FARADAY'S LAWS OF ELECTRO-DEPOSITION
The laws governing the electrolytic processes were formulated by Michael Faraday,
an English Scientist, and are known after his name. These may be stated as below:
Faraday's First Law:
According to this law the chemical deposition due to flow of current through an
electrolyte is directly proportional to the quantity of electricity (coulombs) passed
through it.
i.e., mass of chemical deposition, m ∝ Quantity of electricity or m ∝ It
∵ Q = It or m = ZIt
where I is the steady current in amperes flowing through the electrolyte for t seconds
and Z is a constant of proportionality and is known as the electro-chemical
equivalent of the substance.
Faraday's Second Law:
This law states that when the same quantity of electricity is passed through several
electrolytes, the mass of the substances deposited are proportional to their respective
chemical equivalents or equivalent weights.
PRIMARY AND SECONDARY CELLS :
The electrical energy, in primary as well as secondary cells, is produced from the
chemical energy liberated as a result of the chemical reactions taking place in the
cell.
Primary Cells:
The simple voltage cell is a primary cell. A primary cell supplies current until the
electrolyte is exhausted or the negative electrode is completely dissolved. These
cannot be recharged once decayed. The shelf life of a primary cell is about one year.
Some of the primary cells are:
1. Leclanche cell:
Anode ------------- Carbon rod or plate
Cathode ------------- Zinc plate or container
Electrolyte ---------- An aqueous solution of ammonium chloride
Open circuit e.m.f. ---- About 1.5 V
2. Clark cell:
Anode ------------- Mercurous sulphate
Cathode ------------- Zinc/zinc sulphate
Electrolyte ---------- Saturated zinc sulphate
E.m.f. at 15°C --------- About 1.43 V
3. Weston cell:
Electrodes ------------- Cadmium/cadmium sulphate, and mercurous electrode
Electrolyte ---------- Solution of cadmium sulphate
E.m.f. at 20°C ------ 1.3183 V
4. Other primary cells:
Alkaline primary cells
Water activated primary cells
Primary cells with acid electrolytes
Water activated primary cells
Primary cells with acid electrolytes
Secondary Cells :
Whereas a primary cell (as earlier mentioned) can supply current until the
electrolyte is exhausted or the negative electrode is completely dissolved and the cell
has to be discarded, the secondary cell can be recharged by passing a current in an
opposite direction to the current that normally flows from the cell.
LEAD ACID BATTERY
Components of a Lead Acid Battery:
Positive plate:
PbO2 (lead peroxide), deposited
on a grid frame of antimony
lead alloy. (When the battery is
in fully charged condition, the
positive plate is dark brown in
colour).
Negative plate:
Pb (porous spongy lead), deposited
on a grid frame (similar to the grid
frame of positive plate). When the
battery is in charged condition, the
negative plate is grey in colour.
The number of negative plates in every battery is always one more than the number
of the positive plates so that action occurs on both sides of the positive plate.
Separator:
The function of the separator is to keep the positive and negative plates electrically
apart.
Container:
The container is made of hard glass or hard rubber or other acid resistant materials.
Chemistry of a Lead Acid Battery:
Negative Plate (Pb):
When the cell is producing current, lead atoms on the surface of the plate lose two
electrons each, becoming Pb++ ions. These Pb++ ions do not dissolve into the
liquid, but remain on the plate and attract SO4-- ions from the sulphuric acid
solution, thus forming an invisibly thin layer of PbSO4 on the negative lead plate.
Positive Plate (PbO2):
The positive plate consists of lead peroxide (PbO2), in which each lead particle is
lacking four electrons, which were given to the oxygen when the plate was formed.
Each Pb++++ ion takes two electrons from the external circuit, becoming Pb++.
The energy is obtained from the tendency of neutral lead atoms to give 2 electrons
each to Pb++++ ions, both becoming Pb++ as a result of the transfer.
Incidentally, when Pb++++ ions of lead peroxide pick up the two electrons, they
can no longer hold the oxygen, which goes into the solid solution and combines with
hydrogen ions of the acid, forming water molecules. The lead Pb++ remains on the
plate and picks up SO4-- from the sulphuric acid solution, forming lead sulphate.
These actions are shown in Fig.
