Electrochemical Cells

Types of Electrochemical Cell:

  • An electrochemical cell is a device which is used to generate electrical energy at the expense of spontaneous oxidation-reduction reaction.
  • An electrochemical cell is also known as a galvanic cell or a voltaic cell.  In an electrochemical cell, chemical energy is converted into electrical energy.
  • There are two types of voltaic or galvanic cells used to get an electric current of low voltage.

Primary Cell:

  • A cell in which electrical energy is generated within the cell itself is called as primary cell. e.g. Dry cell, Daniell cell. Such a cell cannot be reused.

Secondary Cell:

  • A cell in which electrical energy is not generated within the cell itself but it previously stored in it from an external source is called the secondary cell. e.g. Lead accumulator.  Secondary cells can be reused.

Primary Cells:

Daniel cell:


  • Due to the spontaneous redox reaction, electrical energy is generated within the cell itself.  So Daniel cell is a primary voltaic cell. When an opposing voltage slightly greater than the cell voltage is applied, the cell reaction is reversed so Daniel cell is a reversible cell. Thus Daniell cell is a primary reversible electrochemical cell.


  • Daniell cell consists of two electrodes or two half cells i.e. zinc half cell and copper half cell. Daniell cell with porous partition consists of a vessel which is divided into two compartments

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  • In one compartment, a solution of zinc sulphate ZnSO4 is placed in which a zinc rod is dipped. This rod acts as negative electrode or anode. This is known as negative half cell or zinc half cell. The other compartment is filled with copper sulphate CuSO4 solution. In this solution, a copper rod is dipped which acts as positive electrode cathode. This is known as positive half cell or copper half cell.
  • The two half-cells are connected externally by a conducting wire and internally by means of a porous partition or salt bridge or porous pot. The porous partition allows the diffusion of ions through it but prevents excessive mixing of the two solutions.

Representation of cell:


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Working of the cell:

  • As long as the circuit is closed, following reactions take place. Oxidation takes place at zinc half cell and reduction takes place at the copper half cell.  Zinc rod acts as anode i.e.   -ve electrode while copper rod acts as cathode i.e. + ve electrode

Cell reactions:

  • At anode (Zn) or – ve electrode: Zinc atoms from rod enter the solution forming Zn ions ( Zn++) leaving behind 2 electrons on the rod.

 Zn  → Zn++  + 2 e   (oxidation)

  • At cathode (Cu) or + ve electrode: Cu++ ions from CuSO4 solution acquire 2 electrons from the copper rod and during this process get deposited on the copper electrode in the form of Cu atoms.

Cd++  + 2 e →   Cd  (reduction)

  • Net cell reaction:

Zn   + Cu++ →  Zn++ + Cu     (redox)

  • Both the oxidation and reduction reaction take place simultaneously and separately. Due to oxidation, zinc rod becomes electron rich and due to reduction copper rod becomes electron deficient. Thus there is a flow of electrons from electron rich zinc rod to electron deficient copper rod through external circuit which constitutes the electric current.
  • The movements of sulphate ( SO4– –) ions from copper sulphate to zinc sulphate through the salt bridge or porous pot completes the Internal circuit and maintain electrical neutrality of the electrolyte.
  • Size of the zinc rod reduces because it dissolves in ZnSO4 solution as zinc ions while the size of copper rod increases because of Cu++ in the CuSO4 solution discharge and copper deposits on the rod. The concentration of ZnSO4 solution increases while the concentration of CuSO4 solution decreases. The solutions in both the compartments remain electrically neutral.

E. M. F. of Daniell cell:

  • If the concentrations of zinc sulphate and copper sulphate solutions are  1 M each then the e.m.f. of Daniel cell is about 1.1 volt. The e.m.f. of cell which is ab

Reversibility of Daniell cell:

  • If an external opposing e.m.f. slightly greater than 1.1 V is applied then the reverse cell reaction takes place.

  Zn++ + Cu  →  Zn   + Cu++   (redox)

  • Now, reduction takes place at zinc electrode and oxidation takes place at the copper electrode. Thus Daniell cell is a reversible cell.


  • Explanation of why there is a gradual decrease in e.m.f. of Daniel cell

EoCell = Eo(ox/cathode) –   E(ox/anode)

EoCell =  Eo(ox/Zn) –  Eo(ox/Cu)

EoCell   =      0.76    –  (- 0.34)

EoCell  =  0.76    +   0.34

EoCell   =      1.1 V

  • As the concentration of Zn++ ions goes on increasing in zinc half cell, the oxidation potential of zinc electrode goes on decreasing. As the concentration of Cu++ ions go on decreasing in the copper half cell, the reduction potential of copper electrode goes on decreasing.
  • Thus the quantity
  • EoCell =  Eo(ox/Zn) + Eo(red/Cu) decreases.

