BATTERY TECHNOLOGY
CELL: A cell designates a single unit. The conversion of chemical energy into electrical energy
is a function of cells or batteries.
BATTERY: A Battery is an electrochemical cell or often several electrochemical cells connected
in series that can be used as a source of direct electric current at a constant voltage.
Uses: Batteries are used in calculators, watches and pacemakers for heart hearing aids,
computers, car engines, stand by power supplies, emergency lightning in hospitals, electroplating
industrial tractions and military and space applications.
Batteries have revolutionized the telecommunication system and are ushering a new era of
transportation with the possible replacement of petrol driven automobiles by the electrical
powered ones. In modern days portability of electronic equipments in the form of handsets has
been made possible by batteries.
Note: The size of the batteries ranges from a fraction of a cubic centimeter to several cubic
decimeters.
Basic concept; the spontaneous redox reaction, which forms the basis of batteries.
Components of Battery: The cell consists of three major components.
1.The anode: The anode selected with the following properties in mind; efficiency as a reducing
agent, high coulombic output (Ah/g) good conductivity , stability ease of fabrication and low
cost .
2.The Cathode: The cathode must be an efficient oxidizing agent ,be stable when in contact with
the electrolyte ,and have a useful working voltage.
3.Electrolyte : The electrolyte must have good ionic conductivity but not be electrically
conductive. AS this would cause internal short circuiting. Electrolyte should be non reactive with
the electrode materials.
4.Containers: Must be resistance to corrosion from both inside and outside the cell.
OPERATION OF A BATTERY DURING DISCHARGE AND CHARGE.
DURING DISCHARGING: The Battery acts as voltaic cell i.e. oxidation takes place at the
negative electrode (anode) and reduction takes place at the positive electrode (cathode).
During charging the Battery acts as an electrolytic cell. The current flow is reversed and
oxidation takes place at the positive electrode (anode) and reduction takes place at the negative
electrode (cathode).
COMMERCIAL CELLS: A useful commercial cell should meet the following basic
requirements.
1.portability.
2.Should be compact and lightweight.
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3.Should provide economically priced, continuous electric supply.
4.Should be capable of recharging.
5.should have long shelf life.
BATTERY CHARACTERSTICS: The performance characteristics of commercial batteries are
given below.
VOLTAGE: According to Nernst equation at 298k
ECELL=Eocell - 2.303RT/ nF log Q
Where Q= [PRODUCT]/[REACTANT]
The emf of the battery depends on the following factors EOCELL, temperature and the
concentration ratio given by Q.
1.The e.m.f of a battery largely depends on E0CELL i.e E0R –E0L the difference in the standard
electrode potentials of the two electrodes. As E0CELL increase the e.m.f also increases for a given
value of Q at constant temperature. In other words, the magnitude of e.m.f is large if the
electrodes used in the cell are far apart in the e.m.f series.
2.Increase in the value of Q marginally decrease the e.m.f . This is because Q decrease
when[products] decreases i.e when current is drawn from the cell.
3.The voltage of the battery depends on the free energy change in the overall cell reaction.
However the Nernst equation is applicable only to reversible systems. A condition for
thermodynamic reversibility is that the system should always be in equilibrium with its
surroundings. Therefore chemical cell behaves reversibly only when the current passing is small.
The system may not behave as a reversible cell if large current are passed since the cell will not
be in a state of equilibrium. The cell reaction selected should have a high negative free energy
change to get a high cell potential. The electrode should be fast and the cell should be designed
with a low internal resistance using high conductivity electrolyte to drive maximum voltage from
the cell.
2.CAPACITY: The capacity is the charge that may be obtained from the battery.It is expressed
in ampere hours and it depends on the size of the battery.
According to faradays law
Capacity C=WnF/M
Where w=weight of the active material.
M=molecular weight of the active material.
It is measured by finding, for a fixed current discharge, the time t taken for the battery to reach a
minimum voltage ,when the cell is said to be dead. A plot of voltage against time at a fixed
current discharge is shown in the fig. The variation of the battery voltage during discharge is
shown by the flatness of the curve. The length of the flat portion of the curve is a measure of the
capacity of the battery. Flatter and longer the curve better is the capacity.
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3.CURRENT: Current is the rate at which the battery is discharging. For an efficient working of
the cell there must be a large quantity of electro active species that help rapid electron transfer
reaction.
