Page 77 - Introduction to Transfer Phenomena in PEM Fuel Cells

66 Introduction to Transfer Phenomena in PEM Fuel Cells
It is sometimes preferable to use the raw power (P gross) in the expression
of oxygen flow rate. The power of the fuel cell is given by the expression:
⋅
⋅
P gross = V J n c [2.51]
and the expression of molar flow rate is:
P
n  = gross [2.52]
4 ⋅ V F ⋅
O 2
–1
For the mass flow rate (q O 2 ) expressed in (kg.s ), we write:

×
M P ⋅ 32 10 − 3 P ⋅ P
O
gross
×
q O 2 = 4V F = 4V F gross = 8.29 10 − 8 ⋅ gross [2.53]
2
⋅
⋅
⋅
⋅
V
–1
where (M O 2 ) is the molar mass of oxygen expressed in kg.mol .
2.3.2. Hydrogen flow rate
The flow rate of hydrogen consumed is deduced in the same way as that
of oxygen.
According to the half-equation describing the reaction at the anode:

+
2H → 4H + 4e − [2.54]
2
Note that two electrons are released per mole of hydrogen and the flow
rate of hydrogen becomes:

J
n  H 2 = 2F n ⋅ c [2.55]

and for the expression containing the gross power, we write:

P
n  = gross [2.56]
2⋅ V F ⋅
H 2