DEPARTMENT OF CHEMISTRY

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Electrochemistry

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Dr.A.Geetha

Associate Professor

Department of Chemistry

Kongu Engineering College

Electrochemistry and Redox

• Oxidation-reduction: “Redox”
• Electrochemistry:
• study of the interchange between chemical change and electrical work (It deals the amount of energy produced or consumed during chemical change)

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• Electrochemical cells:
• systems utilizing a redox reaction (Chemical change) to produce or use electrical energy

Redox Review

• Redox reactions: electron transfer processes

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• Oxidation: loss of one or more e^-
• Reduction: gain of one or more e^-

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• Oxidation numbers: imaginary charges

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Oxidation-reduction

• Oxidation is loss of e^-
• Oxidation Number increases (more positive)

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• Reduction is gain of e^-
• Oxidation Number decreases (more negative)

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• Oxidation involves loss OIL
• Reduction involves gain RIG

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Redox

• Oxidation is loss of e^-

causes reduction

“reducing agent”

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• Reduction is gain of e^-

causes oxidation

“oxidizing agent”

Types of cells

• Voltaic (galvanic) cells:

a spontaneous reaction generates

electrical energy (Chemical change

produces Electrical energy)

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• Electrolytic cells:

absorb free energy from an

electrical source to drive a

nonspontaneous reaction

(Electrical energy utilized to bring chemical change)

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Electrolysis

• Forcing a current through a cell to produce a chemical change for which the cell potential is negative.

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Voltaic (galvanic) cells:

• a spontaneous reaction generates electrical energy (Chemical change produces Electrical energy)

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Common Components

• Electrodes:

conduct electricity between cell and surroundings

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• Electrolyte:

mixture of ions involved in reaction or carrying charge

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• Salt bridge:

completes circuit (provides charge balance)

Electrodes

• Anode:
• Oxidation occurs at the anode (Oxidation half cell)

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• Cathode:
• Reduction occurs at the cathode (reduction half cell)

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• Active electrodes: participate in redox
• Inactive: sites of ox. and red.

Need for batteries

• The batteries store the renewable energy, and when it's needed, they can quickly release the energy into the grid.
• This makes the power supply smoother and more predictable.
• The energy stored in the batteries can also be used in times of peak demand, when more electricity is required.

Batteries

• A battery is a galvanic cell or, more commonly, a group of galvanic cells connected in series.

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Types of Batteries

• Primary Batteries (Non-rechargeable batteries):
• These are basically considered as primary batteries because they can be used only once.
• These batteries cannot be recharged and used again. Let’s see about the regular, daily life primary batteries that we see.

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• Alkaline batteries:
• It is basically constructed with the chemical composition of Zinc (Zn) and Manganese dioxide (MnO₂), as the electrolyte used in it is potassium hydroxide which is purely an alkaline substance the battery is named as alkaline battery having he power density of 100 Wh/Kg.

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Advantages:

• Cycle life is more
• More compatible and efficient for powering up portable devices.
• Shelf life is more.
• Small in size.
• Highly efficient.
• Low internal resistance so that discharge state in idle state is less.
• Leakage is low.

Disadvantages:

• Cost is a bit high. Except it everything is an advantage.

Applications:

• It can used in torches, remotes, wall clocks, small portable gadgets etc.

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Coin cell batteries

• The chemical composition of coil cell batteries is also alkaline in nature.
• Apart from alkaline composition, lithium and silver oxide chemicals will be used to manufacture these batteries which are more efficient in providing steady and stable voltage in such a small sizes.  
• It has Power density of 270 Wh/Kg.

• Advantages:
• Light in weight
• Small in size
• High density
• Low cost
• High nominal voltage (up to 3V)
• Easy to get high voltages by arranging serially
• Long shelf life
• Disadvantages:
• Needs a holder
• Low current draw capability
• Applications:
• Used in watches, wall clocks, miniature electronic products etc.

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�Rechargeable Batteries�

• These are generally called as secondary batteries which can be recharged and can be reused.
• Though the cost is high, but they can be recharged and reused and can have a huge life span when properly used and safely charged.

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Lead-acid batteries

• It consists of lead-acid which is very cheap and seen mostly in cars and vehicles to power the lighting systems in it.
• These are more preferable in the products where the size/space and weight doesn’t matter.
• These comes with the nominal voltage starting 2V to24V and most commonly seen as 2V, 6V, 12V and 24V batteries.
• It has Power density of 7 Wh/Kg.

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Advantages:

1. Cheap in cost
2. Easily rechargeable
3. High power output capability

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Disadvantages:

1. Very heavy
2. Occupies much space
3. Power density is very low

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Applications:

Used in cars, UPS (uninterrupted Power Supply), robotics, heavy machinery etc..

Ni-Cd batteries

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• These batteries are made of Nickel and Cadmium chemical composition.
• Though these are very rarely used, these are very cheap and their discharge rate is very low when compared to NiMH batteries. These are available in all standard sizes like AA, AAA, C and rectangular shapes.
• The nominal voltage is 1.2V, often connected together in a set of 3 which gives 3.6V. It has Power density of 60 Wh/Kg.

Advantages:

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1. Cheap in cost
2. Easy to recharge
3. Can be used in all environments
4. Comes in all standard sizes

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Disadvantages:

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1. Lower power density
2. Contains toxic metal
3. Needs to be charged very frequently in order to avoid growth of crystals on the battery plate.

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Applications:

Used in RC toys, cordless phones, solar lights and mostly in the applications where price is important.

Li-ion batteries

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• These are made up of Lithium metal and are latest in rechargeable technology. As these are compact in size they can be used in most of the portable applications which need high power specifications.

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• These are the best rechargeable batteries available. These have a nominal voltage of 3.7V (most commonly we have 3.6V and 7.2V) and have various ranges of power capacity (starting from 100s of mAh to 1000s of mAh).

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• Even the C-rating ranges from 1C to 10C and Power density of Li-ion batteries is 126 Wh/Kg.

Advantages:

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1. Very light in weight.
2. High C-rating.
3. Power density is very high.
4. Cell voltage is high.

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Disadvantages:

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1. These are a bit expensive.
2. If the terminals are short circuited the battery might explode.
3. Battery protection circuit is needed.

Fuel Cells

• Galvanic cells
• Reactants are continuously supplied.
• 2H_2(g) + O_2(g) → 2H₂O_(l)
• anode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻
• cathode: 4e⁻ + O₂ + 2H₂O → 4OH⁻

THANK YOU