Page 279 - Biofuels Refining and Performance

258 Chapter Nine

The half reactions occurring at each electrode can occur only at a high
rate at the surface of the Pt catalyst. A unique feature of Pt is that it is
sufficiently reactive in bonding H and O intermediates, as required to
facilitate the electrode processes, and is also capable of effectively releas-
ing the intermediate to form the final product. The anode process
requires Pt sites to bond H atoms when the H 2 molecule reacts; next,
these Pt sites release the H atoms, as follows:

2H 2e → H 2
H 2 2Pt → 2(Pt-H)

2(Pt-H) → 2Pt 2H 2e
This optimized bonding to H atoms (neither very weak nor very
strong) is a unique property of the Pt catalyst. To increase the reaction
rate, the catalyst layer is constructed with the highest possible surface
area. This is achieved by using very small Pt particles, about 2 nm in
diameter, resulting in an enormously large total surface area of Pt that
is accessible to gas molecules. The original MEAs for the Gemini space
program used 4 mg of platinum per square centimeter of membrane area
2
(4 mg/cm ). Although the technology varies with the manufacturer, the
2
total platinum loading has decreased from the original 4 mg/cm to
2
about 0.5 mg/cm . Laboratory research now uses platinum loadings of
2
2
0.15 mg/cm . For catalyst layers containing Pt of about 0.15 mg/cm , the
thickness of the catalyst layer is ~10
m; the MEA with a total thickness
~200
m can generate more than half an ampere of current for every
square centimeter of the MEA at a voltage of 0.7 V between the cath-
ode and the anode [2, 3, 10–12]. Recently, scientists at Los Alamos
National Laboratory, USA have developed a new class of hydrogen fuel
cell catalysts that exhibit promising activity and stability. The cata-
lysts, cobalt-polypyrrole-carbon (Co-PPY-XC72) composite, are made of
low-cost metals entrapped in a heteroatomic-polymer structure.
The cell hardware. The hardware of the fuel cell consists of backing
layers, flow fields, and current collectors. These are designed to maxi-
mize the current that can be obtained from an MEA. The backing layers
placed next to the electrodes are made of a porous carbon paper or
carbon cloth, typically 100–300
m thick. The porous nature of the back-
ing material ensures effective diffusion of the reactant gases to the cat-
alyst. The backing layers also assist in water management during the
operation of the fuel cell; too little or too much water can halt the cell
operation. The correct backing material allows the right amount of
water vapor to reach the MEA and keep the membrane humidified.
Carbon is used for backing layers because it can conduct the electrons
leaving the anode and entering the cathode. A piece of hardware, called