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Modelling and Experimental Investigation of the Dynamics in Polymer Electrolyte Fuel Cells
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In polymer electrolyte fuel cells (PEFC) chemical energy, in for example hydrogen, is converted by an electrochemical process into electrical energy. The PEFC has a working temperature generally below 100 °C. Under these conditions water management and transport of oxygen to the cathode are the parameters limiting the performance of the PEFC.

The purpose of this thesis was to better understand the complex processes in different parts of the PEFC. The rate-limiting processes in the cathode were studied using pure oxygen while varying oxygen pressure and humidity. Mass-transport limitations in the gas diffusion layer using oxygen diluted in nitrogen or helium was also studied. A large capacitive loop was seen at 1-10 Hz with 5-20 % oxygen. When nitrogen was changed to helium, which has a higher binary diffusion coefficient, the loop decreased and shifted to a higher frequency.

Steady-state and electrochemical impedance spectroscopy (EIS) models have been developed that accounts for water transport in the membrane and the influence of water on the anode. Due to water drag, the membrane resistance changes with current density. This gives rise to a low frequency loop in the complex plane plot. The loop appeared at a frequency of around 0.1 Hz and varied with D/Lm2, where D is the water diffusion coefficient and Lm is the membrane thickness. The EIS model for the hydrogen electrode gave three to four semicircles in the complex plane plot when taking the influence of water concentration on the anode conductivity and kinetics into account. The high-frequency semicircle is attributed to the Volmer reaction, the medium-frequency semicircle to the pseudocapacitance resulting from the adsorbed hydrogen, and the low-frequency semicircles to variations in electrode performance with water concentration. These low-frequency semicircles appear in a frequency range overlapping with the low-frequency semicircles from the water transport in the membrane. The effects of current density and membrane thickness were studied experimentally. An expected shift in frequency, when varying the membrane thickness was seen. This shift confirms the theory that the low-frequency loop is connected to the water transport in the membrane.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2009. , 46 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2009:6
Keyword [en]
polymer electrolyte fuel cell, modelling, electrochemical impedance spectroscopy, water transport, membrane
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-10087ISBN: 978-91-7415-241-8 (print)OAI: oai:DiVA.org:kth-10087DiVA: diva2:208390
Presentation
2009-04-03, D2, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (Swedish)
Opponent
Supervisors
Note

QC 20121011

Available from: 2009-03-18 Created: 2009-03-11 Last updated: 2012-10-11Bibliographically approved
List of papers
1. Transient techniques for investigating mass-transport limitations in gas diffusion electrodes: II. Experimental characterization of the PEFC cathode
Open this publication in new window or tab >>Transient techniques for investigating mass-transport limitations in gas diffusion electrodes: II. Experimental characterization of the PEFC cathode
2003 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 150, no 12, A1711-A1717 p.Article in journal (Refereed) Published
Abstract [en]

The current-interrupt technique and electrochemical impedance spectroscopy were employed in order to study the behavior of a polymer electrolyte fuel cell (PEFC) cathode containing 30 wt % Nafion and 70 wt % Pt/C. The steady-state polarization curves were also recorded. The experimental results were analyzed with help of the mathematical models developed in Part I of this paper. The effect of a varying oxygen pressure and humidity on the dynamic response of the cathode was investigated. The double-layer capacitance, Tafel slope, oxygen solubility, a group containing the effective O-2 diffusion coefficient and agglomerate size, and finally, the effective proton conductivity in the cathode were obtained. The parameter values were reasonable and attest the robustness of the agglomerate model for describing the PEFC cathode. At low humidity, a second, low-frequency loop was observed that was attributed to the membrane behavior.

Keyword
Capacitance, Cathodes, Diffusion, Electrolytes, Fuel cells, Mass transfer, Mathematical models, Oxygen, Polarization, Pressure effects, Solubility, Spectroscopic analysis
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-10085 (URN)10.1149/1.1624295 (DOI)000186902400023 ()
Note
QC 20101104Available from: 2009-03-16 Created: 2009-03-11 Last updated: 2017-12-13Bibliographically approved
2. Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC
Open this publication in new window or tab >>Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC
Show others...
2005 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 51, no 3, 474-488 p.Article in journal (Refereed) Published
Abstract [en]

In a polymer electrolyte membrane fuel cell (PEMFC), slowdiffusion in the gas diffusion electrode may induce oxygen depletion when using air at the cathode. This work focuses on the behavior of a single PEMFC built with a Nafion® based MEA and an E-TEK gas diffusion layer and fed at the cathode with nitrogen containing 5, 10 and 20% of oxygen and working at different cell temperatures and relative humidities. The purpose is to apply the experimental impedance technique to cells wherein transport limitations at the cathode are significant. In parallel, a model is proposed to interpret the polarization curves and the impedance diagrams of a single PEMFC. The model accounts for mass transport through the gas diffusion electrode. It allows us to qualitatively analyze the experimental polarization curves and the corresponding impedance spectra and highlights the intra-electrode processes and the influence of the gas diffusion layer.

