Andrew R Motz

Quantum Mechanical Study of the Reaction Mechanism for 2π-2π Cycloaddition of Fluorinated Methylene Groups

Perfluorocyclobutyl polymers are thermally and chemically stable, may be produced without a catalyst via thermal 2π-2π cycloaddition, and can form block structures, making them suitable for commercialization of specialty polymers. Thermal 2π-2π cycloaddition is a rare reaction that begins in the singlet state and proceeds through a triplet intermediate to form an energetically stable four-membered ring in the singlet state. This reaction involves two changes in spin state and, thus, two spin-crossover transitions. Presented here are density functional theory calculations that evaluate the energetics and reaction mechanisms for the dimerizations of two different polyfluorinated precursors, 1,1,2-trifluoro-2-(trifluoromethoxy)ethane and hexafluoropropylene. The spin-crossover transition states are thoroughly investigated, revealing important kinetics steps and an activation energy for the gas-phase cycloaddition of two hexafluoropropene molecules of 36.9 kcal/mol, which is in good agreement with the experimentally determined value of 34.3 kcal/mol. It is found that the first carbon-carbon bond formation is the rate-limiting step, followed by a rotation about the newly formed bond in the triplet state that results in the formation of the second carbon-carbon bond. Targeting the rotation of the C-C bond, a set of parameters were obtained that best produce high molecular weight polymers using this chemistry.

V.C.2 Advanced Hybrid Membranes for Next Generation PEMFC Automotive Applications

• Show that heteropoly acid (HPA)-containing films can be fabricated thin and have a low area specific resistance (ASR) at the temperature of an automotive fuel cell stack and at higher temperatures likely to be operational transients whilst also functioning as an electrical resistor. • Increase HPA loading and organization for maximum proton conduction in a functionalized commercial fluoroelastomer manufactured by 3M.

Interplay Between Hydroxyl Density and Relaxations in Poly(vinylbenzyltrimethylammonium)-b-poly(methylbutylene) Membranes for Electrochemical Applications

Anion-exchange membranes (AEMs) consisting of poly(vinyl benzyl trimethylammonium)-b-poly(methylbutylene) of three different ion exchange capacities (IECs), 1.14, 1.64, and 2.03 mequiv g-1, are studied by High-Resolution Thermogravimetry, Modulated Differential Scanning Calorimetry, Dynamic Mechanical Analysis, and Broadband Electrical Spectroscopy in their OH- form. The thermal stability and transitions are elucidated, showing a low temperature Tg and a higher temperature transition assigned to a disorder-order transition, Tδ, which depends on the IEC of the material. The electric response is analyzed in detail, allowing the identification of three polarizations (only two of which contribute significantly to the overall conductivity, σEP and σIP,1) and two dielectric relaxation events (β1 and β2), one associated with the tolyl side groups (β1) and one with the cationic side chains (β2). The obtained results are integrated in a coherent picture and a conductivity mechanism is proposed, involving two distinct conduction pathways, σEP and σIP,1. Importantly, we observed a reordering of the ion pair dipoles which is responsible for the Tδ at temperatures higher than 20 °C, which results in a dramatic decrease of the ionic conductivity. Clustering is highly implicated in the higher IEC membrane in the hydroxyl form, which reduces the efficiency of the anionic transport.

Heteropoly acid functionalized fluoroelastomer with outstanding chemical durability and performance for vehicular fuel cells

To further facilitate commercialization of automotive fuel cells, durability concerns need to be addressed. Currently the addition of a mechanical support in the membrane is able to adequately solve issues of mechanical degradation, but chemical degradation via oxygenated radical attack remains an unsolved challenge. Typical mitigation strategies use cerium or manganese species to serve as radical scavengers, but these ions are able to migrate in the membrane and even leach out of the system. The approach used in this study is to covalently link and immobilize a heteropoly acid (HPA), more specifically 11-silicotungstic acid (HSiW11), a lacunary HPA of the Keggin structure to a fluoroelastomer, serving as both a radical decomposition catalyst and the proton conducting acid. This dual functionality allows for both high content of radical scavenging species and high ion-exchange capacity. An efficient three step, high yield (77%), commercially viable synthesis for this polymer is reported. The synthesis route for making this new heteropoly acid functionalized polymer is confirmed using infrared spectroscopy (IR), nuclear magnetic resonance (NMR) spectroscopy, and thermogravimetric analysis (TGA). The material exhibits clustering of the HSiW11 moieties, resulting in a poorly connected proton conducting phase when dry, but excellent conductivity is achieved at elevated humidities (0.298 S cm−1 at 80 °C and 95% RH). The proton conductivity shows an enhancement above 60 °C due to a softening of the polymer, as shown by DSC. Under an aggressive chemical accelerated stress test (AST), 90 °C, 30% RH, zero current, and pure O2, the PolyHPA losses only 0.05 V of open circuit voltage (OCV) after 500 h, greatly out performing any other material reported in the literature. For comparison, the Nafion® N211 fuel cell drops below 0.8 V after only 76 h under the same conditions. In fuel cell testing the PolyHPAs have outstanding chemical stability and also possess very low in situ high frequency resistance (HFR) leading to high performance (1.14 W cm−2 at 2 A cm−2), compared to 1.11 W cm−2 for the Nafion® N211 fuel cell at the same current. At 75 wt% HSiW11 loading, the fuel cell HFR showed a 22% decrease over N211.