An electrochemical device capable of generating an
electric current by continuous conversion of
chemical energy of a fuel directly into electrical
energy without combustion is called Fuel cell.
Fuel cell systems generally operate on pure
hydrogen and air to produce electricity with water
and heat as the bi-products.
Fuel cells are modular in construction and their
efficiency is independent of size.
A fuel cell requires continuous supply of a fuel
and oxidant to produce D.C. electric power.
The basic difference between battery and fuel cell
battery stores electrical charges, after discharge it
needs recharging while in fuel cells recharging is
In Fuel cells the chemical energy of the reactants is
converted into electrical energy as an Isothermal Process.
Thus heat is not involved in the conversion process and a
high conversion efficiency is possible.
Another reason for the interest in fuel cells is that their
efficiency and cost per kW of power are independent of size
or rating of the fuel cell.
This advantage makes the prospects of fuel cells very
attractive as portable power plants for space crafts,
Fuel cells can be manufactured large or small as necessary
for the particular power application.
Presently, there are micro fuel cells that are the size of a
pencil eraser and generate few milliwatts of power while
there are others large enough to provide large amount of
The power output of fuel cells is fully scalable by stacking
multiple cells in series to obtain the desired voltage.
6. PRINCIPLE OF WORKING
A fuel cell has
- An anode,
- A Cathode,
- An electrolyte,
- A container,
- Fuel supply,
- Oxidant supply etc.
For explaining the principle and operation of fuel cells, the hydrogen-
oxygen fuel cell alkali (Solution of KOH) electrolyte is described. The
electrolyte can be acidic (Solution of H2SO4)
7. HYDROX (H2, O2) CELL
H2 2 H+ + 2e- -------- (i)
½ O2 + H2O + 2e- 2 OH- ---(ii)
In the Electrolyte:
H+ + OH- H2O -----------(iii)
The overall cell reaction:
H2 + ½ O2 H2O -------------(iv)
• Fuel cells can be adopted to a variety of fuels by changing the
electrolyte, but ‘Hydrox’ fuel cells using hydrogen and oxygen as fuel
are the most efficient and highly developed cells.
• A single ‘Hydrox’ fuel cell can produce an e.m.f. of 1.23 V at one
atmospheric pressure at 25 oC.
• It is possible to create useful potentials of 100 to 1000 volts and power
level of 1 kW to 100 MW by connecting a number of cells in series-
• Fuel cells are particularly suited for low voltage and high current
• The Apollo spacecraft used ‘hydrox’ fuel cells for their energy needs
and also as a source of drinking water.
9. Classification of Fuel Cells
Fuel Cells can be classified on various basis as:
Fuel and oxidant combination
Direct fuel or Indirect fuel
The most common classification of Fuel Cells is by electrolyte
Alkaline Fuel Cells (AFC)
Direct Methanol Fuel Cells (DMFC)
Phosphoric Acid Fuel Cells (PAFC)
Proton Exchange Membrane Fuel Cells (PEMFC)
Moltoen Carbonate Fuel Cell (MCFC)
Solid Oxide Fuel Cell (SOFC)
Zinc-Air Fuel Cell (ZAFC)
Regenerative Fuel Cell (RFC)
11. Proton Exchange Membrane Fuel Cells
• In this type of cell electrolyte is a solid polymer membrane (Thin plastic film). This
polymer is permeable to protons when it is saturated with water, but it does not
13. Advantages and Disadvantages of PEMFC
• PEMFC generate more power for a given volume i.e. high power density.
• Rapid start.
• Operating temperature is less than 100 oC.
• Less expensive.
• Because of solid electrolyte, PEMFC has less problems with corrosion.
• Longer Life.
• Best suitable for transportation applications.
• Due to low operating temperature these are not enough to perform useful
14. Phosphoric Acid Fuel Cells (PAFC)
• The electrolyte in this fuel cell is 100% concentrated phosphoric acid (H3PO4). The
ionic conductivity of phosphoric acid is low at low temperature.
• PAFC is similar to PEMFC.
• PAFCs are operated at 150 oC to 220 oC.
• The PAFC operates at greater than 40% efficiency in generating
• The PAFC when operated in cogeneration applications, the overall
efficiency is approximately 85%.
• At present they offer the lowest cost per kW and are used mainly for
plants of 50 to 200 kW capacities.
• In PAFC, the waste heat at operating temperature is capable of heating
the water or generating steam at atmospheric pressure.
16. Molten Carbonate Fuel Cells (MCFC)
Molten carbonate fuel cells use an electrolyte, which is a molten mixture of carbonate
Two mixtures are generally used:
a) Lithium Carbonate and Potassium Carbonate OR
b) Lithium Carbonate and Sodium Carbonate
• Since these salts can act as electrolytes only in liquid phase, the operating
temperature should be as high as 650 oC.
• MCFCs are considered to be second generation fuel cells because they
will reach commercialization after PAFCs.
• Efficiency of MCFCs is more than PAFCs and is around 60%.
• The by-product heat from MCFC can be used to generate high pressure
steam that can be used in many industrial and commercial applications.
18. Direct Methanol Fuel Cells
• In this cell also polymer is an electrolyte and charge carrier is the
• The liquid methanol (CH3OH) is oxidized in the presence of water at
anode and generating carbon dioxide, hydrogen ions and electrons.
