
307Charge and Water Transport in Fuel Cells
them. The number of water molecules dragged by each proton is given by the
electro-osmotic drag coefcient,
nn
drag HO
2
H=
/
. A more detailed discussion of
the composition and construction of a Naon polymer electrolyte membrane
will be given in Chapter 9.
Water transport ux owing to electro-osmotic drag in a cell with operating
current density i is given as
i
HO,esdrag
2
=
, (7.68)
where
= proton ux, which is two times the hydrogen ux.
The major components of water ux in the membrane are shown in Figure 7.9.
Diffusion of water owing to concentration gradient is given as
jDc
HO,d HO
2
=−
. (7.6