Mini Fuel Cell - Instructions, School & College Version:

1. Basic Information

The mini fuel cell consists of two components. A red cathode, and a beaker-like anode, as shown in Figure 1 below.

Figure 1

An electrolyte connects the two electrodes. The electrolyte is about 65 cm³ of potassium hydroxide solution. The concentration of the KOH should be 1M, but this is not critical, and 4M can be used if a higher performance is required.

The fuel is mixed with the electrolyte. The cell can be used with a very wide range of fuels. These include:-

• Sodium tetrahydridoborate, NaBH₄, gives a very high performance.
• Methanol CH₃OH, is more readily available, and gives enough current to show the principle of a fuel cell.
• Methanal (formaldehyde) or methanoic (formic) acid.
• Ethanol C₂H₅OH
• Any ethanol containing beverage!
• Almost any alcohol, or alcohol containing fluid, such as automobile windscreen washer fluid.

To operate the cell, a small amount the fuel is added to the electrolyte, and dissolved in it. The performance depends strongly on temperature, but at about 20 ^oC, typical indicative figures are:-

Clearly the performance with NaBH₄ is vastly superior, but nevertheless the liquid fuels, which all give similar performance, do give enough current and voltage to illustrate fuel cell operation, and are usually more readily available. Only tiny quantities of NaBH₄ are used, and it is not expensive. Small quantities, sufficient for over 100 experiments, are available from Electro-Chem-Technic. (www.i-way.co.uk/~ectechnic)

2. Operating Instructions

The use and care of these mini fuel cells is very simple.

1. Remove the red cathode from the beaker anode.
2. Add potassium hydroxide solution, about 1 M, to the anode beaker till the level reaches the fill line. This will take about 65 cm³.
3. Add fuel. In the case of NaBH₄ this must only be about 20 mg, a small pile on the end of an electrical screwdriver blade or about 0.02 cm³ or 2 mm³. In the case of any liquid fuel about 5 cm³, or one teaspoon full. (Note, fuel concentration is NOT critical, neither is electrolyte concentration.)
4. Gently swirl the anode beaker for a second or two to dissolve the fuel in the electrolyte. (In the case of NaBH₄ this will take a little longer, say 10 seconds. Small bubbles will be seen on the black electrode when this is complete.)TEACHERS' NOTE. It might be simpler to prepare bulk KOH solution and fuel mixture, and keep this in a special bottle. It should not deteriorate, and saves trouble, especially the dispensing of very small amounts of NaBH₄ .
5. Insert the red cathode into the anode beaker. Swirl gently to clear any gas bubbles. The cross section should then be as in Figure 2 below.



Figure 2

6. Connect voltmeter and/or motor or resistor or DC/DC converter or ammeter to the cell to show that it is working and to monitor performance. When using NaBH₄ as fuel gently swirl the cell from time to time to clear any gas bubbles and to remix the fuel.
7. Once you have finished, pour away the electrolyte and fuel mixture, and rinse out the cell using de-min water. The electrodes should preferably be gently dried using paper tissue. N.B. In use the electrolyte/fuel mixture will take some of the black colouring out of the anode - this is inevitable, and gradually declines during the first few uses of the cell.

3 Safety Note

These fuel cells are designed for use in laboratories or suitable lecture locations by scientifically qualified people, or by students under the supervision of appropriately qualified teachers. The fuel cell requires the use of potassium hydroxide (KOH) solution which is corrosive. It can be used with methanol or sodium tetrahydridoborate, both of which are similarly toxic and flammable. All users must make sure they know the appropriate use of such materials. The vendors cannot accept any responsibility for accident or injury arising from their use.

4 Chemistry of the Fuel Cell

4.1 Introduction

At one level the reaction in the cell can be thought of as the reaction:-

HYDROGEN + OXYGEN à WATER + ENERGY

With the energy coming as electrical energy rather than the more familiar heat energy. However, this does not explain where the electric current comes from. To understand that we need to consider the separate reactions that occur at the anode and cathode.

