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Nanotechnology for Energy

Nanotechnology for Energy Foundations

Today, the world’s energy demands are satisfied mainly via the combustion of fossil fuels. Of the  210 million barrels of oil equivalent per day used worldwide, about 85 million barrels come from oil;  the rest comes from coal (23 %), gas (17 %), biomass (17 %), some fission (5 %), a small amount from hydroelectric power (6 %) and almost none from renewable resources:

primary energy world consumption 2018

It is estimated that by 2050, twice the amount of energy that is burned or consumed today  (about  14 Terawatts, TW) will be needed and that most energy will have to come from solar, wind, and geothermal (50 %) energy. By then, it is expected that the world’s population will rise from today’s 6.3 billion to 9 billion people. Relying on fossil fuels (oil, coal and natural gas) to ‘feed’ the world’s future energy needs is not a responsible environmental option since there is some evidence that the combustion of fossil fuels is the main cause of the high levels of greenhouse gases such as carbon dioxide (CO2) that are accumulating in the atmosphere, with consequent dramatic worldwide climatic changes.

But, the problem is not limited to the environmental impact of using fossil fuels. The availability of fossil fuels is limited and the supply is presently decreasing; consequently, as the demand for energy increases, so does the cost of fossil fuels. Affordable energy is instrumental in basically every aspect of our present societies, including preserving global peace. In fact, in the list of the most important problems facing humanity in the next 50 years, energy is at the top (Table 1). The reason is that affordable energy is fundamental for dealing with the other problems listed: the availability of clean water and food right down to poverty and education.

Another alternative energy carrier is hydrogen, but hydrogen fuel cell technology will have to face a number of issues (e.g. hydrogen extraction, hydrogen storage, fuel cell lifetime and cost) before a hydrogen economy can become a reality. Solving the future energy challenges requires not only advances in the field of energy conversion and storage but also energy savings, considering how much energy is wasted today using conventional incandescent lights. As discussed in this document, nanotechnologies not only have the potential to solve many of the issues that the energy sector is facing, but their application to this sector has already resulted in advanced research projects and some commercial realities.

Solar Energy

Among the renewable sources of energy, solar energy holds great potential. Solar light, however, is not constant, it is only available during part of the day, and it is geographically uneven: some countries receive a much higher illumination than others. Figure 2 gives an idea of the area that should be covered worldwide in order to produce a little more than the world’s current total primary energy demand (assuming a conversion efficiency of 8 %). The main problem associated with this form of energy is not its supply, but the development of devices that will allow its efficient and cost-effective conversion into electric current.

The second biggest problem is its storage and efficient transport. The areas that are most irradiated are deserts, therefore, in most cases, remote from the main urban centers, so transporting this energy to where it is most needed becomes a challenge. Making solar energy a viable alternative to fossil fuels, therefore, requires a series of advances that will most likely be possible through fundamental research into solar energy conversion, storage, and transport. Many of these advances are likely to be enabled by nanotechnologies.

Photovoltaics

A photovoltaic (PV) device is a device that converts solar energy into electricity. In a conventional photovoltaic cell, there are two separate material layers, one with a reservoir of electrons that functions as the negative pole of the cell, and the other lacking electrons, the electron holes that function as the positive pole. When sunlight or other light sources are absorbed by the cell, enough energy is provided to the cell to drive the electrons from the negative to the positive pole, creating a voltage difference between them. In this way, the cell can serve as a source of electrical energy. The efficiency of a PV device depends on the type of semiconductor it is made of, and on its absorbing capacity. All semiconductors absorb only a precise ‘energy window’ (the ‘band gap’) which is just a fraction of the entire solar energy available. Presently, maximum energy conversion efficiency (15–20 %) in a PV cell is obtained when it is made of crystalline silicon (Si). This is an excellent conducting material, abundant and widely used in electronics, but has the main drawback of being very expensive to produce, which is reflected in the high cost of current PVs. This has limited their use. Alternative, cheaper materials, such as titanium dioxide, can be used in PV technology. Titanium dioxide is a well-known non-toxic semiconducting material but has the characteristic of absorbing only the UV region of the solar spectrum, which represents only about 5 % of the total solar energy available. This material, therefore, leads to cheaper PVs but with lower energy conversion efficiency.

As will be discussed, nanotechnologies offer the possibility to introduce alternative materials and fabrication methods to produce cells with tailored absorption characteristics in order to absorb a larger portion of the solar energy spectrum. In order to meet the ‘energy challenge’ through solar energy, conversion efficiencies in the order of 45 % are needed, so research in this area is very intense and numerous different types of nanomaterials are being investigated. In order to reach this ambitious goal, devices must be made of materials that absorb the visible part (representing about 46 %) of the solar spectrum.

There are basically two approaches being investigated.

  • Development of silicon nanocrystals engineered to absorb more solar
  • Biomimetic approaches, where the photovoltaic device is engineered to mimic the best known solar-conversion process ever made, the natural photosynthesis molecular

Nanocrystals

 The limitation of silicon is not only related to its processing cost. Due to its indirect band gap, silicon is weak in absorbing light — only a fraction of the solar spectrum is absorbed. This is where nanoscience can help: in sufficiently small nanocrystals, the band gap becomes quasi-direct, which gives rise to strong light absorption. Thus, the optical properties of silicon can be improved by adding nanocrystals. One such example is silicon-based tandem solar cells, where the top cell is based on nanocrystals, while the bottom cell is a standard silicon cell. Inside the solar cells, the nanocrystals are used to increase the generation of current.

