They say mimicry is the greatest form of flattery and in the case of Daniel Nocera and trees, this is true. Nocera, the Patterson Rockwood Professor of Energy at Harvard University and his colleague Pamela Silver, have created an artificial leaf that can produce liquid fuel.
Harnessing the power of the sun, trees create water, carbon dioxide and hydrogen, in a process called photosynthesis. The artificial leaf, once submerged in water and with sunlight directed at it, can produce hydrogen from one side and carbon dioxide from the other.
Hydrogen is seen as an alternative fuel source to fossil fuels and biofuels. When it burns, the sole product is water, making it extremely environmentally friendly. The hydrogen produced by the leaf can be used straight or fed into an engineered organism (bacteria) which then makes liquid fuel by combining it with carbon dioxide.
So what is this leaf made of? “The artificial leaf is made up of a wafer of silicon, coated with two catalysts that I invented. The bionic leaf conveys the integration of the bacteria with the artificial leaf,” Nocera tells me.
Currently hydrogen can be made by reacting steam with coal or natural gas, both of which are non-renewable sources or by passing electricity through water. Production of electricity again, originates from non-renewable resources. Despite this, hydrogen cells are used in cars and the argument goes that it’s easier to control pollution from a large factory then from hundreds of cars.
The leaf created by Nocera is a tremendous improvement on the first which faced a number of challenges, chief among which was the catalyst. The nickel-molybdenum-zinc alloy catalyst creates reactive oxygen species which destroyed the bacteria’s DNA and so in avoidance of this, the scientists had to raise the voltage exceptionally high. This, in turn, vastly reduced efficiency.
The new catalyst – a cobalt-phosphorous alloy – produced no such reactive oxygen species and so enabled better efficiency. In fact it’s so efficient now, that the system can now convert sunlight into fuel at 10 per cent efficiency which is nine percent faster than even plants according to Nocera. Fast enough, he says, to “to keep up with solar fluxes [i.e., the incoming light from the sun]”.
Nocera says it’s efficient enough to be used commercially, but he wants to take it to developing countries, especially India. He tells me:
“We have accomplished a distributed solar fuels process. I believe that there are significant decisions to be made in the future about how emerging economies will build their energy infrastructure. Discoveries like these raise the possibility of a distributed energy infrastructure versus a centralised energy. I would like the scale-up and prototyping to be done with scientists, as partners, in the emerging world.”