4.Power Density: It is the rate at which the storer is able to release the energy per unit weight.
Power density decreases during discharge.
5.Energy efficiency: Energy efficiency for the storage batteries is given by
energy
released
% of energy efficiency=
on
discharge
x 100
energy required for charge
Batteries should have high energy efficiency.
6.Cycle life: is the number of charge and discharge cycles that are possible before failure occurs.
Some of the factors which cause the failure are 1.for a secondary battery it is essential to
charge and discharge cycle to reform the active material in a suitable state for further discharge.
The cycle life depends upon how effectively this is achieved.2.Also depends upon the depth of
each discharge attempts to discharge totally damaging to the electrodes.
The most common form includes corrosion of containers at the contact points. Shedding of the
active materials from the plates.
7.Energy density: This refers to the energy that may be extracted from a given weight of a
substance or device per unit weight.
26.8nv
Energy density=K
watt hours/kg
M
8.Electricity storage density: This is a measure of the total current which can be withdrawn
from the unit weight of a substance when it undergoes an electrochemical reaction.
In other words it is the capacity per unit weight i.e. electricity storage density is the amount of
electricity per unit weight which the accumulator can hold. In addition to the above
requirements, the commercial batteries should have long shelf lives, tolerance to different service
conditions such as temperature, vibration and shock.
CLASSIFICATION OF BATTERIES.
The batteries are classified as
1.Primary battery or primary cells: A primary cell is the one in which electrical energy can be
obtained at the expense of chemical energy only as long as the active materials are still present .
i.e cells in which the cell reaction is not completely reversible. These are not rechargeable
and once discharged have no further electrical use.
Egs; dry cell or Zn-MnO2 Cell
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2.secondary battery: The cell reaction occurring in secondary cells is reversible and these are
rechargeable. They are also referred to as storage cells.
Egs. Lead storage cell and nickel cadmium cell.
3.Reserve Batteries: In these batteries, a key component is separated from the rest of the battery
prior to activation. Usually the electrolyte is the component that is isolated. In this condition,
chemical deterioration or self- discharge is essentially eliminated and the battery is capable of
long- term storage.
Egs: Magnesium water activated batteries, zinc –silver oxide batteries etc.
NOTE: the type of activating medium or mechanism that is involved in the activation can
classify the reserve batteries.
1.Water activated batteries: activated by fresh or seawater.
2.Electrolyte activated batteries: activation by the complete electrolyte or with the electrolyte
solvent.
3.Gas activated batteries: activation by introducing a gas into the cell. The gas can be either the
active cathode material or part of the electrolyte.
4.Heat activated batteries: A solid salt electrolyte is heated to the molten condition and becomes
ionically conductive, this activating the cell.
NOTE: In electrolyte cell cathode is negative and anode is positive.
In galvanic cell cathode is positive and anode is negative.
CLASSICAL BATTERIES:
PRIMARY CELL:
DRY CELL OR LACLANCHE CELL OR Zn-MnO2:
-
Zn/ZnCl2, NH4Cl /MnO2, C+
The cell consists of a zinc anode shaped as a container for the electrolyte and a carbon cathode
surrounded by MnO2 and a paste of ammonium chloride and zinc chloride as an electrolyte.
Manganese dioxide acts as a
Cathodic depolarizer and facilitates the H+ ion discharge reaction by removing the adsorbed
hydrogen atoms.
Carbon is added to increase the conductivity of manganese dioxide. Ammonium chloride is
added to zinc chloride to ensure high rate performance.
The primary reaction at the anode is
Zn
Zn2+ +2eThe overall cathodic reaction is
MnO2+ H2O+2eMn2O3 +2OHThe net cell reaction is
2MnO2+H2O+Zn
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Mn2O3 +Zn2+ +2OH-
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Some of the secondary reactions are
2NH4Cl +2OHZn2+ +2NH3 +2Cl-
2NH3 +2Cl-+2H2O
[Zn(NH3)2]Cl2 OR ZnCl2 .2NH3
The voltage of the cell, which is due to primary reactions is about 1.5V.The secondary reactions
consumes the Zn2+ and OH- ions and once the cell has discharged it cannot be charged again.
CELL PERFORMANCE: 1.This cell gives almost constant voltage for small discharge currents.
2.about 80% of the charge stored can be drawn.
3.Though the cell has a long shelf life, natural deterioration sets in.
