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Modelling and experimental investigation of the porous nickel anode in the molten carbonate fuel cell
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
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 [en]
Chemical engineering, molten carbonate fuel cell, MCFC, mechanism, cell modelling, porous electrode
Keyword [sv]
Kemiteknik
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-374ISBN: 91-7178-117-X (print)OAI: oai:DiVA.org:kth-374DiVA: diva2:9587
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
List of papers
1. Experimental Investigation of the Porous Nickel Anode in the Molten Carbonate Fuel Cell
Open this publication in new window or tab >>Experimental Investigation of the Porous Nickel Anode in the Molten Carbonate Fuel Cell
2001 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 148, no 5, A411-A417 p.Article in journal (Refereed) Published
Abstract [en]

The electrochemical performance of the porous nickel anode in a molten carbonate fuel cell was experimentally investigated in this study. The electrode structure was studied by scanning electron microscope and Hg porosimetry. The effect of electrolyte filling, on overpotential at a constant current density, was also examined. Kinetics for the hydrogen reaction was studied by obtaining stationary polarization curves, for porous nickel anodes at varying temperatures and anode gas compositions. The slopes of these polarization curves were analyzed at low overpotentials, with the assumption that the porous anode was under kinetic control, and the corresponding exchange current densities were determined using a simplified porous electrode model. The obtained partial pressure dependencies of the exchange current density were high, and therefore, difficult to explain by the generally assumed mechanisms.

Keyword
hydrogen oxidation, electrode-kinetics, li/k carbonate, polarization, model, mechanism, impedance, cathodes, oxidant
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5354 (URN)10.1149/1.1359195 (DOI)000168421200003 ()
Note
QC 20101008Available from: 2005-08-05 Created: 2005-08-05 Last updated: 2017-11-21Bibliographically approved
2. A stack model for MCFC system studies for process simulations
Open this publication in new window or tab >>A stack model for MCFC system studies for process simulations
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5355 (URN)
Note

QC 20101008

Available from: 2005-08-05 Created: 2005-08-05 Last updated: 2016-06-01Bibliographically approved
3. Experimental Studies of the Direct Oxidation of Carbon Monoxide in a Small Molten Carbonate Fuel Cell
Open this publication in new window or tab >>Experimental Studies of the Direct Oxidation of Carbon Monoxide in a Small Molten Carbonate Fuel Cell
(English)Manuscript (Other academic)
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5356 (URN)
Note
QC 20101008Available from: 2005-08-05 Created: 2005-08-05 Last updated: 2010-10-08Bibliographically approved
4. 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
5. 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

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