Fuel Cell

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Fuel cell:

Download PPT of Fuel Cell

A fuel cell is an electro chemical device that can convert chemical energy of a fuel directly into electrical energy. It can provide electrical energy for a longer time compare a normal battery, The reactions take place inside the cell and are known as electrochemical cell.

The best suitable fuel is hydrogen, compounds of hydrogen. Hydrogen is fed into Anode of fuel cell Oxygen (or) Air enter the fuel cell through the cathode. Encourage by the catalysts, the hydrogen atom splits into proton and electron.

Proton pass through Electrolyte electron creates a separate current that can be utilized before the electron to cathode and then it reunites with hydrogen from natural gas to methanol and even gasoline.

Fuel cell doesn’t involve large combustion and is one of the cleanest fuel combination process.

1.     Based on the Type of Electolyte

a.   Phosphoric Acid Fuel Cell (PAFC)

b.   Alkaline Fuel Cell (AFC)

c.   Polymer Electrolytic Membrane Fuel Cell (PEMFC)

Or Solid Polymer Fuel Cell (SPFC)

Or Proton Exchange Membrane Fuel Cell (PEMFC)

d.   Molten Carbonate Fuel Cell (MCFC)

e.   Solid Oxide Fuel Cell (SOFC)

Phosphoric Acid Fuel Cell (PAFC)

PAFC was developed in 1980s.It consists of two electrodes of porous conducting material (commonly nickel) to collect charge, with concentrated phosphoric acid filled between them, to work as electrolyte. Pure hydrogen or a hydrogen rich gas is supplied at negative electrode and oxygen or air is supplied at positive electrode. The pores provide an opportunity to gas, electrolyte and electrode to come into contact for electrochemical reaction. The reaction is normally very slow and a catalyst is required in the electrode to accelerate the reaction.

  The basic phosphoric acid fuel cell.

Platinum serves as best catalyst for both electrodes and used for premium fuel cells.

In general, a less expensive material such as nickel (for negative electrode) and silver (for positive electrode) is used wherever possible. Thus finely divided platinum or nickel/silver deposited on the outer surface of electrodes are used as catalyst. During the usage of the cell, the catalyst gradually loses its activity. This loss of Activity is often attributed to “poisoning” (inactivation) of catalyst by the impurities (mostly Sulphur compounds) in the fuel.

At the negative electrode, hydrogen gas is converted to hydrogen ions (H⁺) and an equal number of electrons(e^-).

Thus: H₂ ⇾ 2H ⁺ + 2e^-

The electrons originating at negative electrode flow through the external load to positive electrode. Also the H⁺ ions migrate from negative electrode towards positive electrode through the electrolyte. On reaching the positive electrode they interact with o₂ to produce water.

Thus:  ½ O₂ + 2H⁺ + 2e^-⇾ H₂O

Combining the above equations indicates that a fuel cell combines H₂ and O₂ to produce water (plus electrical energy). The overall reaction is therefore,

H₂ + ½ O₂ ⇾ H₂O

The above reaction is true for any type of hydrogen-oxygen cell. The operating temperature of PAFC is 150⁰C-200⁰C. At atmospheric pressure it produces an ideal emf of 1.23 v at 25⁰C, which reduces to 1.15 v at 200⁰C. The actual value is always less than this and decreases with current. Normally at rated values of current the voltage lies between 0.7 V and 0.8 V.

Alkaline fuel cell (AFC):

Alkaline fuel cell, the oldest of all fuel cells uses 40 percent aqueous KOH as electrolyte. The operating temperature is about 90 ⁰C. The electrodes and other details are same as explained for PAFC. Like PAFC it also works with H₂ and O₂ active materials and same level of emf is produced. The operation and movements of charge carriers at positive electrode oxygen, water (from electrolyte) and returning electrons from the external load combine to produce OH^- ions:

½ O₂ + H₂O + 2e^-  ⇾ 2 OH^-

These OH^- ions migrate from positive to negative electrode through electrolyte. On reaching positive electrode these OH^- ions combine with H₂ to produce water.

An equivalent number of electrons are liberated that flow through external load towards positive electrode.

Thus: H₂ + 2 OH^- ⇾ 2 H₂O + 2 e^-

The overall  reaction is same as that with PAFC. That is:

H₂ + ½ O₂ ⇾ H₂O

The fuel used in AFC must be free from CO₂, because this gas can combine with potassium hydroxide electrolyte to form potassium carbonate. This increase the electrical resistance of the cell, which in turn decreases the available output voltage of the cell. Similarly, if air is used instead of pure oxygen the CO₂must first be removed from the air by scrubbing with lime.

Advantages:

·       It is less pollutant

·       Conversion is higher than diesel or gas engines

·       Plant can be installed near point of use so transmission and distribution losses are avoided.

·       No water is needed for cooling

·       Plants are compact and require less space

·       Low maintenance

·       No noise

·       Reduce the dependency on oil

Dis-Advantages:

·       Fuel cells are expensive because of platinum

·       Require constant fuel supply

·       Hydrogen is very flammable so transportation and storage is difficult

·       Operation of fuel cell requires lot of fuel

·       Pure hydrogen is high cost

Applications:

The application of the fuel cell may be discussed in the following areas

·       Domestic power

·       Central power station.

·       Automotive Vehicles

·       Special Application.