Phase‐Inversion Induced 3D Electrode for Direct Acidic Electroreduction CO₂ to Formic acid

Abstract

Direct electrochemical CO2 reduction to formic acid (FA) instead of formate is a challenging task due to the high acidity of FA and competitive hydrogen evolution reaction. Herein, 3D porous electrode (TDPE) is prepared by a simple phase inversion method, which can electrochemically reduce CO2 to FA in acidic conditions. Owing to interconnected channels, high porosity, and appropriate wettability, TDPE not only improves mass transport, but also realizes pH gradient to build higher local pH micro‐environment under acidic conditions for CO2 reduction compared with planar electrode and gas diffusion electrode. Kinetic isotopic effect experiments demonstrate that the proton transfer becomes the rate‐determining step at the pH of 1.8; however, not significant in neutral solution, suggesting that the proton is aiding the overall kinetics. Maximum FA Faradaic efficiency of 89.2% has been reached at pH 2.7 in a flow cell, generating FA concentration of 0.1 m. Integrating catalyst and gas–liquid partition layer into a single electrode structure by phase inversion method paves a facile avenue for direct production of FA by electrochemical CO2 reduction.

Unveiling pH‐Dependent Adsorption Strength of *CO2− Intermediate over High‐Density Sn Single Atom Catalyst for Acidic CO2‐to‐HCOOH Electroreduction

“…The gaseous and liquid products were verified and quantitatively analysed by gas chromatography (GC) and 1 H nuclear magnetic resonance (NMR) spectroscopy, respectively. As shown in the NMR spectra (Figure 3a), the predominant production of HCOOH takes place at pH=1 and pH=3, while HCOO − production occurs at pH=5, along with a clear chemical shift from 8.17 ppm in the acidic electrolyte to 8.41 ppm in the near neutral electrolyte [19,44] . Figure 3b and S15 shows the FE values of HCOOH, H 2 and CO products obtained over Sn SACs with the applied current densities ranging from −20 to −250 mA cm −2 at pH=3.…”

Unveiling pH‐Dependent Adsorption Strength of *CO2− Intermediate over High‐Density Sn Single Atom Catalyst for Acidic CO2‐to‐HCOOH Electroreduction

“…As shown in the NMR spectra (Figure 3a), the predominant production of HCOOH takes place at pH = 1 and pH = 3, while HCOO À production occurs at pH = 5, along with a clear chemical shift from 8.17 ppm in the acidic electrolyte to 8.41 ppm in the near neutral electrolyte. [19,44] Figure 3b and S15 shows the FE values of HCOOH, H 2 and CO products obtained over Sn SACs with the applied current densities ranging from À 20 to À 250 mA cm À 2 at pH = 3. Sn SACs exhibit high HCOOH selectivity with suppressed H 2 production, which could maintain over 85 % FE for in a wide range of current densities from À 50 to À 150 mA cm À 2 , reaching a maximum of 90.8 % at À 100 mA cm À 2 .…”

“…[16][17][18] Instead, CO 2 RR in acidic conditions allows direct HCOOH production, which circumvents the issues posed under neutral or alkaline electrolyte. [19][20][21] Nevertheless, under acidic conditions, the high proton concentration significantly affects the competition between CO 2 RR and the kinetically-favorable HER, thereby lowering the selectivity of CO 2 RR products. [16,22,23] Several strategies, such as surface structure engineering and alkali cation introduction, have recently been adopted to create a "micro-alkaline" environment on the catalyst surface to favor CO 2 RR over HER.…”

Acidic CO2 Electrolysis Addressing the “Alkalinity Issue” and Achieving High CO2 Utilization

“…For example, Kang et al. prepared a three‐dimensional (3D) porous electrode by a simple phase inversion method, which can electrochemically reduce CO 2 to HCOOH during acidic conditions [9c] . Additionally, the confinement effect of porous structure may also favor the accumulation of *CO, which in turn facilitates the C−C coupling [45] .…”