Electrochemical reduction of CO2 to CO in a flow-through electrolyser

Abstract

Electrochemical reduction of the \ce{CO2} into the chemically valuable \ce{CO} at an industrial scale is a promising way to reverse the worrying rise of \ce{CO2} levels in the atmosphere. Profitable operation at an industrial scale requires high CO partial current densities, producing a CO-rich output, while keeping the energy consumption and the price of the electrolyser low. Conventional \ce{CO2} reduction (\ce{CO2}R) to CO electrolysers suffer from the competing hydrogen evolution (HEV) reaction and mass transport limitations. Therefore, they can produce a maximum CO partial current densities of 20 mA cm$^{-2}$. The current densities can be increased by working at higher pressures or using gas diffusion electrodes, but to date, both require high energy inputs. Therefore, we investigated the potential of using a flow-through electrolyser (FTE) to produce \ce{CO} partial current densities exceeding the 20 mA cm$^{-2}$, while keeping the potential attractive. Porous flow-through electrodes can overcome mass transport limitations by their large reactive surface area and their ability to decrease the diffusion boundary layer thickness. To obtain suitable porous electrodes, we electrodeposited Ag on microporous Ti substrates. We used an aqueous \ce{CO2}-saturated \ce{KHCO_3} solution, as a well-proven electrolyte for \ce{CO2}R to \ce{CO} on a Ag catalyst. Our FTE was capable of overcoming the mass transport limitations. In a one-off test, it produced a \ce{CO} partial current density of 100 mA cm$^{-2}$, at a Faradaic efficiency (FE) of 55 \%. We could reproduce \ce{CO} partial current densities of 60 mA cm$^{-2}$ at a FE of 25 \% in a 0.05 M \ce{KHCO_3} solution. We experienced a decrease in activity towards CO when using more concentrated electrolytes. However, high cell potentials of more than 5 V, were required to exceed \ce{CO} partial current densities of 20 mA cm$^{-2}$. The required high potentials can be ascribed to ohmic losses and low selectivities towards CO. We believe that the insufficient deposition of the Ag on the microporous Ti electrode was the main reason for the low selectivity. Implementing the improvements, that are mentioned in the report, could contribute to make the FTE a promising candidate to reduce \ce{CO2} to CO at an industrial scale effectively.