Ion Management And Mass Transport For (photo-) Electrochemical Conversions
(Photo-) electrochemistry hold great potential for storing the surplus energy of renewably generated electrons in the form of energy-dense chemicals that can be stored for long periods of time at low cost. The efficiency of converting between electrical- and chemical- energy depends on the charge transfer kinetics at the anode and cathode, as well as ion and mass transport; the latter two have been shown to cause energy losses comparable to those of the electrode charge transfer processes. The first three chapters of the thesis explore the use of bipolar membranes (BPMs) for managing ion transport in CO2 electrolysis, fuel cells, and redox flow batteries, while focusing on understanding the fundamental aspects of BPMs. Chapter 1 introduces the potential of using electrolysis for renewable energy storage and summarizes recent progress in BPM research. In Chapter 2, we combine electrochemical impedance spectroscopy and finite element method based numerical modeling to elucidate the relation between the electric field and interfacial catalysis in enhancing water dissociation reaction in BPMs. Chapter 3 presents the results of managing interfacial protons in a BPM-based CO2 electrolyzer. The acidic local environment at the membrane/catalyst interface facilitates the competing hydrogen evolution reaction and leads to a low CO2 reduction efficiency. This problem was mitigated by coating the membrane with a weak acid polyelectrolyte film of ~ 50 nm, the local pH within which was monitored using ratiometric pH indicators covalently attached to the polyelectrolyte. Chapter 4 explores the forward-biased BPM in a redox flow battery that operates the positive/negative electrode in an alkaline/acidic environment. This unique configuration enables a battery potential that is ~ 0.7 V higher than the conventional ones using a single pH condition. The acid-base recombination reaction was found to be inefficient in forward-biased BPMs, being rate-limited by the narrow reaction zone in the junction region. In chapter 5, we designed a novel architecture for the catalyst layer in alkaline fuel cells, which allows for a better control of the microstructure and thus the study of mass transport in a membrane electrode assembly (MEA) configuration. The last chapter summaries the thesis and proposes future directions.