Hydrogen from silicon: Just add water

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  • Published: Feb 1, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Hydrogen from silicon: Just add water

From silicon, hydrogen

On-Demand Hydrogen Generation using Nanosilicon: Splitting Water without Light, Heat, or Electricity

Two techniques, X-ray photoelectron spectroscopy (XPS) and time-of-flight-secondary-ion-mass-spectrometry (TOF-SIMS), have been used in parallel to show how adding water to silicon can generate hydrogen. The work might take us another step closer to the so-called carbon-free hydrogen economy.

Researchers at the University at Buffalo, New York, have demonstrated that silicon nanoparticles can react with water and generate hydrogen spontaneously, and almost instantaneously, without the need to irradiate with ultraviolet light, heat them up, or pass an electric current through the system. The team found that spherical silicon particles just 10 nanometre in diameter would release hydrogen gas from the water and silicic acid as a by-product. The much-touted hydrogen economy of the future will rely on the development of simple and sustainable technologies that can generate the gas with its high energy density and its obvious lack of carbon dioxide generating carbon atoms.

Previous experiments with bulk silicon were 1000 times slower to generate hydrogen gas while 100-nm silicon nanoparticles were still 150 times slower. Writing in the journal Nano Letters in January, the team used the relatively pure hydrogen thus generated to drive a fuel cell, which was capable of powering a fan.

Spontaneous water splitting

"When it comes to splitting water to produce hydrogen, nanosized silicon may be better than more obvious choices that people have been studied for a while, such as aluminium," explains Mark Swihart who, for his sins, runs Buffalo's Strategic Strength in Integrated Nanostructured Systems.

"With further development, this technology could form the basis of a 'just add water' approach to generating hydrogen on demand," adds fellow team leader Paras Prasad. "The most practical application would be for portable energy sources."

The researchers were apparently quite surprised by the rate at which their 10-nm particles reacted with water. They report that within a minute, they had obtained as much hydrogen as 100-nm particles generated in three-quarters of an hour, but with the maximum reaction rate some 150 times higher. Swihart explains that difference is, in effect, down to the geometry of the nanoparticles. As the particle react with the water, the larger particles form non-spherical structures the surfaces of which react then react less rapidly with water. By contrast particles that are a tenth of the diameter of their bulkier counterparts retain reactive characteristics.

The ultimate drawback, which precludes this technology from being entirely sustainable in its present form is that it takes a significant input of energy and resources to make the 10-nm silicon nanoparticles in the first place. The energy requirements are much greater than the energy-equivalent of hydrogen released from the water molecules. Of course, the hydrogen economy is not purely reducing energy demands but is partly about displacing pollution from site of use, so that a hydrogen source would be a cleaner energy supply in a localised region and could be used to give portable power in situations where water is available and portability is important. The military would, by necessity, hang the expense, while glamour campers might also find water-powered camping lanterns and other paraphernalia to be more useful than solar panels, especially on "moister" camp sites.

Nanoscopic fuel delivery

Silicon is an abundant element and water is almost ubiquitous. "Safe storage of hydrogen has been a difficult problem even though hydrogen is an excellent candidate for alternative energy, and one of the practical applications of our work would be supplying hydrogen for fuel cell power. It could be military vehicles or other portable applications that are near water," team member Folarin Erogbogbo explains.

"Perhaps instead of taking a gasoline or diesel generator and fuel tanks or large battery packs with me to the campsite (civilian or military) where water is available, I take a hydrogen fuel cell (much smaller and lighter than the generator) and some plastic cartridges of silicon nanopowder mixed with an activator," Swihart adds. "Then I can power my satellite radio and telephone, GPS, laptop, lighting, etc. If I time things right, I might even be able to use excess heat generated from the reaction to warm up some water and make tea." At some point it might be possible to generate the silicon nanoparticles sustainably too without the energy expenditure, but that aspect of the technology is perhaps for a future phase of development.

The team is now taking the next step with this work, which includes exploration of the hollow nanostructures formed by reaction of the larger silicon particles and studies of hydrogen generation using silicon nanoparticles mixed with other materials, such as alkali metal hydrides," Swihart told SpectroscopyNOW. "The hollow 'nanoballoons' may have interesting applications in other areas such as anodes for lithium-ion batteries," he explains. "Alkali metal hydrides react with water to release hydrogen and produce the alkali metal hydroxides (e.g. NaOH) needed to catalyae the silicon reaction with water. On their own, the metal hydrides are air reactive and unstable, but coating them with silicon nanoparticles might let us increase the hydrogen generation capacity of the system while maintaining an air-stable, easy-to-handle material."

Related Links

Nano Lett, 2013, online: "On-Demand Hydrogen Generation using Nanosilicon: Splitting Water without Light, Heat, or Electricity"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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