Keyword
Gas diffusion layer, Impedance, Mass transport, Modeling, PEMFC
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-10098 (URN)10.1016/j.electacta.2005.05.007 (DOI)000233226100012 ()2-s2.0-26844505918 (Scopus ID)
Note
QC 20101104Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2017-12-13Bibliographically approved
3. Steady-State and EIS Investigations of Hydrogen Electrodes and Membranes in Polymer Electrolyte Fuel Cells: I. Modeling
Open this publication in new window or tab >>Steady-State and EIS Investigations of Hydrogen Electrodes and Membranes in Polymer Electrolyte Fuel Cells: I. Modeling
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 153, no 4, A749-A758 p.Article in journal (Refereed) Published
Abstract [en]

Electrochemical impedance spectroscopy (EIS) and steady-state models have been developed for the porous hydrogen electrode with water concentration dependence and water transport in a polymer electrolyte fuel cell membrane. Because the hydrogen electrode performance is influenced by its water content, the hydrogen electrode model was coupled to the membrane model. The EIS model for the hydrogen electrode gave three to four loops in the complex plane plots. The high-frequency semicircle was attributed to the Volmer reaction and the medium-frequency semicircle to the hydrogen adsorption. The additional low-frequency loops were connected to changes in the hydrogen electrode performance with water concentration, due to changes in kinetics or proton conductivity. Those loops appear in a frequency range depending on the water transport in the membrane, changing with D/L-m(2), where D is the water diffusivity and L-m is the membrane thickness. Modeling of the membrane alone showed that the membrane gives rise to a loop in EIS. The difference between the high- and low-frequency intercepts of the loop is idR/di, where the high-frequency intercept is equal to the membrane resistance. The loop appears in the same frequency range as the hydrogen electrode low-frequency loops and thus overlaps.

Keyword
Adsorption, Diffusion, Electric resistance, Electrodes, Fuel cells, Hydrogen, Polyelectrolytes, Porosity
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-10083 (URN)10.1149/1.2172559 (DOI)000235723600017 ()2-s2.0-33644804381 (Scopus ID)
Note
QC 20101104Available from: 2009-03-12 Created: 2009-03-11 Last updated: 2017-12-13Bibliographically approved
4. Steady-state and EIS investigations of hydrogen electrodes and membranes in polymer electrolyte fuel cells: II. Experimental
Open this publication in new window or tab >>Steady-state and EIS investigations of hydrogen electrodes and membranes in polymer electrolyte fuel cells: II. Experimental
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 153, no 4, A759-A764 p.Article in journal (Refereed) Published
Abstract [en]

Influence of water on membrane and anode performance was studied with steady-state and electrochemical impedance spectroscopy (EIS) measurements using a symmetrical cell with hydrogen on both sides. Both full-cell and half-cell measurements were performed. To obtain half-cell data a new reference electrode approach was demonstrated based on porous references in a four-electrode setup. A varying membrane resistance with current density was obtained using current interrupt and EIS measurements. The EIS measurements showed two semicircles at 10(4) Hz and 0.01-0.1 Hz, respectively. The first corresponds to hydrogen adsorption and the second to the water dependence of the electrode performance and membrane resistance. The low-frequency semicircle appears in a frequency range depending on the membrane thickness. The loop corresponding to the discharge of the double-layer capacitance through the Volmer reaction appears at frequencies too high to be experimentally measurable. The experimental data were in good agreement with the model developed in Part I of this paper. The model was also successfully fitted to experimental full cell data at different current densities and membrane thicknesses. The experiments confirmed that the low-frequency semicircle is attributed to the water dependence of both anode and membrane performance.

Keyword
Current density, Electric currents, Electrodes, Frequencies, Fuel cells, Hydrogen, Polyelectrolytes, Porosity, Water
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-10084 (URN)10.1149/1.2172561 (DOI)000235723600018 ()2-s2.0-33644790292 (Scopus ID)
Note
QC 20101104Available from: 2009-03-12 Created: 2009-03-11 Last updated: 2017-12-13Bibliographically approved

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