Heterojunction

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https://en.wikipedia.org/wiki/Heterojunction

A heterojunction is an interface between two layers or regions of dissimilar semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid-state device applications, including semiconductor lasers, solar cells and transistors. The combination of multiple heterojunctions together in a device is called a heterostructure, although the two terms are commonly used interchangeably. The requirement that each material be a semiconductor with unequal band gaps is somewhat loose, especially on small length scales, where electronic properties depend on spatial properties. A more modern definition of heterojunction is the interface between any two solid-state materials, including crystalline and amorphous structures of metallic, insulating, fast ion conductor and semiconducting materials.

Manufacture and applications

Heterojunction manufacturing generally requires the use of molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) technologies in order to precisely control the deposition thickness and create a cleanly lattice-matched abrupt interface. A recent alternative under research is the mechanical stacking of layered materials into van der Waals heterostructures.

Despite their expense, heterojunctions have found use in a variety of specialized applications where their unique characteristics are critical:

• Solar cells: Heterojunctions are formed through the interface of a crystalline silicon substrate (band gap 1.1 eV) and amorphous silicon thin film (band gap 1.7 eV) in some solar cell architectures. The heterojunction is used to separate charge carriers in a similar way to a p–n junction. The Heterojunction with Intrinsic Thin-Layer (HIT) solar cell structure was first developed in 1983 and commercialised by Sanyo/Panasonic. HIT solar cells now hold the record for the most efficient single-junction silicon solar cell, with a conversion efficiency of 26
• Lasers: Using heterojunctions in lasers was first proposed in 1963 when Herbert Kroemer, a prominent scientist in this field, suggested that population inversion could be greatly enhanced by heterostructures. By incorporating a smaller direct band gap material like GaAs between two larger band gap layers like AlAs, carriers can be confined so that lasing can occur at room temperature with low threshold currents. It took many years for the material science of heterostructure fabrication to catch up with Kroemer's ideas but now it is the industry standard. It was later discovered that the band gap could be controlled by taking advantage of the quantum size effects in quantum well heterostructures. Furthermore, heterostructures can be used as waveguides to the index step which occurs at the interface, another major advantage to their use in semiconductor lasers. Semiconductor diode lasers used in CD and DVD players and fiber optic transceivers are manufactured using alternating layers of various III-V and II-VI compound semiconductors to form lasing heterostructures.
• Bipolar transistors: When a heterojunction is used as the base-emitter junction of a bipolar junction transistor, extremely high forward gain and low reverse gain result. This translates into very good high frequency operation (values in tens to hundreds of GHz) and low leakage currents. This device is called a heterojunction bipolar transistor (HBT).
• Field effect transistors: Heterojunctions are used in high electron mobility transistors (HEMT) which can operate at significantly higher frequencies (over 500 GHz). The proper doping profile and band alignment gives rise to extremely high electron mobilities by creating a two dimensional electron gas within a dopant free region where very little scattering can occur.

Energy band alignment

The three types of semiconductor heterojunctions organized by band alignment.

Band diagram for stradding gap, n-n semiconductor heterojunction at equilibrium.

The behaviour of a semiconductor junction depends crucially on the alignment of the energy bands at the interface. Semiconductor interfaces can be organized into three types of heterojunctions: straddling gap (type I), staggered gap (type II) or broken gap (type III) as seen in the figure. Away from the junction, the band bending can be computed based on the usual procedure of solving Poisson's equation.

Various models exist to predict the band alignment.

• The simplest (and least accurate) model is Anderson's rule, which predicts the band alignment based on the properties of vacuum-semiconductor interfaces (in particular the vacuum electron affinity). The main limitation is its neglect of chemical bonding.
• A common anion rule was proposed which guesses that since the valence band is related to anionic states, materials with the same anions should have very small valence band offsets. This however did not explain the data but is related to the trend that two materials with different anions tend to have larger valence band offsets than conduction band offsets.
• Tersoff proposed a gap state model based on more familiar metal–semiconductor junctions where the conduction band offset is given by the difference in Schottky barrier height. This model includes a dipole layer at the interface between the two semiconductors which arises from electron tunneling from the conduction band of one material into the gap of the other (analogous to metal-induced gap states). This model agrees well with systems where both materials are closely lattice matched such as GaAs/AlGaAs.
• The 60:40 rule is a heuristic for the specific case of junctions between the semiconductor GaAs and the alloy semiconductor Al_xGa_1−xAs. As the x in the Al_xGa_1−xAs side is varied from 0 to 1, the ratio ${\displaystyle \mathrm{\Delta }{E}_C/\mathrm{\Delta }{E}_V}$ tends to maintain the value 60/40. For comparison, Anderson's rule predicts ${\displaystyle \mathrm{\Delta }{E}_C/\mathrm{\Delta }{E}_V=0.73/0.27}$ for a GaAs/AlAs junction (x=1).

The typical method for measuring band offsets is by calculating them from measuring exciton energies in the luminescence spectra.

Effective mass mismatch

When a heterojunction is formed by two different semiconductors, a quantum well can be fabricated due to difference in band structure. In order to calculate the static energy levels within the achieved quantum well, understanding variation or mismatch of the effective mass across the heterojunction becomes substantial. The quantum well defined in the heterojunction can be treated as a finite well potential with width of ${\displaystyle l_w}$. In addition, in 1966, Conley et al. and BenDaniel and Duke reported a boundary condition for the envelope function in a quantum well, known as BenDaniel–Duke boundary condition. According to them, the envelope function in a fabricated quantum well must satisfy a boundary condition which states that ${\displaystyle \psi (z)}$ and ${\displaystyle \frac{1}{m^{\ast }}\frac{\mathrm{\partial }}{\mathrm{\partial }z}\psi (z)}$ are both continuous in interface regions.

Mathematical details worked out for quantum well example.

Nanoscale heterojunctions

Image of a nanoscale heterojunction between iron oxide (Fe₃O₄ — sphere) and cadmium sulfide (CdS — rod) taken with a TEM. This staggered gap (type II) offset junction was synthesized by Hunter McDaniel and Dr. Moonsub Shim at the University of Illinois in Urbana-Champaign in 2007.

In quantum dots the band energies are dependent on crystal size due to the quantum size effects. This enables band offset engineering in nanoscale heterostructures. It is possible to use the same materials but change the type of junction, say from straddling (type I) to staggered (type II), by changing the size or thickness of the crystals involved. The most common nanoscale heterostructure system is ZnS on CdSe (CdSe@ZnS) which has a straddling gap (type I) offset. In this system the much larger band gap ZnS passivates the surface of the fluorescent CdSe core thereby increasing the quantum efficiency of the luminescence. There is an added bonus of increased thermal stability due to the stronger bonds in the ZnS shell as suggested by its larger band gap. Since CdSe and ZnS both grow in the zincblende crystal phase and are closely lattice matched, core shell growth is preferred. In other systems or under different growth conditions it may be possible to grow anisotropic structures such as the one seen in the image on the right.