4.Low temperature will increase the shelf life but the performance efficiency decreases.
5.for higher capacity and longer service life a high % of depolarizer is required, where as for
delivering large current in a short time as in photoflash, a high % of carbon is desirable.
DISADVANTAGES: 1.When current is drawn rapidly from it, products build up on the
electrodes thereby causing drop in voltage.
2. Since the electrolytic medium is acidic, so zinc metal dissolves slowly, thereby the cell
run down slowly, even it is not in use
3. These batteries are not chargeable.
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4. Not intended for heavy duty.
Uses: dry cell finds applications in flashlights, transistors, radios, calculators, tape recorders and
similar electronic devices where small amount of current is required.
SECONDARY BATTERIES:
LEAD-ACID BATTERY:
Pb/PbSO4, H2SO4// PbSO4/PbO2/Pb
(s)
(s)
(s)
It consists of a spongy lead anode and a grid of lead antimony alloy coated with lead dioxide, as
cathode. Both electrodes are dipped in H2SO4 having density 1.25-1.30g/cm3 which is the
electrolyte. Additives such as graphite powder, lignin sulphonate and barium sulphate are added.
Graphite powder increases the conductivity and barium sulphate prevents the reduction in
surface area of the lead.
The electrode reactions that occur during the discharge of the cell, i.e when current is drawn
from the cell are
Anode:
Pb
Pb2+ + 2e-
Pb2+ + SO42Pb + SO42
PbSO4
PbSO4 + 2e-
Cathode:
PbO2 +SO42- +4H+ +2e-
PbSO4 + 2H20
The net cell reaction is
Pb + PbO2 + 2H2SO4
2 PbSO4 + 2H20
The lead sulphate formed gets precipitated on the cathode and in the solution.
CHARGING: When anode and cathode becomes covered with lead sulphate the cell ceases to
function as a voltaic cell. To re charge a lead storage cell the reactions taking place during
discharge are reversed by passing an external e.m.f greater than 2v from a generator. The cell
acts as an electrolytic cell.
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The following reactions takes place are
Reaction at the negative terminal [cathode]
PbSO4 +2e-
Pb +SO42-
Reaction at the positive terminal [anode]
PbSO4 +2H2O
PbO2 +SO42- +4H+ +2e-
Hence the net reaction during charging is
2PbSO4 + 2H20
Pb(s)+ PbO2 (s)+2H2SO4(aq)
The net reaction during charging and discharging can be represented as follows.
Discharging
Pb(s) + PbO2(S) + 2H2SO4
2PbSO4(S) + 2H2O(l)
Charging
During the discharge process the consumption of sulphuric acid is replaced by an equivalent
quantity of water and so as the cell produces electric current the sulpuric acid concentration
decreases. However on charging the reverse reaction takes place. Sulphuric acid being generated
and water consumed. Hence the original acid strength is restored.
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CELL PERFORMANCE: These batteries deteriorate during the idle storage due to shedding of
active materials like lead dioxide and lead sulphate. The corrosion of the grid also occur. This
deterioration can be minimized by frequently charging the battery. The voltage of the cell
depends on the concentration of sulphuric acid. The lead sulphate cloges the pores in the plates
and decrease the penetration of the acid. Overcharging leads to non adhesion and shedding of
active materials. Similarly over discharge leads to the formation of hard crystalline lead sulphate,
which is non conducting and clogs pores. At low temperature the rates of electrochemical
reactions decrease and the capacity of the battery to deliver current also decreases.
APPLICATIONS: The lead storage batteries are extensively used in automobiles to start the
engine. They are also used for supplying current for electrical vehicles, gas engine ignition, in
telephone exchanges, railway trains, in laboratories, hospitals, broadcasting stations, UPS, power
stations etc.
ALKALINE STORAGE BATTERY:
NICKLE CADMIUM BATTERY:
It can be represented as
Cd/CdO/KOH(6M)//Ni(OH)3/Ni(OH)2/Ni
Nickle cadmium cell consists of a cell cup made up of steel coated with nickel acts as cathode
and a cell cap in contact with action anodic materials acts as the anode. The active materials of
the anode are spongy Cd with 78% Cd(OH)2 , 18% Fe, 1% Ni and 1% graphite which are pressed
into a tablet form and wrapped in nickel wire gauge. The cathode contains a mixture of 80%
Ni(OH)2 and Ni(OH)3 , 2% Co(OH)2, 18% graphite and traces of barium compound to increase
the efficiency of active materials.
