Electrochemistry - 1

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Session
Electrochemistry - 1
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Session Objectives

• Conductance of electrolytic solution
• Specific conductance, Equivalent conductance,
  Molar conductance
• Kohlrausch's law

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INTRODUCTION
Electro chemistry is the branch of chemistry
which deals with transformation of electrical
energy into chemical energy vice
versa. Electricity is a flow of electrons
generated by a battery when the circuit is
completed
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Types of Electrolytes
Strong electrolyte are highly ionized in the
solution. Examples are HCl, H2SO4, NaOH, KOH etc
Weak electrolytes are only feebly ionized in the
solution. Examples are H2CO3, CH3COOH, NH4OH etc
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• Conductors
• A substance which allows electric current to pass
  through it is called a conductor.
• These are 2 types
• 1) Metallic conductors
• 2) Electrolytic conductors
• 1.Metalic conductors
• The substances which conduct electricity under
  the influence of an applied electric potential
  through a flow of electrons.
• The flow of electricity does not cause any
  physical or chemical change in the conductors.
• Eg all metals, graphite, human body

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• 2.Electrolytic conductors
• Electrolyte solutions and
• molten salts conduct electricity
• through the migration of ions.
• When the current is passed
• through an electrolyte solutions
• decomposition and changes occur
• in the composition of electrolytes

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Resistance
Resistance refers to the opposition to the flow
of current.
For a conductor of uniform cross section(a) and
length(l) Resistance R,
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Conductance
The reciprocal of the resistance is called
conductance. It is denoted by C.
C1/R Conductors allows electric current to pass
through them. Examples are metals, aqueous
solution of acids, bases and salts etc.
Unit of conductance is ohm-1 or mho or Siemen(S)
Insulators do not allow the electric current
to pass through them. Examples are pure water,
urea, sugar etc.
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Specific Conductivity It is the reciprocal of
specific resistance of an electrolyte.
l/a is known as cell constant
Unit of specific conductance is ohm1cm1SI Unit
of specific conductance is Sm1 where S is
Siemen
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Equivalent conductance Equivalent conductance
is defined as the conductance of all the ions
produced by one gram equivalent of an electrolyte
in a given solution.

• (To understand the manning of equivalent
  conductance, imagine a rectangular trough with
  two opposite sides made of metallic conductor
  (acting as electrodes) exactly 1 cm apart, If 1
  cm3 (1 mL) solution containing 1 gram equivalent
  of an electrolyte is places in this container is
  measured. )
• /\ eq v x specific conductance of 1cm3
  solution (k)
• 
  /\ eq KV
•                                          /\ eq
  k  1000/N
•          Where N
  normality
•                 The unit of equivalent
  conductance is ohm-1 cm-2 equi-1.

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Representation of Equivalent conductance
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Molar conductance
The molar conductance is defined as the
conductance of all the ions produced by
ionization of 1 g mole of an electrolyte when
present in V mL of solution. It is denoted by.
Molar conductance     ? m k
V                                  Where V
is the volume in mL containing 1 g mole of the
electrolyte. If c is the concentration of the
solution in g mole per litre, then

? m k  1000/M
It units are ohm- cm2 mol-1.
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Effect of Dilution on Conductivity
Specific conductivity decreases on
dilution. Equivalent and molar conductance both
increase with dilution and reaches a maximum
value. The conductance of all electrolytes
increases with temperature.
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• Ionic Mobility (u)
• Ionic mobility is defined as the velocity of an
  ion when the potential gradient is 1v/cm.
• Hence the units of u are cm2/v.sec

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Kohlrauschs law of independent ionic mobilities

• At infinite dilution when dissociation complete
  (??m) , the molar conductivity of an electrolyte
  is expressed as the sum of the contributions from
  its individual ions

?8m v ?8 v- ?8-
v and v- are the number of cations and anions
per formula unit of electrolyte respectively
and, ?8 and  ?8- are the molar conductivities
of the cation and anion at infinite dilution
respectively
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Applications of Kohlrausch's law

• Determination of ?8m for weak electrolytes
• Determination of the degree of dissociation of a
  weak electrolyte
• Determination of the solubility of a sparingly
  soluble salt

