Phosphoric Acid Fuel Cells (PAFC)
- A schematic diagram of phosphoric acid fuel cell is shown in Figure. It was developed in 1980s. It uses either pure hydrogen or rich hydrogen gas as fuel and oxygen or air as oxidant.
- The concentrated phosphoric acid (H3PO4) is used as electrolyte. It is a medium temperature type of fuel cell.
- It has two electrodes of porous conducting material, usually of nickel material to collect charge.
- The electrochemical reaction is normally very slow, therefore a catalyst is needed in the electrodes to accelerate the reaction. Finely powdered nickel/platinum/silver coating is provided on the outer surface of electrodes which act as catalyst. Nickel is used since it is comparatively cheaper.
- The working of the fuel cell is similar to H2-O2, fuel cell described above. The chemical reactions in this fuel cell to produce electrical power with heat and H, 0 as by products are as follows :
Anode : H2 → 2 H+ + 2e …(v)
Cathode : 1/2 O2 + 2H+ + 2e → 2OH– …(vi)
Overall reaction : H2 + 1/2 O2 → H2O + heat + power
- The operating temperatures for this type of fuel are in the range of 150°C-220°C.
- At present these are most economical since they have lowest cost per kW. These are used commercially having the plant capacity in the range of 50 KW to 200 kW.
- In case air is used as oxidant instead of O2, the inert gas N2, is discharged along with spent O2, and water vapour formed.