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The anode and the electrolyte in the MCFC
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
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 [en]
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: urn:nbn:se:kth:diva-4382ISBN: 978-91-7178-687-6 (print)OAI: oai:DiVA.org:kth-4382DiVA: diva2:12059
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
List of papers
1. A Steady-State Model of the Porous Molten Carbonate Fuel Cell Anode for Investigation of Kinetics and Mass Transfer
Open this publication in new window or tab >>A Steady-State Model of the Porous Molten Carbonate Fuel Cell Anode for Investigation of Kinetics and Mass Transfer
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, Vol. 153, no 8, A1525-A1532 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this paper was to investigate the effect of gas phase mass transfer and the influence of different reactions on the anode performance and to understand previously made experiments better. This has been done by mathematically modeling how different effects influence the polarization curve of the anode. Some previously obtained experimental data were used as input for the model. In this study, results from using the mechanisms proposed for the hydrogen oxidation by Jewulski and Suski and Ang and Sammels, respectively, show that they are equally likely. Furthermore, the direct electrochemical oxidation of carbon monoxide only slightly influences the anode performance. The concentration gradients in the current collector are larger than inside the electrode for gases not in equilibrium when entering the current collector; this is an effect caused by the shift reaction inside the electrode. However, if the gas compositions correspond to equilibrium at the current collector, the gas composition profiles become almost uniform. The disparities of the partial pressure dependency found in earlier experiments may be explained if the inlet gas composition is assumed to be the one obtained directly after humidification and not in equilibrium, as generally assumed.

Keyword
0on, Phase transitions, Polarization, Gas compositions, Humidification, Inlet gas composition, Polarization curve, Fuel cells
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7134 (URN)10.1149/1.2205198 (DOI)000238470100014 ()2-s2.0-33745514649 (Scopus ID)
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-10-08Bibliographically approved
2. The solubility of Ni in molten Li2CO3–Na2CO3 (52/48) in H2/H2O/CO2 atmosphere
Open this publication in new window or tab >>The solubility of Ni in molten Li2CO3–Na2CO3 (52/48) in H2/H2O/CO2 atmosphere
2007 (English)In: Journal of Power Sources, ISSN 0378-7753, Vol. 166, no 1, 59-63 p.Article in journal (Refereed) Published
Abstract [en]

In this work the solubility of a Ni-Al anode for MCFC has been studied at atmospheric pressure and two different temperatures using various gas compositions containing H-2/H2O/CO2. It is well known that nickel is dissolved at cathode conditions in an MCFC. However, the results in this study show that nickel can be dissolved also at the anode, indicating that the solubility increases with increasing CO2 partial pressure of the inlet gas and decreasing with increasing temperature. This agrees with the results found by other authors concerning the solubility of NiO at cathode conditions. The dissolution of Ni into the melt can proceed in two ways, either by the reduction of water or by the reduction of carbon dioxide.

Keyword
nickel solubility; MCFC; fuel cells
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7135 (URN)10.1016/j.jpowsour.2006.12.079 (DOI)000245527300008 ()2-s2.0-33847621609 (Scopus ID)
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-12-03Bibliographically approved
3. Experimental determination of effective surface area and conductivities in the porous anode of molten carbonate fuel cell
Open this publication in new window or tab >>Experimental determination of effective surface area and conductivities in the porous anode of molten carbonate fuel cell
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.

Keyword
molten carbonate fuel cell, porous anode, electrochemical impedance spectroscopy, effective conductivity
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7136 (URN)10.1016/j.jpowsour.2005.09.038 (DOI)000238964200012 ()2-s2.0-33744998906 (Scopus ID)
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-10-08Bibliographically approved
4. Influence of the anode pore-size distribution and total electrolyte filling degree on the MCFC performance
Open this publication in new window or tab >>Influence of the anode pore-size distribution and total electrolyte filling degree on the MCFC performance
2008 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, Vol. 155, no 2, B172-B179 p.Article in journal (Other academic) Published
Abstract [en]

Experimental data of the total cell reaction resistance as a function of the total electrolyte filling degree was measured to investigate how more electrolyte initially may be added to get as long a cell lifetime as possible. The reaction resistance of each electrode was also measured using two gas compositions and various total electrolyte filling degrees. A theoretical model for the distribution of electrolyte between the anode and the cathode as a function of the total electrolyte filling degree was used to compare the experimental data in this study with data from a symmetrical cell setup. The model takes into account the electrode pore-size distributions and considers two cases for the contact angle between the electrode and the electrolyte for the anode: a zero wetting angle (fully wetted) or reported experimental values for the wetting angle on pure Ni. It was concluded that after the cathode initially has been sufficiently filled with electrolyte the anode pores have to be smaller than the remaining ones of the cathode to allow having the anode act as a reservoir to prolong cell lifetime. The results from the experimental data and the theoretical model for electrolyte distribution were compared with results from a symmetrical setup.

