Search

The proton-conducting hydrated alkali thio-hydroxogermanate's MxGeSx(OH)(4) - x center dot yH(2)O (M = Na and K; x = 1 - 4,y approximate to 0.5 - 2) were investigated by means of neutron diffraction with the aim to elucidate how the structure changes as a function of alkali-ion concentration, x, type of alkali ion, M, and water content, y. For x = 1 - 3 we find that the materials are amorphous and composed of thio-hydroxogermanate anions, water molecules, and charge balancing alkali ions, whereas for x = 4 we find that the materials contain also a crystalline phase, suggesting that it is difficult to prepare purely amorphous materials for the highest alkali-ion concentration, for both the Na and K based materials. For x = 1 - 3, the structure is reflected by an intermediate-range ordering, with a characteristic length-scale ranging from approximately 6 to 9 angstrom, which is dependent on both x and M and which may be related to the separation distance between dimers of thio-hydroxogermanate anions. As x increases, the intermediate-range ordering shortens, possibly as the result of an increasing level of hydration water that may act as a dielectric medium that reduces the repulsive interaction between the negatively charged thio-hydroxogermanate anions and/or between the positively charged alkali ions. A comparison of the structural results to the reported conductivities of the same materials indicates a non-trivial relationship, which depends on both the type and concentration of alkali ions, as well as on the level of hydration water.

Carbon-coated lithium iron phosphate (LiFePO4/C) with uniform carbon coating was synthesized by a mechanical activation method. Silicon negative electrode material was obtained in the form of thin films of amorphous silicon on a Cu foil substrate by vertical deposition technique. The electrochemical performance of the full cell, LiFePO4/C-Si, was tested with 1 M LiPF6 in EC/DMC at 0.5 and 1 C-rates. The cell exhibited an initial discharge capacity of 143.9 mAh g(-1) at 0.5 C-rate at room temperature. A reasonably good cycling performance under a high current density of 1 C-rate could be obtained with the full cell.

The potential of LiFePO4 as cathode material has not been fully exploited due to its intrinsic poor electronic and ionic conductivities. Attempts have been made to improve these properties of which coating of the active carbon on the particle surface is the most viable method so far. Phase-pure LiFePO4 and two LiFePO4/C composites were synthesized by mechanical activation process employing two different methods: (i) direct addition of acetylene black carbon and (ii) addition of sucrose as carbon precursor. The samples were well characterized by various techniques like elemental analysis, Brunauer-Emmett-Teller (BET) method, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and Raman spectroscopy to establish their composition, morphology, particle size and surface area. The structure of these samples is investigated as olivine structure space group Prima by X-ray powder diffraction. Transmission electron microscopy (TEM) confirms that the carbon nanocoating on the LiFePO4 particles has no visible dislocations and fractures. The electrochemical performance of LiFePO4/C is significantly affected by the nature of the carbon nanocoating, which in turn is affected by the choice of synthesis method.

A series of gel polymer electrolytes (GPEs) based on electrospun membranes of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-co-HFP)] incorporating room temperature ionic liquids (RTILs), 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide complexed with lithium bis(trifluoromethylsulfonyl) inside (LiTFSI) as electrolytes have been prepared and their fundamental electrochemical properties were investigated. The morphology of electrospun membranes was examined by a field emission scanning electron microscope (FE-SEM). The membranes show uniform morphology with an average fiber diameter of 780 nm, high porosity and high electrolyte uptake. GPEs were prepared by soaking the electrospun membranes in 1 M LiTFSI in RTILs for 1 h and exhibit a high ionic conductivity of 2.4 x 10(-3)-4.5 x 10(-3) S cm(-1) at 25 degrees C. A Li/GPEs/LiFePO4 cell using these RTILs delivers high discharge capacity (similar to 140 mAh g(-1)) when evaluated at 25 degrees C at 0.1 C-rate and exhibits a very stable discharge capacity under continuous cycling. Among the GPEs, EMITFSI shows the highest electrochemical properties although the solid electrolyte interface (SEI) layer was not formed.

1H NMR was performed on the ceramic proton conductor (NaS)xGe(OH)4 − x·yH2O for x = 2, 3 and y ~ 1–4 over the temperature range of − 100 °C to 400 °C. NMR spectra, nuclear spin–lattice relaxation rates (R1), spin–spin relaxation rates (R2), and rotating frame relaxation rates (R1ρ) were obtained for both hydrated and dehydrated samples. It was found that the low temperature NMR spectra were consistent with simulated H2O lineshapes, and the presence of H3O+ at low temperature was not detected within the experimental uncertainty. Short- and long-range hydrogen motion occurred together, a signature of Grotthuss conduction mechanisms. The long range motion was a combination of H+ and H2O diffusion, with H2O motion limiting H+ dynamics as the onset temperature was approached.

