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PEM Fuel Cells by Ken S. Chen, Yun Wang

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5.5 TRANSIENT PHENOMENA

5.5.1 Transient Terms and Time Constants

In transient operation, species or energy can be stored in or released from bulk materials, which is accounted for by the transient term in the conservation equation. Polymer electrolyte membrane (e.g., Nafion) exhibits a large water holding capacity. Catalyst layers contain several phases including ionomer and gas. Thus, the transient term of the water transport equation is derived to account for their holding capacity of water [15]:

330781.jpg (Eq. 5-41)

The two terms on the left-hand side represent the rates of water storage in the gas and membrane phases, respectively. Assuming that the ...

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