Research Advances in Electrocatalysts, Electrolytes, Reactors and Membranes for the Electrocatalytic Carbon Dioxide Reduction Reaction

Human activities primarily rely on the consumption of the fossil energy, which has led to an energy crisis and environmental pollution. Since the industrial revolution, the atmospheric CO2 concentration has been continuously increasing, and reached 414 x 10(-6) in 2020, which has resulted in global warming and glacial ablation. Converting CO2 into high-value-added fuels and chemicals can alleviate environmental problems, enable the storage of intermittent renewable energy (wind and solar power), and provide a new route for fuel synthesis. The electrochemical CO2 reduction reaction (CO2RR) has attracted extensive attention owing to its mild reaction conditions, controllability, environmental friendliness, and the ability to generate various products. There are four key steps in a typical CO2RR: (1) charge transport (electrons are transported from the conductive substrate to the electrocatalyst); (2) surface conversion (CO2 is adsorbed and activated on the surface of the catalyst); (3) charge transfer (electrons are transferred from the catalyst surface to the CO2 intermediate); and (4) mass transfer (CO2 diffuses from the electrolyte to the catalyst surface, and the products diffuse in the reverse pathway). The former two steps depend on the type of membrane and the development of highly conductive catalysts with abundant active sites, while the latter two steps rely on the properties of the electrolyte and the optimization of the electrolytic cell configuration. To meet the high-selectivity (> 90%), superioractivity (> 200 mA.cm(-2)), and excellent-stability (> 1000 h) requirements of the CO2RR as per industrial standards, the design of efficient electrocatalysts has been a key research area in recent decades. However, other factors have rarely been investigated. In this review, we systematically summarize the development of electrocatalysts, effect of the electrolyte, progress in the development of the reactor, and type of membrane in the CO2RR from industrial and commercial perspectives. First, we discuss how first-principles calculations can be used to determine the chemical rate for CO2 reduction. Additionally, we discuss how in situ or operando techniques such as X-ray absorption measurements can reveal the theoretically proposed reaction pathway. The microenvironment (e.g., pH, anions, and cations) at the three-phase interface plays a vital role in achieving a high CO2RR performance, which can be controlled by changing the electrolyte properties. Further, the suitable design and development of the reactor is very critical for commercial CO2RR technology because CO2RR reactors must efficiently utilize the CO2 feedstock to minimize the cost of upstream CO2 capture. Finally, different types of membranes based on different ion-transfer mechanisms can affect the CO2RR performance. The development opportunities and challenges toward the practical application of the CO2RR are also highlighted.