Strategies to improve the performance of copper-based catalyst for electroreduction of CO<sub>2</sub> to multi-carbon products

Abstract

Electrocatalytic carbon dioxide (CO2) conversion provides an appealing strategy to use renewable electricity to synthesize value-added fuels and chemicals while mitigating CO2 emissions. Copper (Cu) is one of the few metal catalysts that can effectively convert CO2 to high energy density and high-value multi-carbon products (C2+), and thus has received increasing attention. Although significant progress has been made in the field of CO2 electroreduction using copper-based catalysts, there are still some bottlenecks when it comes to practical implementation. To enable the commercial application of electrochemical CO2 electroreduction (CO2RR), the high selectivity of C2+ products, low overpotential and high current density are imperatively required. This paper reviews the state-of-the-art research progress of CO2RR technology with the attention paid to three important aspects, i.e., catalyst development, catalytic environment optimization and CO2 electrolyzer design, that aim to improve the catalytic performance of electrolytic CO2 conversion. In the development of efficient and robust catalyst, we identify that the C−C coupling is the key step of electrocatalytic CO2 conversion to C2+, which rest with the adsorption state of essential intermediate CO* on the Cu catalyst. It is revealed that the selectivity of C2+ is closely related to the adsorption strength of CO* on catalyst while the adsorption configuration and coverage of CO* on the Cu catalyst surface strongly affect the energy barrier for the CO* dimerization. Based on this understanding, a rational design of catalysts was suggested with the principles of strengthening the adsorption between CO* and catalyst, adjusting the adsorption configuration of CO*, increasing the coverage of CO* on Cu surface, and suppressing the adsorption of H* on catalyst to minimize hydrogen evolution. In terms of specific strategy of catalyst development, we recommended the following approaches to regulate the CO* adsorption on the Cu surface: (1) Optimizing the Cu catalyst with crystal facet being Cu(100) and/or high index facet to match an optimized CO* absorption configuration for an improved C2+ selectivity; (2) tuning size of catalyst (>10 nm) and the surface roughness/flexural morphology to suppress the H+ adsorption and promote the adsorption of CO* or other C2+ precursors; (3) creating the oxidation state of Cu by doping or adding a metal oxide support to decrease the C−C coupling barrier; (4) increasing the defects on the Cu surface by destroying the crystal structure or doping effective atoms to create the stronger CO* adsorption sites and sites’ diversity; (5) combining a second catalyst material in tandem with Cu which facilitates the formation of CO and subsequent promotion of the kinetics in C−C coupling; (6) functionalizing the Cu surface with electron-rich groups such as amino group to adjust the CO* adsorption affinity to enhance the CO* dimerization process. In parallel with catalyst design, the catalytic environment including electrolyte properties and operation conditions should be optimized with the following approaches suggested to improve the C2+ selectivity: (1) Using the supporting electrolyte with larger ion radius of cation or anion to enhance the binding between the CO* and Cu surface; (2) increasing the partial pressure of CO2 to increase the aqueous CO2 concentration and subsequently facilitates the CO2RR and the yield of CO* for further reduction; (3) using high-pH electrolyte in the CO2 electrolysis system to promote the CO dimerization whilst suppressing the hydrogen evolution. Lastly, we briefly introduced the emerging CO2 electrolyzer design that incorporates the gas diffusion electrode (GDL) to overcome the challenge of CO2 mass transfer faced by the conventional H-type CO2 electrolysis system. Instead of limited CO2 solubility in aqueous electrolyte, the GDL allows direct contact between gaseous CO2 and catalyst, thus greatly improved the mass transfer of CO2 onto the catalyst surface for CO2RR. This new electrolyzer setup also expand the use of high alkaline electrolyte, that enables the CO2RR to be operated at high selectivity of C2+ under high current density.