(590k) Layer-By-Layer Coated Anion Exchange Membranes for Enhanced Selectivity in CO2 Electrolysis

Alkaline CO₂ electrolyzers offer a promising route for converting CO₂ into C₂₊ fuels (e-fuels) and value-added chemicals with low cell voltages (e.g., <3 V), high current densities (e.g., >200 mA/cm²), and good selectivity (e.g., minimal syngas formation). However, current commercial anion exchange membranes (AEMs) are generally nonselective toward various anions such as OH⁻, HCO₃^-, and CO₃^2-. Moreover, organic products - including ethanol, propanol, and acetic acid - can diffuse from the cathode to the anode, where they may be reoxidized to CO₂, significantly reducing e-fuel productivity and increasing operational costs.

In this work, a series of anionic and cationic polymers, was employed with two-dimensional (2D) materials, to modify commercial AEMs via layer-by-layer deposition techniques. For example, a varying concentration of sulfonated poly(ether ether ketone) (SPEEK) and poly(diallyldimethylammonium chloride) (PDADMAC) solutions were used to deposit one SPEEK/PDADMAC polyelectrolyte bilayer on the membrane surface. The cycle was repeated to form multiple bilayers, yielding a multilayer-coated membrane.

Graphene oxide (GO) was also incorporated as a thin, functional layer due to its unique structural and physicochemical properties. GO can self-assemble into free-standing membranes via strong interactions among its hydroxyl, epoxy, and carboxyl functional groups. These monolayers stack into laminar structures with narrow interlayer spacings that can swell in water to allow ionic transport while forming tortuous pathways that hinder the crossover of gases and organic molecules. A thin GO layer (<100 nm) was deposited onto the membrane surface using an inkjet printer and optimized to minimize the crossover of organic products.

This multilayer modification strategy enables the formation of a thin, selective barrier layer that fine-tunes membrane surface characteristics – such as charge, hydrophilicity, pore size, porosity, and crosslinking density. The resulting AEMs exhibit a 2.4-fold increase in OH^-/HCO₃^- selectivity and a 1.9-fold increase in OH^- conductivity, while effectively suppressing the crossover of organic species (e.g., ethanol, ethylene) to the anode compartment. These enhancements lead to improved e-fuel productivity and reduced system costs, expanding the viability of alkaline CO₂ electrolysis for renewable fuel and chemical production.