Date of Award

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Chair/Advisor

Eric M. Davis

Committee Member

Mark Roberts

Committee Member

Stephen Creager

Committee Member

David Bruce

Abstract

While vanadium redox flow batteries (VRFBs) have garnered significant attention as a promising scalable energy storage technology, the current membrane material used in this technology, Nafion™, suffers from high cross mixing of vanadium ions in the liquid electrolytes, directly impacting battery performance properties such as the voltage and coulombic efficiencies, as well as resulting in a reduction in the overall battery lifetime. The use of ionomer nancomposites has proven to be one promising route of directly addressing the issue of high ion crossover, though the mechanism by which the nanoparticles act to reduce crossover remains in open question. Herein, the vanadium ion (VO2+) dynamics within Nafion nanocomposites were investigated, to provide a better understanding for the tuned vanadium ion permeability. Further, the transport and structural properties of nanocomposites fabricated from a more cost effective ionomer, sulfonated poly(ether ether ketone) (SPEEK) were explored. In addition to traditional nanofillers like silica, the use of organic nanofillers obtained from renewable sources, in this case, lignin, were also explored. Specifically, silica nanoparticles (SiNPs), functionalized with both cationic (i.e., amine groups) and anionic (i.e., sulfonic acid groups) surfaces, as well as lignin of varying molecular weights were incorporated into the ionomer membranes. Finally, the concentration of SiNPs and lignin in the ionomer composite membranes were systematically varied, reaching concentrations as high as 10 mass % and 25 mass % for SiNPs and lignin, respectively.

For the case of ionomer nanocomposites formed from Nafion and SiNPs (Nafion–SiNP), ionomer nanocomposites synthesized via two different fabrication routes were explored. Specifically, Nafion–SiNP ionomer composites were fabricated via: (1) the solution-cast method using a dispersion of Nafion and preformed SiNPs and (2) an in situ sol-gel condensation process, whereby the silica phase is grown directly inside a preformed Nafion membrane. Using a quasi-elastic neutron scattering, the dynamics of hydrated vanadium ions (specifically the vanadyl ion, VO2+) within the Nafion–SiNP nanocomposites that were fabricated via the aforementioned two methods were captured. Notably, the VO2+ self-diffusion coefficient was seen to decrease with introduction of SiNPs, which was accompanied by an increase in the fraction of immobile hydrated vanadyl ions with increasing SiNP content, indicating constrained vanadium ion dynamics with incorporation of SiNPs, helping to explain the reduced vanadium ion permeability observed in these membranes.

As mentioned above, for the case of ionomer nanocomposites formed from SPEEK, we explored both inorganic SiNPs (SPEEK–SiNP), as well as a renewable biopolymer, lignin (SPEEK–lignin). For the case of SPEEK–lignin biocomposites, the impact of both lignin content (ranging from 5 mass % to 25 mass %) and lignin molecular weight (ranging from 5,470 g mol-1 to 34,500 g mol-1) on the final membrane structure and performance properties were studied. Most notably, the proton selectivities (i.e., the proton conductivity divided by the vanadyl ion permeability) of SPEEK–lignin membranes were observed to be as much as four-fold higher than that of both neat SPEEK and Nafion. In conjunction with the enhanced proton selectivity and changes in the ion exchange capacity, data from small-angle neutron scattering suggested that the introduction of lignin resulted in less tortuous, more interconnected channels for proton transport. Lastly, water transport and ionomer swelling kinetics of these SPEEK–lignin composites were characterized using time-resolve infrared spectroscopy, where a direct correlation between water diffusion and proton conductivity were observed. Results from these studies provide insight into the processing-structure-property relationships of ionomer nanocomposites, providing a set of design parameters for the design and synthesis of next-generation ionomer composites for use in redox flow batteries.

Author ORCID Identifier

0000-0001-6468-8279

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