The electrocatalytic CO₂ reduction reaction (CO₂RR) to formic acid has gained significant attention as a potential environmentally friendly approach to reducing CO₂ 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 CO₂RR. 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 CO₂RR 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 CO₂RR to produce pure formic acid (HCOOH) and propose further research to scale up this technology for industrial-scale applications in the future.

The electrochemical reduction of CO ₂ (CO ₂ RR) to liquid fuels and chemicals offers a sustainable route to close the carbon cycle. Solid‐state electrolyte (SSE) systems have recently emerged as a breakthrough platform, enabling the direct synthesis of high‐purity liquid products. Here, a novel Nafion‐based porous solid electrolyte (NPSE) with tunable thickness and porosity is presented, replacing conventional bead‐type ion‐exchange resins. The structural tunability of NPSEs plays a critical role in determining CO ₂ RR performance. Furthermore, a Nafion‐titanium dioxide (TiO ₂ ) composite NPSE (NPSE‐TiO ₂ ) is developed, in which TiO ₂ nanoparticles significantly enhance the selectivity and productivity of formic acid. The NPSE‐TiO ₂ achieves a record‐high Faradaic efficiency (FE _HCOOH ) of 98.6%, the highest value reported to date among SSE systems, along with a high partial current density ( j _HCOOH ) of 431.8 mA cm ⁻² , yielding electrolyte‐free formic acid with a single‐pass concentration of 17.4 wt.%. Mechanistic investigations reveal that TiO ₂ promotes the transport of formate anions (HCOO ⁻ ), suppressing back‐diffusion of formic acid to the cathode and stabilizing cathodic pH.

The application of a nickel foam-based macroporous structure with asymmetric binder engineering significantly enhances electrolyte transport and interfacial CO ₂ regeneration in liquid-fed electrochemical reactive carbon capture.