Comparing Fuel Cells with Batteries and Half Equations Revision Notes | GCSE AQA Higher Chemistry

1. How Fuel Cells Actually Work

Picture a car that never needs charging overnight, just a quick refuel with hydrogen gas. That's the promise of fuel cells - but how do they actually convert hydrogen into electricity?

A hydrogen fuel cell works like a battery that never runs flat, as long as you keep feeding it hydrogen and oxygen. The hydrogen enters at one electrode (the anode), oxygen enters at the other (the cathode), and electricity flows out through an external circuit.

The key difference from a battery is continuous operation. Batteries store energy inside their structure - when that's used up, they're flat. Fuel cells generate energy from external fuel, so they keep working as long as hydrogen flows in.

2. The Chemistry Behind Fuel Cells

Now let's see exactly what happens at each electrode. The overall reaction is deceptively simple: hydrogen plus oxygen makes water and electricity. But the devil is in the detail.

At the anode (negative electrode), hydrogen molecules split apart and lose electrons. At the cathode (positive electrode), oxygen gains those electrons and combines with hydrogen ions to form water. The electrons flow through the external circuit, creating our useful electric current.

Remember the memory trick: AN OX, RED CAT. Anode = Oxidation (lose electrons), Cathode = Reduction (gain electrons). This applies to all electrochemical cells, not just fuel cells.

3. Writing Half Equations for Fuel Cells

Half equations show exactly what happens at each electrode. They're like zooming in on one half of the reaction. For hydrogen fuel cells, we need to write separate equations for the anode and cathode reactions.

Worked example: Writing Half Equations for a Hydrogen Fuel Cell

Write the half equations for the electrode reactions in a hydrogen fuel cell operating in alkaline conditions.

1. Hydrogen is oxidised (loses electrons) — Anode = oxidation. Hydrogen molecules split and lose electrons.
2. $H_2 + 2OH^- \rightarrow 2H_2O + 2e^-$ — Each H₂ molecule loses 2 electrons. In alkaline conditions, OH⁻ ions are present.
3. Oxygen is reduced (gains electrons) — Cathode = reduction. Oxygen gains electrons from the external circuit.
4. $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$ — Each O₂ molecule gains 4 electrons. Water molecules provide hydrogen atoms.
5. Anode produces 2e⁻ per H₂, cathode needs 4e⁻ per O₂ — Need 2 × anode reaction for every 1 × cathode reaction to balance electrons.

Answer: Anode: $H_2 + 2OH^- \rightarrow 2H_2O + 2e^-$ Cathode: $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$

Notice how the electrons balance out - the anode produces exactly the electrons the cathode consumes. This is crucial for any electrochemical cell to work properly.

4. Comparing Fuel Cells with Batteries

Both fuel cells and batteries convert chemical energy to electrical energy, but they do it very differently. Understanding these differences helps explain why we use each technology in different situations.

Think of a battery like a sealed lunch box - it contains all the energy it will ever have. Once you've eaten the lunch, the box is empty until you refill it (recharge). A fuel cell is like having a continuous supply line - as long as fuel keeps arriving, it keeps producing energy.

This fundamental difference affects everything: how long they last, how you refuel them, their environmental impact, and their cost. Let's break down the key comparisons that examiners love to test.

Worked example: Systematic Comparison: Fuel Cells vs Rechargeable Batteries

Compare hydrogen fuel cells with lithium-ion rechargeable batteries across key performance criteria.

1. Fuel cells: External fuel supply Batteries: Internal chemical storage — This is the fundamental difference that drives all other comparisons.
2. Fuel cells: 3-5 minutes (like petrol) Batteries: 30 minutes to 8 hours — Fuel cells win on convenience - just fill up with hydrogen gas.
3. Fuel cells: Continuous (with fuel supply) Batteries: Limited by charge cycles — Batteries degrade after ~1000 charge cycles. Fuel cells don't have this limitation.
4. Fuel cells: Only water as waste product Batteries: Toxic materials, mining impact — Fuel cells can be cleaner in use, but the source of hydrogen also matters.
5. Fuel cells: Hydrogen filling stations needed Batteries: Electrical grid (already exists) — Batteries have the advantage here - we already have electricity everywhere.

Answer: Fuel cells excel in refueling speed and continuous operation, while batteries benefit from existing infrastructure and lower setup costs.

5. Real-World Applications and Limitations

Understanding the theory is one thing, but where do we actually use fuel cells versus batteries? The choice depends on the specific requirements of each application.

Fuel cells excel where you need long-range operation and quick refueling - think buses, trucks, and trains that run all day. Batteries work better for shorter trips with overnight charging - like most cars, phones, and laptops.

The biggest challenge for fuel cells isn't the technology itself - it's the infrastructure. Hydrogen is tricky to store and transport safely, and we need a whole network of hydrogen filling stations. Batteries use the electrical grid we already have.

6. Exam Technique and Common Mistakes

Exam questions on fuel cells often test your ability to evaluate - weighing up advantages and disadvantages. They also love testing half equations, which many students find tricky.

The most common mistake is confusing which electrode does what. Students often write hydrogen being reduced at the cathode - but hydrogen is oxidised at the anode. Use the memory trick and double-check your electron flow.

Another trap is forgetting that fuel cell efficiency depends on how the hydrogen is made. If hydrogen comes from fossil fuels, the overall environmental benefit is much smaller than if it comes from renewable electricity splitting water.