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Investigations of proton conducting polymers and gas diffusion electrodes in the polymer electrolyte fuel cell
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Polymer electrolyte fuel cells (PEFC) convert the chemically bound energy in a fuel, e.g. hydrogen, directly into electricity by an electrochemical process. Examples of future applications are energy conversion such as combined heat and power generation (CHP), zero emission vehicles (ZEV) and consumer electronics. One of the key components in the PEFC is the membrane / electrode assembly (MEA). Both the membrane and the electrodes consist of proton conducting polymers (ionomers). In the membrane, properties such as gas permeability, high proton conductivity and sufficient mechanical and chemical stability are of crucial importance. In the electrodes, the morphology and electrochemical characteristics are strongly affected by the ionomer content. The primary purpose of the present thesis was to develop experimental techniques and to use them to characterise proton conducting polymers and membranes for PEFC applications electrochemically at, or close to, fuel cell operating conditions. The work presented ranges from polymer synthesis to electrochemical characterisation of the MEA performance.

The use of a sulfonated dendritic polymer as the acidic component in proton conducting membranes was demonstrated. Proton conducting membranes were prepared by chemical cross-linking or in conjunction with a basic functionalised polymer, PSU-pyridine, to produce acid-base blend membranes. In order to study gas permeability a new in-situ method based on cylindrical microelectrodes was developed. An advantage of this method is that the measurements can be carried out at close to real fuel cell operating conditions, at elevated temperature and a wide range of relative humidities. The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and electrochemical impedance measurements (EIS). Long-term experiments have been compared to fast ex situ testing in 3 % H2O2 solution. For the direct assessment of membrane degradation, micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF-based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. The influence of ionomer content on the structure and electrochemical characteristics of Nafion-based PEFC cathodes was also demonstrated. The electrodes were thoroughly investigated using various materials and electrochemical characterisation techniques. Electrodes having medium Nafion contents (35<x<45 wt %) showed the best performance. The mass-transport limitation was essentially due to O2 diffusion in the agglomerates. The performance of cathodes with low Nafion content (<30 wt %) is limited by poor kinetics owing to incomplete wetting of platinum (Pt) by Nafion, by proton migration throughout the cathode as well as by O2 diffusion in the agglomerates. At large Nafion content (>45 wt %), the cathode becomes limited by diffusion of O2 both in the agglomerates and throughout the cathode. Furthermore, models for the membrane coupled with kinetics for the hydrogen electrode, including water concentration dependence, were developed. The models were experimentally validated using a new reference electrode approach. The membrane, as well as the hydrogen anode and cathode characteristics, was studied experimentally using steady-state measurements, current interrupt and EIS. Data obtained with the experiments were in good agreement with the modelled results.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , 56 p.
Series
Trita-KET, ISSN 1104-3466 ; 205
Keyword [en]
Theoretical chemistry, polymer electrolyte fuel cell, proton conducting membrane, porous electrode, gas permeability, degradation, water transport
Keyword [sv]
Teoretisk kemi
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-97ISBN: 91-7283-929-5 (print)OAI: oai:DiVA.org:kth-97DiVA: diva2:14973
Public defence
2005-01-14, sal D2, Lindstedtsvägen 5, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101014Available from: 2005-02-02 Created: 2005-02-02 Last updated: 2010-10-14Bibliographically approved
List of papers
1. A novel sulfonated dendritic polymer as the acidic component in proton conducting membranes
Open this publication in new window or tab >>A novel sulfonated dendritic polymer as the acidic component in proton conducting membranes
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2006 (English)In: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 177, no 7-8, 787-794 p.Article in journal (Refereed) Published
Abstract [en]

The present study involves the synthesis of sulfonated poly(3-ethyl-3-(hydroxymethyl)oxetane), sPTMPO, by end-capping the hydroxy-groups in the PTMPO with 1,4-butane sultone. A series of the polymer with different degrees of substitution was investigated. Furthermore, the subsequent use of the sulfonated PTMPO as the acidic component in proton conducting membranes was explored. The membranes were prepared by either a) mixing the partly sulfonated PTMPO with hexamethoxymethyl melamine (HMMM) to form cross-links by ether formation between the methylol groups on HMMM and the remaining hydroxyl groups on the hyperbranched polyether or b) using the sulfonated polyether in conjunction with a pyridine functionalised polysulfone, PSU-pyridine, to produce acid-base blend membranes. Membrane properties such as proton conductivity, water uptake and mechanical properties are discussed.

Keyword
polymer electrolyte membrane, PENT, hyperbranched, dendritic, water uptake, conductivity, polysulfone, blend membranes, PEFC, fuel cell, electrolyte fuel-cells, blend-membranes, exchange membranes, ionomer membranes, terminal groups, polysulfone, dendrimers, morphology, transport, coatings
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-15610 (URN)10.1016/j.ssi.2005.12.031 (DOI)000236892400023 ()2-s2.0-33644962847 (Scopus ID)
Note

QC 20100525

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. In-situ measurements of gas permeability in fuel cell membranes using a cylindrical microelectrode
Open this publication in new window or tab >>In-situ measurements of gas permeability in fuel cell membranes using a cylindrical microelectrode
2002 (English)In: Journal of Electroanalytical Chemistry, ISSN 0022-0728, E-ISSN 1873-2569, Vol. 518, no 2, 115-122 p.Article in journal (Refereed) Published
Abstract [en]

