Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Electrochemical Reactions in Polymer Electrolyte Fuel Cells
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The polymer electrolyte fuel cell converts the chemical energy in a fuel, e.g. hydrogen or methanol, and oxygen into electrical energy. The high efficiency and the possibility to use fuel from renewable sources make them attractive as energy converters in future sustainable energy systems. Great progress has been made in the development of the PEFC during the last decade, but still improved lifetime as well as lowered cost is needed before a broad commercialization can be considered. The electrodes play an important role in this since the cost of platinum used as catalyst constitutes a large part of the total cost for the fuel cell. A large part of the degradation in performance can also be related to the degradation of the porous electrode and a decreased electrochemically active Pt surface.

In this thesis, different fuel cell reactions, catalysts and support materials are investigated with the aim to investigate the possibility to improve the activity, stability and utilisation of platinum in the fuel cell electrodes.

An exchange current density, i0, of 770 mA cm-2Pt was determined for the hydrogen oxidation reaction in the fuel cell with the model electrodes. This is higher than previously found in literature and implies that the kinetic losses on the anode are very small. The anode loading could therefore be reduced without imposing too high potential losses if good mass transport of hydrogen is ensured. It was also shown that the electrochemically active surface area, activity and stability of the electrode can be affected by the support material. An increased activity was observed at higher potentials for Pt deposited on tungsten oxide, which was related to the postponed oxide formation for Pt on WOx. An improved stability was seen for Pt deposited on tungsten oxide and on iridium oxide. A better Pt stability was also observed for Pt on a low surface non-graphitised support compared to a high surface graphitised support. Pt deposited on titanium and tungsten oxide, displayed an enhanced electrochemically active surface area in the cyclic voltammograms, which was explained by the good proton conductivity of the metal oxides. CO-stripping was shown to provide the most reliable measure of the electrochemically active surface area of the electrode in the fuel cell. It was also shown to be a useful tool in characterization of the degradation of the electrodes. In the study of oxidation of small organic compounds, the reaction was shown to be affected by the off transport of reactants and by the addition of chloride impurities. Pt and PtRu were affected differently, which enabled extraction of information about the reaction mechanisms and rate determining steps.

The polymer electrolyte fuel cell converts the chemical energy in a fuel, e.g. hydrogen or methanol, and oxygen into electrical energy. The high efficiency and the possibility to use fuel from renewable sources make them attractive as energy converters in future sustainable energy systems. Great progress has been made in the development of the PEFC during the last decade, but still improved lifetime as well as lowered cost is needed before a broad commercialization can be considered. The electrodes play an important role in this since the cost of platinum used as catalyst constitutes a large part of the total cost for the fuel cell. A large part of the degradation in performance can also be related to the degradation of the porous electrode and a decreased electrochemically active Pt surface.

In this thesis, different fuel cell reactions, catalysts and support materials are investigated with the aim to investigate the possibility to improve the activity, stability and utilisation of platinum in the fuel cell electrodes.

An exchange current density, i0, of 770 mA cm-2Pt was determined for the hydrogen oxidation reaction in the fuel cell with the model electrodes. This is higher than previously found in literature and implies that the kinetic losses on the anode are very small. The anode loading could therefore be reduced without imposing too high potential losses if good mass transport of hydrogen is ensured. It was also shown that the electrochemically active surface area, activity and stability of the electrode can be affected by the support material. An increased activity was observed at higher potentials for Pt deposited on tungsten oxide, which was related to the postponed oxide formation for Pt on WOx. An improved stability was seen for Pt deposited on tungsten oxide and on iridium oxide. A better Pt stability was also observed for Pt on a low surface non-graphitised support compared to a high surface graphitised support. Pt deposited on titanium and tungsten oxide, displayed an enhanced electrochemically active surface area in the cyclic voltammograms, which was explained by the good proton conductivity of the metal oxides. CO-stripping was shown to provide the most reliable measure of the electrochemically active surface area of the electrode in the fuel cell. It was also shown to be a useful tool in characterization of the degradation of the electrodes. In the study of oxidation of small organic compounds, the reaction was shown to be affected by the off transport of reactants and by the addition of chloride impurities. Pt and PtRu were affected differently, which enabled extraction of information about the reaction mechanisms and rate determining steps.

