
113Thermodynamics of Fuel Cells
If n
e
is the number of electrons per molecule of reactant transferred, then n
e
N
a
is the total number of electrons that pass through the external circuit, where
N
a
is the Avogadro number, dened as the number of molecules per mole of sub-
stance and is a constant value, N
a
= 6.022 × 10
23
. If e is the chargeofoneelec-
tron, then the total charge that ows through the circuit is givenby
N
e
= n
e
N
a
e = n
e
F, coulombs (C) (4.13)
where F = N
a
e is Faraday’s constant dened as the charge carried by a mole of
electrons. Considering the value of the charge as e = 1.602 × 10
−19
C, Faraday’s
constant is given as
F = N
a
e