High-Rate and Selective CO₂ Electrolysis to Ethylene via Metal-Organic-Framework-Augmented CO₂ Availability

High-rate conversion of carbon dioxide (CO₂ ) to ethylene (C₂ H₄ ) in the CO₂ reduction reaction (CO₂ RR) requires fine control over the phase boundary of the gas diffusion electrode (GDE) to overcome the limit of CO₂ solubility in aqueous electrolytes. Here, a metal-organic framework (MOF)-functionalized GDE design is presented, based on a catalysts:MOFs:hydrophobic substrate materials layered architecture, that leads to high-rate and selective C₂ H₄ production in flow cells and membrane electrode assembly (MEA) electrolyzers. It is found that using electroanalysis and operando X-ray absorption spectroscopy (XAS), MOF-induced organic layers in GDEs augment the local CO₂ concentration near the active sites of the Cu catalysts. MOFs with different CO₂ adsorption abilities are used, and the stacking ordering of MOFs in the GDE is varied. While sputtering Cu on poly(tetrafluoroethylene) (PTFE) (Cu/PTFE) exhibits 43% C₂ H₄ Faradaic efficiency (FE) at a current density of 200 mA cm^{- 2} in a flow cell, 49% C₂ H₄ FE at 1 A cm^{- 2} is achieved on MOF-augmented GDEs in CO₂ RR. MOF-augmented GDEs are further evaluated in an MEA electrolyzer, achieving a C₂ H₄ partial current density of 220 mA cm^-2 for CO₂ RR and 121 mA cm^-2 for the carbon monoxide reduction reaction (CORR), representing 2.7-fold and 15-fold improvement in C₂ H₄ production rate, compared to those obtained on bare Cu/PTFE.