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Experimental determination of effective surface area and conductivities in the porous anode of molten carbonate fuel cell
Central Research Institute of Electric Power Industry, Energy Engineering Research Laboratory, Kanagawa, Japan.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.ORCID iD: 0000-0001-9203-9313
2006 (English)In: Journal of Power Sources, ISSN 0378-7753, Vol. 158, no 1, 94-102 p.Article in journal (Refereed) Published
Abstract [en]

Stationary polarization curves and electrochemical impedance spectroscopy of a porous nickel anode in a molten carbonate fuel cell were obtained in order to determine the active surface area and conductivities with varying degree of electrolyte filling for two anode feed-gas compositions, one simulating operation with steam reformed natural gas and the other one gasified coal. The active surface area for coal gas is reduced by around 70-80% compared to the standard gas composition in the case of Li/Na carbonate. Moreover, an optimal degree of electrolyte filling was shifted toward higher filling degree in the case of operation with coal gas.In order to evaluate the experimental data a one-dimensional model was used. The reaction rate at the matrix/electrode interface is about five times higher than the average reaction rate in the whole electrode in case of 10% electrolyte filling. This result suggests that the lower limit of the filling degree of the anode should be around 15% in order to avoid non-uniform distribution of the reaction in the electrode. Therefore, in the case of applying Li/Na carbonate in the MCFC, an electrolyte distribution model taking into account the wetting properties of the electrode is required in order to set an optimal electrolyte filling degree in the electrode.

Place, publisher, year, edition, pages
2006. Vol. 158, no 1, 94-102 p.
Keyword [en]
molten carbonate fuel cell, porous anode, electrochemical impedance spectroscopy, effective conductivity
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-7136DOI: 10.1016/j.jpowsour.2005.09.038ISI: 000238964200012Scopus ID: 2-s2.0-33744998906OAI: oai:DiVA.org:kth-7136DiVA: diva2:12055
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-10-08Bibliographically approved
In thesis
1. The anode and the electrolyte in the MCFC
Open this publication in new window or tab >>The anode and the electrolyte in the MCFC
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

A goal of the Swedish government is to increase the usage of renewable fuels and biomass-based fuels. Fuel cells, and especially the MCFC, are useful for these types of fuels. The Swedish market may benefit from the MCFC in two ways: increased efficiency of the biofuels and also utilisation of produced heat in district heating. Most of the commercial MCFC systems today are optimised for use with methane. The possibility to utilise biomass in Sweden makes it important to study how the MCFC may be adapted or optimised for good performance and low degradation with gas produced from biomass or other renewable fuels.

This thesis is focused on methods that may be used to investigate and evaluate MCFC electrodes and electrolytes with renewable fuels i.e. CO2-containing gases. The methods and results are both experimental and mathematically modelled. The objectives of this thesis are to better understand how the performance of the anode is dependent on different fuels. Anode kinetics and the water-gas shift reaction have been investigated as well as the possibility to increase cell lifetime by increasing the initial electrolyte amount by having the anode as a reservoir. The effect of segregation of cations in the electrolyte during operation has also been studied.

It was found that if the gas composition at the current collector inlet is in equilibrium according to the water gas-shift reaction the gas composition inside the electrode is almost uniform. However, if the gas is not in equilibrium then the concentration gradients inside the current collector have a large effect on the gas composition inside the electrode. The conversion of the gas in the gas flow channels according to the water-gas shift reaction depends on the gas flow rate. For an anode used in a gas mixture of humidified hydrogen and carbon dioxide that are not in equilibrium some solubility of Ni in a (Li/Na)2CO3 mixture was found. To have the anode act as an electrolyte reservoir to prolong cell lifetime the anode pore size should be carefully matched with that of the cathode and a bimodal pore-size distribution for the anode is preferable to have as good performance as possible for as large electrolyte filling degree interval as possible. Modelling results of segregation of cations in the electrolyte during operation indicate that the electrolyte composition changes during operation and that the lithium ions are enriched at the anode for both types of electrolyte used for the MCFC. The electrolyte composition changes are small but might have to be considered in long-time operation. The results from this thesis may be used to better understand how the MCFC may be used for operation with renewable fuels and how electrodes may be designed to prolong cell lifetime.

Abstract [sv]

Ett av den svenska regeringens mål är att öka användandet av förnyelsebara bränslen och bränslen från biomassa. Bränsleceller och framförallt MCFC är användbara för dessa typer av bränslen. Den svenska marknaden kan dra fördelar av MCFC på två sätt; ökad bränsleutnyttjandegrad och utnyttjande av producerad värme för fjärrvärme. De flesta kommersiella MCFC-systemen idag är optimerade för användning av metan. Möjligheten att använda biomassa på den svenska marknaden gör det viktigt att studera hur MCFC kan anpassas eller optimeras för bra prestanda och låg degradering för användning med gas från biomassa eller andra förnyelsebara bränslen.

