Tuning the selectivity of the CO2 reduction reaction by addition of zinc to a copper electrocatalyst

Abstract

In order to overcome both the problem of emission of greenhouse gasses and the problem of the depletion of our energy source, alternative energy sources have been investigated. Since CO2 is the most prominent greenhouse gas and an abundant carbon source, the electrochemical CO2 reduction reaction (eCO2RR), to hydrocarbons and alcohols (C2+ products) would be a step forwards in closing the carbon cycle. In this research, copper is used as an electrocatalyst, since it is the only metal that can make C2+ products in significant amounts. However, the selectivity is still a big problem. Zinc is a great candidate as a promoter to improve the selectivity of the copper catalyst, due to its selective CO production, the main intermediate in the CO2RR. The potential synergistic effects of a bimetallic copper zinc catalyst make this electrode an interesting candidate to tune the selectivity of the CO2RR. Two different combinations of copper with zinc were used in this research: galvanic replaced and oxide-derived electrodes. The galvanic replaced electrodes showed an optimum in partial current density of CO when 2.9 - 6.2 atomic% of copper was present. This optimum is most likely caused by a promoting and a deteriorating effect counteracting. An increase in electrochemical surface area is suggested as the promoting effect and the deteriorating effect is likely due to the addition of copper in the Zn/Cu epsilon phase. The addition of ZnO nanorods to Cu2O nanocubes in oxide-derived catalysts causes an increased activity compared to the monometallic electrodes. Unfortunately, reproducibility proves difficult and the stability of the catalysts remains a challenge, observing dissolution of zinc and dendrite formation of the copper particles during catalysis. However, the addition of zinc to the copper electrode resulted in a shift from ethylene to ethanol formation in the CO2RR. Thus, the product formation of the CO2RR can be tuned by changing the ratio of copper and zinc in an electrocatalyst.