Page 80 - Introduction to Transfer Phenomena in PEM Fuel Cells

and:
1
Δ=
⋅
[2.62]
2
2
2 O → O with H 438 kJ mol − 1 Charge Transfer Phenomena 69
–1
that is, an activation energy of 436 + 219 = 685 kJ.mol .
2.4.3. Reaction rate
The use of a catalyst decreases the activation energy required for a
reaction to take place by bringing into play intermediate reactions occurring
on the surface of the catalyst.

Using Arrhenius’ law, which gives the relation between reaction rate and
temperature, it is possible to determine the effect of a variation in the
activation energy (E a) [BAR 05, HIR 98]:

 − E a 
⋅
k = A exp   [2.63]
 RT 
where (k) is the velocity constant, a function of temperature; and (A) is a
constant, a function of the reaction. A small decrease in activationenergy
causes a large increase in the reaction rate: the changing in activation energy
–1
from 100 to 90 kJ.mol increases the reaction rate by approximately 60.

2.4.4. Exchange current

In an open circuit, even in the absence of current flow in a fuel cell, each
electrode/electrolyte interface has a dynamic equilibrium and the charges
(electrons) cross this interface in both directions. The resulting current
density created by the flow of electrons is called the exchange current
–
density (j 0), and it is expressed in amperes per square centimeter (A.cm ²).
This is the measure of the charge transfer rate at equilibrium. The higher it
is, the easier the reaction is to initiate.

For electrochemical systems, (j 0) varies from a few nanoamperes per
square centimeter to a few amperes per square centimeter [BOU 07]. The
values of the exchange current density (j 0) for the H 2/O 2 system are
summarized in Table 2.3.