The promise of artificial photosynthesis
It is still unclear where most of our energy will come from in the longer-term future. Solar power cannot produce industrial quantities of electricity, while the tide is turning against wind turbines because they spoil the landscape and too many would be needed to replace conventional generators. Nuclear energy remains in the doldrums. Fossil fuels continue to threaten global warming.
But a promising new contender is emerging: the harnessing of photosynthesis, the mechanism by which plants derive their energy. The idea is to create artificial systems that exploit the basic chemistry of photosynthesis in order to produce hydrogen or other fuels both for engines and electricity. Hydrogen burns cleanly, yielding just water and energy. There is also the additional benefit that artificial photosynthesis could mop up any excess carbon dioxide left over from our present era of profligate fossil fuel consumption.
As we learned in school, photosynthesis is the process by which plants extract energy from sunlight to produce carbohydrates and ultimately proteins and fats from carbon dioxide and water, releasing oxygen into the atmosphere as a by-product. The evolution of photosynthesis in its current form made animal life possible by producing the oxygen we breathe and the carbon-based foods we eat. Photosynthesis does this on a massive scale, converting about 1,000bn metric tons of carbon dioxide into organic matter each year, yielding about 700bn metric tons of oxygen.
The first problem evolution faced was that the chemical reactions involved in carbohydrate formation are “uphill,” meaning they require energy to drive them forward. Only one source of energy was available on earth—from the sun—but the trouble is that “uphill” chemical reactions need energy in the form of electrons moving at high speeds to power them, in other words an electrical potential or voltage. Plants are in effect solar cells converting light into electrical energy. But for this to be sustainable, plants need a constant source of electrons, and this has to be an element or compound already present in the plant. Evolution tried a variety of chemicals such as hydrogen sulphide early on, and some of these are still used in certain bacteria. But there was a more promising candidate because of its ubiquitous presence — water.
It takes about 2.5 volts to break a single water molecule down into oxygen along with negatively charged electrons and positively charged protons. It is the extraction and separation of these oppositely charged electrons and protons from water molecules that provides the electric power. In plants, chlorophylls evolved to harvest light, and a complex labyrinth of proteins to conduct the photons (units of light energy) to a suitable centre where this crucial water-splitting takes place. In plants, oxygen is the only by-product of this process, but researchers realised some years ago that the reaction could be tweaked to produce hydrogen as well.
Still, tweaking photosynthesis to produce hydrogen rather than electrical energy is the easy bit, and researchers such as Stenbjörn Styring at Lund University in Sweden believe it will be possible to do so in artificial systems within one or two years. The hard part is to replicate the process of splitting water to obtain the electrons and protons in the first place, and this is where a recent breakthrough made by a British team at Imperial College comes in. Through a combination of rigorous analysis and innovative experiment, the team led by professors Jim Barber and So Iwata identified the precise location of just a few critical molecules of manganese, oxygen and calcium within the core of the plant’s photosynthesis engine where the water-splitting is performed.
What is striking about this chemical reaction, to which we owe our existence, is that the critical chemistry is co-ordinated by just a single atom of manganese within the photosynthesis core. The precise geometry of this core is vital to the process, as water molecules are shaped a bit like Mickey Mouse heads, with one oxygen atom bearing a pair of smaller hydrogen atoms forming the ears.
The achievement of Barber and colleagues has been to determine the precise events taking place within water-splitting at the molecular level as each photon of light arrives in the core. This is a level of detail far beyond that known for most chemical reactions in biology.
Following publication of this work in Science in March, leading specialists in artificial photosynthesis such as Styring are eager to start working on mimicking the water-splitting process in the laboratory. Attempts to do so have failed so far because the process is so finely balanced that the geometry has to be just right. Only now do researchers have sufficient detail of the geometry to start building workable systems.
Although such artificial systems will mimic the water-splitting chemistry of natural photosynthesis, they will not look like plants. Artificial systems will use metals such as ruthenium and iron to capture light and provide a scaffold for the water-splitting core. But the core itself would be based on manganese.
These are early days, but the recent breakthrough gives some grounds for optimism. The alternative method of producing hydrogen through water electrolysis powered by solar cells could also work, but photosynthesis promises a more efficient, elegant and economical source of power.
Philip Hunter is a science writer
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