Biomimetic approaches using nanotechnologies

Nature has developed a ‘splendid molecular machine’ (12) that enables the conversion of solar energy into chemical energy through a process called photosynthesis. In this process, solar energy is converted into stored chemical energy (in the form of carbohydrates). The process is an amazing example of natural nanotechnology which serves as an inspiration for the creation of biomimetic devices capable of converting solar energy into other forms of energy. In photovoltaics, the aim is the conversion of solar energy into electricity to be used for powering electrical appliances. In photosynthesis, light is ‘captured’ by light-harvesting antennae (e.g. chlorophyll) in which photons are absorbed, exciting electrons to higher energy states. The pigment in the ‘antenna’ absorbs strongly in most of the visible region of the solar spectrum. The energised electrons are transferred to a series of reaction centres which are called Photosystem I and Photosystem II. In these reaction centres, a series of photochemical electron transfer reactions occur involving quinines. Eventually, the energy is transferred through proton bonds to another reaction centre where the energy is stored in the bonds of the ATP molecule, which reacts further to form nicotinamide adenine dinucleotide phosphate (NADP+). In the final step of the process, hydrogen is taken from water (forming NADPH), releasing oxygen as a by-product. The NADPH stores energy until it is used in the next step of the reaction to provide energy for the formation of C-C bonds, consuming carbon dioxide in the process. The end product is carbohydrates. Therefore, the overall process consumes water and carbon oxide and produces carbohydrates, which are the fixed form of carbon that is the food base for all animal life.

Artificial photosynthesis

Some researchers have been able to extract the complex Photosynthesis I from spinach and use it to power solid-state electronic devices. This represents an example of a biomimetic solid-state photosyn- thetic solar cell. Creating the interface is not trivial because, in the plant, the complex requires salts and water to function, which obviously cannot be used in electronic devices, so surfactants are used instead. The device is made of alternating layers of conducting material (gold), biological material, a semiconducting layer, and a conducting layer on top. The conversion efficiency of the device is 12 %.

Dye-sensitised solar cell

The second approach is a hybrid between conventional photovoltaics, which uses semiconducting materials, and artificial photosynthesis. To date, this seems the most promising approach to improve PV efficiency. In this approach, some other strongly absorbing species (dye) which mimic the function of the chlorophyll are attached to the surface of the semiconductor (e.g. TiO2). These types of cells are called dye-sensitised cells, or Gräztel cells after the name of their inventor, or photoelectrochemical cells (PEC). The complex dye molecules (called sensitisers) are attached to the surface of mesoporous titanium oxide. These are different from a classic thin-film PV in that light is absorbed in a semiconductor layer, whereas in the Gräztel cell, absorption occurs in the dye molecules.

The dye molecules act somewhat like an antenna (mimicking chlorophyll), meaning that more of the light of a particular colour can be captured but also that a wider range of colours of light can be absorbed compared to pure TiO2, thus increasing the efficiency of the device (Figure 3).

In the Gräztel cell, the anode is made of mesoporous dye-sensitised TiO2 and receives electrons from the photo-excited dye which is thereby oxidised. The oxidised dye, in turn, oxidises the mediator,     a redox species that is dissolved in the electrolyte. The mediator is regenerated by reduction at the cathode by the electrons circulated through the external circuit. The mesoporous nature of the titanium oxide provides an enormous internal surface area, thereby reducing the amount of material needed in the cell. The titanium oxide films are produced from a nanoparticle suspension (which is synthesised to form a stable porous material). Specific synthetic dyes are under development to increase light absorption. These cells are extremely promising because they are made of low-cost materials and do not need elaborate apparatus to manufacture. Current scale-up of production utilising polymer materials and roll-to-roll continuous production has the potential to produce the large areas of solar cells required to capture significant amounts of solar energy. Some companies, such as DyeSol, are already producing and selling these types of solar cells, with conversion efficiencies of about 12 % (Figure 4).

Quantum-dot-sensitised solar cells

Another possibility is to use semiconductor nanocrystals (known as quantum dots, QDs) instead of the photosensitive dyes used in the Gräztel cell. For example, CdS, CdSe, InP and InAs quantum dots have been combined with mesoscopic networks of TiO2 nanoparticles to obtain quantum-dot-sensitised solar cells (QDSSCs). In general, QDs offer several advantages compared to organic dyes such as those employed in a Grätzel cell: they provide the ability to match the solar spectrum better because their absorption spectrum can be tuned with particle size. An important nanotechnology discovery with great potential to increase the efficiency of these types of solar cells was reported in May 2006 by a team at the Los Alamos National Laboratory (USA). Researchers in this group found that when nanoparticles of less than 10 nm in diameter made of lead and selenium (PbSe nanoparticles) are illuminated with light they absorb one photon of light but produce up to three electrons. When today’s photovoltaic solar cells absorb one photon of sunlight, the energy is converted to one electron, and the rest of the photon’s energy is lost in heat. Therefore, PbSe nanoparticles produce at least twice the number of electrons compared to conventional semiconductors, a process known as ‘carrier multiplication’. This nanotechnology discovery could boost the efficiency of today’s solar cells from 20–30 to 65 %.

