7.2.1 Energy Production

Solar energy. There is expected to be direct impact on photovoltaic cells converting radiant energy from the sun into electricity. The main primary obstacle to their widespread deployment is the high cost of conventional photovoltaic cells. Devices incorporating particles (e.g., dye-sensitized solar cells (DSSC) or Grätzel cells [4]) offer potentially much lower fabrication costs, especially if inkjet printing technology can be adopted. The potential of incorporating further complexity through mimicry of natural photosynthesis, the original inspiration for the Grätzel cell, is not yet exhausted. The main challenge is to design and fabricate highly efficient and robust catalysts. This requires atomic resolution, however, and therefore must await the development of productive nanosystems (assembler-based fabrication). Robustness is a particularly difficult goal. Unlike natural systems, the components of which are constantly being renewed, the artificial system should remain functional without intervention for many years.

The main secondary obstacle to widespread deployment of photovoltaic cells is that except for a few specialized applications (such as powering air-conditioners in Arabia) the electricity thus generated needs to be stored, hence the interest in simultaneous conversion and storage in chemical form, mimicking much more closely natural energy harvesting. This can also be encompassed within the concept of the Grätzel cell. Undoubtedly natural photosynthesis is only possible through an extremely exact arrangement of atoms within the photosystems working within plant cells, and the more precisely artificial light harvesters can be assembled, the more successful they are likely to be. The problem of robustness is omnipresent.

In view of the intensive research into solar cells, the specific impacts of nanotechnology on progress are not always easy to discern; novel silicon phases also show promise [5]. Reducing the thickness of some of the laminar elements in a photovoltaic cell will save on materials and may enhance efficiency. All applications using catalysts will benefit from rationally designed catalysts constructed atom by atom, but there is no practical fabrication technology for that at present.

Presently photovoltaic electricity generation is rather expensive, and has only managed to achieve a niche in the market because of generous government subsidies. Two key parameters are cost and efficiency. The latter has been relentlessly increasing—albeit linearly rather than exponentially (see Figure 7.1)—but reducing the former has made giant strides more akin to exponential progress through the development of roll-to-roll printing of organic polymer solar cells. Presently these low-cost cells are less efficient and less robust but it seems inevitable that they will become competitive with conventional means of generating electricity.

Fuel cells. Although the scientific basis of this technology, whereby fuel is converted to electricity directly, was established over 150 years ago by Christian Schönbein, it has been very slow to become commercially established. As with photovoltaic cells, the main primary obstacle is the high cost of fabrication. Nanotechnology is expected to contribute through miniaturization of all components (especially reducing the thickness of the various laminar elements), simultaneously reducing inefficiencies and costs, and through realizing better catalysts for oxygen reduction and fuel oxidation. A particular priority is developing fuel cells able to use feedstocks other than ultrapure hydrogen. The catalytic problem is particularly acute here, since there seems to be an inverse relationship between the efficiency of a catalyst and its sensitivity towards inactivation by impurities in the fuel. Until we have routine atom-by-atom assembly, however, it seems that the impact of nanotechnology on catalyst fabrication will be minor since we shall not be able to much improve on existing technology. There is, however, hope that novel catalytic alloys may enable fuel cells to become more commercially viable by diminishing the amount of platinum required. For example, Pt₃Y has recently been found to be more efficient than pure Pt [6].

Nanotechnology in the oil and gas industry. Although according to conventional wisdom the use of oil and gas as an energy source will diminish, its huge significance will remain for many more years, long enough for it to benefit from nanotechnology. Applications are extremely diverse. The extraordinary rheological properties of clay-based drilling muds have, in fact, been exploited for decades. There is much scope for even more advanced sensorial and responsive drilling fluids [7], for example basing them on polymers incorporating nano-objects. Many applications are generic, such as the use of nanocomposites as lightweight, rugged structural materials for drilling rigs, offshore platforms, pipelines, and so forth. Ultrahard nanocomposites can enhance the performance of drilling parts. Further downstream, nanocatalysts should enhance yields of petroleum refining operations, and precisely engineered nanomembranes may achieve a convenient separation of desirable hydrocarbons from impurities. More indirectly, nano-engineered antifouling coatings for oil tankers have the potential to enable significant savings in the energy needed to propel those vessels. Furthermore, the enormous expense of the current generation of giant tankers puts a premium on keeping them almost continuously in service, and their huge size makes servicing their hulls difficult, hence the contribution of nanotechnology to creating maintenance-free hulls will be of great significance.

