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Evaluating Cathode Catalysts in the Polymer Electrolyte Fuel Cell
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The polymer electrolyte membrane fuel cell (PEMFC) converts the chemical energy of hydrogen and oxygen (air) into usable electrical energy. At the cathode (the positive electrode), a considerable amount of platinum is generally required to catalyse the sluggish oxygen reduction reaction (ORR). This has implications regarding the cost in high-power applications, and for making a broad commercialisation of the PEMFC technology possible, it would be desirable to lower the amount of Pt used to catalyse the ORR.

In this thesis a number of techniques are described that have been developed in order to investigate catalytic activity at the cathode of the PEMFC. These methodologies resemble traditional three-electrode research in liquid electrolytes, including cyclic voltammetry in inert gas, but with the advantage of performing the experiments in the true PEMFC environment.

From the porous electrode studies it was seen that it is possible to reach mass activities close to 0.2 gPt/kW at potentials above 0.65 V at 60 ◦C, but that the mass activities may become considerably lower when raising the temperature to 80 ◦C and changing the measurement methodology regarding potential cycling limits and electrode manufacturing.

The model electrode studies rendered some interesting results regarding the ORR at the Pt/Nafion interface. Using a novel measurement setup for measuring on catalysed planar glassy carbon disks, it was seen that humidity has a considerable effect on the ORR kinetics of Pt. The Tafel slopes become steeper and the activity decreases when the humidity level of the inlet gases decreases. Since no change in the the electrochemical area of the Pt/Nafion interface could be seen, these kinetic phenomena were ascribed to a lowered Pt oxide coverage at the lower humidity level, in combination with a lower proton activity.

Using bi-layered nm-thick model electrodes deposited directly on Nafion membranes, the behaviour of TiO2 and other metal oxides in combination with Pt in the PEMFC environment was investigated. Kinetically, no intrinsic effect could be seen for the model electrodes when adding a metal oxide, but compared to porous electrodes, the surface (specific) activity of a 3 nm film of Pt deposited on Nafion seems to be higher than for a porous electrode using ∼4 nm Pt grains deposited on a carbon support. Comparing the cyclic voltammograms in N2, this higher activity could be ascribed to less Pt oxide formation, possibly due to a particle size effect.

For these bi-layered films it was also seen that TiO2 may operate as a proton-conducting electrolyte in the PEMFC.

Abstract [sv]

I polymerelektrolytbränslecellen (PEMFC) omvandlas den kemiska energin hos vätgas och syrgas (luft) direkt till användbar elektrisk energi. På katoden (den positiva elektroden) krävs betydande mängder platina för att katalysera den tröga syrereduktionsreaktionen (ORR). Detta inverkar på kostnaden för högeffektsapplikationer, och för att göra en bred kommersialisering av PEMFC-teknologin möjlig skulle det vara önskvärt att minska den Pt-mängd som används för att katalysera ORR. I denna avhandling beskrivs ett antal tekniker som utvecklats för att undersöka katalytisk aktivitet på katoden i PEMFC. Metodiken liknar traditionella treelektrodexperiment i vätskeformig elektrolyt, med cyklisk voltammetri i inert gas, men med fördelen att försöken utförs i den riktiga PEMFC-miljön. I försök med porösa elektroder visades att det är möjligt att nå massaktiviteter nära 0.2 gPt/kW för potentialer över 0.65 V vid 60 ◦C, men massaktiviteterna kan bli betydligt lägre om temperaturen höjs till 80 ◦C, och om potentialsvepgränser och elektrodentillverkningsmetod ändras. Försök med modellelektroder resulterade i intressanta resultat rörande ORR i gränsskiktet Pt/Nafion. Genom att använda en ny metodik för att mäta på katalyserade plana elektroder av vitröst kol (glassy carbon), var det möjligt att se att gasernas fuktighet har en betydande inverkan på ORR-kinetiken hos Pt. Tafellutningarna blir brantare och aktiviteten minskar när inloppsgasernas fuktighetsgrad minskar. Eftersom den elektrokemiska arean hos Pt/Nafion-gränsskiktet inte ändrades, ansågs dessa kinetiska effekter bero på en lägre täckningsgrad av Ptoxider vid lägre fuktigheter, i kombination med lägre protonaktivitet. Genom att använda Nafionmembran belagda med nm-tjocka tvåskiktsmodellelektroder undersöktes hur Pt i kombination med TiO2 och andra metalloxider verkar i PEMFC-miljön. Kinetiskt sett hade tillsatsen av metalloxider ingen inre påverkan på aktiviteten, men vid jämförelse med porösa elektroder tycks den specifika ytaktiviteten vara högre hos en 3 nm film av Pt på Nafion än för en porös elektrod baserad på ∼4 nm Pt-korn belagda på ett kolbärarmaterial. Jämför man de cykliska voltammogrammen i N2, kan den högre aktiviteten tillskrivas en lägre grad av Pt-oxidbildning, vilket i sin tur kan bero på en storlekseffekt hos Pt-partiklarna. Försöken med dessa tvåskiktselektroder visade också att TiO2 kan verka som protonledande elektrolyt i PEMFC.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , xii, 50 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:39
Keyword [en]
fuel cell, humidity, model electrodes, Nafion, oxygen reduction, PEMFC, platinum, polymer electrolyte, thin film evaporation, titanium oxide
Keyword [sv]
bränslecell, fuktighet, modellelektroder, Nafion, PEMFC, platina, polymerelektrolyt, syrereduktion, tunnfilmsförångning, titanoxid
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4413ISBN: 978-91-7178-714-9 (print)OAI: oai:DiVA.org:kth-4413DiVA: diva2:12197
Public defence
2007-06-11, D2, Lindstedsvägn 5, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100706Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2010-07-06Bibliographically approved
List of papers
1. Alternative catalysts and carbon support material for PEMFC
Open this publication in new window or tab >>Alternative catalysts and carbon support material for PEMFC
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2006 (English)In: Fuel Cells, ISSN 1615-6846, E-ISSN 1615-6854, Vol. 6, no 1, 21-25 p.Article in journal (Refereed) Published
Abstract [en]

