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Electrokinetic transport of water and methanol in Nafion membranes as observed by NMR spectroscopy
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry.
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2010 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 0019-4686, Vol. 55, no 10, 3542-3549 p.Article in journal (Refereed) Published
Abstract [en]
Electrophoretic NMR (eNMR) and pulsed-field-gradient NMR (PFG-NMR) methods were used to study transport processes in situ and in a chemically resolved manner in the electrolyte of an experimental direct methanol fuel cell (DMFC) setup, constituted of several layers of Nation 117. The measurements were conducted at room temperature for membranes fully swollen by methanol-water mixtures over a wide concentration interval. The experimental setup and the experimental protocol for the eNMR experiments are discussed in detail. The magnitude of the water and methanol self-diffusion coefficients show a good agreement with previously published data while the ratio of the two self-diffusion coefficients may indicate an imperfect mixing of the two solvent molecules. On the molecular level, the drag of water and methanol molecules by protons is roughly of the same magnitude, with the drag of methanol molecules increasing with increasing methanol content. The electro-osmotic drag defined on mass-flow basis increased for methanol from a low level with increasing methanol concentration while that of water remained roughly constant. (C) 2010 Elsevier Ltd. All rights reserved.
Place, publisher, year, edition, pages
2010. Vol. 55, no 10, 3542-3549 p.
Keyword [en]
URN: urn:nbn:se:kth:diva-14038DOI: 10.1016/j.electacta.2010.01.064ISI: 000276792000015ScopusID: 2-s2.0-77949270817OAI: diva2:329317
QC20100709Available from: 2010-07-09 Created: 2010-07-09 Last updated: 2010-07-09Bibliographically approved
In thesis
1. Molecular Interactions Studied by Electrophoretic and Diffusion NMR
Open this publication in new window or tab >>Molecular Interactions Studied by Electrophoretic and Diffusion NMR
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Even though electrophoretic NMR (eNMR) experiments may provide unique chemical information and have been performed for three decades, the technique is still rarely applied, mainly because several experimental sources of artifacts have to be controlled to achieve accurate results. In this thesis, new experimental setups and protocols for accurate and precise eNMR experiments are presented. These include a novel eNMR sample cell, a radiofrequency filter and methods to suppress bulk flow effects. These developments improved the signal-to-noise ratio by roughly an order of magnitude compared to the U-tube setup previously used for eNMR. Convection-compensated pulse sequences in combination with a phase correction method were found to efficiently suppress bulk flow effects in the experiments and greatly increase experimental accuracy.

These experimental setups and protocols were applied to probe association of ions and molecules in solution. It is particularly illustrated that the combination of diffusion and eNMR has great potential to provide quantitative results on ionic and molecular association in a variety of systems.

The extent to which ionic surfactants associate with uncharged cyclodextrin probed by eNMR yielded very similar results to those obtained by diffusion NMR experiments. Complexation of a large set of small mono- and polyvalent metal cations to poly(ethylene oxide) was quantified by estimating the effective charge of the polymer through combined diffusion and eNMR information. Significant association was found for cations that have a surface charge density below a critical value.

Ion pairing between tetramethylammonium cations and a series of anions in several solvents was also probed by diffusion NMR and eNMR experiments. For the monovalent anions in ethanol and ethanol-water mixture a dependence on ionic size was demonstrated. In water, dimethylsulfoxide, and methanol no such trend and very little pairing was observed. In acetonitrile, a different pattern was seen that did not correlate well with any single ionic parameter.

An experimental cell and procedures for electrokinetic studies of solvated proton-conducting polymer materials is also presented. Electro-osmotic flow and diffusion were studied for each molecular component in water-methanol mixtures that swell Nafion membranes.

Abstract [sv]

Elektroforetisk NMR (eNMR) är en experimentell metod som funnits i tre decennier och som kan ge unik kemisk information. Ändå används den sällan då flera experimentella artefakter måste korrigeras för, om man ska få korrekta resultat. I denna avhandling presenteras nya experimentella uppställningar och protokoll ämnade att uppnå korrekta och noggranna resultat. Dessa inkluderar en ny mätcell, ett radiofrekvensfilter och metoder för att minimera effekten av samtidiga bulkflöden i provlösningen. Sammantaget uppnås ungefär en storleksordning högre signal-brus-förhållande jämfört med den U-rörsuppställning som tidigare använts. Konvektions-kompenserande pulssekvenser i kombination med en faskorrektionsteknik minskade också bulkflödeseffekter effektivt, vilket ökade resultatens noggrannhet högst avsevärt.

De experimentella uppställningarna och protokollen användes här för att mäta association av joner och molekyler i lösning. Mätningarna visar att kombinationen diffusions- och eNMR har en stor potential att kvantitativt kunna bestämma associationgraden i många olika typer av kemiska system.

Associationsgraden mellan joniska tensider och cyklodextriner undersöktes både med eNMR och diffusions-NMR, och resultaten var mycket lika. Komplex-bildningen mellan en serie enkel- och flerladdade metalljoner och poly-(etylenoxid) kvantifierades genom att uppskatta polymerens effektiva laddning från kombinerad diffusions- och eNMR. Betydande komplexbildning hittades för katjoner med ytladdningstäthet under ett kritiskt värde.

Jonparbildning mellan tetrametylammoniumjoner och en serie av anjoner i flera olika lösningsmedel undersöktes också med diffusions- och eNMR. För de monovalenta anjonerna i etanol och etanol-vatten-blandning påvisades ett samband med jonstorleken. I vatten, dimetylsulfoxid och metanol var däremot jonparbildningen låg och inget liknande samband hittades. I acetonitril observerades ett annat mönster, som inte korrelerade bra med någon av anjonernas normala joniska karakteristika.

Slutligen presenteras en mätcell och procedurer för elektrokinetiska studier i de solvatiserade protonledande polymermaterial som bland annat används i bränsleceller. Elektroosmotiskt flöde och diffusion uppmättes för varje molekylär komponent i Nafion-membran solvatiserade av vatten-metanol-blandningar.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. viii, 59 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2010:7
Physical chemistry, NMR, electrophoretic NMR, Ion pairing, complexation, association, diffusion, pulsed field gradient, Nafion
National Category
Physical Chemistry
urn:nbn:se:kth:diva-12186 (URN)978-91-7415-580-8 (ISBN)
Public defence
2010-04-15, hörsal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
QC20100709Available from: 2010-03-25 Created: 2010-03-24 Last updated: 2010-07-09Bibliographically approved

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