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The Rôle of Side-Chains in Polymer Electrolytes for Batteries and Fuel Cells
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science. (Structural Chemistry)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The subject of this thesis relates to the design of new polymer electrolytes for battery and fuel cell applications. Classical Molecular Dynamics (MD) modelling studies are reported of the nano-structure and the local structure and dynamics for two types of polymer electrolyte host: poly(ethylene oxide) (PEO) for lithium batteries and perfluorosulfonic acid (PFSA) for polymer-based fuel cells. Both polymers have been modified by side-chain substitution, and the effect of this on charge-carrier transport has been investigated. The PEO system contains a 89-343 EO-unit backbone with 3-15 EO-unit side-chains, separated by 5-50 EO backbone units, for LiPF6 salt concentrations corresponding to Li:EO ratios of 1:10 and 1:30; the PFSA systems correspond to commercial Nafion®, Hyflon® (Dow®) and Aciplex® fuel-cell membranes, where the major differences again lie in the side-chain lengths.

The PEO mobility is clearly enhanced by the introduction of side-chains, but is decreased on insertion of Li salts; mobilities differ by a factor of 2-3. At the higher Li concentration, many short side-chains (3-5 EO-units) give the highest ion mobility, while the mobility was greatest for side-chain lengths of 7-9 EO units at the lower concentration. A picture emerges of optimal Li+-ion mobility correlating with an optimal number of Li+ ions in the vicinity of mobile polymer segments, yet not involved in significant cross-linkages within the polymer host.

Mobility in the PFSA-systems is promoted by higher water content. The influence of different side-chain lengths on local structure was minor, with Hyflon® displaying a somewhat lower degree of phase separation than Nafion®. Furthermore, the velocities of the water molecules and hydronium ions increase steadily from the polymer backbone/water interface towards the centre of the proton-conducting water channels. Because of its shorter side-chain length, the number of hydronium ions in the water channels is ~50% higher in Hyflon® than in Nafion® beyond the sulphonate end-groups; their hydronium-ion velocities are also ~10% higher.

MD simulation has thus been shown to be a valuable tool to achieve better understanding of how to promote charge-carrier transport in polymer electrolyte hosts. Side-chains are shown to play a fundamental rôle in promoting local dynamics and influencing the nano-structure of these materials.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2009. , p. 52
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 635
Keywords [en]
molecular dynamics, polymer electrolytes, side-chains, Li-ion batteries, proton exchange membrane fuel cell (PEMFC), PFSA membrane
National Category
Other Chemistry Topics Atom and Molecular Physics and Optics
Research subject
Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-100738ISBN: 978-91-554-7499-7 (print)OAI: oai:DiVA.org:uu-100738DiVA, id: diva2:210891
Public defence
2009-05-11, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2009-04-20 Created: 2009-04-06 Last updated: 2012-10-09
List of papers
1. A molecular dynamics study of the effect of side-chains on mobility in a polymer host
Open this publication in new window or tab >>A molecular dynamics study of the effect of side-chains on mobility in a polymer host
2005 (English)In: Solid State Ionics, Vol. 176, no 39-40, p. 3041-3044Article in journal (Refereed) Published
Abstract [en]

The effect on polymer dynamics of adding methoxy-terminated poly(ethylene oxide) (PEO) side-chains with different lengths and separations to an amorphous long-chain PEO backbone has been studied by Molecular Dynamics (MD) simulation at 293 K and 330 K. The study is seen as having a direct general relevance to the optimal design of ion-conducting polymer hosts for both Li-ion battery and polymer fuel-cell applications. The MD box used contains a long-chain PEO backbone to which side-chains comprising 3, 6, 7, 8, 9 and 15 EO units are added. The chosen separations between the side-chains are 5, 10, 15, 20 and 50 EO units. All potentials used to describe these systems are taken from earlier work (J. Mater. Chem., 13 (2003) 214). The overall mobility of the polymer host system is found to have both minima and maxima at both temperatures for side-chain lengths in the range 6–9 EO units. This is almost totally independent of side-chain separation at 293 K, while the situation is more complex at 330 K.