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• APPLICATIONS OF KOHLRAUSCH LAW
• 1) Determination of molar conductivities of weak
  electrolytes
• It is not possible to determine value of ?8m
  for weak electrolyte like CH3COOH,NH4OH
  ETC. BY THE EXTRAPOLATION OF THE MOLAR
  CONDUCTIVITY VALUES TO ZERO CONCENTRATION. From
  the value of of ?8m HCl, ?8m CH3COONa and ?8m
  NaOH the value of ?8m CH3COOH can be
  calculated.
• ?8CH3 COOH   ?8CH3COONa   ?8HCI  -  ?8NaCI
• ?8m (H) ?8m (Cl-) ?8m ( CH3COO-) ?8m (
  Na) - ?8m ( Na) ?8m (Cl-)
• ?8m (H) ?8m ( CH3COO-)
• ?8CH3 COOH ?8CH3 COOH

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• 2)Determination of degree of dissociation
• Degree of dissociation is the fraction of the
  total number of molecules dissociated into ions.
• Degree of dissociation (8) No. of molecules
  dissociated in to ions
• 
  Total no. of molecules present
• No. of molecules dissociated is directly
  proportional to conductivity of the molecules.
• No. of molecules dissociated in to ions ?8m
  ( molar conductivity at a particular
  concentration.
• Total no. of molecules ?8m (molar conductivity
  at infinite dilution)
• 8(degree of dissociation) ?8m / ?8m

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• 3) Determination of solubility of sparingly
  soluble salt
• The solubility of a sparingly soluble salts such
  as silver chloride, silver chromate, lead
  sulphate, barium sulphate etccan be determined
  from conductance values.
• The solubility S in gram equivalent/ liter is
  related to equivalent conductance ?8 and
  specific conductivity k
• The concentration of sparingly soluble salt is
  the solubility of the salt.
• hence ?8 1000K
• S

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Illustrative Example
Equivalent conductance of NaCl, HCl and
C2H5COONa at infinite dilution are 126.45,
426.16 and 91 ohm1 cm2 respectively.Calculate
the equivalent conductanceof C2H5COOH.
Solution
91 426.16 126.45 390.71 ohm1 cm2
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GALVANIC or ELECTROCHEMICAL CELLS

• Galvanic cell is a device which converts chemical
  energy into electrical energy.

• ex Daniel cell
• Daniel cell consists of zinc and copper
  electrodes. Zn electrode is dipped in ZnSO4
  solution Cu is dipped in CuSO4 solution.

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Cu2 2e- --gt Cu
Zn --gt Zn2 2e-
Reduction Cathode Positive
Oxidation Anode Negative
lt--Anions Cations--gt
Electrons travel thru external wire. Salt bridge
allows anions and cations to move between
electrode compartments.
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• The 2 solutions separated by a porous membrane, a
  current is seen to be flow on connecting the two
  wires externally.
• The cell function due to dissolution of zinc and
  the simultaneous deposition of copper.
• The over all reaction is
• Zn CuSO4 ? ZnSO4 Cu
• The Danial cell may be represented as
• Zn/ZnSO4 // CuSO4/Cu

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• E.M.F
• The potential difference or electrode between the
  two electrodes of the cell which is a driving for
  the fllow of electrons is called the E.M.F of the
  cell.
• Units electron volts

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NERNST EQUATION

• The theoretical relation ship b/w the electro
  chemical reaction and the corresponding cell
  e.m.f, this relation ship is generally known as
  nernest equation.
• Consider a galvanic cell
• a A bB ? c C d D here a,b,c,d are
  represent the number of moles of A,B,C,D
  respectively, the nernest equation is
• Ecell RT/n F ln K - RT/n F ln CcDd /
  AaBb
• Here Ecell e.m.f of the cell,
• R gas constant,
• T Temperature,
• n no. of faraday of current F passed,
• K equilibrium constant,
• RT/n F ln k standard e.m.f of the Eocell

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• Ecell Eo cell - RT/n F ln CcDd /
  AaBb (or)
• Ecell Eo cell - 2.303 RT/ nF log CcDd
  / AaBb
• at R.T T298 K, R8.314 K-1, F96457 C
  substitute the values in above equation
• Ecell Eo cell - 0.05916/ n log CcDd /
  AaBb
• Standard cell e.m.f equal to cell e.m.f when the
  activities of both reactants and products is
  equal to unity.

