Question

Explain how nicad batteries operate. What other uses are made of cadmium?

Short answer

NiCd batteries operate through reversible chemical reactions that convert between chemical and electrical energy. Cadmium is also used in pigments, coatings, and plastics due to its color and corrosion resistance.

Step-by-step solution

Introduction to NiCd Batteries

Nickel-cadmium (NiCd or NiCad) batteries are a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. They are known for their longevity and ability to deliver full charge at high currents.

Chemical Reactions Within NiCd Batteries

During discharge, the cadmium (Cd) at the negative electrode is oxidized to cadmium hydroxide \[ \text{Cd} + 2 \text{OH}^- \rightarrow \text{Cd(OH)}_2 + 2 \text{e}^- \]while the nickel oxyhydroxide (NiO(OH)) at the positive electrode is reduced to nickel hydroxide (Ni(OH)₂): \[ \text{2 NiO(OH)} + 2 \text{H}_2\text{O} + 2 \text{e}^- \rightarrow 2 \text{Ni(OH)}_2 + 2 \text{OH}^- \]. This converts chemical energy to electrical energy.

Charging Process of NiCd Batteries

When the battery is recharged, the chemical reactions reverse. Cadmium hydroxide is reduced back to cadmium, and nickel hydroxide is oxidized back to nickel oxyhydroxide, restoring the electrodes to their original state for another cycle of use.

Uses of Cadmium Outside of Batteries

Beyond batteries, cadmium is used in various applications such as in pigments, coatings, and as a stabilizer in plastics. It is utilized due to its corrosion-resistant properties and ability to produce vibrant colors.

Environmental Concerns and Alternatives

Cadmium is toxic and poses environmental and health risks, prompting a decline in its use and an increase in the development of alternative technologies such as nickel-metal hydride (NiMH) and lithium-ion batteries which do not contain cadmium.

Key concepts

Chemical Reactions

Nickel-cadmium batteries operate through carefully balanced chemical reactions. During discharge, the reactions take place at two electrodes. At the negative electrode, cadmium (Cd) is oxidized to form cadmium hydroxide (Cd(OH)_2). This reaction releases electrons that travel through the circuit to perform work:

• \( \text{Cd} + 2 \text{OH}^- \rightarrow \text{Cd(OH)}_2 + 2 \text{e}^- \)

At the positive electrode, nickel oxyhydroxide (NiO(OH)) is reduced to nickel hydroxide (Ni(OH)_2):

• \( \text{2 NiO(OH)} + 2 \text{H}_2\text{O} + 2 \text{e}^- \rightarrow 2 \text{Ni(OH)}_2 + 2 \text{OH}^- \)

This conversion of chemical energy into electrical energy is what powers devices. When the chemical reactions occur, the battery releases electrical energy, allowing electronic devices to function smoothly.

Charging Process

Charging a nickel-cadmium battery involves reversing the chemical reactions that occurred during discharge. During charging:

• The cadmium hydroxide at the negative electrode is converted back to cadmium.
• Nickel hydroxide at the positive electrode is oxidized back to nickel oxyhydroxide.

These reverse reactions are achieved by applying an electrical current from a charger through the battery. This process restores the electrodes to their initial state, readying the battery for another cycle of use. The ability to recharge multiple times is what makes NiCd batteries so valuable in portable applications. However, it's important to avoid overcharging as it can reduce battery life. Modern chargers typically include safeguards to prevent overcharging, ensuring battery longevity.

Environmental Concerns

Cadmium is a heavy metal and poses significant environmental and health risks when not handled properly. The NiCd batteries are among the key contributors to cadmium pollution. If these batteries are not disposed of correctly, cadmium can leach into the soil, contaminating water supplies and endangering ecosystems.

Given these concerns:

• Recycling programs have been established to responsibly manage NiCd battery disposal.
• Strict regulations often govern cadmium emissions in industrial processes.

Additionally, the development of alternative rechargeable battery technologies like nickel-metal hydride (NiMH) and lithium-ion batteries, which do not use cadmium, is reducing dependence on NiCd batteries. These alternatives provide similar or even superior performance without the associated environmental risks.

Cadmium Applications

While cadmium's use in batteries is well-documented, this metal has several other applications due to its unique properties. Its corrosion-resistant qualities make cadmium valuable for:

• Protective coatings on metals, which help prevent rust and extend the lifespan of products.
• In pigments, cadmium provides vibrant colors used in plastics, ceramics, and glasses.
• As stabilizers in plastics, cadmium compounds help maintain material strength and flexibility.

Despite these advantages, its toxicity has led to decreased use, particularly in consumer products. Safer alternatives are being sought to minimize cadmium exposure while still achieving reliable production quality in various industries.