Lotus-mimicking PV coatings

The lotus leaf is characterised by being extremely water-repellent, so much so that water simply rolls off its surface dragging dirt with it. The consequence is the extreme cleanliness of the leaf,  which is a symbol of purity in some Indian cultures. The superhydrophobic properties of the lotus leaf are a consequence of its micro and nano-topography, as well as its surface chemistry.

Superhydrophobic surfaces are useful in improving the performance of solar cells. One of the problems with this technology is that the cells are outside and, therefore, prone to becoming very dirty. This layer of dirt ‘masks’ the catalytic areas of the solar cells and so reduces their efficiency and lifetime. Coating the solar panel with a superhydrophobic coating keeps the panel considerably cleaner. Because of the nano-surface roughness, the coating is transparent to UV light, a necessity for these types of devices. The superhydrophobic coating is also durable, which further improves the solar panel lifetime.

In one research project, PV cells have been coated with a nanostructured coating which increases the absorption of light. The coating mimics the lotus leaf, so it is superhydrophobic, with the effect of also conferring self-cleaning properties on the PV surface.

Solar energy storage

Storage of electrical power is critical for making solar energy a primary power source. The best place to provide this storage is local, near the point of use. Ideally, every house, business and building should have its own local electrical energy storage device, an uninterruptible power supply capable of handling the entire needs of the occupiers for 24 hours. If this were done using today’s lead-acid storage batteries, such a unit for a typical house capable of storing 100 kWh electrical energy would take up a small room and cost over USD 10 000. Through advances in nanotechnology, it may be possible to shrink an equivalent unit to the size of a washing machine and reduce the cost to less than USD 1 000.

Solar heating

Solar energy can also be used as a heating source to produce hot water, and heat homes and offices. Current systems are able to convert 25–40 % solar radiation into heat. The principle of solar heating is straightforward: a material absorbs sunlight energy and releases it in the form of heat directly to a water source or a heat exchange element (heat pump). Any material that can enhance the surface area or have improved absorption properties would improve this technique: numerous nanomaterials do so. Since the sun is a variable source that produces diffuse energy, controlling the incident solar radiation is difficult because of its changing position. Nanotechnologies can be used to fabricate complex nano-structured mirrors and lenses to optimise solar thermal collection. Furthermore, aerogels with nanopores are used as transparent and thermally isolating materials for the cover material of solar collectors.

Hydrogen Society

Hydrogen (H2) can, in principle, be a future environmentally friendly energy carrier when it is produced from renewable energy. The ideal scheme would be to produce hydrogen by splitting water molecules using solar light (Figure 5). Hydrogen could then be used as an energy carrier to provide electricity in our homes, fuel our cars, etc.

The idea of a fuel cell was first conceived in 1839 by Sir William Grove who thought that electricity could be produced by reversing the process of electrolysis, in which hydrogen and oxygen are produced by the electrolysis of water.

Hydrogen fuel cells use hydrogen and oxygen as fuel to generate electricity. The fuel molecules in the cell must be ionized to react. The ionization must be catalyzed by the electrodes, and an electrolyte must conduct the formed ionic species so that they can react (Figure 6). The sub-product of this reaction is only water no CO2 is produced during the conversion of hydrogen to electricity. The result of the electrochemical process is a maximum of 1.2 V and 1 W cm-2 of power.

The three fundamental elements of a hydrogen fuel cell are therefore the fuel (H2 and O2), the catalyst and the electrolyte. At present, there are problems associated with each of these elements, making the fabrication and operation of hydrogen fuel cells technically challenging and very expensive. However, the technology is developed enough, and worldwide research so intense that consumer goods powered by fuel cells are likely on a large scale.

Hydrogen production

The first problem is associated with the nature of the fuel, hydrogen. Although hydrogen is abundant in nature it is not freely available, it needs to be extracted from a  source, such as hydrocarbons  (e.g. methane), which produce CO2 on extraction, or water. The extraction of hydrogen from water is better. Ideally, hydrogen should be extracted using a renewable energy source (solar, wind, geothermal, etc.). One of the most promising methods of hydrogen generation is its photochemical extraction from viable. Therefore, fundamental research is necessary to overcome the limitations of photochemical water decomposition to produce hydrogen.

In 1972, A. Fujishima and K. Honda demonstrated the photoelectrolysis of water with a TiO2 photoanode using platinum as a counter-electrode. Although the reaction is possible, before it can become viable (i.e. both economic and efficient) as a source of hydrogen, two main problems need to be solved.

The first is the limited light absorption of wideband gap semiconductors (such as TiO2) in the visible range of the solar spectrum. This problem has already been mentioned in the section on photovoltaics as it applies to both technologies. Basically, photovoltaics and photoinduced water splitting implement the same concept of using sunlight to excite electrons but they differ in how the excited electron (e-h pairs) are used: to drive a current (in PVs) or to drive a chemical redox reaction (in photoinduced water splitting).