In order to investigate the possibility of increasing the reactivity for oxygen reduction reaction (ORR) of the cathode in a PEMFC a series of Pt/C catalysts was prepared using water-in-oil microemulsions for synthesizing Pt nanoparticles. The Pt nanoparticles were deposited on porous carbon support (Vulcan XC-72 or a mesoporous carbon) and the catalysts were processed into MEAs. The MEA samples were evaluated and compared with a commercial sample and with Pt/C catalyst samples prepared using a conventional direct impregnation method. The mesoporous carbon support investigated as a potential alternative to Vulcan XC72 has a very high specific surface area and a narrow pore size distribution. The materials were characterized with XRD, TEM, SEM-EDX, N-2 sorption and steady state polarization. It was found that it is possible to increase the ORR reactivity using the microemulsion route for formation of Pt nanoparticles. It was concluded that the MEA processing conditions for the mesoporous carbon support have to be modified to reach improved ORR reactivity, likely due to the large differences in specific surface area, porosity and conductivity compared to the Vulcan carbon.

Keyword
catalyst preparation; mesoporous; carbon; microemulsion; platinum nanoparticles
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7238 (URN)10.1002/fuce.200500092 (DOI)000235648000005 ()2-s2.0-33644585520 (Scopus ID)
Note
QC 20100706Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2010-07-06Bibliographically approved
2. On the influence of Pt particle size on PEMFC cathode performance
Open this publication in new window or tab >>On the influence of Pt particle size on PEMFC cathode performance
2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 0019-4686, Vol. 52, no 24, 6848-6855 p.Article in journal (Refereed) Published
Abstract [en]

Colloidal suspensions of almost spherical and crystalline Pt nanoparticles between 1.6 and 2.6 nm in diameter and with narrow size distribution were synthesized using the phase transfer method (PTM) with alkylamines, CnNH2, as stabilizing agents. Batches of such homogenous Pt-CnNH2 (n =8, 12) nanocrystals were deposited onto Vulcan XC-72 carbon powder, and the activity for the oxygen reduction reaction (ORR) of this series of Pt/C materials was evaluated under PEMFC conditions. The aim was to elucidate whether this type of stabilized Pt nanoparticles were as active for the ORR as a corresponding commercial Pt/C material, and if any difference in mass activity could be observed between catalysts with different Pt particle size. In the PEMFC experiments, i.e. voltammetry in oxygen and nitrogen, it was found that, after an initial electrode activation, the ORR activity of the catalysts prepared from the alkylamine-stabilized Pt nanoparticles deposited on carbon was as high as that of the employed commercial reference catalyst. In fact, all samples in the Pt/C series showed high and very similar ORR activity normalized to Pt-loading, without significant dependence on the initial Pt particle size. However, pre- and post-electrochemical characterization of the Pt/C material series with TEM showed that structural changes of the Pt nanoparticles occurred during electrochemical evaluation. In all samples studied the mean Pt particle size increased during the electrochemical evaluation resulting in decreased differences between the samples explaining the observed similar ORR performance of the different materials. These results emphasize the necessity of post-operation characterization of fuel cell catalysts when discussing electrocatalytic activity. In addition, employing complex preparation efforts for lowering the Pt particle size below 3 ran may have limited practical value unless the particles are stabilized from electrochemical sintering.

