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Membrane Electrode Assemblies Based on Hydrocarbon Ionomers and New Catalyst Supports for PEM Fuel Cells
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The proton exchange membrane fuel cell (PEMFC) is a potential electrochemicalpower device for vehicles, auxiliary power units and small-scale power plants. In themembrane electrode assembly (MEA), which is the core of the PEMFC single cell,oxygen in air and hydrogen electrochemically react on separate sides of a membraneand electrical energy is generated. The main challenges of the technology are associatedwith cost and lifetime. To meet these demands, firstly, the component expensesought to be reduced. Secondly, enabling system operation at elevated temperatures,i.e. up to 120 °C, would decrease the complexity of the system and subsequentlyresult in decreased system cost. These aspects and the demand for sufficientlifetime are the strong motives for development of new materials in the field.In this thesis, MEAs based on alternative materials are investigatedwith focus on hydrocarbon proton-conducting polymers, i.e. ionomers, and newcatalyst supports. The materials are evaluated by electrochemical methods, such ascyclic voltammetry, polarisation and impedance measurements; morphological studiesare also undertaken. The choice of ionomers, used in the porous electrodes andmembrane, is crucial in the development of high-performing stable MEAs for dynamicoperating conditions. The MEAs are optimised in terms of electrode compositionand preparation, as these parameters influence the electrode structure andthus the MEA performance. The successfully developed MEAs, based on the hydrocarbonionomer sulfonated polysulfone (sPSU), show promising fuel cell performancein a wide temperature range. Yet, these membranes induce mass-transportlimitations in the electrodes, resulting in deteriorated MEA performance. Further,the structure of the hydrated membranes is examined by nuclear magnetic resonancecryoporometry, revealing a relation between water domain size distributionand mechanical stability of the sPSU membranes. The sPSU electrodes possessproperties similar to those of the Nafion electrode, resulting in high fuel cell performancewhen combined with a high-performing membrane. Also, new catalystsupports are investigated; composite electrodes, in which deposition of platinum(Pt) onto titanium dioxide reduces the direct contact between Pt and carbon, showpromising performance and ex-situ stability. Use of graphitised carbon as catalystsupport improves the electrode stability as revealed by a fuel cell degradation study.The thesis reveals the importance of a precise MEA developmentstrategy, involving a broad methodology for investigating new materials both as integratedMEAs and as separate components. As the MEA components and processesinteract, a holistic approach is required to enable successful design of newMEAs and ultimately development of high-performing low-cost PEMFC systems.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 69 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:64
Keyword [en]
Catalyst support, Carbon, High Temperature Proton Exchange Membrane Fuel Cell, Hydrocarbon Ionomer, Membrane Electrode Assembly, Nafion, PEFC, PEMFC, Porous Electrode, Sulfonated Polysulfone, Titanium Dioxide
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-9208ISBN: 978-91-7415-124-4 (print)OAI: oai:DiVA.org:kth-9208DiVA: diva2:37372
Public defence
2008-10-24, F3, Lindstedtsvägen 26, Stockholm, 09:30 (English)
Opponent
Supervisors
Note
QC 20100922Available from: 2008-10-13 Created: 2008-10-03 Last updated: 2010-09-22Bibliographically approved
List of papers
1. Pore Size Distribution and Water Uptake in Hydrocarbon and Perfluorinated Proton-Exchange Membranes as Studied by NMR Cryoporometry
Open this publication in new window or tab >>Pore Size Distribution and Water Uptake in Hydrocarbon and Perfluorinated Proton-Exchange Membranes as Studied by NMR Cryoporometry
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2008 (English)In: Fuel Cells, ISSN 1615-6846, E-ISSN 1615-6854, Vol. 8, no 3-4, 262-269 p.Article in journal (Refereed) Published
Abstract [en]

