Which one of the following pairs of materials serves as electrodes in chargeable batteries commonly used in devices such as torchlights, electric shavers, etc?

1. Fundamentals of Electrochemical Cells (basic)

At its heart, an electrochemical cell is a clever device that converts stored chemical energy into electrical energy. Imagine it as a bridge between the world of matter and the world of power. Every cell, regardless of its size or shape, requires three fundamental components to function: two electrodes (solid conductors) and an electrolyte (a substance that allows charge to flow). When these components interact, a chemical reaction occurs that pushes electrons through a circuit, creating the electricity we use to power everything from wall clocks to satellites.

The simplest version is the Voltaic cell (also called a Galvanic cell). It consists of two plates made of different materials, such as copper and zinc, partially dipped into a liquid electrolyte like a weak acid or salt solution Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p. 55. Because the two metals have different "appetites" for electrons, a chemical tug-of-war begins. This creates a potential difference, causing current to flow from the positive terminal to the negative terminal through the external circuit. However, once the chemicals are exhausted, the cell becomes 'dead' and cannot be used again.

To make cells portable and safer for everyday use, scientists developed the dry cell. Instead of a messy liquid, it uses a thick, moist paste as an electrolyte Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p. 57. In a standard dry cell, a zinc container acts as the negative terminal, while a carbon rod at the center serves as the positive terminal. While the chemistry is more contained, the principle remains the same: a chemical reaction facilitates the movement of charge.

Feature	Voltaic (Wet) Cell	Dry Cell
Electrolyte Form	Liquid (e.g., dilute acid)	Moist paste
Portability	Low (risk of leakage/spillage)	High (sealed and compact)
Common Use	Laboratory demonstrations	Torches, remotes, toys

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.55; Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.57

2. Classification: Primary vs. Secondary Batteries (basic)

In our journey through everyday chemistry, we must understand that a battery is essentially a portable chemical factory. It converts chemical energy into electrical energy through reactions occurring at its electrodes. As we see in fundamental physics, a chemical reaction within the cell generates a potential difference between its two terminals, which sets electrons in motion to create an electric current Science, Class X (NCERT 2025 ed.), Electricity, p.188. However, not all chemical reactions are created equal; some are "one-way streets," while others can be reversed.

Batteries are classified into two primary categories based on this reversibility:

• Primary Batteries: These are designed for single use. The chemical reaction occurs only once, and after a period of use, the battery becomes "dead" because the reactants are exhausted. Common examples include the dry cells used in wall clocks or TV remotes.
• Secondary Batteries: These are rechargeable. By passing an external electric current through the battery in the opposite direction, the internal chemical reaction is reversed, restoring the cell's ability to provide power. This makes them ideal for high-drain, modern devices.

A classic example of a secondary battery is the Nickel-Cadmium (Ni-Cd) cell. In this system, nickel oxide/plate serves as the positive electrode and cadmium serves as the negative electrode. Historically, these were the go-to power sources for portable tools, torchlights, and electric shavers because they could be recharged hundreds of times Science, Class VIII (NCERT 2025 ed.), Electricity: Magnetic and Heating Effects, p.61. While newer technologies like Lithium-ion are now more common, understanding Ni-Cd is vital because cadmium is a heavy metal, making the proper recycling of these batteries a significant environmental concern.

Feature	Primary Batteries	Secondary Batteries
Rechargeability	Non-rechargeable (Use-and-throw)	Rechargeable (Multiple cycles)
Chemical Reaction	Irreversible	Reversible
Common Examples	Alkaline cells, Zinc-Carbon cells	Lead-acid, Li-ion, Ni-Cd

Sources: Science, Class X (NCERT 2025 ed.), Electricity, p.188; Science, Class VIII (NCERT 2025 ed.), Electricity: Magnetic and Heating Effects, p.61

3. Common Non-Rechargeable Cells (Dry Cells) (intermediate)

In our journey through everyday chemistry, we must understand how we transitioned from the bulky, liquid-filled Voltaic cells to the portable powerhouses we use in remote controls and wall clocks today. While early cells used liquid electrolytes that were prone to leaking, the Dry Cell revolutionized portability by using a thick moist paste instead of a free-flowing liquid Science, Class VIII, Electricity: Magnetic and Heating Effects, p.57. These are classified as Primary Cells, meaning the chemical reactions inside are irreversible; once the chemicals are exhausted, the cell is 'dead' and cannot be recharged Science, Class VIII, Electricity: Magnetic and Heating Effects, p.55.

