Energy, naturally

Plants and trees do it. Algae do it. Even primitive bacteria do it. They capture energy from the Sun and use it to convert water and carbon dioxide into oxygen and carbohydrates, such as glucose. 

6CO₂ + 12H₂O + Energy → C₆H₁₂O₆ + 6O₂ + 6H₂O  

This is photosynthesis, and began on Earth over two billion years ago, changing our atmosphere from an oxygen-poor to an oxygen-rich one. In a two-stage process, water is first split into hydrogen and oxygen (reaction (i), sometimes referred to as the 'light' reaction because light is needed) and the hydrogen then reduces carbon dioxide to carbohydrates and other organic molecules (reaction (ii), the 'dark' reaction because no light is needed).  

H₂O → O₂ + H₂ (i) 

nCO₂ + nH₂n → (H-COOH) carbohydrates + organic molecules (ii) 

Today many scientists around the world believe that if this process could be reproduced in the laboratory and developed on an industrial scale, we would have the perfect solution to our energy problem. By 2050 scientists estimate that there will be an energy shortfall of 14 TW (1 TW = 10¹² W). With over 100,000 TW falling on the Earth as sunlight, the potential of 'artificial photosynthesis' is huge. Not only does it offer a route to sustainable hydrogen production and other useful fuels such as methane and ethanol, but it also removes CO₂ from the atmosphere, a key process with the potential to build other useful chemicals.  

A challenging problem 

By the 1970s, interest in artificial photosynthesis began in earnest. Scientists knew from earlier work on 'photogalvanic cells' (basically a dye in water plus a couple of electrodes) that they could generate electricity via charge separation and electron transfer reactions, but they had severe limitations. The amount of electricity produced was very small and the cells had to be extremely thin otherwise the ions would react in solution. So sustaining the output from such cells was difficult and scientists looked towards Nature for a better solution.  

But it's not that simple. Natural photo-synthesis is incredibly complex. The energy of sunlight is transferred in the form of electrons and positive charges throughout a pathway of myriad steps before the final products - carbohydrates - are formed. Crucially, these steps often require up to four electrons. This presents an enormous challenge to chemists because they need to be able to store electrons and use them at just the right time. All chemical reactions done in the laboratory involve transferring just one electron at any one time. To stand any chance of replicating and exploiting photosynthesis, scientists need to know exactly what is going on in the natural systems.  

By this time they knew that natural photosynthesis occurs in specific membranes - chloroplasts - in plant cells. These high surface area membranes contain proteins and light-absorbing pigments such as chlorophyll and carotenoids. Chlorophyll captures the light and transfers it to the reaction centre, ie the catalytic site where the water-splitting reaction takes place. Chlorophyll is regenerated frequently during this process. This, Tony Harriman, professor of physical chemistry at the University of Newcastle, explains to InfoChem, raises another big challenge for an artificial system based on organic molecules. 'Organic molecules, such as dyes, when exposed to sunlight for an extended period will degrade. How such molecules can be renewed or repaired in an artificial system presents a huge challenge for chemists', he says. 

Understanding Nature's masterpiece 

Originally chemists thought that photo-synthesis was occurring through a random distribution of chlorophyll in the chloroplasts, which they reasoned should be possible to replicate in the lab. However, over the past 30 years scientists have uncovered a different picture. They now know that Nature has learnt to position chlorophyll molecules in precise positions in the chloroplasts - at exactly the same angle and same orientation in every chloroplast. This was confirmed by Professor Richard Cogdell and his team at the University of Glasgow in the 1990s by using X-ray diffraction studies.  

Harriman explains, 'Within the chloroplast, there are a series of precisely placed chlorophyll molecules in a protein matrix which is ca Å 60 (1 Å = 10-10m) thick. It is a masterpiece of engineering. Shine light on one side and an electron will move in a series of jumps until it reaches the other side of the membrane. The result is a battery with a positive charge on the top of the membrane and a negative charge on the bottom. The catalyst is able to store these until it gets either four electrons or four positive charges, and then it will make oxygen on one side and reduce carbon dioxide on the other side. The way the chlorophyll species are arranged ensures that the positive and negative charges never come together otherwise they would annihilate each other and there would be no potential to do work. When you try to design this system in the laboratory it is impossible to arrange the chlorophyll species in precisely the correct positions so that these electron transfer reactions take place to split water'.  

Another breakthrough in our understanding of natural photosynthesis was the elucidation of the structure of the catalytic reaction centres in chlorophyll in 2004 by Professor Jim Barber and his team at Imperial College, London. There are two very complex enzyme systems which carry out the light reaction - called photosystem 1 (PSI) and photosystem 2 (PSII). Barber identified the precise location and geometry of a few critical ions - manganese, oxygen and calcium - within the core of PSII, and this has led to research teams around the world looking for more robust catalysts that will do the same job outside of the natural environment. The problem with the natural systems, explains Harriman, is that they are very elaborate and totally unstable once they are removed from their protein environment.  

Most of the current research in this area is focusing on ruthenium and iridium oxides as potential catalysts for an artificial system based on dye molecules. So far, however, scientists have only been able to achieve efficiencies of ca 0.3 per cent with such laboratory-based systems.  

Moving to the dark side 

Mimicking the dark reaction is also highly desirable. Again, in natural systems this involves a very elaborate mechanism - the Calvin cycle, named after Melvin Calvin one of the chemists at the University of California, Berkeley who discovered it. In this process a co-enzyme system, NADP⁺/NADPH, moves back and forth, picking up a proton and two electrons and depositing them for use in the production of carbohydrates.  

Chemists, notably in the US and Germany, are looking for a catalyst that will mimic the natural co-enzyme in the lab. Etsuko Fujita and her team at the Brookhaven National Laboratory in New York, recently reported a new catalyst for CO₂ reduction based on ruthenium, which holds much promise for fuel production.  

At the present time, despite having taken huge steps in our understanding of the natural process, artificial photosynthesis is still considered as 'blue-sky' research. Its further development and exploitation lie in the hands of policy makers and funding agencies, and a supply of high quality chemists and engineers in the future to deliver the breakthroughs.

This article was originally published in The Mole