It has been shown that the driving force for charge transfer between conduction bands in these structures is the conduction band offset. By decreasing the size of CdSe nanocrystals grown on TiO₂, Robel et al. found that electrons transferred faster from the higher CdSe conduction band into TiO₂. In CdSe the quantum size effect is much more pronounced in the conduction band due to the smaller effective mass than in the valence band, and this is the case with most semiconductors. Consequently, engineering the conduction band offset is typically much easier with nanoscale heterojunctions. For staggered (type II) offset nanoscale heterojunctions, photoinduced charge separation can occur since there the lowest energy state for holes may be on one side of the junction whereas the lowest energy for electrons is on the opposite side. It has been suggested that anisotropic staggered gap (type II) nanoscale heterojunctions may be used for photocatalysis, specifically for water splitting with solar energy.

Nanoremediation

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Nanoremediation

Nanoremediation is the use of nanoparticles for environmental remediation. It is being explored to treat ground water, wastewater, soil, sediment, or other contaminated environmental materials.Nanoremediation is an emerging industry; by 2009, nanoremediation technologies had been documented in at least 44 cleanup sites around the world, predominantly in the United States. In Europe, nanoremediation is being investigated by the EC funded NanoRem Project. A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale. During nanoremediation, a nanoparticle agent must be brought into contact with the target contaminant under conditions that allow a detoxifying or immobilizing reaction. This process typically involves a pump-and-treat process or in situ application.

Some nanoremediation methods, particularly the use of nano zero-valent iron for groundwater cleanup, have been deployed at full-scale cleanup sites. Other methods remain in research phases.

Applications

Nanoremediation has been most widely used for groundwater treatment, with additional extensive research in wastewater treatment. Nanoremediation has also been tested for soil and sediment cleanup. Even more preliminary research is exploring the use of nanoparticles to remove toxic materials from gases.

Groundwater remediation

Currently, groundwater remediation is the most common commercial application of nanoremediation technologies. Using nanomaterials, especially zero-valent metals (ZVMs), for groundwater remediation is an emerging approach that is promising due to the availability and effectiveness of many nanomaterials for degrading or sequestering contaminants.

Nanotechnology offers the potential to effectively treat contaminants in situ, avoiding excavation or the need to pump contaminated water out of the ground. The process begins with nanoparticles being injected into a contaminated aquifer via an injection well. The nanoparticles are then transported by groundwater flow to the source of contamination. Upon contact, nanoparticles can sequester contaminants (via adsorption or complexation), immobilizing them, or they can degrade the contaminants to less harmful compounds. Contaminant transformations are typically redox reactions. When the nanoparticle is the oxidant or reductant, it is considered reactive.

The ability to inject nanoparticles to the subsurface and transport them to the contaminant source is imperative for successful treatment. Reactive nanoparticles can be injected into a well where they will then be transported down gradient to the contaminated area. Drilling and packing a well is quite expensive. Direct push wells cost less than drilled wells and are the most often used delivery tool for remediation with nanoiron. A nanoparticle slurry can be injected along the vertical range of the probe to provide treatment to specific aquifer regions.

Surface water treatment

The use of various nanomaterials, including carbon nanotubes and TiO₂, shows promise for treatment of surface water, including for purification, disinfection, and desalination. Target contaminants in surface waters include heavy metals, organic contaminants, and pathogens. In this context, nanoparticles may be used as sorbents, as reactive agents (photocatalysts or redox agents), or in membranes used for nanofiltration.

Trace contaminant detection

Nanoparticles may assist in detecting trace levels of contaminants in field settings, contributing to effective remediation. Instruments that can operate outside of a laboratory often are not sensitive enough to detect trace contaminants. Rapid, portable, and cost-effective measurement systems for trace contaminants in groundwater and other environmental media would thus enhance contaminant detection and cleanup. One potential method is to separate the analyte from the sample and concentrate them to a smaller volume, easing detection and measurement. When small quantities of solid sorbents are used to absorb the target for concentration, this method is referred to as solid-phase microextraction.

With their high reactivity and large surface area, nanoparticles may be effective sorbents to help concentrate target contaminants for solid-phase microextraction, particularly in the form of self-assembled monolayers on mesoporous supports. The mesoporous silica structure, made through a surfactant templated sol-gel process, gives these self-assembled monolayers high surface area and a rigid open pore structure. This material may be an effective sorbent for many targets, including heavy metals such as mercury, lead, and cadmium, chromate and arsenate, and radionuclides such as ⁹⁹Tc, ¹³⁷CS, uranium, and the actinides.

Mechanism

The small size of nanoparticles leads to several characteristics that may enhance remediation. Nanomaterials are highly reactive because of their high surface area per unit mass. Their small particle size also allows nanoparticles to enter small pores in soil or sediment that larger particles might not penetrate, granting them access to contaminants sorbed to soil and increasing the likelihood of contact with the target contaminant.

Because nanomaterials are so tiny, their movement is largely governed by Brownian motion as compared to gravity. Thus, the flow of groundwater can be sufficient to transport the particles. Nanoparticles then can remain suspended in solution longer to establish an in situ treatment zone.

Once a nanoparticle contacts the contaminant, it may degrade the contaminant, typically through a redox reaction, or adsorb to the contaminant to immobilize it. In some cases, such as with magnetic nano-iron, adsorbed complexes may be separated from the treated substrate, removing the contaminant. Target contaminants include organic molecules such as pesticides or organic solvents and metals such as arsenic or lead. Some research is also exploring the use of nanoparticles to remove excessive nutrients such as nitrogen and phosphorus.

Materials

A variety of compounds, including some that are used as macro-sized particles for remediation, are being studied for use in nanoremediation. These materials include zero-valent metals like zero-valent iron, calcium carbonate, carbon-based compounds such as graphene or carbon nanotubes, and metal oxides such as titanium dioxide and iron oxide.