The short-range structures of hydrated and dry samples of the proton-conducting, cubic-structured, acceptor-doped perovskites BaM0.10Zr0.90O2.95 (M=Y, In, Sc and Ga), and, for comparison, BaZrO3, have been investigated using infrared and Raman spectroscopy. The results show that the acceptor-doping leads to local structural distortions of the average cubic structure, most likely related to tilts of (M/Zr)O-6 octahedra, and that the distortions are most pronounced in the Ga-doped material for which we also find additional distortions upon hydration. For the Y-, In- and Sc-doped equivalents the distortions are somewhat weaker and hydration is found to have a reversed, although weak, effect. A comparison of the spectroscopic results to the reported proton diffusivities of the investigated materials would suggest that the much lower proton diffusivity in the Ga-doped material is associated to its stronger structural distortions. (C) 2009 Elsevier B.V. All rights reserved.

Proton motions in hydrated proton conducting perovskites BaZr(0.90)A(0.10)O(2.95) (A = Y and Sc) have been investigated using quasielastic neutron scattering. The results reveal a localized motion on the ps time scale and with an activation energy of similar to 10-30 meV, in both materials. The temperature dependence of the total mean square displacement of the protons shows an onset of this motion at a temperature of about 300 K. The low activation energy, much lower than the activation energy for the macroscopic proton conductivity, suggests that this motion is not the rate-limiting process for the long-range proton diffusion, i.e. it is not linked to the two materials significantly different proton conductivities. In fact, a comparison of the QENS results with density functional theory calculations indicates that for both materials the observed motion may be ascribed to intra-octahedral proton transfers occurring close to a dopant atom. (C) 2008 Elsevier B.V. All rights reserved.

Traditional solid state sintering has been used to prepare the perovskite BaZr0.9Yb0.1O3-delta. Analysis of X-ray powder diffraction data shows that an increase of the unit cell parameter, a, was observed compared to undoped BaZrO3. Rietveld analysis of room temperature neutron powder diffraction data confirmed cubic symmetry (space group Pm-3m). Dynamic thermogravimetric analysis (TGA) indicates that the hydration process occurs below 200 degrees C. The strong O-H stretch band, 2500-3500 cm(-1), in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity was investigated on hydrated and as-prepared samples under dry and wet atmospheres, respectively. Keywords: X-ray diffraction; neutron diffraction; Rietveld refinement; proton conductor; perovskite

Polymer composite proton conducting membranes with increasing amount of tungstophosphoric acid (PWA) have been synthesized starting from poly(vinylidene) fluoride and alumina. Membranes could be prepared with an acid loading up to 16.7 wt.%. The conductivity of the membranes increases with acid loading up to 10- 3 Scm- 1 at the highest loading. The membrane matrices have a good thermal stability up to 240 °C, however a loss of water absorbed during the preparation procedure is observed at a considerably lower temperature. Raman spectroscopy showed that in the membrane the polymer adopts a conformation that could promote a separation into hydrophilic/hydrophobic sites. An interaction between alumina and PWA is also found that might limit the conductivity of the membranes.

The structure of Cs 2 GeS 2 (OH) 2 · y H 2 O in hydrous and anhydrous states has been investigated by means of neutron diffraction and infrared spectroscopy. This material belongs to a new class of intermediate temperature highly proton conducting alkali thio-hydroxogermanates. The results show that this material is amorphous, and that both the short and intermediate range structure of the hydrated and dehydrated states of the materials are largely the same. Moreover, the results point to a porous structure built up of thio-oxoanions and alkali ions. In the hydrated state, the water molecules appear to be confined to cages with a characteristic length of 7 A.

The proton conductivity and structural features of In3+ substituted BaZrO3 samples, i.e., BaZr1−xInxO3−δ, were investigated. Rietveld analysis of low temperature (10 K) neutron powder diffraction data collected on as-prepared and deuterated samples confirmed cubic symmetry (space group Pm-3m) for all compositions. The level of oxygen vacancies refined in the as-prepared samples were in good agreement with the values expected to conserve charge neutrality, whilst an increase in oxygen occupancy, reflecting the incorporation of OD− species, was obtained for the deuterated materials. An expansion of the unit cell parameter, a, was observed as a function of In3+ doping as well as after the deuteration reaction. The conductivity of pre-hydrated and dry samples was measured using impedance methods. For 25% In-doped BaZrO3, the low T (300 °C) conductivity of the heating cycle of the dried sample was greater than that of the cooling cycle of the pre-hydrated sample indicating a greater number of protons in the nominally dry sample. In contrast, the conductivity values were similar at higher temperatures e.g. T > 500 °C where proton conduction is not dominant.