7.2.2 Energy Storage

The primary means of storing energy is as fuel, but unless photoelectrochemical cells generating fuel from sunlight receive renewed impetus, renewable sources will mainly produce electricity and except for some special cases (such as photovoltaic-driven air-conditioning installations in Arabia as already mentioned) at least some of the electricity will have to be stored in a buffer to enable supply to match demand. High energy and power densities have become particularly relevant as a consequence of the search for electric replacements for the fossil fuel-powered internal combustion engines for vehicles. The main contenders are supercapacitors and accumulators. The proliferation of portable electronic devices has also greatly increased demand for small and lightweight power sources for goods such as laptop computers and cellphones, especially in the absence of any viable energy harvesting technology.

Supercapacitors based on carbon nanotubes have attracted especial interest for rapidly responsive energy storage, but the impact of nanotechnology is likely to be small since using ordinary carbon black has already enabled over 90% of the theoretical maximum charge storage capacity to be achieved, at much lower cost. In any case, the classic photoelectrochemical process generates hydrogen (from water), the storage of which is problematical. The same problem besets hydrogen-fed fuel cells—unless the storage problem solved effectively and economically, the “hydrogen economy” will not be able to get established. Through the design and fabrication of rational storage matrix materials, nanotechnology should be able to contribute to effective storage, although whether this will tip the balance in favor of the hydrogen economy is still questionable. Conventional routes to materials synthesis had become highly developed without making use of nanotechnology, except in a secondary fashion by exploiting nanometrology tools during the discovery process.

Typically, energy storage devices such as batteries can be nanified by making internal layers thinner and more accurately, which not only reduces costs of materials (provided they can be manufactured) but also improves their performance. The actual achievement, on an industrial scale, of nanoscale internal structure might well require a revolutionary change in manufacturing technology. Clearly achieving it through assemblers would be revolutionary. Otherwise, the most revolutionary contribution seems to be through the use of nanoparticles rather than thin films. This enables well-established printing technologies to be used for fabricating devices.

Mechanical storage devices, such as large flywheels, are obviously the antithesis of nanotechnology, although it can help indirectly with low-friction sliding or rotating surfaces and so forth. The largest flywheel installations can now store about 100 MJ and deliver about 1 MW pf peak power.

Electrical Storage Devices

Electrical capacity depends on the distance separating the oppositely charged plates in a condenser, and interfacial area contributes to the performance of chemically-fuelled storage batteries. It is obvious that such systems can be structurally enhanced with respect to performance by nanification. A great deal of work is currently being undertaken in many laboratories worldwide (but especially in the USA, Japan, and some European countries) on these topics, and on the related one of hydrogen storage. The feasibility of implementing any technically appropriate solution will depend on issues such as compatibility and total life cycle. Previously, it has been accepted that the relatively small achievable improvements in, say, supercapacitor performance through replacing carbon black, which is a very imperfect nanomaterial, by precisely fabricated carbon nanotube arrays were not worth the great increase in expense. However, significant improvements in carbon nanotube synthesis are constantly taking place and the feasibility of any proposed application will need to be assessed in terms of the very latest technology developments.

Table 7.1 compares the specific energies and energy densities of some current and emerging technologies. A third parameter is specific power (“gravimetric power density”). Here, capacitors are as good as combustion fuels (around 1 MW/kg), which are much better than batteries and fuel cells (around 100 W/kg).

Table 7.1

Specific energies $\mathfrak{E}$ and energy densities $\mathfrak{D}$ of existing storage systems,^a ranked by $\mathfrak{E}$

Type	$\mathfrak{E}$/MJ kg−1	$\mathfrak{D}$/GJ m−3
Gasolineb	46	34
Aluminiumb	31	84
Al–air battery	4.6	?
Thermite	4	18
Fuel cellc	1	wide range
Li-ion batteryc	0.7	2
Na–NiCl batteryc	0.43	?
Lead–acid batteryc	0.14	0.36
Supercapacitorc	0.1	0.2

a Specific energy is energy per unit mass—sometimes, confusingly, called energy density, which is energy per unit volume. The latter parameter is less well defined than the former and also liable to increase as new (nano)materials are developed.

b Not self-contained—requires external oxidizer and a combustion apparatus, the output from which might only be 10% of the given figure.

c Approximate, from laboratory experiments (there is a wide range depending on design and other considerations).

