Here is More then most want to know about Fuel Cells.


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In principle, a fuel cell operates like a battery. Unlike a battery, a
fuel cell does not run down or require recharging. It will produce
energy in the form of electricity and heat as long as fuel is supplied.

A fuel cell consists of two electrodes sandwiched around an electrolyte.
Oxygen passes over one electrode and hydrogen over the other, generating
electricity, water and heat.

Hydrogen fuel is fed into the "anode" of the fuel cell. Oxygen (or air)
enters the fuel cell through the cathode. Encouraged by a catalyst, the
hydrogen atom splits into a proton and an electron, which take different
paths to the cathode. The proton passes through the electrolyte. The
electrons create a separate current that can be utilized before they
return to the cathode, to be reunited with the hydrogen and oxygen in a
molecule of water.

A fuel cell system which includes a "fuel reformer" can utilize the
hydrogen from any hydrocarbon fuel - from natural gas to methanol, and
even gasoline. Since the fuel cell relies on chemistry and not
combustion, emissions from this type of a system would still be much
smaller than emissions from the cleanest fuel combustion processes.

Types of Fuel Cells:

*Phosphoric Acid (PAFC). This type of fuel cell is commercially
available today. More than 200 fuel cell systems have been installed all
over the world - in hospitals, nursing homes, hotels, office buildings,
schools, utility power plants, an airport terminal, landfills and waste
water treatment plants. PAFCs generate electricity at more than 40%
efficiency -- and nearly 85% of the steam this fuel cell produces is
used for cogeneration -- this compares to about 35% for the utility
power grid in the United States. Operating temperatures are in the range
of 300 to 400 degrees F (150 - 200 degrees C). At lower temperatures,
phosphoric acid is a poor ionic conductor, and carbon monoxide (CO)
poisoning of the Platinum (Pt) electro-catalyst in the anode becomes
severe. The electrolyte is liquid phosphoric acid soaked in a matrix.
One of the main advantages to this type of fuel cell, besides the nearly
85% cogeneration efficiency, is that it can use impure hydrogen as fuel.
PAFCs can tolerate a CO concentration of about 1.5 percent, which
broadens the choice of fuels they can use. If gasoline is used, the
sulfur must be removed. Disadvantages of PAFCs include: it uses
expensive platinum as a catalyst, it generates low current and power
comparably to other types of fuel cells, and it generally has a large
size and weight. PAFCs, however, are the most mature fuel cell
technology. Through organizational linkages with Gas Research Institute
(GRI), electronic utilities, energy service companies, and user groups,
the Department of Energy (DOE) helped in bringing about the
commercialization of a PAFC, produced by ONSI (now UTC Fuel Cells).
Existing PAFCs have outputs up to 200 kW, and 1 MW units have been
tested. 

Anode: H2(g) -> 2H+(aq)+ 2e- 

Cathode: 保2(g) + 2H+(aq) + 2e- -> H2O(l) 


------------------------------------------------------------------------
--------

Cell: H2(g) + 保2(g)+ CO2 -> H2O(l) + CO2 

*Proton Exchange Membrane (PEM). These cells operate at relatively low
temperatures (about 175 degrees F or 80 degrees C), have high power
density, can vary their output quickly to meet shifts in power demand,
and are suited for applications, -- such as in automobiles -- where
quick startup is required. According to DOE, "they are the primary
candidates for light-duty vehicles, for buildings, and potentially for
much smaller applications such as replacements for rechargeable
batteries." The proton exchange membrane is a thin plastic sheet that
allows hydrogen ions to pass through it. The membrane is coated on both
sides with highly dispersed metal alloy particles (mostly platinum) that
are active catalysts. The electrolyte used is a solid organic polymer
poly-perflourosulfonic acid. The solid electrolyte is an advantage
because it reduces corrosion and management problems. Hydrogen is fed to
the anode side of the fuel cell where the catalyst encourages the
hydrogen atoms to release electrons and become hydrogen ions (protons).
The electrons travel in the form of an electric current that can be
utilized before it returns to the cathode side of the fuel cell where
oxygen has been fed. At the same time, the protons diffuse through the
membrane (electrolyte) to the cathode, where the hydrogen atom is
recombined and reacted with oxygen to produce water, thus completing the
overall process. This type of fuel cell is, however, sensitive to fuel
impurities. Cell outputs generally range from 50 to 250 kW. 

