ENERGY
GREEN ENERGY
A
t the dawn of the third millennium, two major challenges face the human race. First, carbon in the
atmosphere must be decreased lest we initiate a climate change event that our species cannot survive; and secondly, we need clean, cheap plentiful energy. One technology offers a solution
to both problems: photosynthesis. The past two decades have seen an explosion of research into how to replicate this process either as a one or two step - sunlight capture and synthesis - process. These artificial photosynthesis
approaches can each be classed according to their material types. They all involve the four basic steps as seen in natural photosynthesis: light harvesting, charge separation, water splitting and fuel production.
Biomimetics and biology Molecular systems are biomimetic; they copy biology. Studies of energy conversion and storage in natural enzymes provide inspiration for the copying of the complex chemistry of natural photosynthesis by mimicking enzymatic catalytic functions. Entirely molecular systems are
difficult to develop, but have the advantage of allowing a modular approach where the four phases of the photosynthetic process are tackled individually before being merged together into a structure that should do the whole job. However, the absence of knowledge of how the system spontaneously functions in nature currently renders a molecular assembly approach impractical. The crucial missing link is how the so-called ‘responsive matrix’ – the
Researchers are finally closing in on a route to green, cheap energy – one that our planet has been using for over 2bn years, Richard Corfield reports
structural framework that organises molecular reactivity, self-assembly and self-repair– actually works. Another problem with mimicking the natural processes of photosynthesis is that most organic molecules tend to degrade quickly under extended exposure to sunlight. Natural photosynthesis is also inefficient: less than 1% of sunlight in converted into biomass energy. Conventional Si/Ti based solar cells, by contrast, have an efficiency of about 40%.
One of the goals of artificial
photosynthesis is to make it more efficient and modify the output from carbohydrates to a fuel with a higher energy density. Combustible products such as hydrogen or alcohols are preferable. Hydrogen can be used directly as liquid fuel or channelled into a fuel cell. Getting the process to produce hydrogen is not a problem, since it’s already there in the water molecules. And alcohols can be interconverted on an industrial scale to provide the fuel of choice. The complexity of photosynthesis is splitting the water molecules to get the electrons necessary to facilitate the chemical process to produce hydrogen. This means the process requires a catalyst.
But mimicking key functions of
the photosynthetic centre, where specialised biomolecules carry out photosynthesis, has proved challenging. Artificial photosynthesis requires designing a system that can absorb light, transport and separate electrical charge, and catalyse fuel-producing reactions - complex processes that must operate synchronously to achieve high energy-conversion.
Semiconductor cells Inorganic systems include established photovoltaic cell technology where semiconductors absorb sunlight and separate electrical charges. Semiconductors are therefore obvious candidates for incorporation into artificial photosynthetic systems; they can split water and produce fuel at the surface of the electrodes. They are also stable under prolonged exposure to sunlight. Titanium-based photovoltaic cells
have been around for almost 40 years. But titanium is not a common element nor is it as efficient as other solar converters could be. One problem is that many
semiconductors with the right electronic properties to split water either absorb only UV light, or require
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