Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y6: Towards Medium Temperature Proton Conductors for Fuel Cell Applications |
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Sponsoring Units: FIAP Chair: Ali Sayir, NASA Glenn Research Center, Cleveland, Ohio Room: Colorado Convention Center 207 |
Friday, March 9, 2007 11:15AM - 11:51AM |
Y6.00001: On the question of ``dry'' proton motion in ionic liquids and plastic crystals Invited Speaker: In supercooled water containing 0.01MHCl, the ionic conductivity at -32$^{o}$C is more than an order of magnitude higher than would be predicted from its fluidity. The developing tetrahedral order and the associated high vib-librational anharmonicity, permits efficient ``dry'' proton ``hopping'' transfer of protons between favorable sites. Reproducing this transport mechanism in non-aqueous (and preferably also solid) phases is a leading aim of current research. We report progress in this direction substituting water by spinning protonated cations such as NH$_{4}^{+}$ and CH$_{3}$NH$_{3}^{+}$, and anions such as HSO$_{4}^{-}$, HPO$_{3}$F$^{- }$and H$_{2}$PO$_{4}^{-}$ for the water molecules, studying both liquid and plastic crystal phases. We use pulsed field gradient NMR to distinguish proton motion from host $^{31}$P species motion, and use double quantum techniques to study $^{1}$H{\ldots}$^{31}$P separation kinetics. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y6.00002: Polymic protic salt membranes, a new approach to the Holy Grail of a solid state proton conductor Invited Speaker: Electrons are readily transported in solids through the electronic conduction band in metals and semiconductors, but ion conduction is not as simple. Most proton conductors are aqueous solutions. Water plays two roles: i) water ionizes dissolved acids (and bases), and ionic conductivity results from the diffusion of protons and anions, a vehicular mechanism; ii) water also has an accessible~``proton hopping path'' (proton transport via hydrogen bonding and rotations) resulting in higher solution conductivity than by diffusion of ions alone. Some liquids, like phosphoric acid, have been know to conduct only protons with no co-transport of other species, but until recently these have been few in number. Some solids, like polyguanine, conduct only protons, but there have been even fewer reports of these. A proton transfer salt is an equimolar mixture of an acid and a base that internally transfer a proton. Recently, a number of proton transfer salts in the liquid state have been found that can transport proton without water, even at temperatures well above the boiling point of water. Whether a vehicular or hopping transport mechanism operates for these liquid proton transfer salts is under study. Vehicular transport is not possible in a solid membrane made for proton transfer salt formed from a solid polymer with one moiety (e.g., base) covalently fixed into the polymer and with the other moiety (e.g., acid) electrostatically bound after proton transfer. Synthetic strategies and characterization of solid proton conducting membranes, including solid protic transfer salt membranes, will be presented. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y6.00003: Polybenzimidazole-phosphoric acid complexes and proton conducting membranes Invited Speaker: Polybenzimidazole (PBI) polymers are excellent candidates for PEM fuel cell membranes capable of operating at temperatures up to 200$^{\circ}$C. The ability to operate at high temperatures provides benefits such as faster electrode kinetics and greater tolerance to impurities in the fuel stream. In addition, PBI membranes doped with phosphoric acid can operate efficiently without the need for external humidification and the related engineering hardware to monitor and control the hydration levels in the membrane. A new sol-gel process was developed to produce PBI membranes loaded with high levels of phosphoric acid. This process uses polyphosphoric acid as the condensing agent for the polymerization and the membrane casting solvent. Upon hydrolysis of the solvent to phosphoric acid, a sol-gel transition occurs to provide membranes with an attractive set of properties. PBI membranes are currently being investigated as candidates for portable, stationary, and transportation PEM fuel cell applications. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y6.00004: Protonic Conducting Ceramic for 300-400 \r{ }C Invited Speaker: Combining high protonic conductivity with thermodynamic stability is considered to be a key problem for high temperature protonic conducting ceramic (HTPC) membranes for electrochemical applications. The objective was to gain an understanding of the relationship of crystallite size, grain boundaries and defect chemistry on proton conduction and thermodynamic stability. We developed an analytical method using concurrent techniques of high-resolution transmission electron microscopy, impedance spectroscopy and nuclear microprobe to reveal spatial distribution of hydrogen. In our recent work, we shown that high-density of defects exist in the microstructure can reduce the level of proton incorporation. The results showed that hydrogen is concentrated at the grain boundaries where the hydrogen mobility is low. Perovskite structure BaCe$_{1-x}$Y$_{x}$O$_{3-\delta }$ (BCY) and BaZr$_{1-x}$Y$_{x}$O$_{3-\delta }$ films were deposited using pulsed laser deposition system on porous structures to provide mechanical strength. The ease with which the stoichiometry of a multi-component system can be maintained in the deposited films using pulsed laser deposition approach offered a significant advantage over other conventional film deposition techniques. Impedance spectroscopy was used to investigate protonic conductivity of high-density BaCe$_{1-x}$Y$_{x}$O$_{3-\delta }$ (BCY) and BaZr$_{1-x}$Y$_{x}$O$_{3-\delta }$ (BZY) films (2 - 5 $\mu $m). The crystallite size, grain boundaries and defect chemistry were characterized by XRD, SEM, TEM and HRTEM which showed the adverse effect of grain and domain boundaries. Variation of the process parameters, in particular of the substrate temperature, induced changes in the microstructure of the films and in their conductivity. Columnar grains enclosing reduced density of defects were seen to yield the best proton conductivities. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y6.00005: Understanding microstructure-induced limitations of hydrogen transport in high temperature proton conductors: can nuclear microanalysis give an answer? Invited Speaker: High temperature protonic conductors (HTPC) are envisioned as electrolytes for fuel cells working at intermediate temperature (400\r{ }C -- 600\r{ }C) to complement Y:ZrO$_{2}$ electrolytes operating at 800\r{ }C -- 1000\r{ }C. The most mature HTPC are doped perovskites (ABO$_{3})$ where tetravalent cation B is partially substituted by a trivalent one. Protons can be introduced in the lattice as point defects corresponding to hydroxyl groups on oxygen ion sites. In the temperature region of interest for technological applications, lattice vibrations allow the diffusion of protons by jumping and reorientation of O-H bonds (hoping mechanism). BaCeO$_{3}$ or SrCeO$_{3}$-based perovskites doped with a rare earth are the most widely studied compounds. However the proton conductance of these ceramics and their chemical stability are lower than the calculated values on single crystals and not sufficient to fulfill technological requirements. In most cases, the reasons for these discrepancies lie in uncontrolled microstructures with inter- and intra-granular defects that act as barriers for hydrogen diffusion but are preferential paths for chemical degradation by hydrolysis or carbonatation. Despite this crucial point, very few efforts are devoted to the optimization of microstructure of HTPC. Microstructure induced limitations are usually evidenced via impedance measurements which enable determination of respective contributions of bulk and grain boundaries to overall conductivity. Further information on hydrogen transport relevant for improvement of microstructure design requires local methods for hydrogen concentration measurement. Nuclear microanalysis, based on the use of MeV light ions microbeam, meets this demand. According to the chosen technique, nuclear reaction, elastic recoil or forward coincident scattering, the nuclear microprobe gives 2D-3D quantitative information on hydrogen distribution and diffusion within microstructure and enables to identify barriers and short-circuits. [Preview Abstract] |
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