Daniel Cell with Porous Pot:

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Daniel Cell with Copper Vessel as Cathode:

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Daniel Cell with Salt Bridge:

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Dry Cell:

  • The dry cell is actually a modification of Leclanche cell. A dry cell is primary cell because electrical energy is generated within the cell itself.
  • It is an irreversible cell as cell reaction doesn’t reverse if higher external emf is applied to it.
  • Since it doesn’t contain any liquid it is called the dry cell.  It is very convenient to carry.

Construction :

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  • A dry cell consists of an outer zinc vessel (can) which works as an anode or negative electrode.
  • In the centre of the zinc vessel, there is a stout graphite rod surrounded by compressed is an aqueous paste of MnO2, carbon powder and ammonium chloride. This compressed solid is placed at the centre of the vessel.
  • The gap between zinc can and compressed mixture is filled up with a jelly paste prepared from zinc chloride, ammonium chloride and starch. Starch jelly helps to keep ammonium chloride moist for a long time. These Two different pastes are separated by means of a cotton partition.
  • The top of the can is sealed with sealing wax or resin or plastic to avoid drying of the paste. The whole cell is covered with insulating safety cover.


  • On connecting carbon (+) and zinc (-) terminals through an external circuit, the cell starts working.

When cell functions, oxidation takes place at zinc vessel and reduction takes place at graphite.

  • Reactions at negative electrode (Zn) :

 Zn  → Zn++  + 2 e   (oxidation)

  • Reactions at positive electrode (C) :

2 MnO2 +   2 NH4+ +   2e  → Mn2O3 + 2NH3(g) + H2O(l)   (reduction)

  • Net Cell Reaction:

The overall reaction is,

Zn + 2 MnO2 +   2 NH4+    →  Zn++ +Mn2O3 + 2NH3(g) + H2O(l) 

  • Thus zinc is consumed continuously during working and holes are formed in the container which results in the leakage of the cell contents.
  • Side Reaction :

Zn++ +  NH3 →   [Zn(NH3]++

The EMF of Dry Cell: The EMF of this cell is 1.5 V.

Reversibility of Cell: A dry cell is non-reversible cell.

Uses of the dry cell.

  • The dry cell is used in torches, calculators and toys.
  • Dry cells are used in flashlights.
  • They are extensively used in many electronic appliances like clocks, transistor-radios, tape recorders, calculators etc.

Disadvantages of dry cell:

  • The acidic ammonium chloride corrodes the zinc container and thus the dry cell does not have a long time.
  • Zinc is consumed continuously during working and holes are formed in the container which results in the leakage of the cell contents.
  • This cell cannot be recharged by applying external potential. This is because Zn++ ions and NH3 molecule react each other to form a complex [Zn(NH3)]++ and they are not available for reverse reaction. Therefore the dry cell is an irreversible cell.

Irreversibility of the dry cell:

  • During discharging of a dry cell following reaction takes place.

 Zn  → Zn++  + 2 e   (oxidation)

  • Thus zinc of vessel dissolves in the surrounding electrolyte forming zinc ions. These Zn++ ions and NH3 molecule react each other to form complex {Zn(NH3)]++
  • Thus if we apply an external e.m.f. zinc ions are not available for reverse reaction. Hence instead of the reduction of zinc, the reduction of ammonium ion takes place as follows.

2 NH4+  +2  e  →   2NH3(g) +  H2 (reduction)

  • Hence due to the absence of reverse reaction. The cell is irreversible.

Button Cell:

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  • Anode can: Usually it contains zinc metal and electrolyte mixture (zinc powder + mercury). This forms the negative terminal of the cell.
  • Cathode can: It contains the cathode material/electrolyte mixture (ZnO + KOH). This forms the positive terminal of the cell.
  • Separator: It contains porous material with electrolyte.


  • On connecting cathode (+) and anode (-) terminals through an external circuit, the cell starts working.

Cell Reactions:

  • Reactions at anode:

Zn   + 2OH–  →   ZnO(s)  + H2O(l)   +  2 e          (oxidation)

  • Reactions at cathode:

HgO(s) + H2O(l)  +  2 e → Hg(l) + 2OH     (reduction)

  • Net Cell Reaction:

The overall reaction is,

Zn   + HgO(s)    →  ZnO(s) +  Hg(l)

The EMF of cell:

  • The EMF of this cell is 1.35 V. Practically this e.m.f. remains constant because no ions are involved whose concentration can change during use.