Nano zero-valent iron

As of 2012, nano zero-valent iron (nZVI) was the nanoscale material most commonly used in bench and field remediation tests.^[2] nZVI may be mixed or coated with another metal, such as palladium, silver, or copper, that acts as a catalyst in what is called a bimetallic nanoparticle. nZVI may also be emulsified with a surfactant and an oil, creating a membrane that enhances the nanoparticle's ability to interact with hydrophobic liquids and protects it against reactions with materials dissolved in water. Commercial nZVI particle sizes may sometimes exceed true “nano” dimensions (100 nm or less in diameter).

nZVI appears to be useful for degrading organic contaminants, including chlorinated organic compounds such as polychlorinated biphenyls (PCBs) and trichloroethene (TCE), as well as immobilizing or removing metals. nZVI and other nanoparticles that do not require light can be injected belowground into the contaminated zone for in situ groundwater remediation and, potentially, soil remediation.

nZVI nanoparticles can be prepared by using sodium borohydride as the key reductant. NaBH₄ (0.2 M) is added into FeCl₃•6H₂ (0.05 M) solution (~1:1 volume ratio). Ferric iron is reduced via the following reaction:

4Fe³⁺ + 3BH⁻
₄ + 9H₂O → 4Fe⁰ + 3H₂BO⁻
₃ + 12H⁺ + 6H₂

Palladized Fe particles are prepared by soaking the nanoscale iron particles with an ethanol solution of 1wt% of palladium acetate ([Pd(C₂H₃O₂)2]₃). This causes the reduction and deposition of Pd on the Fe surface:

Pd²⁺ + Fe ⁰ → Pd⁰ + Fe²⁺

Similar methods may be used to prepared Fe/Pt, Fe/Ag, Fe/Ni, Fe/Co, and Fe/Cu bimetallic particles. With the above methods, nanoparticles of diameter 50-70 nm may be produced. The average specific surface area of Pd/Fe particles is about 35 m²/g. Ferrous iron salt has also been successfully used as the precursor.

Titanium dioxide

Titanium dioxide (TiO₂) is also a leading candidate for nanoremediation and wastewater treatment, although as of 2010 it is reported to have not yet been expanded to full-scale commercialization. When exposed to ultraviolet light, such as in sunlight, titanium dioxide produces hydroxyl radicals, which are highly reactive and can oxidize contaminants. Hydroxyl radicals are used for water treatment in methods generally termed advanced oxidation processes. Because light is required for this reaction, TiO₂ is not appropriate for underground in situ remediation, but it may be used for wastewater treatment or pump-and-treat groundwater remediation.

TiO₂ is inexpensive, chemically stable, and insoluble in water. TiO₂ has a wide band gap energy (3.2 eV) that requires the use of UV light, as opposed to visible light only, for photocatalytic activation. To enhance the efficiency of its photocatalysis, research has investigated modifications to TiO₂ or alternative photocatalysts that might use a greater portion of photons in the visible light spectrum. Potential modifications include doping TiO₂ with metals, nitrogen, or carbon.

Challenges

When using in situ remediation the reactive products must be considered for two reasons. One reason is that a reactive product might be more harmful or mobile than the parent compound. Another reason is that the products can affect the effectiveness and/or cost of remediation. TCE (trichloroethylene), under reducing conditions by nanoiron, may sequentially dechlorinate to DCE (dichloroethene) and VC (vinyl chloride). VC is known to be more harmful than TCE, meaning this process would be undesirable.

Nanoparticles also react with non-target compounds. Bare nanoparticles tend to clump together and also react rapidly with soil, sediment, or other material in ground water. For in situ remediation, this action inhibits the particles from dispersing in the contaminated area, reducing their effectiveness for remediation. Coatings or other treatment may allow nanoparticles to disperse farther and potentially reach a greater portion of the contaminated zone. Coatings for nZVI include surfactants, polyelectrolyte coatings, emulsification layers, and protective shells made from silica or carbon.

Such designs may also affect the nanoparticles’ ability to react with contaminants, their uptake by organisms, and their toxicity. A continuing area of research involves the potential for nanoparticles used for remediation to disperse widely and harm wildlife, plants, or people.

In some cases, bioremediation may be used deliberately at the same site or with the same material as nanoremediation. Ongoing research is investigating how nanoparticles may interact with simultaneous biological remediation.

Nanobatteries

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Nanobatteries 

Image left: shows what a nanosized battery looks like under Transmission Electron Spectrometry (TEM) Image center and right: NIST was able to use TEM to view nanosized batteries and discovered that there probably exists a limit to how thin an electrolyte layer can be until the battery malfunctions. Credit: Talin/NIST Author: National Institute of Standards and Technology

Nanobatteries are fabricated batteries employing technology at the nanoscale, particles that measure less than 100 nanometers or 10⁻⁷ meters. These batteries may be nano in size or may use nanotechnology in a macro scale battery. Nanoscale batteries can be combined to function as a macrobattery such as within a nanopore battery.

Traditional lithium-ion battery technology uses active materials, such as cobalt-oxide or manganese oxide, with particles that range in size between 5 and 20 micrometers (5000 and 20000 nanometers – over 100 times nanoscale). It is hoped that nano-engineering will improve many of the shortcomings of present battery technology, such as volume expansion and power density.

Background

A basic schematic of how an ion battery works. The blue arrows indicate discharging. If both arrows reversed direction, the battery would be charging and this battery would then be considered a secondary (rechargeable) battery.

Main article: Battery (electricity)

See also: History of nanotechnology

A battery converts chemical energy to electrical energy and is composed of three general parts:

• Anode (positive electrode)
• Cathode (negative electrode)
• Electrolyte

The anode and cathode have two different chemical potentials, which depend on the reactions that occur at either terminus. The electrolyte can be a solid or liquid that is ionically conductive. The boundary between the electrode and electrolyte is called the solid-electrolyte interphase (SEI). Connecting a circuit across the electrodes causes the chemical energy stored in the battery to be converted to electrical energy.

Limitations of current battery technology

See also: Lithium-ion battery

A battery's ability to store charge is dependent on its energy density and power density. It is important that charge can remain stored and that a maximum amount of charge can be stored within a battery. Cycling and volume expansion are also important considerations as well. While many other types of batteries exist, current battery technology is based on lithium-ion intercalation technology for its high power and energy densities, long cycle life and no memory effects. These characteristics have led lithium-ion batteries to be preferred over other battery types. To improve a battery technology, cycling ability and energy and power density must be maximized and volume expansion must be minimized.

During lithium intercalation, the volume of the electrode expands, causing mechanical strain. The mechanical strain compromises the structural integrity of the electrode, causing it to crack. Nanoparticles can decrease the amount of strain placed on a material when the battery undergoes cycling, as the volume expansion associated with nanoparticles is less than the volume expansion associated with microparticles. The little volume expansion associated with nanoparticles also improves the reversibility capability of the battery: the ability of the battery to undergo many cycles without losing charge.

In current lithium-ion battery technology, lithium diffusion rates are slow. Through nanotechnology, faster diffusion rates can be achieved. Nanoparticles require shorter distances for the transport of electrons, which leads to faster diffusion rates and a higher conductivity, which ultimately leads to a greater power density.

