1. Fuel Cell Electric Prime Movers
P M V Subbarao
Professor
Mechanical Engineering Department
A Single Stage Energy Conversion Systems .....

2. The Future of Vehicles – Various Technologies for Automobiles
Vehicle weight
challenges
2nd generation biofuels
high
transportation
Long range public transport
Fuel cell vehicles
Public transport
Economy of fuel
Plug-in hybrids
City LDVs
Everyday use
Battery vehicle
2nd car
Economy of propulsion system
acceptance
Energy density
Electro cycle
safety
low
distance
Short trips (city)
Long trips (highway)

3. Fuel cell Electric Prime Mover

4. Fuel Cells
“Fuel cells are the electrochemical device that converts chemical energy of the fuel directly into the electrical energy". H2 O2
H2O
2e-
Electrolyte
(ion conductor)
Cathode
(positive electrode)
Anode
(negative electrode)
Air flow channel
Fuel flow channel
Heat +
Fuel in
Depleted air
Negative Ion
Load
Air
Positive ion or
Depleted fuel
and product
gases out
Electrolyte layer is in contact with a porous anode and cathode on sides.
Hydrogen flows along anode side.
Oxygen flows along cathode. O2 H2
H2O

5. Electrochemical Reaction
Electrochemical reaction produces electricity
At cathode ½ O2 + 2e-  O2-
At anode H2 + O  H2O + 2e-
Overall reaction H2+ ½ O2  H2O
Emits steam and heat
The chemical reaction in a fuel cell is similar to that in a chemical battery.
The thermodynamic voltage of a fuel cell depends on the energy released and the number of electrons transferred in the reaction.
The energy released by the battery cell reaction is given by the change in Gibbs free energy, ΔG.
The change in Gibbs free energy in a chemical reaction can be expressed as

6. Ideal Fuel Cell Capacity
The “ideal” efficiency of a reversible galvanic cell is related to the enthalpy for the cell reaction by
ηid will be 100% if the electrochemical reaction involves no change in the number of gas moles, that is, when ΔS is zero.

7. Fuel Cell Technologies
There are six major types of fuel cells depending on the type of their electrolyte.
Proton exchange membrane (PEM) or Polymer exchange membrane fuel cells (PEMFCs)
Alkaline fuel cells (AFCs)
Phosphoric acid fuel cells (PAFCs)
Molten carbonate fuel cells (MCFCs),
Solid oxide fuel cells (SOFCs)
Direct methanol fuel cells (DMFCs)

8. Classification based on electrolyte usedV
AFC
PEMFC
PAFC
MCFC
SOFC H2
OH -
Fuel
H2O
H +
DMFC
O 2-
CO3 2-
600C kW
600C kW
1900C MW
6500C 2 MW
10000C 1 MW
800C 2 MW O2
CO2
CH3OH
Anode
Electrolyte
Cathode
Oxygen e
Max attained
production

9. Ideal Thermodynamic Potential
The maximum thermodynamic work output obtained from the above reaction is related to the free-energy change of the reaction.
The chemical reactions occur at a constant temperature and pressure.
The above reaction is spontaneous and thermodynamically favoured because the free energy of the products is less than that of the reactants.
The standard free energy change of the fuel cell reaction is indicated by the equation
Where ΔG is the free energy change,
n is the number of moles of electrons involved,
E is the reversible potential, and
F is Faraday’s constant

10. The Nernst Equation
The Nernst Equation enables the determination of cell potential under non-standard conditions.
It relates the measured cell potential to the reaction quotient and allows the accurate determination of equilibrium constants.
General form of Nernst Expression
Ideal cell potential

11. Other Electro-chemical Reactions
C+1/2O2 CO
CO+1/2O2 CO2
C+O2 CO2
Reforming Reactions for Methane : CH4 + H2O 3H2+CO
This is an endothermic reaction

12. Electrode Potential and Current–Voltage Curve

13. Actual Fuel Cell Efficiency
Actual cell potential (Vactual )
= Ideal cell potential (E) – losses or polarization

14. Polarization Distinctiveness
Actual potential of the cell is less than the equilibrium potential due to irreversible losses or polarization.
Losses or polarizations in Actual Performance
Vact
Vohm
Vcon

15. Polarization Distinctiveness
Actual potential of the cell is less than the equilibrium potential due to irreversible losses or polarization.
Losses or polarizations in Actual Performance
V-I Characteristics of the Fuel cell
Activation over potential (Vact)
Flow of ions should overcome the electronic barrier.
Ohmic over potential (Vohm)
Resistance offered by the total cell components to the flow.
Concentration over potential (Vcon)
Gas transport losses, dilution of fuel as the reactions progress.

16. A hydrogen–air fuel cell system