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Example: Maxwell-Stefan Diffusion in a Proton Exchange Membrane Fuel Cell

Example: Maxwell-Stefan Diffusion in a Proton Exchange Membrane Fuel Cell. Maxwell-Stefan diffusion in a fuel cell - Geometry.

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Example: Maxwell-Stefan Diffusion in a Proton Exchange Membrane Fuel Cell

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  1. Example:Maxwell-Stefan Diffusion in a Proton Exchange Membrane Fuel Cell

  2. Maxwell-Stefan diffusion in a fuel cell - Geometry This example models the cross-section of a proton exchange membranefuel cell cathode. The cross-section includes equal parts of the current collector and the channel in the bipolar plate as upper boundary.The active layer defines the lower boundary.

  3. Maxwell-Stefan diffusion in a fuel cell - Equation Equation: • Mass balance; divergence of the mass flux through diffusion • M - total molar mass of the mixture • Mj - molar mass of species j • wj - mass fraction of species j

  4. Maxwell-Stefan diffusion in a fuel cell - Equation • The symmetric diffusivities are strongly dependent on the composition and are given by the following expressions: • xj is the molar fraction of species j • Dij is the Maxwell-Stefan diffusivity for the pair ij • Subsequent entries can be obtained by permutation of the indices

  5. Maxwell-Stefan diffusion in a fuel cell - Diffusivities • The Maxwell-Stefan diffusivities can be described with asemi-empirical equation based on kinetic gas theory: • Resulting diffusivities, in m2 s-1, at T=353 K and p=101 kPa are: DO2N2 = DN2O2 = 9.7e-6 DO2H2O= DH2OO2= 1.20e-5 DH2ON2= DN2H2O= 1.24e-5

  6. Maxwell-Stefan diffusion in a fuel cell - BC’s • At the reactive boundary, oxygen is consumed according to the following expression • Subscript 0 in the mass fraction for oxygen represents the reference state, and kis the reaction rate given by the Tafel expression: • Combining all constants, the flux can be written as: -0.0442*wO2

  7. Maxwell-Stefan diffusion in a fuel cell - Results Results – mass fraction of oxygen, wO2. • Oxygen transport limits the rate of the reaction. • The concentration of oxygen in the active layer is less than 50% of the concentration at the position of the channel.

  8. Maxwell-Stefan diffusion in a fuel cell - Results Results – concentration dependent diffusion coeficcients • Large variations in concentration have a direct influence on the value of the diffusion coefficients. • DD11, reflects the interaction between oxygen molecules.

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