A new method to study permeation of gases in proton conducting membranes using a cylindrical microelectrode is presented. The focus of this work was to develop an in-situ method to study transport properties of hydrogen and oxygen close to real fuel cell operating conditions. The gas permeability is strongly affected by the change of water content in the membrane and it is therefore of advantage that, by using this method, measurements can be carried out over a wide range of relative humidities. The numerical method makes it possible to separate the diffusion coefficient and the concentration of dissolved gas in the membrane and also allows kinetic limitations to be taken into account. Chronoamperometric measurements on Nafion(R) 117 were successfully evaluated numerically. Experiments at temperatures of 25 and 60 degreesC with respect to oxygen permeation and at 60 degreesC for hydrogen permeation at relative humidities in the range 30-94% are presented. The reproducibility of data was excellent when measuring with different microelectrodes, on the same membrane sample, but differed when measuring on different samples. In general, the permeability increases with increasing temperature and relative humidity.

Keyword
proton conducting membrane, microelectrode, oxygen gas, hydrogen gas, gas permeability, polymer electrolyte membrane fuel cell, oxygen reduction, microcylinder electrodes, permeation properties, hydrogen permeation, nafion interface, water-uptake, spe method, diffusion, chronoamperometry, voltammetry
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-21296 (URN)10.1016/S0022-0728(01)00698-2 (DOI)000173639700005 ()
Note
QC 20100525Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved
3. Membrane Durability in a PEM Fuel Cell Studied Using PVDF Based Radiation Grafted Membranes
Open this publication in new window or tab >>Membrane Durability in a PEM Fuel Cell Studied Using PVDF Based Radiation Grafted Membranes
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2003 (English)In: Fuel Cells, ISSN 1615-6846, E-ISSN 1615-6854, Vol. 3, no 1-2, 21-27 p.Article in journal (Refereed) Published
Abstract [en]

The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and impedance measurements. The PEFC measurements, which are time consuming, have been compared to fast ex situ testing in 3% H2O 2 solution. For the direct assessment of membrane degradation micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. By using low degrees of grafting, below about 35%, the durability of this type of membrane can be increased. Degradation in the fuel cell was found to be highly localised. It was found that in situ measurements in the PEFC alone are not sufficient. Measurement of the cell resistance via impedance is not always a reliable indicator of changes in membrane resistance because other resistance changes in the cell can easily interfere and cannot be separated from those caused by the membrane. Micro-Raman is an ideal complementary method to in situ testing, but it is time consuming. For fast pre-screening of membrane durability mass loss measurements during exposure to 3% H2O2 solution combined with the determination of changes in the IEC can be performed.

Keyword
Degradation, Durability, Membrane, PEM Fuel Cell
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-25257 (URN)10.1002/fuce.200320239 (DOI)
Note
QC 20101014Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2017-12-12Bibliographically approved
4. Influence of the composition on the structure and electrochemical characteristics of the PEFC cathode
Open this publication in new window or tab >>Influence of the composition on the structure and electrochemical characteristics of the PEFC cathode
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2003 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 48, no 28, 4175-4187 p.Article in journal (Refereed) Published
Abstract [en]

The influence the composition of the cathode has on its structure and electrochemical performance was investigated for a Nafion content spanning from 10 to 70 wt.%. The cathodes were formed on a Nafion membrane by the spray method and using 20 wt.% Pt on Vulcan (E-TEK). Materials characterisation (SEM, STEM, gas and mercury porosimetry, electron conductivity) and electrochemical characterisation (steady-state polarisation curve, impedance spectroscopy in O-2 and current-pulse measurements in N-2) were performed. The impedance spectra were analysed using our dynamic agglomerate model. The results indicate that the agglomerate model is valid until a Nafion content of about 45 wt.%. Pt/C and Nation are homogeneously mixed for any composition and no Nafion film was observed. The cathodes containing 36-43 wt.% Nation display a single or double Tafel slope behaviour ascribed to diffusion limitations in the agglomerates. At larger Nation content, the agglomerate model can describe the curves only by assuming a diffusion coefficient 3-4 decades smaller than that of gases. At such compositions, the porosity was only 10%. These results were interpreted as a blocking of the pores and a non-percolating pore system for too large Nafion contents.

Keyword
PEFC cathode, nafion content, electrochemical characterisation, material characterisation, agglomerate model, electrolyte fuel-cells, gas-diffusion electrodes, mass-transport limitations, catalyst layer, performance
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-22970 (URN)10.1016/s0013-4686(03)00603-0 (DOI)000186645500009 ()
Note
QC 20100525Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved
5. Modelling and experimental studies of the membrane and hydrogen electrode in the PEFC: steady state and electrochemical impedance investigations
Open this publication in new window or tab >>Modelling and experimental studies of the membrane and hydrogen electrode in the PEFC: steady state and electrochemical impedance investigations
(English)Manuscript (preprint) (Other academic)
Keyword
Polymer electrolyte membrane, PEM, water transport, conductivity, PEFC, fuel cell, hydrogen kinetics, electrochemical impedance spectroscopy, modelling
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-25261 (URN)
Note
QC 20101014Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2010-10-14Bibliographically approved

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