Three Research Projects Focus On Potential Of Hydrogen

• Aug 01, 2005

Researchers Examine Potential For 'Refilling' Hydrogen Storage Material

Performing quantum calculations on a supercomputer, scientists at Pacific Northwest National Laboratory (PNNL) have characterized a material that might allow on-board refueling of hydrogen-powered vehicles, PNLL announced on Aug. 29.

Researchers, led by Maciej Gutowski, looked at different crystalline structures of a compound made up of nitrogen, boron and hydrogen (NBH6) and found one that might be more stable compared to ammonia borane. Ammonia borane can hold a lot of hydrogen but isn't easily reversible -- able to be refilled with hydrogen. Ammonia borane, as a storage material, would likely have to be removed from the vehicle and be sent to some sort of processing plant and undergo a reaction to be refilled, the researchers said.

The more stable compound, diammoniate of diborane (DADB), holds more promise for reversibility. Initial thermodynamic properties for the compound indicate that it might spontaneously uptake hydrogen fuel.

This work is performed under the Grand Challenge Project "Computational studies of materials to hydrogen storage" in the Molecular Sciences Computing Facility at PNNL. Researchers plan to perform additional calculations, synthesize the diammoniate of diborane compound and test their theories on the material in the coming year.

Chemical Could Revolutionize Polymer Fuel Cells

Heat has always been a problem for fuel cells. There's usually either too much (ceramic fuel cells) for certain portable uses, such as automobiles or electronics, or too little (polymer fuel cells) to be efficient.

While polymer electrolyte membrane (PEM) fuel cells are widely considered the most promising fuel cells for portable use, their low operating temperature and consequent low efficiency have blocked their jump from promising technology to practical technology.

But researchers at the Georgia Institute of Technology have pinpointed a chemical that could allow PEM fuel cells to operate at a much higher temperature without moisture, potentially meaning that polymer fuel cells could be made much more cheaply than ever before and finally run at temperatures high enough to make them practical for use in cars and small electronics.

A team lead by Dr. Meilin Liu, a professor in the School of Materials Science and Engineering at Georgia Tech, has discovered that a chemical called triazole is significantly more effective than similar chemicals researchers have explored to increase conductivity and reduce moisture dependence in polymer membranes, according to an Aug. 24 announcement.

"Triazole will greatly reduce many of the problems that have prevented polymer fuel cells from making their way into things like cars, cell phones and laptops," said Liu. "It's going to have a dramatic effect."

A fuel cell essentially produces electricity by converting the chemicals hydrogen and oxygen into water. To do this, the fuel cell needs a proton exchange membrane, a specially treated material that looks a lot like plastic wrap, to conduct protons (positively charged ions) but block electrons. This membrane is the key to building a better fuel cell.

Current PEMs used in fuel cells have several problems that prevent them from wide use. First, their operating temperature is so low that even trace amounts of carbon monoxide in hydrogen fuel will poison the fuel cell's platinum catalyst. To avoid this contamination, the hydrogen fuel must go through a very expensive purification process that makes fuel cells a pricey alternative to conventional batteries or gasoline-fueled engines. At higher temperatures, like those allowed by a membrane containing triazole, the fuel cell can tolerate much higher levels of carbon monoxide in the hydrogen fuel.

The use of triazole also solves one of the most persistent problems of fuel cells -- heat. Ceramic fuel cells currently on the market run at a very high temperature (about 800 degrees Celsius) and are too hot for most portable applications such as small electronics.

While existing PEM fuel cells can operate at much lower temperatures, they are much less efficient than ceramic fuel cells. Polymer fuel cell membranes must be kept relatively cool so that membranes can retain the moisture they need to conduct protons. To do this, polymer fuel cells were previously forced to operate at temperatures below 100 degrees Celsius.

Heat must be removed from the fuel cells to keep them cool, and a water balance has to be maintained to ensure the required hydration of the PEMs. This increases the complexity of the fuel cell system and significantly reduces its overall efficiency. But by using triazole-containing PEMs, Liu's team has been able to increase their PEM fuel cell operating temperatures to above 120 degrees Celsius, eliminating the need for a water management system and dramatically simplifying the cooling system.

"We're using the triazole to replace water," Liu said. "By doing so, we can bring up the temperature significantly."

Triazole also is a very stable chemical and fosters stable fuel cell operating conditions.

While they have pushed their polymer fuel cells to 120 degrees Celsius with triazole, Liu's team is looking into better polymers to get those temperatures even higher, he said.

Georgia Tech School of Materials Science and Engineering: http://www.mse.gatech.edu.

