Cells and Batteries (Basics)

Basic Structure of Cells

A chemical cell is a device that converts chemical energy into electrical energy through chemical reactions. It consists of:

• Two electrodes: These are conductors, usually metals, where the chemical reactions occur. The electrode where oxidation happens is called the anode, and the electrode where reduction happens is the cathode.
• Electrolyte solution: This is a liquid or paste containing ions that allow the flow of charge between the electrodes, completing the circuit.

During the chemical reactions, electrons are transferred from one electrode to the other through an external circuit, producing an electric current.

For example, in a simple zinc-carbon cell, zinc acts as the anode and carbon as the cathode, with an acidic paste as the electrolyte.

Example: If a single cell has a voltage of 1.5 V and you connect 2 cells in series, the total voltage is: $1.5 \times 2 = 3.0\, \text{V}$.

How Batteries Work

A battery is made by connecting multiple cells together to increase the voltage and provide more electrical energy. The total voltage of a battery depends on:

• The type of cell used (different chemical reactions produce different voltages).
• The number of cells connected in series (voltages add up).

Each cell converts chemical energy stored in its chemicals into electrical energy until the chemicals are used up.

For instance, a 12 V car battery contains six 2 V cells connected in series.

The chemical reactions inside the cells cause electrons to flow through the external circuit, powering devices like torches, phones, or cars.

The voltage of a cell is a measure of the energy transferred per unit charge passed through the circuit.

Example: If a single cell has a voltage of 1.5 V and you connect 4 cells in series, the total voltage is:

$1.5 \times 4 = 6.0\, \text{V}$

Common Types of Cells

Cells can be divided into two main types:

• Non-rechargeable cells (primary cells): These cells cannot be recharged because the chemical reactions are irreversible. Once the chemicals are used up, the cell stops working. An example is the alkaline cell commonly used in remote controls and clocks.
• Rechargeable cells (secondary cells): These cells can be recharged by passing an electric current through them, reversing the chemical reactions. Examples include lithium-ion cells used in mobile phones and laptops.

The main difference lies in the reversibility of the chemical reactions inside the cell.

Rechargeable cells are more expensive but can be used many times, making them more economical and environmentally friendly in the long run.

Example: In an alkaline cell, zinc is oxidised and manganese dioxide is reduced, but the reaction cannot be reversed. In a lithium-ion cell, lithium ions move between electrodes during discharge and recharge.

Simple Fuel Cells

A simple fuel cell is a device that produces electrical energy from the reaction of a fuel with oxygen, without burning the fuel. The most common example is the hydrogen-oxygen fuel cell.

• Fuel: Hydrogen gas (H₂)
• Oxidant: Oxygen gas (O₂) from the air
• Products: Water (H₂O) and electrical energy

The hydrogen fuel cell works by oxidising hydrogen at the anode, producing electrons and hydrogen ions. The electrons flow through an external circuit, providing electricity, while the hydrogen ions move through the electrolyte to the cathode, where they react with oxygen and electrons to form water.

The overall reaction is:

$2H_2 + O_2 \rightarrow 2H_2O$

The half-equations are:

Anode (oxidation): $2H_2 \rightarrow 4H^+ + 4e^-$

Cathode (reduction): $O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$

Advantages of fuel cells compared to batteries:

• They can produce electricity continuously as long as fuel (hydrogen) and oxygen are supplied.
• They are more efficient and produce less pollution (only water as a by-product).
• They are lighter and can be refuelled quickly, unlike batteries which need recharging.

Fuel cells are used in some electric vehicles and spacecraft due to these benefits.

Example: In a hydrogen fuel cell, hydrogen gas is supplied at the anode where it loses electrons (oxidation). The electrons flow through the circuit to the cathode, while hydrogen ions pass through the electrolyte. At the cathode, oxygen gas combines with electrons and hydrogen ions to form water.