Environmental Engineering Reference
In-Depth Information
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Fig. 4.6
Structure of a proton exchange membrane
More recently, a fuel cell made of ceramic materials (solid oxide fuel cell: SOFC)
offering an efficiency of more than 50 % has entered practical use. An SOFC-gas
turbine combined system with even higher efficiency is under development. SOFC
is also applied to smaller systems as a household fuel cell and for a mobile system.
A further type, the polymer based fuel cell (polymer electrolyte fuel cell: PEFC)
has made rapid progress in recent decades for fuel cell vehicles as well as for house-
hold applications. Developments in fuel cell vehicles and their associated infra-
structure are now at a turning point for widespread installation into society and
the start of mass production. In the following sections, we introduce the PEFC and
SOFC and consider their detailed mechanisms, technology status, and challenges.
4.4
Mechanism and Characteristics of the Polymer
Electrolyte Fuel Cell (PEFC)
Currently under development as a power source for automobiles is the fuel cell
which uses a polymer electrolyte membrane which passes hydrogen ions (protons).
This type is referred to as a polymer electrolyte fuel cell (PEFC) or proton exchange
membrane fuel cell (PEMFC). Nafion® (developed by Dupont Company) or a simi-
lar polymer is generally used as the electrolyte membrane.
Figure
4.6
is a schematic diagram of the electrolyte membrane in the PEFC. The
polymer has a backbone of a chemically stable fluorocarbon polymer structure in
which there are branches with SO
3
−
H
+
(sulfo group) attached. These parts gather to-
gether to form a local structure (inverse micelle structure), take in water easily, mak-
ing proton-conducting channels. Proton conductivity becomes higher as humidity in-
creases. Protons are known to transport by 'hopping' along the framework of hydro-
gen bonds in the absorbed water (Grötthus mechanism), or migrate as H
3
O
+
together
with a water molecule (vehicle mechanism). Recent reports of molecular dynamics
simulation suggest the former mechanism is dominant in a Nafion® membrane.
The electrodes used in the PEFC are made of porous carbon material encrusted
with fine particles of platinum. They are attached to both sides of the electrolyte
