Influence of Surface Composition of AgSn Films on the Selectivity and Electrokinetics of CO 2 Reduction in the Presence of Protic Organic [DBU–H] + Cations

Electrodeposited composite film electrodes prepared from electroplating baths with varying ratios of Ag + and Sn 2+ triflates were studied to understand how the performance of such materials varies as a function of composition. X-ray photoelectron spectroscopy (XPS) analysis confirms that for each composite, Ag existed in the metallic (Ag 0 ) state, while Sn was mainly oxidized (Sn 2+/4+ ). The AgSn composite films studied herein are therefore best considered as AgSnO x cathodes with varying ratios of Ag 0 /Sn 2+/4+ . These systems were assessed as CO 2 reduction reaction (CO 2 RR) electrocatalysts and were found to promote the 2e – /2H + reductions to deliver CO and HCOOH with fast kinetics and high efficiencies from electrolyte solutions containing the protic organic cation [DBU–H] + (i.e., protonated 1,8-diazabicyclo[5.4.0]­undec-7-ene). While Sn-rich composite films showed poor selectivities for CO vs HCO 2 H, a significant increase in CO vs HCO 2 H selectivity (up to 99%) was achieved for composite film electrodes in which the Ag content ranged from 25 to 75%. Tuning the ratio of Ag 0 to SnO x delivered composite films that support quantitative current efficiencies for generation of CO with geometric current densities approaching 30 mA/cm 2 at applied overpotentials that are less than 750 mV were realized. Additionally, electrochemical impedance spectroscopy (EIS) coupled with analysis of the distribution of relaxation times (DRT) was used to better understand factors important to the composites’ activity under CO 2 RR conditions. Probing the dynamics with DRT analysis revealed that multiple processes relating to both adsorption and diffusion-controlled events are important to the activity of the electrocatalysts considered in this work. The collection of electroanalytical investigations suggest that synergistic interactions between Ag and SnO x give rise to rough films that support enhanced CO 2 RR kinetics and that mixing of Ag with SnO x enhances the efficacy of adsorption and stabilization of reduced CO ...