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New Materials for the Molten Carbonate Fuel Cell
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [sv]

Smältkarbonatbränslecellen (MCFC) är en högtemperaturbränslecell för stationära applikationer. Den har samma höga totalverkningsgrad som konventionella kraftvärme-anläggningar, men kan byggas i mindre moduler (från 250 kWe). De små modulerna och den bränsleflexibilitet (naturgas, biogas, etanol, diesel) som MCFC har, gör den intressant för exempelvis industrier med organiska restprodukter och höga krav på tillförlitlighet. Den höga temperaturen och närvaron av en saltsmälta gör dock materialdegradering till en viktig faktor för forskning och utveckling inom området. För även om de fälttester som nyligen gjorts har visat på att vissa av degraderingsprocesserna är mindre allvarliga än förväntat, finns fortfarande ett behov av utveckling för att sänka kostnaderna och förlänga livstiden.

I första delen av detta arbete undersöktes material för olika delar av cellen inom ramarna för EU-projektet IRMATECH. Materialen ansågs vara interessanta alternativ till de nuvarande materialen på grund av deras lägre kostnad och/eller bättre prestanda. Två alternativa anodströmtilledarmaterial undersöktes. För anodströmtilledaren är korrosionen och den elektriska resistansen av det eventuella oxidlagret nyckelparametrar. Dessa parametrar undersöktes och utvärderades. Fastän de båda alternativa materialen hade oxidlager med låg resistans, fanns indikationer på korrosionsprocesser som kan äventyra materialets långtidsstabilitet.

För katodmaterialet, NiO, har upplösningen varit problemet. De upplösta nickeljonerna fälls ut i elektrolyten och bildar dendriter som kan kortsluta cellen. Därför undersöktes nickelupplösningen hos tre alternativa katodmaterial. Det mest lovande materialet, en nickeloxid-katod dopad med magnesium och järn testades i en singelcell för att studera elektrokemisk prestanda, morfologi och områden där nickelutfällning skett. Resultaten visade att prestandan var jämförbar med NiO, men att den mekaniska stabiliteten måste undersökas ytterligare.

I ”wet-seal”-området är det rostfria stålet belagt med ett aluminiumskikt för att skydda det från den mycket korrosiva miljön. Tillverkningsprocesserna för dessa aluminiumbeläggningar har hittills varit dyra och komplexa. Därför utvärderades en alternativ tillverkningsprocess. Beläggningen, studerad i både reducerande och oxiderande miljö visade en tendens till att spricka och därmed exponera det underliggande rostfria stålet. Detta berodde troligtvis på en manuell beläggningsprocess som resulterade i ett inhomogent ytskikt.

I den andra delen av arbetet föreslogs en alternativ tillverkningsmetod, baserad på nyligen publicerade resultat där man elektrodeponerat aluminium från jonvätskor. Dessa har ett större katodiskt fönster än vatten och möjliggör därför elektrodeponering av elektropositiva material. För att göra processen industrivänlig provades ett alternativ till den vanligen använda aluminiumtrikloriden. Det visade sig dock att påverkan av miljön på stabiliteten hos jonvätskan behövde undersökas innan några material kunde tillverkas. Vatten i kombination med syre visade sig ha en stor inverkan på den katodiska strömtätheten. I frånvaro av dessa komponenter var jonvätskan mycket stabil.

Abstract [en]

The Molten Carbonate Fuel Cell (MCFC) is a high temperature fuel cell for stationary applications. It has the same high over-all efficiency (90%) as traditional combined heat and power plants, but MCFC can be built in small modules (from 250 kWe). The small modules in combination with fuel flexibility (natural gas, biogas, ethanol, diesel) makes MCFC an interesting alternative for industries with organic waste and high demands for reliability. The high temperature (650 °C) and the presence of molten salt result however in material degradation. Corrosion and dissolution of the materials used have been the challenge for MCFC. Although long-term field trials have shown that some of the material problems are not as severe as first believed, further material development is necessary to decrease the cost and prolong the life-time.

