Recent Advances in Integrated Electrode for Electrocatalytic Carbon Dioxide Reduction

The electrocatalytic carbon dioxide reduction reaction (E-CO2RR) has attracted attention in recent years for its ability to effectively alleviate the environmental problems caused by the rapid increase of CO2 in the atmosphere and transform CO2 into high value-added fuels or chemicals (e.g., CO, HCOOH, CH4, CH3OH, C2H4, C2H5OH, etc.) under mild conditions. In addition, clean energy sources, such as solar and wind energy, can provide electrical energy for the electrochemical CO2 conversion technology used in large-scale industrial applications. One limitation of the E-CO2RR is that CO2 is a thermodynamically stable linear molecule with a slow kinetic reaction rate. In addition, the E-CO2RR involves complex processes, such as gas diffusion and multi-electron transfer, making its selectivity problematic. Therefore, constructing highly efficient and stable catalytic electrodes has become a core research topic in the field of E-CO2RR. Unfortunately, the traditional method of coating electrodes with binders (e.g., Nafion, polyvinylidene fluoride, and polytetrafluoroethylene) usually results in a low utilization ratio of active sites due to the easy aggregation of the catalysts themselves. This could result in the severe embedding of active sites and limited mass transfer. Moreover, the dissolution of the catalyst layer during the electrocatalytic process also reduces the activity and stability of the electrodes, making it difficult to reuse. Therefore, it is necessary to regulate the electrode reaction interface to improve the utilization ratio of active sites. The integrated electrodes, where the catalyst is grown directly on the current collector, can avoid the use of binders to facilitate the exposure of active sites and transfer of electrons. The integrated structure can also enhance the bonding strength between the active material and current collector and improve the cycling stability of the electrodes. Meanwhile, the micro-environment (e.g., pH, concentration of CO2, and intermediates) at the three-phase interface can be effectively controlled on the integrated electrodes, which can enhance the performance of the E-CO2RR. In recent years, encouraging progress has been achieved in the study of the E-CO2RR. However, current reviews of the E-CO2RR mainly focus on the regulation of the intrinsic activity of catalysts; discussions and reviews from the perspective of the electrodes are rarely reported. This article reviews the latest research of the integrated electrodes for the E-CO2RR with a focus on the application of different types of integrated electrodes (e.g., metal, alloy, metal oxide, metal sulfide/phosphide, and metal single atom). It also analyzes the effects of morphology, surface, and interface regulation on the electrocatalytic performance of the E-CO2RR. Finally, it highlights the challenges that still exist in this field and discusses the future development of the integrated electrodes.