From observation to control
A remarkable string of advances over the last
half-century has allowed us to probe and
understand energy-conversion phenomena at
ever smaller lengths and shorter timescales.
Aberration-corrected transmission electron
microscopy, a wealth of scanning probe
microscopies, laser- and accelerator-based
ultrafast photon pulses, intense neutron
beams and massively parallel teraflop
computing are all helping us to unravel
the structures and dynamics of the
macromolecules, proteins and complex
materials architectures that carry out energy
conversion on the nano-scale.
The next step is to not only observe, but also
control these fundamental energy-conversion
phenomena. To do this, researchers need to
exploit the remarkable progress that has been
made in nanoscience, numerical modelling and
complex materials. Nanoscience has given us
techniques for nearly atom-by-atom construction
of complex atomic and molecular architectures,
including “top-down” techniques such as optical
and electron beam lithography and “bottom-up”
approaches like molecular beam epitaxy, ink-jet
printing and the rich potential of directed
self-assembly – as exemplified by DNA-driven
biological construction. These fabrication tools
give us the means to make nano-scale structures
with the complexity, precision and functionality
of computer chips, which we now routinely
manufacture on the micro-scale.
Nanotubes offer versatile and promising
opportunities for controlling energy conversion at
the nano-scale. TiO2 nanotubes like those
pictured above are inexpensive, chemically inert,
photostable, provide high surface-to-volume
ratio and have band gaps that support
sustainable energy technologies like solar water
splitting, dye-sensitized solar cells and
transparent conducting electrodes. They can be
prepared by a variety of electrochemical
processes, doped to tune their band gaps and
Promising materials TiO2 nanotubes like these can
catalyse processes that produce hydrogen from water
using sunlight.
decorated to promote surface catalytic activity.
On the modelling side, the past decade has
seen computational speeds increase from
teraflops (1012 operations per second) to
petaflops, with exaflops on the horizon. These
remarkable hardware advances allow the
simulation of million-atom assemblies and the
detailed nano-scale energy conversions they
perform. The potential of this large-scale
atomistic simulation is not just incrementally
better, but game-changing.
With these tools in hand, our challenge is to
design materials and molecular assemblies that
can convert energy among photons, electrons
and chemical bonds with minimum losses. One
guiding example is nature. Green plants
manufacture their internal nano-scale
architectures from abundant, environmentally
friendly elements and recycle them harmlessly to
the environment at the end of their useful life.
They split water and carbon dioxide at room
temperature using sunlight and use the liberated
carbon and hydrogen to synthesize sugar to fuel
their growth and reproduction. Plants emit
unwanted oxygen to the atmosphere, where it is
recycled to carbon dioxide by respiration in
animals and other living things, closing the
chemical cycle and leaving no change. Cellulose
in the leaves and stalks of plants is rich in sugars
that can be converted to ethanol and other
biofuels, provided we can break down the
protective coating of lignin that shields cellulose
from physical and chemical attack during its life.
We can learn from biology by observing how
lignin breaks down on the forest floor and how
sunlight splits water and carbon dioxide in the
growing plant, and adapt these processes to
create and control our own sustainable
hydrocarbon fuel cycle.
Another emerging guide is numerical
materials simulation, which allows us to imagine
complex nano-scale structures and then
simulate their behaviour to see if they function
as we intend. Such simulations dramatically
shorten nature’s design process, which proceeds
by incremental random mutations of existing
structures. Once filtered and refined by computer
simulation, the most promising nano-scale
energy-processing structures can then provide
inspiration for fabrication.
Complexity is an essential ingredient in any
functional design. Like information processing,
energy processing requires many sequential
steps, including manipulating quanta of energy
in photonic, electronic and molecular
excitations, as well as chemical transformation.
Not only must each step in the sequence be
understood and controlled, but also the
individual steps must be integrated into a
seamlessly functioning assembly that efficiently
links the functions of its constituent parts.
Information technology and biology provide two
shining examples of the value and impact of
complexity on functionality. We have mastered
the micro-scale complexity of information
technology; the next frontier is the nano-scale
complexity of biological function and
sustainable energy technology.
higher energy density and longer life-cycles; non-aque-ous electrolytes for higher operating voltages; and
entirely new electrochemistry approaches such as
lithium–air electrodes or doubly ionized cations that
can lower the battery’s charge-to-mass ratio.
Fuel cells offer an alternative to batteries by generating electricity onboard via hydrogen oxidation.
Developing this alternative requires scientific break-
throughs in catalysis for the oxygen-reduction reaction
at fuel-cell cathodes, high-density storage of hydrogen
in lightweight solid compounds and the production
of hydrogen from renewable resources. Substantial
progress has been made in the last five years towards
overcoming these barriers to using hydrogen as an
energy carrier, as indicated by the rise in the number
of researchers and published papers in the field. Such
advances raise the probability that fuel cells will be a
viable long-term option for transportation.
The sustainability profile of electric transportation
is potentially high because electricity, once produced,
is environmentally benign and leaves no chemical
change. The primary sustainability issue is the large-scale production of electricity for battery-powered
vehicles or of hydrogen for fuel-cell vehicles. Existing
production routes use fossil fuels to generate electricity and the reformation of natural gas to produce
hydrogen; both deplete finite natural resources and
emit substantial amounts of carbon dioxide. In contrast, using renewable electricity produced by solar and
wind to charge batteries or solar-powered water splitting to produce hydrogen has the potential to last a long
time, do no harm and leave no change. Achieving these
renewable production routes will require breakthroughs in discovery and use-inspired basic science.
