3. A fuel cell is a device that converts the chemical
energy from a fuel into electricity through a
chemical reaction with oxygen or another oxidizing
agent.
Hydrogen is the most common fuel, but
hydrocarbons such as natural gas and alcohols
like methanol are sometimes used.
Fuel cells are different from batteries in that they
require a constant source of fuel and oxygen to
run, but they can produce electricity continually
for as long as these inputs are supplied.
4. There are many types of fuel cells, but they all consist of
an anode (negative side), a cathode (positive side) and
an electrolyte that allows charges to move between the two sides of
the fuel cell.
Electrons are drawn from the anode to the cathode through an
external circuit, producing direct current electricity.
As the main difference among fuel cell types is the electrolyte, fuel
cells are classified by the type of electrolyte they use.
Fuel cells come in a variety of sizes. Individual fuel cells produce
relatively small electrical potentials, about 0.7 volts, so cells are
"stacked", or placed in series, to increase the voltage and meet an
application's requirements.
In addition to electricity, fuel cells produce water, heat and,
depending on the fuel source, very small amounts of nitrogen
dioxide and other emissions.
The energy efficiency of a fuel cell is generally between 40–60%, or
up to 85% efficient if waste heat is captured for use.
5.
6. Anode:The electrode at which oxidation (a loss of
electrons) takes place. For fuel cells and other galvanic
cells, the anode is the negative terminal; for electrolytic
cells (where electrolysis occurs), the anode is the positive
terminal.
Aqueous solution: a: of, relating to, or resembling
water b : made from, with, or by water.
Catalyst: A chemical substance that increases the rate of a
reaction without being consumed;
In a fuel cell, the catalyst facilitates the reaction of oxygen
and hydrogen. It is usually made of platinum powder very
thinly coated onto carbon paper or cloth. The catalyst is
rough and porous so the maximum surface area of the
platinum can be exposed to the hydrogen or oxygen. The
platinum-coated side of the catalyst faces the membrane
7. Cathode: The electrode at which reduction (a gain of electrons) occurs. For fuel cells and
other galvanic cells, the cathode is the positive terminal; for electrolytic cells (where
electrolysis occurs), the cathode is the negative terminal.[34]
Electrolyte: A substance that conducts charged ions from one electrode to the other in a
fuel cell, battery, or electrolyzer.[34]
Fuel Cell Stack: Individual fuel cells connected in a series. Fuel cells are stacked to increase
voltage.[34]
Matrix: something within or from which something else originates, develops, or takes
form.[36]
Membrane: The separating layer in a fuel cell that acts as electrolyte (an ion-exchanger) as
well as a barrier film separating the gases in the anode and cathode compartments of the
fuel cell.[34]
8. Molten Carbonate Fuel Cell (MCFC): A type of fuel cell that contains
a molten carbonate electrolyte. Carbonate ions (CO3
2−) are
transported from the cathode to the anode. Operating
temperatures are typically near 650 °C.
Phosphoric Acid Fuel Cell (PAFC): A type of fuel cell in which the
electrolyte consists of concentrated phosphoric acid (H3PO4).
Protons (H+) are transported from the anode to the cathode. The
operating temperature range is generally 160–220 °C
Polymer Electrolyte Membrane (PEM): A fuel cell incorporating a
solid polymer membrane used as its electrolyte. Protons (H+) are
transported from the anode to the cathode. The operating
temperature range is generally 60–100 °C
Solid Oxide Fuel Cell (SOFC): A type of fuel cell in which the
electrolyte is a solid, nonporous metal oxide, typically zirconium
oxide (ZrO2) treated with Y2O3, and O2− is transported from the
cathode to the anode. Any CO in the reformate gas is oxidized to
CO2 at the anode. Temperatures of operation are typically 800–
9.
10. Thermionic power plant
Thermionic power generator (TPG) is a device that
converts heat energy into electrical energy.
Thermionic emission is the basis for the working of
this system.
The thermionic emission is the emission of
electrons from metal surface due to heat.
11. The idea of electrons leaving the surface is shown in figure
12. PH 0101 Unit-5 Lecture-4 12
Thermionic effect
Thermionic effect is the ejection of electron from the heated metal
surface and forms as electron cloud at the cathode.
The number of electron emitted from the metal surface depends on
temperature and work function.
13. 13
Principle, Construction and working of
Thermionic power generator (TPG)
Principle
Thermionic power generator is based on the principles of
Thermionic effect that the electrons are emitted from a hot
metal surface and responsible for the production of
electricity.
Construction
• The TPG consist of tungsten metal, which is negatively
charged cathode acts as an emitter.
• There is positively charged electrode is called
collector. It is collecting the ejected electrons. The
emitter and collector are kept in a vacuum quartz tube.
14. 14
Emitter Quartz tube Collector
Thermal energy
Electrical energy
VL
RL
Thermionic Power generator
15. 15
Working
• The electrons within a metal can be treated as
"electron gas" in which individual outer most
electrons are capable of moving freely under the
influence of a field.
• This movement of electrons is responsible for
the function of electric circuits.
• At the surface of a metal, a potential barrier
exists which prevents the electrons from escaping
unless certain conditions are met. This concept can
be explained as follows.
16. 16
Surface of cathode
Electrons escaped from cathode
Electrons Promoted
from EF
EF
A
B
C
• It is known from the free electron theory, at 0K, all the energy
levels up to EF (fermi energy) are completely filled and all the
energy level above the EF are completely empty.
17. 17
• The energy level from the surface of metallic cathode to
the level of EF (BC in figure) is the potential energy
barrier called work function (.).
• If any electron wants to escape from the surface of the
metallic cathode, they should cross this potential barrier.
• At 0K, all the electrons are bound within fermi energy level
and cannot escape from the surface of cathode (emitter).
• When the thermal energy is supplied on the emitter side,
some of the electrons are promoted to above the fermi level.
18. PH 0101 Unit-5 Lecture-4 18
• These activated electrons can cross the potential energy
barrier and escape from the surface of cathode and
responsible for the current production.
• As long as the temperature increased, the number of electrons
escapes from the surface of emitter increases. Collector collects
the emitted electrons and there is an external circuit through
which the current flows
• The thermionic emission current density is determined by the
'work function' of the material, which is basically the magnitude
of the potential energy barrier.
• Good emitters should have low work functions.
19. 19
• It is generally accepted that the thoriated tungsten is the
best cathode metal because of its lower value of work function
• The metal can be heated in two different ways.
• One is direct heating and second is the indirect heating.
• In the direct heating where the filament itself is the cathode
and the indirect heating where the cathode is heated by a
separate filament.
• Pure tungsten is used as main metal in the case of direct
heating method
• Nickel (or) Nickel alloys are used as main metal in the case of
indirect heating.
20. 20
5. Advantage, disadvantage and applications
Advantages
• Higher efficiency and high power density
• Compact to use
Disadvantages
• There is a possibility of vaporization of emitter surface
• Thermal breaking is possible during operation
• The sealing is often gets failure
21. 21
Applications
• They are used in space power application for spacecraft
•They are used to power submarines and boats.
• They used in water pump for irrigation,
• They used in power plant for industry and domestic purpose
