Hydrogen

Dave - With us this evening we have Fraser Armstrong from Oxford University. You work on fuel cells - can you explain what a hydrogen fuel cell is?

Fraser - Well first of all hydrogen is a energy carrier much like petrol or any sort of oil or coal. But it's a very different type of energy carrier because it's a gas, and it's not necessarily a very convenient energy carrier because hydrogen is a gas all the way down to something like 20 Kelvin. This is about minus 250 degrees Celsius. So it's not a very convenient fuel, but when combined with oxygen, hydrogen and oxygen make a bang together and give off water.

Chris - The people on the Hindenburg knew a bit about that.

Fraser - Well they did unfortunately, yes. Hydrogen is a very good and light fuel; that's why it's used in spacecraft even if it's not very useful in terms of being able to transport it efficiently.

Chris - Just as aside on the Hindenburg disaster, it was actually a bit of a myth that it was the hydrogen, although that didn't help. When the Germans built it they thought it looked nice as a silver colour because it showed up the Nazi swastika very nicely. In order to get that colour they sprayed it with aluminium, and aluminium particles burn beautifully.

Fraser - That was a bad choice.

Dave - So what does an actual fuel cell look like?

Fraser - Well a fuel cell consists of two electrodes: one on which hydrogen is oxidised to protons. Of course, as we've just heard from Emma, this needs a catalyst and the catalyst in this case is generally platinum or platinum with other precious metals. Hydrogen is oxidised to protons and at the other electrode, oxygen is reduced to oxide. The oxide and the protons combine to form water. We find that we have a large amount of energy produced from this, and it's the same amount of energy as would be produced if we deliberately burned hydrogen and oxygen and got an explosion. Now the energy is converted directly into electricity, which can be used to power devices.

Dave - I guess there's a problem if the hydrogen gets on the wrong side and the hydrogen gets on the wrong side. How do you normally solve that problem?

Fraser - Well normally the anode and the cathode as the two electrodes are called, are separated by a membrane called the proton exchange membrane. Hydrogen is directed at one of the electrodes and air is directed at the other electrode. Generally there is very little in the way of cross-over, which is the mixing of gases.

Chris - Since this show is about catalysts, I've got to ask, what is the catalyst that's doing this in your fuel cells?

Fraser - In the conventional fuel cell, which is called the proton exchange membrane fuel cell, the catalyst is platinum, as we heard from Emma. My research group is investigating the possibility of other types of catalysts for this type of technology, particularly ones that are based on enzymes that occur in microbes. These particular enzymes do not of course contain platinum at their active centre but contain other elements that are much more familiar: in particular iron and most often nickel as well.

Dave - I guess that this is a big advantage because if you powered all the cars with platinum fuel cells you'd run out of platinum quite quickly.

Fraser - Well either we'd run out or the price would go up and up. There's always a good point to having catalysts that are on the market.

Chris - So why do bacteria need to be able to do this with hydrogen? Why do we need to do that?

Fraser - Very interestingly the bacteria have used hydrogen as a fuel for over 2.5 billion years. If we go back in time to the earliest life forms, at the particular time there was no oxygen on the Earth and many microbes would use the proton as an oxidant. Of course, when one reduces a proton, we obtain hydrogen. So many bacteria have the ability to make hydrogen from protons, that is, from water. Equally, other bacteria have the ability to use hydrogen as a fuel. So there's a kind of cycling that's possible in the microbial world.

Chris - Is it possible for us to co-opt this efficiently enough to run our cars though?

Fraser - No I don't see this ever running cars because as it stands at the moment, the problem with enzymes is that they're not designed to last forever and they're not designed to withstand very high temperatures and reaction conditions. However, we can learn a considerable amount by studying the active sites of the enzymes and the molecular structure.

Chris - In other words the business end that does the catalysis.

Fraser - Yes the business end at which catalysis occurs.

Chris - And what, you'd hope to make a model of that or reproduce that more stabley?

Fraser - For the purposes of high energy orhigh power, it may be possible in the future to make catalysts which are alternatives to platinum that use the chemistry of the active sites of enzymes as we currently understand them. It may also be possible to actually use enzymes themselves for power production, which is much less demanding than the automotive industry.

Dave - So the advantage of your design with the enzymes is that you don't need to keep the oxygen separate any more.

Fraser - In principle that may be quite correct. It is possible to mix hydrogen and oxygen to get non-explosive mixtures. However, the amount of hydrogen that one requires for this is less than 4% in air in order to avoid hazardous mixtures.