Fuelled up for a liberated future

orts Painted-on cells that use sunlight to directly split water in order to liberate hydrogen – the favoured fuel for fuel cell-powered vehicles – is one vision of the future. And it may not be that far away from reality. It is certainly being given shape through research at the University of Greenwich and could well lead to buildings, and perhaps even cars, extracting fuel from rain directed into the spaces between panes of glass in specially constructed windows. The basic idea is not new – there is a plethora of published patents and papers about photocatalytic cells based on titanium oxide, starting with a paper by A. Fujishima and K. Honda in ‘Nature’ in 1972. But Dr Bruce Alexander, a lecturer in physical chemistry on the university’s Medway campus, is looking at materials that gather their energy from a much wider part of the sun’s spectrum than is possible with titania. At present, the vast majority of hydrogen is produced by reforming natural gas, which, as well as being from a fossil fuel source, contains contaminants that have to be removed, if they are not to poison the fuel cells. Hence the hydrogen to fuel the predicted hydrogen economy is going to have to come from other sources, either by electrolysing water or from water directly, which is the point of the photocatalytic cells. They also have other potential uses, such as to remove the last traces of organic pollutants from drinking water. And, by working in a slightly different manner, they could photo reduce carbon dioxide to methanol, which can also be used as a fuel. In addition to meeting the electrical requirements of the hydrogen producing cells, Alexander is restricting himself to materials that can be deposited by painting solutions on window glass, such as tungsten trioxide, or by spraying them on, such as iron oxide. “I have kept away from chemical vapour deposition, because that would be essentially pointless in terms of manufacturing because of the cost,” he explains. The basic process involves photons of light striking a very thin layer of semiconductor, typically only one micron thick, which produces electrons and holes – the same principle as that used in a photovoltaic cell. In a photocatalytic cell, however, the electrons combine with hydrogen ions in water at the cathode surface to produce hydrogen gas molecules, whereas the holes interact with water molecules at the anode surface, in order to produce oxygen and hydrogen ions, which have to be able to migrate through the material to the cathode. The cells can be made planar, as is the case with photovoltaic cells, or as particles, dispersed in water. Whereas it is quite easy to make a device that produces small amounts of hydrogen, producing larger amounts has so far defeated many expert minds. The problem is that the material must not react with water; the conduction band carrying the electrons has to be sufficiently negative to liberate hydrogen, while the band gap should be sufficiently small that photons from the visible part of the spectrum, as opposed to the ultra violet part (as is the case with titanium oxide), can produce the electrons and holes. Alexander has found that materials such as tungsten trioxide and iron oxide have sufficiently narrow band gaps to work with visible light, but then have to have a small voltage applied to them to liberate hydrogen. He has made cells, based on tungsten trioxide, that produced 50 micro moles per hour per square centimetre, equating to one gramme or 11.2 litres of hydrogen gas per square metre per hour. This is about as good as the best titanium oxide based cells made so far and much better than the best ‘artificial leaf’ cells that produce hydrogen from stressed algae. The need to bias cells leads to the idea of what is called a tandem cell, in which the bulk of the light is caught by the hydrogen producing layer, but red light passes through it into a photovoltaic layer to produce a small voltage, sufficient to provide the bias. This then poses the additional requirement that the photocatalytic layer has to be transparent, and makes the cell more complicated and expensive, although not excessively so. The frustrating aspect of all this is not that there are very few materials to try, but rather that, if one includes all the doping possibilities, a great many. This could mean that the ideal material could take decades to discover or somebody could stumble on it next week. Alexander describes the process as being “educated guesswork”. So far, he has found that iron oxide can harness about 40% of the energy in sunlight, as opposed to about 3% for titanium oxide, but it suffers from what he calls “sluggish hole conduction - the holes are too slow at getting to the catalyst surface and so don't get a chance to react to split water”. His investigations, should they prove fruitful, could turn out to be the breakthrough everyone is looking for, providing cheap, limitless sources of hydrogen fuel for everything we use. The water-powered car, which conspiracy theorists constantly insist is a technology that has been suppressed by oil companies, could then become a reality. The other applications of photocatalytic devices are in the removal of traces of organic pollutants that are not easily removed from drinking water by conventional methods, as well as reducing carbon dioxide to methanol. Photocatalytic air purifiers equipped with ultra violet lamps are, of course, already commercially available and are widely used, particularly in Japan and the Far East, to remove odours in crowded apartments and restaurants. We have also seen Japanese-made photocatalytic artificial flowers, which accomplish the same goal, if placed in ultraviolet containing light. The current generation of materials being studied for hydrogen production are all ‘n’ type semiconductors. However, as Alexander points out: “People are beginning to look at ‘p’ type semiconductors that could photo reduce carbon dioxide to something like methanol, although there are only a handful of reports and most of these suggest that there are difficulties in producing methanol in the final step of the reaction. But the challenge is there.” Success with that idea would solve, at one blow, both the problem of what to do with sequestered CO2 from burning fossil fuels and the problem of replacing them with a replacement ‘green’ fuels. Pointers * New materials offer the possibility that photocatalytic conversion of water to hydrogen for fuel can be accomplished using a much wider part of the solar spectrum than at present * The ideal material has not yet been found, but progress is being made and there are many more material combinations to try * Photocatalytic destruction of organic pollutants and organisms is already a proven commercial technology, using ultra violet light, but can also be accomplished using sunlight * There is also the possibility of using photocatalytic reduction of carbon dioxide to methanol