Liquid–Solid Boundaries Dominate Activity of CO₂ Reduction on Gas-Diffusion Electrodes

Abstract

Electrochemical CO2 electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO 2 gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO2, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst's surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase vs. two-phase reaction has substantial implications for the design of catalysts, gasdiffusion layers and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here, we outline the macro, micro and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO2 electrolysis. From this information, we comment on the outlook for the broader CO2 electro-reduction GDE cell architecture.

Visualisation and quantification of flooding phenomena in gas diffusion electrodes (GDEs) used for electrochemical CO2 reduction: A combined EDX/ICP–MS approach

“…The wettability of the GDL plays a crucial role in determining the performance of gas-fed zero-gap electrolysers. The presence of some amount of water, based on Equation (1), is essential; however, if too much water enters the GDL structure, it may block the access of CO 2 to the catalyst, shifting the balance of the cathode reaction from the preferred ec-CO 2 RR towards the parasitic HER [24].…”

Hydrophobic thiol coatings to facilitate a triphasic interface for carbon dioxide reduction to ethylene at gas diffusion electrodes

“…However, it is possible that the porous catalyst layer will be fully wetted with electrolyte, and so CO 2 entering the GDE will still have to dissolve into the electrolyte and diffuse to the catalyst, especially to reach the catalyst material further away from the carbon support. 23 Recently, hydrophobic thiol coatings of nanostructured copper electrodes were shown to signicantly increase the yield of C 2+ products in a liquid phase cell by trapping gases on the nanoscale. 24 Deposition of a hydrophobic long chain thiol trapped gases at the catalyst surface, providing a triphasic environment where one would not normally be favoured.…”

Regulating the Electron Localization of Metallic Bismuth for Boosting CO2 Electroreduction

“…Interestingly, the free energy of *H is 0.48 eV on Bi-B, significantly lower than on pure Bi (0.82 eV), indicating a higher water dissociation activity on the B modulated surface [ 58 ]. At higher cathodic potentials in CO 2 RR, the H + is considered to be supplied by H 2 O dissociation [ 59 – 61 ]. When the binding energy of *H intermediate is too strong, the *H will occupy the active sites on the catalyst surface.…”