Nature is not always the best role model, however,as the evolutionary process has something of a ‘fit for purpose’ approach to design. It tends to cobble things together from older bits of kit, in a Heath Robinson type way, with the emphasis on functionality ratherthan efficiency.
Take photosynthesis, for example. The corner stone of life on Earth, photosynthesis comprises a suite of pigments, protein structures and enzymes that use sunlight to split water into oxygen, electrons and protons.These electrons and protons are then shuttled along a complex network of pathways, ultimately providing the energy and hydrogen required to convert carbondioxide into a wide range of carbohydrates. In this way,photosynthesis converts sunlight into chemical bonds,providing a mechanism for storing the sun’s energy.
Nevertheless, despite its impressive complexity,photosynthesis is not an overly efficient process for capturing energy from the sun. Even the fastest growing plants convert less than 1% of the solarradiation falling on them into biomass.
But the sun can theoretically provide all the energy we could ever need. It’s an oft repeated truism that more energy from the sun hits the earth in one hour than is currently consumed by human kind over the course of a whole year. If we could convert this sunlight into fuel more efficiently than plants (at levels of around10%) then our energy worries would beover, without having to blanket the world in energy crops.
Achieving this will demand artificial versions of photosynthesis that are more efficient than the natural version, and the development of such artificial photosynthesis systems is becoming an increasingly active area of research.
In the US, the National Science Foundation has established a Center for Chemical Innovation to develop artificial photosynthesis systems. Entitled Powering the Planet, this centre comprises a network of research groups from various different institutions, including Pennsylvania State Universityand Massachusetts Institute of Technology. In Europe, the European Science Foundation recently called for a substantial increase in funding of solar energy technologies, particularly artificial photosynthesis.
Scientists now understand the process of photosynthesis in quite some detail. But they clearly need to go beyond this if they are to develop improved versions. So they are adding the latest findings and developments in catalysis, material science and nanotechnology. And these efforts are now starting to bear fruit.
Synthetic shortcut
One shortcut that scientists are able to take advantage of is that they don’t need to replicate the full photosynthesis process. In natural photosynthesis, the protons and electrons generated by splitting water are used to produce the energy storage molecule ATP (adenosine triphosphate) and to reduce NADP (nicotinamideadenine dincleotide phosphate) to NADPH. The ATP and NADPH then provide the energy and hydrogen, respectively, needed to convert carbondioxide into carbohydrates.
But for energy purposes, the protons and electrons could simply be combined to produce hydrogen, which can then be used to generate electricity in a fuel cell. So scientists only really need to concentrate on the water splitting part of photosynthesis, and that’s where most of the action is focused.
Light-powered catalysts that can generate hydrogen from water have been around for many years. The problem is that they contain expensive metals such as platinum and often only work with high-powered ultraviolet (UV) light rather than natural sunlight.But scientists are now stumbling across alternatives that are both cheaper and better. In early 2009, a team led by Xinchen Wang from the Max-Planck Institute of Colloids and Interfaces in Potsdam, Germany, reported that a polymeric form of carbon nitride originally discovered in 1834is able to generate hydrogen from water when exposed to sunlight.
‘The special thing about carbon nitride is thatit is stable in water, even under extremely acidic oralkali conditions,’ says Wang. ‘Apart from that, it is very easy and inexpensive to produce.’
Unfortunately, it’s not particularly productive,generating only four micromoles of hydrogen an hour. Doping the carbon nitride with platinum increased this rate by a factor of seven, but meant it no longer had any cost advantages over existing catalysts. However, Wang and his team have since found that the rate of hydrogen production can be improved by almost a factor of 10 by creating a highly porous version of the carbon nitride polymer,greatly increasing its catalytic surface area.
Nevertheless, carbon nitride still comes up against a problem that besets every catalyst that produces hydrogen from water: what to dowith the left-over oxygen atoms. If you’re not careful, these loneoxygen atoms will simply recombine with the hydrogenatoms, ruining the efficiency of the wholeprocess. Wang and his team got around this problem by employing a compound to soak up the oxygen atoms, but this is hardly ideal.
The real issue is that hydrogen production needs to be driven by the production of oxygen,specifically the production of molecular oxygen(O2) via the oxidation of water, as is the case in natural photosynthesis.
