Scientists Explore How the Humble Leaf Could Power the Planet

It is one of evolution's crowning achievements - a mini green power
station and organic factory combined and the source of almost all of
the energy that fuels every living thing on the planet.

Now
scientists developing the next generation of clean power sources are
working out how to copy, and ultimately improve upon, the humble leaf.
The intricate chemistry involved in photosynthesis, the process where plants
use sunlight to convert water and carbon dioxide into sugar, is the
most effective solar energy conversion process on Earth. And
researchers believe that mimicking parts of it could be the ticket to a
limitless supply of clean power.

The untapped potential for using
the sun's rays is huge. All human activity for a whole year could be
powered by the energy contained in the sunlight hitting the Earth in
just one hour. Harnessing even a small amount of this to make
electricity or useful fuels could satisfy the world's increasing need
for energy, predicted to double by 2050, without further endangering
the climate.

Most solar power
systems use silicon wafers to generate electricity directly. But
although costs are coming down, these are still too expensive in many
cases when compared with fossil fuels such as coal, oil and gas.
Scientists are keen to develop more efficient and cheaper alternatives
sources of energy.

At Imperial College London, researchers have
embarked on a PS1m project to study, and eventually mimic,
photosynthesis. Part of a project called the "artificial leaf",
involves working out exactly how leaves use sunlight to make useful
molecules. The team then plans to build artificial systems that can do
the same to generate clean fuels such as hydrogen and methanol. These
would then be used in fuel cells to make electricity or directly to
power super-clean vehicles.

Similar projects are gathering pace
around the world: the US is poised to approve a federal research budget
of around $35m a year for ideas that could create fuels from sunlight
and the Dutch government has allocated EUR40m for similar research.

According
to James Barber, a biologist at Imperial College London and leader of
the artificial leaf project, if artificial photosynthesis systems could
use around 10% of the sunlight falling on them, they would only need to
cover 0.16% of the Earth's surface to satisfy a global energy
consumption rate of 20 terawatts, the amount it is predicted that the
world will need in 2030. And unlike a biological leaf, the artificial
equivalent could be placed in the arid desert areas of the world, where
it would not compete for space agricultural land.

Ultimately,
Barber hopes to improve on nature's own solar cell. "If the leaf can do
it, we can do it but even better," he said. "[But] it doesn't mean that
you try to build exactly what the leaf has. Leonardo da Vinci tried to
design flying machines with feathers that flapped up and down. But in
the end we built 747s and Airbus 380s, completely different to a bird
and, in fact, even better than a bird."

Photosynthesis starts
with a chemical reaction where sunlight is used to split water into
hydrogen and oxygen. The oxygen is released into the atmosphere while
the hydrogen is used to create sugars and other organic molecules for
the plant. The aim of Barber's artificial leaf project is to find an
efficient way of mimicking that water-splitting reaction to create a
clean and limitless source hydrogen. Unlike normal leaves, the new
devices would not suck CO2 out of the atmosphere.

Hydrogen is a
clean, energy-rich fuel that could be used in fuel cells to make
electricity or else combined with carbon dioxide from the atmosphere
(or from the exhaust of fossil-fuel power stations) to make methanol, a
fuel that could be dropped into vehicles without the need for any
engine modifications. "The challenge is to get hydrogen out of water
using a ready supply of energy," said Barber.

For domestic
purposes, Dan Nocera, a chemist at the Massachusetts Institute of
Technology, has calculated that using artificial leaf to split a few
litres of water a day into hydrogen and oxygen would be enough to
supply all a home's energy needs.

Scientists can already produce
hydrogen by splitting water but current techniques are expensive, use
harsh chemicals and need carefully controlled environments in which to
operate. The critical part of the artificial leaf project is developing
catalysts made from cheap materials that can be used to split water in
everyday conditions.

John Loughhead, executive director of the UK
Energy Research Centre, described the artificial leaf idea as very
promising because "we know that plants have already evolved to do it
and we know that, fundamentally, it's a workable process on a large
scale."

He added: "Ultimately, the only sustainable form of
energy we've got is the sun. From a strategic viewpoint, you have to
think this looks really interesting because we know we're starting from
a base of feasibility."

Barber's colleagues at Imperial, led by
chemist James Durrant, have recently developed a catalyst from rust
that carried out part of the water-splitting reaction. So far the
process is not very efficient, so Durrant's team is looking at
improving this by engineering the surface of the rust. "We're looking
at adding small catalytic amounts of cobalt onto the surface of the
iron oxide to make it more efficient."

Nocera is also working on
a catalyst made from cobalt and phosphorus that can split water at room
temperature. Speaking last year, when he published his preliminary
results in the journal Science, he said efficient water-splitting
technology would be useful as a way of storing solar energy,which is a
major problem for anyone who wants to use large amounts of solar power.
During the day, an artificial leaf could use sunlight to split water
and, at night, the stored hydrogen would be used to make electricity as
it was needed. Chemical fuels such as hydrogen can store far more
energy per unit mass than even the most advanced batteries.

Both Durrant's and Nocera's catalysts are many years from becoming commercial products.