In the first part of this work, materials for different parts of the cell were tested within the EU project IRMATECH. The materials were interesting alternatives to the state-of-the-art materials due to their lower cost and/or better performance. Two alternative anode current collector materials were tested. For the anode current collector the corrosion and electrical resistance of the possible oxide layer are key parameters. These parameters were investigated and evaluated. Although both the materials showed a low resistance, there were indications of corrosion processes which could affect the life-time of the material.

For the cathode material, NiO, the dissolution of the material has been a problem. The dissolved nickel ions precipitate in the electrolyte and form conductive nickel dendrites that eventually short-circuit the cell. Therefore, the nickel dissolution of three alternative cathode materials was tested. The most promising material, a NiO doped with magnesium and iron, was tested in a single cell to study the electrical performance, the morphology after operation and the area where nickel had precipitated. The results showed that the performance was comparable to NiO, but it is necessary to investigate the mechanical strength of the material further.

In the wet-seal area, the stainless steel is coated with an aluminium coating to protect the material from a severe corrosion environment. The production of aluminium coatings has so far been expensive and complex and an alternative coating process was evaluated. The alternative coating, tested in both reducing and oxidising environments showed a tendency to crack and expose the stainless steel to the corrosive environment. This was suggested being due to the manual coating process that resulted in inhomogeneous coatings.

In the second part, an alternative process to coat the wet-seal was suggested, based on recently published results where aluminium had been electrodeposited from ionic liquids. These solvents have a wider electrochemical window than water, and electropositive materials can therefore be deposited. To make the coating process suitable for industrial applications, an alternative to the commonly used AlCl3 was tested. It was shown however, that the influence of the environment had to be investigated before any materials could be produced. The environment, especially water in combination with oxygen was shown to influence the cathodic current density. In absence of these components, the ionic liquid was shown to be very stable.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 53 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:6
Keyword [en]
Molten Carbonate Fuel Cell, Anode current collector, Cathode, Wet-seal, Ionic liquid, TFSI
Keyword [sv]
Smältkarbonatbränslecell, Anodströmtilledare, Katod, Wet-seal, Jonvätska, TFSI
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4636ISBN: 978-91-7178-858-0 (print)OAI: oai:DiVA.org:kth-4636DiVA: diva2:13180
Public defence
2008-03-07, D3, Lindstedtsvägen 5, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100906Available from: 2008-02-12 Created: 2008-02-12 Last updated: 2010-09-06Bibliographically approved
List of papers
1. Corrosion of anode current collectors in molten carbonate fuel cells
Open this publication in new window or tab >>Corrosion of anode current collectors in molten carbonate fuel cells
2006 (English)In: Journal of Power Sources, ISSN 0378-7753, Vol. 160, no 2, 782-788 p.Article in journal (Refereed) Published
Abstract [en]

Corrosion of metallic parts is one of the life-time limiting factors in the molten carbonate fuel cell. In the reducing environment at the anode side of the cell, the corrosion agent is water. As anode current collector, a widely used material is nickel clad on stainless steel since nickel is stable in anode environment, but a cheaper material is desired to reduce the cost of the fuel cell stack. When using the material as current collector one important factor is a low resistance of the oxide layer formed between the electrode and the current collector in order not to decrease the cell efficiency. In this study, some candidates for anode current collectors have been tested in single cell molten carbonate fuel cells and the resistance of the oxide layer has been measured. Afterwards, the current collector was analysed in scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). The results show that the resistances of the formed oxide layers give a small potential drop compared to that of the cathode current collector.

Keyword
molten carbonate fuel cell (MCFC), corrosion, reducing environment, anode current collector
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7979 (URN)10.1016/j.jpowsour.2006.04.069 (DOI)000241411600003 ()2-s2.0-33748969393 (Scopus ID)
Note
QC 20100906Available from: 2008-02-12 Created: 2008-02-12 Last updated: 2011-09-28Bibliographically approved
2. Investigation of a Ni(Mg,Fe)O Cathode for Molten Carbonate Fuel Cell Applications
Open this publication in new window or tab >>Investigation of a Ni(Mg,Fe)O Cathode for Molten Carbonate Fuel Cell Applications
2007 (English)In: Fuel Cells, ISSN 1615-6846, E-ISSN 1615-6854, Vol. 7, no 3, 218-224 p.Article in journal (Refereed) Published
Abstract [en]