Advantages of nanotechnology

Using nanotechnology to manufacture of batteries offers the following benefits:

• Increasing the available power from a battery and decreasing the time required to recharge a battery. These benefits are achieved by coating the surface of an electrode with nanoparticles, increasing the surface area of the electrode thereby allowing more current to flow between the electrode and the chemicals inside the battery.
• Nanomaterials can be used as a coating to separate the electrodes from any liquids in the battery, when the battery is not in use. In the current battery technology, the liquids and solids interact, causing a low level discharge. This decreases the shelf life of a battery.

Disadvantages of nanotechnology

Nanotechnology provides its own challenges in batteries:

• Nanoparticles have low density and high surface area. The greater the surface area, the more likely reactions are to occur at the surface with the air. This serves to destabilize the materials in the battery.
• Owing to nanoparticle's low density, a higher interparticle resistance exists, decreasing the electrical conductivity of the material.
• Nanomaterials can be difficult to manufacture, increasing their cost. While nanomaterials may greatly improve the abilities of a battery, they may be cost-prohibitive to make.

Active and past research

Much research has been performed surrounding lithium-ion batteries to maximize their potential. In order to properly harness clean energy resources, such as solar power, wind power and tidal energy, batteries capable of storing massive amounts of energy used in grid energy storage are required. Lithium iron phosphate electrodes are being researched for potential applications to grid energy storage.

Electric vehicles are another technology requiring improved batteries. Electric vehicle batteries currently require large charge times, effectively prohibiting the use for long-distance electric cars.

Nanostructured anode materials

Graphite and SEI

The anode in lithium-ion batteries is almost always graphite. Graphite anodes need to improve their thermal stability and create a higher power capability. Graphite and certain other electrolytes can undergo reactions that reduce the electrolyte and create an SEI (Solid Electrolyte Interphase), effectively reducing the potential of the battery. Nanocoatings at the SEI are currently being researched to stop these reactions from occurring.

In Li-ion batteries, the SEI is necessary for thermal stability, but hinders the flow of lithium ions from the electrode to the electrolyte. Park et al. have developed a nanoscale polydopamine coating such that the SEI no longer interferes with the electrode; instead the SEI interacts with the polydopamine coating.

Graphene and other carbon materials

Graphene has been studied extensively for its use in electrochemical systems such as batteries since its first isolation in 2004. Graphene offers high surface area and good conductivity. In current lithium-ion battery technology, the 2D networks of graphite inhibit smooth lithium-ion intercalation; the lithium ions must travel around the 2D graphite sheets to reach the electrolyte. This slows the charging rates of the battery. Porous graphene materials are currently being studied to improve this problem. Porous graphene involves either formation of defects in the 2D sheet or the creation of a 3D graphene-based porous superstructure.

As an anode, graphene would provide space for expansion such that the problem of volume expansion does not occur. 3D graphene has shown extremely high lithium ion extraction rates, indicating a high reversible capacity. As well, the random "house-of-cards" visualization seen below of the graphene anode would allow lithium ions to be stored not only on the internal surface of graphene, but also on the nanopores that exist between the single layers of graphene.

Raccichini et al. also outlined the drawbacks of graphene and graphene-based composites. Graphene has a large irreversible mechanism during the first lithiation step. As graphene has a large surface area, this will result in a large initial irreversibility capacity. He proposed that this drawback was so large that graphene-based cells are “unfeasible”. Research is still being done on graphene in anodes.

Carbon nanotubes have been used as electrodes for batteries that use intercalation, like lithium-ion batteries, in an effort to improve capacity.

Titanium oxides

Titanium oxides are another anode material that have been researched for their applications to electric vehicles and grid energy storage. However, low electronic and ionic capabilities, as well as the high cost of titanium oxides have proven this material to be unfavorable to other anode materials.

Silicon-based anodes

Silicon-based anodes have also been researched, namely for their higher theoretical capacity than that of graphite. Silicon-based anodes have high reaction rates with the electrolyte, low volumetric capacity and an extremely large volume expansion during cycling. However, recent work has been done to decrease volume expansion in silicon-based anodes. By creating a sphere of conductive carbon around the silicon atom, Liu et al. has proven that this small structural change leaves enough room for the silicon to expand and contract without providing mechanical strain on the electrode.

Nanostructured cathode materials

Carbon nanostructures have been used to increase the capability of electrodes, namely the cathode. In LiSO₂ batteries, carbon nanostructuring was able to theoretically increase the energy density of the battery by 70% from the current lithium-ion battery technology. In general, lithium alloys have been found to have an increased theoretical energy density than lithium ions.

Traditionally, LiCoO₂ has been used as the cathode in lithium-ion batteries. The first successful alternative cathode for use in electric vehicles has been LiFePO₄. LiFePO₄ has shown increased power density, a longer lifetime and improved safety over LiCoO₂.

Graphene

During intercalation, a) lithium ions into a graphite lattice, b) lithium ions into a graphene lattice, c) sodium ions unable to fit into a graphite lattice, d) sodium ions into a graphene lattice.

Graphene could be used to improve the electrical conductivity of cathode materials. LiCoO₂, LiMn₂O₄and LiFePO₄ are all commonly used cathode materials in lithium-ion batteries. These cathode materials have typically mixed with other carbon-composite materials to improve their rate capability. As graphene has a higher electrical conductivity than these other carbon-composite materials, like carbon black, graphene has a greater ability to improve these cathode materials more than other carbon-composite additives.

Piao et al. has specifically studied porous graphene in relation to just graphene. Porous graphene combined with LiFePO₄ was advantageous over just graphene combined with LiFePO₄, for improved cycle stability. Porous graphene created good pore channels for the diffusion of lithium ions and prevented the buildup of LiFePO₄ particles.

Raccichini et al. suggested graphene-based composites as cathodes in sodium-ion batteries. Sodium ions are too large to fit into the typical graphite lattice, so graphene would allow sodium ions to intercalate. Graphene has also been suggested to fix some of the problems related to lithium-sulphur batteries. Problems associated with lithium sulphur batteries include dissolution of the intermediate in the electrolyte, large volume expansion and poor electrical conductivity. Graphene has been mixed with sulphur at the cathode in an attempt to improve the capacity, stability and conductivity of these batteries.

Conversion electrodes

Conversion electrodes are electrodes where chemical ionic bonds are broken and reformed. A transformation of the crystalline structure of the molecules also occurs. In conversion electrodes, three lithium ions can be accommodated for every metal ion, whereas the current intercalation technology can only accommodate one lithium ion for every metal ion. Larger lithium to metal ion ratios indicate increased battery capacity. A disadvantage of conversion electrodes is its large voltage hysteresis.

