Fuel cells - electricity from without burning:

A series of experiments comparing fuel cell operation with ordinary burning.

Introduction

The purpose of these demonstrations or experiments is to compare what is going on in the fuel cell with ordinary burning. The point to get over is that the fuel is chemically reacting with oxygen in the air, but that electricity is being produced instead of heat and light.

These experiments can be done by the pupils, if a suitable oxygen supply is available. Alternatively they can be done as a teacher demonstration.

The experiment/demonstrations are very short, and even with the pupils doing their own experiments, and then writing them up, all should be finished within 35 minutes.

The current produced during this experiment will be strongly dependent on the resistance of the meter used. However, in all cases the proportional change in current will be much the same whatever meter is used. An important feature of this fuel cell is that no harm will be done if the cell is "short circuited" by an ammeter, even if the "short circuit" is maintained for weeks on end.

Apart from the electrolyte, and fuel, the other apparatus and materials needed are:-

• liquid fuel anode
• air cathode
• ammeter (0-1 A)
• 2 connecting wires
• water
• a supply of oxygen
• a supply of carbon dioxide (optional)

Instructions

Take care, this experiment involves the use of methanol (which is poisonous), and KOH solution (which is corrosive).

1) Fill the liquid fuel anode up to the fill line KOH electrolyte (~1 M) BUT DO NOT PUT ANY FUEL IN.

2) Connect the ammeter (0-1A) to the cell, and record the current. (It should be zero.)

3) Add about 10 cm³ of methanol to the cell. Observe the current. (It should rise to about 0.3A). Conclusion. The current is caused by some reaction involving the methanol.

4) Cut off the air supply to the upper electrode (the cathode) by pouring a little water into the air cathode, as shown in the diagram below. Record the effect on the current. (It should fall, after a few seconds, to just above zero.) Conclusion. The reaction causing the current also involves air, just like burning.

Note. The current does not fall completely to zero because of the dissolved oxygen in the electrolyte and the water. Note also that the current does not fall instantly because of the electric charge stored in the pores of the electrodes.

5) Pour the water out of the cathode holder, taking care to do this over the sink, so that any drips of electolyte from the cathode do no damage.

6) Get the cell working again. Direct some oxygen into the space above the cathode. Being denser than air, the oxygen will stay in the tube above the electrode. Observe the effect on the current. It should increase noticeably. Conclusion. The reaction causing the current is between the methanol and the oxygen in the air.

7) Discuss with the students the comparisons between this reaction and the burning of methanol. There are similarities, but electricity is produced instead of heat and flame. If necessary repeat one or two of the dramatic "burning things in oxygen " type demonstrations.

8) The comparison with burning could be reinforced by "extinguishing" the fuel cell with CO₂. Direct CO₂ gently into the space above the cathode. The current will be observed to fall to zero, though it will rise again when the CO₂ disperses.