It will be noted that the Al–air battery (a rechargeable device developed by Europositron) comes close to the practical output from the gasoline-fueled internal combustion engine. The best fuel cells are still operating far below their theoretical limits, but given that the field has already been researched for about 170 years it must be conceded that the barriers to further progress are considerable. Nanotechnology can in principle contribute in many ways. Lightweight refractory nanocomposites could increase the effective specific energy of gasoline, and nano-engineered catalysts would benefit fuel cells. Nano-engineering will have the most direct impact on supercapacitors, whose stored energy depends directly on their internal surface area.

A bald list of specific energies might not be adequate to convey the potential of the technologies. Very often, it is through judicious combinations that really important benefits can be achieved. As an example, consider remote equipment such as miniature sensors needing small amounts of electrical power round-the-clock. The combination of a small photovoltaic solar cell together with a supercapacitor capable of storing surplus energy during the day and releasing it at night allows the equipment to be completely autonomous regarding its energy requirements. This is the current state-of-the-art.

Furthermore, fuel cells, while not electrical storage devices in the usual sense of the term, are still being actively developed after almost 170 years of effort and now offer attractively high specific energies of several MJ/kg (including the mass of stored fuel). The drawback is, however, the extremely low power density (a few tens of watts per kilogram).

Supercapacitors are undergoing intense development. Rather than relying on random nanostructures (e.g., carbon black) regular arrays of nanofeatures may offer an increase of one to two orders of magnitude. This would put the supercapacitors ahead of the experimental Al–air battery and the practical output from gasoline. An intermediate, semistructured, development is to incorporate carbon nanotubes into cellulose-based paper [8]. Hitherto the main exploitation of nanotechnology for capacitors has been to increase the plate area. Nanoscale plate separation limits the voltage that can be stored on the plates because of arcing. There are now attempts to examine the theory more deeply, taking quantum effects explicitly into account, in order to understand how capacitor plates with separation distances in the nanoscale could be designed to avoid arcing at high voltages.

Obviously, specific energies can only be compared if all other things are equal. These “other things” include all the infrastructure required to sustain the chosen system, including its cost, as well as charge/discharge characteristics, lifetime, etc. At least there seems to be no shortage of lithium as a raw material. Apart from efforts to increase power and energy densities, increasing the rapidity of charging and discharging would also be useful. Here too, regular nanostructures might be the key. Some of the technologies, notably fuel cells, have been hampered by large minimum practicable sizes. Here again, micro- and nanotechnologies may be helpful.

The novel nanomaterial graphene is currently being explored for a variety of energy storage applications. Clearly it is a candidate for capacitor plates, provided it can be stably configured. Carbon nanotubes are also finding novel applications, notably as high-capacity anodes in lithium ion batteries. A more exotic application is as a scaffold for fuel coatings which, when ignited, create a thermal (combustion) wave or thermowave that entrains electrons as it moves along the carbon nanotube, generating electrical current.

Apart from the above technologies, there is also design work being done on improving the nanoscale integration of the disparate components of electrical storage media. One area ready for nano-enabled improvement is lessening the large proportion of battery volume (currently around 50%) taken up by separators.

The Li-ion battery market is growing rapidly. In 2005, global production amounted to the equivalent of about 5 TJ storage capacity. In 2005 it was about 20 TJ, and in 2015 exceeded 125 TJ. The growth is largely driven by the demand for electric vehicles, although large batteries are also being used as backup power supplies for critical installations, such as data processing and storage centers.

New battery systems are constantly emerging and being subjected to scrutiny. Silicon is being investigated as an alternative to carbon for the anode, but cycling lifetime is poor [9]. The lithium–air batteries being developed at the National Institute for Materials Science (NIMS) in Japan have an electrical storage capacity at least an order of magnitude greater than the best lithium-ion batteries, which are now being realized close to the theoretical limit [10,11]. In contrast, a potential successor to Li-ion, lithium–sulfur, has only been realized at a fraction of the theoretical limit [12]. Progress is, however, been made by observing the charge and discharge processes in situ using X-ray diffraction [13].