Solid state sintering has been used to prepare the cubic perovskite structured compounds BaZr1−xInxO3−δ (0.0 ≤ x ≤ 0.75). Analysis of X-ray powder diffraction (XRPD) data reveals that the unit cell parameter, a, increases linearly with an increased Indium concentration. XRPD data was also used to demonstrate the completion of sample hydration, which was reached when the materials showed a set of single-phase Bragg-peaks. Dynamic thermogravimetric analysis (TGA) data showed that approx. 89% of the total number of available oxygen vacancies can be filled in BaZr1−xInxO3−δ for x = 0.50, and that the maximum water uptake occurs below 300 °C. Rietveld analysis of the room temperature neutron powder diffraction (NPD) data confirmed the average cubic symmetry (space group Pm-3m), and an expansion of the unit cell parameter after the hydration reaction. The strong O–H stretch band, 2500–3500 cm− 1, in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity of hydrated BaZr1−xInxO3−δ, x = 0.75 was investigated during heating and cooling cycles under dry argon atmosphere. The total conductivity during the heating cycle was nearly two orders of magnitude greater than that of cooling cycle at 300 °C, whilst these values were similar at higher temperatures i.e. T > 600 °C.

We have investigated the thermal stability and reversibility upon dehydration and re-hydration of a novel class of proton conducting alkali thio-hydroxogermanates. The results indicate that no phase transitions or structural degradation occur in the temperature range 25 to 300 °C, and that repeated dehydration and subsequent re-hydration is a reversible process. The dehydration occurs gradually with increasing temperature, starting at about 80 °C. For temperatures above 180 °C the materials are dry, as all molecular water has been dried off. The dehydration process is shown to be reversible and the material can be rehydrated by exposure to air. The thermal stability and reversibility of the dehydration–rehydration process are attractive properties of functional materials, making the proton conducting alkali thio-hydroxogermanates to potential fuel cell electrolytes.

Raman and infrared spectroscopy results of protonated and dry samples of the proton conducting double perovskites Sr 3 CaZr 0.5 Ta 1.5 O 8.75 and Ba 3 Ca 1.18 Nb 1.82 O 8.73 (BCN18) are presented. As confirmed by the Raman spectra, the incorporation of protons leads to only small distortions in the host lattice of the perovskites. However, the intensity growth of a band at approximately 650 cm − 1 in the Raman spectrum of Sr 3 CaZr 0.5 Ta 1.5 O 8.75 after protonation may be related to a symmetry reduction towards a monoclinic structure. The O–H stretch region in the infrared spectra of Sr 3 CaZr 0.5 Ta 1.5 O 8.75 and BCN18 are quite different. Stronger bands in the 2000–2500 cm − 1 range in Sr 3 CaZr 0.5 Ta 1.5 O 8.75 may be related to the higher concentration of oxygen vacancies in this material after protonation.

The structures of ion conducting mixed-alkali metaphosphate glasses (A 2 O) x –(B 2 O) 1− x –P 2 O 5 (A,B=Li,Na,Rb) have been examined by neutron diffraction and Raman spectroscopy. The results show that the short range order of the phosphorous–oxygen network is almost independent of the composition. The length-scale of the intermediate range order correlations increases with increasing size of the cation. For all the investigated compositions the nearest Li–O, Na–O and Rb–O distances have been estimated from the atomic pair correlation functions to be 2.0±0.1, 2.35±0.1 (Na–O I), 2.6±0.1 (Na–O II) and 2.9±0.1 A, and the corresponding average coordination numbers are approximately 4, 3 (I)+2.5 (II) and 5, respectively. Thus, the cation environments are distinctly different for Li + , Na + and Rb + and each of them remain the same for all compositions within experimental errors. The alkali ions are quasi-uniformly distributed and we were not able to observe any significant preference for either similar or dissimilar pairs. This implies that the mixed-alkali effect can be qualitatively understood on a purely structural basis. The A sites are energetically unfavourable to the B + ions (and vice versa) and jumps to dissimilar sites are then unlikely to occur. Thus, the A sites will have the tendency of blocking the pathways for the B + ions and vice versa.