Abstract [sv]

Polymerelektrolytbränslecellen omvandlar den kemiska energin i ett bränsle, exv. vätgas eller metanol, och syrgas  till elektrisk energi. Den höga verkningsgraden samt möjligheten att använda bränsle från förnyelsebara källor gör dem attraktiva som energiomvandlare i framtida hållbara energisystem. En enorm utveckling har skett under det senaste årtiondet men för att kunna introducera polymerelektrolytbränslecellen på marknaden i en större skala måste livstiden öka och kostnaden minska. Elektroderna har en central del i detta då den platina som används som katalysator står för en stor del av kostnaden för bränslecellen. En stor del av prestandaförsämringen med tiden hos bränslecellen kan också relateras till en degradering av den porösa elektroden och en minskad elektrokemiskt aktiv platinayta.

I denna avhandling studeras olika bränslecellsreaktioner samt olika katalysatorer och supportmaterial med målet att undersöka möjligheten att förbättra platinakatalysatorns aktivitet, stabilitet och utnyttjandegrad i bränslecellselektroder.

Utbytesströmtätheten, i0, för vätgasoxidationen i bränslecell bestämdes till 770 mA cm-2Pt genom försök med modellelektroderna. Denna var högre än vad som framkommit tidigare i litteratur, vilket visar att de kinetiska förlusterna på anoden är mycket små. Katalysatormängden på anoden borde därför kunna minskas utan några större potentialförluster så länge masstransporten av vätgas är tillräcklig. Den elektrokemiskt aktiva ytan, aktiviteten och stabiliteten hos elektroden visade sig kunna påverkas av supportmaterialet. Platina deponerad på volfram oxid hade en högre aktivitet vid höga potentialer vilket relaterades till den förskjutna oxidbildningen på ytan. Elektroder med platina på volframoxid och iridiumoxid var mer stabila än elektroder med platina på kol. Det var även platina på ett icke grafitiserat kol med låg yta jämfört med platina på grafitiserade kol med en hög yta. Platina på metalloxidskikt av volfram och titan visade en högre elektrokemiskt aktiv yta i de cykliska voltamogrammen än platina på kol, vilket förklarades med att båda metalloxiderna har en bra protonledningsförmåga. CO-stripping gav det säkraste måttet på den elektrokemiskt aktiva ytan i en elektrod i bränslecell. CO-stripping visade sig även vara användbart för karaktärisering av degraderingen av en elektrod. Oxidationen av små organiska föreningar påverkades av borttransporten av intermediärer samt av kloridföroreningar. Pt aoch PtRu påverkades olika vilket gjorde det möjligt att få fram information om reaktionsmekanismer och hastighetsbestämmande steg.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , 55 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:40
Keyword [en]
Fuel cell, model electrodes, oxygen reduction, methanol oxidation, formic acid oxidation, hydrogen oxidation, CO oxidation, degradation, tungsten oxide, carbon support
Keyword [sv]
Bränslecell, modellelektroder, syrgasreduktion, metanoloxidation, myrsyraoxidation, vätgasoxidation, CO oxidation, degradering, wolfram oxid, kolsupport
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-25267ISBN: 978-91-7415-747-5 (print)OAI: oai:DiVA.org:kth-25267DiVA: diva2:356885
Public defence
2010-10-25, F3, Lindstedts väg 26, KTH, Stockholm, 10:00 (English)
Opponent
Note
QC 20101014Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2010-12-10Bibliographically approved
List of papers
1. Hydrogen oxidation reaction on thin platinum electrodes in the polymer electrolyte fuel cell
Open this publication in new window or tab >>Hydrogen oxidation reaction on thin platinum electrodes in the polymer electrolyte fuel cell
2010 (English)In: Electrochemistry communications, ISSN 1388-2481, E-ISSN 1873-1902, Vol. 12, no 11, 1585-1588 p.Article in journal (Refereed) Published
Abstract [en]

A method for measuring the kinetics of the hydrogen oxidation reaction (HOR) in a fuel cell under enhanced mass transport conditions is presented. The measured limiting current density was roughly 1600 mA cmPt− 2, corresponding to a rate constant of the forward reaction in the Tafel step of 0.14 mol m− 2 s− 1 at 80 °C and 90% RH. The exchange current density for the HOR was determined using the slope at low overvoltages and was found to be 770 mA cmPt− 2. The high values for the limiting and exchange current densities suggest that the Pt loading in the anode catalyst can be reduced further without imposing measurable voltage loss.