Sulfonated polysulfone (sPSU) membranes were analysed by nuclear magnetic resonance (NMR) cryoporometry, conventional gravimetric water uptake measurements as well as by differential scanning calorimetry (DSC). NMR cryoporometry is based on the relation between the pore size and the melting point depression of the pore-filling liquid, i.e. water in fuel cell membranes; thus providing a relation between the amount of molten water and the temperature shift, i.e. the pore size, in hydrated membranes. An sPSU membrane with high ion-exchange capacity (IEC 1.45 mequiv. g –1) possessed a significant amount of large pores after hydrothermal pretreatment at 80

Keyword
Differential Scanning Calorimetry, Nafion, Nuclear Magnetic Resonance Cryoporometry, ProtonExchange Membrane Fuel Cell, PEMFC, Sulfonated Polysulfone, Water Domain Size, Water Uptake
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-9165 (URN)10.1002/fuce.200800010 (DOI)000258086400013 ()2-s2.0-55349126100 (Scopus ID)
Projects
MISTRAs bränslecellsprogram
Note
QC 20100921Available from: 2008-10-13 Created: 2008-09-29 Last updated: 2017-12-13Bibliographically approved
2. Gas Diffusion Electrodes and Membrane Electrode Assemblies Based on a Sulfonated Polysulfone for High-Temperature PEMFC
Open this publication in new window or tab >>Gas Diffusion Electrodes and Membrane Electrode Assemblies Based on a Sulfonated Polysulfone for High-Temperature PEMFC
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2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 153, no 11, A2077-A2084 p.Article in journal (Refereed) Published
Abstract [en]

Membrane electrode assemblies MEAs with a sulfonated polysulfone sPSU as the proton-conducting phase were fuel cellevaluated at varying temperatures in over-humidified conditions. The sPSU was prepared by a direct polycondensation involvinga commercially available sulfonated naphthalene diol monomer. The gas diffusion electrodes GDEs and MEAs were successfullyfabricated and a thorough morphological study was subsequently carried out on GDEs with varying sPSU contents and inksolvents. The scanning electron microscopy and porosimetry studies revealed highly porous GDE morphologies at sPSU contentsbelow 20 wt %. Double-layer capacitance measurements showed an almost fully sPSU-wetted electronic phase when the sPSUcontent was 10 wt %. The MEAs were prepared by applying the GDEs directly onto sPSU membranes. MEAs with a total Ptloading of 0.2 mg/cm2 were successfully fuel cell operated at 120°C. The MEAs showed mass-transport limitations in the rangeof 600–800 mA/cm2, most probably caused by abundant water due to the overhumidified measuring conditions. The low resistanceof the MEAs indicated a well-integrated structure between the GDEs and the membrane.

Keyword
Gas diffusion electrodes (GDE), Membrane electrode assemblies (MEA), Sulfonated polysulfone (sPSU), Diffusion, High temperature effects, Membranes, Monomers, Morphology, Polysulfones
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-9178 (URN)10.1149/1.2335979 (DOI)000241057000011 ()2-s2.0-33749612550 (Scopus ID)
Projects
MISTRAS bränslecellsprogram
Note
QC 20100922Available from: 2008-10-13 Created: 2008-09-30 Last updated: 2017-12-13Bibliographically approved
3. Substitution of Nafion with Sulfonated Polysulfone in Membrane-Electrode Assembly Components for 60-120 °C PEMFC Operation
Open this publication in new window or tab >>Substitution of Nafion with Sulfonated Polysulfone in Membrane-Electrode Assembly Components for 60-120 °C PEMFC Operation
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2008 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 155, no 10, B1001-B1007 p.Article in journal (Refereed) Published
Abstract [en]

