Page 65 - Introduction to Transfer Phenomena in PEM Fuel Cells

54 Introduction to Transfer Phenomena in PEM Fuel Cells
Finally, the different models that we present allow the calculation of the
operating voltage of a cell according to the supply conditions, the
temperature and the desired current density [RAM 05].
2.2. Thermodynamics and chemistry of the PEM fuel cell
The PEM fuel cell is an electrochemical system involving electron
transfer to the surface of the electrodes; the produced current is proportional
to the surface of the electrode and it is expressed as current density.
Understanding the phenomena occurring within the fuel cell requires
knowledge of thermodynamics, chemistry, electrochemistry, hydraulics and
electricity [HIR 94, LAR 03].

2.2.1. The base reaction

The electrochemical reaction, in the case of a fuel cell, occurs
simultaneously at both sides of the membrane (anodeand cathode) as
follows:

– anode:
+
H ⎯⎯→ 2H +2e - [2.1]
2
– cathode:

1 O+2H +2e ⎯⎯→ H O [2.2]
+
-
2 2 2
– overall reaction:

H + 1 O ⎯⎯→ H O [2.3]
2
2 2 2
These reactions may have other intermediate steps giving rise to
undesirable reactions [OHA 09].

2.2.2. Heat reaction

The overall reaction given by equation [2.3] is the same as the hydrogen
combustion reaction, which is exothermic. The enthalpy of the chemical