Macroscopic Modeling of Transport Phenomena in Direct Methanol Fuel Cells

Research output: ResearchPh.D. thesis

Abstract

An increasing need for energy efficiency and high energy density has sparked a growing interest in direct methanol fuel cells for portable power applications. This type of fuel cell directly generates electricity from a fuel mixture consisting of methanol and water. Although this technology surpasses batteries in important areas, fundamental research is still required to improve durability and performance. Particularly the transport of methanol and water within the cell structure is difficult to study in-situ. A demand therefore exist for the fundamental development of mathematical models for studying their transport.
In this PhD dissertation the macroscopic transport phenomena governing direct methanol fuel cell operation are analyzed, discussed and modeled using the two-fluid approach in the computational fluid dynamics framework of CFX 14. The overall objective of this work is to extend the present fundamental understanding of direct methanol fuel cell operation by developing a three-dimensional, two-phase, multi-component, non-isotherm mathematical model including detailed non-ideal thermodynamics, non-equilibrium phase change and non-equilibrium sorption-desorption of methanol and water between fluid phases and the polymer electrolyte membrane. In addition to the performed modeling work, experiments are devised and constructed in order to provide data for a parameter assessment and modeling validation.
Throughout this work different studies have been carried out, addressing various issues of importance for direct methanol fuel cell operation and its modeling. In one study, the effect of inhomogeneous gas diffusion layer compression on cell performance was investigated. This was done to elucidate modeling capabilities with regard to liquid phase flooding of porous media assemblies and its effect on oxygen transport towards the catalyst layer. It was demonstrated that inhomogeneous compression enhances the extent of flooding under the land area, hereby significantly decreasing oxygen transport towards the catalyst layer. Moreover, it was shown that gas diffusion layer compression also affects liquid water transport in the catalyst layer inhomogeneously.
In another study the effect of membrane hydration on the diffusivity of water in Nafion was examined to discuss the alleged existence of a local maximum. Based on state-of-the-art knowledge on water sorption isotherms and self-diffusivities of water, a new relation for the Fickian diffusivity of water was derived. This diffusivity model did not exhibit a characteristic spike as reported in other studies. Furthermore, it was shown that the existence of a local maximum cannot be validated by merely comparing water flux measurements, unless the exact sorption/desorption kinetics are known even for fairly thick membranes. Similarly, it was shown that permeation experiments falsely can predict a local maximum if care is not put on the formulation of the sorption isotherm used in its conversion.
In a final study, a complete direct methanol fuel cell was partially validated and used for investigating the coupling between the volume porosity of the gas diffusion layer and the capillary pressure boundary condition and its impact on electrochemical performance. In this study, it was shown how a pressure based boundary condition predicts considerable differences in the phase distribution of the GDL when changing its volume porosity, as opposed to a constant liquid volume fraction boundary condition, commonly found in the literature. Moreover, it was shown how this imposed difference in phase distribution causes substantial differences in the predicted limiting current density.
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An increasing need for energy efficiency and high energy density has sparked a growing interest in direct methanol fuel cells for portable power applications. This type of fuel cell directly generates electricity from a fuel mixture consisting of methanol and water. Although this technology surpasses batteries in important areas, fundamental research is still required to improve durability and performance. Particularly the transport of methanol and water within the cell structure is difficult to study in-situ. A demand therefore exist for the fundamental development of mathematical models for studying their transport.
In this PhD dissertation the macroscopic transport phenomena governing direct methanol fuel cell operation are analyzed, discussed and modeled using the two-fluid approach in the computational fluid dynamics framework of CFX 14. The overall objective of this work is to extend the present fundamental understanding of direct methanol fuel cell operation by developing a three-dimensional, two-phase, multi-component, non-isotherm mathematical model including detailed non-ideal thermodynamics, non-equilibrium phase change and non-equilibrium sorption-desorption of methanol and water between fluid phases and the polymer electrolyte membrane. In addition to the performed modeling work, experiments are devised and constructed in order to provide data for a parameter assessment and modeling validation.
Throughout this work different studies have been carried out, addressing various issues of importance for direct methanol fuel cell operation and its modeling. In one study, the effect of inhomogeneous gas diffusion layer compression on cell performance was investigated. This was done to elucidate modeling capabilities with regard to liquid phase flooding of porous media assemblies and its effect on oxygen transport towards the catalyst layer. It was demonstrated that inhomogeneous compression enhances the extent of flooding under the land area, hereby significantly decreasing oxygen transport towards the catalyst layer. Moreover, it was shown that gas diffusion layer compression also affects liquid water transport in the catalyst layer inhomogeneously.
In another study the effect of membrane hydration on the diffusivity of water in Nafion was examined to discuss the alleged existence of a local maximum. Based on state-of-the-art knowledge on water sorption isotherms and self-diffusivities of water, a new relation for the Fickian diffusivity of water was derived. This diffusivity model did not exhibit a characteristic spike as reported in other studies. Furthermore, it was shown that the existence of a local maximum cannot be validated by merely comparing water flux measurements, unless the exact sorption/desorption kinetics are known even for fairly thick membranes. Similarly, it was shown that permeation experiments falsely can predict a local maximum if care is not put on the formulation of the sorption isotherm used in its conversion.
In a final study, a complete direct methanol fuel cell was partially validated and used for investigating the coupling between the volume porosity of the gas diffusion layer and the capillary pressure boundary condition and its impact on electrochemical performance. In this study, it was shown how a pressure based boundary condition predicts considerable differences in the phase distribution of the GDL when changing its volume porosity, as opposed to a constant liquid volume fraction boundary condition, commonly found in the literature. Moreover, it was shown how this imposed difference in phase distribution causes substantial differences in the predicted limiting current density.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages134
ISBN (Print)978-87-92846-26-6
StatePublished - 2013
Publication categoryResearch

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