Keyword
Hydrogen oxidation reaction, Limiting current density, Exchange current density, PEMFC, Fuel cell, Thermal evaporation
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-25239 (URN)10.1016/j.elecom.2010.08.037 (DOI)000284386100033 ()2-s2.0-78049244427 (Scopus ID)
Funder
StandUp
Note

QC 20150716

Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2017-12-12Bibliographically approved
2. Electrochemical performance and stability of thin film electrodes with metal oxides in polymer electrolyte fuel cells
Open this publication in new window or tab >>Electrochemical performance and stability of thin film electrodes with metal oxides in polymer electrolyte fuel cells
2010 (English)In: Electrochimica Acta, ISSN 0013-4686, Vol. 55, no 26, 7590-7596 p.Article in journal (Refereed) Published
Abstract [en]

Thin film electrodes are prepared by thermal evaporation of nanometer thick layers of metal oxide and platinum on a gas diffusion layer (GDL), in order to evaluate different metal oxides' impact on the activity and stability of the platinum cathode catalyst in the polymer electrolyte fuel cell. Platinum deposited on tin, tantalum, titanium, tungsten and zirconium oxide is investigated and the morphology and chemistry of the catalysts are examined with scanning electron microscopy and X-ray photoelectron spectroscopy. Cyclic sweeps in oxygen and nitrogen are performed prior and after potential cycling degradation tests. Platinum seems to disperse better on the metal oxides than on the GDL and increased electrochemically active surface area (ECSA) of platinum is observed on tin, titanium and tungsten oxide. A thicker layer metal oxide results in a higher ECSA. Platinum deposited on tungsten performs better than sole platinum in the polarisation curves and displays higher Tafel slopes at higher current densities than all other samples. The stability does also seem to be improved by the addition of tungsten oxide, electrodes with 3 nm platinum on 3, 10 and 20 nm tungsten oxide, performs better than all other electrodes after the accelerated degradation tests.

Keyword
Fuel cell, Oxygen reduction, PEMFC, Thermal evaporation, Transition metal oxides
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-25226 (URN)10.1016/j.electacta.2009.12.040 (DOI)000283209800007 ()2-s2.0-77957236254 (Scopus ID)
Funder
StandUpMistra - The Swedish Foundation for Strategic Environmental Research
Note

QC 20150716

Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2015-07-16Bibliographically approved
3. Tungsten oxide in polymer electrolyte fuel cell: A thin-film model electrode study
Open this publication in new window or tab >>Tungsten oxide in polymer electrolyte fuel cell: A thin-film model electrode study
2011 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 56, no 25, 9496-9503 p.Article in journal (Refereed) Published
Abstract [en]

Thin films of WO(x) and Pt on WO(x) were evaporated onto the microporous layer of a gas diffusion layer (GDL) and served as model electrodes in the polymer electrolyte fuel cell (PEFC) as well as in liquid electrolyte measurements. In order to study the effects of introducing WO, in PEFC electrodes, precise amounts of WO(x) (films ranging from 0 to 40 nm) with or without a top layer of Pt (3 nm) were prepared. The structure of the thin-film model electrodes was characterized by scanning electron microscopy and X-ray photoelectron spectroscopy prior to the electrochemical investigations. The electrodes were analyzed by cyclic voltammetry and the electrocatalytic activity for hydrogen oxidation reaction (HOR) and CO oxidation was examined. The impact of Nafion in the electrode structure was examined by comparing samples with and without Nafion solution sprayed onto the electrode. Fuel cell measurements showed an increased amount of hydrogen tungsten bronzes formed for increasing WO(x) thicknesses and that Pt affected the intercalation/deintercalation process, but not the total amount of bronzes. The oxidation of pre-adsorbed CO was shifted to lower potentials for WO(x) containing electrodes, suggesting that Pt-WO(x) is a more CO-tolerant catalyst than Pt. For the HOR. Pt on thicker films of WO(x) showed an increased limiting current, most likely originating from the increased electrochemically active surface area due to proton conductivity and hydrogen permeability in the WO(x) film. From measurements in liquid electrolyte it was seen that the system behaved very differently compared to the fuel cell measurements. This exemplifies the large differences between the liquid electrolyte and fuel cell systems. The thin-film model electrodes are shown to be a very useful tool to study the effects of introducing new materials in the PEFC catalysts. The fact that a variety of different measurements can be performed with the same electrode structure is a particular strength.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
PEMFC, Fuel cell, Tungsten oxide, Platinum, Hydrogen oxidation, CO oxidation, Thermal evaporation
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-25264 (URN)10.1016/j.electacta.2011.08.046 (DOI)000295997000069 ()2-s2.0-80053254751 (Scopus ID)
Funder
StandUpSwedish Energy AgencySwedish National Space Board
Note

