Electric Cell : Properties | Types

What is Electric Cell?

Ans: An electric cell (or electrochemical cell) is a device that converts chemical energy (or light/thermal energy) into electrical energy. It acts as a source of direct current (DC) by maintaining a steady potential difference across a closed circuit.

1. Electrical properties of a Cell:

There are three fundamental electrical properties of electric cell:

i) Electromotive Force (EMF, $E$): The work done by source in taking a unit charge once round the complete circuit.

$E= \frac{W}{q}$

ii) Terminal Voltage ($V$): The actual potential difference across the cell's terminals when it supplies current $I$ to an external load resistance $R$ (closed circuit).

iii) Internal Resistance ($r$): The inherent opposition offered by the electrolyte and electrodes inside the cell to the flow of current.Discharging Equation: When a cell supplies current $I$ to an external circuit:

$V = E - I r$

Charging Equation: When an external source charges the cell:

$V = E + I r$

2. Types of Cells

Cells are classified based on their underlying mechanism, chemical reversibility, and application.

A. Primary Cells (Non-Rechargeable): In primary cells, the internal chemical reaction is irreversible. Once the active chemical reactants are consumed, the cell can no longer generate electricity.

Working Principle: Direct conversion of chemical energy into electrical energy via non-reversible redox reactions.

Characteristics: High energy density, low initial cost, lightweight, but cannot be recharged.

Examples:

Simple Voltaic Cell: Copper anode (+) and Zinc cathode (-) in dilute sulfuric acid ($\text{H}_2\text{SO}_4$).

Daniell Cell: Copper in $\text{CuSO}_4$ solution and Zinc in $ZnSO_4$ solution separated by a porous pot or salt bridge.

Dry Cell (Leclanché Cell): Zinc container (-) with a central carbon rod (+) surrounded by manganese dioxide ($\text{MnO}_2$) and ammonium chloride ($\text{NH}_4\text{Cl}$) paste. Used in remotes and clocks.

B. Secondary Cells (Rechargeable / Accumulators): In secondary cells, the chemical reaction is reversible. Applying an external electrical current forces the reaction in reverse, restoring the chemical state (charging).

Working Principle: Reversible energy conversion (Electrical $\rightarrow$ Chemical during charging; Chemical $\rightarrow$ Electrical during discharging).

Characteristics: Reusable for hundreds to thousands of cycles, lower internal resistance, capable of supplying high currents.

Examples:

Lead-Acid Accumulator: Used in automobiles and home backup power (inverters).

Lithium-Ion (Li-ion) Cell: Used in smartphones, laptops, and electric vehicles (EVs).

Nickel-Cadmium (NiCd) & NiMH Cells: Common in power tools and rechargeable AA/AAA batteries.

C. Fuel Cells: Fuel cells continuously convert the chemical energy of a fuel supplied from an external tank (such as hydrogen) and an oxidant (oxygen) directly into electricity.

Working Principle: Continuous supply of fuel $\rightarrow$ continuous electric current and water byproduct.

Characteristics: High operating efficiency ($\approx 60-70\%$), zero greenhouse gas emissions.

D. Photovoltaic (Solar) Cells: Unlike electrochemical cells, solar cells convert light energy (photons) directly into electrical energy via the photovoltaic effect in semiconductor $p\text{-}n$ junctions (typically silicon), without relying on chemical reactions.