Types of fuel cells

Types of Fuel Cells
By
Duddu Sampurna rao, M.Sc.,
Lecturer in Physics,
KRK Govt. Degree College,
Addanki, Prakasam Dist.

Topics
• Introduction
• Alkaline Fuel Cell
• Polymer Electrolyte Fuel Cell
• Phosphoric Acid Fuel Cell
• Molten Carbonate Fuel Cell
• Solid Oxide Fuel Cell
• Applications Of Fuel Cells

Introduction
• We know that voltaic cells involve oxidation-
reduction reaction.
• A workable cell forces the electrons that
leave the substance being oxidized through
an external conductor before they are
received by the oxidizing agent.
• We also know that all ordinary combustion
reactions are redox reactions.

Introduction(Conti.)
• In a fuel cell, electric energy is obtained
without combustion from oxygen and a gas
that can be oxidized. Hence, a fuel cell
converts the chemical energy of the fuels
directly to electricity.
• The essential process in a fuel cell is
roductsOxidationpyElectricitOxygenFuel 

Alkaline Fuel Cell
• One of the simplest and most successful fuel
cell is alkaline fuel cell.
• It consists essentially of an electrolytic
solution such as 25% KOH solution and two
inert porous electrodes.
• Hydrogen and oxygen gases are bubbled
through the anode and cathode
compartment respectively, where the
following reactions take place:

Alkaline Fuel Cell(Conti.)
• Reaction at Anode:
• Reaction at Cathode:
• Net Reaction:
eHOHH O 222 22


OHeHO O 442 22


OHOH 22 222


• The standard emf of the cell,
• In actual practice, the emf of cell is 0.8 to
1.0V. It may be noted that the only product
discharged by the cell is water.
• Usually, a large number of these cells are
stacked together in series to make a battery,
called fuel cell battery or fuel battery.
VVVEEE reductionoxidation
23.140.083.0
00
0


• It may be pointed have that the electrodes
must meet stringent requirements.
• They must be good conductors,
• Good electron-sources or sinks, and
• Not consumed or deteriorated by the
electrolyte heat or electrode reactions.
Moreover, they must be excellent catalysts
for the reactions that take place on their
surfaces.

• When hydrogen is used as the fuel, the
electrodes are made of either graphite
impregnated with finely divided platinum,
or a 75/25 alloy of palladium and silver or
nickel.
• The secret of successful fuel cells probably
lies in the development of inexpensive
electrodes that are powerful catalysts for
the electrode reactions.

• Electrolytes used for most often are aqueous
or ion-exchange resin saturated with water.
• For low-temperature operating fuel battery
(- 54°C to 72°C), potassium thiocyanate
dissolved in liquid ammonia: is employed.

• Applications: Hydrogen-oxygen fuel cell is used
as auxiliary energy source in space vehicles,
submarines or other military-vehicles.
• The weight of the fuel battery sufficient for 15
days in space is approximately 250 kg. This
may be compared to the several tonnes that
would have been required for an engine-
generator set.
• In case of fuel cells, the product water proved
to be a valuable source of fresh water by the
astronauts.

Proton Exchange Membrane Fuel
Cell
• proton exchange membrane fuel cell (PEMFC)
offers an higher power density than any other
fuel cell system.
• The PEFC can operate on reformed
hydrocarbon fuels, with pretreatment on air.
• The use of a solid polymer electrolyte
eliminates the corrosion and safety concerns
associated with liquid electrolyte fuel cells.
• Its low operating temperature provides instant
start-up and requires no thermal shielding to
protect personnel.

Cell(Conti.)
• Recent advances in performance and design
offer the possibility of lower cost than any
other fuel cell system.
• The PEFC uses as its electrolyte a polymer
membrane, which is an electronic insulator,
but an excellent conductor of hydrogen ions.
• The materials used consist of a fluorocarbon
polymer backbone, similar to Teflon, to
which are attached sulfonic acid groups.

Cell(Conti.)
• The acid molecules are fixed to the polymer
and cannot leak out, but the protons on these
acid groups are free to migrate through the
membrane.
• With the solid polymer electrolyte, electrolyte
loss is not an issue with regard to stack life.
• The electrolyte membrane looks rather like a
thick sheet of food wrap and can be handled
easily and safely.

