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Evaluating Cathode Catalysts in the Polymer Electrolyte Fuel Cell

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Evaluating Cathode Catalysts in the Polymer Electrolyte Fuel Cell Evaluating Cathode Catalysts in the Polymer Electrolyte Fuel Cell Henrik Ekström Doctoral Thesis Applied Electrochemistry, School of Chemical Science and Engineering, Kungliga Tekniska Högskolan, Stockholm, 2007 Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm, framlägges till offentlig granskning för avläggande av teknologie doktorsexamen måndagen den 11:e juni 2007, kl. 13.00 i sal D2, Lindstedtsvägen 5, Entréplan ii All rights reserved © Henrik Ekström 2007 Printed in Sweden Universitetsservice US-AB, Stockholm, 2007 TRITA-CHE-Report 2007:39 ISSN 1654-1081 ISBN 978-91-7178-714-9 Abstract The polymer electrolyte membrane fuel cell (PEMFC) converts the chemical energy of hydrogen and oxygen (air) into usable electrical energy. At the cathode (the posi- tive 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 method- ologies resemble traditional three-electrode research in liquid electrolytes, includ- ing cyclic voltammetry in inert gas, but with the advantage of performing the ex- periments 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 mem- branes, 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. Keywords: fuel cell, humidity, model electrodes, Nafion, oxygen reduction, PEMFC, platinum, polymer electrolyte, thin film evaporation, titanium oxide iii Sammanfattning I polymerelektrolytbränslecellen (PEMFC) omvandlas den kemiska energin hos vät- gas och syrgas (luft) direkt till användbar elektrisk energi. På katoden (den positiva elektroden) krävs betydande mängder platina för att katalysera den tröga syrere- duktionsreaktionen (ORR). Detta inverkar på kostnaden för högeffektsapplikatio- ner, 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 under- söka katalytisk aktivitet på katoden i PEMFC. Metodiken liknar traditionella tre- elektrodexperiment 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 ka- talyserade plana elektroder av vitröst kol (glassy carbon), var det möjligt att se att gasernas fuktighet har en betydande inverkan på ORR-kinetiken hos Pt. Tafel- lutningarna blir brantare och aktiviteten minskar när inloppsgasernas fuktighets- grad minskar. Eftersom den elektrokemiska arean hos Pt/Nafion-gränsskiktet in- te ändrades, ansågs dessa kinetiska effekter bero på en lägre täckningsgrad av Pt- oxider vid lägre fuktigheter, i kombination med lägre protonaktivitet. Genom att använda Nafionmembran belagda med nm-tjocka tvåskiktsmodell- elektroder undersöktes hur Pt i kombination med TiO2 och andra metalloxider ver- kar i PEMFC-miljön. Kinetiskt sett hade tillsatsen av metalloxider ingen inre påver- kan 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 cyklis- ka 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. Nyckelord: bränslecell, fuktighet, modellelektroder, Nafion, PEMFC, platina, poly- merelektrolyt, syrereduktion, tunnfilmsförångning, titanoxid iv List of Papers This thesis is based on the following papers: Paper 1 Alternative catalysts and carbon support material for PEMFC K. Wik- ander, H. Ekström, A.E.C. Palmqvist, A. Lundblad, K. Holmberg and G. Lindbergh Fuel Cells 06 (2006) 21–25 Paper 2 On the influence of Pt particle size on PEMFC cathode performance K. Wikander, H. Ekström, A.E.C. Palmqvist and G. Lindbergh Electro- chimica Acta (2007) doi:10.1016/j.electacta.2007.04.106 Paper 3 On the activity and stability of Sr3NiPtO6 and Sr3CuPtO6 as electro- catalysts for the oxygen reduction reaction in a polymer electrolyte fuel cell P. Kjellin, H. Ekström, G. Lindbergh and A.E.C. Palmqvist Journal of Power Sources 168 (2007) 346-350 Paper 4 A Novel Approach for Measuring Catalytic Activity of Planar Model Cat- alysts in the Polymer Electrolyte Fuel Cell Environment H. Ekström, P. Ha- narp, M. Gustavsson, E. Fridell, A. Lundblad and G. Lindbergh Jour- nal of The Electrochemical Society 153 (2006) A724–A730 Paper 5 Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell M. Gus- tavsson, H. Ekström, P. Hanarp, L. Eurenius, G. Lindbergh, E. Olsson and B. Kasemo Journal of Power Sources 163 (2007) 671–678 Paper 6 Nanometer-thick films of titanium oxide acting as electrolyte in the poly- mer electrolyte fuel cell H. Ekström, B. Wickman, M. Gustavsson, P. Ha- narp, L. Eurenius, E. Olsson and G. Lindbergh Electrochimica Acta 52 (2007) 4239–4245 In general, all manufacturing and non-electrochemical characterisation of the catalyst materials were performed by co-writers of these papers, rather than by the author of this thesis. Therefore the thesis has its focus on the planning, performing and evaluation of the electrochemical measurements of the above papers. The modelling work in Paper 4 was also done by the author. v vi LIST OF PAPERS The following publications, containing contributions from the author, are not included in this thesis, but were also published/compiled during the thesis work: Reduced two-dimensional one-phase model for analysis of the anode of a DMFC E. Birgersson, J. Nordlund, H. Ekström, M. Vynnycky and G. Lindbergh Jour- nal of The Electrochemical Society 150 (2003) A1368–A1376 Evaluation of a sulfophenylated polysulfone membrane in a fuel cell at 60 to 110 ◦C H. Ekström, B. Lafitte, A. Lundblad, P. Jannasch and G. Lindbergh Solid State Ionics (2007) doi:10.1016/j.ssi.2007.04.002 Acknowledgements First I would like to thank Professor Göran Lindbergh and Dr. Anders Lund- blad for supervising me, and the Swedish Foundation for Environmental Research (MISTRA) as well as Autobrane (a part of the 6th Framework Pro- gramme of the European Union) for financial support. Further it can not be stressed enough that none of this work would have been possible without the contribution of my colleagues at Chalmers — Per Hanarp, Kjell Wikander, Marie Gustavsson, Björn Wickman and Per Kjellin, who patiently have been providing me with catalytic materials to investigate throughout this work. Thank you. Other major scientific contributions that need to acknowledged are the work of the Mistra Phase 1 people: Peter Gode, Frédéric Jaouen, and Jari Ihonen, and the hints, tricks and tips from Dan Petterson that saved me a vast amount of time. At this point I also thank my diploma worker Gokul Ramamurthy, who participated in the the early development of the pipette method. In continuation I would like to thank all my other colleagues at Applied Electrochemistry during the years for providing a nice social atmosphere and a good working environment. For contributing to the advent of the thesis some of you deserve some extra recognition: Mårten, for sharing office with me and giving theoretical support in times of doubt, Sophie, for sharing bench and distress in the lab, Linda, for cheering me up, and Andreas for coaching. Finally, I would also express my gratitude for the emotional support from friends and family throughout the years. Thank you all. vii viii ACKNOWLEDGEMENTS Contents Abstract iii List of Papers v Acknowledgements vii 1 Introduction 1 2 Theoretical Background 3 2.1 The polymer electrolyte fuel cell .
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