Keyword
platinum; nanoparticle size; PEMFC; oxygen reduction; phase transfer method
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7239 (URN)10.1016/j.electacta.2007.04.106 (DOI)000248783300033 ()2-s2.0-34447095322 (Scopus ID)
Note
QC 20100706. Uppdaterad från Accepted till Published 20100706.Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2010-07-06Bibliographically approved
3. On the activity and stability of Sr3NiPtO6 and Sr3CuPtO6 as electrocatalysts for the oxygen reduction reaction in a polymer electrolyte fuel cell
Open this publication in new window or tab >>On the activity and stability of Sr3NiPtO6 and Sr3CuPtO6 as electrocatalysts for the oxygen reduction reaction in a polymer electrolyte fuel cell
2007 (English)In: Journal of Power Sources, ISSN 0378-7753, Vol. 168, no 2, 346-350 p.Article in journal (Refereed) Published
Abstract [en]

Sr3NiPtO6 and Sr3CUPtO6 were evaluated as low-platinum alternative oxygen reduction catalysts in a solid polymer electrolyte fuel cell at 80 degrees C. The oxides were synthesised using a new method based on an organometallic precursor route. The electrochemical evaluation showed similar oxygen reduction performance for Sr3NiPtO6 and Sr3CUPtO6, with a slightly higher activity for Sr3NiPtO6. In comparison with the oxides, the oxygen reduction activity for a commercial Pt/C catalyst was approximately 10 times higher. XRD analysis of the used electrodes revealed that the oxides were not stable in the PEMFC environment, and converted into platinum during operation. Elemental analysis of the used electrodes also showed a difference in platinum formation, where the platinum content on the surface of the electrode facing the gas diffusion layer was several times higher for Sr3NiPtO6 than Sr3CUPtO6. This indicates that the Sr3NiPtO6 electrode may be more susceptible to platinum migration.

Keyword
platinum oxide; Sr3NiPtO6; Sr3CuPtO6; fuel cell; PEMFC
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7240 (URN)10.1016/j.jpowsour.2007.02.051 (DOI)000247055200007 ()2-s2.0-34247632579 (Scopus ID)
Note
QC 20100706Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2010-07-06Bibliographically approved
4. A Novel Approach for Measuring Catalytic Activity of Planar Model Catalysts in the Polymer Electrolyte Fuel Cell Environment
Open this publication in new window or tab >>A Novel Approach for Measuring Catalytic Activity of Planar Model Catalysts in the Polymer Electrolyte Fuel Cell Environment
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2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, Vol. 153, no 4, A724-A730 p.Article in journal (Refereed) Published
Abstract [en]

The electrochemical oxygen reduction reaction on nanostructured supported platinum electrodes is measured using a newly developed solid-state polymer electrolyte electrochemical cell. Measurements were made on three types of catalytic surfaces on glassy carbon supports: nanostructured model electrodes prepared by colloidal lithography, a thin thermally evaporated Pt film, and a pure glassy carbon surface. Measurements in nitrogen and oxygen at several different humidities were performed at 60 degrees C in a fuel-cell-like environment. Lowering humidity showed a higher Tafel slope at high potentials for oxygen reduction on the nanostructured catalyst. Good agreement between the electrochemical active area from the hydrogen adsorption peaks and the catalytic area determined from scanning electron microscopy images was found. No significant change of the electrochemically active area with humidity could be found. Double-layer capacitance and oxygen reduction currents increased with increased humidification temperatures.

Keyword
Atmospheric humidity; Capacitance; Electrochemistry; Electrodes; Fuel cells; Nanostructured materials; Platinum; Polyelectrolytes; Scanning electron microscopy
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7241 (URN)10.1149/1.2170578 (DOI)000235723600013 ()2-s2.0-33644806890 (Scopus ID)
Note

QC 20100706

Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2016-12-21Bibliographically approved
5. Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell
Open this publication in new window or tab >>Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell
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2007 (English)In: Journal of Power Sources, ISSN 0378-7753, Vol. 163, no 2, 671-678 p.Article in journal (Refereed) Published
Abstract [en]

Thin film Pt/TiO2 catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO2, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3-6 mu g cm(-2)). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO2 layer between the platinum and the Nation increases the performance compared to a Pt film deposited directly on Nation. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO2 compared to on Nalion and in addition, substantial proton conduction through the thin Ti02 layer. It is also shown that deposition order and the film thickness affects the performance.

Keyword
fuel cells; polymer electrolyte; oxygen reduction; electrocatalyst; Pt; TiO2
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7242 (URN)10.1016/j.jpowsour.2006.10.005 (DOI)000243650800007 ()2-s2.0-33845722564 (Scopus ID)
Note
QC 20100706Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2010-07-06Bibliographically approved
6. Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell
Open this publication in new window or tab >>Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell
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2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 0019-4686, Vol. 52, no 12, 4239-4245 p.Article in journal (Refereed) Published
Abstract [en]

0-18nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nation 117 membranes, and used as thin spacer electrolyte layers between the Nation and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 degrees C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO2 may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO2 as electrolyte.

Keyword
proton conduction; titanium oxide; fuel cell; oxygen reduction; thermal evaporation
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7243 (URN)10.1016/j.electacta.2006.12.002 (DOI)000245365900043 ()2-s2.0-33847294164 (Scopus ID)
Note
QC 20100706Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2010-07-06Bibliographically approved

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