Anode: H2(g) -> 2H+(aq) + 2e- 

Cathode: 保2(g) + 2H+(aq) + 2e- -> H2O(l) 


------------------------------------------------------------------------
--------

Cell: H2(g) + 保2(g) -> H2O(l) 


*Molten Carbonate (MCFC). These fuel cells use a liquid solution of
lithium, sodium and/or potassium carbonates, soaked in a matrix for an
electrolyte. They promise high fuel-to-electricity efficiencies, about
60% normally or 85% with cogeneration, and operate at about 1,200
degrees F or 650 degrees C. The high operating temperature is needed to
achieve sufficient conductivity of the electrolyte. Because of this high
temperature, noble metal catalysts are not required for the cell's
electrochemical oxidation and reduction processes. To date, MCFCs have
been operated on hydrogen, carbon monoxide, natural gas, propane,
landfill gas, marine diesel, and simulated coal gasification products.
10 kW to 2 MW MCFCs have been tested on a variety of fuels and are
primarily targeted to electric utility applications. Carbonate fuel
cells for stationary applications have been successfully demonstrated in
Japan and Italy. The high operating temperature serves as a big
advantage because this implies higher efficiency and the flexibility to
use more types of fuels and inexpensive catalysts as the reactions
involving breaking of carbon bonds in larger hydrocarbon fuels occur
much faster as the temperature is increased. A disadvantage to this,
however, is that high temperatures enhance corrosion and the breakdown
of cell components.

Anode: H2(g) + CO32- -> H2O(g) + CO2(g) + 2e- 

Cathode: 保2(g) + CO2(g) + 2e- -> CO32- 


------------------------------------------------------------------------
--------

Cell: H2(g) + 保2(g) + CO2(g) -> H2O(g) + CO2(g) 


*Solid Oxide (SOFC). Another highly promising fuel cell, this type could
be used in big, high-power applications including industrial and
large-scale central electricity generating stations. Some developers
also see SOFC use in motor vehicles and are developing fuel cell
auxiliary power units (APUs) with SOFCs. A solid oxide system usually
uses a hard ceramic material of solid zirconium oxide and a small amount
of ytrria, instead of a liquid electrolyte, allowing operating
temperatures to reach 1,800 degrees F or 1000 degrees C. Power
generating efficiencies could reach 60% and 85% with cogeneration and
cell output is up to 100 kW. One type of SOFC uses an array of
meter-long tubes, and other variations include a compressed disc that
resembles the top of a soup can. Tubular SOFC designs are closer to
commercialization and are being produced by several companies around the
world. Demonstrations of tubular SOFC technology have produced as much
as 220 kW. Japan has two 25 kW units online and a 100 kW plant being
testing in Europe.

Anode: H2(g) + O2- -> H2O(g) + 2e- 

Cathode: 保2(g) + 2e- -> O2- 


------------------------------------------------------------------------
--------

Cell: H2(g) + 保2(g) -> H2O(g) 


*Alkaline. Long used by NASA on space missions, these cells can achieve
power generating efficiencies of up to 70 percent. They were used on the
Apollo spacecraft to provide both electricity and drinking water. Their
operating temperature is 150 to 200 degrees C (about 300 to 400 degrees
F). They use an aqueous solution of alkaline potassium hydroxide soaked
in a matrix as the electrolyte. This is advantageous because the cathode
reaction is faster in the alkaline electrolyte, which means higher
performance. Until recently they were too costly for commercial
applications, but several companies are examining ways to reduce costs
and improve operating flexibility. They typically have a cell output
from 300 watts to 5 kW. 

Anode: H2(g) + 2(OH)-(aq) -> 2H2O(l) + 2e- 

Cathode: 保2(g) + H2O(l) + 2e- -> 2(OH)-(aq) 


------------------------------------------------------------------------
--------

Cell: H2(g) + 保2(g) -> H2O(l) 


*Direct Methanol Fuel Cells (DMFC). These cells are similar to the PEM
cells in that they both use a polymer membrane as the electrolyte.
However, in the DMFC, the anode catalyst itself draws the hydrogen from
the liquid methanol, eliminating the need for a fuel reformer.
Efficiencies of about 40% are expected with this type of fuel cell,
which would typically operate at a temperature between 120-190 degrees F
or 50 -100 degrees C. This is a relatively low range, making this fuel
cell attractive for tiny to mid-sized applications, to power cellular
phones and laptops. Higher efficiencies are achieved at higher
temperatures. A major problem, however, is fuel crossing over from the
anode to the cathode without producing electricity. Many companies have
said they solved this problem, however. They are working on DMFC
prototypes used by the military for powering electronic equipment in the
field.

