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Active Area Determination of Porous Pt Electrodes Used in Polymer Electrolyte Fuel Cells: Temperature and Humidity Effects
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0002-0452-0703
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0002-6019-6485
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0002-1626-1067
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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.

Place, publisher, year, edition, pages
2010. Vol. 157, no 12, B1795-B1801 p.
Keyword [en]
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: urn:nbn:se:kth:diva-25266DOI: 10.1149/1.3494220ISI: 000283938300022Scopus ID: 2-s2.0-78449298256OAI: oai:DiVA.org:kth-25266DiVA: diva2:356873
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
In thesis
1. Electrochemical Reactions in Polymer Electrolyte Fuel Cells
Open this publication in new window or tab >>Electrochemical Reactions in Polymer Electrolyte Fuel Cells
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
Fuel cell, model electrodes, oxygen reduction, methanol oxidation, formic acid oxidation, hydrogen oxidation, CO oxidation, degradation, tungsten oxide, carbon support, Bränslecell, modellelektroder, syrgasreduktion, metanoloxidation, myrsyraoxidation, vätgasoxidation, CO oxidation, degradering, wolfram oxid, kolsupport
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-25267 (URN)978-91-7415-747-5 (ISBN)
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
2. Electrode degradation in proton exchange membrane fuel cells
Open this publication in new window or tab >>Electrode degradation in proton exchange membrane fuel cells
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The topic of this thesis is the degradation of fuel cell electrodes in proton exchange membrane fuel cells (PEMFCs). In particular, the degradation associated with localized fuel starvation, which is often encountered during start-ups and shut-downs (SUs/SDs) of PEMFCs. At SU/SD, O2 and H2 usually coexist in the anode compartment. This situation forces the opposite electrode, i.e. the cathode, to very high potentials, resulting in the corrosion of the carbon supporting the catalyst, referred to as carbon corrosion. The aim of this thesis has been to develop methods, materials and strategies to address the issues associated to carbon corrosion in PEMFC.The extent of catalyst degradation is commonly evaluated determining the electrochemically active surface area (ECSA) of fuel cell electrode. Therefore, it was considered important to study the effect of RH, temperature and type of accelerated degradation test (ADT) on the ECSA. Low RH decreases the ECSA of the electrode, attributed to re-structuring the ionomer and loss of contact with the catalyst.In the search for more durable supports, we evaluated different accelerated degradation tests (ADTs) for carbon corrosion. Potentiostatic holds at 1.2 V vs. RHE were found to be too mild. Potentiostatic holds at 1.4 V vs. RHE were found to induce a large degree of reversibility, also attributed to ionomer re-structuring. Triangle-wave potential cycling was found to irreversibly degrade the electrode within a reasonable amount of time, closely simulating SU/SD conditions.Corrosion of carbon-based supports not only degrades the catalyst by lowering the ECSA, but also has a profound effect on the electrode morphology. Decreased electrode porosity, increased agglomerate size and ionomer enrichment all contribute to the degradation of the mass-transport properties of the cathode. Graphitized carbon fibers were found to be 5 times more corrosion resistant than conventional carbons, primarily attributed to their lower surface area. Furthermore, fibers were found to better maintain the integrity of the electrode morphology, generally showing less degradation of the mass-transport losses. Different system strategies for shut-down were evaluated. Not doing anything to the fuel cell during shut-downs is detrimental for the fuel cell. O2 consumption with a load and H2 purge of the cathode were found to give around 100 times lower degradation rates compared to not doing anything and almost 10 times lower degradation rate than a simple air purge of the anode. Finally, in-situ measurements of contact resistance showed that the contact resistance between GDL and BPP is highly dynamic and changes with operating conditions.