Fokus i denna avhandling är på metoder som kan användas för att undersöka och utvärdera MCFC-elektroder och -elektrolyter med förnyelsebara bränslen, dvs. gaser innehållande CO2. Metoderna och resultaten är både experimentella och matematiskt modellerade. Målet med denna avhandling är att bättre förstå hur anodens prestanda beror på användningen av olika bränslen. Anodens kinetik och vattengasskiftreaktionen har studerats liksom möjligheten att förlänga cellens livstid genom att öka den initiala mängden elektrolyt medelst användning av anoden som reservoar. Effekten av segregation av katjoner i elektrolyten under last har också undersökts.

Om gassammansättningen är i jämvikt enligt vattengasskiftreaktionen vid inloppet till strömtilledaren kommer gassammansättningen att vara nära uniform inuti elektroden. Om ingående gas inte är i jämvikt kommer stora koncentrationsgradienter uppkomma i strömtilledaren och påverka gassammansättningen i elektroden. Omsättningen med avseende på vattenskiftreaktionen av gasen i flödeskanalen verkar vara beroende av gasens flödeshastighet. För en anod som används i en uppfuktad blandning av vätgas och koldioxid som inte är i jämvikt befanns det att Ni har en viss löslighet i (Li/Na)2CO3. För att kunna använda anoden som reservoar för elektrolyt för att förlänga livstiden för MCFC skall anodens porstorleksfördelning överensstämma med katodens och ha en bimodal porstorleksfördelning för att ge en tillräckligt god prestanda i ett så stort elektrolytfyllnadsgradsintervall som möjligt. Modelleringsresultat för segregering av katjoner i elektrolyten under drift visar att litiumjoner anrikas i anoden för båda typerna av elektrolyt som används i MCFC. Elektrolytkoncentrationsförändringarna är små men kan behövas tas i beaktande vid långa driftstider. Denna avhandlings resultat kan användas för att bättre förstå hur MCFC skall anpassas för drift med förnyelsebara bränslen och hur elektroder kan utformas för att förlänga livstiden.

Place, publisher, year, edition, pages
Stockholm: Kemiteknik, 2007. [7], 47 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:32
Keyword
composite electrolytes, electrode kinetics, nickel solubility, porous electrode, reformate, water-gas shift reaction, electrolyte, electrolyte distribution, hydrogen oxidation reaction, ion segregation, mass transport, mathematical modelling, MCFC, molten carbonate fuel cell
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-4382 (URN)978-91-7178-687-6 (ISBN)
Public defence
2007-06-01, F3, KTH, Lindstedtsvägen 26, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2012-03-19Bibliographically approved
2. Modelling and experimental investigation of the porous nickel anode in the molten carbonate fuel cell
Open this publication in new window or tab >>Modelling and experimental investigation of the porous nickel anode in the molten carbonate fuel cell
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The thesis is focussed on the performance of the fuel cell and the design of the cell for operation with natural gas and renewable fuels, e.g. biogas or gasified biomass. The performance is one of the important issues for the development and commercialisation of fuel cell stacks. In order to operate fuel cell on renewable fuels, without preceding reforming of the fuel, a high temperature fuel cell is needed, i.e. a solid oxide fuel cell (SOFC) or a molten carbonate fuel cell (MCFC). At present, the latter fuel cell type is much more mature when regarding the technical aspects than is the solid oxide fuel cell. The German company MTU has up to date installed about thirty MCFC plants, mainly in Europe and the USA but also in Japan. Moreover the European Commission has decided that the use of renewable fuels must increase at the expense of fossil fuels. This decision is one step towards a smaller dependence on fossil energy sources and limited emissions of greenhouse gases.

The objective of this work is to better understand the factors that influence the cell performance: to determine the kinetic parameters of the hydrogen oxidation and the carbon monoxide oxidation and to get more information about the reaction mechanism, even when dealing with gases of low hydrogen content. The latter is of special importance when operating the cells on biogas or gasified biomass. These fuels also contain higher concentrations of carbon monoxide and carbon dioxide.

It was found that the hydrogen mechanism proposed by Jewulski and Suski describes the anode performance even at lower concentrations of hydrogen, i.e. gases corresponding to gasified biomass. Furthermore, the carbon monoxide reaction will only slightly influence the anode performance but if the rate of the shift reaction is small the influence of direct oxidation of carbon monoxide will increase. Experimental investigations have shown that mass transfer limitations in the gas phase exist. By mathematical modelling it was found that the current collector has a larger affect on the concentration gradients than the porous electrode. The concentrations gradients in the current collector are caused by the shift reaction that mainly takes place at the electrode. However, if the gas corresponds to equilibrium at the current collector the profiles will become almost uniform. Furthermore the influence of wetting properties, the pore structure and pore size distribution have also been investigated in this thesis. The outcome of this thesis may be used for electrode development and design, as well as for input to reliable cell and stack models for system simulations.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 58 p.
Series
Trita-KET, ISSN 1104-3466 ; 218
Keyword
Chemical engineering, molten carbonate fuel cell, MCFC, mechanism, cell modelling, porous electrode, Kemiteknik
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-374 (URN)91-7178-117-X (ISBN)
Public defence
2005-08-24, Sal D3, Lindstedtsvägen 5, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101008Available from: 2005-08-05 Created: 2005-08-05 Last updated: 2010-10-08Bibliographically approved

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