| Summary: | The electrocatalytic CO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR) to formic acid has gained significant attention as a potential environmentally friendly approach to reducing CO<sub>2</sub> emissions and producing carbon-neutral liquid fuels. However, several challenges must be addressed to achieve the production of high-purity and high-concentration formic acid through CO<sub>2</sub>RR. One major challenge is the formation of a formate mixture instead of pure formic acid in conventional reactors. This requires costly downstream purification and concentration processes to obtain pure formic acid. To overcome this problem, a three-compartment reactor design has been proposed where a solid-state electrolyte (SSE) is inserted between the anode and cathode compartments to recover pure formic acid directly. This reactor design involves the use of an anion exchange membrane (AEM) and a cation exchange membrane (CEM) to separate the anode and cathode compartments, and a center compartment filled with high-conductivity SSE to minimize ohmic resistance. Several studies have implemented this reactor design for continuous CO<sub>2</sub>RR and have reported remarkable improvements in the concentration and purity of the formic acid product. In this review, we summarize the recent progress of the SSE reactor design for CO<sub>2</sub>RR to produce pure formic acid (HCOOH) and propose further research to scale up this technology for industrial-scale applications in the future.
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