To investigate the influence of sulfonated polysulfone (sPSU) in membrane–electrode assemblies (MEAs), sPSU-based gas diffusion electrodes (GDEs) and sPSU membranes were studied both as complete MEAs and as separate components in assembled MEAs at 60–120°C. The complete sPSU MEAs showed mass-transport limitations, irrespective of ion exchange capacity, compared to Nafion MEAs. Cyclic voltammetry and low-current impedance analysis revealed comparable electrochemically active catalyst areas and kinetic properties in the sPSU and Nafion GDEs, while gas-crossover measurements showed a lower gas permeability in sPSU compared to Nafion. The sPSU and Nafion GDEs, deposited on Nafion membranes, possessed comparable fuel cell characteristics at 120°C and 100% relative humidity, demonstrating no considerable limitations when utilizing sPSU as an alternative to Nafion in the GDE, thus implying a sufficient gas permeability in the sPSU GDE at high humidity. Furthermore, the results clearly showed that the sPSU membrane induced mass-transport limitations in both sPSU and Nafion GDEs, revealing that the limiting factor of the sPSU MEAs was primarily the membrane-induced cathode flooding due to unoptimized water transport in the sPSU membrane. The work demonstrates the importance of electrochemical evaluation of ionomers as complete MEAs and as separate components when studying MEAs.

Keyword
Atmospheric humidity, Bioelectric phenomena, Capillarity, Cell membranes, Cyclic voltammetry, Diffusion in gases, Electric batteries, Electrolysis, Fuel cells, Gas permeability, Gases, Ion exchange, Ion exchange membranes, Ion exchangers, Ionomers, Liquids, Membranes, Metallizing, Meteorology, Moisture, Polymers, Silicones, Voltammetry, Active catalysts, Cathode flooding, Electrochemical evaluations, Gas Diffusion Electrodes, High humidity, Impedance analysis, Ion-exchange capacities, Kinetic properties, Limiting factors, Mass-transport limitations, Membrane-electrode assemblies, Nafion membranes, Relative humidities, Sulfonated polysulfone, Water transport
National Category
Chemical Engineering Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-9180 (URN)10.1149/1.2959113 (DOI)000258976500019 ()2-s2.0-51849165611 (Scopus ID)
Projects
MISTRAs bränslecellsprogram
Note
QC 20100922Available from: 2008-10-13 Created: 2008-09-30 Last updated: 2017-12-13Bibliographically approved
4. Evaluation of TiO2 as catalyst support in Pt-TiO2/C composite cathodes for the proton exchange membrane fuel cell
Open this publication in new window or tab >>Evaluation of TiO2 as catalyst support in Pt-TiO2/C composite cathodes for the proton exchange membrane fuel cell
Show others...
2008 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 180, no 1, 185-190 p.Article in journal (Refereed) Published
Abstract [en]

Anatase TiO2 is evaluated as catalyst support material in authentic Pt-TiO2/C composite gas diffusion electrodes (GDEs), as a different approach in the context of improving the proton exchange membrane fuel cell (PEMFC) cathode stability. A thermal stability study shows high carbon stability as Pt nanoparticles are supported on TiO2 instead of carbon in the Pt-TiO2/C composite material, presumably due to a reduced direct contact between Pt and C. The performance of Pt-TiO2/C cathodes is investigated electrochemically in assembled membrane-electrode assemblies (MEAs) considering the added carbon fraction and Pt concentration deposited on TiO2. The O-2 reduction current for the Pt-TiO2 alone is expectedly low due to the low electronic conductivity in bulk TiO2. However, the Pt-TiO2/C composite cathodes show enhanced fuel cell cathode performance with growing carbon fraction and increasing Pt concentration deposited on TiO2. The proposed reasons for these observations are improved macroscopic and local electronic conductivity, respectively. Electron micrographs of fuel cell tested Pt-TiO2/C composite cathodes illustrate only a minor Pt migration in the Pt-TiO2/C structure, in which anatase TiO2 is used as Pt support. On the whole, the study demonstrates a stable Pt-TiO2/C Composite material possessing a performance comparable to conventional Pt-C materials when incorporated in a PEMFC cathode.

Keyword
titanium dioxide, carbon degradation, cathode performance, proton, exchange membrane fuel cell, polymer electrolyte fuel cell, electrochemical characteristics, electrolyte, pemfc, electrocatalysts, degradation, situ, size, pefc
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-17547 (URN)10.1016/j.jpowsour.2008.02.023 (DOI)000256069900023 ()
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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