At the anode the fuel is oxidised - electrons are removed. These electrons pass round the external circuit to the cathode, where the oxygen reacts, using these electrons. The fuel electrode, the source of electrons, is thus the electrically negative electrode. The air or oxygen electrode is the electrically positive electrode – it attracts the negative electrons. (Some people find the fact that the cathode is positive confusing, but it is the case for all primary cells, batteries as well as fuel cells. The cathode is the terminal into which electrons flow. This makes it the negative terminal for electrolytic cells and diodes, but the positive for sources of electrical power.)

At the anode, the reaction is different for each fuel. However, at the cathode the reaction is the same in all cases. So, it is with the cathode reaction that we will start.

4.2 Reaction at the cathode

Electrons from the external circuit are absorbed at the cathode, reacting with oxygen from the air and water in the electrolyte, according to the equation:-

4e- + O₂ + 2H₂O à 4 OH-

The creation of the OH- ions maintains the alakline concentration of the electrolyte.

4.3 Reaction at the anode with hydrogen fuel

This is the simplest anode reaction, and occurs when either sodium tetrahydridoborate or hydrazine are used. The reation is:-

H₂ + 2 OH- à 2H₂O + 2e-

The electrons released form the electric current when they flow to the cathode round the external circuit. As with all the fuel cell reactions, this reaction is only possible because of the special catalysts that make up the structure of the catalysts.

4.4 Anode reaction when using NaBH₄ (sodium tetrahydridoborate) fuel

There are two possible routes when using NaBH₄ fuel. In the first the NaBH₄ is directly oxidised, according to the equation:-

NaBH₄ + 8OH- à NaBO₂ + 6H₂O + 8e-

This reaction is promoted by the platinum catalyst on the electrode. However, this catalyst also promotes a reaction that produces hydrogen:-

NaBH₄ + 2H₂O à NaBO₂ + 4H₂

These four hydrogen molecules then react to produce eight electrons, as shown in section 4.3. In either case the remarkable thing is that ONE molecule of NaBH₄ gives EIGHT electrons. This is why it is such a good fuel. When using this fuel the hydrogen production rate can exceed the rate of use, especially at low currents, and if too much fuel is put in the electrolyte. This is a waste.

4.5 Anode reactions when using methanol fuel.

The methanol fuel reaction is a three-stage reaction, with each reaction releasing 2 electrons. In the first the methanol reacts to form methanal (formaldehyde).

CH₃OH + 2OH- à HCHO + 2H₂O + 2e-

The next stage is the reaction of the methanal to methanoic (formic) acid.

HCHO + 2OH- à HCOOH + H₂O + 2e-

The methanoic acid is finally oxidised to carbon dioxide, releasing a further two electrons.

HCOOH + 2OH- à CO₂ + 2H₂O + 2e-

The multistage nature of this reaction leads to an interesting experiment that can be done with this cell, which is described at our web based Manual of Experiments, at www.i-way.co.uk/~ectechnic.

This fuel gives SIX electrons for each molecule of fuel, which is very good, though not quite as good as NaBH₄. Nevertheless, a major problem with this fuel, when using an alkaline electrolyte, is that the carbon dioxide produced reacts with the electrolyte, as shown below, forming potassium carbonate. This gradually destroys the electrolyte. (Note, this does not occur when using methanol with certain other types of fuel cell.)

CO₂ + 2KOH à K₂CO₃ + H₂O

4.6 Anode reaction with ethanol fuel

It is sometimes more convenient, and creates an interesting demonstration, if ethanol, or an ethanol containing beverage, is used as a fuel. The cell works reasonably well, but it is not a viable fuel, as only one stage of oxidation occurs, releasing only two electrons per molecule of fuel.

C₂H₅OH + 2OH- à CH₃CHO + 2H₂O + 2e-

Note that in all these anode reactions OH- ions are involved. These are available because the electrolyte is alkaline.

5 Further Information

For more information about fuel cells and their technology and applications, together with detailed instructions about many further experiments that can be done with these cells, visit our WWW site. The is THE site for those who want to find out about fuel cells.

http://www.ectechnic.co.uk

In addition, the book "Fuel Cell Systems Explained",(2nd edition) by Larminie & Dicks, published by Wiley, ISBN 0470 84857 X, is very strongly recommended.