The cell reactions are
At anode: Cd(s) + 2OH-(aq)
cathode: 2Ni(OH)3 +2e-
Cd(OH)2(s) + 2e2Ni(OH)2 + 2OH(s)
The overall reaction is
Cd(s) +2Ni(OH)3
Cd(OH)2 +2Ni(OH)2
(s)
(s)
The reaction can be readily reversed because the reaction products Ni(OH) 2 and Cd(OH)2 adhere
to the electrode surfaces.
Ni-Cd battery is a portable, rechargeable cell and its cell voltage is fairly constant (1.4v). It can
be left for long periods of time without any appreciable deterioration, since no gases are
produced during discharging or charging.
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USES: It is used in electronic calculators, electronic flash units, cordless electronic shavers,
transistors and other battery powered small tools. Phones, alarms systems, transmitters, receivers,
computers, emergency lighting, hearing aids, telemeter etc.
MODERN BATTERIES:
Zinc air battery:
It is a metal –air battery, which use oxygen directly from the atmosphere to produce
electrochemical energy. The air diffuses into the cell, as it is needed. The air cathode acts only as
a reaction site and is not consumed. The zinc air cell consists of an anode, made up of loose
granulated powder of zinc mixed with electrolyte [KOH] and a gelling agent to immobilize the
composite and to ensure adequate electrolyte contact with zinc granules. The structure includes
the separators, catalyst layer, metallic mesh diffusion membrane, air distribution layer. The
catalyst layer contains carbon blended with oxides of manganese to form a conducting medium.
The outer metal acts as the cathode of the battery and a plastic gasket insulates the anode active
materials and the cathode. An air access hole on the positive terminal of a zinc air cell provides a
path for oxygen to enter the cell and diffuse to the cathode catalyst site. A schematic
representation of a typical zinc air cell is given in fig.
Electrode reaction:
At the anode: Zn
Zn2+ +2e-
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Zn2+ +2OHZn(OH)2
Zn(OH)2
ZnO +H2O
Overall reaction: Zn +2OH-
ZnO +H2O +2e-
At the cathode: ½ O2+ H2O + 2e-
cell reaction : Zn + ½ O2
2OH-
ZnO
The cell produces an open circuit potential of 1.4v.
ADVANTAGES: 1.High energy density.
2.Flat discharge voltage.
3.Long shelf life.
4.No ecological problems.
5.Low cost.
6.Capacity independent of load and temperature when within operating range.
DISADVANTAGES: 1.Not independent of environmental conditions.
2.Drying out limits shelf life once opened to air.
3.Short activated life.
4.Flooding limits power output.
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APLLICATIONS: 1. As a power source for hearing aids, electronic pagers, telemetry (voice
transmitters) portable battery charges, used in medical devices. It is served as a portable power
source for the wireless crew communicator system used by astronauts aboard the space shuttle.
Nickle metal hydride batteries: The rechargeable sealed nickel metal hydride battery is similar to
those of the nickel –cadmium battery. The principal difference is that the nickel –metal hydride
battery uses hydrogen, adsorbed in a metal alloy for the active negative material in place the
cadmium used in the nickel-cadmium battery.
The active material at the cathode of the nickel –metal hydride battery, is nickel oxyhydroxide
and at the anode is hydrogen in the form of a metal hydride. This metal alloy is capable of
undergoing a reversible hydrogen adsorbing, desorbing reaction as the battery is charged and
discharged.
An aqueous solution of potassium hydroxide is the major component of the electrolyte.
The cathode in the cylindrical nickel –metal hydride cell is a highly porous sintered, or felt
nickel substrate into which the nickel compounds are impregnated or pasted and converted into
the active material by electrode deposition. The anode is highly porous structure using a
perforated nickel foil or grid onto which the plastic bonded active hydrogen storage alloy is
coated. The electrodes are separated with a synthetic non -woven material, which serves as an
insulator between the two electrodes and as a medium for absorbing the electrolyte.
Electrode reactions that occurs during discharge are,
At the anode: MH +OHM +H2O +eAt the cathode: NiOOH + H2O +e-
Ni(OH)2 + OH-
The overall reaction on discharge is,
MH +NiOOH
M +Ni(OH)2
The process is reversed during charge.