Reversibility of Cell: 

  • Button cell is a non-reversible cell.

Uses of the button cell.

  • It is small in size hence can be used in digital and electronic instruments like watches, computers, hearing aids etc.
  • The extremely constant voltage over its useful life. Suitable for low drain and intermittent high drain applications.
  • It has a long shelf life – up to 3 years.

Disadvantages of the button cell.

  • Contains mercury, which in certain forms is highly toxic to humans and animals.

Fuel cells:

  • These are galvanic cells in which fuel, in the form of gases, which constitute electrode material are constantly supplied to the cell and electrochemical reaction takes place to produce electricity. The galvanic cells in which the energy of combustion of fuels is directly converted into electrical energy are called fuel cells.
  • As it is direct method to convert chemical energy into electrical energy, the cell has the efficiency of 70%. In another method by burning the fossil fuel like coal, coke, petrol, diesel heat is generated, which is used to raise steam, which in turn used to rotate generators to produce electricity. This method has the efficiency of 40 % only.

Construction and working of hydrogen-oxygen fuel cell:

  • These cells are used in space shuttle and space stations to produce electricity and water for consumption for onboard astronauts.


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  • It consists of a cylindrical container of alkali resistant material. It is divided into three compartments. Middle compartment contains a hot concentrated solution of KOH. Pure and dry hydrogen gas enters the anodic compartment from one side and pure dry oxygen enters the cathodic compartment from the other side.
  • The anode is porous carbon electrode impregnated with platinum or palladium which acts as a catalyst and provides electrical contact. The cathode is also porous carbon electrode impregnated with cobalt oxide, platinum or silver which acts as a catalyst and provides electrical contact.


  • Anode reaction: Hydrogen gas under high pressure diffuses through porous carbon anode

2 H2(g) + 4OH(aq)→ 4H2O(l)  + 4e

  • Cathode reaction: The electron generated at anode reach cathode through external circuit and oxygen adsorbed on the surface of cathode get reduced to hydroxyl ions.

O2(g) + H2O(l)  +  4e →  4OH

Thus the net reaction of the cell is

2 H2(g) + O2(g) →   2H2O(l)

  • Thus water is the by-product of the reaction. These cells do not pollute the atmosphere.

E.M.F. of a Cell:

EoCell = Eo(ox/cathode) –   E(ox/anode)

EoCell    =      0.83    +   0.40

EoCell    =      1.23 V

Advantages of Fuel Cells:

  • The reacting substances are continuously supplied to the electrodes. Hence unlike conventional cells, the fuel cells do not have to be discharged when the chemicals are consumed.
  • Thus water is the by-product of the reaction. These cells do not pollute the atmosphere.
  • As it is direct method to convert chemical energy into electrical energy, the cell has the efficiency of 70%.

Drawbacks of Fuel Cells:

  • The calculated cell voltage is 1.23 V. Due to the reversibility of the reaction the cell voltage is less than 1.23 V.
  • Hydrogen gas is hazardous and its production is expensive.

Uses Fuel Cells:

  • These cells have been used in automobiles on an experimental basis.
  • These cells are used in space shuttle and space stations to produce electricity and water for consumption for onboard astronauts.
  • They may be used in Hospitals, hotels and homes for production of electricity.

Secondary Cells:

Lead Accumulator:

  • A lead accumulator is a secondary cell because electrical energy is not generated within the cell itself but it is previously stored in it from the external source.
  • It is reversible cell because cell reactions are reversed if external emf just greater than the emf of this cell is applied. Thus in this cell net cell reaction can be reversed by applying external opposing e.m.f. greater than the cell e.m.f.
  • This cell can store electrical energy in form of from external source (charging) and can supply it during discharging.  The energy is stored in form of chemical energy. So this cell is a storage cell or accumulator or storage battery.
  • Its voltage does not depend on the size of electrodes or size of its cell but depends upon the strength of a sulphuric acid solution.


Net cell reaction

  • Lead accumulator consists of lead plates as the negative electrode. Lead plates impregnated with lead oxide act as the positive electrode. Negative and positive electrodes are arranged in an alternate manner.
  • This assembly of lead plates is dipped in a non-conducting vessel made up of glass or plastic or ebonite and containing 38% H2SO4 (sp. gr. 1.215).
  • All positive plates are connected to each other and all negative plates are connected to each other.

