
240 Fuel Cells
where
= mass diffusion ux of species i (kg/m
2
· s)
ρ
i
= mass concentration or density of species i (kg/m
3
)
ρ = total mass density (kg/m
3
)
x
i
= mass fraction of i
Diffusion mass transfer exists in all phases, that is, in gases, liquids, and
solids, with a higher order of magnitude in gases than in liquids and a higher
order of magnitude in liquids than in solids.
6.3.1.1.1 Binary Diffusion
For binary diffusion with only two species a and b present in a mixture, the
diffusion coefcient is expressed as D
ab
. For example, if a humidied hydro-
gen gas is supplied as fuel, then the binary diffusion involves two species,
hydrogen ...