Designing Electrolyzers for Electrocatalytic CO2 Reduction

The electrocatalytic CO2 reduction reaction (CO2RR) driven by renewable energy is an efficient approach to achieve the conversion and utilization of CO2. In this context, CO2RR has become an emerging research focus in the field of electrocatalysis over the past decade. While a large number of nanostructured catalysts have been developed to accelerate CO2RR, the tradeoff between activity and selectivity usually renders the overall electrocatalytic performance very poor. Beyond catalyst design, rationally designing electrolyzers is also of substantial importance for improving the CO2RR performance and achieving its scale-up for practical applications. To a large extent, the electrolyzer configuration determines the local reaction environment near an electrode by affecting the process conditions, thereby resulting in remarkably different electrocatalytic performances. To be techno-economically viable, the performance of CO2 electrolyzers is expected to be at least comparable to that of the current state-of-the-art proton exchange membrane (PEM) water electrolyzers, with regard to their activity, selectivity, and stability. Researchers have made great progress in the development of CO2 electrolyzers over the past few years, but they are also facing many issues and challenges. This review aims to provide an in-depth analysis of the research progress and status of current CO2 electrolyzers including Hcell, flow-cell, and membrane electrode assembly cell (MEA-cell) electrolyzers. Herein, operation at industrial current densities (> 200 mA.cm(-2)) is set as a basis when these electrolyzers are discussed and compared in terms of the four main figures of merit (current density, Faradic efficiency, energy efficiency and stability) that describe the CO2RR performance of an electrolyzer. The advantages and drawbacks of each electrolyzer are discussed and highlighted with emphasis on the key achievements reported to date. Compared to conventional H-cell electrolyzers that work well in mechanistic studies, the newly developed electrolyzers using gas diffusion electrodes, both flow-cell and MEA-cell electrolyzers, are able to break the limitation of CO2 solubility in water and acquire industrial current densities. Although flow-cell electrolyzers have achieved current densities exceeding 1 A.cm(-2), they suffer from low energy efficiencies because of the significant iR drop and poor stability owing to the use of alkaline electrolytes. These issues can be overcome in the case of zero-gap MEA-cell electrolyzers with ion exchange membranes being as solid electrolytes. The anion exchange membrane (AEM)-based CO2 electrolyzers are at the center of the current research, as they demonstrate promising activity and selectivity toward specific CO2RR products and exhibit excellent stability for over thousands of hours in few cases. Meanwhile, the crossover of CO2 and liquid products from the cathode to the anode through the membrane tends to lower the utilization efficiency of the CO2 supplied to the AEM electrolyzers. MEA-cell electrolyzers using cation exchange membranes and bipolar membranes have also been explored; however, neither of them have shown satisfactory CO2RR performance. The development of new polymer electrolyte membranes and ionomers would help address these problems. While issues and challenges still exist, MEA-cell electrolyzers hold the greatest promise for practical applications. As concluding remarks, research strategies and opportunities for the future have been proposed to accelerate the development of CO2RR technology for practical applications and to deepen the mechanistic understanding behind improved performance. This review provides new insights into rational electrolyzer design and guidelines for researchers in this field.

Automated summary

The electrocatalytic reduction of CO2 is an efficient approach driven by renewable energy, but there is a tradeoff between activity and selectivity. Rational design of electrolyzers is crucial for improving CO2RR performance.