Abstract [sv]

Denna doktorsavhandling behandlar degraderingen av polymerelektrolytbränslecellselektroder. polymerelektrolytbränslecellselektroder. Den handlar särskilt om nedbrytningen av elektroden kopplad till en degraderingsmekanism som heter ”localized fuel starvation” oftast närvarande vid uppstart och nedstängning av bränslecellen. Vid start och stopp kan syrgas och vätgas förekomma samtidigt i anoden. Detta leder till väldigt höga elektrodpotentialer i katoden. Resultatet av detta är att kolbaserade katalysatorbärare korroderar och att bränslecellens livslängd förkortas. Målet med avhandlingen har varit att utveckla metoder, material och strategier för att både öka förståelsen av denna degraderingsmekanism och för att maximera katalysatorbärarens livslängd.Ett vanligt tillvägagångsätt för att bestämma graden av katalysatorns degradering är genom mätning av den elektrokemiskt aktiva ytan hos bränslecellselektroderna. I denna avhandling har dessutom effekten av temperatur och relativ fukthalt studerats. Låga fukthalter minskar den aktiva ytan hos elektroden, vilket sannolikt orsakas av en omstrukturering av jonomeren och av kontaktförlust mellan jonomer och katalysator.Olika accelererade degraderingstester för kolkorrosion har använts. Potentiostatiska tester vid 1.2 V mot RHE visade sig vara för milda. Potentiostatiska tester vid 1.4 V mot RHE visade sig däremot medföra en hög grad av reversibilitet, som också den tros vara orsakad av en omstrukturering av jonomeren. Cykling av elektrodpotentialen degraderade istället elektroden irreversibelt, inom rimlig tid och kunde väldigt nära simulera förhållandena vid uppstart och nedstängning.Korrosionen av katalysatorbäraren medför degradering av katalysatorn och har också en stor inverkan på elektrodens morfologi. En minskad elektrodporositet, en ökad agglomeratstorlek och en anrikning av jonomeren gör att elektrodens masstransportegenskaper försämras. Grafitiska kolfibrer visade sig vara mer resistenta mot kolkorrosion än konventionella kol, främst p.g.a. deras låga ytarea. Grafitiska kolfibrer visade också en förmåga att bättre bibehålla elektrodens morfologi efter accelererade tester, vilket resulterade i lägre masstransportförluster.Olika systemstrategier för nedstängning jämfördes. Att inte göra något under nedstängning är mycket skadligt för bränslecellen. Förbrukning av syre med en last och spolning av katoden med vätgas visade 100 gånger lägre degraderingshastighet av bränslecellsprestanda jämfört med att inte göra något alls och 10 gånger lägre degraderingshastighet jämfört med spolning av anoden med luft. In-situ kontaktresistansmätningar visade att kontaktresistansen mellan bipolära plattor och GDL är dynamisk och kan ändras beroende på driftförhållandena.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 77 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:43
Keyword
PEMFC; PEFC; Porous electrodes; Carbon corrosion; Carbon nanotubes; Carbon fibers; Polyhedral carbon nanofoams; Phosphonated hydrocarbon ionomer; Mass-transport losses; Electrode morphology; Electrode collapse; Start-up and shut-down; System strategies; Stainless steel; Bi-polar plates; In-situ contact resistance.
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-133437 (URN)978-91-7501-890-4 (ISBN)
Public defence
2013-11-22, Kollegisalen, Brinellvägen 8, plan 4, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131104

Available from: 2013-11-04 Created: 2013-11-01 Last updated: 2013-11-04Bibliographically approved
3. The Impact of Hydrocarbon and Carbon Oxide Impuritiesin the Hydrogen Feed of a PEM Fuel Cell
Open this publication in new window or tab >>The Impact of Hydrocarbon and Carbon Oxide Impuritiesin the Hydrogen Feed of a PEM Fuel Cell
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The proton exchange membrane fuel cell generates electricity from hydrogen and oxygen (from air) through electrocatalytic reactions in an electrochemical cell. The Pt/C catalyst, commonly used in PEM fuel cells, is very sensitive to impurities that can interact with the active catalyst sites and limit fuel cell performance. Unfortunately, most hydrogen is currently produced from fossil sources, and inevitably contains impurities.

The subject of this thesis is the effect of hydrogen impurities on the operation of a PEM fuel cell using a Pt/C anode. The impurities studied are carbon monoxide (CO), carbon dioxide (CO2), and selected hydrocarbons. Particular focus is given to the interaction between the impurities studied and the anode catalyst. The main method used in the study involved performing cyclic voltammetry and mass spectrometry, simultaneously. Other electrochemical techniques are also employed.

