Effects of mass transfer on the electrocatalytic {CO2} reduction on Cu

By Lim, C. F. C.; Harrington, D. A.; Marshall, A. T.
Published in Electrochimica Acta NULL 2017

Abstract

Abstract The effects of mass transfer on the electrocatalytic reduction of {CO2} on a polycrystalline Cu rotating cylinder electrode were investigated. When the rotation rate was increased, the current efficiency toward {CO2} reduction products decreased while that for the hydrogen evolution reaction increased. Furthermore, the product selectivity switched from {CH4} to {CO} as the rotation rate was increased. This observation is generally inconsistent with the widely reported dependence of the electrocatalytic {CO2} reduction on interfacial pH and {CO2} concentration. As increasing the rotation rate improves mass transfer of species to and from the electrode surface, the interfacial pH becomes closer to the bulk pH while the interfacial concentration of {CO2} at the electrode surface increases. However, increasing the rotation rate significantly decreased the {CO2} reduction activity for constant current electrochemical {CO2} reduction despite the increased availability of {CO2} at the electrode surface. As the changes in interfacial pH and {CO2} concentration with rotation rate cannot adequately explain the results, it is instead suggested that the enhanced mass transfer of dissolved {CO} away from the electrode surface at high rotation rates is the main reason behind the observed effects. We propose that this enhanced mass transfer of {CO} away from the electrode surface decreases the surface coverage of {COads} (due to the equilibrium between {COads} and dissolved {CO} at the electrode-electrolyte interface) and limits the further reduction of {COads} to hydrocarbons.

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