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PEM Fuel Cells by Ken S. Chen, Yun Wang

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5.2 DIFFUSION

Diffusion transports matter as a result of the random walks of diffusing particles. It can be explained through the molecular view: the constituent molecules move constantly, usually referred to as “random walks,” driven by thermal energy or the particle’s kinetic energy. At room temperature, the intermolecular distance of air or hydrogen gas, which is an ideal gas, is around 10 times larger than the molecular diameter; consequently constituent molecules travel randomly from higher to lower concentration regions.

5.2.1 Diffusivity

Another common process, as a result of random molecular motions, is heat transfer by conduction. There is an obvious analogy between diffusion and conduction, as illustrated by the lattice model presented ...

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