Grain Boundary Engineering of Cu–Ag Thin-Film Catalysts for Selective (Photo)Electrochemical CO₂ Reduction to CO and CH₄

We investigated the relationship between grain boundary (GB) oxidation of Cu–Ag thin-film catalysts and selectivity of the (photo)electrochemical CO2 reduction reaction (CO2 RR). The change in the thickness of the Cu thin film accompanies the variation of GB density, and the Ag layer (3 nm) has an island-like morphology on the Cu thin film. Therefore, oxygen from ambient air penetrates into the Cu thin film through the GB of Cu and binds with it because the uncoordinated Cu atoms at the GBs are unstable. It was found that the Cu thin film with a small grain size was susceptible to spontaneous oxidation and degraded the faradaic efficiency (FE) of CO and CH4. However, a relatively thick (≥80 nm) Cu layer was effective in preventing the GB oxidation and realized catalytic properties similar to those of bulk Cu–Ag catalysts. The optimized Cu (100 nm)–Ag (3 nm) thin film exhibited a unique bifunctional characteristic, which enables selective production of both CO (FECO = 79.8%) and CH4 (FECH4 = 59.3%) at a reductive potential of −1.0 and −1.4 VRHE, respectively. Moreover, the Cu–Ag thin film was used as a cocatalyst for photo-electrochemical CO2 reduction by patterning the Cu–Ag thin film and a SiO2 passivation layer on a p-type Si photocathode. This novel architecture improved the selectivity of CO and CH4 under light illumination (100 mW/cm2).