The results show that all the impurities studied adsorb to some extent on the Pt/C catalyst surface, and require potentials comparable to that of CO oxidation, i.e., about 0.6V, or higher to be removed by oxidation to CO2. For complete oxidation of propene, and toluene, potentials of above 0.8, and 1.0V, respectively, are required. The unsaturated hydrocarbons can be desorbed to some extent by reduction, but oxidation is required for complete removal. Adsorption of ethene, propene, and CO2 is dependent on the presence of adsorbed or gaseous hydrogen. Hydrogen inhibits ethene and propene adsorption, but facilitates CO2 adsorption. Adsorption of methane and propane is very limited and high concentrations of methane cause dilution effects only.

The adlayer formed on the Pt/C anode catalyst in the presence of CO2, or moderate amounts of hydrocarbons, is found to be insffuciently complete to notably interfere with the hydrogen oxidation reaction. Higher concentrations of toluene do, however, limit the reaction.

Abstract [sv]

Polymerelektrolytbränslecellen genererar elektricitet fran vätgas och syrgas (fran luft) genom elektrokatalytiska reaktioner i en elektrokemisk cell. Den platina-baserade katalysator som oftast används i dessa bränsleceller är känslig mot föroreningar, då dessa kan interagera med katalysatorns aktiva yta, och därmed begränsna bränslecellens prestanda. Tyvärr produceras dagens vätgas huvudsakligen fran fossila källor och innehåller därför oundvikligen föroreningar.

Denna avhandling behandlar hur olika vätgasföroreningar påverkar katalysatorns aktivitet och bränslecellens drift. De föroreningar som studeras är kolmonoxid (CO) och koldioxid (CO2), samt ett antal mindre kolväten. Störst fokus ligger på hur dessa föroreningar interagerar med anodens Pt/C katalysator. Den metod som huvudsakligen används är cyklisk voltammetri kombinerat med masspektrometri, men flera elektrokemiska metoder har använts.

Resultaten visar att alla undersökta föroreningar adsorberar på Pt/C katalysatorns yta i större eller mindre utstreckning. For att avlägsna det adsoberade skiktet genom oxidation till CO2 krävs potentialer jämförbara med CO oxidation, dvs ca 0,6V, eller högre. Fullständig oxidation av propen eller toluen kräver potentialer högre än 0,8V respektive 1,0V. De omättade kolvätena kan delvis avlägsnas genom reduktion, men fullständig avlägsning kräver oxidation. Närvaron av väte, i gasform eller adsorberat pa katalysatorn, hämmar adsorptionen av eten och propen, men främjar CO2 adsorption. Metan och propan adsorberar i mycket begränsad utstreckning på Pt/C katalysatorns yta. De prestandaförluster som uppstår av höga koncentrationer av metan förklaras av utspädning av vätgasen.

Det adsorberade skiktet som bildas när Pt/C katalysatorn exponeras för CO2 eller måttliga koncentrationer av studerade kolväten, är inte tillräckligt heltäckande for att märkbart påverka vätgasreduktionen. Däremot kan höga koncentrationer av toluen begränsa reaktionen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 91 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:26
Keyword
Fuel Cell, Hydrogen Impurities, Carbon Monoxide, Carbon Dioxide, Ethene, Propene, Methane, Propane, Toluene, Electrochemically Active Surface Area, Cyclic Voltammetry, Mass Spectrometry, bränslecell, vätgasföroreningar, kolmonoxid, koldioxid, eten, propen, metan, propan, toluen, elektrokemisk aktiv yta, cyklisk voltammetri, masspektrometri
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-193694 (URN)978-91-7729-008-7 (ISBN)
Public defence
2016-10-28, K2, Teknikringen 28, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20161010

Available from: 2016-10-10 Created: 2016-10-07 Last updated: 2016-10-10Bibliographically approved

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Wreland Lindström, RakelOyarce, AlejandroLagergren, CarinaLindbergh, Göran

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