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Electrochemical Characterisation of Porous Cathodes in the Polymer Electrolyte Fuel Cell

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Electrochemical Characterisation of Porous Cathodes in the Polymer Electrolyte Fuel Cell ELECTROCHEMICAL CHARACTERISATION OF POROUS CATHODES IN THE POLYMER ELECTROLYTE FUEL CELL Frédéric Jaouen Doctoral Thesis Department of Chemical Engineering and Technology Applied Electrochemistry Kungl Tekniska Högskolan Stockholm 2003 TRITA-KET R175 ISSN 1104-3466 ISRN KTH/KET/R-175-SE ISBN 91-7283-450-1 ELECTROCHEMICAL CHARACTERISATION OF POROUS CATHODES IN THE POLYMER ELECTROLYTE FUEL CELL Frédéric Jaouen Doctoral Thesis Department of Chemical Engineering and Technology Applied Electrochemistry Kungl Tekniska Högskolan Stockholm 2003 AKADEMISK AVHANDLING som med tillstånd av Kungl Tekniska Högskolan i Stockholm, framlägges till offentlig granskning för avläggande av teknisk doktorsexamen fredagen den 25 april 2003, kl. 10.15 i Kollegiesalen, Valhallavägen 79, Kungl Tekniska Högskolan, Stockholm. Abstract Polymer electrolyte fuel cells (PEFC) convert chemical energy into electrical energy with higher efficiency than internal combustion engines. They are particularly suited for transportation applications or portable devices owing to their high power density and low operating temperature. The latter is however detrimental to the kinetics of electrochemical reactions and in particular to the reduction of oxygen at the cathode. The latter reaction requires enhancing by the very best catalyst, today platinum. Even so, the cathode is responsible for the main loss of voltage in the cell. Moreover, the scarce and expensive nature of platinum craves the optimisation of its use. The purpose of this thesis was to better understand the functioning of the porous cathode in the PEFC. This was achieved by developing physical models to predict the response of the cathode to steady-state polarisation, current interruption (CI) and electrochemical impedance spectroscopy (EIS), and by comparing these results to experimental ones. The models account for the kinetics of the oxygen reduction as well as for the transport of the reactants throughout the cathode, i.e. diffusion of gases and proton migration. The agglomerate structure was assumed for the descrip- tion of the internal structure of the cathode. The electrochemical experiments were performed on electrodes having a surface of 0.5 cm2 using a laboratory fuel cell. The response of the cathode to various electrode compositions, thickness, oxygen pressure and relative humidity was experimentally investigated with steady-state polarisation, EIS and CI techniques. It is shown that a content in the cathode of 35- 43 wt % of Nafion, the polymer electrolyte, gave the best performance. Such cathodes display a doubling of the apparent Tafel slope at high current density. In this region, the current is proportional to the cathode thickness and to the oxygen pressure, which, according to the agglomerate model, corresponds to limitation by oxygen diffusion in the agglomerates. The same analysis was made using EIS. Moreover, experimental results showed that the Tafel slope increases for decreasing relative humidity. For Nafion contents lower than 35 wt %, the cathode becomes limited by proton migration too. For Nafion contents larger than 40 wt %, the cathode performance at high current density decreases again owing to an additional mass transport. The latter is believed to be oxygen diffusion throughout the cathode. The activity for oxygen reduction of catalysts based on iron acetate adsorbed on a carbon powder and pyrolysed at 900°C in ammonia atmosphere was also investigated. It was shown that the choice of carbon has a tremendous effect. The best catalysts were, on a weight basis, as active as platinum. Keywords: polymer electrolyte fuel cell, cathode, mass transport, porous electrode, modelling, agglomerate model, electrochemical impedance spectroscopy, current interrupt, transient techniques, non-noble catalysts i Sammanfattning Polymerelektrolytbränsleceller (PEFC) förvandlar kemisk energi till elektrisk energi med en högre verkningsgrad än förbränningsmotorer. De är särskilt lämpliga i fordon eller bärbara tillämpningar på grund av sin höga effekttäthet och låga driftstemperatur. Den sistnämnda utgör dock en nackdel för de elektrokemiska reaktionernas kinetik och i synnerhet för syrereduktion vid katoden. För att höja syrereduktionens hastighet måste den bästa katalysatorn användas, idag platina. Trots detta står katoden för det huvudsakliga spänningsfallet i cellen. Dessutom är platina ett kostsamt material med begränsad tillgänglighet vilket innebär att dess användning måste optimeras. Syftet med avhandlingen var att öka förståelsen för den porösa katodens beteende i PEFC. Detta möjliggjordes med hjälp av fysikaliska modeller som simulerar katodens respons vid