Mapping

Balke et al. is aiming to understand the intercalation mechanism for lithium-ion batteries at the nanoscale. This mechanism is understood at the microscale, but behavior of matter changes depending on the size of the material. Zhu et al. are also mapping the intercalation of lithium ions at the nanoscale using scanning probe microscopy.

Mathematical models for lithium battery intercalation have been calculated and are still under investigation. Whittingham suggested that there was no single mechanism by which lithium ions move through the electrolyte of the battery. The movement depended on a variety of factors including, but not limited to, particle size, the thermodynamic state or metastable state of the battery and whether the reaction operated continuously. Their experimental data for LiFePO₄ – FePO₄ suggested the movement of Li-ions in a curved path rather than a linear straight jump within the electrolyte.

Intercalation mechanisms have been studied for polyvalent cations as well. Lee et al. has studied and determined the proper intercalation mechanism for rechargeable zinc batteries.

Stretchable electronics

These fiber-like electrodes are wound like springs to get their flexibility. a) is an unstretched spring and b) is a partially stretched spring, showing how pliant these fibers are.

Research has also been done to use carbon nanotube fiber springs as electrodes. LiMn₂O₄ and Li₄Ti₅O₁₂ are the nanoparticles that have been used as the cathode and anode respectively, and have demonstrated the ability to stretch 300% of their original length. Applications for stretchable electronics include energy storage devices and solar cells.

Printable batteries

Researchers at the University of California, Los Angeles have successfully developed a "nanotube ink" for manufacturing flexible batteries using printed electronics techniques. A network of carbon nanotubes has been used as a form of electronically conducting nanowires in the cathode of a zinc-carbon battery. Using nanotube ink, the carbon cathode tube and manganese oxide electrolyte components of the zinc-carbon battery can be printed as different layers on a surface, over which an anode layer of zinc foil can be printed. This technology replaces charge collectors like metal sheets or films with a random array of carbon nanotubes. The carbon nanotubes add conductance. Thin and flexible batteries can be manufactured that are less than a millimeter thick.

Although discharge currents of the batteries are at present below the level of practical use, the nanotubes in the ink allow the charge to conduct more efficiently than in a conventional battery, such that the nanotube technology could lead to improvements in battery performance. Technology like this is applicable to solar cells, supercapacitors, light-emitting diodes and smart radio frequency identification (RFID) tags.

Researching companies

Toshiba

By using nanomaterial, Toshiba has increased the surface area of the lithium and widened the bottleneck, allowing the particles to pass through the liquid and recharge the battery more quickly. Toshiba states that it tested a new battery by discharging and fully recharging one thousand times at 77 °C and found that it lost only one percent of its capacity, an indication of a long battery life. °C Toshiba's battery is 3.8 mm thick, 62 mm high and 35 mm deep.

A123Systems

A123Systems has also developed a commercial nano Li-Ion battery. A123 Systems claims their battery has the widest temperature range at -30 .. +70 °C. Much like Toshiba's nanobattery, A123 Li-Ion batteries charge to "high capacity" in five minutes. Safety is a key feature touted by the A123 technology, with a video on their website of a nail drive test, in which a nail is driven through a traditional Li-Ion battery and an A123 Li-Ion battery, where the traditional battery flames up and bubbles at one end, the A123 battery simply emits a wisp of smoke at the penetration site. Thermal conductivity is another selling point for the A123 battery, with the claim that the A123 battery offers 4 times higher thermal conductivity than conventional Lithium-Ion cylindrical cells. The nanotechnology they employ is a patented nanophosphate technology.

Valence

Also in the market is Valence Technology, Inc. The technology they are marketing is Saphion Li-Ion Technology. Like A123, they are using a nanophosphate technology, and different active materials than traditional Li-Ion batteries.

Altair

AltairNano has also developed a nanobattery with a one-minute recharge. The advance that Altair claims to have made is in the optimization of nano-structured lithium titanate spinel oxide (LTO).

U.S. Photonics

U.S. Photonics is in the process of developing a nanobattery utilizing "environmentally friendly" nanomaterials for both the anode and cathode as well as arrays of individual nano-sized cell containers for the solid polymer electrolyte. U.S. Photonics has received a National Science Foundation SBIR phase I grant for development of nanobattery technology.

Sony

Produced the first cobalt-based lithium-ion battery in 1991. Since the inception of this first Li-ion battery, the research of nanobatteries has been underway with Sony continuing their strides into the nanobattery field.

Clean technology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Clean_technology

Fully electric car charging its battery at a public charging station.

Renewable transportation fuel composed of organic waste. Alternative fuel strategies drastically lowers carbon emissions and air pollution.

Clean technology, in short cleantech or climatetech, is any process, product, or service that reduces negative environmental impacts through significant energy efficiency improvements, the sustainable use of resources, or environmental protection activities. Clean technology includes a broad range of technology related to recycling, renewable energy, information technology, green transportation, electric motors, green chemistry, lighting, grey water, and more. Environmental finance is a method by which new clean technology projects can obtain financing through the generation of carbon credits. A project that is developed with concern for climate change mitigation is also known as a carbon project.

Clean Edge, a clean technology research firm, describes clean technology "a diverse range of products, services, and processes that harness renewable materials and energy sources, dramatically reduce the use of natural resources, and cut or eliminate emissions and wastes." Clean Edge notes that, "Clean technologies are competitive with, if not superior to, their conventional counterparts. Many also offer significant additional benefits, notably their ability to improve the lives of those in both developed and developing countries."

Wind turbines in a field in Spain.

Investments in clean technology have grown considerably since coming into the spotlight around 2000. According to the United Nations Environment Program, wind, solar, and biofuel companies received a record $148 billion in new funding in 2007 as rising oil prices and climate change policies encouraged investment in renewable energy. $50 billion of that funding went to wind power. Overall, investment in clean-energy and energy-efficiency industries rose 60 percent from 2006 to 2007. In 2009, Clean Edge forecasted that the three main clean technology sectors, solar photovoltaics, wind power, and biofuels, would have revenues of $325.1 billion by 2018.

According to an MIT Energy Initiative Working Paper published in July 2016, about a half of over $25 billion funding provided by venture capital to cleantech from 2006 to 2011 was never recovered. The report cited cleantech's dismal risk/return profiles and the inability of companies developing new materials, chemistries, or processes to achieve manufacturing scale as contributing factors to its flop.

