by Prof. Mark Biggs
Various learned-bodies1,2 and
even Nobel Laureates3 have over the past recent
years identified a range of Grand Challenges that Humanity must address in the
coming decades in order to ensure the survival of our way of life - tackling the
root causes of climate change whilst meeting huge increases in energy demand,
reducing our impact on the wider environment, exploiting our natural resources
more efficiently, ensuring adequate supplies of safe drinking water, and defence
against terrorism are just a few of these key challenges.
Nanoporous carbons have long played a role in the areas associated with these
Grand Challenges (e.g. purification of drinking water; capture of volatile
organic compounds from industry; gas masks), but they have an ever larger role
to play into the future. For example, nanoporous carbons are at the core of
Lithium-ion batteries,4 which are to be used to
substantially increase the range of the next generation of hybrid
vehicles5 - this is essential to reducing
CO2 emissions from transport,6 which
accounts for a third of all such emissions.7
These batteries and nanoporous carbon-based supercapacitors are being
developed for energy storage from intermittent renewable sources such as wind
too8 - such energy storage is essential to large
scale use of renewable energy.6 Nanoporous carbons
are also a serious alternative for separating CO2 from exhaust gas
streams,9 which is part of the so-called 'carbon
capture and sequestration' strategy that many are pursuing in an effort to
ensure the future use of coal as a fuel.6 Finally,
impregnated nanoporous carbons may be effective hydrogen storage media10 - the current absence of any such technology is a major
barrier to the realisation of the 'hydrogen economy'.6
So what is this magical material we call 'nanoporous carbon'? Some may
recognise the more old fashion terms of 'activated carbon' or 'microporous
carbon' - nanoporous carbon encompass these non-crystalline carbonaceous
materials as well as more novel forms of porous carbon such as carbon nanotubes
and templated carbons. Nanoporous carbons are highly porous carbon-dominated
materials that almost always contain small quantities of heteroatoms such as
oxygen, hydrogen, and, depending on their origin, nitrogen, sulphur and even
'heavy metal' atoms.
The pore 'widths' in these materials typically range from less than a
nanometre through to 10s of nanometres and even larger. The size and geometry of
the pores combined with the nature of the solid carbon skeleton means nanoporous
carbons have large surface areas11 - typically 1 to
15 tennis courts worth per gram of material - compared to many other porous
materials. The pore sizes and large surface areas, the ease with which they can
both be modified, and the relative inertness of nanoporous carbon are just a few
of the reasons why it is such a popular material.
Despite their long history of exploitation, many may be surprised to learn
that experimental trial-and-error still dominates the development of many
nanoporous carbons, especially the less crystalline forms, at least from a
molecular perspective. The limited use of molecular modelling in the design of
less crystalline forms of nanoporous carbon can be contrasted with its use for
zeolites12 and other crystalline materials such as
metal-organic frameworks13 where an identifiable
unit cell greatly facilitates modelling.
For many nanoporous carbons, the so-called 'slit-pore' model (see Figure 1) -
which dates back to at least Emmett in the 1940s14
- has been used as a proxy unit cell. The utility of this model more generally
has been proven many times. For example, it continues to underpin many of the
experimental carbon characterization methods used today. It was also used in the
early 1990s in conjunction with molecular simulation of methane adsorption to
identify the optimal pore size and conditions for adsorbed natural gas
|Figure. 1. The
slit-pore model of Emmett14: a pore width h is defined by the basal surfaces of
two opposed semi-infinite blocks of graphite. As the review of Bandosz et
al.22 indicates, this model has been used
extensively with molecular simulation to study adsorption, diffusion and
reaction in nanoporous carbons. © Mark J. Biggs
Despite the popularity of the slit-pore model of Emmett, it also omits many
details of carbons that can play important roles in many circumstances. For
example, adsorption in pores with the thin walls suggested by experiment differs
from that of the slit pore model.16 The model also
does not admit finite pore lengths, which can be a significant source of surface
area,11 nor the pore system topology that is known
to be important in diffusion.17 Finally, it does
not permit inclusion of heteroatoms in a realistic manner, which are central to
hydrogen storage,10 adsorption of polar and ionic
fluids,18 and catalysis19 and amongst other phenomena and technologies.
The shortcomings of the slit-pore model encouraged Biggs in the 1990s20,21 to develop a model of nanoporous
carbons, termed Virtual Porous Carbon (VPC), that captures at least
qualitatively some of the things that were missing from the slit pore model (see
Figure 2). As recent invited reviews indicate,23
this approach has been used extensively to better understand the fundamentals of
adsorption and diffusion in carbons, and assess and develop improved
adsorption-based characterisation methods and, most recently, models for
diffusion in carbons.24
Figure 2. Snapshot
from a Monte-Carlo simulation of adsorption (top) and non-equilibrium molecular
dynamics simulation of mass transport (bottom) on a Virtual Porous Carbon (VPC)
of Biggs. In the top image, the carbon atoms and fluid molecules are shown in
grey and blue respectively. In the bottom image, the pathways taken by the fluid
through the VPC under a pressure gradient (acting from the right to left) are
shown by the blue envelope, which has been cut-open in places to reveal the
fluid velocity field (red highest to dark blue lowest speed). © Mark J. Biggs
Other VPC models have appeared in more recent years, including a class of
models that use Reverse Monte Carlo, a so-called inverse method, to force the
models to match direct measures of the microscopic structure of the target
carbon such as the carbon-carbon radial distribution function obtained from
X-ray diffraction.25-31 Despite these advances,
even the most sophisticated VPC models of today are missing many details that
are often central to the performance of carbons including adequate treatment of
heteroatoms and structural order beyond 1-2 nm. These issues along with the
uniqueness problem that attends all inverse methods must be addressed before
VPCs can be used in the design context - this work is currently underway in the
laboratory of Professor
Biggs at The University of
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Copyright AZoNano.com, Professor Mark J. Biggs (The University