7.3.1 Lighting

Lighting appears here because it is an indispensable part of civilization, and is so widely used that improvements (e.g., more light output for the same input of electrical energy) have the potential to make a significant impact on energy consumption. For all applications where collateral heat production is not required, nanotechnology-enabled light-emitting diodes can replace incandescent filaments. Lighting based on light-emitting semiconductor diodes can achieve a similar luminous output for much less power than incandescent filament lamps. The heat produced by the latter may be of value in a domestic context (e.g., contributing to space heating in winter) but is simply wasted in the case of outdoor street lighting (although the esthetic effect is agreeable [15]). In fact, the actual operational efficiency of a device in a given functional context typically represents only a fraction of the overall effectiveness in achieving the intended function. For example, if the intended function of street lighting is to reduce road accidents, there is probably an ergonomic limit of the number of lamps per unit length of street above which the reduction becomes insignificant. Data is hard to come by and it seems that few amenities have been properly analyzed in this manner. It may well be that the number of lamps could be halved without diminution of the functional effect. Such a reduction would correspond to a really significant technological advance in the device technology.

7.3.2 Computation

Miniaturizing computer chips diminishes the heat dissipated per floating point operation (but, at present, not by as much as the increased number of floating point operations per unit area enabled by the miniaturization). Devices in which bits are represented as electron spins rather than as electron charges will dissipate practically no heat. If all digital information processors used single spin logic, which is most likely to be realized using nanotechnology [16], given the growing ubiquity of such processors, the contribution to energy economy is likely to be very significant.

7.3.3 Electrical Cabling

As long as electricity is produced at locations remote from its use, it must be transmitted along cables, in which process a considerable amount of energy is dissipated as heat. There is, therefore, much interest in lowering the resistance of electrical conductors.

Carbon nanotubes (CNT) have, in certain forms, remarkable electrical attributes (Table 7.2). In the so-called “armchair” structure (in which the chiral vectors are equal) the band structure is metallic and the current capacity of the nanotubes is several orders of magnitude greater than that of copper (Table 7.2), in contrast to nanotubes having the so-called “zigzag” structure, which is semiconducting. The difficulty in creating a macroscopic electrical current-carrying cable, whose mass per unit length would be far smaller than that of an equivalent copper cable, is essentially one of assembling and connecting up the nanotubes to make a macroscopic conductor. A single nanotube has a diameter of the order of 1 nm. Until relatively recently they could only be manipulated individually using the laborious procedures associated with scanning probe ultramicroscopies. An early approach was to randomly disperse them in a polymer matrix. Because of their extreme elongation, the percolation threshold is rather low (typically much less than 1 vol-%) but these composite materials had disappointingly low conductivities (Table 7.2), presumably because each nanotube may only contact other nanotubes at two points and the contact resistance is high. Some improvement was noted when the nanotubes were clustered [17]. A significant breakthrough occurred with the demonstration of the ability to spin nanotubes into a yarn [18]. In such a material each nanotube may make hundreds or thousands of contacts with other nanotubes and the overall conductivity is correspondingly higher (Table 7.2). A single strand of yarn may have a diameter of some micrometers, and such strands may be braided or woven to create even larger structures.

Various improvements in the spinning process have been reported. For example, spinning from a dispersion of single-walled carbon nanotubes (SWNT) to which surfactant had been added resulted in a resistivity of $2.8\times {10}^{-3}\mathrm{\Omega }\text{ cm}$ [19]. Even more dramatic improvements were achieved by doping (double-walled) carbon nanotubes with iodine, enabling resistivities of as low as ${10}^{-5}\mathrm{\Omega }\text{ cm}$ to be reached [20]. Another difficulty in realizing CNT-based cabling is the fact that known synthetic procedures for making the nanotubes produce a mixture of semiconducting and metallic ones, and of single-walled and multiwalled. One way of overcoming the problem is to refine the synthesis parameters. Methods of purifying the mixture are also being devised, including centrifugation and affinity separation using DNA.