| 1. |
- Schantz Zackrisson, Anna, 1973, et al.
(författare)
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Concentration effects on irreversible colloid cluster aggregation and gelation of silica dispersions.
- 2006
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Ingår i: Journal of colloid and interface science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 301:1, s. 137-44
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Tidskriftsartikel (refereegranskat)abstract
- Effects of particle concentration on the irreversible aggregation of colloidal silica are studied using in situ destabilization via the ionic strength increase derived from the enzymatic hydrolysis of urea by urease. Aggregation is monitored by time-resolved optical density and dynamic light scattering measurements. It terminates at a gel boundary, signaled by a prominent increase of the optical density and incipient non-ergodicity. Raman scattering is used to demonstrate that the enzymatic reaction continues, well beyond gelation for the compositions studied here, until the urea is consumed. Calibration of the ionic conductivity permits for constructing stability diagrams in terms of particle and salt concentration. As with reversible gelation, the process exhibits a collective character in that lower ionic strengths are required for gelation of concentrated dispersions and vice versa. However, light scattering demonstrates that the gel boundary is preceded here by a line marking the transition from reversible to irreversible cluster formation, with the two transition boundaries tracking each other. Comparisons are made with dispersions destabilized by direct addition of salt solutions, which gel under very different conditions.
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| 2. |
- Holomb, Roman, et al.
(författare)
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Ionic liquid structure: the conformational isomerism in 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4])
- 2008
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486 .- 1097-4555. ; 39:7, s. 793-805
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Tidskriftsartikel (refereegranskat)abstract
- As a probe of local structure, the vibrational properties of the 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] ionic liquid were studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four [bmim]+ conformers (GG, GA, TA, and AA) at room temperature was established through unique spectral responses. The Raman modes characteristic of the two most stable [bmim] + conformers, GA and AA, according to the ab initio calculations, increase in intensity with decreasing temperature. To assess the total spectral behavior of the ionic liquid both the contributions of different [bmim] + conformers and the [bmim]+- [BF4]- interactions to the vibrational spectra are discussed.
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| 3. |
- Martinelli, Anna, 1978, et al.
(författare)
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A structural study on ionic-liquid-based polymer electrolyte membranes
- 2007
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 1945-7111 .- 0013-4651. ; 154:8, s. G183-G187
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated novel proton conducting membranes synthesized through the gelification of poly(vinylidene fluoride-co-hexafluoropropylene) in aprotic ionic liquids. Mobile protons were introduced by doping the system with the strong bis(trifluoromethanesulfonyl)imide acid (HTFSI), which is chemically compatible with the ionic liquids through the common TFSI- anion. The obtained membranes are thermally stable up to 115°C set by the melting of the polymer phase. At this temperature, the conductivity is on the order of 10-2 S cm-1. Raman and infrared spectroscopy show no chemical interactions between the components, indicating that the bulklike nature of the doped ionic liquids preserved within the membrane, as is the thermal stability and the high conductivity. © 2007 The Electrochemical Society.
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| 4. |
- Martinelli, Anna, 1978, et al.
(författare)
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A study on the state of PWA in PVDF-based proton conducting membranes by Raman spectroscopy
- 2007
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 178:7-10, s. 527-531
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Martinelli, Anna, 1978, et al.
