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FUEL CELL FUNDAMENTALS

In 1839, the English physicist William R. Grove, working from the knowledge that running an electric current through water would produce hydrogen and oxygen, showed that combining hydrogen and oxygen could produce water, and an electric current.

Grove's demonstration opened the way to a new electrical power source, the fuel cell, but little was done with the concept for well over a century. Fuel cells were used to provide electrical power to the Apollo Moon capsule and other spacecraft, but failed to reach a wider market. They now are undergoing rapid development, however.

The fuel cell is conceptually simple. It has been known since the beginning of modern chemistry that running an electrical current through water yields hydrogen and oxygen gas:

2 * H2O -> 2 * H2 + O2

This reaction is known as "electrolysis", and as Grove discovered, it can be performed in reverse, combining hydrogen and oxygen to generate an electric current, plus water as a by-product.

2 * H2 + O2 -> 2 * H2O

Fuel cells are based on reverse electrolysis. They resemble batteries in that their DC electrical output is due to an electrochemical process. However, unlike batteries, fuel cells operate off a continuous stream of air as a source of oxygen, and a source of hydrogen fuel. While straight diatomic hydrogen can be used, in practice this is not a very convenient fuel, and so in general fossil fuels, such as methane, methanol, naptha, coal gas, and other hydrocarbons, are broken down to provide hydrogen.

Fuel cells are also unlike batteries in that their active elements are not consumed by the chemical reaction. This means that fuel cells in principle have much longer service lifetimes than batteries.

In general form, a fuel cell consists of a porous anode and a porous cathode, with these two electrodes separated by a electrolyte. An oxidant is fed to the cathode to supply hydrogen, while a fuel is fed to the anode to supply hydrogen. The electrolyte supports the transfer of ions between anode and cathode to support the reverse electrolysis reaction.

The anode and cathode may be patterned with channels to allow distribution of oxygen or hydrogen. An individual fuel cell generates from 0.6 to 0.8 volts DC, and large numbers of such cells have to be stacked in a fuel cell system and connected in series to provide a useful power output.

Different types of fuel cells operate at different temperatures, from under 100 degrees Celsius to over 1,000 degrees Celsius (210 degrees Fahrenheit to 1,800 degrees Fahrenheit). The anode and cathode may also have channels to allow the distribution of coolants, such as water. The waste heat provided by fuel cells that operate at high temperatures can be used for heating, or the fuel cell can act as a "combustor" to drive a gas turbine for generating power. Such "cogenerating" systems can have high overall efficiencies.

A catalyst is often used to help accelerate the reverse electrolysis reaction, particularly in fuel cells that operate at low temperatures. The catalyst is platinum for some types of fuel cells, a factor that strongly influences their cost.

Although the only output of reverse electrolysis itself is water, the fact that most fuel cells break down hydrocarbon fuels to obtain hydrogen means that fuel cell systems generally exhaust carbon dioxide, some sulfur dioxide, and nitrous oxides along with the water. Nonetheless, fuel cells are relatively nonpolluting, and are in principle quiet, easy to maintain as they have no moving parts, and very efficient, with conversion efficiencies of roughly 50%. Cogenerating systems can approach overall efficiencies of up to 80% in ideal circumstances.

A workable fuel cell system consists of more than just fuel cells. It will always include a "power conditioner" output subsystem to provide electrical power at the proper DC or AC voltages required by the equipment being driven. Fuel cells that use hydrocarbon fuels also require a "fuel processor" input subsystem to convert the hydrocarbons into hydrogen gas.

Fuel processing is based on methods familiar from industrial chemical plants, traditionally known as "fuel reformation". Typical fuel processing steps include:

Desulfurization, where a catalyst is used to remove sulfur contaminants in the fuel. Sulfur compounds are noxious, and they can also bind catalysts used in later stages of fuel reformation and make them useless.
Reformation, where the fuel is mixed with steam and then passed over a catalyst to break it down into hydrogen, as well as carbon dioxide and carbon monoxide,
Shift conversion, where the carbon monoxide reacts with steam over a catalyst to produce more hydrogen and carbon dioxide.

Fuel processing can obtain heat by burning some of the hydrogen fuel, and may use a catalytic system to enhance the reaction. Some types of fuel cells are able to break down hydrocarbons in hydrogen directly at the anode using catalysts and do not need a separate fuel processing system.

* There are two general classes of fuel cells, based on whether the electrolyte is alkaline (basic) or acidic. Resistance in the electrolyte is a source of power loss, but this problem can be reduced by making the electrolyte either very alkaline or very acidic.

There is only one type of alkaline fuel cell, and it is the oldest fuel cell technology. It is still in use in aerospace applications. There are four types of acidic fuel cells:

The phosphoric acid fuel cell (PAFC).
The proton exchange membrane (PEM) fuel cell.
The molten carbonate fuel cell (MCFC).
The solid oxide fuel cell (SOFC).

The PAFC and the PEM fuel cells are the best developed acidic fuel cells. The PAFC is in modest use as a fixed AC power source for buildings and sites, while the PEM is under intense development as a power source for automobiles. The MCFC and SOFC are also under investigation as fixed AC power sources, but their development is not as far advanced as that of the PAFC.

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