Feature

Surprising air bubbles could cut costs for automotive industry

A somewhat per chance discovery by Prof Len Barbour, chemistry professor of the University of Stellenbosch, and two American colleagues could ensure that purified hydrogen can be produced much cheaper in future. This could have a cost-saving impact on the implementation of fuel cells in the automotive industry.

Photograph showing air escaping from platelike crystals of TBC4. After exposure of sublimed single crystals to the atmosphere for two days, they were immersed in nitrobenzene. Air bubbles immediately began to emanate from the sides of the crystals as nitrobenzene enters the lattice, expelling the gas.

Because of the cost involved, fuel cells, such as those used in space craft, are currently not used on a commercial basis where consumers have ready access to the technology.

The research was done by Prof Len Barbour from the University of Stellenbosch's Chemistry Department in the Faculty of Science, and two Americans, chemistry professor Jerry Atwood and graduate student Agoston Jerga from the University of Missouri in Columbia. The research was done in Missouri shortly before Prof Barbour recently returned to South Africa.

While experimenting with organic crystals of a material called calixarene (a large cup-shaped molecule), the team realized that the crystals unexpectedly soak up gases when stored in air. 

This follows on their discovery in 2002 that the crystals could absorb liquids, such as vinyl bromide, by confining single molecules in the molecular cavities. These earlier studies also revealed that the open ends of the cup-shaped molecules can join together via weak van der Waals interactions to form a cavity in the shape of an hourglass.

Prof Barbour says that because these cavities are closed off, with no pores leading out, it was a surprise to notice gas bubbles escape from the crystals after they were submerged in nitrobenzene. The group of doubting Thomases went on to prove that the gas bubbles consist of air that seeps into the cavities when the calixarene crystals are stored in the open.

Two p-tert-butylcalix[4]arene molecules (red and blue, shown in cross section, structure at right) face one another to form a cavity that can trap CO2 but doesn't retain H2. The cavity is partially sealed off by tert-butyl groups (green) from neighbouring calixarenes.

Because calixarene absorbs nitrobenzene molecules more strongly, the gas molecules are literally pushed out of the cavities when the crystals are submerged in this liquid.

As a rule, hydrogen produced from water and CO in the water-gas shift reaction is contaminated with carbon dioxide. Through experimentation it was found that carbon dioxide (CO2) is absorbed much quicker than hydrogen (H2) in a CO2-H2 mixture, thus leaving purified hydrogen behind.
"It seems that materials need not be porous at the molecular level in order to be good containers for small molecules," explains Prof Barbour. Currently there are a lot of researchers designing new materials to store gases, but they are focused on making the materials porous, even to the extent of tailoring the exact size of the molecular pores. "My coworkers and I are showing that pore size is perhaps much less of an issue than the void space where the molecules eventually have to go when they have entered the material."

"Separating hydrogen is complicated and expensive," says Prof Barbour, who believes that using these organic crystals could be a more cost-effective way to produce high-quality purified hydrogen needed to ensure efficient fuel cells with a long working life.

"A reusable H2 purification system for fuel cells could be an important development for the automotive industry", said noted inorganic chemist Jonathan Steed from the University of Durham in England in Chemical and Engineering News. Steed said that although a number of materials are known that will absorb CO and CO2, new and better materials are always needed.

Although the infrastructure for refilling them is still not in place, several pilot projects are currently underway worldwide on how to implement hydrogen fuel cells in cars.

According to Prof Barbour, his research is also aimed at addressing issues such as storing hydrogen for mobile applications, as well as finding better separation technologies that can be useful as the demand for hydrogen increases in future. 

More information:

Department of Chemistry, Faculty of Science
University of Stellenbosch
Tel: +27 - 21 - 808 3335
Fax: +27 - 21 - 808 3849

ljb@sun.ac.za 

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