Q.Suggest two materials other than hydrogen that can be used as fuels in fuel cells.

Concept: Fuel Cell Fuels – A fuel cell requires a substance that can be oxidised at the anode to release electrons. While hydrogen is the most common, other fuels can be used if they can be efficiently oxidised and their byproducts managed.

Reasoning:

1. A fuel must be able to donate electrons at the anode, typically through a reaction with oxygen or air at the cathode.
2. The fuel should be readily available, have a high energy density, and produce manageable waste products (ideally water or carbon dioxide). …

Fuel cells work like a continuous electrochemical battery — they don’t store energy internally but consume a fuel and an oxidant (usually oxygen from air) to produce electricity, water, and heat. The key is that the fuel must be oxidised at the anode, releasing electrons that flow through an external circuit.

Hydrogen is the most famous fuel because it gives only water as a product, but it’s not the only option. Any substance that can be oxidised electrochemically at the anode can serve as a fuel, provided the cell’s catalyst and membrane are compatible. The choice of fuel affects the cell’s operating temperature, efficiency, and practical applications.

Let’s look at two widely used alternatives.

1. Methanol ($CH_3OH$) Methanol is a liquid at room temperature, which makes it far easier to store and transport than hydrogen gas. In a Direct Methanol Fuel Cell (DMFC), methanol reacts with water at the anode:

The protons travel through the membrane, and at the cathode oxygen combines with them and the returning electrons to form water:

The overall reaction is:

Methanol has a high energy density (about 4.8 kWh/L) and is used in portable electronics and small vehicles. A common pitfall is that methanol can cross over the membrane, reducing efficiency — this is why DMFCs often use thicker membranes or dilute methanol solutions.

2. Methane ($CH_4$) — the main component of natural gas …

Method: Fuel Selection Based on Electrochemical Reactivity & Storage Feasibility

This method evaluates candidate fuels by their oxidation potential, energy density, and practical handling at standard conditions.

Steps

1. Identify the core requirement

   The fuel must undergo electrochemical oxidation at the anode, releasing electrons to the external circuit. It should not require extreme temperatures or pressures for storage.

2. Screen for high energy density

   Prefer fuels with high hydrogen content per unit mass/volume (since most fuel cells oxidize hydrogen indirectly).

3. Check for direct or indirect oxidation

   Some fuels can be fed directly (e.g., methanol in DMFC), others need a reformer to produce hydrogen first.

4. Verify safety & infrastructure

   The fuel should be storable, transportable, and non-toxic enough for practical use.

Two Alternative Fuels

1. Methanol ($\text{CH}_3\text{OH}$)

• Used in: Direct Methanol Fuel Cells (DMFC)
• Why it works:
  • Liquid at room temperature → easy to store and handle
  • High energy density (~$4.8\ \text{kWh/L}$)
  • Anode reaction:

• Limitation: Lower efficiency than hydrogen; crossover through membrane reduces performance.

2. Methane ($\text{CH}_4$, natural gas)

• Used in: Solid Oxide Fuel Cells (SOFC), operating at 600-1000°C
• Why it works: …

Here are the common mistakes students make on this question, along with clear strategies to avoid each.

Mistake 1: Naming fuels that cannot be used directly in a fuel cell

Students often write coal, petrol, diesel, or wood. These are combustion fuels — they burn with oxygen, but they do not undergo the electrochemical oxidation required inside a fuel cell.

How to avoid:

Remember that a fuel cell needs a fuel that can be oxidised electrochemically at the anode. The fuel must be a gas or a liquid that can be fed continuously and react without burning.

✓ Correct examples: Methanol ($CH_3OH$), ethanol ($C_2H_5OH$), methane ($CH_4$), or even ammonia ($NH_3$).

Mistake 2: Writing only hydrogen and then repeating it

Some students write “hydrogen” as one answer and “hydrogen gas” as the second — this is the same material and will get zero marks for the second.

How to avoid:

List two distinct substances. The question explicitly says “other than hydrogen”.

✓ Correct pair: Methanol and methane (or ethanol and ammonia, etc.).

Mistake 3: Confusing fuel with oxidiser

Students sometimes write “oxygen” or “air” as a fuel. Oxygen is the oxidiser at the cathode, not the fuel.

How to avoid:

Use the mnemonic: Fuel goes to the Anode, Oxidiser goes to the Cathode.

If it’s not consumed at the anode, it’s not a fuel.

Mistake 4: Giving materials that are impractical or not standard in exams …