QC 20150716. Updated from manuscript to article in journal. Previous title: ON THE ROLE OF TUNGSTEN OXIDE IN POLYMER ELECTROLYTE FUEL CELL ELECTRODES. QC 20160222

Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2017-12-12Bibliographically approved
4. Active Area Determination of Porous Pt Electrodes Used in Polymer Electrolyte Fuel Cells: Temperature and Humidity Effects
Open this publication in new window or tab >>Active Area Determination of Porous Pt Electrodes Used in Polymer Electrolyte Fuel Cells: Temperature and Humidity Effects
Show others...
2010 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 157, no 12, B1795-B1801 p.Article in journal (Refereed) Published
Abstract [en]

This paper discusses the proper measure of the electrochemically active area (ECA)of carbon supported Pt catalyst in PEM fuel cells employing in situ cyclic voltammetry. The charges of the hydrogen underpotential deposition (Hupd) and CO stripping peak obtained in situ are compared, and the influence of operation temperature (25–80°C) and relative humidity (40%–90%) is discussed. The results show that the charges of the Hupd decrease with rising temperature, while the corresponding charges of the CO stripping peak are essentially independent of temperature, at least at high relative humidity. The unexpectedly small Hupd charges are explained by the significant overlap with the hydrogen evolution reaction in a fuel cell at elevated temperatures. According to our results, it is proposed that a more reliable value of Pt ECA is estimated from the CO stripping charge. However, with decreasing humidity the charges of both Hupd and CO stripping peaks decrease, which is probably an effect of increasing blockage of Pt active sites by hydrophobic domains in the electrode ionomer. Some implications of varying cell conditions on the estimated Pt ECA and its correlation with fuel cell activity are discussed in an example from a fuel cell degradation test.

Keyword
SINGLE-CRYSTAL SURFACES, CARBON-MONOXIDE, UNDERPOTENTIAL DEPOSITION, HYDROGEN ADSORPTION, MEMBRANE INTERFACE, PT(100) ELECTRODES, AQUEOUS H2SO4, CO OXIDATION, PLATINUM, ELECTROOXIDATION
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-25266 (URN)10.1149/1.3494220 (DOI)000283938300022 ()2-s2.0-78449298256 (Scopus ID)
Funder
StandUp
Note

QC 20101014. Tidigare titel: Active Area Determination of Porous Pt Electrodes Used inPEM Fuel Cells: Temperature and Humidity Effects

Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2017-12-12Bibliographically approved
5. Degradation studies of PEMFC cathodes based on different types of carbon
Open this publication in new window or tab >>Degradation studies of PEMFC cathodes based on different types of carbon
2009 (English)In: ECS Transactions, 2009, Vol. 25, no 1 PART 2, 1241-1250 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this study different accelerated degradation tests were used evaluating three different carbon supports as well as a thin model electrode. Cyclic ADTs, by 1000 cycles beween 0.6 and 1.2 V in nitrogen, did not degrade the porous electrodes to any larger extent in terms of oxygen reduction activity, whereas a significant loss of electrochemical surface area was seen, often more than 50%. Potentiostatic hold at 1.4 V during 3 h, did not permanently degrade the electrodes but instead an improved activity was obtained after rest during night. A correlation of increase in double layer capacitance and improved performance was seen and believed to be caused by the good proton conductivity of carbon surface oxides. CO-stripping peaks revealed that the humidity and wetting of Nafion™ may have caused the observed temporary changes during the potentiostatic hold. ©The Electrochemical Society.