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• Cell formulation
• A short hand notation for representing a cell is
  called cell formulation
• In this notation the state of the element ,a
  single stroke for separation of two different
  phases, a double stroke for separation of the two
  electrodes.
• Eg H2(g) / Pt/H(1M) // Cu2(1M) / Cu (s)
• SHE anode SCE cathode
• Classification of electrodes
• metal-metal ion electrode. Eg Cu2 /Cu
• Metal-metal insoluble salt electrode. Eg calomel
  electrode.
• Gas electrode. Eg hydrogen electrode.
• Redox electrode. Eg
  Pt(s)/Fe2 (1M),Fe 3(1M)

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metal-metal ion electrode. Eg Cu2 /Cuit
consists of a pure metal (M) in contact with a
solution of its ion(Mn)It is represented as Mn
(aq) ne- ? M(S)
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Metal-metal insoluble salt electrode. Eg calomel
electrode.It consists of a metal (M) covered by
layer of sparingly soluble salt(MX) immersed in a
solution containing a common ion (X-)it is
represented as X-(aq)// MX/ M(S)MX(s) ne- ?
M(s) X-(aq)
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c) Gas electrode. Eg hydrogen electrode.It is
represented as X(aq)/ X2(P atm) PtX2(p)
2e- ? 2X(aq)
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Reference electrodes

• Reference electrodes are electrodes at which the
  oxidation or reduction occurs reversibly.
• Eg standard hydrogen electrode, calomel
  electrode.
• 1)STANDARD CALOMEL ELECTRODE (S.C.E)
• The calomel electrode undergoes the spontaneous
  process of reduction with respect to the hydrogen
  electrode represented as
• The cell is represented as
• Pt/Hg,Hg2Cl2(s) /KCl

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• Hg2Cl22e- ? 2 Hg (l) 2Cl(aq)
  
  
• The emf of the calomel electrode varies with the
  concentration of the chloride ions and three
  concentrations of chloride ions are normally
  used.
• The decinormal calomel electrode with 0.1N KCl
  having a potential of 0.3338v
• Normal calomel electrode with 1.0 N KCl having a
  potential of 0.28 v
• Saturated calomel electrode with saturated KCl
  having a potential of 0.2415 v on the SHE scale
  at R.T

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• The electrode can be coupled with hydrogen
  electrode containing solution of unknown PH
• The emf of the cell
• Ecell E right- Eleft 0.2422V 0.0592V PH
• PH Ecell- 0.2422V
• 0.0592V

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Quinhydrone electrode (Redox electrode)

• This is a redox electrode reversible to protons
  often replaces the hydrogen electrode
• This is 11 molar mixture of quinone
  hydroquinone
• Electrode consist of a Pt electrode dipped in a
  test solution which is saturated with quinhydrone
• The electrode reaction is given by

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• Q 2H 2e- ? QH2
• (quinone) (
  hydroquinone)
• The electrode potential at 25oc is given by
• E Pt/Q,,H,QH2
• E0 Pt/Q,H,QH2 0.0592/2 log aQH2/aQa2 H
• Since Q,,QH2 are in equimolar amounts i.e. a Qa
  QH2
• So, EE0 0.0592 log a H
• EQ,QH2 E0 Q,QH2 0.0592 p H
• Quinhydrone electrode can be measure p H of a
  solution.
• This electrode cant be used at p H gt8
• Even this electrode fails in the presence of
  strong oxidizing reducing agents

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ION SELECTIVE ELECTRODES

• This electrodes consist of specially prepared
  membranes placed between two electrolytes.
• Ecellk- (0.059/n)log(a1/a2)
• a1,a2 are the activities of the ion to be
  measured in the external and internal solutions
  respectively.
• The ion selective electrodes is coupled to a SCE
  and immersed in the sample or test solution
  containing the ion to be monitored.
• The potential developed across the membrane is
  related to the activities of the ion of interest
  in the gel and sample solution.

