Abstract:

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 × 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%), superior-activity (> 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.

Key words: Electrochemical CO₂ reduction reaction, Electrocatalyst, Electrolyte, Membrane, Reactor, Industrialization