As discussed in the previous section, nanotechnology is leading the way in solving some of the problems associated with solar energy conversion with the introduction of nanostructured materials that have high solar energy absorption rates. Along with this approach, the group under the direction of Dr. Misra at the University of Nevada has developed titanium dioxide (TiO2) nanotube arrays having a modified bandgap for generating hydrogen by splitting water using sunlight.

The second problem in photoinduced water splitting is the fast electron-hole recombination, which lowers the efficiency of the process. In simple terms, photocatalysis involves harvesting solar photons in a semiconductor (the TiO2 surface) and subsequent conversion of these photons to electronic excitations, which then induce the desired water-splitting reaction. The excited electron-hole pair, though, has a high tendency to recombine. Nanostructures offer the opportunity to minimize the distances (and thus the times) over which charges have to survive and be transported after excitation. The deposition of small noble metal islands (< 5 nm) or metal nanoparticles enhances the photocatalytic activity of systems that use TiO2  as the photocatalytic surface. This effect is due to the charge separation across the metal-semiconductor interface. More recent nanotechnology approaches include the use of carbon-doped titania nanotubes arrays, SWCNTs and nanostructured hematite films.

Hydrogen storage

The combustion of hydrogen is straightforward with no detrimental by-products: H2 + ½ O2 → H2O. However, a problem exists that relates to its storage and transport which need to be both efficient and safe. The problem is easily seen by comparing the energy-to-volume ratio for gaseous hydrogen (3.0 MJ/L) to that of conventional gasoline (32.0 MJ/L). This means that, given the same volume, the energy produced by hydrogen is about 10 times lower than that from conventional gasoline. This obvi- ously represents a problem for storing hydrogen in a vehicle: a big, heavy tank would be required to store and transport the required amount of hydrogen (Figure 7). Some possible solutions are to use liquid hydrogen (8.5 MJ/L), compressed hydrogen or to store hydrogen in solid metallic support such as metal complexes (hydrides).

The use of compressed hydrogen implies using liquid tanks that must be made of very strong yet lightweight material. This material should also have outstanding insulating and pressurization properties in order to avoid hydrogen leakage. This problem can potentially be solved using nanotechnology to develop new materials with exceptional properties in terms of strength and density.

Solid metallic nanostructured supports

Solid metallic supports are probably the most viable option for hydrogen storage. In this approach hydrogen is ‘loaded’ to a solid support and extracted from it when needed. The main challenges here are the material loading capacity and the regeneration kinetics to re-extract the hydrogen from the support The best material would achieve an optimum compromise between having hydrogen too weakly bonded to the storage material, which means a low storage capacity, and a too strongly bonded to the storage material, which would require high temperatures to release hydrogen. Nanotechnology can contribute to this field by developing new molecules that allow high hydrogen loading capacity and acceptable regeneration kinetics. Researchers aim to develop nanomaterials that are light in weight, low in volume, have high loading capacities, good regeneration kinetics, and are low in cost.

Two candidate materials are complex metal hydrides, which have an intermediate bonding of hydrogen, and nanostructured carbon-based materials, such as carbon nanotubes. The properties of some complex metal hydrides as hydrogen storage materials, such as LiBH4, NaBH4  and NaAlH4, are summarised in Table 3. In Figure 8, the schematic representation of sodium aluminum hydride (NaAlH4) is shown. The structure can be visualized as a salt made of sodium ions (Na+, yellow) and a complexion of aluminum (Al, orange) and hydrogen (blue), AlH4- (other examples of complexions are the ion sulfate SO42- and the ion phosphate PO43).

  • Hydrogen pressure in a gas tank (700 bar is the target).
  • Temperature for H2 release at p(H2) = 1 bar, without

Adapted from T. R. Jensen, ‘Hydrogen Fuel Cells’, Aktuel Naturvidenskab, 2004 (http://www.ird.dk), reprinted with the permission of the author

The hydrogen fuel cell

In order for hydrogen fuel cells to become an economically viable alternative to combustion chambers, two other main problems need to be addressed: the nature of the catalyst and that of the electrolyte in the cell.

Currently, the electrodes in a fuel cell are made of a metal such as platinum (Pt), which is a rare, expensive metal, also sensitive to the CO and sulfur species that are dispersed in the atmosphere. These deactivate the platinum surface (a phenomenon called ‘poisoning of the catalyst’). Fuel cells operate at high temperatures (> 70 °C) since the poisoning agents at these temperatures tend to de- adsorb. Nanotechnology is already actively involved in addressing some of the issues concerning fuel cell catalysts. Improvements in this area through nanotechnology concern (a) increasing the material catalytic activity, and (b) reducing the use of rare metals. Since the current generated at an electrode is proportional to the active surface of the catalyst, fuel cells that have higher power density can be formed from nanomaterials, which have a higher surface area to volume ratio. Electrocatalytic material properties are also proportional to particle size, so nanoparticles and nanomaterials have increased catalytic activity compared to bulk materials. This characteristic can lead to a reduction in the use of rare metals, for example by using carbon nanomaterials as a support for the dispersions of nano-sized platinum, thereby reducing the weight of platinum needed to produce the same surface area of active  Pt catalyst. Carbon nanomaterials are particularly suitable since they act both as a support for the platinum nanomaterial and as a conductor. Suitable carbon nanomaterials are carbon foams containing nanopores, different types of nanotubes and single-walled nanohorns.

