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On the influence of Pt particle size on PEMFC cathode performance
Chalmers tekniska högskola, Göteborg.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
Chalmers tekniska högskola, Göteborg.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0001-9203-9313
2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, 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.

Place, publisher, year, edition, pages
2007. Vol. 52, no 24, 6848-6855 p.
Keyword [en]
platinum; nanoparticle size; PEMFC; oxygen reduction; phase transfer method
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-7239DOI: 10.1016/j.electacta.2007.04.106ISI: 000248783300033Scopus ID: 2-s2.0-34447095322OAI: oai:DiVA.org:kth-7239DiVA: diva2:12192
Note
QC 20100706. Uppdaterad från Accepted till Published 20100706.Available from: 2007-05-31 Created: 2007-05-31 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Evaluating Cathode Catalysts in the Polymer Electrolyte Fuel Cell
Open this publication in new window or tab >>Evaluating Cathode Catalysts in the Polymer Electrolyte Fuel Cell
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
fuel cell, humidity, model electrodes, Nafion, oxygen reduction, PEMFC, platinum, polymer electrolyte, thin film evaporation, titanium oxide, bränslecell, fuktighet, modellelektroder, Nafion, PEMFC, platina, polymerelektrolyt, syrereduktion, tunnfilmsförångning, titanoxid
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-4413 (URN)978-91-7178-714-9 (ISBN)
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

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