Page 81 - Introduction to Transfer Phenomena in PEM Fuel Cells

70 Introduction to Transfer Phenomena in PEM Fuel Cells

–3
10
Hydrogen
Platinum
Acidic
–4
Basic
10
Electrolyte membrane Catalyst Environment J 0 (A/cm²)
−
–9
Oxygen Platinum Acidic 10
Table 2.3. Current density values
The exchange current depends on the concentration and also on
temperature; the effective exchange current density (per unit of geometric
surface area of the electrode). It is also a function of the charge
(concentration) of the catalyst and the surface area of the electrode
[GAS 03]. The following equation illustrates an expression of the exchange
current density as a function of the reference value:
 P  γ  E  T  
j = j a L  ref  r  exp −  c  1−   [2.64]
ref
c c 
0
 P r    RT  T ref   
where:
ref
– j 0 is the reference exchange current density (at the reference
temperature and pressure, typically 25°C and 101.25 kPa) per unit area of
–
the catalyst (platinum “Pt” expressed in A.cm ² of Pt);
– a c is the specific surface area of the catalyst (the theoretical limit of the
–1
–1
platinum catalyst is 2,400 cm².mg , between 600 and 1,000 cm².mg can be
found; up to 30% of the platinum can be incorporated into the electrode;
–
– L c is the catalyst charge (between 0.3 and 0.5 mg of Pt.cm ²), lower
values are possible but they give rise to small voltages in the fuel cell;
– P r is the partial pressure of the reactant (kPa);
ref
– P r is the reference pressure (kPa);
– γ is the pressure coefficient (between 0.5 and 1.0);

–1
– E c is the activation energy, approximately 66 kj.mol (for the reduction
of oxygen);
–1
–1
– R is the ideal gas constant (8.314 j. mol k );
– T is the temperature (K);
– T ref is the reference temperature (298.15 K).