Tuesday
Sep292009
Fuel Cell Theory and Application
Tuesday, September 29, 2009 at 12:01AM | Share Article
By Steve Auger
Part II: Fuel cell theory, applications and different types available.
The five main types of fuel cell, their properties and typical application are summarized in the table below. Each type of fuel cell has advantages and drawbacks. In general, either the fuel cell has to be operated at high temperature to allow impure fuels or operate at lower temperatures with pure fuel only and precious metal (platinum) catalysts. The platinum catalyst makes the fuel cell more expensive than some markets can tolerate (automobile market for example). The high temperature fuel cells tend to have slow start-up, a higher corrosion rate and faster breakdown of cell components.
Fuel cells have either liquid or solid electrolytes. The liquid electrolytes generally require pumps or more complex electrolyte management whereas the solid electrolytes have less corrosion and do not suffer the same management problems as liquid electrolytes.
Part II: Fuel cell theory, applications and different types available.
Fuel Cell Theory
Fuel cells generate electricity by means of a chemical reaction. The reaction takes place at two electrodes similar to a battery with one positive electrode (anode) and one negative (cathode). The electrodes are separated by an electrolyte membrane. In a hydrogen fuel cell, hydrogen gas is passed over the anode and reacts to a catalyst resulting in negatively charged electrons (e-) and positively charged ions (H+). The electrons are used as electrical energy outside the fuel cell. The hydrogen ions pass through the membrane to the cathode where they combine with oxygen to produce water.
Some cells require pure fuel as input
such as pure hydrogen for example. If a source of pure hydrogen
is not available then a “reformer” is used to purify the fuel.
Some fuel cells can handle a certain amount of impurities. In
this case the electrolyte is generally operated at higher temperatures
to maintain efficiency.
Fuel Cell Applications
Scientists have developed many different types of fuel cell, hydrogen being only one. All fuel cells have in common an anode, cathode, catalysts and an electrolyte. The electrode design, materials and technical details are generally constrained by the choice of electrolyte. The main types of electrolyte used today are alkaline, molten carbonate, phosphoric acid, proton exchange membrane (PEM) and solid oxide. A single fuel cell generates only a small amount of electricity and applications generally require a stack of cells to be useful.The five main types of fuel cell, their properties and typical application are summarized in the table below. Each type of fuel cell has advantages and drawbacks. In general, either the fuel cell has to be operated at high temperature to allow impure fuels or operate at lower temperatures with pure fuel only and precious metal (platinum) catalysts. The platinum catalyst makes the fuel cell more expensive than some markets can tolerate (automobile market for example). The high temperature fuel cells tend to have slow start-up, a higher corrosion rate and faster breakdown of cell components.
Fuel cells have either liquid or solid electrolytes. The liquid electrolytes generally require pumps or more complex electrolyte management whereas the solid electrolytes have less corrosion and do not suffer the same management problems as liquid electrolytes.
| Type |
Electrolyte |
Catalyst |
Fuel |
Oper. Temp. |
Cell
Output |
Typical
Application |
| Alkaline | Potassium
hydroxide (liquid) |
Platinum |
Compressed
hydrogen and oxygen |
150
- 200OC |
Electricity
300W- 5kW, drinking water |
NASA
Apollo and Gemini spacecraft |
| Molten
Carbonate (MCFC) |
Sodium
or magnesium Carbonate (liquid) |
Nickel |
Hydrogen,
carbon monoxide, natural gas, propane, ... |
650OC | Electricity
10 kW - 2 MW, waste heat is recycled |
Electric
utility applications |
| Phosphoric
Acid (PAFC) |
Phosphoric
acid (liquid) |
Platinum |
Impure
hydrogen, gasoline (ex-sulpher) |
150
- 200OC |
Electricity
200 kW - 20 MW, steam used for cogeneration |
Emergency
back-up power in hospitals, nursing homes, businesses, ... |
| Proton
Exchange Membrane (PEM) |
Polymer
(solid) |
Platinum |
Pure
hydrogen, oxygen |
80OC | Electricity
50 watts to 75 kW, water vapor |
Light
vehicles, buildings |
| Solid
Oxide (SOFC) |
Calcium
or zirconium oxide (solid) |
Variety |
Impure
hydrogen |
1,000OC | Electricity
up to 100 kW, waste heat is recycled |
Stationary
applications, auxiliary power units |
Other Types of Fuel Cells
Some other types of fuel cell in development are:- Direct Methanol - liquid methanol is input to the fuel cell and no reformer is necessary. This type of fuel cell is attractive for small applications such as cell phones and laptop computers.
- Regenerative - This is a closed loop system whereby water is electrolyzed into hydrogen and oxygen using solar power. The hydrogen and oxygen are fed into the fuel cell, where electricity and water are produced. The water is fed back to the electrolyzer.
- Zinc Air - From a user perspective this fuel cell operates similar to a battery. Oxygen and zinc are input to the fuel cell which produces electricity and zinc oxide. Once the fuel is spent, the system is connected to the grid and the process is reversed, leaving zinc. The advantage over batteries is the high energy density and fast reversal time.
- Protonic Ceramic - designed to be operated with fossil fuels directly this fuel cell operates at elevated temperatures.
- Microbial - Uses microorganisms such as bacteria to convert organic material into fuel at room temperature. Potential applications could be medical devices fueled by blood glucose, or water treatment plants.
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