The open circuit potential of the cell ranges from 1.25-1.35V.
ADVANTAGES. 1. Higher capacity than nickel cadmium batteries.
2.No maintenance required.
3. Rapid recharge capabilities.
5.
Cadmium free, minimal environmental problems.
6.
5. Long cycle life.
7.
6. Long shelf life.
DISADVANTAGES:
1.High rate performance not as good as with nickel cadmium batteries.
2.Moderate memory effect.
3.Poor charge retention.
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APPLICATIONS: Used in computers, cellular phones and other portable and consumer
electronic applications where the higher specific energy is desired. They also used in electronic
vehicles Etc.
LITHIUM CELLS: Lithium metal is attractive as a battery anode material because of its
lightweight, high voltage, high electrical equivalence and good conductivity. Because of these
outstanding features, the use of lithium has predominated in the development of high
performance primary and secondary batteries.
ADVANTAGES OF LITHIUM CELLS:
1. High voltage.
2. High energy density.
3. Operation over a wide range of temperature. Many of the lithium cells will perform over
a temperature range from about 70-40oC.
4. Good power density.
5. Flat discharge characteristics.
6. Superior shelf life.
CLASSIFICATIONS OF LITHIUM PRIMARY CELLS:
Lithium primary cells can be classified into several categories, based on the type of electrolyte or
solvent and cathode material used.
1.Soluble cathode cells: These cells use liquid or gaseous cathode material, such as sulphur
dioxide or thionyl chloride, that dissolves in the electrolyte or are the electrolyte solvent.
2. Solid cathode cells: Uses solid material for the cathode such as MnO2, CuS and V2O5 ETC.
3. Solid electrolyte cells: This type of cells use electrolytes in the solid form itself as the
cathode.PbI2, PbS etc
LITHIUM MANGANESE DIOXIDE CELL: Fig. shows the illustration of a typical coin cell.
The manganese dioxide pellet faces the lithium anode disk and is separated by a non- woven
polypropylene separator impregnated with the electrolyte. The cell is seald with the can serving
as the positive terminal and the cap as the negative terminal. The Li/MnO2 cell uses lithium for
the anode, an electrolyte containing lithium salts in a mixed organic solvent (propylene carbonate
and 1,2-dimethoxyethane) and a specially prepared heat- treated form of MnO2 for the active
cathode material.
The cell reactions for the system are
Anode: Li
Li+ + e-
Cathode: MnO2 + Li+ + e-
LiMnO2
The overall reaction is Li + MnO2
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LiMnO2
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Manganese dioxide is reduced from the tetravalent to the trivalent state by lithium. LiMnO2
signifies that the Li+ ion enters into the MnO2 crystal lattice. The theoretical voltage of the total
cell reaction is about 3.5V.
APPLICATIONS: Used as long -term memory backup, safety and security devices, cameras,
lighting equipment and many consumer electronic devices.
FUELS CELLS:
DEFINITION: A fuel cell is an electrochemical device, which can continuously convert the
chemical energy of a reducing agent and an oxidant fuel stored externally by a process involving
an essentially invariant electrode electrolyte system.
A fuel cell consists of two electrodes and an electrolyte. However, the fuel and the oxidizing
agents are continuously and separately supplied to the two electrodes of the cell, at which they
undergo reactions. These cells are capable of supplying current as long as they are supplied with
the reactants.
A fuel cell may be represented as
Fuel/electrode/electrolyte/electrode/oxidant
At the anode fuel undergoes oxidation and at the cathode oxidant undergoes reduction.
ADVANTAGES OF FUEL CELL SYSTEM:
1.Savings in fossil fuels due to the high efficiency of electrochemical energy conversion.
2.Low pollution level, no noxious exhaust gases formed.
3.Production of water of drinking quality in hydrogen-oxygen system.
4.only a small number of moving parts.
5.Low noise level.
6.Low maintenance, exchangeable parts.
7.No need of charging.
DISADVANTAGES:
1.High initial cost of the system (catalyst, membranes etc).
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2.Large weight and volume of gas fuel storage systems.
3.High price of clean hydrogen.
4.Lifetimes of the cells are estimated but not accurately known (40,000hrs for acidic and
10,000hrs for alkaline cells).
5.Present lack of infrastructure to distribute hydrogen.
Classification of fuel cells:
They are classified as
1.Indirect fuel cells: use organic fuels or biochemical substance, which is decomposed by using
enzymes to a simple fuel like hydrogen.