‘The water oxidation reaction is generally believed to be the “limiting” process, meaning that if it is notcatalysed efficiently, it limits hydrogen production,’explains James Muckerman, a chemist at the USDepartment of Energy’s Brookhaven National Laboratory, who studies water splitting catalysts.‘You can’t sustain hydrogen production without the protons and electrons generated by water oxidation.’
Unfortunately, generating O2 from water is amuch more complex process, although catalysts based on ruthenium show some promise. Indeed,when Wang and his team doped their carbonnitride catalyst with ruthenium dioxide it could generate both hydrogen and oxygen, albeit at very low levels.
Catalysis chemistry
In natural photosynthesis, water is oxidised by amolecular structure known as the oxygen-evolving complex (OEC), which is based on a central calciumatom surrounded by four manganese atoms. In2006, an international team of researchers used a combination of X-ray absorption spectroscopy and crystallography to determine the precise structureof the OEC. Using this blueprint, scientists are now beginning to produce the first synthetic copies.
In 2008, a team of US and Australian chemistsled by Leone Spiccia from Monash University synthesised a molecular structure in which four manganese and four oxygen atoms are bound together in a cube shape (C&I 2009, 1, 15).Their calculations predicted that this manganese structure should be able to oxidise water, but they were initially unable test this prediction because the structure is insoluble in water.
So Spiccia and his team incorporated the structure into the ionic polymer known as Nafion, which is commonly used as the proton conducting membrane in fuel cells. ‘When we bound the catalyst within the pores of the Nafion membrane, it was stabilised against decomposition and, importantly, water could reach the catalyst where it was oxidised on exposure to light,’ says Spiccia.
Other researchers are taking their inspiration more from catalysis chemistry than from photosynthesis, and as such have developed water splitting structures that look very different from the OEC.
In April 2009, a team from the Weizmann Institute of Science in Rehovot, Israel, led by David Milstein reported the development of a totally novel molecular construct based on ruthenium that generates hydrogen from water via a heat driven process and then generates O2 via a light driven process.
The inspiration for this construct came from the team’s previous work on catalytic reactions with alcohols, in which they used a similar ruthenium construct to activate oxygen-hydrogen bonds.‘We thought that the activation of the oxygenhydrogenbond in water should also work,’explains Milstein.
What Milstein and his team have now come up with represents a totally novel strategy for splitting water. Their construct can produce O2 and hydrogen without requiring any electron or protondonors or any compounds to soak up oxygenatoms. During the course of the water splitting process, the construct undergoes a number ofchemical changes but always reverts back to its original form, much like the OEC, but it appears to generate O2 via hydrogen peroxide (H2O2), which is unprecedented in water splitting reactions.
So far, this work is very much proof-of principle,as the construct is not actually catalytic, but Milstein intends to investigate further. ‘We are planning to study the mechanisms of this reaction, modify the complex structure to make it robust towards oxidation, and combine the individual steps into a catalytic cycle,’ he says.
Nanotechnology also offers novel strategies forsplitting water. For instance, titanium dioxidebecomes a powerful oxidising agent whenirradiated with UV light. If this oxidising abilitycould be triggered with natural sunlight then itcould also be used to split water. This is whatCraig Grimes of Pennsylvania State University hasnow achieved.
In 2008, he developed an array containing two different types of titanium dioxide nanotubes and showed that it could split water when exposed to natural sunlight, with hydrogen produced by one of the nanotube types and oxygen generated by the other. Although the conversion efficiency of this array is fairly low, at 0.3%, Grimes believes it could be increased to 5–10%.
Then, at the beginning of 2009, he showed that titanium dioxide nanotubes could form the basis of a system that replicates almost all the features of natural photosynthesis, generating organic molecules from carbon dioxide, watervapour and sunlight.
This involved first coating titanium dioxidenanotubes with nanoparticles of copper and platinum to enhance their catalytic ability. Then Grimes placed an array of these nanotubes ina specially-made metal chamber with a central window, pumped a mixture of carbon dioxide and water vapour into the chamber and placed it outside on a sunny day for two to three hours.
The array transformed the carbon dioxide and water vapour into methane and related organic compounds such as ethane and propane at rates 20 times higher than any previous method. Nevertheless,the conversion efficiency is still quite low.
‘If you tried to build a commercial systemusing what we have accomplished to date, you’d go broke,’ admits Grimes. ‘However, we believe,running the numbers, that solar-to-fuel conversion efficiencies of about 8% could be achieved in the near future.’
With this level of conversion efficiency, artificial photosynthesis systems looking very different from the natural version could start to take bloom.
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