with common Cu catalysts in terms of ec-CO2RR product selectivity and catalyst degradation behavior.