Proton exchange membrane fuel cell

The electrolyte conventionally used in fuel cells is aqueous KOH operating at about 70 °C (liquid alkaline cell). It leads to corrosion of the electrodes, reducing the cell lifetime, and operates at high temperatures, decreasing the thermodynamic efficiency of the cell. A solid electrolyte is preferred, therefore modern hydrogen fuel cells use a proton-permeable membrane made of a polymer (e.g. Nafion®), which has a high proton conductivity due to the presence of water molecules in its structure. In a Proton Exchange Membrane Fuel Cell (PEMFC), the proton ions generated at the platinum anode pass through the proton-conducting membrane to the platinum cathode, where they combine with oxygen and form water (Figure 9 illustrates the operating principle of a PEMFC).

Proton Exchange Membrane Fuel cells (PEMFC) are the most likely fuel cells to achieve commercialization for automotive purposes as a result of their low operating temperatures and their inherent properties of being lightweight, producing high current densities and containing no corrosive materials.

An area where improvement is urgently needed, however, is the nature of the solid membrane. Nafion®, for example, is expensive, subject to degradation through dehydration at operating temperatures above 100 °C, and is not fabricated with nanoscale control; therefore, it has pores that are not uniform in size and distribution, so that the active sites on the membrane surface (directly involved in proton binding) are randomly exposed. Other 3D solid electrolytes have been investigated, but they have the problem of either very low conductivity (reducing the efficiency of the cell) or requiring high temperatures to operate.

In this context, nanotechnologies can aid in the development of nanostructured solid membranes to increase proton conductivity, cell efficiency, and durability. These include ceramic electrolyte membranes (e.g. metal-oxane membranes) and nanostructured solid electrolytes or fillers fabricated with nanoscale control. In addition, fuel cell assembly, durability, and cost could, in principle, be improved by employing nanotechnology to fabricate sturdier cells able to withstand the large changes in temperature required in some applications, such as automotive operation.

Thermoelectricity

Thermoelectric materials (TE) are functional materials that have the double property of being able to convert heat to electricity and vice versa. Thermoelectricity can be generated in all conductive materials. When a temperature gradient is applied across a wire, electrons diffuse from the hot to the cold part due to the larger thermal speed of the electrons in the hot region. Consequently, a charge difference builds up between the hot and cold regions, creating a voltage and producing an electric current. Alternatively, a current can be applied to the wire to carry heat away from a hot section to cooler areas.

Thermoelectric materials can, therefore, be used either for cooling or power generation. Although current devices have a low conversion efficiency of around 10 %, they are strongly advantageous compared to conventional energy technologies, since the converters have no moving parts and are thus both reliable and durable. Furthermore, they are scalable and hence ideal for miniature power generation, and no pollutants are released into the environment. If significantly improved thermoelectric materials can be developed, thermoelectric devices may replace the traditional cooling systems in refrigerators. They could also make power generators in cars obsolete by utilizing heat from the exhaust gases, or they could possibly be used to convert huge amounts of industrial waste heat into electricity.

Despite their enormous potential, thermoelectric materials have not yet fulfilled their huge promise, and are currently only employed in niche applications, most notably by NASA to generate electricity for spacecraft that are too far from the sun for solar cells to operate (Figure 10). The problem is that for the process to be efficient, the thermoelectric materials need to be good electrical conductors but poor thermal conductors, so that the temperature difference inside the material remains. More specifically, thermoelectric materials are ranked by their figure of merit, ZT, which is defined as ZT = S2σT/k, where S is the thermopower (or Seebeck coefficient), σ is the electrical conductivity, k is the thermal conductivity and T is the absolute temperature. To be competitive with conventional refrigerators and generators, thermoelectric materials with ZT > 3 must be developed. Yet in five decades of research, the room temperature ZT of bulk semiconductors has increased only marginally, from 0.6 to about 1.0. The challenge lies in the fact that S, T, and k are interdependent: changing one alters the others, making optimization extremely difficult.

However, in the last years, there have been reports of dramatic increases in the properties of thermoelectric materials. In all these cases, the material has been found to be nanostructured. Researchers are studying these nanomaterials in detail to understand whether this is a result of quantum confinement or photon dynamics and transport. What is clear is that to have optimum properties, the material needs to have high symmetry at the nanoscale level, and needs to incorporate heavy elements because of its low thermal conductivity.

Some examples of nanostructured thermoelectric materials are half-heister alloys (ZrNiSn), Zn4Sb3, skutterudites, and novel PbTematerials (e.g. LAST-18, SALT-20 and LASTT).

