Copper sulfide as the cation exchange template for synthesis of bimetallic catalysts for CO 2 electroreduction.

Among metals used for CO 2 electroreduction in water, Cu appears to be unique in its ability to produce C2+ products like ethylene. Bimetallic combinations of Cu with other metals have been investigated with the goal of steering selectivity via creating a tandem pathway through the CO intermediate or by changing the surface electronic structure. Here, we demonstrate a facile cation exchange method to synthesize Ag/Cu electrocatalysts for CO 2 reduction using Cu sulfides as a growth template. Beginning with Cu 2- x S nanosheets (C-nano-0, 100 nm lateral dimension, 14 nm thick), varying the Ag + concentration in the exchange solution produces a gradual change in crystal structure from Cu 7 S 4 to Ag 2 S, as the Ag/Cu mass ratio varies from 0.3 to 25 (CA-nano- x , x indicating increasing Ag fraction). After cation exchange, the nanosheet morphology remains but with increased shape distortion as the Ag fraction is increased. Interestingly, the control (C-nano-0) and cation exchanged nanosheets have very high faradaic efficiency for producing formate at low overpotential (-0.2 V vs. RHE). The primary effect of Ag incorporation is increased production of C2+ products at -1.0 V vs. RHE compared with C-nano-0, which primarily produces formate. Cation exchange can also be used to modify the surface of Cu foils. A two-step electro-oxidation/sulfurization process was used to form Cu sulfides on Cu foil (C-foil- x ) to a depth of a few 10 s of microns. With lower Ag + concentrations, cation exchange produces uniformly dispersed Ag; however, at higher concentrations, Ag particles nucleate on the surface. During CO 2 electroreduction testing, the product distribution for Ag/Cu sulfides on Cu foil (CA-foil- x-y ) changes in time with an initial increase in ethylene and methane production followed by a decrease as more H 2 is produced. The catalysts undergo a morphology evolution towards a nest-like structure which could be responsible for the change in selectivity. For cation-exchanged nanosheets (CA-nano- x ), pre-reduction at negative potentials increases the CO 2 reduction selectivity compared to tests of as-synthesized material, although this led to the aggregation of nanosheets into filaments. Both types of bimetallic catalysts are capable of selective reduction of CO 2 to multi-carbon products, although the optimal configurations appear to be metastable.