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Influence of Surface Composition of AgSn Films on the Selectivity and Electrokinetics of CO2 Reduction in the Presence of Protic Organic [DBU–H]+ Cations

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posted on 2021-12-01, 17:40 authored by Thabiso Kunene, Abderrahman Atifi, Joel Rosenthal
Electrodeposited composite film electrodes prepared from electroplating baths with varying ratios of Ag+ and Sn2+ 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 (Ag0) state, while Sn was mainly oxidized (Sn2+/4+). The AgSn composite films studied herein are therefore best considered as AgSnOx cathodes with varying ratios of Ag0/Sn2+/4+. These systems were assessed as CO2 reduction reaction (CO2RR) 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 HCO2H, a significant increase in CO vs HCO2H selectivity (up to 99%) was achieved for composite film electrodes in which the Ag content ranged from 25 to 75%. Tuning the ratio of Ag0 to SnOx delivered composite films that support quantitative current efficiencies for generation of CO with geometric current densities approaching 30 mA/cm2 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 CO2RR 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 SnOx give rise to rough films that support enhanced CO2RR kinetics and that mixing of Ag with SnOx enhances the efficacy of adsorption and stabilization of reduced CO2 intermediates and [DBU–H]+ cations to facilitate CO evolution at the cathode/electrolyte interface.

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