The most common type of dry cell is the Zinc-Carbon cell. Its anatomy is a masterpiece of simple engineering. The outer Zinc container serves a dual purpose: it holds the ingredients and acts as the negative terminal (anode). At the very center sits a Carbon (graphite) rod, which acts as the positive terminal (cathode). Surrounding this rod is a mixture of Manganese Dioxide (MnO₂) and powdered carbon. The Manganese Dioxide is crucial because it prevents the buildup of hydrogen gas bubbles, which would otherwise insulate the rod and stop the current Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.29.

The space between the zinc walls and the central rod is filled with an electrolyte paste, typically consisting of ammonium chloride (NH₄Cl) and zinc chloride. When you flip a switch, a chemical reaction triggers: the zinc atoms lose electrons (oxidation), which flow through your device toward the carbon rod. This flow of electrons is the electricity that powers your gadgets. Because the zinc container is actually consumed during this process, very old batteries can sometimes 'leak' as the outer shell thins out and develops holes.

Component	Material Used	Function
Negative Terminal	Zinc (Zn) Container	Source of electrons (Anode)
Positive Terminal	Carbon (Graphite) Rod	Current collector (Cathode)
Electrolyte	Ammonium Chloride Paste	Medium for ion movement
Depolarizer	Manganese Dioxide (MnO₂)	Prevents gas buildup

Sources: Science, Class VIII . NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.57; Science, Class VIII . NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.55; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Distribution of World Natural Resources, p.29

4. Modern Energy Storage: Lithium-ion and Lead-Acid (intermediate)

To understand modern energy storage, we must first distinguish between Primary cells (single-use) and Secondary cells (rechargeable). Secondary batteries operate on reversible chemical reactions, allowing us to restore their electrical potential by passing a current through them in the opposite direction. Two technologies dominate this space: the legacy Lead-Acid battery and the modern Lithium-ion (Li-ion) battery.

Lead-Acid batteries have been the reliable workhorse of the industrial world for over a century. They typically consist of electrodes made of Lead (Pb) and Lead Oxide (PbO₂) submerged in an electrolyte of sulfuric acid (H₂SO₄). While they are heavy and have low energy density (they store little energy relative to their weight), they are excellent at providing high "surge" currents, which is why they are still the standard for starting internal combustion engines in cars Science, Class VIII, p. 61. However, lead is a toxic heavy metal, making proper disposal critical to prevent environmental contamination.

In contrast, Lithium-ion (Li-ion) batteries represent the cutting edge of portable power. These batteries rely on the movement of lithium ions between a cathode (often containing metals like Cobalt, Nickel, or Manganese) and an anode (usually graphite) Science, Class VIII, p. 58. Their primary advantage is high energy density—they can store a large amount of energy in a very small, lightweight package, which is why they power almost all modern smartphones, laptops, and Electric Vehicles (EVs).

Feature	Lead-Acid Battery	Lithium-ion (Li-ion) Battery
Primary Use	Car starters, UPS systems, Backup power	Phones, Laptops, Electric Vehicles
Energy Density	Low (Heavy/Bulky)	High (Light/Compact)
Key Materials	Lead, Sulfuric Acid	Lithium, Cobalt, Graphite
Environmental Impact	High (Toxic Lead)	Moderate (Mining issues, but recyclable)

As we transition toward green energy, the focus is shifting to battery sustainability. A battery is never truly "dead"; even when it stops holding a charge, it contains valuable and potentially hazardous metals like lithium, cadmium, or lead Science, Class VIII, p. 61. Scientists are now developing Solid-state batteries, which replace liquid electrolytes with solid materials to make batteries safer, faster-charging, and longer-lasting Science, Class VIII, p. 58.

Sources: Science, Class VIII, NCERT (2025), Electricity: Magnetic and Heating Effects, p.58, 61

5. Environmental Impact and E-Waste Management (exam-level)

Electronic waste, or e-waste, represents one of the most complex challenges in applied chemistry because it isn't just "trash"—it is a concentrated mix of hazardous toxins and valuable resources. In India, we generate approximately 17 lakh tonnes of e-waste annually, a figure growing by 5% each year Environment, Shankar IAS Academy, Environmental Pollution, p.94. When we discard devices like old phones or shavers, we are releasing heavy metals like Lead, Cadmium, and Nickel into the ecosystem. For instance, even when a battery seems "dead," it remains a chemical reservoir. Rechargeable batteries, such as Nickel-Cadmium (Ni-Cd) cells, utilize these specific metals as electrodes. If these reach a landfill, they don't just sit there; they can leach into groundwater. Specifically, if the soil becomes slightly acidic (a pH drop from 6.5 to 4.5), the leaching of Cadmium can increase fivefold, potentially leading to severe renal tubular damage in humans who consume contaminated water Environment, Shankar IAS Academy, Environmental Pollution, p.105.