Anode: CH3OH(aq) + H2O(l) -> CO4(g) + 6H+(aq) + 6e- 

Cathode: 6H+(aq) + 6e- + 3/2O2(g) -> 3H2O(l) 


------------------------------------------------------------------------
--------

Cell: CH3OH(aq) + 3/2O2(g) -> CO4(g) + 2H2O(l) 


Regenerative Fuel Cells. 
Still a very young member of the fuel cell family, regenerative fuel
cells would be attractive as a closed-loop form of power generation.
Water is separated into hydrogen and oxygen by a solar-powered
electrolyser. The hydrogen and oxygen are fed into the fuel cell which
generates electricity, heat and water. The water is then recirculated
back to the solar-powered electrolyser and the process begins again.
These types of fuel cells are currently being researched by NASA and
others worldwide.

*Zinc-Air Fuel Cells (ZAFC). In a typical zinc/air fuel cell, there is a
gas diffusion electrode (GDE), a zinc anode separated by electrolyte,
and some form of mechanical separators. The GDE is a permeable membrane
that allows atmospheric oxygen to pass through. After the oxygen has
converted into hydroxyl ions and water, the hydroxyl ions will travel
through an electrolyte, and reaches the zinc anode. Here, it reacts with
the zinc, and forms zinc oxide. This process creates an electrical
potential; when a set of ZAFC cells are connected, the combined
electrical potential of these cells can be used as a source of electric
power. This electrochemical process is very similar to that of a PEM
fuel cell, but the refueling is very different and shares
characteristics with batteries. Metallic Power is working on ZAFCs
containing a zinc "fuel tank" and a zinc refrigerator that automatically
and silently regenerates the fuel. In this closed-loop system,
electricity is created as zinc and oxygen are mixed in the presence of
an electrolyte (like a PEMFC), creating zinc oxide. Once fuel is used
up, the system is connected to the grid and the process is reversed,
leaving once again pure zinc fuel pellets. The key is that this
reversing process takes only about 5 minutes to complete, so the battery
recharging time hang up is not an issue. The chief advantage zinc-air
technology has over other battery technologies is its high specific
energy, which is a key factor that determines the running duration of a
battery relative to its weight. When ZAFCs are used to power EVs, they
have proven to deliver longer driving distances between refuels than any
other EV batteries of similar weight. Moreover, due to the abundance of
zinc on earth, the material costs for ZAFCs and zinc-air batteries are
low. Hence, zinc-air technology has a potential wide range of
applications, ranging from EVs, consumer electronics to military.
Powerzinc in southern California is currently commercializing their
zinc/air technology for a number of different applications. 

*Protonic Ceramic Fuel Cell (PCFC). This new type of fuel cell is based
on a ceramic electrolyte material that exhibits high protonic
conductivity at elevated temperatures. PCFCs share the thermal and
kinetic advantages of high temperature operation at 700 degrees Celsius
with molten carbonate and solid oxide fuel cells, while exhibiting all
of the intrinsic benefits of proton conduction in polymer electrolyte
and phosphoric acid fuel cells (PAFCs). The high operating temperature
is necessary to achieve very high electrical fuel efficiency with
hydrocarbon fuels. PCFCs can operate at high temperatures and
electrochemically oxidize fossil fuels directly to the anode. This
eliminates the intermediate step of producing hydrogen through the
costly reforming process. Gaseous molecules of the hydrocarbon fuel are
absorbed on the surface of the anode in the presence of water vapor, and
hydrogen atoms are efficiently stripped off to be absorbed into the
electrolyte, with carbon dioxide as the primary reaction product.
Additionally, PCFCs have a solid electrolyte so the membrane cannot dry
out as with PEM fuel cells, or liquid can't leak out as with PAFCs.
Protonetics International Inc. is primarily researching this type of
fuel cell.


























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