Exploring conductive, selective and stable polymeric membranes for redox flow batteries

Yan, YushanRedox flow batteries (RFBs) are currently considered as the most appealing energy storage options for the large-scale grid integration of the intermittent renewable energy sources such as solar and wind. Favorable features of RFBs include higher cell voltage, which leads to higher power and energy density and thus lower capital costs, and redox couples based on the same element, which allows easy crossover remediation. At present, zinc-cerium double membrane RFBs have the highest reported cell voltage in aqueous electrolytes, while vanadium RFBs (VRFBs) are the only commercially demonstrated systems with redox couples based on the same element. The goal of this thesis research is to explore highly conductive, selective and stable ion-exchange membranes (IEMs) for RFB systems. For Zn-Ce double membrane RFBs, the focus is the oxidative resistance, while for VRFBs, it is the conductivity. The work began with the examination of four commercial polymers for oxidation resistance, and among the four aromatic polymers examined, hexafluoroisopropylidene polybenzimidazole (F6PBI) showed the highest oxidative resistance in the cerium electrolyte. F6PBI was then functionalized to provide anion conductivity with tris(2,4,6- trimethylphenyl)phosphonium (TTMePP+ ) and tris(2,4,6- trimethoxylphenyl)phosphonium (TTMPP+ ). The tris(2,4,6- trimethoxylphenyl)phosphonium-functionalized polybenzimidazole (TTMPP-PBI) membrane exhibited a good balance of oxidation resistance and anion conductivity. Anion-exchange membranes (AEMs) are also excellent proton exchangers in polyprotic acid electrolytes. The TTMPP-PBI membrane was acid doped, and an excellent acid doping level was achieved due to the favorable acid-base interaction at the lone-pair nitrogen and oxygen sites, and cation-anion interaction at the positive cations. The high acid doping level led to low area specific resistance which was exploited in VRFBs for better voltage and energy efficiency compared with Nafion 115 and F6PBI membranes. An alternative pathway to functionalize the F6PBI was also explored by using methyl iodide to produce methylated hexafluoroisopropylidene polybenzimidazolium (DMF6PBI) membrane. The DMF6PBI membrane showed higher acid doping level and higher voltage and energy efficiency than F6PBI and Nafion 115 for VRFBs. However, the DMF6PBI membrane was not chemically stable, and the degradation was investigated for the future design of robust membranes.University of Delaware, Department of Chemical and Biomolecular EngineeringPh.D