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Proton Diffusion (proton + diffusion)
Selected AbstractsDensity and Temperature Dependence of Proton Diffusion in Water: A First-Principles Molecular Dynamics StudyCHEMPHYSCHEM, Issue 9 2005Mauro Boero Prof. New insights into proton propagation: The authors present a detailed report on the diffusion of a proton in different thermodynamic states of water. At high temperature and intermediate density, the proton,which spends more time as an Eigen complex than as a Zundel complex (see figure),is destabilized, its migration is faster, and an incomplete Eigen complex acts as an acid with high chemical reactivity. [source] NMR Studies of Proton Transport in Anhydrous Polymer Electrolytes for High Temperature Fuel Cells,FUEL CELLS, Issue 3-4 2008H. A. Every Abstract This paper presents an NMR study of the dynamic processes related to proton transport in a new polymer consisting of two blocks , poly(2,6-diphenylphenol) (P3O) and an imidazole functionalised poly(2,6-dimethylphenol) (imi-PPE) , and subsequently doped with polyphosphoric acid (PPA). From 1H and 31P NMR relaxation and diffusion measurements of the individual homopolymers and block copolymer, it was observed that addition of PPA significantly enhanced the mobility of imi-PPE and the imi-block copolymer, but not of P3O. The similarity in 1H T2 values between imi-PPE and the imi-block copolymer suggests that the relaxation behaviour in the block copolymer is dominated by the imi-PPE phase. 1H diffusion in P3O and the imi-block copolymer were comparable to pure PPA, suggesting that the proton diffusion is similar in each case. For imi-PPE, the diffusion coefficients were several orders of magnitude lower, reflecting a restricted diffusion process that is not indicative of the proton mobility. For all three polymers, the 31P T2 relaxation behaviour and inability to measure 31P diffusion coefficients imply hindered translational motion of the phosphonate groups. From these results, it can be concluded that hydrogen bonds between the phosphoric acid and the polymer form a network that facilitates proton transport via a hopping mechanism. [source] Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized PolysiloxaneFUEL CELLS, Issue 3-4 2006G. Scharfenberger Abstract Intrinsically proton conducting polymers with imidazole as proton solvent tethered to a polysiloxane backbone via a flexible spacer have been synthesized. Apart from the standard characterization also their thermal properties and transport behavior have been investigated. The materials exhibit proton conductivity as a consequence of self-dissociation of the imidazole moieties and "structure diffusion" of the resulting defects. In particular, no liquid phase such as water or monomeric imidazole is needed for the observed proton conductivities. To study the influence of the tether structure on the transport properties, cyclic oligomers and open chain polymers with different spacer lengths have been synthesized. The materials are thermally stable up to 200,°C and become soft around room temperature. The conductivity exhibits VTF and WLF behavior with maximum conductivities around ,,=,1.5.10,3,S,cm,1 at T,=,160,°C. The activation volume of the conductivity as derived from pressure dependent measurements is found to be unusually high. The lowest activation volumes and the highest conductivities are observed for the materials with the highest segmental mobilities, i.e. the longest spacers. Proton self-diffusion coefficients as obtained from PFG NMR diffusion measurements are significantly higher than expected from the proton conductivities obtained by dielectric spectroscopy. This corresponds to unusually high Haven ratios which have been interpreted by correlated proton transfers allowing for fast proton diffusion while minimizing the separation of ionic charge carriers. [source] The effect of water content on proton transport in polymer electrolyte membranesFUEL CELLS, Issue 3-4 2002P. Commer Abstract We investigate proton transport in a polymer electrolyte membrane using continuum theory and molecular dynamics (MD) computer simulations. Specifically our goal is to understand the possible molecular origin of the effect of water content on the activation energy (AE) and pre-exponential factor of proton conductivity, in comparison with experimental observations reported for Nafion, where a decrease of AE with increasing water content has been observed. We study proton diffusion in a single pore, using a slab-like model. We find that although the average proton diffusion coefficient is several times smaller in a narrow pore than in a wide water-rich pore, its AE is almost unaffected by the pore width. This contradicts an earlier proposed conjecture that the sizable Coulomb potential energy barriers near the lattice of immobile point-like SO3, groups increase the AE in a narrow pore. Here we show that these barriers become smeared out by thermal motion of SO3, groups and by the spatial charge distribution over their atoms. This effect strongly diminishes the variation of the AE with pore width, which is also found in MD simulations. The pre-exponential factor for the diffusion process, however, decreases, indicating a limited number of pathways for proton transfer and the freezing out of degrees of freedom that contribute to the effective frequency of transfer. Decreasing the pore size diminishes bulk-like water regions in the pore, with only less mobile surface water molecules remaining. This hampers proton transfer. The increase of AE takes place only if the thermal motion of the SO3, head groups freezes out simultaneously with decreasing water content, but the effect is not profound. The stronger effect observed experimentally may thus be associated with some other rate-determining consecutive process, concerned with polymer dynamics, such as opening and closing of connections (bridges) between aqueous domains in the membrane under low water content. [source] Transport Processes at ,-Quartz,Water Interfaces: Insights from First-Principles Molecular Dynamics SimulationsCHEMPHYSCHEM, Issue 7 2008Waheed A. Adeagbo Dr. Abstract Car,Parrinello molecular dynamics (CP,MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the ,-quartz,water interface. The model system initially consists of a periodically repeated quartz slab with O-terminated and Si-terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H2O molecules into H+ and OH, is observed at the Si-terminated surface. The OH, fragments immediately bind chemically to the Si-terminated surface while Grotthus-type proton diffusion through the water film leads to protonation of the O-terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO2 present as Si(OH)4 in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)4 molecule as would have been indicated by a substantial lowering of the Si(OH)4 diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP,MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two SiO bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new SiO bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz SiO bond. The final solute species is Si(OH)4. [source] Copolymerization of Divinylsilyl-11-silicotungstic Acid with Butyl Acrylate and Hexanediol Diacrylate: Synthesis of a Highly Proton-Conductive Membrane for Fuel-Cell ApplicationsCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 3 2009James Abstract Highly conducive to high conductivity: Polyoxometalates were incorporated in the backbone of a hydrocarbon polymer to produce proton-conducting films. These first-generation materials contain large, dispersed clusters of polyoxometalates. Although the morphology of these films is not yet optimal, they already demonstrate practical proton conductivities and proton diffusion within the clusters appears to be very high. [source] | ||||||||||