Fuel Cell Theory and Application

By Steve Auger
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.

Part III will cover the current state of fuel cell technology as applied to vehicles.

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