Research Impact

Alternative energy generation and a healthier environment
With the increasing trend of crude oil prices over the past several years, the Department of Energy and many companies and research centers have placed an emphasis on the generation of alternative energy.   Even with this emphasis and some alternative products on the market, the U.S. demand of crude oil does not appear to be slowing.   According to energy statistcs kept by the Energy Information Administration (EIA), the world's approximate petroleum consumption has increased from 21.34 million barrels per day (bpd) in 1960 to 83.56 million bpd in 2005.   In 2005, the U.S. was responsible for about 25% of the total consumption.   Products to counter the consumption such as hybrid and ethanol vehicles, fuel cell technology, and solar cells have garnered some attention for reducing dependance on oil.   However, many present products are not cost-effective to the manufacturer or the customer.   Fuel cells have promise, especially in the automotive industry, but the cost of production plague the commercialization of the technology.   Many hybrid vehicles cost more than a normal vehicle of the same type.   Adding up potential fuel costs over the lifetime of both cars, the amount of cost savings is insignificant.   Ethanol vehicles, at this point in time, are impractical because the vast majority of fueling pumps across the country are not yet equipped with ethanol.   Energy research and development with nanotechnology has peaked interest as a possible solution.  

Nano solar cells
Conventional solar cells are composed of a semiconducting material such as silicon.   Solar cells (or photovoltaic cells) utilize the semiconductor's optical properties to produce charge carriers for conducting current in the presence of light.   The incoming light to the solar cell must have enough energy to move an electron from the valence band to the conduction band.   The energy bandgap of silicon is 1.12 eV, so any light with energy greater than or equal to 1.12 eV will generate an electron-hole pair.   In actuality, the energy bandgap depends somewhat on temperature, but in most operating environments the changes are insignificant.   Since the bandgap of silicon is fairly small, normal amounts of sunlight or indoor lighting have more than enough energy to generate electrons.   Any light which does not have sufficient energy will not be absorbed by the material.   Instead, the light which just transmit through the silicon.   From the equation E = hf = hc/λ, the wavelengths of light which give the required amount of energy can be determined.   Given that Planck's constant in terms of eV is 4.135 × 10-15 eV-s and c is 3 × 108 m/s, the cutoff wavelength is about 1.108 μm, which is in the infrared spectrum.   Once the electron-hole pairs are generated by sufficient amounts of energy, they must be swept out of the semiconductor to the load connected to the solar cell.   Electrons and holes have the ability to move through a material in one of two ways: drift or diffusion.   In traditional solar cells which are basically flat pn junctions (much like a diode), the mode of transport is drifting.   When electrons and holes are generated by light, an electric field is induced across the middle of the pn junction which causes the electrons to exit the semiconductor in one direction and the holes to exit in the opposite direction.   The problem of current semiconductor-based solar cells is the cost.   If solar cells are used in low-current, small applications, then the cost is reasonable compared to replacing batteries.   However, cost usually becomes unbearable when a large number of cells must be used to generate enough current to power the load (i.e. covering an entire roof or valuable land space with solar cells for residential power).   Studies show that over the lifespan of a solar cell, the cost is about $0.40 to $0.50 per kilowatt hour (kWh).   According to the EIA, in 2005 consumer electricity cost averaged across the residential, commercial, and business sectors was $0.0814.  

