Page 64 - Introduction to Transfer Phenomena in PEM Fuel Cells

2

Charge Transfer Phenomena

2.1. Introduction

The movement of the charged species within the PEM cell occurs due to
the creation of an electric current; the charges involved are known as protons
and electrons. Their production at the anode and their recombination at the
cathode are ensured by the catalyzed reactions of oxidation of hydrogen and
the reduction of oxygen, respectively. These two half-reactions, located at
the electrodes, allow the conversion of chemical energies into electrical
energy. In this chapter, we will describe the transfer of charges within the
PEM cell and its electrical performance.
First, we will give an overview of the theoretical characterization of the
reversible electric power for such a system. The introduction of the
thermodynamic variables H, S and G will make it possible to define
respectively the total energy supplied to the system, the part dissipated as
heat, and finally, the electric energy available. The variations in these
variables with temperature and pressure are detailed.

Then, the real (non-reversible) operation of the fuel cell is studied. The
different overvoltages in the PEM fuel cell are identified and quantified. For
this, some electrochemical reminders are given to introduce the Nernst and
Butler–Volmer equations.

Using the correlations of the membrane proton conductivity, it is possible
to calculate the ohmic, activation and concentration overvoltages for a cell in
operation.