Rechargeable Batteries

Rechargeable batteries are energy storage devices used to power small electronic equipment such as mobile phones and personal computers, where high power and lightweight are important. These same attributes are required for electric vehicles, hybrid electric vehicles, power tools and backup power subsystems. In simple terms, a battery is an electrochemical device that generates a direct current through a coupled set of reduction-oxidation (redox) reactions. The positive electrode is reduced (captures electrons) and the negative electrode is oxidized (releases electrons). The battery consists of a positive electrode facing a negative electrode divided by a porous separator that prevents the electrodes from touching, and anionic electrolyte, which is a conducting medium that ensures movement of the ions from one electrode to the other. Intercalation-based batteries using the small lithium (Li+) ion are the most used (Figure 11). These batteries have at least one redox-active electrode with an open crystal structure with ‘holes’ capable of intercalating Li+. For example, oxidation of cobalt in  LiCoO2  expels  Li+ which is taken up in a graphite electrode. When the battery is charged, the Li+ moves from the positive electrode to the negative one via the electrolyte. On discharge, the opposite occurs, releasing energy in the process. Ideally, the structure of the redox-active crystal should be capable of reversibly intercalating the small Li+ ion.

Nanotechnologies to impact energy capacity, battery power, charge rate and lifetime

Current problems with lithium rechargeable batteries involve a number of issues, the first being the battery energy capacity: in order to allow ions and electrons to move quickly into and out of the active material (allowing fast charging and discharging), the material must be deposited as a thin film. This limits the amount of active material that can be incorporated into the battery (energy capacity). For high-capacity batteries, the thickness is increased in order to provide more energy storage but with the drawback of slower charging.

The second issue concerns the battery power: an important attribute of large format batteries is their capability to deliver power quickly. Power is restricted by the ion removal capability in lithium batteries, which depends on the electrochemical properties of the battery. Then there is the problem of charge rate: batteries need to be recharged, and recharging times are now in the order of hours. The time of charge is restricted by the incorporation rate capabilities of Li+ inside the graphite electrode.

Lithium battery lifetime also needs to be improved: in current batteries, every time Li+ enters/exits the graphite electrode, the pores of the electrode need to expand or shrink. This repeated expansion and shrinkage fatigues the graphite particles, which break apart as a result, reducing battery performance.

Nanomaterials as alternatives to conventional electrodes

Nanocrystalline composite materials and nanotubes can be used to replace the conventional graphite or Li-graphite electrode. These can be fabricated to house voids having the same size as the lithium ions they have to accommodate. This allows much more active material to be packed into an electrode, increasing energy capacity. A nanostructured electrode with voids having the same size as the lithium ions increase the battery life and also ensures high charge rates. In the future, nanotechnology will also allow a move away from flat layers of electrode materials to positive and negative electrodes that interpenetrate. This 3D nano-architecture could improve the mobility of ions and electrons, thereby increasing battery power.

In this context, it is interesting to note the work reported in December 2007 by Yi Cui et al. at Stanford University (USA), on the use of silicon nanowires as anode material. Bulk silicon has been investigated in the past as an alternative material to graphite since it has a low discharge potential and the high-     est theoretical charge capacity (more than 10 times that of existing graphite anodes). However, silicon bulk anodes (containing silicon films or large silicon particles) have shown short battery lifetime and capacity fading due to pulverization and loss of electrical contact between the active material and the current collector. These problems arise from the fact that the volume of silicon anodes changes by about 400 % during battery cycling as a result of the anode swelling (battery charging) and shrinking (battery discharging) as lithium ions enter and exit the anode.

The group at Stanford University replaced a conventional bulk silicon anode with one formed of silicon nanowires (SiNW), grown directly on the metallic current collector. In this way, they were able to achieve the theoretical charge capacity of silicon anodes (10 times that of current ion-lithium batteries)  and to maintain a discharge capacity close to 75 % of this maximum. The work has been patented and the discovery has great potential for commercial high-performance lithium batteries.

Paper battery’

Some exciting work recently reported by scientists at  Rensselaer Polytechnic Institute (USA) uses a composite material that combines high energy capacity with flexibility. The researchers found that they could combine nanotubes (which are highly conductive) with a layer of cellulose, the material used to make paper. In this way, they were able to obtain ‘paper batteries’ which can be rolled or folded just like paper without any loss of efficiency. This opens the door to batteries molded to assume a particular form. Like all batteries, the paper version comprises electrodes, electrolyte, and a separator. The first electrode is formed by vertically aligned multi-walled carbon nanotubes, deposited on silicon substrates. Plant cellulose is cast on top of this layer, solidified, and dried to form the porous separator. The middle paper layer is then impregnated with an ionic liquid which acts as the electrolyte; this can be an organic salt that is liquid at room temperature. The ionic liquid contains no water, so there is nothing in the batteries to freeze or evaporate. This expands the working temperature range of the battery, which can withstand extreme temperatures from 195 K to 450 K. To make a battery, the second electrode is formed by coating the paper side with lithium oxide. Interestingly, the same material can be used to make a supercapacitor simply by folding the paper in half, so that there is a carbon electrode at both the top and bottom. The team was also able to fabricate dual-storage devices containing three electrodes that act as both supercapacitors and batteries.

Battery operation range, lifetime and safety

Lithium batteries are, at present, limited in their operating temperature range. Below 0 °C and above  50 °C the batteries cannot be recharged, and above 130 °C they become unsafe due to thermal run- away. Thermal runaway, which is due to the reaction of the graphite with the electrolyte, can also occur due to battery impurities. Finally, lithium batteries are made of toxic metals and are, therefore, harmful to the environment.

