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

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.