1. Instructions

1.1    Getting started

There is no easier hydrogen generator to use with small fuel cells than this one! The procedure is simple:-

1.   Half fill the generator with water – up to the 15 ml mark. Ordinary tap water can by used.

2.   Add about 0.1g of sodium borohydride to the water. The concentration is not important, but even 0.1 g will last for about an hour. A tenth of a gram is a small pile of about a dozen grains on the end of an electrical screwdriver. A ‘pinch’ in culinary terms. The sodium borohydride may slowly dissolve, but there will be no significant bubbling.

3.   Add the reactor(s) to the water. You will immediately observe a gas bubbling off the reactors. This is due to a catalytic action – the reactors are not actually taking part in the reaction – they are just promoting the reaction between the water and the sodium borohydride.

4.   Place the stopper in the top of the generator.

5.   Attach the pipe, which is connected to the stopper, to the fuel cell.

6.   WAIT ! It will take a few minutes (typically 3) for the air in the pipes and the fuel cell to be driven out by the hydrogen. You can watch this by connecting a voltmeter to the fuel cell. When the open circuit voltage of the fuel cell has reached about 0.75 – 0.80 volts, it can be used.

1.2    Optional extras

1.   The HGEN unit generates hydrogen at a rate that is suitable for fuel cells of power about 1.5 Watts. If you are using less power than this, much of the hydrogen will be passing straight through the cell. This can be saved by trapping it in a balloon. When the HGEN unit stops, the balloon will then supply the hydrogen. In this way it can run for a couple of hours.

2.  If you need to power a more powerful fuel cell, then the rate of hydrogen production can be greatly increased by using hot water in the generator.

3.   If you require hydrogen to be produced more slowly, for longer, then only put one of the two reactors into the generator.

1.3    To Finish

1.   Leave the reactors in the solution till bubbling has stopped. This means the solution contains harmless borates, which can safely be disposed of down the drains.

2.  The reactors are fragile, handle with great care. Gently dry them with a paper handkerchief, and replace in the box provided.

3.  The reactors are 90% of the cost of the generator – don’t loose them! In fact, it does not matter if they break – they work just as well in pieces. However, they must not be lost down the drain, or accidentally thrown away.

The proper systematic name for the compound NaBH4 is sodium tetrahydridoborate. However, it is normally called by the shorter name sodium borohydride. The basic hydrogen producing (hydrolysis) reaction is:-

NaBH₄ + 2H₂O à NaBO₂ + 4H₂

This reaction does not normally proceed spontaneously, and solutions of NaBH₄ in water are quite stable. Some form of catalyst is needed. That is provided by ultra-finely divided catalyst on the surface of the reactors that are placed in the generator.

·  The reaction is highly controllable – remove the catalyst and it stops

·  The reactor needed to release the hydrogen requires no energy, and can operate at ambient temperature and pressure.

·   For small quantities of gas, it is by far the simplest way of generating hydrogen.

·   For small quantities of hydrogen, it is by far the cheapest method, other than those using calcium/water or zinc/acid. However, these also produce acidic or caustic vapors that will certainly damage a PEM fuel cell.

·  It is easy to ensure that no high pressures are produced, which could damage a fuel cell. (Note: this is obviously a problem with pressurized hydrogen, but is also very difficult when using metal hydride stores.)

·  Hydrogen is the only gas produced, it is not diluted with carbon dioxide

·  If the system is warm, then water vapor will be mixed with the hydrogen, which is highly desirable for PEM fuel cell systems.

·  The rate of reaction is very steady – H₂ is produced slowly and steadily over a period. This is exactly what is required for small demonstration systems

·  No acidic or caustic vapors are produced which can damage a PEM fuel cell – this is not the case with calcium/water and zinc/sulphuric acid chemical generators.

In short, this method is ideal for very small systems such as demonstration fuel cells. However, it can also be used with larger systems, and companies such as Millennium Cell are developing its use for larger systems. Here the advantages are not so clear cut, as the cost of the sodium borohydride becomes a factor, as do the problems of disposing of large amounts of sodium borate solution. For a full and clear discussion of the different ways of producing hydrogen, see Chapter 8 of Fuel Cell Systems Explained (2^nd edition) by Larminie and Dicks, published by Wiley, ISBN 0 470 84857X.

2.3    Background

Although rather overlooked in recent years, NaBH₄ has been known as a viable hydrogen generator since 1943. The compound was discovered by the Nobel laureate Herbert C. Brown, and the story is full of interest and charm, but is well told by Prof. Brown himself, and can be found at http:// www.chem.purdue.edu/hcbrown/Lecture.htm. Suffice to say that shortly before the end of the 1939-1945 war plans were well advanced to bulk manufacture the compound for use in hydrogen generators by the US Army Signals Corps, presumably for hydrogen balloons. The end of the war caused these plans to be shelved. However, in the following years many other uses of sodium borohydride, notably in the paper processing industries, were discovered, and it is produced at the rate of about 5,000 tonnes per year, mostly using Brown’s method, by Morton International (merged with Rohm and Haas in 1999).

The essentials of our generator are those of the original generators built by Brown in the 1940s. However, others are working on systems to produce systems that produce hydrogen at much higher rates, and Millennium Cell in the USA (see http:// www.millenniumcell.com) are the leaders in this field. The main problem is the cost of sodium borohydride. If this is solved, then this method will certainly become very widely used for supplying hydrogen to fuel cells.