Andreas Späth

Reinventing the leaf

2014-06-23 07:53

Andreas Wilson-Späth

One of the holy grails of solar power technology might soon become a reality. Researchers around the globe are working on designs for an "artificial leaf" that would mimic the ability of plants to convert the energy contained in sunlight into a form that can be stored and used whenever it’s needed.

Among those who've made significant progress in copying nature's photosynthetic powers are MIT chemistry professor Daniel Nocera and his co-workers. They made one of their initial breakthroughs in 2008 when they came up with a cheap and effective chemical catalyst that allowed them to split water into its constituent parts and generate oxygen gas.

Previously, this feat could only be accomplished using expensive materials and relatively large amounts of energy under very acidic or alkaline conditions. Nocera's innovation meant that water molecules could be ripped apart efficiently, cheaply, at room temperature and in water.

As an added bonus, their catalyst, which is made of cobalt and phosphorus, is "self-healing" and can be re-used indefinitely. What's more, Nocera suggested that the energy required to drive the chemical reaction involved could be supplied by a conventional photovoltaic solar cell.

In 2011, two separate MIT teams under Nocera's guidance managed to deposit the catalyst onto thin silicon wafers. When submerged in water and exposed to light, these devices split the water and produced bubbles of oxygen. As in conventional solar panels, the semiconducting silicon turns the energy of light photons into a flow of electrons, which then power the water-splitting reaction with the help of the catalyst.

Next, Nocera and his colleagues produced a silicon wafer covered with their cobalt and phosphorous catalyst on one side and a different, nickel-molybdenum-zinc alloy catalyst on the other. The former produces oxygen gas as previously, while the role of the second catalytic layer is to combine the remaining products of the water-splitting process (protons and electrons which would otherwise just dissipate into the water) into hydrogen gas.

Provided with sunlight, this thin, catalyst-coated silicon wafer successfully splits water into two separate gases – oxygen and hydrogen (you can see it in action in this video clip).

So what’s the big deal about all that?

Collected separately, these gases can be compressed, stored, transported and used at a later stage to generate energy, for instance by feeding them into a fuel cell that recombines them into water while simultaneously producing electricity.

The process that happens on the surface of these artificial leafs may not be exactly the same as photosynthesis (in which plants use sunlight to split water molecules to produce oxygen as a waste product and hydrogen, which reacts with carbon dioxide from the air to eventually make energy-storing sugar molecules), but it is a viable way of capturing the energy of sunlight in a stable form that can be used when needed.

The last and still unresolved step involves the engineering challenge of effectively collecting, storing and using the gases. Unfortunately, finding a cost-effective method to scale up the action of artificial leafs to a commercially feasible level has presented a major stumbling block for companies such as Sun Catalytix, which was started on the back of Nocera’s research.

But there are others, including a number of start-up companies and a Japanese consortium that are trying to crack the remaining issues.

In California, a large research programme housed at Caltech – the Joint Centre for Artificial Photosynthesis – is being funded by the US Department of Energy to the tune of $116m. Using a slightly different approach to that pursued by Nocera, those involved have the stated aim of producing a working artificial leaf prototype by next year.

So maybe it’s not too optimistic to expect to see this very promising technology become a workable reality in the near future. Natural photosynthesis already provides us with almost all of our nutritional energy through the food we eat, as well as with the oxygenated air we breathe.

Now imagine a world with effectively harnessed artificial photosynthesis in which water is turned into storable, clean, renewable energy using only sunlight as a source and generating only water as waste.

- Andreas is a freelance writer with a PhD in geochemistry. Follow him on Twitter: @Andreas_Spath
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