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, we present a novel Nafion-based porous solid electrolyte (NPSE) with tunable thickness and porosity, replacing conventional bead-type ion-exchange resins. The structural tunability of NPSEs plays a critical role in determining CO₂RR performance. Furthermore, we develop a Nafion-TiO₂ composite NPSE (NPSE-TiO₂), 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 system, 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 TiO₂promotes the transport of formate anions (HCOO⁻), suppressing back-diffusion of formic acid to cathode and stabilizing cathodic pH.