Tandem Electrochemical Conversion of CO2 to Liquid Fuels and Chemical Feedstocks

Abstract

Increasing demand for energy has led to a high dependence on fossil fuels, which are limited and have been closely associated with major environmental issues such as groundwater pollution and imbalances in the carbon cycle. A more sustainable and cleaner method of producing chemicals and fuels is the electrochemical CO2 reduction reaction (CO2RR). When coupled with renewable electricity, CO2 can be converted to high energy density fuels and commodity chemicals, such as ethanol, propanol, and ethylene, in an environmentally friendly way.Recently, the use of CO feedstocks, instead of CO2, was shown to enhance the selectivity toward multi-carbon products on copper-based electrocatalysts. Technoeconomic analyses have also demonstrated that CO can be produced from CO2RR cost-effectively. This has led to increased efforts in developing tandem electrocatalytic systems. However, state-of-the-art CO2-to-CO electrocatalysts are based on expensive noble metals such as Ag and Au, while earth-abundant Zn displays relatively poorer selectivity and activity. Herein, we show that oxide-derived Zn with high surface area can reduce CO2 to CO with a Faradaic efficiency of 86% and a partial current density (j CO) of −201 mA cm-2. While oxygen vacancies were previously implicated for CO2RR to CO, we pinpointed by detailed experiments and density functional theory calculations that highly undercoordinated Zn sites provide even higher activity, in view of their nearly optimal *COOH adsorption energies. These findings indicate that suitably engineered ZnO-derived materials can potentially be an alternative to the more costly Ag and Au electrocatalysts, and that the main guideline for their design is to increase the undercoordination of the catalytic sites.We also investigate the electrochemical CO reduction reaction (CORR) to liquid fuels and industrial feedstocks on copper-based and mixed copper-silver catalysts. To further enhance the CORR activity and product selectivities, a systematic optimization of the experimental environment, such as the use of various supports, catalyst binder and electrolytes, was done. We develop a set of experimental conditions for optimal CORR performance to value-added products.