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

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 (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 TiO₂ promotes the transport of formate anions (HCOO^-), suppressing back-diffusion of formic acid to the cathode and stabilizing cathodic pH.