Highly Selective CO2 Reduction to Pure Formic Acid Using a Nafion‐TiO2 Composite Porous Solid Electrolyte

A Nafion‐based porous solid electrolyte (NPSE) is reported, optimized by thickness, porosity, and titanium dioxide (TiO2) incorporation for selective electrochemical reduction of CO2 to formic acid. The NPSE–TiO2 composite achieves a FEHCOOH of 98.6%, jHCOOH of 431.8 mA cm−2, and single‐pass concentration of 17.4 wt.%. Mechanistic insights reveal that TiO2 enhances formate transport, suppressing back‐diffusion of formic acid and stabilizing cathodic pH. Abstract The electrochemical reduction of CO2 (CO2RR) 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 CO2RR performance. Furthermore, a Nafion‐titanium dioxide (TiO2) composite NPSE (NPSE‐TiO2) is developed, in which TiO2 nanoparticles significantly enhance the selectivity and productivity of formic acid. The NPSE‐TiO2 achieves a record‐high Faradaic efficiency (FEHCOOH) of 98.6%, the highest value reported to date among SSE systems, along with a high partial current density (jHCOOH) of 431.8 mA cm−2, yielding electrolyte‐free formic acid with a single‐pass concentration of 17.4 wt.%. Mechanistic investigations reveal that TiO2 promotes the transport of formate anions (HCOO−), suppressing back‐diffusion of formic acid to the cathode and stabilizing cathodic pH. A Nafion-based porous solid electrolyte (NPSE) is reported, optimized by thickness, porosity, and titanium dioxide (TiO 2 ) incorporation for selective electrochemical reduction of CO 2 to formic acid. The NPSE–TiO 2 composite achieves a FE HCOOH of 98.6%, j HCOOH of 431.8 mA cm −2, and single-pass concentration of 17.4 wt.%. Mechanistic insights reveal that TiO 2 enhances formate transport, suppressing back-diffusion of formic acid and stabilizing cathodic pH. Abstract 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. Advanced Science, Volume 13, Issue 2, 9 January 2026.