Battery safety can be increased if the graphite electrode in a lithium battery is replaced with a nanostructured material that is inert towards the electrolyte. Nanotechnology can also be employed to use alternative active materials which are less expensive and non-toxic to the environment. For example, the non-toxic magnetite (Fe3O4) has been employed as the active material in a high-capacity Cu nano- architectured electrode (Figure 12). Nanostructured Lithium batteries are becoming a commercial reality, such as the Altamirano NanoSafe™ and 123 Systems Batteries.

Supercapacitors

Supercapacitors are another way of storing electricity that can benefit from nanotechnology. They are needed in devices that require rapid storage and release of energy, for instance hybrid-electric and fuel cell-powered vehicles. They are constructed of two electrodes immersed in an electrolyte, with an ion-permeable separator between them. Each electrode-electrolyte interface represents a capacitor, so the complete cell can be considered as two capacitors in series. The focus in the development of these devices has been on achieving high surface area with low matrix resistivity. The most remarkable property of a supercapacitor is its high power density, about 10 times that of a secondary battery. The maximum power density of a supercapacitor is proportional to the reciprocal of its internal resistance.  A number of sources contribute to the internal resistance and are collectively referred to as Equivalent Series Resistance (ESR). Contributors to the ESR include the electronic resistance of the electrode material and the interfacial resistance between the electrode and the current collector. Carbon, in its various forms, is currently the most extensively used electrode material in supercapacitors. A typical commercial supercapacitor can produce a power density of approximately 4  kW/kg. Nanotubes can be used to increase the power density of supercapacitors since the nanoscale tubular morphology of these materials offers a unique combination of low electrical resistivity and high porosity in a  readily accessible structure. Single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) are under investigation. Research has shown that the use of thin-film electrodes with multi-walled aligned nanotubes increases the specific power density (laboratory results of 30 kW/kg have been reported),  as a result of the reduction in.

Energy Savings

Energy savings can be achieved in numerous ways, such as improving the insulation of residential homes and offices; more efficient lighting; and using lighter and stronger materials to build devices that would then require less energy to operate. Moreover, a large portion of the energy is lost during its transport, so there is a need for a more efficient electric grid to transport energy. Nanotechnologies can potentially be applied to all of these energy-saving materials and technologies.

Catalysis

Catalysis is of vital importance in our society and constitutes a cornerstone of life from biological processes to the large-scale production of bulk chemicals. The availability of plentiful and inexpensive chemicals relies on industrial catalytic processes and, without them, it would be impossible to maintain the current living standard of the present human population. Other technologies also depend on catalysis, including the production of pharmaceuticals, means of environmental protection, and the production and distribution of sustainable energy. As already discussed in some of the previous sections, many technological advances required to make alternative energy carriers to fossil fuels such as sunlight and hydrogen an economically viable option rely on optimizing a catalytic process.

For example, for water to be split into hydrogen and oxygen with sunlight to feed a fuel cell, researchers need to improve the activity of the catalyst (e.g. TiO2).

A typical heterogeneous catalyst consists of a few nanometre-wide catalytically active nanoparticles dispersed on a highly porous support material which can have surface areas up to 250 m2/g. The manufacturing of structures on the nanometre scale has been a central issue in catalysis research and development for decades. This fact relates to the structure of a heterogeneous catalyst, which requires the control of materials ranging from macroscopic dimensions down to the nanoscale. Heterogenous catalysis, therefore, has, in a sense, always had a nanoscience component. Since catalytic action takes place at a surface, and catalytic materials are often very expensive (as they use rare materials such as platinum), the goal for chemists has always been to fabricate catalysts with as high a surface-to-volume ratio as possible, so as to maximize the surface exposed to the reaction and minimize the amount of catalyst required.

Advanced materials

Nanotechnology can offer some indirect energy-saving solutions by developing materials with better properties. One example is materials with improved strength which make constructions leaner and thus lighter, with an indirect energy saving, for example in the transport sector (both on the road and in the air). Since a large fraction of the fuel consumption in a car is weight-related, making cars with lighter materials would be a  very efficient way of saving energy. The higher tensile strength can be exploited as well as higher possible loads so that with the same amount of material, stronger components can be built. For example, wind turbines could be capable of sustaining higher wind speeds if they were made of high-strength nanomaterials. Better creep resistance is an advantage in virtually any system for thermal power generation due to the higher operating temperature allowed and the concurrent higher efficiency. Nanocoatings with improved corrosion properties have a longer service life in aggressive environments and thus have the potential for energy saving throughout their entire life cycle (e.g. extraction, production, operation, disposal and recycling).

