Abstract: The storage of electrical energy from renewable sources as chemical energy in the products of CO₂ reduction is an attractive strategy for renewable energy storage. However, limited understanding of heterogeneous CO₂ reduction electrocatalysts has allowed for the design of efficient systems for conversion of CO₂ into commodity chemicals such as CO and HCOOH. Toward improving our understanding the interaction between the electrode and electrolyte, we have proposed a relatively straightforward approach whereby we use electrodeposited multimetallic oxides with varying ratios of silver and tin to investigate how the electrode composition will affect the electrocatalytic performance of such composite materials.

We have developed a class of electrodeposited silver-tin oxide (AgSnO_x) composite thin films that promote the electrocatalytic reduction of CO₂ to CO and HCOO — with fast kinetics and high efficiencies in the presence of a protic ionic liquid. We demonstrate that via tuning the concentration of silver, a significant increase in CO vs. HCOOH selectivity (up to 99%) is achieved for silver content from 25 - 75%. Increased AgO_x content also promotes an anodic shift in the onset potential for CO₂ compared with the monometallic films hence increased total current geometric current density up to 30 mA/cm² at less than 800 mV overpotentials. Furthermore, using ex situ XPS analysis, we show that the presence of silver promotes the growth and stability of Sn(IV) surface species, which are well known to promote both low onset potentials and high selectivity for CO₂ reduction over the often parasitic HER.

We also use electrochemical impedance spectroscopy to probe the dynamics between the metal-oxide, IL and CO₂ at the electrode-electrolyte interface and provide insight into the pathway(s) by which the oxide species drive CO₂ reduction. Overall, we propose that the parallel galvanic reaction between Sn(II) and Ag(I) salts during electrodeposition of the composite films promotes the growth of highly porous films compared with the monometallic counterparts.