10 August 2019 | New Scientist | 37mature, fully integrated industry,” he says.
“I think artificial photosynthesis is at the
same stage the photovoltaic industry was
at when I was a student.”
As well as artificial photosynthesis, scientists
at Chalmers University of Technology in
Sweden are working on a liquid that traps solar
energy and releases it as heat. It can already
store energy about as effectively, in terms of
joules per kilogram, as a lithium-ion battery,
and for months at a time. It is less efficient
than a solar panel, but team leader Kasper
Moth-Poulsen is hopeful it will improve. “For it
to absorb energy, all you need do is put it in the
sun,” he says. Then, when you need to release
the heat, you just add a catalyst. An efficient
solar heating system that works all year round
could have a huge impact on emissions.
With luck, commercial artificial
photosynthesis and solar liquids could be
with us in a couple of decades. Beyond that,
Atwater is imagining the ultimate form of
energy from the sun: space-based solar
panels (see “Orbiting power plants”, left).
As exciting as such developments could
be, on its own, solar won’t be enough.
“We need to be more efficient,” says Chase.
“We need to use more public transport, more
cycling.” Still, though, the growth of solar is a
reason for cautious optimism. “The progress
has made me start to think that we can
maintain some sort of developed-world
lifestyle using solar,” she says. ❚many complex molecules, including plastics.
At the moment, Atwater’s lab is able to turn
atmospheric CO 2 into a combination of about
70 per cent ethanol, a fuel, and ethene, the
basis for many plastics. The other 30 per cent
is mainly hydrogen, which is itself useful. It is
a far less efficient process than water-splitting,
but progress is being made.Copying nature
The difficulty is that artificial photosynthesis
relies on catalysts, but existing ones rely on
external energy and are often toxic. So some
other groups are looking to nature for
alternatives. Erwin Reisner and his team at the
University of Cambridge have made progress
with “semi-artificial” photosynthesis, using
natural enzymes as the catalysts. These proteins
are much more effective catalysts and are better
at creating precisely the products you want with
no extra energy push. The downside is that they
break down in a few hours or days. “In
organisms, they are constantly repaired and
replaced,” says Reisner. He has two hopes: that
this semi-artificial technology can be harnessed
to improve fully artificial catalysts or, further
in the future, that bioengineered algae could
create the required hydrocarbons, using natural
enzymes that they repair in the traditional way.
Artificial photosynthesis will allow solar
energy to be stored through the winter and
make it easier to power systems that are hard
to electrify, such as heavy vehicles and air
transport, says Atwater. And although the
technology is still new, he believes it could
improve rapidly. When talking to his grad
students, he tells them about the photovoltaic
sector when he started out. “Now it’s aTom Chivers (@TomChivers)
is a science journalist based
in London. His book The AI
Does Not Hate You is out nowJAMEY STILLINGS
Solar has become
a priority in Japan“ Artificial
photosynthesis
will allow solar
energy to be
stored through
the winter”
turn inert chemicals into energy-filled ones.
The most basic version of this splits water to
produce hydrogen. A photovoltaic cell creates a
current that passes through water via a
positively charged anode and a negative
cathode. Oxygen ions gather at the anode and
hydrogen ions at the cathode. “A lab prototype
here reported a world record of 19.3 per cent
efficiency,” says Atwater: that is, nearly
20 per cent of the solar energy hitting
the photovoltaic cell was stored in usable
hydrogen fuel. “It’s an expensive device,
but it’s a demonstration of what’s possible.”
It is relatively easy to make hydrogen from
water. “We did it as high school students with
a copper wire and a platinum wire,” says
Atwater. “Your feedstock is inexpensive.”
One complication is that pure water is needed
to avoid the cathode becoming fouled by
impurities, but it is straightforward to produce
very pure hydrogen that can be used in fuel cells.
A bigger hurdle is making hydrocarbon fuels
from atmospheric carbon dioxide, says Atwater.
“It’s more difficult for fundamental reasons.
The chemistry of carbon is very rich: diamond,
graphite, polymers, methane – all are based on
carbon.” Carbon dioxide can be turned into
many different forms, he says. “The challenge
is designing a process where if you want to
make one thing, you get exclusively that. If you
want to make ethanol, you don’t want to end
up with carbon monoxide.” But although it is
harder, it has two advantages over water-
splitting: it strips a greenhouse gas from the
atmosphere and, alongside the fuel, it results
in forms of carbon that can be used to make