Research status and trend of electrolytes in the CO<sub>2</sub> electrochemical reduction

Abstract

The concentration of CO2 in the atmosphere increases with utilizing fossil energy, which caused serious environmental problems. Owing to the advantage of using renewable electricity, the issue of carbon recycling and energy recovery based on the electrochemical CO2 reduction reaction (CO2RR) presents a potentially sustainable way to transform CO2 into a variety of valuable products, such as carbon monoxide (CO), methanol (CH3OH), formic acid (HCOOH/HCOO−), and so on. The challenges of CO2RR mainly include high overpotential, low current density and low Faraday efficiency (FE). Therefore, the development of new electrochemical reduction systems to reduce overpotential and improve reaction selectivity is the research hotpot in CO2RR. The selection of the electrolyte is very important to design efficient electrochemical reduction systems, which provides the reaction environment, transports ions and serves a conductive medium in CO2RR system. Under the same condition, the species, concentration, and pH of electrolytes influence the local reaction environment, the current density and the selectivity of products. So far, various electrolytes, such as alkali metal and ionic liquid electrolytes have been applied to enhance the performance of CO2RR. Therefore, in comprehensive understanding of the electrolyte effects on CO2RR is crucial in selecting proper electrolytes for the reduction of CO2 to desired products. This paper mainly reviews the recent processes in the alkali metal and ionic liquid electrolytes, and summarizes their effects in CO2RR as well. Firstly, we summarize the product distribution and current density of CO2RR in the different alkali metal aqueous. Alkali metal aqueous is the most common solvent for CO2RR, because alkali metal aqueous are widely available at low cost, relatively easy to prepare, and store safely. Many studies reported that alkali metal aqueous effects CO2RR current density and product distribution. The effects of metal aqueous on the CO2RR based on preferential hydrolysis of hydrated cations near the cathode surface, which to buffer the electrolyte near the electrode surface, offsetting the polarization losses caused by concentration gradient. At the same time, hydrated alkali metal cations in the outer Helmholtz plane can create a dipole field, which can stabilize the adsorption the intermediates to enhance the selectivity of the CO2RR on the intrinsic activity. Secondly, ionic liquids as electrolytes have also attracted extensive interest due to their high electrical conductivity, high CO2 adsorption capacity, and high selectivity. Because the ionic liquid can be adsorption on the electrode surface to reduce the overpotential and change the reaction microenvironment. Using different ionic liquid electrolytes can promote CO2 electroreduction to different products, such as CO, HCOO−/HCOOH, CH3OH and so on, especially in the performance of production selectively. Although important progress has been made in CO2RR with ionic liquid electrolyte, the mechanism of CO2RR to C2+ products is unclear, especially the interaction between the liquid and the electrode surface. Finally, we briefly prospect the future research priorities and directions of CO2RR systems in ionic liquid electrolyte.