Experimental studies and microkinetic analysis of electrochemical CO₂ reduction to CO on silver electrode

The electrochemical reduction of CO₂ on silver in aqueous electrolytes under ambient conditions yields CO and is accompanied by the production of H₂ from water reduction. In this work, a detailed multistep kinetics of the electrochemical conversion of CO₂ to CO on a planar silver catalyst is examined using linear sweep voltammetry and chronoamperometry. The experiments were conducted in 0.1 M potassium bicarbonate solutions saturated with either CO₂ or N₂ in a wide potential window (from 0 V to -1.99 V vs. RHE). The production rate of CO reached a maximum at approximately -1.6 V vs. RHE. A four-step mechanism involving three intermediate species, viz. \({}^{*}C{O}_{2}^{-}\) $_{}^{\ast }C{O}_2^{-}$ , \({}^{*}COOH\) $_{}^{\ast }COOH$ and \({}^{*}CO\) $_{}^{\ast }CO$ , was proposed to describe the CO₂ reduction to CO. The hydrogen evolution reaction is shown to occur via Volmer–Heyrovsky mechanism involving \({}^{*}H\) $_{}^{\ast }H$ species. The model predicts that, in CO₂ bubbled electrolytes, the surface coverage values of \({}^{*}C{O}_{2}^{-}\) $_{}^{\ast }C{O}_2^{-}$ and \({}^{*}CO\) $_{}^{\ast }CO$ species gradually increase with cathodic overpotential until -1.62 V vs. RHE, and decrease at higher overpotentials. On the other hand, the fractional coverage of \({}^{*}COOH\) $_{}^{\ast }COOH$ exhibits a maximum at -1.06 V vs. RHE. The water reduction mechanism reveals that, in both CO₂ and N₂-bubbled electrolytes, \({}^{*}H\) $_{}^{\ast }H$ coverage is very low at low overpotentials, increases continuously at medium overpotentials, and almost saturates at unity at high cathodic overpotentials, explaining the low Faradaic efficiency of CO production at high cathodic overpotentials.