PbO₂+ 2H2SO4 + Pb ⇔ PbS04 + 2H2O + PbSO4 + electrical energy
EDISON AND NICKEL STORAGE BATTERIES :
Edison (Nickel-iron) Storage Battery:
The Edison is structurally stronger and lighter in weight than lead cells of the same
current rating. The negative plates consist of a nickeled-steel grid containing
powdered iron, with some FeO and Fe(OH)2. The iron is the source of the electrons,
which are attracted through the external circuit toward nickel ions, Ni++ and
Ni+++ on the positive plate. The positive plates are nickel-plated tubes containing a
mixture of nickel oxides and hydroxides, with flakes of pure nickel for increased
conductivity. The electrolyte is a 21% solution of KOH (potassium hydroxide,
caustic potash) which is chemically a base rather than an acid. The Edison and
nickel-cadmium cells are often called alkaline cells, referring to the nature of the
electrolyte.
The disadvantages of the Edison cell are (1) high initial cost, (2) high internal
resistance that limits maximum current, especially so when the cell is cold. These
disadvantages are enough to prevent its use in most situations. It is not damaged by
remaining in a discharged condition.
It is used in some portable lighting equipment and in a few marine installations,
where it neither gets nor needs the attention that lead cells would.
The Edison battery is appropriate for running electrical traction equipment, such as
mine locomotives and fork-lift trucks, but not appropriate for starting gasoline and
Diesel engines, because its internal resistance limits the current too much.
Nickel-cadmium battery:
The nickel-cadmium battery followed a line of development that produced a battery
not intended for frequent cycling, but rather a more general-purpose battery that
enabled the user to draw as many amperes as possible from a battery of given
ampere-hour rating, without excessive falling-off of voltage. The starting of gasoline
and Diesel engines and the operation of signals, relays and controls are jobs for
which the nickel-cadmium battery is suited.
CAPACITY OF A BATTERY:
The capacity of a battery is given in terms of ampere-hours on discharge. This is
determined by the following factors:
- Final limiting voltage of the cell
- Discharge rate
- Number,design and dimensions of plates
- Design of separators
- Quantity of electrolyte
- Density of electrolyte
- Temperature etc
EFFICIENCY OF A BATTERY:
The efficiency of a battery is defined as,
"the ratio of the output of a cell or a
battery to the input required to restore the initial state of charge under specified
conditions of temperature, current rate and final voltage".
The following points are worth noting:
1. Matching a cell means making the lead resistance equal to the generator's
internal resistance. The result is maximum power delivered to the load from the cell.
2. A constant-voltage source has a very low internal resistance. Output voltage is
relatively constant with changing values of load because of small internal voltage
drops.
3. A constant-current source has a very high internal resistance. This determines the
constant value of current in the source circuit relatively independent of the load
resistance.
| SNO. | Aspects | Lead acid cell | Alkaline Cell |
|---|---|---|---|
| 1. | Positive plate | Lead peroxide (PbO2). Dark chocolate brown in colour | Perforated steel tubes into which is placed nickel hydroxide |
| 2. | Negative plate | Spongy lead (Pb). Dark grey in colour | Steel grid into the pocket of which is placed powdered iron oxide |
| 3. | Electrolyte | Dilute solutionof sulphuricacid (H2SO4) | Dilute solution of caustic potash (KOH) into which a small quantity of lithium hydroxide is added |
| 4. | Average e.m.f. of the cell | 2.0 V | 1.2 V |
| 5. | Life | About 12 years | About 5 years. |
| 6. | Cost | Cheaper than alkaline cell | Costlier than the lead-acid cell |
| 7. | Internal resistance | Low | High |
| 8. | Trickle charge | Cells when not in use must be put on trickle charge | No need of trickle charge |
| 9. | Weight per kWh | More weight | Lighter |
| 10. | Discharged condition | Should not be left in the discharged condition | Can be left in the discharged condition |
| 11. | Short circuits | With short circuits the life of the cell decrease to much low value | Short circuits do not reduce the life |
| 12. | Advantages | Used more in practice because of higher ampere-hour capacities and voltages and higher efficiencies | (i) Mechanically more sturdy
(ii) Do not evolve obnoxious fumes (iii) Less maintenance (iv) The plates do not buckle or swell |