Clean technology has also emerged as an essential topic among businesses and companies. It can reduce pollutants and dirty fuels for every company, regardless of which industry they are in, and using clean technology has become a competitive advantage. Through building their Corporate Social Responsibility (CSR) goals, they participate in using clean technology and other means by promoting Sustainability. Fortune Global 500 firms spend around $20 billion a year on CSR activities in 2018.

Definition

Farmer using crops for biofuel

Cleantech products or services are those that improve operational performance, productivity, or efficiency while reducing costs, inputs, energy consumption, waste, or environmental pollution. Its origin is the increased consumer, regulatory, and industry interest in clean forms of energy generation—specifically, perhaps, the rise in awareness of global warming, climate change, and the impact on the natural environment from the burning of fossil fuels. Cleantech is often associated with venture capital funds and land use organizations. The term has historically been differentiated from various definitions of green business, sustainability, or triple bottom line industries by its origins in the venture capital investment community and has grown to define a business sector that includes significant and high growth industries such as solar, wind, water purification, and biofuels.

Nomenclature

While the expanding industry has grown rapidly in recent years and attracted billions of dollars of capital, the clean technology space has not settled on an agreed-upon term. Cleantech, is used fairly widely, although variant spellings include ⟨clean-tech⟩ and ⟨clean tech⟩. In recent years, some clean technology companies have de-emphasized that aspect of their business to tap into broader trends, such as smart cities.

Origins of the concept

The idea of cleantech first emerged among a group of emerging technologies and industries, based on principles of biology, resource efficiency, and second-generation production concepts in basic industries. Examples include: energy efficiency, selective catalytic reduction, non-toxic materials, water purification, solar energy, wind energy, and new paradigms in energy conservation. Since the 1990s, interest in these technologies has increased with two trends: a decline in the relative cost of these technologies and a growing understanding of the link between industrial design used in the 19th century and early 20th century, such as fossil fuel power plants, the internal combustion engine, and chemical manufacturing, and an emerging understanding of human-caused impact on earth systems resulting from their use (see articles: ozone hole, acid rain, desertification, climate change, and global warming).

Investment worldwide

Annual cleantech investment in North America, Europe, Israel, China, India

Year	Investment ($mil)
2001	506.8
2002	883.2
2003	1,258.6
2004	1,398.3
2005	2,077.5
2006	4,520.2
2007	6,087.2
2008*	8,414.3
*2008 data preliminary
Source: Cleantech Group

In 2008, clean technology venture investments in North America, Europe, China, and India totaled a record $8.4 billion. Cleantech Venture Capital firms include NTEC, Cleantech Ventures, and Foundation Capital. The preliminary 2008 total represents the seventh consecutive year of growth in venture investing, widely recognized as a leading indicator of overall investment patterns. China is seen as a major growth market for cleantech investments currently, with a focus on renewable energy technologies. In 2014, Israel, Finland and the US were leading the Global Cleantech Innovation Index, out of 40 countries assessed, while Russia and Greece were last. With regards to private investments, the investment group Element 8 has received the 2014 CleanTech Achievement award from the CleanTech Alliance, a trade association focused on clean tech in the State of Washington, for its contribution in Washington State's cleantech industry.

According to the published research, the top clean technology sectors in 2008 were solar, biofuels, transportation, and wind. Solar accounted for almost 40% of total clean technology investment dollars in 2008, followed by biofuels at 11%. In 2019, sovereign wealth funds directly invested just under US$3 billion in renewable energy .

The 2009 United Nations Climate Change Conference in Copenhagen, Denmark was expected to create a framework whereby limits would eventually be placed on greenhouse gas emissions. Many proponents of the cleantech industry hoped for an agreement to be established there to replace the Kyoto Protocol. As this treaty was expected, scholars had suggested a profound and inevitable shift from "business as usual." However, the participating States failed to provide a global framework for clean technologies. The outburst of the 2008 economic crisis then hampered private investments in clean technologies, which were back at their 2007 level only in 2014. The 2015 United Nations Climate Change Conference in Paris is expected to achieve a universal agreement on climate, which would foster clean technologies development. On 23 September 2019, the Secretary-General of the United Nations hosted a Climate Action Summit in New York.

In 2022 the investment in cleantech (also called climatetech) boomed. "In fact, climate tech investment in the 12 months to Q3 2022 represented more than a quarter of every venture dollar invested, a greater proportion than 12 of the prior 16 quarters."

Implementation worldwide

India is one of the countries that have achieved remarkable success in sustainable development by implementing clean technology, and it became a global clean energy powerhouse. India, who was the third-largest emitter of greenhouse gases, advanced a scheme of converting to renewable energy with sun and wind from fossil fuels. This continuous effort has created an increase in the country's renewable energy capacity (around 80 gigawatts of installed renewable energy capacity, 2019), with a compound annual growth rate of over 20%. By steadily increasing India's renewable capacity, India is achieving the Paris Agreement with a significant reduction in producing carbon emissions. Adopting renewable energy not only brought technological advances to India, but it also impacted employment by creating around 330,000 new jobs by 2022 and more than 24 million new jobs by 2030, according to the International Labour Organization in the renewable energy sector.

Germany has been one of the renewable energy leaders in the world, and their efforts have expedited the progress after the nuclear power plant meltdown in Japan in 2011, by deciding to switch off all 17 reactors by 2022. Still, this is just one of Germany's ultimate goals; and Germany is aiming to set the usage of renewable energy at 80% by 2050, which is currently 47% (2020). Also, Germany is investing in renewable energy from offshore wind and anticipating its investment to result in one-third of total wind energy in Germany. The importance of clean technology also impacted the transportation sector of Germany, which produces 17 percent of its emission. The famous car-producing companies, Mercedes-Benz, BMW, Volkswagen, and Audi, in Germany, are also providing new electric cars to meet Germany's energy transition movement.

Africa and the Middle East has drawn worldwide attention for its potential share and new market of solar electricity. Notably, the countries in the Middle East have been utilizing their natural resources, an abundant amount of oil and gas, to develop solar electricity. Also, to practice the renewable energy, the energy ministers from 14 Arab countries signed a Memorandum of Understanding for an Arab Common Market for electricity by committing to the development of the electricity supply system with renewable energy.

United Nations: Sustainable Development Goals

United Nations: 17 Sustainable Development Goals

The United Nations has set goals for the 2030 Agenda for Sustainable Development, which is called "Sustainable Development Goals" composed of 17 goals and 232 indicators total. These goals are designed to build a sustainable future and to implement in the countries (member states) in the UN. Many parts of the 17 goals are related to the usage of clean technology since it is eventually an essential part of designing a sustainable future in various areas such as land, cities, industries, climate, etc.

