Copper Sulfide as the Cation Exchange Template for Synthesis of Bimetallic Catalysts for CO2 Electroreduction
Published Web Location
https://chemrxiv.org/articles/preprint/Copper_Sulfide_as_the_Cation_Exchange_Template_for_Synthesis_of_Bimetallic_Catalysts_for_CO2_Electroreduction/14479836/1Abstract
Among metals used for CO2 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 CO2
reduction using Cu sulfides as a growth template. Beginning with Cu2−xS nanosheets (C?nano-0, 100 nm lateral dimension, 10 nm thick), varying the Ag+
concentration in the
exchange solution produces a gradual change in crystal structure from Cu7S4 to Ag2S, as
the Ag/Cu mass ratio varies from 0.1 to 10 (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 10s of microns. With lower Ag+
concentrations, cation
exchange produces uniformly dispersed Ag; however, at higher concentrations, Ag
particles nucleate on the surface. During CO2 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 H2 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 CO2 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 CO2 to multi-carbon products, although the optimal configurations appear to
be metastable.
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