Enhancements of the Synthesis and Fabrication of Anion Exchange Membranes
Open Access
- Author:
- Matta, Megan
- Graduate Program:
- Chemistry (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 06, 2023
- Committee Members:
- Ramesh Giri, Major Field Member
Elizabeth Elacqua, Major Field Member
Michael Hickner, Chair & Dissertation Advisor
Philip Bevilacqua, Program Head/Chair
Enrique Gomez, Outside Unit & Field Member - Keywords:
- Ion exchange membranes
Anion exchange membranes
AEM
IEM
Ionic Polymers
Fabrication of Anion Exchange Membrane
Fabrication of AEM
Ziegler-Natta Polymerization
AEM for ROMP
Quaternary Ammonium polymer - Abstract:
- Ion exchange membranes (IEMs) have emerged as important components in a wide variety of electrochemical processes and applications, playing a vital role in facilitating the selective transport of ions while preventing species crossover between the anolyte and catholyte. These versatile polymer or polymer composite membranes find uses in diverse fields such as energy conversion and storage, water treatment, chemical synthesis, and biotechnology. At the core of their functionality, ion exchange membranes exhibit a unique property known as "selective permeability." This property allows them to permit the controlled movement of specific ions across their structure based on differences in charge and size, while restricting the transport of other species. Diffusion of other molecules, such as water, can also be controlled simultaneously by optimizing the ion content, water uptake, and polymer structure of these materials. The design and synthesis of ion exchange membranes involve the incorporation of charged functional groups capable of exchanging ions with the surrounding solution. This functional group can be embedded in the backbone of the polymer, in an ionene-type structure, or attached to the backbone by tethered side chains. IEMs are broadly classified by the charge of the functional group into cation exchange membranes (CEMs) and anion exchange membranes (AEMs). CEMs have anionic functional groups and permit the transport of cations, whereas AEMs have cationic functional groups and permit the transport of anions. Bipolar membranes (BPM) are a type of composite IEM consisting of an AEM and CEM laminated together and an interesting topic of future research. Most AEMs produced today contain chemically unstable arylene ether backbones, have poor mechanical properties, and require toxic reagents for functionalization. This dissertation seeks to synthesize new anion exchange membranes that are stable in alkaline conditions while maintaining the high conductivities required for the operation of electrochemical devices. To develop a chemically stable aliphatic polyolefin AEM, a library of polymers synthesized via Ziegler-Natta polymerization was examined. This library consisted of quaternized poly(11-bromo-1-undecene-co-4-phenyl-2-butene) with degrees of functionalization of 20 mol% to 50 mol% — and was made utilizing a different catalyst system than previously explored. A TiCl3ˑAA catalyst was reacted with triisobutylaluminum (TiBA) co-catalyst to synthesize a more robust catalyst complex than the traditional AlEt2Cl catalyst. It was found that this new system could incorporate a higher amount of halogenated monomer (up to 50 mol %) on a multi-gram scale (50 g) at high yield without poisoning and deactivating the catalyst. The monomer incorporation was equal to the monomer feed and allowed for the targeting of specific ion exchange capacities. A large-scale synthesis (100 g) was conducted to assess the feasibility of pilot scale experiments. The dissolved polymers were solvent cast onto an ePTFE support and heterogeneously quaternized by immersing the alkylbromide-functionalized membrane in trimethyl amine solution. This procedure resulted in uneven wrinkled membranes, and a new method of casting supported membranes was subsequently developed, as described below. The hydroxide conductivity of the membranes was measured at room temperature, with conductivities up to 32 mS/cm reported. Given the issues with deformities in the supported membranes, further work focused on developing more efficient and consistent methods of membrane fabrication. It is speculated from initial experiments that high molecular weight polymer was being filtered from the sample during solution processing – decreasing the mechanical strength of cast membranes. The quaternary ammonium-functionalized polyolefins were cryomilled to retain high molecular weight polymer without aggregation. Casting membranes from the cryomilled powder increased the mechanical properties of the membrane such that the mechanical membrane support was no longer needed. Moreover, cryomilling facilitated bulk heterogenous quaternization of the polymer prior to membrane fabrication, reducing the amount of trimethylamine solution needed. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) measurements confirmed that the Menshutkin reaction went to completion and all bromine moieties were quaternized for all degrees of functionalization. The quaternized polymer resins were cryomilled to yield a free-flowing powder that could be used for AEM fabrication. The polymers were examined with thermogravimetric analysis, and the degradation temperature of the ammonium was found to be 215 °C in the chloride form. Suggesting that thermal processing, such as heat pressing, is a viable option for membrane fabrication. Environmentally benign ethanol as a solvent was utilized to disperse the powder and cast membranes unsupported. The resulting AEMs were smooth, with even appearance across the sample, and thin, with thicknesses ranging between 15 µm and 20 µm. Preliminary synthesis on a monomer for ring opening metathesis polymerization (ROMP) was attempted in an effort to examine the effects of well-defined backbone architecture on conductivity and chemical durability. The monomer, 3-(N,N’-dimethylpropyl-1-amine)-cyclooctene, was synthesized in high purity and characterized by proton NMR. Unfortunately attempts to polymerize the monomer via ROMP were unsuccessful as of the writing of this dissertation.