Development of electrochemical CO2 reduction to gas and liquid fuels in aqueous media with scale-up potential

Abstract

Electrochemical CO2 reduction reaction (eCO2RR) in CO2 utilization field attracts large interests as it only consumes water and electricity as the inputs to build valuable carbonaceous fuels and chemicals (i.e., CO, formate, low-carbon oxygen-contained and oxygen-free hydrocarbons). However, in aqueous media, hydrogen evolution reaction (HER) competes with eCO2RR and theoretically much easier to be carried out resulting in low Faradaic efficiency (FE) towards eCO2RR. Aiming at developing eCO2RR system with supressed HER, this study enhanced CO2 mass transfer and reaction kinetics through cell configuration and electrolyte engineering, and developed the FE of valuable carbonaceous products by catalysts design. eCO2RR has been generally carried out by a two-chamber (2C) cell using CO2-saturated electrolyte, mass transfer is challenged by the CO2 solubility. This study constructed a gas diffusion electrode (GDE) cell that CO2 could be directly supplied to reaction interface with less resistance from the electrolyte, which greatly enhanced the overall FE for carbonaceous products compared with the 2C cell. The use of high alkalinity catholytes in GDE cell improved reaction kinetics and further increased FE of carbonaceous products, especially for C2 (ethanol and ethylene). C2 FE achieved 40% and current density reached -234 mA cm-2 at -1.17 V (RHE) with using 2.0 M KOH. Targeting to enhance the selectivity of CO, a simple synthesis procedure for Cu-In catalyst was developed by electrochemical spontaneous precipitation (ESP) method. The material possessed a hybrid structure of amorphous In(OH)3 nano-layer capping on polycrystalline CuxO, which structurally facilitated Cu-In interaction. CO production using Cu-In catalyst reached ~90% FE and -200 mA cm-2 current density at -1.17 V (RHE). Syngas with tunable CO/H2 ratio could be also produced by changing the ESP condition. Carbon supported SnO2 catalyst was studied for selective formate production (>80% FE). High concentration (0.5 M) of formate solution was produced within 1-hour CO2 reduction, which was utilized for a direct formate fuel cell (DFFC). The DFFC used Pd-CeO2/C anode and FeCo/C cathode, producing 92 mW cm-2 peak power density at 30% energy efficiency. A close loop of “electricity-formate-electricity” was realized, illustrating the important potential of synthetic liquid fuels from CO2 for energy storage and transportation applications.