Types of Fuel Cells
PEMFC -Polymer exchange membrane fuel cell
The Department of Energy (DOE) is focusing on the
PEMFC as the most likely candidate for transportation
applications. The PEMFC has a high power density and a
relatively low operating temperature (ranging from 60 to
80 degrees Celsius, or 140 to 176 degrees Fahrenheit). The
low operating temperature means that it doesn’t take very
long for the fuel cell to warm up and begin generating
electricity.
SOFC - Solid oxide fuel cell
These fuel cells are best suited for large-scale
stationary power generators that could provide electricity
for factories or towns. This type of fuel cell operates at
very high temperatures (between 700 and 1,000 degrees
Celsius) which makes reliability a problem because parts
of the fuel cell can break down after cycling on and off
repeatedly. However, solid oxide fuel cells are very stable
when in continuous use. In fact, the SOFC has
demonstrated the longest operating life of any fuel cell
under certain operating conditions. The high temperature
also has an advantage in that the steam produced by the
fuel cell can be channeled into turbines to generate more
electricity. This process is called co-generation of heat and
power (CHP) and it improves the overall efficiency of the
system. The Bloom Box is one type of SOFC.
AFC - Alkaline fuel cell
This is one of the oldest designs for fuel cells; the
United States space program has used them since the
1960s. The AFC is very susceptible to contamination, thus
it requires pure hydrogen and oxygen. It is also very
expensive, so this type of fuel cell is unlikely to be
commercialized anytime soon but some companies are
still trying.
MCFC - Molten-carbonate fuel cell
Like the SOFC, these fuel cells are also best suited for
large stationary power generators. They operate at 600
degrees Celsius, so they can generate steam that can be
used to generate more power. They have a lower
operating temperature than solid oxide fuel cells which
means they don’t need such exotic materials. This makes
the design a little less expensive.
PACF - Phosphoric-acid fuel cell
The phosphoric-acid fuel cell has potential for use in
small stationary power-generation systems. It operates at
a higher temperature than polymer exchange membrane
fuel cells, so it has a longer warm-up time. This makes it
unsuitable for use in most vehicles.
DMFC - Direct-methanol fuel cell
Methanol fuel cells are comparable to a PEMFC in
regards to operating temperature, but are not as efficient.
Also, the DMFC requires a relatively large amount of
platinum to act as a catalyst which makes these fuel cells
very expensive.
Source - www1.eere.energy.gov/hydrogenandfuelcells/
fuelcells/ fc_types.html
Electrolysis Mode and Fuel Cell
Mode. There are also two chemical
processes involved: oxidation and
reduction.
In the Electrolysis Mode (Figure
2), water is introduced to both sides
of the MEA where it is split into
hydrogen at the cathode (negative)
and oxygen at the anode (positive)
by a small voltage level less than 1.5
volts DC called the water
decomposition voltage that you’ll
learn more about later. Hydrogen
collects at the cathode and oxygen is
created at the anode. Electrolysis of
pure water requires excess energy in
the form of overpotential to
overcome various activation barriers.
Without the excess energy, the
electrolysis of pure water occurs very
slowly if at all. This is, in part, due to
the limited self-ionization of water.
Pure water has an electrical
conductivity about one millionth that
of seawater. Nevertheless, electrolysis
can be accomplished. If you’re a
chemistry buff, here are the chemical
reactions:
Electrolysis Mode Reactions
• Anode reaction: 2H2O → 4H+ +
4e- + O2
• Cathode reaction: 4H+ + 4e- → 2H2
MEA - Membrane Electrode Assembly
The membrane electrode assembly — or MEA — is the heart of a hydrogen fuel
cell. It consists of the ion-exchange membrane, platinum anode and cathode
electrodes, and gas diffusion and catalyst layers. Here’s basically how it works
with hydrogen and oxygen to create electricity:
The Anode — the negative side of the fuel cell — conducts the electrons that are
freed from the hydrogen molecules so they can be used in an external circuit.
Channels etched into the anode disperse the hydrogen gas equally over the
surface of the catalyst.
The Cathode — the positive side of the fuel cell — also contains channels that
distribute the oxygen to the surface of the catalyst. It conducts the electrons back
from the external circuit to the catalyst where they can recombine with the
hydrogen ions and oxygen to form water.
The Polymer Electrolyte Membrane — or PEM — is a specially treated material that looks something like ordinary
kitchen plastic wrap that conducts only positively charged ions and blocks the negatively charged electrons. The PEM is
the key to fuel cell technology as it permits only the necessary ions (molecules stripped of their electrons) to pass
between the anode and cathode. The thickness of the membrane varies, depending on the catalyst and the amount of
platinum (Pt) that is used in each electrode.
Image Credit - Wikipedia
48
May 2010