Egs. Reformer fuel cells and biochemical fuel cells.
2.Direct fuel cells: The products of the reaction are discarded.
Direct fuel cells are classified as
1.Low temperature fuel cell: egs H2-O2, N2 compound –O2 and Metal-oxygen fuel cells etc
[<100oc]
2.Intermediate temperature fuel cell: egs. Organic compound - O2 and H2-O2 fuel cells etc [100500oc]
3.High temperature fuel cell: egs. H2-O2, CO-O2 fuel cells etc [500-1000oc]
4.Very high temperature fuel cell: egs, H2-O2, CO-O2 fuel cells etc [> 1000oc]
Classification based on the electrolyte used:
TYPE
OPERATION
ELECTROLYTE USED
1.ALKALINE FUEL CELL
[AFC]
Aq. KOH solution
2.POLYMER
ELECTROLYTE
MEMBRANE FUEL CELL
[PEMFC]
3.MOLTEN
CARBONATE
FUEL CELL [MCFC]
4.PHOSPHORIC ACID
FUEL CELL [PAFC]
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polymer membrane
TEMPERATURE
OF
60-120oc
60-120oc
mixture of molten carbonates
60-650oc
concentrated phosphoric acid
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180-220oc
14
5.SOLID OXIDE FUEL CELL
[SOFC]
ceramic solid ZrO2 (Y2O3)
900-1000oc
ALKALINE FUEL CELL [AFC]: Alkaline fuel cell uses aqueous solution of KOH or NaOH as
electrolyte. Pure oxygen or air is used as the oxidant. The fuels of the cells are hydrogen or any
hydrocarbons. The cell and electrodes can be built from low cost carbon and plastic.
Reaction: At anode H2 + 2OH-
2H2O + 2e-
At cathode ½ O2 + H2O + 2e-
H2 + ½ O2 + H2O
2OH-
2H2O
Disadvantage: Carbonation of electrolyte by CO2, poisoning of catalyst by CO2.
Advantages: AFC offer the best prospect for usage of non-noble metals catalysts such as raney
Ni and raney Ag etc.
Polymer electrolyte membrane fuel cell [PEMFC]: The cell employs a thin cation- exchange
membrane [polystyrene sulphonic acid mixed with triflouroethylene resin as plasticizer] as
electrolyte. A Ti screen coated with a pt catalyst covers each side of the rectangular membrane.
The electrodes used in this type of cells are typical gas diffusion electrodes, made of porous
carbon and impregnated with platinum catalyst. Hydrogen gas is supplied at anode and oxygen at
cathode.
Reaction takes place are
At anode 2H2
4H+ + 4eAt cathode O2 + 4H+ + 4e-
2H2 + O2
2H2O
2H20
A storage system built of these cells having an average power of 900W and a maximum power
of 2KW was used in the first two mars Gemini spacecraft.
DISADVANTAGES:
1.The water balance is considered to be the main problem.
2.Dehydration of the membrane is a problem therefore complicated cooling systems have to be
used in addition to humidification of feed gases.
3.Reactant gases should be free from CO, as it is a catalytic poison.
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ADVANTAGES:
1.Long life.
2.Simple to fabricate the cell.
3.No free corrosive liquid in the cell.
4.Able to withstand large pressure.
MOLTEN CARBONATE FUEL CELL [MCFC]:
Here instead of pure hydrogen and oxygen, CO or natural gas largely methane (as source of
hydrogen) and air (as source of oxygen) are used. Mixed with steam methane is reformed into
hydrogen and carbon monoxide with a Ni catalyst heated to 600-750oc. The electrolyte in the
form of a paste with MgO is a molten mixture of Li2CO3, Na2CO3 and K2CO3.Mg is added to
increase the conductivity and hold the mixture in the form of paste. A thin layer of porous Ni is
the anode while a finely divided Ag constitutes the cathode.
Reaction:
At anode: CO + CO32-
2CO2 + 2e-
H2 + CO32-
CO2 + H2O +2e-
At cathode: 2CO2 + O2 + 4e-
2 CO32-
Single cells are able to sustain current density of 100mA Cm2 at 0.6-0.7v for continuous periods
of more than one year.
ADVANTAGE: Reduction of polarization of oxygen cathode due to high temperature. However
electrode lifetime is short due to high temperature corrosion.