Beyond the immediate toxicity, the chemistry of e-waste management involves a "cradle-to-grave" approach known as Extended Producer Responsibility (EPR). Under the E-Waste Management Rules, the burden of collection and safe disposal is shifted from the consumer to the producers, manufacturers, and refurbishers Environment, Shankar IAS Academy, Environmental Pollution, p.95. This is vital because many components, like Lithium and Nickel, are rare and valuable. By creating specialized e-waste recycling facilities, we can recover these materials for reuse, preventing the environmental degradation caused by primary mining and improper disposal Science, Class VIII NCERT, Electricity: Magnetic and Heating Effects, p.61.

Heavy Metal	Common Source	Environmental/Health Impact
Lead	Solder, Lead-acid batteries	Neurophysiological dysfunction, especially in children.
Cadmium	Rechargeable Ni-Cd batteries	Kidney (renal) damage; leaching increases in acidic water.
Nickel	Rechargeable battery electrodes	Potential carcinogen and environmental toxin if mismanaged.

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.94, 95, 105; Science, Class VIII NCERT, Electricity: Magnetic and Heating Effects, p.61

6. The Nickel-Cadmium (Ni-Cd) System (exam-level)

At its heart, the Nickel-Cadmium (Ni-Cd) cell is a type of secondary cell, meaning it is a rechargeable battery where the chemical reactions can be reversed by applying an external electrical current. Unlike primary (single-use) batteries where the chemicals are simply 'used up' Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p. 55, a Ni-Cd battery allows us to restore its energy potential, making it a sustainable choice for high-drain portable devices like electric shavers and older cordless power tools.

The system operates using two specific metal-based electrodes. The negative electrode (anode) is made of metallic Cadmium (Cd), while the positive electrode (cathode) consists of Nickel oxide-hydroxide (NiO(OH)). These are immersed in an alkaline electrolyte, typically Potassium Hydroxide (KOH). When you use the device, a chemical reaction moves electrons from the cadmium to the nickel; when you charge it, this process is pushed in reverse to reset the chemistry Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p. 57.

Component	Material Used	Function
Positive Electrode	Nickel (NiO(OH))	Attracts electrons during discharge.
Negative Electrode	Cadmium (Cd)	Releases electrons during discharge.
Electrolyte	Potassium Hydroxide (KOH)	Facilitates ion movement.

While Ni-Cd batteries are incredibly robust and can handle high discharge rates, they have largely been superseded by Lithium-ion technology Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p. 58. A primary reason for this shift is environmental safety. Cadmium is a toxic heavy metal that poses significant risks to the environment and human health if the batteries are thrown into regular garbage instead of being processed at e-waste recycling facilities Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p. 61.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.55; Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.57; Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.58; Science, Class VIII. NCERT (Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.61

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of electrochemical cells and the distinction between primary and secondary batteries, this question asks you to apply that knowledge to real-world technology. The key lies in identifying which chemical pair allows for reversibility—the ability to be recharged—within the compact size required for portable devices like shavers. As we discussed, a secondary cell must have stable electrodes that can undergo repeated oxidation and reduction cycles without significant degradation.

To arrive at the correct answer, think about the specific constraints of the devices mentioned. While several combinations can store energy, the Nickel and cadmium (Ni–Cd) system was the pioneering technology for small-scale rechargeable power. According to Science, Class VIII, NCERT (Revised ed 2025), the nickel-cadmium cell uses a nickel-based positive electrode and a cadmium negative electrode. Because of its long cycle life and ability to deliver high current, it became the historical gold standard for handheld electronics. Therefore, (A) Nickel and cadmium is the correct choice.

UPSC often includes distractors that are correct in other contexts to test your precision. For instance, Zinc and carbon (Option B) are the components of the standard Leclanché dry cell, which is non-rechargeable (primary). Lead peroxide and lead (Option C) are indeed used in rechargeable batteries, but these are lead-acid batteries—far too heavy and bulky for a torch or a shaver, typically reserved for automobiles or inverters. Finally, Iron and cadmium (Option D) is a distractor designed to trip up students who have only partially memorized the specific secondary battery pairs.