A new technique for producing hydrogen gas using sunlight and water has been developed by researchers at the University of Colorado Boulder. The method is more efficient and produces more hydrogen than previous water-splitting methods.
Hydrogen is an extremely important gas, used in everything from the production of the petrol we use in our cars to the creation of ammonia, which is used to fertilise the food that we eat.
65 million tonnes of hydrogen gas are produced every year [PDF], according to the International Energy Agency, most of it created by burning natural gas. "Instead of using natural gas to produce hydrogen [...] imagine using sunlight and water," says Chris Muhich, lead author of a paper published in Science on 2 August.
In that paper, Muhich and his colleagues describe a process for creating hydrogen that uses the heat of the sun to split water into hydrogen and oxygen.
Crucially, unlike previous techniques, their approach produces between three and 12 times more hydrogen than previous water-splitting techniques. Additionally, their process is more efficient and puts less stress on the materials in the reaction.
Water is made up of two hydrogen atoms and one oxygen. Given that two-thirds of the Earth's surface is covered in water, there is an abundant supply of clean energy literally lapping on our shores. The trick is unlocking the hydrogen trapped inside water molecules.
Scientists do this using a metal oxide, which is a metal that has reacted and bonded with oxygen. For example, rust is a metal oxide -- the iron has reacted with the oxygen in the air. "When you heat up [the metal oxide] really hot, you end up driving some of the oxygen out of that metal oxide," says Muhich.
By heating up the metal oxide to temperatures over 1,000 degrees Celsius, the metal oxide can be forced to let go of the oxygen.
While the metal is in this oxygen-deficient state, water is passed over it in the form of steam. In this final step, the metal grabs the oxygen from the water, leaving behind two hydrogen atoms, which form hydrogen gas.
It has previously been assumed that a temperature change between step one and step two was required, in order to balance the thermodynamic processes in the reaction. However, Muhich found that the same balancing could be done by altering the pressure, allowing the reaction to be carried out at higher temperatures -- and therefore faster reaction rates. "You don't need a temperature change. And when you don't have that temperature change, we end up producing more hydrogen than when we have those temperature swings," says Muhich.
Additionally, previous changes in temperature involved a 400-degree swing, which not only reduces the efficiency of the process -- you have to expend energy changing the temperature -- but also puts a strain on the materials in the reaction, as the temperature change causes rapid expansion and contraction.
Producing hydrogen gas with this clean method, as opposed to burning natural gas, is still some way off. "Natural gas is just so cheap, it becomes really difficult to compete with that economically," says Muhich.
There is also a lot more research to be done into finding out the optimum temperatures at which to carry out the reactions, as well as finding the best metal oxide mixtures and also understanding what a hydrogen-producing solar reactor would look like.
But one day techniques like the one Muhich has developed could unlock the abundant fuel stored in our oceans.
This article was originally published by WIRED UK