(författare)
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Phase behavior and ionic conductivity in LiTFSI doped ionic liquids of the pyrrolidinium cation and TFSI anion
- 2009
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 113:32, s. 11247-11251
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Tidskriftsartikel (refereegranskat)abstract
- The phase behavior and the ionic conductivity of ionic liquids (ILs) of the N-alkyl-N-alkylpyrrolidinium (PYR xy ) cation and the bis(trifluoromethanesulfonyl)imide (TFSI) anion are investigated upon addition of LiTFSI salt. We compare the case of two new ILs of the PYR 2y cation (where 2 is ethyl and y is butyl or propyl) with that of the PYR 14 (where 1 is methyl and 4 is butyl). We find that the addition of LiTFSI increases the glass transition temperature, decreases the melting temperature and the heat of fusion and, in the ILs of the PYR 2y family, suppresses crystallization. In the solid state, significant ionic conductivities are found, being as high as 10 -5 Scm -1 , strongly increasing with Li + concentration. The opposite trend is found in the liquid state, where the conductivity is on the order of 10 -3 -10 -2 Scm -1 at room temperature. A T g -scaled Arrhenius plot shows that the liquid-state ionic conductivity in these systems is mainly governed by viscosity and that the fragility of the liquids is slightly influenced by the structural modifications on the cation. © 2009 American Chemical Society.
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| 6. |
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| 7. |
- Martinelli, Anna, 1978, et al.
(författare)
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Structural analysis of PVA-based proton conducting membranes
- 2006
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 177:26-32, s. 2431-2435
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Tidskriftsartikel (refereegranskat)abstract
- We have synthesized and characterized a new family of proton conducting membranes based on cross-linked poly(vinyl alcohol), PVA, and functionalized silica filler. Glutaraldehyde, GLA, was used as the cross-linking agent in order to improve chemical and thermal stabilities. The functionalization of the silica particles is such that terminal -SO 3 H groups are formed during membrane preparation, thus possibly providing additional mobile protons. We find that the crystallinity of the PVA-based membranes is enhanced by the presence of the functionalized silica particles, whereas it is reduced by means of cross-linking. The thermal stability of the ternary system PVA:GLA:silica is improved due to the additive contribution of GLA and silica. The conductivity of membranes swelled in a sulfuric acid solution was found to be of the order of 10 - 1 S cm - 1 . © 2006 Elsevier B.V. All rights reserved.
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| 8. |
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| 9. |
- Martinelli, Anna, 1978
(författare)
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Structure, interactions and functionality in novel electrolyte materials for fuel cell applications
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is today well known that fossil fuels will sooner or later be depleted, and the need to find alternative energy sources is now accepted. Among the diverse technologies developed to produce energy, considerable interest is currently directed to the hydrogen fuel cell, a device that converts chemical into electrical energy, is versatile and has a low environmental impact. However, in order to meet the requirements for mass-commercialization, several technical aspects must be improved. For instance, the performance of the polymer electrolyte membrane, which is a central part of the fuel cell, should be enhanced with respect to the operational temperature window and the cost of production.In this thesis, several approaches are explored with the aim of finding new polymer electrolyte materials with high proton conductivities in addition to thermal, mechanical and chemical stability. The main idea is to confine a liquid electrolyte in a polymer based membrane, where the function of the latter is mainly to act as a solid container. We investigate different electrolyte systems, such as aqueous solutions of acids, solid acid hydrates and the new generation of ionic liquids, which are incorporated in membranes based on the polymer poly(vinylidene fluoride) (PVDF). PVDF is only partially fluorinated hence relatively cheap, is both chemically and thermally resistant, and can be used as a starting material to form self-standing porous membranes.The key-issue in this thesis is to understand the microscopic properties that relate to the functionality of the membrane. The investigations were made using vibrational spectroscopy (Raman and infrared), a powerful technique to probe the local coordination of atoms in materials. The results show that PVDF adopts different crystalline forms, depending on the thermal treatment as well as on the type of electrolyte introduced into the pores. In addition, the properties of the liquid electrolyte, such as acid dissociation and thermal dependence of conductivity, have been addressed. We find that these properties are affected upon confinement into a solid matrix. We also add valuable information for a better understanding of the structure--property relations in bis(trifluoromethanesulfonyl)imide (TFSI-) based ionic liquids. The configuration and the conformation of the constituting ions is determined, and the nature of ion--ion interaction is addressed.Keywords: Polymer electrolyte membrane (PEM), fuel cell, vibrational spectroscopy, ionic liquid, proton conductivity
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| 10. |
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