One of the largest supplies of fuel that we have on this planet is all around us, and it is already piped to the great majority of homes in vast amounts – more than we could ever possibly use. That fuel is water, and most people don’t think of that substance as being a particularly useful energy source; it doesn’t burn, we can’t make a fire out of it and cook food with it. However, water is actually an extremely rich source of energy. It just needs a little pretreatment before we can access it. Water, as most people know, is a molecule made up of two hydrogen atoms and one oxygen atom – H2O. If you take a mixture of hydrogen and oxygen gases and ignite them with a spark, you get a pretty spectacular explosion and form the product, which is water. This reaction is one of the most energetic known to chemistry; two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water.
One of the central principles of chemistry is that any reaction which proceeds in one direction must, in theory at least, be able to proceed in the reverse direction. The reverse reaction may be very difficult to accomplish, and may take years to complete under normal conditions, but it has to at least be possible. Anything that is possible can be accomplished given enough effort, and lots of scientists are investigating how to run this particular water reaction in reverse. Instead of starting with hydrogen and oxygen gases and producing water, they want to start with water and produce hydrogen and oxygen gas. One method scientists have already developed to accomplish this is electroysis. Passing an electric current through a solution of water provides a large jolt of energy to the molecules, a large enough amount of energy to overcome the energy barrier necessary for the reaction to proceed and so the water molecules (which consist of hydrogen and oxygen atoms) split up into hydrogen and oxygen gas. These gases can be siphoned off and used to power a hydrogen fuel cell, which generates electricity.
The problem with electrolysis is that a lot of energy is required to break up the water molecules, so much energy that the benefits of generating the hydrogen are lost. You spend more money on the electricity than you gain from selling the hydrogen. So, electrolysis is interesting but it’s never going to be a commercially viable process. Luckily there is another method chemists can use to overcome a high energy barrier in a reaction pathway: bringing in a catalyst. Catalysts do not supply energy by themselves, but they lower the energy barrier by providing alternative pathways for the reaction to succeed. A reaction which might take years to complete under normal conditions could be complete in seconds in the presence of the right catalyst. This was the approach that British chemists have described in a recent article in the journal Chemical Communications.
Their approach was very clever; I found it pretty admirable. The scientists attach a luminescent dye to particles of titanium dioxide, which is a common component of paint and which acts as a support for the dye molecule. They then attach a biological enzyme called a “hydrogenase” to the titanium dioxide molecules. Enzymes are Natures catalysts – extremely complex and extremely efficient catalysts. Normally enzymes are “powered” by the energy gained from the breakdown of glucose molecules in the body. However, once out of the body the enzymes lose their normal power supply. That is where the innovation of this new catalyst design comes into play. Regular sunlight is introduced to a solution of the catalyst in water. The sunlight illuminates the dye molecule, which converts the incoming light energy into a weak stream of electrons. This electrons are transferred through the titanium dioxide particles and channeled into the enzyme. This provides the energy needed for the enzyme to go to work, and it transforms the water molecules in the flask into the component hydrogen and oxygen gases.
This is an extremely creative approach. If this technology could be commercialized, everyone would have the two basic components – sunlight and water – that they need to produce the fuel for a hydrogen fuel cell. All of the electronic components would fit into a small box. This process is a fantastic way of using a catalyst from Nature – which has been evolving for millions of years, and is much better than any catalyst we could design from scratch – and powering it using a natural energy source. This combination of organic synthesis (the dye), materials chemistry (the nanoparticles), and biochemistry (the enzyme) represents the very best in interdisciplinary collaboration.
The source for this article can be found at: http://www.rsc.org/Publishing/Journals/CC/article.asp?doi=b817371k