• Goal 6: "Ensure availability and sustainable management of water and sanitation for all"
  • Various kinds of clean water technology are used to fulfill this goal, such as filters, technology for desalination, filtered water fountains for communities, etc.
• Goal 7: "Ensure access to affordable, reliable, sustainable and modern energy for all"
  • Promoting countries for implementing renewable energy is making remarkable progress, such as:
    • "From 2012 to 2014, three quarters of the world’s 20 largest energy-consuming countries had reduced their energy intensity — the ratio of energy used per unit of GDP. The reduction was driven mainly by greater efficiencies in the industry and transport sectors. However, that progress is still not sufficient to meet the target of doubling the global rate of improvement in energy efficiency."
• Goal 11: "Make cities and human settlements inclusive, safe, resilient and sustainable"
  • By designing sustainable cities and communities, clean technology takes parts in the architectural aspect, transportation, and city environment. For example:
    • Global Fuel Economy Initiative (GFEI) - Relaunched to accelerate progress on decarbonizing road transport. Its main goal for passenger vehicles, in line with SDG 7.3, is to double the energy efficiency of new vehicles by 2030. This will also help mitigate climate change by reducing harmful CO₂ emissions.
• Goal 13: "Take urgent action to combat climate change and its impacts*"
  • Greenhouse gas emissions have significantly impacted the climate, and this results in a rapid solution for consistently increasing emission levels. United Nations held the "Paris Agreement" for dealing with greenhouse gas emissions mainly within countries and for finding solutions and setting goals.

Energy applications of nanotechnology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Energy_applications_of_nanotechnology

 

As the world's energy demand continues to grow, the development of more efficient and sustainable technologies for generating and storing energy is becoming increasingly important. According to Dr. Wade Adams from Rice University, energy will be the most pressing problem facing humanity in the next 50 years and nanotechnology has potential to solve this issue. Nanotechnology, a relatively new field of science and engineering, has shown promise to have a significant impact on the energy industry. Nanotechnology is defined as any technology that contains particles with one dimension under 100 nanometers in length. For scale, a single virus particle is about 100 nanometers wide.

People in the fields of science and engineering have already begun developing ways of utilizing nanotechnology for the development of consumer products. Benefits already observed from the design of these products are an increased efficiency of lighting and heating, increased electrical storage capacity, and a decrease in the amount of pollution from the use of energy. Benefits such as these make the investment of capital in the research and development of nanotechnology a top priority.

Commonly used nanomaterials in energy

An important sub-field of nanotechnology related to energy is nanofabrication, the process of designing and creating devices on the nanoscale. The ability to create devices smaller than 100 nanometers opens many doors for the development of new ways to capture, store, and transfer energy. Improvements in the precision of nanofabrication technologies are critical to solving many energy related problems that the world is currently facing.

Graphene-based materials

There is enormous interest in the use of graphene-based materials for energy storage. The research on the use of graphene for energy storage began very recently, but the growth rate of relative research is rapid.

Graphene recently emerged as a promising material for energy storage because of several properties, such as low weight, chemical inertness and low price. Graphene is an allotrope of carbon that exists as a two-dimensional sheet of carbon atoms organized in a hexagonal lattice. A family of graphene-related materials, called "graphenes" by the research community, consists of structural or chemical derivatives of graphene. The most important chemically derived graphene is graphene oxide (defined as single layer of graphite oxide, Graphite oxide can be obtained by reacting graphite with strong oxidizers, for example, a mixture of sulfuric acid, sodium nitrate, and potassium permanganate) which is usually prepared from graphite by oxidization to graphite oxide and consequent exfoliation. The properties of graphene depend greatly on the method of fabrication. For example, reduction of graphene oxide to graphene results in a graphene structure that is also one-atom thick but contains a high concentration of defects, such as nanoholes and Stone–Wales defects. Moreover, carbon materials, which have relatively high electrical conductivity and variable structures are extensively used in the modification of sulfur. Sulfur–carbon composites with diverse structures have been synthesized and exhibited remarkably improved electrochemical performance than pure sulfur, which is crucial for battery design. Graphene has great potential in the modification of a sulfur cathode for high performance Li-S batteries, which has been broadly investigated in recent years.

Silicon-based nano semiconductors

Silicon-based nano semiconductors have the most useful application in solar energy and it also has been extensively studied at many places, such as Kyoto University. They utilize silicon nanoparticles in order to absorb a greater range of wavelengths from the electromagnetic spectrum. This can be done by putting many identical and equally spaced silicon rods on the surface. Also, the height and length of spacing have to be optimized for reaching the best results. This arrangement of silicon particles allows solar energy to be reabsorbed by many different particles, exciting electrons and resulting in much of the energy being converted to heat. Then, the heat can be converted to electricity. Researchers from Kyoto University have shown that these nano-scale semiconductors can increase efficiency by at least 40%, compared to the regular solar cells.

Nanocellulose‐based materials

Cellulose is the most abundant natural polymer on earth. Currently, nanocellulose‐based mesoporous structures, flexible thin films, fibers, and networks are developed and used in photovoltaic (PV) devices, energy storage systems, mechanical energy harvesters, and catalysts components. Inclusion of nanocellulose in those energy‐related devices largely raises the portion of eco‐friendly materials and is very promising in addressing the relevant environmental concerns. Furthermore, cellulose manifests itself in the low cost and large‐scale promises.

Nanostructures in energy

One-dimensional nanomaterials

One-dimensional nanostructures have shown promise to increase energy density, safety, and cycling-life of energy storage systems, an area in need of improvement for Li-ion batteries. These nanostructures are mainly used in battery electrodes because of their shorter bi-continuous ion and electron transport pathways, which results in higher battery performance.

Additionally, 1D nanostructures are capable of increasing charge storage by double layering, and can also be used on supercapacitors because of their fast pseudocapacitive surface redox processes. In the future, novel design and controllable synthesis of these materials will be developed much more in-depth. 1D nanomaterials are also environmentally friendly and cost-effective.

Two-dimensional nanomaterials

The most important feature of two dimensional nanomaterials is that their properties can be precisely controlled. This means that 2D nanomaterials can be easily modified and engineered on nanostructures. The interlayer space can also be manipulated for nonlayered materials, called 2D nanofluidic channels. 2D nanomaterials can also be engineered into porous structures in order to be used for energy storage and catalytic applications by applying facile charge and mass transport.

