Page 50 - Introduction to Transfer Phenomena in PEM Fuel Cells

Introduction to Hydrogen Technology 39
Depending on the materials used in the MEA, the gases must be injected
with a certain humidity; their humidification can be done actively or
passively [CHU 09]. Finally, gas purification accessories are sometimes
necessary, especially in the case of hydrogen produced from the reforming
of hydrocarbons. The main components required for the proper functioning
of a PEMFC cell are presented in the following sections.
1.3.1.1. The electrolyte layer
The electrolyte layer (electrolytic polymer membrane) is at the core of a
fuel cell. It allows electrons to travel in the required manner by attracting
protons and allowing them to pass through the membrane while maintaining
their proton state. In fact, the membrane behaves like an acid solution in
–
which there are negatively charged fixed sulfonic sites (SO 3). These sites
+
dissociate water molecules to create H protons. The membrane transports
ions and water through diffusion and osmotic drive effects (these will be
discussed in detail in Chapter 3 on mass transfer). The electrons flow
through the external circuit to supply the charge in the form of an electric
current. The hydrogen protons thus cross the electrolyte layer to join the
cathode, and then recombine with the oxygen to form water. The membrane
is a complex polymer that must have the following properties: it must be
both a good ionic conductor, an electronic insulator, impervious to gas and
stable mechanically and chemically.

1.3.1.1.1. Types of membranes
The membrane used in PEMFC is composed of a fluorocarbon skeleton
(based on perfluorocarbon sulfonic acid ionomer (PSA) similar to Teflon
(hydrophobic tetrafluoroethylene (TFE)) with hydrophilic sulfonic groups
–
(SO 3)), and even several perfluorosulfonate monomers. These membranes
TM
are most commonly known as Nafion and is in the form of poly(perfluoro-
sulfonylethoxy propylene vinyl ether) (PSEPVE). It was developed in the
1970s by DuPont de Nemours. It is generally characterized by its
concentration in fixed charges: the higher it is, the better the conduction of
protons. Its chemical structure is shown in Figure 1.16 [COL 08].