Purdue Creates New Method To Drive Fuel Cells For Portable Electronics

Engineers at Purdue University have developed a new way of producing hydrogen for fuel cells to automatically recharge batteries in portable electronics, such as notebook computers, and eliminate the need to use a wall outlet.

The findings were presented Aug. 28 during the annual meeting of the American Chemical Society in Washington, D.C., and also will be detailed in a peer-reviewed paper to appear in an upcoming issue of the journal Combustion and Flame. The paper was written by research scientist Evgeny Shafirovich, postdoctoral research associate Victor Diakov and Arvind Varma, the R. Games Slayter Distinguished Professor of Chemical Engineering and head of Purdue's School of Chemical Engineering.

The researchers developed the new method earlier this year and envision a future system in which pellets of hydrogen-releasing material would be contained in disposable credit-card-size cartridges. Once the pellets were used up, a new cartridge would be inserted into devices such as cell phones, personal digital assistants, notebook computers, digital cameras, handheld medical diagnostic devices and defibrillators.

The method also might have military applications in portable electronics for soldiers and for equipment in spacecraft and submarines, Varma said.

The new technique combines two previously known methods for producing hydrogen. The previous methods have limitations making them impractical when used alone, but those drawbacks are overcome when the methods are combined, Varma said.

One of the methods was invented by Herbert C. Brown, a chemist and Nobel laureate from Purdue who discovered a compound called sodium borohydride during World War II. The compound contains sodium, boron and hydrogen. He later developed a technique for producing hydrogen by combining sodium borohydride with water and a catalyst. The method, however, has a major drawback because it requires expensive catalysts such as ruthenium.

The other method involves a chemical reaction in which tiny particles of aluminum are combined with water in such a way that the aluminum ignites, releasing hydrogen during the combustion process. This method does not require an expensive catalyst, but it yields insufficient quantities of hydrogen to be practical for fuel cell applications.

"Our solution is to combine both methods by using what we call a triple borohydride-metal-water mixture, which does not require a catalyst and has a high enough hydrogen yield to make the method promising for fuel cell applications," Varma said. "So far we have shown in experiments that we can convert 6.7 percent of the mixture to hydrogen, which means that for every 100 grams of mixture we can produce nearly 7 grams of hydrogen, and that yield is already better than alternative methods on the market."

The researchers have filed a provisional patent application for the technique and hope to increase the yield to about 10 percent through additional experiments, Shafirovich said.

Hydrogen produced by the method could be used to drive a fuel cell, which then would produce electricity to charge a battery. A computer chip would automatically detect when the battery needed to be recharged, activating a new pellet until all of the pellets on the cartridge were consumed. Byproducts from the reaction are environmentally benign and can either be safely discarded or recycled, Diakov said.

In addition to its potential use in portable electronics, the technology offers promise as an energy source to power hardware in spacecraft.

"The Apollo 13 accident was caused by an explosion involving liquid oxygen, which is needed along with liquid hydrogen to feed a fuel cell in spacecraft," Shafirovich said. "Use of chemical mixtures, such as ours, for generation of hydrogen and oxygen would eliminate the possibility of such an explosion."

A key step in the hydrogen-producing reaction is the use of tiny particles of aluminum only about as wide as 100 nanometers, or 100 billionths of a meter.

"You don't want to use large lumps of aluminum because then you only get reactions on the outer surfaces of those lumps, so you don't produce enough hydrogen," Varma said. "What you would rather use is tiny particles that have a high surface area, which enables them to completely react, leaving no waste and producing more hydrogen."

Another crucial component is a special gel created by combining water with a material called polyacrylamide.

"If you want to ignite a mixture of aluminum with water, the problem is that water boils at 100 degrees Centigrade and aluminum ignites at a much higher temperature," Shafirovich said. "So, if you try to ignite the mixture you just vaporize water and the aluminum doesn't ignite.

"When we use this gel, water boils at a much higher temperature, and the nanoscale powder also decreases the ignition temperature of aluminum. So you are both increasing the boiling point of water and decreasing the ignition temperature of aluminum, making the reaction possible."

The researchers believe they will be able to safely dissipate the heat produced by the reaction, making the technology practical for portable electronics.

Related Web sites: Arvind Varma: https://engineering.purdue.edu/ChE/News_and_Events/News/Archive/031024_NewHead.html American Chemical Society meeting: http://acswebcontent.acs.org/nationalmeeting/dc05/home.html

This article originally appeared in the 08/01/2005 issue of Environmental Protection.