Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution

Abstract

Electroreduction of CO₂ at Cu in aqueous inorganic electrolytes was studied by means of voltammetric, coulometric and chronopotentiometric measurements. CO, CH₄, C₂H₄, EtOH and PrⁿOH are produced at ambient temperatures. Formation of CO predominates at less negative potentials (more positive than –1.2 V vs. NHE); hydrocarbons and alcohols are favourably produced below –1.3 V vs. NHE, where the Faradaic efficiency of CO drops. CO, formed as an intermediate from CO₂, is adsorbed on the Cu electrode, interfering with cathodic hydrogen formation. The adsorption strength of CO on Cu is very weak as compared with that on Pt. Adsorbed CO is reduced to Hydrocarbons and alcohols at more negative potentials. The product distribution from CO₂ depends strongly upon the electrolytes employed. Formation of C₂H₄ and alcohols is favoured in KCl, K₂SO₄, KClO₄ and dilute HCO^–₃ solutions, whereas CH₄ is preferentially produced in relatively concentrated HCO^–₃ and phosphate solutions. The product selectivity depends upon availability of hydrogen or protons on the surface, which is controlled by pH at the electrode. The pH at the electrode is greatly affected by the electrolyte, since OH^– is released in the electrode reactions. The production of hydrocarbons and alcohols is discussed in comparison with the mechanism of the Fishcher–Tropsch reaction.