Driving into the H2 Age
Scientists are close to overcoming a major problem in the path of hydrogen cars — storing the gas in a viable form.
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It is touted as the stone that could kill two birds. Hydrogen gas, if tapped as an energy source, could help address the earth’s depleting fossil fuel reserves as well as the accumulation of heat-trapping gases — such as carbon dioxide –– in the atmosphere, thereby offering a much-needed solution to the overarching problem of climate change.
If fuel cells using hydrogen gas are used to power vehicles, the only waste discharged from the tail pipe would be water.
The promise of hydrogen as fuel never looked better. Compared to 20 years ago, technology has advanced so much that the cost of production of hydrogen fuel has reduced by several times. Generally hydrogen is made from natural gas. This, however, undermines its green credentials as it releases carbon dioxide into the atmosphere. But proponents of hydrogen energy are confident that the process would soon become eco-friendly, as alternative routes of hydrogen production such as microbial treatment of biomass and simply splitting water using solar or nuclear energy are becoming increasingly viable.
There are, however, other hurdles — its storage and distribution. Hydrogen contains more energy by weight than any other substance, but because it is the lightest gas it has very low energy by volume. At 200 times the atmospheric pressure, one litre of natural gas contains five times more energy than hydrogen gas does. As a result, storing hydrogen gas in a compact form is a major problem. For example, for a 500-km trip, a car would require a fuel tank the size of a double-decker bus!
To what extent scientists are occupied by these issues is evident from the fact that nearly 50 per cent of the 300 odd papers presented at the recently concluded 17th World Hydrogen Energy Conference at Brisbane, Australia, dealt with them.
One method many car manufacturers making prototypes of hydrogen-fuelled vehicles are resorting to is storing the gas in liquid form under very high pressure and extremely low temperature. This is not only expensive but also potentially dangerous as hydrogen is very volatile.
Another option being toyed with is fuel cells made of materials called metal hydrides. Metal hydrides, that have emerged as the leading contender for hydrogen storage, are capable of releasing hydrogen at will.
Very little is known about the mechanism by which the metal particles in metal hydrides — such as of magnesium, sodium or calcium — absorb hydrogen. Their advantage, which varies from metal to metal, is that they can load and unload hydrogen very fast. But there is a practical problem: they operate at very high temperatures.
However, we may finally have an answer to this, thanks to the efforts of an international team of scientists. Rajeev Ahuja of the University of Uppasala, Sweden, and others have theoretically resolved the problem, paving the way for better manipulation of the storage material.
Their work, reported recently in the online version of the Proceedings of the National Academy of Sciences, shows that the temperatures at which these metal hydrides release hydrogen can be lowered substantially if suitable catalysts are used.
Working on magnesium hydride, which is capable of holding up to 7.7 weight per cent of hydrogen, the researchers found that when the metal is used in the nanoparticle form, the metal ions are capable of facilitating easier movement of hydrogen. As a result, very high temperatures will not be required for releasing the fuel. “We are now close to overcoming a major bottleneck in the way of a hydrogen economy — storing hydrogen in a safe form,” Ahuja told KnowHow.
In metal hydrides, metals like magnesium bind to hydrogen atoms chemically. The bonds between the two are normally strong. When exposed to high temperature, the bonds become weaker and hydrogen is released. But Ahuja and his colleagues found that the energy required to make the bond weaker is much lower when the metal particles are very tiny or spiked with transitional metals like iron or vanadium in small quantities.
“Finding hydrogen storage materials that can operate at near ambient temperatures and pressures is the key to realising the hydrogen energy dream,” says Purusottam Jena, a scientist at Virginia Commonwealth University in the US. Jena, who collaborated with Ahuja, says that their work has provided a fundamental understanding of the mechanism that will enable experimentalists to synthesise the desired material.
The major problem with metal hydrides like magnesium hydride is “the temperature at which hydrogen can be released, and the role catalysts may have in lowering this temperature. We have been able to provide an answer to this,” Jena told KnowHow.
Fuel cell technology is already here and hydrogen cars are also commercially available. However, to make it affordable and acceptable to a larger market, ways must be found to store and release hydrogen at lower pressures and temperatures than are currently possible, he observes.
“We need lightweight materials with favourable thermodynamic properties. The search is still on to find these,” says Jena.
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Sources: The Telegraph Kolkata, India