PHOSPHORIC ACID FUEL CELL [PAFC]:
This is based on the immobilized phosphoric acid electrolyte. The cell operated to 180-220oc and
uses air for the source of oxygen. Hydrogen is used as a fuel. The most desirable fuel system is
one, which operates on hydrocarbons directly because of the low cost of the hydrocarbon fuel.
The most important property required for an electrolyte in a cell is that it must be able to
withstand temperature above 100oc. It must posses good electronic conductivity. It must support
complete electrochemical oxidation of carbon containing fuel cell and it must not contain anion,
which must adsorbed on the electrode and block the reaction site. The electrolyte, which satisfies
several of these criteria, is concentrated phosphoric acid. Each cell consists of two Teflon bonded
gas diffusion electrodes on a porous conducting support. Pt is deposited on both the electrodes
which acts as catalyst.
ADVANTAGES:
1.Excellent thermal, chemical and electrochemical stability.
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2.Low volatility of phosphoric acid above 150oc.
3.Simple construction.
DISADVANTAGE: Only Pt catalyst can be used.
SOLID OXIDE FUEL CELL [SOFC]:
It uses a solid electrolyte (ZrO2) 0.85 (CaO) 0.15 or (ZrO2) 0.9 (Y2O3) 0.1
The electrolyte is an impervious ceramic and its electronic conductivity is very high at
temperature above 1000oc
The current carriers in the electrolyte are oxide ion. The single cell is fabricated by applying a
porous Pt electrode on both sides of a sharp cylindrical electrolyte segment. The fuel gas (H2)
passes through the tube and the outer surface is exposed to air. The anode is CO-ZrO2 or Ni-ZrO2
cermet and the cathode is Sr doped LaMnO3.
Reaction: The oxygen from the air reduced at the cathode
O2 + 4e-
2O2-
The oxide ions formed migrate through the electrolyte and reach the anode (inner surface) where
they are reconverted to oxygen. The oxygen then combines with hydrogen the reaction being
catalyzed by Pt to form H2O.
2O2-
O2 + 2H2
O2 + 4e-
2H2O
Open circuit voltage is 1.15 V.
ADVANTAGE: SOFC do not suffer from the poisoning, leakage or evaporation problems
experienced by other fuel cell system.
DISADVANTAGE: The main problems are brittleness, thermal expansion and corrosion of the
cell intermediates.
H2 – O2 FUEL CELL: The cell consists of a porous carbon impregnated with Ni/Pt catalyst as
anode. The cathode is also a porous carbon impregnated with silver catalyst. The electrolyte is an
aqueous solution of KOH.
In this cell there are three comportments separated from one another by porous electrodes. The
hydrogen gas is fed into another comportment. These gases diffuse through the electrodes and
react with an electrolyte solution in the center comportment.
At the cathode the oxygen undergoes reduction, producing OH- ions.
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O2 (g) + 2H2O (l) + 4e-
4OH-(aq)
At the anode, hydroxide ions react with hydrogen gas by the equation
2H2 (g) + 4OH- (aq)
4H2O (l) + 4e-
The net cell reaction is 2H2 (g) + O2 (g)
2H2O (l)
The fuel cell is operated at high temperature, so the water that is produced is removed as steam.
H2- O2 fuel cell was used as the primary source of electrical energy on the Appolomoon flights.
A secondary advantage is that the water produced as a product of the cell reaction can be
condensed and used as drinking water for the astronauts.
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METHYLALCOHOL FUEL CELL: It is an egs. of a liquid fuel cell. Platinised porous carbon
acts as anode and hollow porous carbon tube impregnated with mixed oxides of Ag, Co and Al
acts as cathode. Methanol dissolved in potassium hydroxide acts as a fuel. The oxidant is the
oxygen or air. Electrolyte is alkaline KOH.
The electrode reactions are
At anode
CH3OH + 6OH-
CO2 + 5H2O + 6e- X 2
At cathode
O2 + 2H2O + 4e-
4OH-
Net reaction
2 CH3OH + 3 O2
X3
2 CO2 + 4 H2O
The use of alkali as electrolyte presents problems. The carbon dioxide is absorbed by the
electrolyte and the electrolyte is gradually converted into carbonate. This decreases the cell
efficiency because of the increasing concentration polarization at the electrode surface and the
decreasing conductivity of the electrolyte.
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