Membrane-Electrolyte System Studies for Aqueous Redox Flow Reactors

Abstract

Electrochemical flow reactors are promising for electrochemistry at scale. Reactants are flowed continuously through typically porous electrodes where redox occurs, and ions transport through the electrolyte and typically at least one ion exchange membrane to provide current in the electrochemical circuit and balance charge. Ion exchange membranes are made of charged polymers that take up solvent to create ionically conductive pathways while blocking bulk mixing of adjacent electrolyte solutions. This thesis examines a crucial interface in electrochemical flow reactors, where the membrane and contacting electrolyte interact and exchange chemical species, and we find that the interaction of membrane and electrolyte governs the structure and transport of the membrane phase, which affects device scale performance. In redox flow batteries, the membrane must block crossover of reactants while enabling high conductivity. A combination of reactant size and charge effects influence permeation rates through charged membranes, with charge exerting especially strong influence under dilute conditions. The overall concentration and composition of the battery electrolyte influences the membrane hydration and hence transport, and we use conclusions from a systematic study of these effects to design crossover-free membrane-electrolyte systems with both commercial and novel membranes. In bipolar membranes, we find that the composition of impure strong electrolytes affects the local composition at the bipolar junction, which determines open circuit voltage and polarization behavior. Finally, we involve a series of ion exchange membranes including a bipolar membrane in a redox electrodialysis process for pH-driven separations, and untangle the concentration- and current-dependent membrane transport phenomena that limit the process efficiency.