Fundamental Understanding of the Cathodic Catalyst Layer of an Anion Exchange Membrane-Based CO2 Electrolyzer: Catalyst Optimization and Ionomer Characterization
CO2 electrolysis exhibits significant potential in addressing climate change and facilitating the transformation of captured CO2 into valuable chemicals and fuels. Recent years have witnessed remarkable advancement in the design of CO2 electrolyzers, offering future commercial viability. These design advancements encompass cell configuration, catalytic materials for the cathode and anode, as well as suitable membranes and ionomers. This thesis is focused on the cathodic catalyst layer (CCL) of a gas-fed anion exchange membrane-based CO2 electrolyzer, a pivotal component. The CCL consists of a porous nanocomposite structure comprising electrocatalyst coated with a nanothin ionomer film. The pores facilitate CO2 gas transport, the connected network of catalyst allow electron transport, the percolated distribution of ionomer ensures ion diffusion, the ionomer coverage of catalyst provides the required electrochemical interface and together these features provide for efficient electrochemical reactions. Specifically, the work targeted on optimizing an Ag-based catalyst for CO production and characterizing an imidazolium-based anion exchange ionomer within the CO2 electrolysis environment. While prior research mostly employed highly loaded (1-3 mg.cm-2) commercial Ag nanoparticles, which is spray-coated on the electrode with a maximum mass-specific activity of around 100-150 mA.mgAg-1, this study introduces an electrodeposition technique to synthesize Ag catalysts. This technique not only enhances the electrical contact with the substrate, but also enables control over catalyst size, morphology, and crystallography, which influences catalyst performance and catalyst utilization. Various Ag electrodes with distinct sizes and structures, ranging from polycrystalline to dendritic, were synthesized and assessed for CO2 reduction. An optimized dendritic Ag catalyst, with 0.29 mg.cm-2 Ag loading and maximum (220)/(111) facet ratio, exhibited a high mass-specific activity of 362 mA.mgAg-1, current density of 105 mA.cm-2, and 94% CO selectivity at a 3 V cell potential, maintaining robust performance over extended 100 h CO2 reduction reaction. The catalyst/ionomer interface plays a pivotal role in CO2 reduction, necessitating not only highly electrochemically-active sites to maximize catalyst performance, but also optimized CCL microstructure for ion transport to minimize catalyst utilization. The characteristics of an imidazolium-based anion exchange ionomer (Sustainion, XA-9) were probed under CO2 electrolysis conditions. The influence of relative humidity (0-95%), film thickness (8-61 nm), chemical environment (N2 vs CO2 exposure), and ionomer counterion exchange (Cl- to OH-) on hydration properties, ionic conductivity, and CO2 adsorption/reduction was investigated. Results reveal that swelling, water content (λ), and ionic conductivity of the ionomer films in Cl- form (XA9-Cl), increase with rising relative humidity, with a confinement behavior observed below 26 nm thickness; (22% swelling, 32 wt.% water content, and 13 mS.cm-1 ionic conductivity, at 95% RH on a 26 nm thick film). Although the CO2 exposure did not change the swelling properties of the XA9-Cl ionomer thin film, the water content was slightly increased due to the carbonate/bicarbonate formation in the ionomer. The ionic conductivity of the XA9-Cl thin film responds differently in the N2 vs CO2 chemical environment, displaying a crossover at λ > 6. CO2-exposed ionomer films exhibit higher conductivity at lower λ but lower conductivity at higher λ compared to N2-exposed films. This phenomenon could arise from bicarbonate species enhancing ion conductivity at lower λ, while exhibiting reduced mobility and conductivity compared to OH- ions under high humidity conditions. The transformation of the ionomer to its OH- form improves swelling, water content, and ionic conductivity, reaching levels of 26%, 36 wt.%, and 33 mS.cm-1, respectively, at 95% RH on a 26 nm thin film. Moreover, the OH- form ionomer demonstrates improved CO2 adsorption and reduction on an Au electrode, confirmed by CV analysis. Nevertheless, thicker films manifest postponed CO2 reduction owing to transport limitations in the ionomer film.
Electrocatalysis, CO2 electrolyzer, Ag catalyst, Anion exchnage ionomer, Thin film
Alihosseinzadeh, A. (2023). Fundamental understanding of the cathodic catalyst layer of an anion exchange membrane-based CO2 electrolyzer: catalyst optimization and ionomer characterization (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.