A record efficiency for the production of hydrogen from sunlight using an artificial leaf has been achieved by scientists in Germany.
Although sunlight is readily available, we are only able to harness its energy during certain hours of the day. This means we can't use the Sun as a direct, continuous energy source. But the problem can be solved by developing a means of storing the energy captured during daylight hours so that supplies can be maintained, even when it is dark. One way to do this is to use chemical bonds, by splitting water into hydrogen and oxygen.
Researchers in Berlin have taken a step towards realising this with a device that can be placed directly into water and, under sunlit conditions, produces hydrogen.
This kind of water splitting is how plants use the Sun's energy, combining the hydrogen and oxygen with carbon dioxide from the air, to make sugars, which are in turn used to build and power the plant.
But rather than make sugars, the German approach is to use the hydrogen as an energy source. Combined with oxygen in a fuel cell, it can produce electricity with the only emission being water.
Such devices have been made previously, but problems with corrosion, chemical instabilities and poor efficiency have held up progress and implementation.
One earlier model, made in 1998, had a maximum efficiency value of just 12.4% and nothing has been able to beat it since. Now Matthias May and his group have finally broken this barrier.
Writing in Nature Communications this week, the team describe how, thanks to modifications made to the surface of the material, they were able to produce hydrogen at a record efficiency of 14%. Hydrogen can currently be produced cheaply from natural gas, but, according to May, "starting from 15% efficiency, this kind of device could produce competitive solar hydrogen."
Their surface treatment involved submerging the device in a solution of rhodium chloride, connecting it to an electrical circuit and then shining light on it. The reaction of the surface with the solution formed a stable layer to prevent corrosion. Onto this, they deposited the rhodium catalyst by pulsing an electric voltage through the cell first in the dark and then under flashing light.
The ability to do both the surface stabilisation and catalyst deposition in the same solution means that the technique could be used industrially. To scale up these devices, it is not just a case of bigger is better, May says. A single cell needs to remain as small as a few centimetres to allow the charged particles essential to its function to move effectively.
However, in the same way as plants work by connecting many leaves, these small cells can be connected together to produce a large module. Just one of these modules could produce litres of hydrogen per day from the sun: enough to fill the tank of a hydrogen car...