Researchers at Pennsylvania State University are developing a green, cost-effective system for generating hydrogen using solar energy. According to Science Daily, the method splits water into hydrogen and oxygen through photolysis. Electrical Engineering Professor Craig A. Grimes and his team use nanotube groups and something called a photoelectrochemical diode to make photolysis happen:
In Grimes' photoelectrochemical diode, one side is a nanotube array of electron donor material -- n-type material -- titanium dioxide, and the other is a nanotube array that has holes that accept electrons - p-type material -- cuprous oxide titanium dioxide mixture. P and n-type materials are common in the semiconductor industry. . . . While titanium dioxide is very absorbing in the ultraviolet portion of the sun's spectrum, many p-type materials are unstable in sunlight and damaged by ultraviolet light, they photo-corrode. To solve this problem, the researchers made the titanium dioxide side of the diode transparent to visible light by adding iron and exposed this side of the diode to natural sunlight. The titanium dioxide nanotubes soak up the ultraviolet between 300 and 400 nanometers. The light then passes to the copper titanium side of the diode where visible light from 400 to 885 nanometers is used, covering the light spectrum.
The photoelectrochemical diodes function the same way that green leaves do, only not quite as well. They convert the energy from the sun into electrical energy that then breaks up water molecules. The titanium dioxide side of the diode produces oxygen and the copper titanium side produces hydrogen.
Grimes and his team report that their photocorrosion-stable diodes "generate a photocurrent of approximately 0.25 milliampere per centimeter square, at a photoconversion efficiency of 0.30 percent" using "incident sunlight." Making enough hydrogen to power an SUV would probably take a while at that efficiency level, but Grimes anticipates that the technology could mature to allow efficiencies of 5-10%. He adds, "These devices are inexpensive and because they are photo-stable could last for years."
Science Daily mentions that the U.S. Department of Energy is backing the research.