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Materials Science and Engineering


Michael Arnold

Assistant Professor Michael Arnold (large image)

Harvesting sunlight with carbon nanotubes

A new alternative energy technology relies on the element most associated with climate change: carbon. Assistant Professor Michael Arnold is researching how to create inexpensive, efficient solar cells from carbon nanotubes, which are 1-nanometer sheets of carbon rolled into seamless cylinders. Many researchers are studying how to use nanotubes for mechanical and electronics applications, but Arnold is one of the first to apply them to solar energy.

“We are developing new materials and methods to create scalable, inexpensive, stable and efficient photovoltaic solar-cell technologies,” Arnold says. “Semiconducting carbon nanotubes have remarkable electronic and optical properties that are ideally suited for photovoltaics, so this is a good starting point.”

Carbon is a promising choice for solar cells because it is an abundant element, and carbon nanotubes have excellent electrical conductivity and strong optical absorptivity. Most current solar cells use silicon, which converts 10 to 30 percent of sunlight absorbed into electricity. This is a good rate, but silicon cells are expensive. Arnold hopes to achieve comparable efficiency for less cost.

To create the new solar cells, Arnold and his students grow nanotube structures and then separate the useful semiconducting nanotubes from undesirable metallic ones. During the process, they wrap the tubes in a polymer to make them soluble and put them into a solution, which they can spray in a thin film onto transparent indium-tin-oxide coated glass substrates. Then, they deposit an electron-accepting semiconductor and a negative electrode on top of the nanotubes to complete the entire cell.

Arnold, whose work is funded by the National Science Foundation, is currently studying how charge is generated in the nanotubes in response to light and how different electron-accepting materials affect the efficiency and speed of the separation of that charge.

Permanently polarized materials: Potential power for tiny devices

For his PhD at Georgia Institute of Technology, Assistant Professor Xudong Wang created a piezoelectric nanogenerator that potentially could run small devices. Now at UW-Madison, Wang is continuing his work by researching a new material that could make the nanogenerator more powerful and efficient.

Wang’s nanogenerator currently is composed of zinc oxide nanowires that produce 10 nanowatts per square centimeter with a very low efficiency. With support from the National Science Foundation, Wang is now developing ferroelectric materials that could produce nanowires with 10 times the electric potential of the original zinc oxide ones. The increase occurs because the crystal of a ferroelectric material is made of spatially unbalanced atoms that produce automatic, permanent polarization in the material. When Wang introduces strain inside this unbalanced crystal, the polarization is enhanced, creating a significant electric potential.

This high electrical potential could convert mechanical energy from sources as varied as wind, car engines, breathing or body movements into electricity that could then power a small device. Very little mechanical energy would be needed to power the new nanogenerator because even a small amount of displacement has a large effect at the nanoscale — a theory Wang intends to prove in his lab.

Fabricating the new nanowires is more challenging than fabricating zinc oxide nanowires. To grow the new materials, Wang uses chemical vapor deposition, which involves vaporizing a source material in a furnace and condensing it over a substrate, and hydrothermal techniques, which involve mixing and sealing crystals in a water solution and heating the solution until it decomposes into the desired crystal.

Despite the complicated processes, Wang has confidence in the new nanowires, each one of which is 10,000 times smaller than a human hair. “The goal is to make a real nanodevice to power things like microelectromechanical systems, transistors, biomedical devices, sensors or robots,” he says. “The new generator could serve as an unlimited battery.”

Smaller isn’t always better: Catalyst simulations could lower fuel cell cost

Assistant Professor Dane Morgan and Materials Science Program PhD student Edward Holby have found that smaller isn’t always better. Morgan is researching how particle size relates to the overall stability of materials, and he has developed a computational model to help reduce catalyst degradation, an important step in making fuel cells a viable, widespread technology.

Fuel cells are electrochemical devices that facilitate a reaction between oxygen and hydrogen, producing electrical power and forming water. In the type of fuel cells Morgan is researching, called proton exchange membrane fuel cells, or PEMFCs, hydrogen is split into a proton and electron at one side of the fuel cell (the anode). The proton moves through the device while the electron is forced to travel in an external circuit, where it can perform useful work.

At the other side of the fuel cell (the cathode) the protons, electrons and oxygen are combined to form water, which is the only waste product.

The degradation of the catalyst used to aid the reaction between hydrogen and oxygen is one of the hurdles in producing commercial fuel cells. Current fuel cells use platinum and platinum alloys as a catalyst, but the metal is expensive and not very abundant. To maximize platinum use, researchers use catalysts made with metal nanoparticles, sometimes only a couple of nanometers in diameter. These tiny structures have a lot of surface area to aid the fuel cell reaction.

However, platinum catalysts this small degrade very quickly. This means the fuel cell doesn’t last long, and current estimates project that fuel cells have to function for 5,000 hours to be practical. Morgan’s model shows that if the particle size of a platinum catalyst is increased to 4 or 5 nanometers, which is still only approximately 20 atoms across, the level of degradation significantly decreases. This means the catalyst and the fuel cell as a whole can continue to function for much longer.

Morgan is working in collaboration with Professor Yang Shao-Horn from Massachusetts Institute of Technology. 3M and the U.S. Department of Energy fund the model, which will be especially useful to scientists exploring platinum alloy catalysts.

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