Identify incorrect statement about $H_2-O_2$ fuel cell from the following:

The cell runs continuously even if reactants are not supplied

The correct answer is Option 3. The cell runs continuously even if reactants are not supplied.

A hydrogen-oxygen fuel cell is an electrochemical device that converts the chemical energy from hydrogen and oxygen into electrical energy, with water as the only byproduct. This technology is commonly used in various applications, including powering vehicles, stationary power generation, and portable electronic devices.

Basic Components of the Fuel Cell

Anode: The electrode where oxidation occurs (loss of electrons).

Cathode: The electrode where reduction takes place (gain of electrons).

Electrolyte: A medium (liquid or solid) that conducts ions, allowing movement between the anode and cathode.

The Electrochemical Reactions

At the Anode (Oxidation Reaction):

Hydrogen gas (H₂) is introduced at the anode:

\(2H_2 \rightarrow 4H^+ + 4e^-\)

In this reaction, hydrogen molecules are oxidized to form protons (H⁺) and electrons (e⁻). The electrons flow through an external circuit, generating electricity.

At the Cathode (Reduction Reaction):

At the cathode, oxygen (O₂) is introduced, where it undergoes a reduction reaction:

\(O_2 + 4e^- + 2H_2O \rightarrow 4OH^-\)

Oxygen combines with electrons and water to form hydroxide ions (OH⁻) in alkaline fuel cells.

Importance of Reactants

Continuous Supply: For the fuel cell to operate, hydrogen and oxygen must be supplied continuously. If either reactant runs out, the reaction halts, and the fuel cell stops producing electricity. This characteristic is crucial and differentiates fuel cells from batteries, which store energy until used.

Flow of Electrons: The flow of electrons from the anode to the cathode through an external circuit is what generates electrical energy. The chemical reactions at both electrodes facilitate this flow.

Analyzing the Statements

1. At cathode O₂ is reduced to OH⁻:

This statement accurately describes what occurs at the cathode in alkaline fuel cells. Oxygen is indeed reduced to hydroxide ions.

2. At anode H₂ is oxidised to form water:

This statement is true but may be misleading. While hydrogen is oxidized at the anode, it primarily generates protons and electrons. Water is formed at the cathode when the protons combine with oxygen.

3. The cell runs continuously even if reactants are not supplied:

This statement is false. Fuel cells need a continuous supply of hydrogen and oxygen to keep functioning. Once the reactants are depleted, the fuel cell will stop operating, as the electrochemical reactions cannot occur without them.

4. Water is formed as a byproduct:

The statement is correct. Water is the main product of the reaction, which is one of the advantages of using hydrogen fuel cells, as they produce no harmful emissions.

Conclusion

The operation of an H₂-O₂ fuel cell relies entirely on a continuous supply of hydrogen and oxygen to generate electricity. The incorrect statement highlights a common misconception about fuel cells; they cannot operate indefinitely without reactants. Understanding this is key to appreciating how fuel cells work and their applications in energy generation.