Browsing by Subject "Polymer electrolyte membranes"
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Item Atomistic and molecular simulations of novel acid-base blend membranes for direct methanol fuel cells(2013-08) Mahajan, Chetan Vasant; Ganesan, VenkatOne of the main challenges to transform highly useful Direct Methanol Fuel Cells (DMFC) into a commercially viable technology has been to develop a low cost polymer electrolyte membrane (PEM) with high proton conductivity, high stability and low methanol crossover under operating conditions desirably including high temperatures. Nafion, the widely used PEM, fails to meet all of these criteria simultaneously. Recently developed acid-base polymer blend membranes constitute a promising class of PEMs alternative to Nafion on above criteria. Even though some of these membranes produce better performance than Nafion, they still present numerous opportunities for maximizing high temperature proton conductivity and dimensional stability with concomitant minimization of methanol crossover. Our contribution embarks on the fundamental study of one such novel class of blend membranes viz., sulfonated poly (ether ether ketone) (SPEEK)(95 % by weight) blended with polysulfone tethered with base (5 % by weight) such as 2-aminobenzimidazole (ABIm), 5-amino-benzotriazole (BTraz) and 1H-perimidine (PImd), developed by Manthiram group at The University of Texas at Austin. In this work, we report extensive all-atom classical as well as ab-initio molecular dynamics (MD) simulations of such water-methanol solvated blend membranes (as well as pure SPEEK and Nafion) the first time. Our approach consists of three steps: (1) Predict dynamical properties such as diffusivities of water, methanol and proton in such membranes (2) Validate against experiments (3) Develop understanding on the interplay between basic chemistry, structure and properties, the knowledge that can potentially be used to develop better candidate membranes. In particular, we elucidate the impact of simple, fundamental physiochemical features of the polymeric membranes such as hydrophilicity, hydrophobicity, structure or the size of the base on the structural manifestations on the bigger scale such as nanophase segregation, hydrogen bonding or pore sizes, which ultimately affect the permeant transport through such systems.Item Synthesis of cross-linked sulfonated polysulfone and mechanical properties of SPEEK-based membranes for direct methanol fuel cells(2011-05) Zieren, Shelley Marie; Manthiram, Arumugam; Meyers, Jeremy P.Direct methanol fuel cells (DMFC) are being investigated for use as low-power electrochemical energy conversion devices. These types of fuel cells can be useful for portable electronics. The polymer electrolyte membrane plays a critical role in the overall performance of DMFC. The commercially available membrane, Nafion, suffers from high methanol permeability and a resulting methanol crossover from the anode to the cathode; it is also expensive. Accordingly, alternative membrane materials, such as sulfonated hydrocarbons, are intensively pursued for DMFC. For example, sulfonated poly (ether ether ketone) (SPEEK) and sulfonated polysulfone (SPsf) are two such candidates. This thesis focuses first on a simple synthesis method for a cross-linked sulfonated polysulfone membrane. Sulfonated polysulfone (Psf) membranes, with high IEC (1.4 - 2.2 meq/g), were characterized by nuclear magnetic resonance spectroscopy (NMR), proton conductivity, and water uptake. The degree of sulfonation was calculated by NMR and verified by acid-base titration analysis. Although the membranes showed good proton conductivity, they suffered from excessive swelling at high temperatures. Furthermore, the post-sulfonation of a carboxyl-substituted polysulfone (Psf-COOH) was carried out with trimethylsilyl chlorosulfonate, and solubility issues of the Psf-COOH in chlorinated solvents led to difficulty in controlling the degree of sulfonation (DS) and in purification. Accordingly, this approach to cross-linking sulfonated polysulfone was rejected as a viable method. This thesis then focused on the investigation of the mechanical properties of acid-base blend membranes based on SPEEK and heterocycle-tethered Psf and cross-linked membranes based on SPEEK that were previously reported by our group; these membranes were known to exhibit good performance in DMFC. However, the assessment of the mechanical stability of any new membranes developed is critical for their practical viability in DMFC. Accordingly, the mechanical strength and ductility of these membranes were investigated and compared for various membrane compositions. The acid-base blend membranes investigated consisted of SPEEK (acidic polymer) and a heterocycle-tethered Psf (basic polymer); for example, blends consisting of SPEEK and amino-benzimidazole-tethered Psf (SPEEK/Psf-ABIm) and SPEEK and benzotriazole tethered Psf (SPEEK/Psf-Btraz) were investigated. The cross-linked SPEEK was made by Friedel-Craft acylation with Psf-COOH (DS = 1 or 2). The two blend membranes showed superior mechanical properties compared to Nafion 115 and comparable to plain SPEEK. The crosslinked membranes showed good mechanical properties and better strength than Nafion 115, but they were more brittle than both Nafion 115 and plain SPEEK. Further optimization of cross-linking conditions is necessary to produce the best performing membrane.