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• The response of the membrane is usually highly
  selective to only one ion or a small number of
  ions.
• The construction is similar to that of the glass
  electrode and consists of a tube, one end of
  which is fused to an electrically conducting
  membrane.
• The tube consists with a gel incorporating the
  ion to which the electrode is sensitive and inert
  electrolyte such as KCl.
• A silver wire in contact with the gel together
  with the inert electrolyte constitutes the
  Ag-AgCl reference electrode.

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• Different types of ion-selective electrodes have
  been developed as the above principle.
• 1) glass electrodes for the determination of
  cations other than H
• 2) Solid state electrodes
• 3) liquid-ion exchange membrane electrodes
  heterogeneous membrane electrodes
• 4) Gas sensing electrodes
• glass electrodes with high
  selectivity for cations such as Na,NH4,Ag
  Li consists glass membranes whose composition
  determines the selectivity to individual cations.
  

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• POTENTIOMETRIC TITRATIONS
• The detection of the end point of a volumetric
  titration by the use of potentiometer is an
  important application of measurement of emf.
• The emf of cell consisting of an indicator
  electrode responsive to the analyte ions and a
  reference electrode is measured as a function of
  the volume of titrant added to the analyte
  solution.

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BATTERIES

• A device which stores Chemical energy for later
  release as electricity is called battery.
• It is an electrochemical cell or often
  electrochemical cells connected in series ,can be
  used as a source of direct electric current at
  a constant voltage.
• Batteries are classified into two categories
  depending on their recharging capacities.
• 1) Primary batteries
• 2) secondary batteries.

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1)PRIMARYCELL

• In primary cells, a chemical reaction proceeds
  spontaneously and the free energy of the reaction
  is converted into electrical energy.
• The production of electrical energy at the
  expense of the free energy of the cell reaction
  is called DISCHARGEING of the cell.
• In a primary cell the chemical reactions cant be
  reversed by passing electricity through the cell
  and hence a discharged cell cant be used again
  and the battery become dead.
• In a primary cell the cathode at which reduction
  occurs is designated positive by conversion. Eg
  voltaic cell, Daniel cell, leclanche cell (or)
  dry cell, lithium cell.

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LITHIUM CELL

• Li battery chemistry comprises a number
• of cell designs, in that Li is used as anode
  due to its light weight and highest standard
  potential greater than 3V.

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• Types of lithium cells
• Three types
• 1)Lithium primary cell with Liq cathode
• 2) Lithium primary cell with solid cathode
• 3) Lithium primary cell with solid electrolyte

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• Liq cathode cells
• Anode Li
• Cathode SOCl2
• Electrolyte LiAlCl2
• Anode Li --?Li e-
• Cathode 4Li 4e- SOCl2 --? 4 LiCl SO2 S
• Overall rxn 4Li 2SOCl2 --? 4 LiCl SO2 S
• They perfom best in low current applications and
  have a very long service life. For this reason
  they are used in pacemaker.

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• These cells offer higher discharge rates due to
  the rxns occur at the cathode surface.
• The direct contact between the liq cathode n the
  Li forms a film over the Li,called solid
  electrolyte interface.(SEI)
• This prevents further chemical rxn when not in
  use,thus preserving the shells life.
• The thick film causes an initial voltage delay.

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• solid cathode (LiMnO2) cell
• Anode Li
• Cathode MnO2
• Electrolyte propylene carbonate n 1,2-dimethoxy
  ethane
• Anode Li --?Li e-
• 
  4 3
• Cathode Li e- MnO2 --? MnO2 (Li)
• Li MnO2 --? MnO2 (Li)
• 4 3
• Uses
• Low rate cells are used commercially for small
  electronics and memory back up.

