Design and Investigation of Membrane-Electrode Assemblies for the Electrochemical Reduction of CO2
The electrochemical conversion of carbon dioxide is of increasing interest because it offers a means of achieving the generation of value-added products, mitigation of greenhouse gas (GHG)-emissions, and storage of intermittent renewable energy. Aqueous gas-diffusion electrode (GDE) systems for electrochemical CO2 reduction allow for an order-of-magnitude increase in obtainable limiting current densities compared to planar systems due to their inherent increase in CO2 flux. Despite this improvement, aqueous GDEs exhibit significant ohmic resistances that limit the current densities that can be achieved at applied overpotentials, making them impractical for industrial implementation.
Membrane-electrode assemblies (MEAs), consisting of humidified gaseous feeds at one or both porous electrodes and no aqueous electrolyte (only a solid ion-conducting polymer or ionomer), can overcome the limitations of aqueous GDEs. Commercial-scale generation of carbon-containing products by means of electrochemical CO2 reduction (CO2R) requires electrolyzers operating at high current densities and product selectivities. MEAs have been shown to be suitable for this purpose. In this dissertation, various MEA designs are considered: the Full-MEA, the H2O-MEA, and the Exchange-MEA. Based on recent modeling and experimental findings presented and cited within this dissertation, the Exchange-MEA emerges as the cell design of choice for ultimately developing an efficient and scalable CO2 reduction device because it enables good ionomer and membrane hydration, as well as high catalyst activity and selectivity. However, despite its outwardly simple design, the MEA is a complicated system that warrants further investigation into its structure and function.
There is significant complexity in the CO2R MEA, specifically in its ionomer-based catalyst layers. In particular, the cathode catalyst layer and the multiple phenomena occurring in this region contribute extensively to the rate of CO2R, overall current-voltage performance, and product selectivity profiles in the MEA. Thus, the theme of this dissertation is to elucidate how key components in the cathode catalyst-layer region (ionomer, catalyst, water, etc.) interact so that a systematic understanding of how these microscale interactions contribute to underlying, limiting phenomena can be achieved and ultimately used to improve macroscale CO2R MEA performance.
Chapter 1 of this dissertation introduces the field of CO2R and discusses current progress towards understanding the complexity of the catalyst layer. Chapter 2 documents the experimental and engineering design work done on MEAs within this dissertation project, presenting a systematic exploration of various system factors, such as relative humidity and temperature, as well as the development of MEA fabrication and operation best practices and guiding principles. In Chapter 3, the influence of the ionomer-to-catalyst ratio (I:Cat), the catalyst loading, and the catalyst-layer thickness – as well as the anode exchange solution concentration, the MEA cell design, and the degree of hydration – on the performance of a cathode catalyst layer containing Ag nanoparticles supported on carbon is investigated, with the goal of establishing how and why these parameters affect the total current density and the partial current densities of H2 and CO. Chapter 4 delves deeper into more fundamental aspects of the Ag/ionomer interface, its chemistry/pH, and its effect on kinetic behavior, interfacial capacitance, catalyst-layer morphology, CO2 crossover, CO2R selectivity, and activity. Lastly, Chapter 5 takes a forward-looking approach and outlines key open questions and emerging challenges in CO2 electrochemical synthesis towards improved optimization and understanding for scale-up and technological deployment.