steady-state polarisationskurvor, strömbrytning (CI) och elektrokemisk impedansspektroskopi (EIS) samt genom att jämföra teoretiska och experimentella resultat. Modellerna tar hänsyn till syrereduktionskinetik samt reaktanternas transport genom elektroden, d.v.s. gasdiffusion och protonmigration. Katodens inre struktur beskrivs i modellerna som agglomerat. Elektrokemiska experiment genomfördes på elektroder med en yta av 0.5 cm2 i en laboratorie- bränslecell. Katodens sätt att reagera vid olika elektrodsammansättningar, tjocklek, syrgas partialtryck och relativ fuktighet undersöktes experimentellt med steady-state, EIS och CI tekniker. Det framgick att bäst prestanda erhålls när andelen polymerelektrolyt i katoden, Nafion, ligger mellan 35 och 43 viktsprocent. Då visar katoden en tydlig fördubbling av den synbara Tafellutningen vid hög strömtäthet. I detta område är strömmen proportionell med katodens tjocklek och syrgastryck, vilket, enligt agglomeratmodellen, stämmer överens med begränsning p.g.a långsam syrediffusion i agglomeraten. Samma analys gjordes med EIS. Dessutom visade de experimentella resultaten att Tafellutningen ökar med minskad relativ fuktighet. Utöver det blir en katod som har en Nafionhalt lägre än 35 viktsprocent begränsad p.g.a protonmigration. När Nafionhalten överstiger 40 viktsprocent försämras katodens prestanda vid hög strömtäthet. Detta tyder på en ytterligare masstransportbegränsning som visar flera tecken på att vara långsam syrediffusion genom elektroden. Järnacetat adsorberat på olika kolpulver och pyrolyserat vid 900°C i ammoniakatmosfär undersöktes som katalysator för syrereduktion. Studien visar att kolet har en väldig effekt. De bästa erhållna katalysatorer var lika aktiva, per vikt, som platina. Nyckelord: Polymerelektrolytbränslecell, katod, masstransport, porös elektrod, modellering, agglomeratmodell, elektrokemisk impedansspektroskopi, strömbrytning, transienta tekniker, icke ädelmetalkatalysator ii List of Papers This thesis is by far and large based on the following papers, though a handful of completing results appear in this thesis only. I. Jari Ihonen, Frédéric Jaouen, Göran Lindbergh, Göran Sundholm A novel polymer electrolyte fuel cell for laboratory investigations and in-situ contact resistance measurements Electrochimica Acta 46, 2899 (2001) II. Frédéric Jaouen, Göran Lindbergh, Göran Sundholm Investigation of Mass-Transport Limitations in the Solid Polymer Fuel Cell Cathode. I. Mathematical Model Journal of The Electrochemical Society 149, A437 (2002) III. Jari Ihonen, Frédéric Jaouen, Göran Lindbergh, Anders Lundblad, Göran Sundholm Investigation of Mass-Transport Limitations in the Solid Polymer Fuel Cell Cathode. II. Experimental Journal of The Electrochemical Society 149, A448 (2002) IV. Frédéric Jaouen, Göran Lindbergh Transient Techniques for Investigating Mass-Transport Limitations in Gas Diffusion Electrodes. I. Modeling the PEFC Cathode Submitted to Journal of The Electrochemical Society V. Frédéric Jaouen, Göran Lindbergh, Katarina Wiezell Transient Techniques for Investigating Mass-Transport Limitations in Gas Diffusion Electrodes. II. Experimental Characterization of the PEFC Cathode Submitted to Journal of The Electrochemical Society VI. Peter Gode, Frédéric Jaouen, Göran Lindbergh, Anders Lundblad, Göran Sundholm Influence of the ink composition on the structure and electrochemical characteristics of the PEFC cathode Manuscript VII. Frédéric Jaouen, Sébastien Marcotte, Jean-Pol Dodelet, Göran Lindbergh Oxygen reduction catalysts for polymer electrolyte fuel cells from the pyrolysis of iron acetate adsorbed on various carbon supports Journal of Physical Chemistry B 107, 1376 (2003) iii My contribution to the different papers of the thesis It is difficult to specify the contribution of each author in a paper's conclusions. At an earlier stage, it is easier to define the practical tasks performed by each author. This is done below, not in too great detail, and focusing on those papers containing data that I did not measure or evaluate myself. My contribution to paper I was minor. I was involved in the development of the first version of the "small polymer fuel cell" as well as with the testing of the humidifiers. In papers III and VI, I was neither involved in the non-electrochemical materials characterisation nor in the electrode fabrication. In paper V, the electrochemical investigation as well as the elaboration of the fitting algorithms were shared with Katarina Wiezell. The writing of paper VII is, I fear, an inextricable network of Jean-Pol Dodelet's and my own work. However, my contribution to the introduction section was minor. As for the concrete work, I synthesised all catalysts and studied them using the rotating disk electrode (RDE), x-ray photoelectron spectroscopy (XPS) and gas porosimetry. In order to obtain more statistical results, the RDE experiments
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