Keywords
Molecular dynamics, Polymer side-chains, Chain separation, Ion conduction, Mobility, Polymer fuel cells
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-75441 (URN)doi:10.1016/j.ssi.2005.10.003 (DOI)
Available from: 2006-02-07 Created: 2006-02-07 Last updated: 2011-01-11
2. A Molecular Dynamics study of the influence of side-chain length and spacing on lithium mobility in non-crystalline LiPF6·PEOx; x = 10 and 30
Open this publication in new window or tab >>A Molecular Dynamics study of the influence of side-chain length and spacing on lithium mobility in non-crystalline LiPF6·PEOx; x = 10 and 30
2009 (English)In: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 180, no 23-25, p. 1272-1284Article in journal (Refereed) Published
Abstract [en]

Molecular Dynamics (MD) simulation techniques have been used to investigate systematically how the length and spacing of poly(ethylene oxide) (PEO) side-chains along a PEO backbone influence ion mobility for two different salt concentrations. This is of fundamental relevance to the design of new polymer electrolytes for battery applications. The salt used has been LiPF6 in concentrations corresponding to Li:EO ratios of 1:30 and 1:10. The MD box contained PEO backbones of 89-343 EO units to which 3, 6, 7, 8, 9 and 15 EO unit side-chains were added. The selected spacings along the backbone between the PEO side-chains attachment points were 5, 10, 15, 20 and 50 EO units. The backbone and all side-chains were methoxy end-capped, and the simulations were all made at 293 K. Ion mobilities have been estimated from the variation of mean-square-displacement with time, and have been analysed in relation to chain dynamics, cross-linking and ion pairing. Comparisons are also made with the results of simulated PEO systems without side-chains and/or without salt. It is found that, at a higher concentration, many short side-chains give the highest ion mobility, while the mobility is highest for side-chain lengths of 7-9 EO units at the lower concentration.

Keywords
Molecular Dynamics; Ion conduction; Polymer electrolyte; Salt ions; Side-chain; Li-ion batteries
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-101259 (URN)10.1016/j.ssi.2009.07.009 (DOI)000273493500008 ()
Available from: 2009-04-21 Created: 2009-04-21 Last updated: 2017-12-13Bibliographically approved
3. Molecular dynamics studies of the Nafion®, Dow® and Aciplex® fuel-cell polymer membrane systems
Open this publication in new window or tab >>Molecular dynamics studies of the Nafion®, Dow® and Aciplex® fuel-cell polymer membrane systems
2007 (English)In: Journal of Molecular Modeling, ISSN 1610-2940, E-ISSN 0948-5023, Vol. 13, no 10, p. 1039-1046Article in journal (Refereed) Published
Abstract [en]

The Nafion, Dow and Aciplex systems – where the prime differences lies in the side-chain length – have been studied by molecular dynamics (MD) simulation under standard pressure and temperature conditions for two different levels of hydration: 5 and 15 water molecules per (H)SO3 end-group. Structural features such as water clustering, water-channel dimensions and topology, and the dynamics of the hydronium ions and water molecules have all been analysed in relation to the dynamical properties of the polymer backbone and side-chains. It is generally found that mobility is promoted by a high water content, with the side-chains participating actively in the H3O+/H2O transport mechanism. Nafion, whose side-chain length is intermediate of the three polymers studied, is found to have the most mobile polymer side-chains at the higher level of hydration, suggesting that there could be an optimal side-chain length in these systems. There are also some indications that the water-channel network connectivity is optimal for high water-content Nafion system, and that this could explain why Nafion appears to exhibit the most favourable overall hydronium/water mobility.

Keywords
Molecular dynamics, Nafion membrane, Proton exchange membrane fuel cell (PEMFC), Side-chain length
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-13970 (URN)10.1007/s00894-007-0230-7 (DOI)000248912300002 ()17665227 (PubMedID)
Available from: 2008-01-28 Created: 2008-01-28 Last updated: 2018-01-03Bibliographically approved
4. Molecular Dynamics Modelling of Proton Transport in Nafion® and Hyflon® Nano-Structures
Open this publication in new window or tab >>Molecular Dynamics Modelling of Proton Transport in Nafion® and Hyflon® Nano-Structures
(English)Manuscript (Other academic)
Abstract
Identifiers
urn:nbn:se:uu:diva-101260 (URN)
Available from: 2009-04-21 Created: 2009-04-21 Last updated: 2018-01-03

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