2D nanomaterials also have a few challenges. There are some side effects of modifying the properties of the materials, such as activity and structural stability, which can be compromised when they are engineered. For example, creating some defects can increase the number of active sites for higher catalytic performance, but side reactions may also happen, which could possibly damage the catalyst's structure. Another example is that interlayer expansion can lower the ion diffusion barrier in the catalytic reaction, but it can also potentially lower its structural stability. Because of this, there is a tradeoff between performance and stability. A second issue is consistency in design methods. For example, heterostructures are the main structures of the catalyst in interlayer space and energy storage devices, but these structures may lack the understanding of mechanism on the catalytic reaction or charge storage mechanisms. A deeper understanding of 2D nanomaterial design is required, because fundamental knowledge will lead to consistent and efficient methods of designing these structures. A third challenge is the practical application of these technologies. There is a huge difference between lab-scale and industry-scale applications of 2D nanomaterials due to their intrinsic instability during storage and processing. For example, porous 2D nanomaterial structures have low packing densities, which makes them difficult to pack into dense films. New processes are still being developed for the application of these materials on an industrial scale.

Applications

Lithium-sulfur based high-performance batteries

The Li-ion battery is currently one of the most popular electrochemical energy storage systems and has been widely used in areas from portable electronics to electric vehicles. However, the gravimetric energy density of Li-ion batteries is limited and less than that of fossil fuels. The lithium sulfur (Li-S) battery, which has a much higher energy density than the Li-ion battery, has been attracting worldwide attention in recent years. A group of researches from the National Natural Science Foundation of China (Grant No. 21371176 and 21201173) and the Ningbo Science and Technology Innovation Team (Grant No. 2012B82001) have developed a nanostructure-based lithium-sulfur battery consisting of graphene/sulfur/carbon nano-composite multilayer structures. Nanomodification of sulfur can increase the electrical conductivity of the battery and improve electron transportation in the sulfur cathode. A graphene/sulfur/carbon nanocomposite with a multilayer structure (G/S/C), in which nanosized sulfur is layered on both sides of chemically reduced graphene sheets and covered with amorphous carbon layers, can be designed and successfully prepared. This structure achieves high conductivity, and surface protection of sulfur simultaneously, and thus gives rise to excellent charge/discharge performance. The G/S/C composite shows promising characteristics as a high performance cathode material for Li-S batteries.

Nanomaterials in solar cells

Engineered nanomaterials are key building blocks of the current generation solar cells. Today's best solar cells have layers of several different semiconductors stacked together to absorb light at different energies but still only manage to use approximately 40% of the Sun's energy. Commercially available solar cells have much lower efficiencies (15-20%). Nanostructuring has been used to improve the efficiencies of established photovoltaic (PV) technologies, for example, by improving current collection in amorphous silicon devices, plasmonic enhancement in dye-sensitized solar cells, and improved light trapping in crystalline silicon. Furthermore, nanotechnology could help increase the efficiency of light conversion by utilizing the flexible bandgaps of nanomaterials, or by controlling the directivity and photon escape probability of photovoltaic devices. Titanium dioxide (TiO₂) is one of the most widely investigated metal oxides for use in PV cells in the past few decades because of its low cost, environmental benignity, plentiful polymorphs, good stability, and excellent electronic and optical properties. However, their performances are greatly limited by the properties of the TiO₂ materials themselves. One limitation is the wide band gap, making TiO₂ only sensitive to ultraviolet (UV) light, which just occupies less than 5% of the solar spectrum. Recently, core–shell structured nanomaterials have attracted a great deal of attention as they represent the integration of individual components into a functional system, showing improved physical and chemical properties (e.g., stability, non-toxicity, dispersibility, multi-functionality), which are unavailable from the isolated components. For TiO₂ nanomaterials, this core–shell structured design would provide a promising way to overcome their disadvantages, thus resulting in improved performances. Compared to sole TiO₂ material, core–shell structured TiO₂ composites show tunable optical and electrical properties, even new functions, which are originated from the unique core–shell structures.

Nanoparticle fuel additives

Nanomaterials can be used in a variety of ways to reduce energy consumption. Nanoparticle fuel additives can also be of great use in reducing carbon emissions and increasing the efficiency of combustion fuels. Cerium oxide nanoparticles have been shown to be very good at catalyzing the decomposition of unburnt hydrocarbons and other small particle emissions due to their high surface area to volume ratio, as well as lowering the pressure within the combustion chamber of engines to increase engine efficiency and curb NO_x emissions. Addition of carbon nanoparticles has also successfully increased burning rate and ignition delay in jet fuel. Iron nanoparticle additives to biodiesel and diesel fuels have also shown a decrease in fuel consumption and volumetric emissions of hydrocarbons by 3-6%, carbon monoxide by 6-12% and nitrogen oxides by 4-11% in one study.

Environmental and health impacts of fuel additives

While nanomaterials can increase energy efficiency of fuel in several ways, a drawback of their use lies in the effect of nanoparticles on the environment. With cerium oxide nanoparticle additives in fuel, trace amounts of these toxic particles can be emitted within the exhaust. Cerium oxide additives in diesel fuel have been shown to cause lung inflammation and increased bronchial alveolar lavage fluid in rats. This is concerning, especially in areas with high road traffic, where these particles are likely to accumulate and cause adverse health effects. Naturally occurring nanoparticles created by the incomplete combustion of diesel fuels are also large contributors to toxicity of diesel fumes. More research needs to be conducted to determine whether the addition of artificial nanoparticles to fuels decreases the net amount of toxic particle emissions due to combustion.

Economic benefits

The relatively recent shift toward using nanotechnology with respect to the capture, transfer, and storage of energy has and will continue to have many positive economic impacts on society. The control of materials that nanotechnology offers to scientists and engineers of consumer products is one of the most important aspects of nanotechnology and allows for efficiency improvements of a variety of products. More efficient capture and storage of energy by use of nanotechnology may lead to decreased energy costs in the future, as preparation costs of nanomaterials becomes less expensive with more development.

A major issue with current energy generation is the generation of waste heat as a by-product of combustion. A common example of this is in an internal combustion engine. The internal combustion engine loses about 64% of the energy from gasoline as heat and an improvement of this alone could have a significant economic impact. However, improving the internal combustion engine in this respect has proven to be extremely difficult without sacrificing performance. Improving the efficiency of fuel cells through the use of nanotechnology appears to be more plausible by using molecularly tailored catalysts, polymer membranes, and improved fuel storage.

In order for a fuel cell to operate, particularly of the hydrogen variant, a noble-metal catalyst (usually platinum, which is very expensive) is needed to separate the electrons from the protons of the hydrogen atoms. However, catalysts of this type are extremely sensitive to carbon monoxide reactions. In order to combat this, alcohols or hydrocarbons compounds are used to lower the carbon monoxide concentration in the system. Using nanotechnology, catalysts can be designed through nanofabrication that limit incomplete combustion and thus decrease the amount of carbon monoxide, improving the efficiency of the process.