Judges’ Queries and Presenter’s Replies

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Presentation Discussion

  • May 21, 2012 | 11:02 p.m.

    Nice presentation.

    The basic question I have is why did you choose Meta Bis Acid, Ortho Bis Acid, and PFIA as the “test” molecules for your model. Also, although you nicely explained the behavior of these molecules, what was the basic objective of the study or hypothesis?

    Thanks.

  • Icon for: Jeffrey Clark

    Jeffrey Clark

    Presenter
    May 23, 2012 | 06:32 p.m.

    Thank you very much. The meta bis acid, ortho bis acid, and PFIA ionomers are currently developed by 3M with whom I collaborate. Currently the most widely used proton exchange membrane ionomer for fuel cell application is Nafion which only exhibits high proton conductivity at high levels of hydration which places restrictions and adversely affects various aspects of the fuel cell. As such, immense efforts have been made in the development of membrane materials that show high proton conductivity at low levels of hydration. The materials in this study have been experimentally shown to exhibit higher proton conductivity at lower hydration levels than Nafion, but the exact molecular-level features that contribute to the improved proton mobility in these materials are not entirely understood. Since few experimental techniques are capable of resolving information such as the formation and breaking of hydrogen bonds in these highly complex systems, ab initio molecular modeling techniques have been extensively employed in attempts to uncover the underlying characteristics contributing to proton mobility. The primary objective of this study was to provide some fundamental relative insight into how protogenic group separation, specific side chain chemistry, and local hydration have in facilitating proton dissociation and transfer within materials of this type.

    Thank you
    Jeffrey

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Icon for: Jeffrey Clark

JEFFREY CLARK

University of Tennessee at Knoxville
Years in Grad School: 3

Proton Dissociation and Transfer in PEM Ionomers with Multiple and Distinct Pendant Acid Groups: An Ab Initio Study

Proton Exchange Membrane (PEM) fuel cells offer tremendous potential as clean alternative energy conversion systems. Critical to these devices is the proton-conducting polymer electrolyte membrane. A molecular-level understanding of the factors that contribute to proton dissociation and transport in PEMs at low hydration levels is of vital importance in the development of novel proton conducting electrolyte materials for fuel cell application. Ab initio electronic structure calculations were performed to study the effects local hydration, protogenic group separation, and specific side chain chemistry have in facilitating proton dissociation and transfer in fragments of 3M bis(sulfonyl imide)-based PEM ionomers under conditions of low hydration. Three ionomers containing multiple acid groups per pendant side chain with structural and chemical differences mediating protogenic group separation were considered (side chains: –O(CF2)4SO2(NH)-SO2C6H4SO3H) with the sulfonic acid group located in either the meta or the ortho position on the phenyl ring and –O(CF2)4SO2(NH)SO23SO3H). Fully optimized structures of these fragments with the addition of water molecules revealed that both side chain chemistry and protogenic group separation are key contributors to proton dissociation and the energetics of proton transfer in these materials. Specifically, cooperative interaction between protogenic groups through hydrogen bonding and electron withdrawing –CF2– groups were found to be critical for first proton dissociation and the state of the dissociated proton at low levels of hydration. These factors, along with the surrounding hydrogen bond network, were also observed to have strong influence on the energetic penalty associated with proton transfer.