Keyword
Electrolytes; Mathematical models; Molten carbonate fuel cells (MCFC); Pore size; Size distribution; Wetting; Cell reaction resistance; Electrolyte filling; Anodes
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7137 (URN)10.1149/1.2816213 (DOI)000251906800023 ()2-s2.0-37549036724 (Scopus ID)
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-12-03Bibliographically approved
5. A Model for Mass Transport of Molten Alkali Carbonate Mixtures Applied to the MCFC
Open this publication in new window or tab >>A Model for Mass Transport of Molten Alkali Carbonate Mixtures Applied to the MCFC
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, Vol. 153, no 11, A2111-A2119 p.Article in journal (Refereed) Published
Abstract [en]

A one-dimensional model based on the Stefan-Maxwell formulation for mass transfer of the main components of a binary molten carbonate electrolyte, including all of the nonidealities, was formulated and applied to the molten carbonate fuel cell (MCFC). The Stefan-Maxwell diffusion coefficients were determined from literature transport data; still, a narrow parameter window in electrolyte composition and temperature had to be used to keep the integrity of the fits. The model for calculation of the electrolyte composition was combined with equations describing the current distribution in the electrodes and the electrolyte. The calculated results of the electrolyte composition changes show that they depend predominantly on the current density and the total electrolyte filling degree. It was also concluded that the electrolyte composition changes are less then two percent for Li/K and five percent for Li/Na. This model demonstrates how experimental data measured at equilibrium conditions may be used to determine Stefan-Maxwell diffusion coefficients and then applied to a transport model for the electrolyte, in this case an MCFC.

Keyword
Electrolyte filling; Molten carbonate fuel cell (MCFC); Stefan Maxwell diffusion coefficients; Transport models
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7138 (URN)10.1149/1.2338653 (DOI)000241057000016 ()2-s2.0-33749614140 (Scopus ID)
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-12-03Bibliographically approved
6. Conductivity of SDC and (Li/Na)2CO3 composite electrolytes in reducing and oxidising atmospheres
Open this publication in new window or tab >>Conductivity of SDC and (Li/Na)2CO3 composite electrolytes in reducing and oxidising atmospheres
Show others...
2007 (English)In: Journal of Power Sources, ISSN 0378-7753, Vol. 172, no 2, 520-529 p.Article in journal (Refereed) Published
Abstract [en]

Composite electrolytes made of samarium-doped cerium oxide and a mixture of lithium carbonate and sodium carbonate salts are investigated with respect to their structure, morphology and ionic conductivity. The composite electrolytes are considered promising for use in so called intermediate temperature solid oxide fuel cells (IT-SOFC), operating at 400-600 degrees C. The electrolytes are tested in both gaseous anode (reducing) and cathode (oxidising) environments and at different humidities and carbon dioxide partial pressures. For the structure and morphology measurements, it was concluded that no changes occur to the materials after usage. From measurements of melting energies, it was concluded that the melting point of the carbonate salt phase decreases with decreasing fraction of carbonate salt and that a partial melting occurs before the bulk melting point of the salt is reached. For all the composites, two regions may be observed for the conductivity, one below the carbonate salt melting point and one above the melting point. The conductivity is higher when electrolytes are tested in anode gas than when tested in cathode gas, at least for electrolytes with less than half the volume fraction consisting of carbonate salt. The higher the content of carbonate salt phase, the higher the conductivity of the composite for the temperature region above the carbonate melting point. Below the melting point, though, the conductivity does not follow this trend. Calculations on activation energies for the conductivity show no trend or value that indicates a certain transport mechanism for ion transport, either when changing between the different composites or between different gas environments.

Keyword
Composite; Conductivity; Electrolyte; Fuel cell; ITSOFC; SDC
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7139 (URN)10.1016/j.jpowsour.2007.07.065 (DOI)000250654700004 ()2-s2.0-34748894956 (Scopus ID)
Note
QC 20100630Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-12-03Bibliographically approved

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