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• solid electrolyte cells
• Anode Li
• Cathode poly -2- vinyl pyridine(P2VP)
• Electrolyte solid Li
• Anode 2Li --?2Li 2e-
• Cathode 2Li 2e- P2VP.n I2 --? P2VP.(n-1)
  I2 2LiI
• Overall rxn 2Li P2VP.n I2 --? P2VP.(n-1) I2
  2LiI

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• These cells cant be used in high drain
  applications and dont perform well under low
  temp conditions.
• They are used generally for memory back up
  ,watches and portable electronic devices

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2)SECONDARY CELLS (ACCUMULATORS)

• In secondary cells chemical reactions proceeds
  both in the forward and reverse directions
  depending on weather electrical energy is
  supplied from an external source
• Electrical energy is passed into the cell to
  induce a chemical reaction and the products
  remained at the electrodes, this process is
  called charging the cell.
• Secondary cells can accumulate electrical energy
  in the form of chemical reaction and later on the
  reaction is reversed to liberate electrical
  energy. Hence these cells are called accumulators
  or storage batteries. The cathode at which
  reduction occurs during the discharge of the cell
  is designated ve. while it becomes anode during
  charging.
• Eg lead-acid battery, alkaline storage battery,
  nickel cadmium battery.

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• LEAD ACID BATTERY
• This battery consists of a number of spongy lead
  anodes and a grid of lead dioxide coated
  lead-antimony alloy cathode.
• The electrode pairs separated by inert porus
  partitions are kept immersed in the electrolyte
  sulphuric acid.

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• At the anode Pb ? Pb2 2e-
• Pb2 SO42-?PbSO4
• At the cathode
• PbO2 4H 2e- ?Pb2 2H2O
• Pb2 SO4 2- ?PbSO4
• The net reaction of the cell is
• PbO2 4H 2e- SO42- ?PbSO4 2H2O
• The lead sulphate formed precipitates on the
  cathode
• .
• Pb/PbSO4(s)/H2SO4(aq),PbSO4(s)/Pb
• The net reaction of the cell is
• PbO2Pb2H2SO4?2 PbSO4 2H2O
• ?

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NICKEL-CADMIUM CELL

• It consists of a steel grid containing cadmium
  powder as the anode and cathode made of Ni2O3
  mixed with Ni supported on steel grid.
• An aqueous solution of KOH placed in an inert
  steel container is used as the electrolyte. The
  cell generates the voltage of 1.35v and the
  reaction may be represented as
• Cd 2OH- ? Cd(OH)2 2 e- (at
  anode)
• 2NIO(OH) 2H2O 2e- ?2 Ni(OH)2
  2OH- (at cathode)
• The net reaction is
• Cd 2Ni(OH)3 ? Cd(OH)2 2NI(OH)2
• The disadvantage of this battery is the reaction
  can be reversed, bcos the reaction products
  Cd(OH)2 Ni(OH)2 remain adhered to the
  electrodes called MEMORY EFFECT or FALSE BOTTOM.

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Nickel-Cadmium battery
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FUEL CELLS

• A fuel cell is an electrochemical cell which
  converts chemical energy contained in a easily
  available fuel oxidant system into electrical
  energy.
• The basic principle of a fuel cell is the
  chemical energy is provided by a fuel and an
  oxidant stored outside the cell.
• The fuel and the oxidizing agent are continuously
  and separately supplied to the electrodes of the
  cell at which they undergo reactions.
• These are also primary cells they are capable
  of current as long as the reactants are supplied.

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HYDROGEN OXYGEN FUEL CELL

• It consists of two inert porous electrodes made
  of either graphite impregnated with finely
  divided Pt or a 75/25 alloy of Pd with Ag(Ni) and
  an electrolyte solution which is 25 KOH
  solution.
• Through the anode, H2 gas is bubbled and through
  the cathode O2 gas bubbled.
• At anode 2H2(g) 4OH-(g) ? 4H2O(l) 4e-
• At cathode O2(g) 2H2O(l) 4e- ? 4OH-(aq)
• Net rxn 2H2(g) O2(g) ? 2H2O(l)
• The emf of the cell is 0.8 to 1.0v
• A no of such cells are stacked together in series
  to make a battery, called fuel cell battery.

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Hydrogen-oxygen fuel cell
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Uses of H2- O2 fuel cell

• They are used as auxiliary energy source in space
  vehicles (ex Apollo space craft), submarines
  other military vehicles.
• For space craft, they are preferred due to their
  lightness product water is available as source
  of fresh water for astronauts.
• Fuel cells are categorized on the basis of
  electrolyte used
• 1)Proton exchange membrane fuel cell
• 2)Alkaline Fuel cell
• 3) Molten carbonate Fuel cell
• 4) Phosphoric acid fuel cell
• 5) Solid oxide fuel cell

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