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

## TL;DR

This paper introduces a new solid electrolyte that efficiently converts CO2 into pure formic acid with high selectivity and performance.

## Contribution

The novel Nafion-TiO2 composite electrolyte achieves record selectivity and productivity for CO2 reduction to formic acid.

## Key findings

- The NPSE-TiO2 composite achieves a Faradaic efficiency of 98.6% for formic acid production.
- It delivers a high partial current density of 431.8 mA cm−2 and a single-pass concentration of 17.4 wt.% formic acid.
- TiO2 enhances formate transport and stabilizes cathodic pH, improving overall performance.

## 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 (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.

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%, j
HCOOH 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.