The Molten Carbonate Fuel Cell (MCFC) converts chemical energy into electrical energy and heat. Since the working temperature is high, less expensive materials can be used compared to low temperature fuel cells. However, the components of the fuel cell still need to be improved. The dissolution of the NiO cathode has, for a long time, been a problem for the Molten Carbonate Fuel Cell (MCFC) and this area is still the focus for MCFC component research. In this study, solubility measurements for a NiC) cathode material doped with magnesium and iron are carried out and the electrochemical performance of this cathode material is tested under the standard conditions of the MCFC over 2,000 hours and compared with the performance of a standard NiO cathode. After operation, nickel precipitation in the matrices is investigated. It is concluded that a NiO cathode with magnesium and iron could be a viable candidate material for the MCFC.

Keyword
cathode material, electrochemical performance, MCFC, precipitation, solubility
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7980 (URN)10.1002/fuce.200600011 (DOI)000247394400007 ()2-s2.0-34547471530 (Scopus ID)
Note
QC 20100906Available from: 2008-02-12 Created: 2008-02-12 Last updated: 2010-09-06Bibliographically approved
3. The influence of air and its components on the cathodic stability of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide
Open this publication in new window or tab >>The influence of air and its components on the cathodic stability of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide
Show others...
2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 0019-4686, Vol. 53, no 4, 1837-1842 p.Article in journal (Refereed) Published
Abstract [en]

Although water- and air-stable ionic liquids have been in use for some years, experiments found in the literature are still per-formed in inert gas with ppm levels of oxygen and water. In this study, the influence of different environments (vacuum, argon, nitrogen, air and oxygen and water) on the cathodic electrochemical window of the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) is reported and compared with investigations and processes found in the literature. The investigation indicates that this ionic liquid is highly stable in a vacuum and under argon flow. However, its cathodic stability is reduced in nitrogen and dry air. The simultaneous presence of water and air strongly affected the useful electrochemical window, as seen previously for imidazolium-based ionic liquids.

Keyword
ionic liquid, PYR14TFSI, electrochemical cathodic stability, reduction, environment
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-7981 (URN)10.1016/j.electacta.2007.08.029 (DOI)000251759900058 ()2-s2.0-35548987452 (Scopus ID)
Available from: 2008-02-12 Created: 2008-02-12 Last updated: 2011-09-12Bibliographically approved
4. Effect of water and oxygen traces on the cathodic stability of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide
Open this publication in new window or tab >>Effect of water and oxygen traces on the cathodic stability of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide
Show others...
2008 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 53, no 22, 6397-6401 p.Article in journal (Refereed) Published
Abstract [en]

Although research in the field of ionic liquids for electrochemical applications has led to a deeper knowledge in their electrochemical properties, doubts in the interpretation of the experimental results are still encountered in the literature due to the poor control of the experimental conditions and/or to the limited number of experiments conducted. In this work, the effect of water and oxygen traces on the cathodic stability window of hydrophobic, air-stable ionic liquids composed of N-alkyl-N-methylpyrrolidinium (PYR1A') cations and bis(trifluoromethanesulfonyl)imide (TFSI-) anion, is reported. The extensive investigation performed by linear sweep voltarnmetry (LSV) and cyclic voltarnmetry (CV) indicates that the TFSl- anion is cathodically stable if the ionic liquid is pure and dry. The N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids investigated showed featureless cathodic linear sweep voltarnmetry curves before the massive cation decomposition took place at very low potentials.

Keyword
pyrrolidinium, TFSI, N(Tf)(2), ionic liquid, cathodic stability, temperature ionic liquids, acetonitrile solutions, organic-solvents, molten-salt, lithium, impurities, electrochemistry, electrolytes, chloride, systems
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-17732 (URN)10.1016/j.electacta.2008.04.058 (DOI)000258009800015 ()2-s2.0-54249105423 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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