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Fuel Cells 257

The H 2 half reaction. At the anode, hydrogen (H ) gas molecules diffuse
2
through the porous electrode until they encounter a platinum (Pt) par-
ticle. Pt catalyzes the dissociation of the H molecule into two hydrogen
2
atoms (H) bonded to two neighboring Pt atoms; here each H atom

releases an electron to form a hydrogen ion (H ). These H ions move
through the hydrated membrane to the cathode while the electrons pass
from the anode through the external circuit to the cathode, resulting in
a flow of current in the circuit.
The O 2 half reaction. The reaction of one oxygen (O ) molecule at the
2
cathode is a four-electron reduction process that occurs in a multistep
sequence. The catalysts capable of generating high rates of O reduction
2
at relatively low temperatures (~80 C) appear to be the Pt-based expen-
sive catalysts. The performance of the PEMFCs is limited primarily by
the slow rate of the O reduction half reaction, which is many times
2
slower than the H oxidation half reaction.
2
Electrolyte. The polymer electrolyte membrane is a solid organic poly-
mer, usually poly-[perfluorosulfonic] acid. A typical membrane material
used in the PEMFC is Nafion [11, 12]. It consists of three regions:

1. The teflon-like fluorocarbon backbone, hundreds of repeating
–CF –CF–CF – units in length
2
2
2. The side chains, –O–CF –CF–O–CF –CF –, which connect the molec-
2
2
2
ular backbone to the third region

3. The ion clusters consisting of sulfonic acid ions, SO 3 H

The negative ion SO 3 is permanently attached to the side chain and
cannot move. However, when the membrane becomes hydrated by
absorbing water, the hydrogen ion becomes mobile. Ion movement occurs

by protons (H ) bonded to water molecules, hopping from one SO 3 site
to another within the membrane. Because of this mechanism, the solid
hydrated electrolyte is an excellent conductor of hydrogen ions.
Electrodes. The anode and the cathode are separated from each other
by the electrolyte, the PEM. Each electrode consists of porous carbon to
which very small Pt particles are bonded. The porous electrodes allow the
reactant gases to diffuse through each electrode to reach the catalyst.
Both platinum and carbon are good conductors, so electrons are able to
move freely through the electrode [13, 14].
Catalyst. The two half reactions occur very slowly under normal con-
ditions at the low operating temperature (~80 C) of the PEMFC.
Therefore, catalysts are needed on both the anode and cathode to
increase the rates of each half reaction. Although platinum is a very
expensive metal, it is the best material for a catalyst on each electrode.