Nanoporous Carbons for Today's Grand Challenges - Opportunities and Barriers

Various learned-bodies^1,2 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 storage.¹⁵

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 2010.

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 1990s^20,21 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 2010.

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 Adelaide.

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