Importance of Proton Concentration on the Product Selectivity of the CO2 Reduction Reaction

Abstract

The electrochemical CO2 reduction reaction (CO2RR) is seen as promising route for converting waste CO2 and excess renewable energy into valuable chemicals and synthetic fuels. During this process a variety of carbon-based products such as CO, HCOOH and hydrocarbons can be formed. The cathode material plays a crucial role on determining the product distribution during the CO2RR. For example, Cu has been found to be an interesting material on which CO2 is directly reduced to alcohols and hydrocarbons. (1) While, Ag based catalyst (2) and metal nitrogen doped materials (MNCs) are considered promising catalyst for the reduction of CO2 to CO.(3) In addition to the catalysts material, the pH and local pH are known to be key on determining the selectivity of the reaction. In the case of Cu catalysts, we have shown that the ratio between methane and ethylene prodiuction is strongly dependent on the local pH as ethylene formation is not affected by the proton concentration. As result, in conditions in which a high local pH is favored ethylene is the predominant product, while at low pH values CH4 is favored. (4) In the case of carbon base materials, however, the influence of the reaction conditions on the catalytic performance has not been addressed. Herein we looked into the role of the electrolyte on the selectivity of FeNC materials on CO2RR selectivity. (5) Unlike hydrogen or methane generation, CO production is independent on proton concentration on the NHE scale, suggesting a decoupled proton-electron transfer mechanism for CO formation. The similarity of the reaction mechanism with that of Metal-N macrocyclic complexes is an indication that the Fe centers incorporated to our carbon material indeed behave as the active centers of complex based catalysts. Furthermore, our results show that on doped carbon catalysts, such as FeNC, the selectivity towards the CO2RR can be conveniently tuned by the proton availability in both aqueous and non-aqueous electrolytes.