Various companies have explored different ways of integrating nanotechnology into solar cells.   The advantages to applying nanotechnology to solar cells are not just limited to potential cost reduction.   The next generation of solar cells will be even lighter and smaller.   In addition, performance can improve through more surface area.   To illustrate the role of surface area, consider the rate at which different sizes of ice melt in a drink.   With all other conditions the same, a larger chunk of ice will melt much slower than many smaller chunks of ice.   When a larger piece is broken into smaller sections, each individual section has additional surfaces in contact with the liquid.   The same principle holds true in working with nanoparticles — smaller particles contribute to a greater amount of total surface area.   The concept of electron-hole generation used in normal solar cells also governs the operation of nano solar cells.   Research involving solar cells surrounds the performance of different materials as well as integrating solar cells into everyday items such as fabric and glass.   The performance of a solar cells is related to two things: absorption of photons and the lifetime of charge carriers.   Certain materials absorb photons better than others, and a larger percentage of photon absorption creates more electron-hole pairs.   Ideally, once charge carriers are produced, all of them should proceed to the electrodes for dissipation into the electrical load.   However, some of the electrons recombine with the holes in the valence band before reaching the electrodes.   Materials with high photon absorption as well as longer charge carrier lifetimes are desired.   Photon absorption depends on a photon's wavelength and the width of the material's energy bandgap as discussed above.   A good semiconductor for solar cells will absorb many photons over a short distance, while a poor material will allow most of the photons to transmit through the material.   Consider the difference in photon intensity between the surface of the semiconductor and at some length of dx in the material.   As more photons are absorbed when traveling in the semiconductor, the total flux of photons will decrease.   An absorption coefficient (α) is used to show the relative number of photons absorbed in a unit of distance.   A large absorption coeffecient indicates that only a short distance is needed to absorb photons.   Since the absoprtion coeffecient depends on the energy of the photons, the width of the semiconductor plays a factor in the number of absorbed photons.   High energy photons (i.e. UV light) may be absorbed in a semiconductor with small width, but larger energy photons (i.e. infrared light) will have minimal absorption.   Therefore, traditional solar cells were made quite bulky so the semiconductors could utilize the majority of the spectrum from infrared light to visible light as well as UV light.   Recent advances with the application of nanotechnology to solar cells show that solar cells can be made small and flexible with thin films of nanoparticles and ink dyes.   Though the new generation of solar cells does not have as high of a yield, the cost of production is most lower than the rigorous process to produce silicon wafers.   A video of Nanosolar's production process demonstrates the thin film tehcnology.   Thin film solar cells use non-silicon based semiconductors to absorb photons.   Among the non-silicon materials, CIGS (Copper Indium Galluim DiSelenide) has proved to be the most competitive with the efficiency of silicon-based solar cells.   The benefit of CIGS nanoparticles is that the nanoparticles can be printed on rolls of a flexible substrate to reduce cost of mass production.   Furthermore, electrodes for the CIGS solar cells can be printed and also made transparent.   The tranparent electrodes can be made from zinc oxide (ZnO2) nanoparticles doped with a metal, such as aluminum.   Tranparent electrodes allow the light to transmit to the CIGS particles while conducting the excited electrons to the connected electrical load.   The direct interface between the ZnO2 and the CIGS nanoparticles aids in the transport of the charge carriers to the electrode.   The CIGS semiconductor's energy bandgap can also be tuned by changing the proportions of the group III and group V elements in the compound.   Tuning the bandgap allows for certain photon energies to transmit through the material.  

Another thin film technology uses a light-sensitive dye coupled to a semiconducting nanocrystalline oxide.   The particular oxide which has drawn most of the research attention has been titanium dioxide (TiO2).   Conventional solar cells rely on silicon to both produce the electron hole-pair as well as transport the charge carriers to the electrodes.   Dye-sensitized solar cells (DSSC) separate the two processes to increase the efficiency by decreasing the likelihood of electron-hole recombination.   In DSSCs, the excited electron is generated by a dye which surrounds the arrangment of nanocrystalline TiO2.   If the electron has enough energy to overcome the energy bandgap of TiO2, the electron will move from the dye into the conduction band of the oxide.   Once in the conduction band, the electron is free to diffuse to the electrode.   TiO2 has a larger bandgap than silicon, so electrons produced from smaller photon energies in the infrared spectrum will most likely not reach the conduction band.   The efficiency of the DSSC is due in part to both the size of oxide nanoparticles and the nature of the barrier between the oxide and surrounding dye.   Again, the size of the oxide nanocrystals greatly increases the surface area contact with the dye to maximize the amount of electrons injected into the TiO2.   The very nature of the barrier between the dye and TiO2 allows electrons to pass from the dye to the oxide while inhibiting recombination from the oxide to the dye.  

Nanotechnology in fuel cells
The potential applications of a hydrogen fuel cell are far reaching, but two things stand in the way of making it cost-effective: the efficient, clean production of hydrogen and hydrogen storage.   Hydrogen is an ideal alternative fuel because its energy density is comparable to or better than fossil fuels.   Both hydrogen and petroleum can generate large amounts of power from a small amount of material.   Other energy sources with low densities are impractical because a larger amount of fuel is needed to produce a similar power output.   The production of hydrogen can be accomplished by methods of steam reforming and electrolysis.   However, steam reforming requires natural gas and produces carbon dioxide as a byproduct, and though conventional electrolysis uses electricity to break down water into hydrogen and oxygen, the electricity must be produced from fossil fuel.   Research is being conducted on a photoelectrochemical (PEC) cell as a new method of hydrogen production.   The operation of the PEC cell relies on nanotechnology in two areas: the production of electricity and the separation of water molecules.   The PEC cell converts light into electricity which is used to break apart water molecules; therefore, the process has been called photoelectrolysis.   Research with PEC cells has taken advantage of recent advances in the area of photovoltaics, particularly with the use of TiO2.   A PEC cell contains two electrodes which are electrically connected together.   The water or a more commonly used electrolyte spans the gap between the two electrodes.   The electrodes are typically made of conductive glass, but one of the electrodes is coated with the metal oxide semiconducting nanoparticles.   When photons strike the nanoparticles, electrons are knocked into the conductive glass (provided that it doesn't recombine with a hole before reach the glass).   Since the two electrodes are connected, electrons diffuse to the uncoated electrode to make it negatively charged.   The nanoparticle layer now has some missing electrons indicating the presence of positively charge holes.   The water molecules touching the nanoparticles donate electrons to the nanoparticles and in the process are broken down into positively charged hydrogen atoms and stable oxygen molecules (O2). The negatively charge electrodes attract the positive hydrogen ions to its surface where the electrode transfers electrons to form H2 molecules.   Again, the nanoparticle layer's contribution to the total surface area increases the number of water molecules in contact with the nanoparticles.  

