4.7 Battery — A Source of Electrical Energy

Introduction

A battery is one of the most common applications of electrochemistry in everyday life. It is a galvanic cell (or a group of galvanic cells) that generates electric current through a redox reaction.

Batteries power many devices we use daily — from flashlights and remote controls to cars and spacecraft. This section explains what a battery is and describes a special type of galvanic cell called a fuel cell.

Definition

Battery: A battery is a galvanic cell, or a group of galvanic cells joined in series. It generates electric current by a redox reaction.

Fuel Cell: A fuel cell is a special type of galvanic cell which is based upon the reaction between oxygen and a gaseous fuel hydrogen.

In simple words:

• Battery = a galvanic cell (or group of cells) that produces electricity from a chemical reaction.
• Fuel cell = a special battery that uses hydrogen and oxygen to produce electricity, with water as the only chemical product.

Main Idea

• A battery is a galvanic cell or a group of galvanic cells joined in series.
• When connected in a circuit:
• Its anode is oxidized, releasing electrons.
• These electrons flow through the external circuit towards the cathode.
• At the cathode, these electrons reduce the oxidizing agent present in the electrolyte.
• Examples of batteries: dry cells, storage cells, mercury batteries, etc.

Fuel Cell:

• A fuel cell is a special type of galvanic cell based on the reaction between oxygen and hydrogen.
• When hydrogen burns in air, an exothermic reaction occurs and a lot of chemical energy is released in the form of heat and light.
• In this reaction, hydrogen is oxidized to water.

2H_2(g) + O_2(g) → 2H₂O_(g) + heat

- In a fuel cell, the energy released from the reaction of hydrogen with oxygen to form water is converted to electrical energy (not heat).

Core Concept

Key point: A fuel cell converts the chemical energy of the hydrogen-oxygen reaction directly into electrical energy, with water as the only chemical product.

Detailed Explanation

What is a Battery?

A battery is a galvanic cell, or a group of galvanic cells joined in series.

How it works:

• When connected in a circuit, its anode is oxidized, releasing electrons.
• These electrons flow through the external circuit towards the cathode.
• At the cathode, these electrons reduce the oxidizing agent present in the electrolyte.

Examples of batteries:

• Dry cells
• Storage cells
• Mercury batteries

What is a Fuel Cell?

A fuel cell is a special type of galvanic cell which is based upon the reaction between oxygen and a gaseous fuel hydrogen.

The reaction:

When hydrogen burns in air, an exothermic reaction occurs and a lot of chemical energy is released in the form of heat and light. In this reaction, hydrogen is oxidized to water.

2H_2(g) + O_2(g) → 2H₂O_(g) + heat

In a fuel cell, the energy released from the reaction of hydrogen with oxygen to form water is converted to electrical energy.

Structure of a Hydrogen-Oxygen Fuel Cell

A hydrogen-oxygen fuel cell has three compartments separated from one another by porous carbon electrodes.

• These electrodes contain platinum as a catalyst.
• The middle compartment contains a hot aqueous solution of KOH.
• Hydrogen gas is passed through the anode compartment.
• Oxygen gas is passed through the cathode compartment.

At the anode: Hydrogen is oxidized to water.

At the cathode: Oxygen is reduced to hydroxide ions.

Anode reaction: 2H_2(g) + 4OH^-_(aq) → 4H₂O_(g) + 4e^-

Key point: Hydrogen is oxidized at the anode, and oxygen is reduced at the cathode. The only chemical product is water.