ATLANTIC designs and manufactures Proton Exchange Membrane (PEM)
hydrogen
fuel cells. This information deals mainly with that type.
There are several types of devices available know as
fuel cells. Among them
are PEM, Solid Oxide and Alkaline. The PEM cells are
the potentially the type
that will be the most widely used due to their low temperature
operation as
well as quick start up time and simpler balance of plant
requirements.
We will attempt to answer some of the questions that
someone interested
in fuel cells and their operation may have in trying
to get one to actually
work.
A great deal of press has been given fuel cells and their
potential appli-
cations in the last few years but most press releases
usually don't contain
any technical information as to why they are not already
running our cell
phones, homes and automobiles.
The fuel cell is a conceptually simple device. The problems
arise in that
they are temperature and humidity sensitive as to output
power. They also
require what is generally refered to as balance of plant
which includes
among other things air pumps filters, humidifiers, valves
and power
processing circuitry to regulate the fairly unregulated
output voltage of
the fuel cell itself.
There are two basic types of PEM cells. The first is
the direct hydrogen which
runs on hydrogen gas and the second is commonly refered
to as DMFC or Direct
Methanol Hydrogen Fuel Cell. This document covers mainly
direct hydrogen
fuel cells or cells that run on compressed hydrogen gas.
DMFC's reform
methanol directly at the membrane but are still generally
under development.
PEM cells are really batteries with an external supply
of fuel. Unlike conven-
tional batteries which have an internal source of electrons
(the chemical or
fuel), fuel cells are fed from a continuous supply of
hydrogen (containing
electrons).
A cell consists of a thin plastic like membrane material
coated on each side
with a chemical catalyst such as platinum. In order to
gain as much surface
area as possible, the platinum is usually coated on microscopic
carbon
particles which have a very large surface area. The catalyst
aDsorbes the
chemical that you want to react such as hydrogen or oxygen.
Adsorption is the
process of capturing something on a surface as opposed
to aBsorption which is
basically a sponge like operation which "soaks up" something
to its interior.
How the Fuel Cell works
A hydrogen atom entering one side of the cell first encounters
the platinum
catalyst that dissociates the electron from its proton.
(hydrogen has no
neutrons).
The electron is conducted away through the electrode
(anode) to the electrical
load (lightbulb, motor, etc). Its proton travels through
the membrane (proton
exchange membrane) material to the other side. The hydrogen
gas itself is not
able to penetrate the plastic membrane material.
Upon emerging on the other side of the membrane, the
proton waits for the
electron it gave up on the other side. Once the electron
finds a proton,
the catalyst on that side re-attaches the electron and
proton back together
to reform a hydrogen atom. As soon as the hydrogen atom
finds an oxygen atom;
two of them combine to form a water molecule which is
the only bi-product of
the fuel cell.
This reaction continues as long as hydrogen and air are
present. Each hydrogen
atom supplies one electron. Each mole of hydrogen carries
6.02E23 electrons
(Avagadro's number).
FUEL
Proton Exchange Membrane fuel cells generally operate
from low pressure hydrogen
gas. At present, the most convenient supply is from industrial
and welding gas
suppliers. The gas is usually supplied in steel cylinders
at relatively high
pressures on the order or 2000 to 5000 psi. A cylinder
regulator must be used
to reduce the high cylinder pressures to the pressure
required by the fuel cell.
Most fuel cells operate in the 2 to 20 psig range.
Other sources of hydrogen gas are electrolyzers which
can separate water
into hydrogen and oxygen. The oxygen is usually discarded
and air used in the
fuel cell to simplify the plumbing. Electrolyzers are
essentially reverse fuel
cells. When a current is passed through the water it
caused the atomic bonds
between the O and H atoms to break. The hydrogen ends
up at the anode and the
oxygen ends up at the cathode.
Another source of hydrogen gas is from the reforming
of hydrocarbons such as
methanol, natural gas, propane, etc. Reforming is the
term used to describe
the operation of separating hydrocarbons into individual
elements.
In the case of methanol (wood alcohol) a molecule of
methanol is one carbon
connected to 3 hydrogen atoms and one hydroxyl (OH).
The reformer breaks the
bonds between the atoms and allows the removal of the
hydrogen by itself. the
biproduct is usually CO2.
In either case the main requirement of the hydrogen for
the fuel cell is that
it must be essentially free of CO (carbon monoxide).
Current catalysts can
only tolerate a few parts per million CO molecules in
the fuel stream feeding
the fuel cell.
AIR
Air is supplied to the fuel cell by some form of compressor
pump. Some
fuel cells use atmospheric air pressure (14.7psia) but
most still use a
more controlled source of air due to humidification requirements
described
below.
The air must be absolutely free of oils and like the
hydrogen, contain less
than about 10 parts per million CO.
HUMIDIFICATION
Currently, most PEM fuel cells use membrane materials
that conduct the
hydrogen's proton through the material by way of a water
molecule (H2O).
This means that the higher the water content in the material
the better
it conducts the proton. If the gas supplies contain water
(humidity) the cell
output is potentially much higher.
Relative Humidity (RH) depends upon the temperature of
the gas in order
to maintain a near saturated gas supply. Also, there
can't be too much humidity
as long as the saturated gas vapor doesn't encounter
a surface that is colder
than the gas stream. If it does, the water vapor will
condense out and form
water dropplets in the plumbing or worse still the fuel
cell itself. Water
droplets in the fuel cell can block platinum catalyst
sites which will keep
those sites from performing their chemical reaction duties.
ELECTRICAL OUTPUT
The fuel cell's output is unregulated in that the output
voltage is dependent
upon how much it's loaded. The following is a graph of
the output of
our HFC1201 cell.
Typical Voltage/Current
Load Line(12cm2 25 Degree C 70%RH)
STARTUP
The fuel cell has four gas ports; two for fuel (hydrogen)
and two for air
(oxygen).
HYDROGEN PORTS- One is the inlet and one is the outlet.
The gas is fed into the
inlet. The outlet is used upon startup to allow the gas
to completely purge
the flow field in the cell. Once filled with hydrogen;
this port can be blocked.
Purging usually only takes a few seconds.
AIR PORTS- The air is pumped continuously through the
air side of the cell.
Water vapor and air will emerge from the air outlet port.
This water vapor
can be used to humidify the incoming gases.
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