Insulators and ‘smart’ coatings

Insulation is a very effective way of minimizing energy consumption, for example in homes and offices. Nanotechnology offers the possibility of developing new materials with improved insulating properties. One example is nanoporous aerogels to improve thermal insulation. A commercial example is represented by Aspen Aerogels products. This company produces flexible aerogel nanoporous insulation blankets (e.g. Cryogel™) designed for cryogenic applications (e.g. insulating pipes and tanker ships). These insulation blankets can be cut just like normal textiles and installed faster than traditional materials, and their low thermal conductivity requires less material to be used. Additionally, Aspen’s products are resistant to compression and inherently hydrophobic so they can be exposed to water for long periods without damaging the products’ outstanding thermal properties. Nanotechnology applied to indirect energy saving can be found in the form of ‘smart’ materials such as electrochromic and photochromic coatings used for the darkening window. They reduce indoor heating in summer, so less air- conditioning is required to keep the atmosphere cool, with consequent energy saving. Another example of nanotechnology applied to smart coatings is the use of a family of wavelength-selective films used to manufacture ‘heat mirrors’.

One of these materials is indium tin oxide (ITO), an infrared absorber. A 0.3 nm ITO coating on glass provides more than 80 % transmission for the wavelengths predominant in sunlight. The transmission properties of the window can be varied by changing the thickness and material composition of the coating so that a combination of materials could be used to produce smart windows that reflect solar energy in summer but transmit solar energy in winter.

Energy-harvesting materials

Numerous innovative electronic devices under development have nanoscale components (see this module, Chapter 4: Information and Communication Technologies). Nanodevices do not use much energy, and if the little they need could be scavenged from vibrations associated with footsteps, heartbeats, noises, and airflow, a whole range of applications in personal electronics, sensing and defense technologies would open up. In order to do this, an ‘energy-scavenger’ having nanoscale dimensions would have to be included in the device. Furthermore, the energy gathering of this type requires a technology that works at a low-frequency range (below 10 Hz), ideally based on soft, flexible materials.  A group working at Georgia Institute of Technology (USA) has now come up with a system that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibers. A piezoelectric material that makes uses of piezoelectricity was discovered in 1883 by Pierre Curie and his brother Jacques. They showed that electricity was produced when pressure was applied to selected crystallographic orientations. Piezoelectricity is thus the induction of electrical polarisation in certain types of crystals due to mechanical stress. Zinc Oxide nanowires are such a type of piezoelectric nanomaterial. In the work just mentioned, researchers have grown ZnO nanowires radially around fiber of Kevlar, which is a material known for its strength and stability (Figure 13). By entangling two fibers and moving them by sliding them back and forth, a relative ‘brushing motion’ is created, which in turn produces an output current.

The mechanical energy (sliding motion) is converted into electricity via a coupled piezoelectric-semi- conductor process. This work shows a potential method for creating fabrics that scavenge energy from light winds and body movement. In the future, these types of nano-energy scavengers could be incorporated in textiles to power personal electronics.

Efficient lighting

Another important application of nanotechnology in the area of energy-saving is the production of more efficient lighting devices. Conventional incandescent lights are not energy efficient, a large portion of their energy being dispersed in heat. Solid-state light devices in the form of light-emitting diodes (LEDs) are attracting serious attention now as low-energy alternatives to conventional lamps. The need is to engineer white-light LEDs as a more efficient replacement for conventional lighting sources. One proposed solution is to use a mixture of semiconductor nanocrystals as the intrinsic emitting layer in an LED device. Simply mixing several colors of nanocrystals together to achieve white light is a possibility, but this would result in an overall reduction in device efficiency through self-absorption between the various sizes of the nanocrystal. An important result that can potentially resolve this problem has recently come from the work of some researchers at Vanderbilt University (USA). They found that crystals of cadmium and selenium of a certain size (‘magic-sized’ CdSe) emit white light when excited by a UV laser, a property that is the direct result of the extreme surface-to-volume ratio of the crystal. This material could, therefore, be ideal for solid-state lighting applications.

Organic light-emitting diodes (OLEDs) represent a promising solution for lighting applications as well as for low cost, full-colour flat panel displays quantum dots (QD) are another class of nanomaterials that are under investigation for the manufacture of more efficient displays and light sources (QD-LEDs). Quantum dots are characterized by emitting saturated and monochromatic light; the color emitted depends on the size of the quantum dot and the light is emitted under certain conditions (e.g. when current passes to them via conductive polymer films). Recently, even white-emitting quantum dots have been fabricated. Therefore, quantum dot-based LEDs are promising light sources and could be useful for use in flat-panel displays. The structure and properties of OLEDs and of QD-LEDs is described in detail in this module, Chapter 4: Information and Communication Technologies.

Efficient energy transport

One area where there is a large margin for improvement is in the transport of electric current. As the world’s power demand increases, the burden on the electricity infrastructure grows. This has been shown recently in some nationwide blackouts such as those that occurred in the north-east USA in 2003, and in Italy in the same year. Therefore, a major challenge is to develop new transmission- line materials that are lighter and have less energy loss than copper. Single carbon nanotubes (CNT) have the remarkable property of weighing one-sixth as much as copper but with similar or even better conductivity and negligible eddy current loss. This material thus has the potential to overcome some of the limitations of current transmission materials. Before this can become a reality, however, advances in the production of CNTs are needed. At present, scientists produce CNTs often less than 100 nm in length and with widely varying electrical conduction properties. The challenge for the future is, therefore, to produce nanotubes with controlled properties. Moreover, the manufacturing must be cost-effective and able to produce cables of fibers with the desired electrical properties. At present, therefore, this application remains a vision.

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