Cell(Conti.)
• The anode and cathode are prepared by applying a
small amount of platinum black to one surface of a
thin sheet of porous, graphitized paper which has
previously been wet-proofed with Teflon.
• The electrolyte is then sandwiched between the
anode and cathode and the three components are
sealed together under heat and pressure to produce
a single membrane electrode assembly (MEA).
• This assembly, which is the heart of the fuel cell, is
less than a millimeter thick.

Cell(Conti.)
• The anode and cathode are contacted on the back
side by flow field plates made of graphite in which
channels have been formed.
• The ridges between the channels make electrical
contact with the backs of the electrodes and conduct
the current to the external circuit. The channels
supply fuel to the anode and oxidant to the-cathode.
• Reactions: Hydrogen from the fuel gas stream is
consumed at the anode, yielding electrons to the
anode and producing hydrogen ions, which enter the
electrolyte.

Cell(Conti.)
• At the cathode, oxygen combines with
electrons (from the cathode) and hydrogen
ions (from the electrolyte) to produce water.
The water does not dissolve in the electrolyte
and is, instead, rejected from the back of the
cathode info the oxidant gas stream.
• As the PEFC operates at about 80° C, the water
is produced as liquid water and is carried out
of the fuel cell by excess oxidant flow.

Cell(Conti.)
• Net Reaction:
eHH 222


OHeHO 224 22


OHOH 22 222


Cell(Conti.)

Phosphoric Acid Fuel Cells
• Phosphoric acid fuel cells were the first fuel
cells to cross the commercial threshold in the
electric power industry.
• More than 200 of these first generation power
units are now operating in stationary power
applications in the United States and overseas.
• Most are the 200-kilowatt fuel cell. The largest
phosphoric acid fuel cell to be tested is an 12-
megawatt power plant sited in Japan.

Phosphoric Acid Fuel Cells(Conti.)
• As the name implies, these fuel cells use liquid
phosphoric acid as the electrolyte.
• The electrodes are made of carbon paper
coated with a finely-dispersed platinum
catalyst.
• The catalyst strips electrons off the hydrogen
rich fuel at the anode.
• Positively charged hydrogen ions then migrate
through the electrolyte from the anode to the
cathode.

• Electrons generated at the anode travel through an
external circuit, providing direct current electric
power and return to the cathode. There the
electrons, hydrogen ions and oxygen form water,
which is discharged from the cell.
• Phosphoric acid fuel cells operate at around 150 to
200°C above the boiling point of water
• This is one reason why phosphoric acid is preferred,
although it is a less efficient conductor of electricity
than other acidic electrolytes.
• Other acid electrolytes that require water for
conductivity don’t have this capability.

• At a phosphoric acid fuel cell's operating
temperatures, the expelled water can be converted to
steam for space and water heating. In this combined
heat and power application, overall efficiencies can
approach 80 percent. Yet the actual electricity-
generating efficiency is relatively low, only 37 to 42%.
• Also, hydrogen must be extracted from fuels such as
natural gas outside the fuel cell (a process called
“external reforming”). If the hydrocarbon fuel is
gasoline, sulphur must be removed or it will damage
the platinum catalyst.

• Net Reaction:
eHH 222


OHeHO 224 22


OHOH 22 222


• Applications: These fuel cells are being used
in hotels, hospitals, office building and large
vehicles, buses
• Nonetheless, for applications where on-site
high-quality and reliable power is needed,
phosphoric acid fuel cells have been
installed and shown to be reliable suppliers
of premium power

Molten carbonate fuel cells
• Molten carbonate fuel cells are a class of “second
generation” fuel cells, designed to operate at higher
temperatures than phosphoric acid or proton
exchange membrane fuel cells.
• Molten carbonate technology can achieve higher
fuel-to-electricity and overall energy use efficiencies
than lower temperature cells.
• In a molten carbonate fuel cell, the electrolyte is
made up of lithium-potassium carbonate salts
heated to about .
• At these temperatures, the salts melt into a molten
state that can conduct charged particles, called ions,
between two porous electrodes.
650


Molten carbonate fuel cells(Conti.)
• Molten carbonate fuel cells eliminate the external
fuel processors that other fuel cells need to extract
hydrogen from the fuel.
• When natural gas is the fuel, methane (the main
ingredient of natural gas) and steam are converted
into a hydrogen-rich gas inside the fuel cell stack (a
process called “internal reforming”).
• At the anode, hydrogen reacts with the carbonate
ions ( ) to produce water, Carbon dioxide, and
electrons. The electrons travel through an external
circuit creating electricity and return to the cathode.
CO3
2