• Efficiency of these cells is approximately 40% at operating temperature
50oC - 120oC.
• The main disadvantage of this cell is that at low temperature oxidation of
methanol to hydrogen ions and carbon dioxide requires more active
catalyst, which increases the cost and weight.
19. Zinc-Air Fuel Cells (ZAFC)
• In this type of cells electrolyte is a ceramic solid and charge carries are
hydroxyl ions OH-.
• The operating temperature of this cell is high and remain around 700oC.
• The anode is composed of zinc and is supplied with hydrogen or
• The cathode is separated from the air supply with gas diffusion electrode,
a permeable membrane that allows atmospheric oxygen to pass through.
• The by-product heat can be used to generate high pressure steam which
can be used for industrial or commercial applications.
20. Regenerative Fuel Cells
In regenerative fuel cells reactants are regenerated from the products
Regenerative fuel cells operate in a closed loop.
Fuel cells generate the electricity, heat and water from Hydrogen and
The hydrogen would be generated from the electrolysis of water by
splitting it into hydrogen and oxygen by using renewable energy
source as solar, wind etc.
The hydrogen thus generated is reused as fuel, oxygen as can be used
as oxidant and water is re-circulated for electrolysis.
21. Solid Oxide Fuel Cells
• SOFC operates at high temperature between 650oC to 1000oC.
• The electrolyte in this cell is solid, non-porous metal oxide that is
conductive to oxygen ions.
• The charge carrier in SOFC is the oxygen ion.
• At the cathode, the oxygen molecules from the air are split
into oxygen ions with the addition of four electrons.
• The oxygen ions are conducted through the electrolyte and
combine with hydrogen at the anode releasing four
• The electrons move through the external circuit producing
electric power and by product heat.
23. Type of fuel cell Electrolyte Temp. in °C Fuel
AFC Potassium Hydroxide (KOH) 70-100 H2 + O2
PEM Proton Exchange Membrane (Nafion, Gore) 50-100 H2 + O2/Air
PAFC Phosphoric acid 160-210 H2 Hydrogen rich
gas + Air
MCFC High temperature compounds of salt
carbonates CO3 (Sodium or Magnesium)
650 H2 Hydrogen rich
gas + Air
SOFC Solid Ceramic Compound (Calcium or
800-1000 H2 Hydrogen rich
gas + Air
DMFC Proton Exchange Membrane (Nafion, Gore) 50-100 Methanol/Ethanol +
24. ELECTRODE MATERIALS & CATALYSTS
• Porous Nickel Electrode (Commercial application)
• Porous Carbon Electrode (Commercial application)
• Platinum Electrode (Special applications in military and space)
Incorporated with electrode materials for speeding the reactions.
• Finely divided platinum (very costly)
• Nickel (for active material as H2)
• Silver (for active material as O2)
25. Performance Analysis of a Fuel Cell
The Electromotive Force that will drive the electrons through external
load is proportional to Gibbs Free Energy change and given as
E = (-ΔG/nF) Volts
Where E = EMF
ΔG = Change in Gibbs free energy (J/mol)
n = Number of electrons per mole of fuel
= 2 for hydrogen
F = Faraday’s constant
= 96487 coulombs / mole
Gibbs free energy is defined as
ΔG = ΔH – TΔs kcal/mol
Where ΔH = Heat of Reaction
Δs = Change of entropy
TΔs = Isothermal Heat Transfer
ηth = ΔG/ΔH
= 1 – T Δs/ΔH
For reversible e.m.f of the cell, the
efficiency is given as
ηi = -nFE/ΔH
= -ItE/ ΔH
Where I = current and
t = time for which current flows
The overall efficiency of fuel cell is
ηoverall = ηth x Loss Factor
The power output of a reversible fuel cell,
Prev = ΔGm/(Molar Mass of Hydrogen)
Molar mass of hydrogen = 2.016 kg/mole
Actual electrical power output ,
P = Prev x ηoverall
The rate of heat released,
Q = (Prev – P) (In Watts)
Fuel Cells Losses
a) Activation losses
b) Fuel cross-over losses
c) Ohmic or resistance losses
d) Mass transport losses
28. ADVANTAGES OF FUEL CELLS
High conversion efficiency as high as 70%
It can be installed near the use point reducing transmission losses.
Because of very less mechanical components, its operation is fairly
It requires less attention and less maintenance.
Creates very less or no pollution.
No cooling water is required as in conventional steam power plant.
They can be readily accepted in residential areas because of noise free
It takes a little time to start its operation.
Space requirement is considerably less as compared to conventional
29. DISADVANTAGES OF FUEL CELLS
• High initial cost.
• Development costs are very high.
• Use costly catalysts for the reaction to takes place.
• Low service life.
• Low Voltage.
30. Applications of Fuel Cells
3. Portable Power Plants
4. Central Base Load Power Plants
7. Defense Applications
• Fuel cells are particularly suited for low voltage and high current
• Hydrogen-Oxygen fuel cells have been proposed for propulsion of
electric vehicles, with metal hydride as the source of hydrogen.
• At present the use of hydrogen-oxygen cell is restricted to manned
• Fuel cells with porous Nickel electrodes and Potassium hydroxide
electrolyte have been used to provide electric power for the Apollo and
• The hydrogen and oxygen for operating the cell are stored in liquid
form to minimize the volume occupied.