Browsing by Author "Wagemaker, M"

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    Direct view on nanoionic proton mobility
    (Wiley-Blackwell, 2011-04-22) Chan, WK; Haverkate, LA; Borghols, WJH; Wagemaker, M; Picken, SJ; van Eck, ERH; Kentgens, APM; Johnson, MR; Kearley, GJ; Mulder, FM
    The field of nanoionics is of great importance for the development of superior materials for devices that rely on the transport of charged ions, like fuel cells, batteries, and sensors. Often nanostructuring leads to enhanced ionic mobilities due to the induced space-charge effects. Here these large space-charge effects occurring in composites of the proton-donating solid acid CsHSO4 and the proton-accepting TiO2 or SiO2 are studied. CsHSO4 is chosen for this study because it can operate effectively as a fuel-cell electrolyte at elevated temperature while its low-temperature conductivity is increased upon nanostructuring. The composites have a negative enthalpy of formation for defects involving the transfer of protons from the acid to the acceptor. Very high defect densities of up to 10% of the available sites are observed by neutron diffraction. The effect on the mobility of the protons is observed directly using quasielastic neutron scattering and nuclear magnetic resonance spectroscopy. Surprisingly large fractions of up to 25% of the hydrogen ions show orders-of-magnitude enhanced mobility in the nanostructured composites of TiO2 or SiO2, both in crystalline CsHSO4 and an amorphous fraction.© 2011, Wiley-Blackwell. The definitive version is available at www3.interscience.wiley.com
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    Dynamic solubility limits in nanosized olivine LiFePO(4)
    (American Chemical Society, 2011-07-06) Wagemaker, M; Singh, DP; Borghols, WJH; Lafont, U; Haverkate, LA; Peterson, VK; Mulder, FM
    Because of its stability, nanosized olivine LiFePO(4) opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO(4). In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties. © 2011, American Chemical Society
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    Hydrogen in porous tetrahydrofuran clathrate hydrate
    (Wiley-VCH Verlag Berlin, 2008-06-23) Mulder, FM; Wagemaker, M; van Eijck, L; Kearley, GJ
    The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier.([1]) Storage within solid-state clathrate hydrates,([2-4]) and in the clathrote hydrate of tetrohydrofuran (THF), has been recently reported.([5,6]) In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that-in contrast to previous reports([5,6]) - hydrogen (deuterium) occupies the small cages of the clathrote hydrate only to 30% (at 274 K and 90.5 bar). Such a D-2 load is equivalent to 0.27 wt. % of stored H-2. In addition, we show that a surplus of D2O results in the formation of additional D2O ice Ih instead of in the production of sub-stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. [6]). Structure-refinement studies show that [D-8]THF is dynamically disordered, while it fills each of the large cages of [D-8]THF center dot 17D(2)O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above approximate to 220 K, the H-storage characteristics of the clathrate hydrate have similarities with those of surface-adsorption materials, such as nanoporous zeolites and metal-organic frameworks,([7,8]) but at lower temperatures, the adsorption rates slow down because of reduced D-2 diffusion between the small cages. © 2008, Wiley-VCH Verlag Berlin