• There, oxygen from the air and carbon dioxide
recycled from the anode react with the
electrons to form ions that replenish the
electrolyte and transfer current through the
fuel cell, completing the circuit.
• Molten carbonate fuel cells can reach fuel-to-
electricity efficiencies approaching 60%,
considerably higher than the 37-42%
efficiencies of a phosphoric acid fuel cell plant.
• When the waste heat is captured and used,
overall fuel efficiencies can be as high as 85%.
CO3
2

• Reaction at Anode
• Reaction at Cathode
• Net Reaction
eCOHCOH O 222
2
2 3


COeCOO 242 3
2
22


OHOH 22 222


Solid oxide fuel cells
• Solid oxide fuel cells (SEFC) differ in many respects
from other fuel cell technologies.
• First, they are composed of all solid state materials -
the anode, cathode and electrolyte are all made from
ceramic substances.
• Second, because of the all-ceramic make-up, the cells
can operate at temperatures as high as 1,000°C
significantly hotter than any other major category of
fuel cell.
• This produces exhaust gases at temperatures ideal
for cogeneration applications for use in combined-
cycle electric power plants.

Solid oxide fuel cells(Conti.)
• Third, the cells can be configured either as
rolled tubes or as flat plates and manufactured
using many of the techniques now employed
today by the electronics industry.
• Although, a variety of oxide combinations have
been used for solid oxide electrolytes the most
common to date has been a mixture of
zirconium oxide and calcium oxide.
• Formed as a crystal lattice, the hard ceramic
electrolyte is coated on both sides with
specialized porous electrode materials.

• At the high operating temperatures, oxygen ions are
formed at the “air electrode” (the cathode).
• When a fuel gas containing hydrogen is passes over
the “fuel electrode” (the anode), the oxygen ions
migrate through the crystal lattice to oxidize the
fuel.
• Electrons generated at the anode move out through
an external circuit, creating electricity.
• Reforming natural gas or water gas or other
hydro carbon fuels to extract the necessary
hydrogen can be accomplished within the fuel cell,
eliminating the need for an external reformer.
 COH 2

• In such a fuel, reformate gas is used as
a fuel and oxygen as the oxidant.
• Reactions at Anode
• Reactions at Cathode
• Overall Reaction
 COH 2
OeOyx yxyx
21
2
)(2)(4)(


eCOxHOxH yxyOyxyCO
1
22
2
2
)(2)(


COHOyxxH yOxyCO 2222
222 )(2  

• The reaction rate at the operating temperature
(about 1,000° C) is quite high, so no noble metal
catalyst is needed. Moreover, carbon monoxide does
not poison the electrodes.
• The fuel-to-electricity efficiencies of solid oxide fuel
cells are expected to be around 50 percent. If the hot
exhaust of the cells is used in a hybrid combination
with gas turbines, the electrical generating efficiency
is likely to approach 60%.
• In applications designed to capture and utilize the
system’s waste heat, overall fuel use efficiencies
could top 80 - 85%.

Applications
• One characteristic feature of fuel cell systems is
that their efficiency is nearly unaffected by size.
This means that small, relatively high efficient
power plants can be developed, thus avoiding
the higher cost exposure associated with large
plant development.
• Distributed generation involves small, modular
power systems that are sited at or near their
point of use. The typical system is less than 30
MW, used for generation or storage, and
extremely clean.

Applications(Conti.)
• Fuel cells are also used in light-duty and heavy-
duty vehicle propulsion. Further, such vehicles
offer the advantages of electric drive and low
maintenance because of few moving parts.
• In space applications the fuel cells are used
because they consume pure reactant gases.
Ballard Power Systems has produced an 80 kW
PEFC fuel cell unit for submarine use
(methanol fueled) and for portable power
systems.

Applications(Conti.)
• In addition to high-profile fuel cell applications
such as automotive propulsion and distributed
power generation, the use of fuel cells as
auxiliary power units (APUs) for vehicles has
received considerable attention.
• Because of the modular nature of fuel cells,
they are attractive for use in small portable
units, ranging in size from 5 W or smaller to
100 W power levels.

Types of fuel cells

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Types of fuel cells

Similar to Types of fuel cells (20)

Recently uploaded

Recently uploaded (20)

Types of fuel cells