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Gaurav Singh Ppt

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    NAME: GAURAV S SINGH

    ROLL NO: 934

    CLASS : BE-4 SUBJECT: E.M.C.

    SEMINAR ON OVER POTENTIAL ANDPERFORMANCE OF CELL

    GUIDED BY: PROF V.V. MATHANE

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    INTRODUCTION:

    OVER VOLTAGE:Overpotential meansthat even though you have yourelectrodes at the "right" potential for a

    reaction to occur (according to thecharts), the reaction doesn'tnecessarily take place to a noticable

    degree, and you have to go to a higherpotential before anything happens.

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    Over potential is an electrochemical termwhich refers to the potential (voltage)

    difference between a half-reaction'sthermodynamically determined reductionpotential and the potential at which the redox event is experimentally observed.

    Overpotential is a potential that must beapplied in an electrolytic cell in addition to

    the theoretical potential required to liberatea given substance at an electrode. Thevalue depends on the electrode materialand on the current density.

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    Expressed as:n=Ei-Erevv;where Ei=voltage impressed ,

    Errevv=E reversible,Erevv=Ecathode-Eanode..

    EXAMPLE:

    The most commonly cited example is theoxidation of water, where two molecules of H2Ocome together to make an O2. In order to get thereaction to happen, you have to go to a higherpotential than expected. The effect is related to

    the rates (kinetics) of the reactions that arehappening at the electrodes, so overpotentialsare most commonly needed when two or moremolecules have to take part in a reaction at anelectrode.

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    Due to overpotential :

    o An electrolytic cell's anode is morepositive using more energy thanthermodynamics require.

    o An electrolytic cell's cathode is morenegative using more energy than

    thermodynamics require.o A galvanic cell's anode is less negative

    supplying less energy thanthermodynamically possible.

    o A galvanic cell's cathode is less positivesupplying less energy thanthermodynamically possible.

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    Activation over potential for the evolution of selected gases on various electrode materials at 25C

    Material of the electrode Hydrogen Oxygen Chlorine

    Platinum (platinized) 0.07 V +0.77 V +0.08 V

    Palladium 0.07 V +0.93 V

    Gold 0.09 V +1.02 V

    Iron 0.15 V +0.75 V

    Platinum (shiny) 0.16 V +0.95 V +0.10 V

    Silver 0.22 V +0.91 V

    Nickel 0.28 V +0.56 V

    Graphite

    0.62 V

    +0.95 V

    +0.12 V

    Lead 0.71 V +0.81 V

    Zinc 0.77 V

    Mercury 0.85 V

    http://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Chlorinehttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Platinized_platinumhttp://en.wikipedia.org/wiki/Palladiumhttp://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Silverhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Leadhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Mercury_(element)http://en.wikipedia.org/wiki/Mercury_(element)http://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Leadhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Silverhttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Palladiumhttp://en.wikipedia.org/wiki/Platinized_platinumhttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Chlorinehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydrogen
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    Varieties of over potential: Over potentials can be judged into three

    categories: activation,

    concentration, and

    resistance.

    Activation over potential: Depends on the activation energy of the redox

    event.

    Refers exclusively to the activation energynecessary to transfer an electron from an electrodeto an electrolyte.

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    Reaction over potential :o Related to chemical reactions that must formally

    precede electron transfer.o The reaction over potential can be reduced or

    eliminated with the use of homogeneous orheterogeneous electro catalysts .

    Concentration over potential:o All involve the depletion of charge-carriers at the

    electrode surface.

    o The concentration of charge-carriers is depleted by

    the physical formation of a bubble.o Caused by differences in concentration of the

    charge-carriers between bulk solution and on theelectrode surface.

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    Bubble over potential:

    It is due to the evolution of gas at either the anodeor cathode.

    This reduces the effective area for current andincreases the local current density.

    Example would be the electrolysis of an aqueoussodium chloride solution

    Resistance over potential :

    Includes junction over potentials,

    cell design,

    surface polarization , other sources of counter electromotive forces .

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    Over voltage concerned with two basicsreaction.

    1.Cathodic reaction

    2.Anodic reaction

    1.CATHODIC REACTION:

    o Deposition processo Metal over voltage

    o Hydrogen over voltage.

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    HYDROGEN OVER POTENTIAL:

    In most cases the difference between electrodepotential arising from the passage of the current on

    hydrogen evolution and the potential of theequilibrium hydrogen under the same conditions isidentified with activation polarization sinceconcentration polarization is small here and may be

    disregarded.When hydrogen is evolved from alkaline solution,concentration polarization must be lower owing tovery high concentration of discharging particles.

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    2.ANODIC REACTION:

    Here main concern is oxygen over voltage.

    OXYGEN OVER POTENTIAL:The anodic evolution of oxygen is nearly an important asthe cathodic hydrogen evolution reaction. The reaction

    constituting the decomposition of water is;

    2H2O=2H2+O2

    The oxygen over potential represents a considerablefraction of the total over potential in a cell for electrolysis

    of water and affects the consumption of electric power inthe industrial electrolytic production of hydrogen and

    oxygen.

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    REFERENCE:

    1.INTERNET

    2.THEORETICAL ELECTROCHEMISTRY by-

    L.ANTROPOV

    3. CLASS DISCUSSION

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    THANK

    YOU


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