Web Date: January 24, 2013
Hydrogen Fuel On Demand With Silicon Nanoparticles
Hydrogen fuel cells produce electricity without harmful emissions, but the devices face a significant hurdle to widespread use: Transporting the flammable fuel poses safety risks. So scientists have been working on systems for making the fuel where it’s needed, by splitting water into hydrogen and oxygen gas. Now researchers report that silicon nanoparticles could help. The nanoparticles react with water to produce hydrogen much faster than larger forms of silicon can (Nano Lett., DOI: 10.1021/nl304680w).
Some prototype hydrogen-generating systems use metal materials such as magnesium hydride, aluminum, or zinc to react with water. The compounds don’t require light, catalysts, or heat to push the reactions along. But these materials often react slowly, which isn’t ideal for applications that need a quick burst of electrical power.
Paras N. Prasad and Mark T. Swihart, of the University at Buffalo, wanted to test whether silicon nanoparticles might perform better. They knew that bulk silicon reacted slowly with water to produce hydrogen and that, as the reaction proceeds, oxides form on the silicon surface, halting the reaction. But the chemists and their colleagues thought the high surface-area-to-volume ratio of silicon in nanoparticle form would lead to speedy reaction rates.
The team studied the reaction of slightly basic water with three types of roughly spherical silicon particles: 10-nm-diameter particles made in their lab along with commercially available 100-nm and 40-µm ones. The smallest particles are surprisingly fast, the scientists report. The 10-nm particles produce 1 mmol of hydrogen in five seconds, while the 100-nm and 40-µm ones take 811 and 3,075 seconds, respectively. The reaction rate for the 10-nm particles was six times quicker than that of aluminum and zinc nanoparticles, the previous fastest rates reported in the literature, Swihart says.
The Buffalo group then tested the particles in a hydrogen fuel cell loaded with water, to ensure that the reactions produce hydrogen without creating any side products that could damage the fuel cell. During a small test, a nanoparticle-water mix generated enough hydrogen to run a commercial fuel cell for about four minutes. The cell ran normally, as it would if fed pure hydrogen, with no apparent damage.
However, John Turner, a research fellow who specializes in hydrogen fuel cells at the National Renewable Energy Laboratory in Golden, Colorado, is not convinced that nanosilicon is a good means to generate hydrogen, especially if the goal is to produce electricity with low emissions. Prasad’s group makes the nanoparticles from silane gas, and producing the gas, Turner says, requires a lot of energy and generates carbon dioxide. “You have to take a holistic look at the energy that goes into the whole process.”
Prasad says his group plans to address these practical issues after further basic research into how conditions such as particle shape affect the silicon-water reaction.
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