Once hydrogen is produced, it needs to be stored.   Currently, hydrogen is usually stored as a liquid.   Since a liquid is much more dense than gas, much more hydrogen can be stored per unit volume.   However, to obtain enough hydrogen in liquid form to power a fuel cell, a large thick-walled, heavy tank would be required.   In addition, large pressures or extremely small temperatures are needed to keep hydrogen stored as a liquid; thus, liquid storage is deemed impractical.   Research in hydrogen storage involves the development of nanomaterials which will absorb hydrogen and release hydrogen on demand.   To be able to use hydrogen effectively for fuel cells, the storage capacity of the material should be at least 6% (by weight) in normal operating conditions.   The U.S. Department of Energy (DOE) has actually set the current target for hydrogen storage system development to 6.0% by weight and 9% by 2015.   Restrictions on the storage system, however, limit research to just a few different types of materials.   New materials must be lightweight to keep the storage system from being too heavy.   Furthermore, the desorption and absorption of hydrogen must occur at a normal range of temperatures (typically 60-120° C) to prevent the need for further energy losses.   Proper measures must also be taken to ensure that the complex used to store H2 is not toxic.   For example, ammonia (NH3) is known to store adequates amount of hydrogen.   Ammonia can be split by a catalyst into its nitrogen and hydrogen components, but it has also been tagged by the EPA to be highly toxic to human health.   The rate of hydrogen absorption and desorption is also of concern when developing new materials.   If hydrogen is slowly transferred out of the storage material by desorption, then the fuel cell will not provide ample electrical output.   Research has shown that the most viable ways of storing hydrogen are solids such as metal ammines (Mg(NH3)6Cl2) ammonia borane (NH3BH3) and metal hydrides (MgH2, AlH3, and NaAlH4).   Metal ammines, in essence, are solid tablet forms of ammonia which have been demonstrated to store hydrogen at more than 9 wt.%.   By combining the ammonia with a metal into a solid lattice, the toxicity and smell of the ammonia disappear altogether at room temperature.   Since the ammonia is not handled directly, the risk of exposure is limited.   In the metal ammine storage system, the ammonia is the source of hydrogen.   Therefore, the most of the fuel cost is associated with the production of ammonia.   Metal ammines have the form M(NH3)mXn, where M is a metal cation and X is a nonmetal anion and values of m and n depend on cation and the stoichiometry of the ammine decomposition.   In the case of Mg(NH3)6Cl2, Mg+2 is the cation and Cl- is the anion.   The first process to extract the hydrogen from metal ammines is to thermally decompose the ammine into a salt (MgCl2) and ammonia.   When ammonia is desorbed from the ammine, the ammine keeps it shape but contains nanopores around the size of 80nm.   The nanopores are an advantage in reabsorbing ammonia into the remaining salt (the nanopores increase the surface area contact with the ammonia).   The ammonia can then be decomposed into its nitrogen and hydrogen components through the addition of heat and a catalyst.   One of the main concerns with solid hydrogen storge is the propensity for hydrogen to be restored in the host material to make a complete cycle.   In the metal ammine system, hydrogen must combine with nitrogen to form ammonia.   With the extraction of heat, the ammonia and salt form the metal ammine once again.  

The drawback of the Mg(NH3)6Cl2 ammine is the heat required for the thermal decomposition of the ammine and ammonia to obtain all of the hydrogen.   Temperatures of about 600 K are necessary to break down the all of the ammine.   A third of the hydrogen is released at about 420 K, and two-thirds are released by 500 K.   For the metal ammine system to be a complete solution, changes must be made to reduce the decomposition temperature.   Research has already shown that by changing the dimensions of the crystalline (i.e. the lattice constant), the operation temperatures are reduced.   In the metal ammine system, the hydrogen is extracted from ammonia, but ammonia must be produced in large factories which create CO2 pollution.   Producing the hydrogen from PEC cells instead will elliminate the need for hydrocarbons (i.e. CH4 or other fossil fuels).   Nitrogen is abundant in air, so relatively simple air processing techniques can be used to extract the nitrogen needed for ammonia production.