Hydrogen Cars

How can we overcome these obstacles? Simple: put rocks in your gas tank.

Not ordinary rocks. Zeolites. Sacco explains: "Zeolites are porous, rocky substances that act like molecular sponges. In their crystalline form, zeolites are threaded by a network of interconnected tunnels and cages, similar to a honeycomb." A fuel tank lined with such crystals might be able to trap and store hydrogen gas "in a liquid-like state - without heavy cryogenics." With support from NASA's Space Product Development program at the Marshall Space Flight Centre, Sacco and colleagues at CAMMP are working to make zeolite gas tanks a reality.

The name zeolite comes from the Greek words "zeo" (to boil) and "lithos" (stone), literally meaning "the rock that boils." This is because zeolites give up their contents when heated.

Sacco described how a temperature-controlled zeolite gas tank might work: "We would add some negatively-charged ions to the zeolite. These ions act like caps, just like caps on an ink bottle; they block the zeolite's crystalline pores. By heating the tank - just a little - we can make the ions move away from the pores. We fill the zeolite with hydrogen, drop the temperature back to normal, and the ions slide back in place, sealing off the exits."

Image credit and copyright: CamaroMuscle.com The gas tank of a Chevy Camaro. Automakers would like hydrogen fuel tanks to be about the same size and weight - and hold the same amount of energy.

Nearly 50 kinds of zeolites with different chemical compositions and crystal-structures are found in nature, and chemists have figured out how to synthesize many more. Anyone with a cat has seen some: they act as odour-absorbers in kitty litter. "The zeolites we have now can store quite a bit of hydrogen," notes Sacco. "But not enough."

How much is enough?

Picture this: Your car's fuel tank is lined with crystallized, porous rock and that "rock" weighs 93 pounds. You pull into a hydrogen fueling station and the attendant forces 7 pounds of hydrogen into the zeolite-lined walls of the tank. This, theoretically, would be the hydrogen equivalent to a full tank of gasoline - in both total weight and energy content.