30
electro-chemical
storage
gasoline
energy/volume (MJ/l)
20
10
electrical storage
ethanol
uct, helium, is likewise kind to the environment and
climate. The fusion process, however, is even more
extreme than fission, requiring neutron fluxes approximately 100 times greater than in fission reactors.
Designing materials to withstand these exceptional
irradiation conditions is a major scientific challenge.
hydrogen
compounds
(target)
methanol
chemical storage
and combustion
compressed
hydrogen gas
0
0
supercapacitors
10
batteries
20
energy/weight (MJ/kg)
30
Biofuels and electric cars
Replacing conventional oil with biofuels has the potential to achieve greater sustainability by recycling carbon
dioxide and enabling fossil resources to last longer.
However, it is now generally accepted that the energy
balance and carbon footprint of corn ethanol and gasoline are only marginally different. In terms of lasting
a long time and doing no harm, therefore, the two are
approximately equally sustainable. Cellulosic ethanol
made from the stalks and leaves of plants offers more
hope. The scientific challenge for cellulosic ethanol is
to discover or design a better chemical-conversion
route from cellulose, nature’s evolution-hardened construction material, to fermentable sugar or liquid fuel.
The known chemical and enzymatic routes are too
expensive and inefficient to be competitive. Biofuels
from algae offer an alternative route, since cultivating
algae requires far less land than other biofuel crops,
but a cost-competitive conversion route is not yet available and the science is still in its infancy.
Recycling carbon dioxide and water to produce fuel
can also be done without biology, by using concentrated
solar heat to drive high-temperature thermochemical
reactions or electronic excitation from solar photons
to drive photochemical reactions at room temperature.
Both routes face materials challenges, and neither is
scientifically ready to deploy. Thermochemical recycling requires advanced hybrid materials that can
physically withstand and chemically promote the targeted reactions at high temperatures, and photochemical recycling requires new cost-effective catalysts that
split carbon dioxide and water to drive the synthesis of
hydrocarbon fuel. These challenges are firmly in the
realm of discovery. Overcoming them will require use-inspired basic research (see box opposite).
The sustainability profile of recycling carbon dioxide through biofuels (other than corn ethanol) and
thermochemical or photochemical cycles is promising.
These technologies can be fully or partially renewable
and thus last a long time. They reduce harm to the environment by lowering greenhouse-gas emissions, and
they have the potential to close the chemical cycle and
leave no change.
Electrifying transportation breaks the exclusive dependence of transportation on oil, thereby enabling
flexibility in fuelling as more sustainable alternatives
for electricity generation become available. Electric
motors are also far more efficient (over 90%) and
mechanically much simpler than gasoline engines, and
so are able to transport people and goods at a much
lower cost per mile. The challenge is onboard storage
or generation of electricity to power the electric motor.
For batteries, the energy density must be increased by a
factor of two to five from current levels before the journey range of electric vehicles becomes competitive with
multipurpose gasoline vehicles (see figure above). The
materials challenges are to develop electrodes for
40
How to store it Chemicals like gasoline and ethanol store energy at much higher densities than
batteries. With scientific advances, the gap can be filled with electro-chemical storage where
chemical energy is converted to electricity in fuel cells.
kinetic energy from an incident particle is transferred
successively to electronic, atomic, vibrational and structural systems, requiring a diverse mix of theoretical formulations appropriate for different spatial scales.
To create new extreme materials that operate effectively in reactor environments we will have to go beyond
observation and modelling to controlling the evolution
of defect structures and interrupting their development
before they can degrade performance. Introducing
designer interfaces that collect and trap nano-scale
defects before they cluster is one strategy for making
materials defect-tolerant or self-healing. Treating defected regions with targeted photon or particle beams
to anneal out the damage before it grows beyond a critical size is another. Although developed for nuclear
applications, next-generation materials with these features should find wide application in other areas, including high-temperature turbines and coal-fired
boilers, thus bringing overall energy efficiency closer to
thermodynamic limits.
The sustainability profile of nuclear electricity shares
many qualitative features with carbon sequestration.
Like coal, terrestrial uranium resources will last a few
hundred years, longer than oil but not as long as the
Sun, and nuclear reactors emit no carbon dioxide into
the atmosphere. The threat to the atmosphere is, however, replaced by a new potential danger: spent fuel
that must be stored, perhaps underground, for hundreds or thousands of years to reduce its radiation level
by natural decay. Developing fuels that fission a larger
fraction of the available nuclei in a fuel rod – the current average is 4% – would mean that uranium supplies
would last longer, and that the storage requirements
for spent fuel would be less. Still, nuclear waste, like sequestered carbon dioxide, has the potential to leak,
threatening water supplies and human health. New
methods of treating, containing, monitoring and modelling nuclear waste are needed. Finally, like fossil-fuel
electricity, nuclear electricity leaves substantial chemical change by removing uranium from the Earth and
replacing it with radioactive waste.
Fusion electricity is more sustainable than fission
because it replaces hazardous heavy-element fuel with
benign, light and abundant hydrogen. The fusion prod-
