| (Media-Newswire.com) - University Park, Pa. -- A greener, lessexpensive method to produce hydrogen for fuel may eventually bepossible with the help of water, solar energy and nanotube diodes that use the entire spectrum of the sun's energy, according to PennState researchers.
"Other researchers have developed ways to produce hydrogen withmind-boggling efficiency, but their approaches are very high cost,"says Craig A. Grimes, professor of electrical engineering. "We areworking toward something that is cost effective."
Currently, the steam reforming of natural gas produces most of ourhydrogen. As a fuel source, this produces two problems. The processuses natural gas and so does not reduce reliance on fossil fuels;and, because one byproduct is carbon dioxide, the processcontributes to the carbon dioxide in the atmosphere, the carbonfootprint.
Grimes' process splits water into its two components, hydrogen andoxygen, and collects the products separately using commonlyavailable titanium and copper. Splitting water for hydrogenproduction is an old and proven method, but in its conventionalform, it requires previously generated electricity. Photolysis ofwater solar splitting of water has also been explored, but is not acommercial method yet.
Grimes and his team produce hydrogen from solar energy, using twodifferent groups of nanotubes in a photoelectrochemical diode. Theyreport in the July issue of Nano Letters that using incidentsunlight, "such photocorrosion-stable diodes generate aphotocurrent of approximately 0.25 milliampere per centimetersquare, at a photoconversion efficiency of 0.30 percent."
"It seems that nanotube geometry is the best geometry forproduction of hydrogen from photolysis of water," says Grimes
In Grimes' photoelectrochemical diode, one side is a nanotube arrayof electron donor material n-type material titanium dioxide,and the other is a nanotube array that has holes that acceptelectrons - p-type material cuprous oxide titanium dioxidemixture. P and n-type materials are common in the semiconductorindustry. Grimes has been making n-type nanotube arrays fromtitanium by sputtering titanium onto a surface, anodizing thetitanium with electricity to form titanium dioxide and thenannealing the material to form the nanotubes used in other solarapplications. He makes the cuprous oxide titanium dioxide nanotubearray in the same way and can alter the proportions of each metal.
While titanium dioxide is very absorbing in the ultraviolet portionof the sun's spectrum, many p-type materials are unstable insunlight and damaged by ultraviolet light, they photo-corrode. Tosolve this problem, the researchers made the titanium dioxide sideof the diode transparent to visible light by adding iron andexposed this side of the diode to natural sunlight. The titaniumdioxide nanotubes soak up the ultraviolet between 300 and 400nanometers. The light then passes to the copper titanium side ofthe diode where visible light from 400 to 885 nanometers is used,covering the light spectrum. |
