Macroscopic Modeling of Polymer-Electrolyte Membranes

In order to accomplish the objective of studying and optimizing the flow channel geometries and dimensions for high-temperature proton-exchange-membrane (PEM) fuel cells (with operating temperatures above 120°C), a mathematical model has been developed in this work. As the major step of the modeling, the average concentrations of gas species in bulk flows as well as in the layers of electrodes are calculated through mass transfer analysis in one-dimensional direction normal to the membrane-electrodes layers. Therefore, the concentration and activation polarizations are simulated with much less computational work compared to a three-dimensional numerical model. The ohmic loss is taken into consideration through analysis of a representative network circuit simulating the electron and proton conduction in the elements of electrodes and electrolyte, respectively. The simulated results for high-temperature PEM fuel cells were compared with experimental results from literature. The results from the simulation and experimental tests showed good agreement, which validated the mathematical model. As the model requires less computational work, it was used to analyze a large number of cases with different gas flow channel dimensions and operating conditions, and optimization to the dimensions of channels and ribs was accomplished.

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“…Three mechanisms contribute to mass transfer in the porous media in a fuel cell based on the Nernst-Planck equation [40], as shown in Eq. (11).…”

Section: Mass Transfer In Porous Mediamentioning

confidence: 99%

Effects of geometry/dimensions of gas flow channels and operating conditions on high-temperature PEM fuel cells

Liu

Hartz

et al. 2014

Int J Energy Environ Eng

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“…, Alternatively, diffusion can be viewed from a thermodynamic point of view and be expressed in terms of a chemical potential gradient (6,7). This leads to following flux expression:…”

Section: Chemical Vs Fickian Diffusivitymentioning

confidence: 99%

On the Diffusion Coefficient of Water in Polymer Electrolyte Membranes

2013

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The Fickian diffusivity of water in Nafion has been shown to exhibit a local maximum. In the present research effort it is shown that this spike vanishes if the equilibrium water content and chemical diffusivity are modeled carefully. Further, it is shown that permeation experiments falsely can generate a local maximum, if a wrong equilibrium water content relation is used. Finally, in order to study the obtained diffusivity model a parameter variation is carried out using a one-dimensional steady-state model. The effect of diffusivity model, surface roughness and water content driving force is studied. IntroductionAn issue of great importance in fuel cell commercialization is the understanding of water transport in the membrane. Water transport is governed by several mechanisms: diffusion, electro-osmotic drag (EOD), hydraulic permeation, swelling and interfacial resistance. Particularly, the Fickian diffusivity of water has been intensively discussed during the last decade due to inconsistencies in the reported values and dependencies on water content. Differences in the order of three decades have been reported, depending on the measurement technique employed (i.e. NMR-relaxation, mass sorption/desorption and permeation). In the study by Majsztrik et al. (1) it was shown that the latter two techniques essentially measure a combination of transport mechanisms, i.e. swelling, non-equilibrium interfacial resistance and diffusion. For permeation experiments the limiting resistance was shown to be the interfacial resistance, and for mass sorption/desorption experiments the limiting resistance was shown to be a combination of interfacial resistance and membrane swelling.Meanwhile, NMR-relaxation avoids these issues by directly measuring the selfdiffusion inside a membrane. However, when one wants to relate the chemical diffusivity to a Fickian, the diffusivity changes it apparent shape quiet drastically in that it suddenly contains a large spike (2).In the present research effort it will be argued that the very existence of such a spike is merely a mathematical artifact of its derivation. Moreover, a new diffusivity model without the spike present is validated against experimental measurements of the water content distribution through a one-dimensional diffusion model with non-equilibrium sorption/desorption based on the uptake model by Ge et al. (3). Moreover, using the onedimensional model, it is shown that permeation experiments falsely can generate a local maximum. Finally, the proposed diffusivity model is compared with models published in the literature through a parameter study. In particular the interaction between the diffusivity and the sorption/desorption is analyzed. 10.1149/05002.0979ecst ©The Electrochemical Society ECS Transactions, 50 (2) 979-991 (2012) 979 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.111.121.42 Downloaded on 2015-07-05 to IP TheoryThere are two terms that contribute to the observation of a local m...

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“…(30), (A.1), and (A.2) it is possible to cast the dimensionless equations in a similar manner to eqs. (22) and (23).…”

Section: Appendix Amentioning

confidence: 99%

“…This simulation method is described by Weber and Newman [23] to be a combined computational approach involving the superposition of hydraulic and diffusive water transport components via linear combination. This is a typical method of calculating water transport in a fuel cell which usually involves the addition of Fickian diffusion, Darcy permeation and electroosmotic drag components.…”

Section: Introductionmentioning

confidence: 99%