Representation of Cell:

– Pb(s)|PbSO4(s)|38%H2SO4(aq)| PbSO4 (s)| PbO2(s) |Pb(s) +


  • Discharging of Cell: When cell starts working, oxidation takes place at lead plates and reduction takes place at lead plates with lead oxide.  This is called as discharging of cell.

2 H2SO4(aq)  → 4H+ +  2 SO4 – 

Reaction at negative electrode (Anode)

Pb  →    Pb++ +   2 e

Pb++ + SO4– – → PbSO4(s)

Pb(s) + SO4– –   →   PbSO4(s) + 2e         (oxidation)

Reaction at positive electrode (Cathode)

PbO2(s) +  4H+ + SO4– –  + 2e–  → PbSO4(s)+ 2H2O(l)

Net cell  reaction during discharging

Pb(s) + PbO2(s) +  2H2SO4(aq)   → 2 PbSO4(s) +  2H2O(l)

  • Thus during discharging sulphuric acid is converted into water and hence specific gravity of sulphuric acid solution falls to 1.17.
  • Charging of cell: When external emf greater than the emf of this cell is applied, exact reverse reactions take place.  At positive electrode oxidation takes place and a negative electrode reduction takes place.  This is called charging of cell.

Reaction at negative electrode (Cathode)

PbSO4(s) + 2e  →   Pb(s) + SO4– –   

Reaction at positive electrode (Anode)

 PbSO4(s)+ 2H2O(l)      →  PbO2(s) +  4H+ + SO4– –  + 2e– 

Net Cell  Reaction During Charging

 2 PbSO4(s) +  2H2O(l)

Pb(s) + PbO2(s) +  2 H2SO4(aq)

Pb(s) + PbO2(s) +  2H2SO4(aq)   → 2 PbSO4(s) +  2H2O(l)

E.M.F. of Cell:

  • A fully charged accumulator with 38% H2SO4 has a voltage about 2.041 V i.e. approx 2 V.

Uses of Lead Accumulator:

  • Lead accumulators are used in motor cars and other automotive vehicles and therefore popularly known as car batteries. Here generally six cells are connected in series to give a voltage of 12 V.
  • They are used in laboratories as a source of constant DC voltage.
  • They are used in telephone and telegraph offices.
  • They are used in electric clocks, radio sets, burglar’s alarms etc.
  • They are also used as a source of electricity in the initial stages of the rocket.

Maintenance of Lead Accumulator:

  • It is kept erect and should not be tilted.
  • When the cell is not in use for a longer time, the terminals should be disconnected and acid from the cell should be completely removed and it is to be kept dry.
  • The concentration of sulphuric acid should always be maintained to get the optimum value of cell potential.
  • The cell should not be allowed to run down much. When in use the e.m.f. of the cell decreases. When the voltage is about 1.8 V, the cell should be recharged.
  • The cell should not be overcharged i.e. e.m.f. of the cell should not be more than 2.1 V.
  • Acid level should be maintained properly by adding distilled water.
  • Positive and negative terminals of lead accumulator should be cleaned to remove insoluble lead sulphate from time to time for good electrical contact.
  • The cell should be charged regularly and never left in discharge condition.

Nickel Cadmium Cells:

  • The first Ni–Cd battery was created by Waldemar Jungner of Sweden in 1899.
  • Nickel-Cadmium battery is a rechargeable battery with Nickel Oxide-Hydroxide (cathode) and Cadmium as a cathode and Alkaline Potassium Hydroxide as the electrolyte.
  • Reaction at anode (Oxidation):

Cd(s) + 2OH (aq) →  Cd(OH)2(s) + 2e

  • Reaction at cathode (Reduction):

NiO2(s)+2H2O(l) + 2e →   Ni(OH)2(s)  + 2OH (aq)

  • Net Reaction:

Cd(s) + 2H2O(l) + 2e   →  Cd(OH)2(s)  + Ni(OH)2(s) 

  • The reaction products are solid which get adhered to the respective electrode surface, hence these cells can be recharged.
  • E.m.f obtained is 1.4 V. These cells have much more life than other dry cells. They are used in electronic watches, calculators and photographic equipment. They have Low internal resistance, Can tolerate deep discharge cycle and can also get charged rapidly.  These batteries also have the long lifetime and low maintenance cost.
  • Nickle-Cadmium batteries are costlier than lead-acid battery also the Cadmium used in the battery causes severe environmental pollution.

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