Manufacturing Bits: July 29

Measuring hydrogen fuel for cars
The use of fuel cells to power cars and buses is still in its infancy. Fuel cell vehicles are electric-based systems powered by hydrogen.

A fuel cell uses a certain type of proton exchange membrane (PEM). The PEM fuel cells are stacked together to form a fuel cell stack. All told, fuel cell systems are about 60% efficient, or roughly two to three times more efficient than combustion engines, according to the California Fuel Cell Partnership.

Fuel cell electric vehicles (FCEVs) offer performance, range and refill time similar to combustion vehicles, and the quiet operation, zero emissions and power characteristic of battery electric vehicles, according to the California Fuel Cell Partnership.

But the technology is also expensive. The target cost for fuel cells is $30/kilowatt (kW) by 2017, according to the organization. In 2012, the DOEreported that fuel cell cost is $47/kW, a more than 135% reduction from $275/kW in 2002 when the fuel cell program began, it said.

Today, FCEVs are on the road in various parts of the world. Mercedes-Benz and Honda, in fact, directly lease FCEVs to customers in Southern California. California has more hydrogen filling stations than any other region in the world. The state has opened nine refueling stations. By 2016, California hopes to have 68 stations in the state.

To support the fair sale of gaseous hydrogen as a vehicle fuel, the National Institute of Standards and Technology (NIST) has developed a prototype field meter test standard to test the accuracy of hydrogen fuel dispensers. The next challenge is to determine what accuracy is achievable in field installations of hydrogen dispensing systems.

NIST has developed this prototype field test standard to test the accuracy of hydrogen fuel dispensers. (Source: NIST)

To help its cause, NIST and the FCEV community will turn to the so-called NIST Handbook 44. This is a reference text for weights and measurements for use in everything from gasoline dispensers to grocery store scales.

Handbook 44, which has been adopted by all states, stipulates that hydrogen will be sold by the kilogram. Hydrogen-dispensing pumps must be accurate to within 2%, or 20 grams, per kilogram. “It’s much more difficult to measure hydrogen gas delivered at 5,000 to 10,000 psi than it is to measure a product that is a liquid at atmospheric temperatures and pressures,” said NIST’s Juana Williams on the agency’s Web site. “While a kilogram of hydrogen has approximately the same energy content as a gallon of gasoline, the allowable error is slightly less stringent than for gasoline.”

Even with the larger allowance, some have suggested these tolerances are too tight and proposed alternatives as high as 10% or 20%. “We’ve shown that the master meter in our lab is capable of dispensing helium from a simulated hydrogen dispenser with errors of 1% or less,” added NIST’s Jodie Pope. “So we can extrapolate that it is possible to measure hydrogen with accuracy sufficient for a fair marketplace.”

Debunking silicene
Silicene, a 2D material that is similar to graphene, is the subject of interest. Silicene is a 2D sheet of silicon atoms, which can be created by heating silicon and evaporating atoms onto a silver platform. Like graphene and other 2D materials, silicene could enable futuristic FET devices.

But the U.S. Department of Energy’s Argonne National Laboratory has called into question the existence of silicene. Supposedly, silicene consists of only one layer of silicon atoms. So, silicene must be handled in a vacuum. Exposure to any amount of oxygen would supposedly destroy the material.

In the lab, researchers deposited silicene on a silver platform. In tests, the material would “precipitate” out. This, in turn, meant the material was not silicene, according to researchers. “Everybody assumed the sample would immediately decay as soon as they pulled it out of the chamber,” said Northwestern University graduate student Brian Kiraly, on Argonne’s Web site.

“We found out that what previous researchers identified as silicene is really just a combination of the silicon and the silver,” added Northwestern graduate student Andrew Mannix.

Ice templates for DSA
The University of Science and Technology of China (USTC) has developed a self-assembly process that would transform 1D nanomaterials into 3D structures with electrical properties. The secret? Researchers use ice as the template.

Researchers from USTC have also found a way to regulate the process of ice growth. In doing so, they can dictate the microstructure of the macroscopic assemblies. With this method, USTC has assembled 1D silver nanowires (AgNW) into 3D hierarchical structures. The 3D-based AgNW assemblies had electrical conductivity and electromechanical stability, according to researchers.

In fact, USTC made stretchable and foldable conductors, based on AgNWs/PDMS composites. “The composite remained conductive with a resistance change of only about 2Ω until it broke at 140% strain,” according to researchers. “The resistance of the AgNWs/PDMS composite increased with increasing tensile strain during the first cycle of stretching, while it became nearly stable from the second stretch-release cycle.”

The irreversible resistance increase was only about 0.3Ω under 50% stretching. “The resistance showed a slight increase of less than 0.1Ω at a bending radius of 2.0mm during the first bending cycle, and almost entirely recovered after straightening,” according to researchers. “Even after 5,000 cycles with a bend radius of 2.0mm, the resistance variation of composites was just less than 0.2Ω.”

The tunability of the network structure, coupled with the electrical properties, can make the macroscopic 3D AgNW architectures and their composites possible candidates in electronic applications.