Keyword
Accelerated degradation tests, Carbon support, Carbon surface oxides, Degradation study, Double-layer capacitance, Electrochemical surface area, Oxygen Reduction, PEMFC cathode, Porous electrodes, Potentiostatics, Degradation, Electrolytic reduction, Membranes, Oxygen, Proton exchange membrane fuel cells (PEMFC), Protons, Stripping (dyes), Electrochemical electrodes
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-25234 (URN)10.1149/1.3210679 (DOI)000329585500126 ()2-s2.0-77649265443 (Scopus ID)978-156677738-4 (ISBN)
Conference
9th Proton Exchange Membrane Fuel Cell Symposium (PEMFC 9) - 216th Meeting of the Electrochemical Society; Vienna; 4 October 2009 through 9 October 2009
Note

QC 20101014

Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2014-10-06Bibliographically approved
6. Methanol oxidation as anode reaction in zinc electrowinning
Open this publication in new window or tab >>Methanol oxidation as anode reaction in zinc electrowinning
2005 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 152, no 11, D201-D207 p.Article in journal (Refereed) Published
Abstract [en]

In this study, different types of Pt- and PtRu-based electrodes were compared regarding their activity and stability for methanol oxidation in an acid zinc sulfate solution. The lead anodes used in zinc electrowinning today are not dimensionally stable and have a high overvoltage for oxygen evolution. By replacing the oxygen evolution in sulfate-based electrolytes with methanol oxidation, the anode potential could be significantly lowered. This would reduce the energy consumption and also enable the use of new and more stable types of electrodes. The activities of the electrodes studied were found sufficient for electrowinning. The highest activity was obtained with porous PtRu, while platinized titanium was the most stable electrode. All electrodes were deactivated with time, and the deactivation rate of PtRu was influenced by mass transport. The dominating reaction mechanism seemed to be different on Pt and on PtRu. Due to deactivation of the electrodes with time, a reactivation method was needed. The best method tested in this study was to periodically reverse the current. After assessing the electrodes with respect to important properties, the platinized titanium electrode was considered to be most suitable for use in electrowinning processes.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-24976 (URN)10.1149/1.2039567 (DOI)000233133700044 ()2-s2.0-27944434717 (Scopus ID)
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2017-12-12Bibliographically approved
7. Methanol and formic acid oxidation in zinc electrowinning under process conditions
Open this publication in new window or tab >>Methanol and formic acid oxidation in zinc electrowinning under process conditions
2008 (English)In: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 38, no 1, 17-24 p.Article in journal (Refereed) Published
Abstract [en]

The possibility of using methanol or formic acid oxidation as the anode process in zinc electrowinning was examined. The activity for methanol and formic acid oxidation on Pt coated high surface area electrodes was investigated over 36 h, at a current density used in industry. The activity could be maintained at a constant potential level in a synthetic electrowinning electrolyte if the current was reversed for short periods. During the tests, the anode potential was, more than 1.2 V below the potential for the oxygen evolving lead anodes used in modern zinc electrowinning. The lowered anode potential would lead to a significant energy reduction. However, tests in industrial electrolyte resulted in a very low activity for both methanol and formic acid oxidation. The low activity was shown to be caused mainly by chloride impurities. A reduction of the chloride content below 10(-5) M is needed in order to obtain sufficient activity for methanol oxidation on Pt for use in zinc electrowinning. Pt and PtRu electrodes were compared regarding their activity for methanol oxidation and the latter was shown to be more affected by chloride impurities. However, at a potential of 0.7 V vs NHE, with a chloride content of 10(-4) M, formic acid oxidation on PtRu gives the highest current density.

Keyword
anode reaction, zinc electrowinning, formic acid, methanol
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-24975 (URN)10.1007/s10800-007-9387-0 (DOI)000251370900003 ()2-s2.0-36749007865 (Scopus ID)
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2017-12-12Bibliographically approved

Open Access in DiVA

fulltext(3338 kB)1818 downloads
File information
File name FULLTEXT01.pdfFile size 3338 kBChecksum SHA-512
904047de766cae60558b5917b6c0f2fd7b75f64031092511373bb926ceb13bb13e2c8c3b12ff33cb50b198640171d3af54f103e892353a77defed14fef847ebb
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Wesselmark, Maria
By organisation
Applied Electrochemistry
Chemical Engineering

Search outside of DiVA

GoogleGoogle Scholar
Total: 1818 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 697 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf