The term “solar energy” usually conjures up visions of blue glass rectangles, absorbing sunlight and turning it into electricity. But there’s another way to take advantage of the sun’s power—use it to create hydrogen fuel.
A group of researchers at the University of California, Berkeley, has found a way to make the devices that use sunlight to split water into hydrogen and oxygen much more efficient. They mix two different materials–one that strongly absorbs sunlight and one that efficiently conducts electricity—to build a device with the strengths of each. They describe their results in the current issue of ACS Central Science.
The device is a photoelectrochemical cell, which captures sunlight to generate electric current, and then uses the current to split water. The resulting hydrogen can be stored, and later used to power a car’s fuel cell, for instance.
Back in 2013, Peidong Yang, a chemist at the University of California Berkeley, demonstrated just such a PEC cell, which he and his colleagues built with semiconductor nanowires. The problem was the cell didn’t generate very much current, and therefore the splitting happened very slowly. It turned out the bottleneck lay in the photoanode, which was made of titanium dioxide nanowire. TiO2 has a large bandgap, which leads it to absorb ultraviolet light very well. It’s not so good with visible light.
Other materials—bismuth vanadate is one—are better at absorbing sunlight but don’t conduct electricity nearly as well as titanium dioxide. But Yang thought he could achieve the best of both worlds by making the two work together. He built nanowires out of tantalum and TiO2, then coated them with particles of bismuth vanadate. The BiVO4 absorbs the photons of sunlight, and passes electrons to the TiO2.
“It’s still not as good as a photocathode, but it’s progress,” Yang says.
The silicon nanowire photocathode has currents greater than 25 milliamps per square centimeter, but the photoanode only reaches 2.1 mA/cm2. The smallest current determines the efficiency of the cell.
Yang is hopeful he can find a combination that will work even better. “We are looking at a couple other material compositions,” he says. “Many research groups are looking into this very challenging area.”
Indeed, plenty of researchers would like to make a marriage between solar energy and hydrogen. Both Toyota and Hyundai have introduced SUVs that operate on hydrogen-based fuel cells, but owners still face the challenge of gassing up; hydrogen is flammable and can be difficult to transport, and there just aren’t that many fueling stations. The solution may be to rely on different fuels that are easier to handle and convert them to hydrogen right at the fuel cell, says Nico Hotz, a materials scientist at Duke University. “Instead of using, transporting, and storing pure hydrogen, we want to use any kind of fuel that is easy to use, transport, and store,” Hotz told a session of the Material Research Society’s fall meeting in Boston in December.
The idea, he explained, is to take a fuel such as methanol, ethanol, or methane, mix it with water, and heat it up to between 300°C and 400°C to create a reaction that produces hydrogen, which the fuel cell can then burn. Traditionally, this steam reforming process has burned some of the fuel in order to produce the necessary heat; running the process with natural gas typically burns about half of the gas. “We say that’s a waste of fuel,” Hotz said. “We want to convert all our fuel to hydrogen.”
He and his colleagues are developing catalysts that can help the process along. They created nanoparticles made of copper oxide, zinc oxide, and aluminum nitrate. With enough copper oxide in the mix, and with reactor temperatures between 220°C and 295°C, and methanol-water mixes flowing into the reactor at rates between 2 and 50 microliters per minute, they achieved complete conversion of the fuel.
A solar collector can heat the reactor to 250°C without any sort of sunlight concentration, Hotz said. A collector that adds lenses or mirrors to concentrate light can reach temperatures from 500°C to 700°C, which could allow the use of fuels with higher energy density.
One problem with the process is that the reaction produces about 1 percent carbon monoxide. “If you run that 1 percent CO directly into the fuel cell you will kill it,” Hotz said. So the researchers have also developed a gold/iron-oxide catalyst to convert the carbon monoxide to less harmful carbon dioxide.
Like Berkeley’s Yang, Alfred Tok ling Yoong, head of the nanomaterials group at Nanyang Technical University in Singapore, is working on developing photoelectrochemical cells to produce hydrogen. The key to his work is building a better semiconductor photoelectrode that can capture a wide swatch of the solar spectrum, efficiently convert the hydrogen, not corrode in the water, and not cost too much. To that end, he told an MRS audience, he is building titanium dioxide photonic crystals with a so-called “inverse opal” structure.
Scientists discovered in the 1990s that the opals, a gemstone made of amorphous silica, had a periodic structure that made them natural photonic crystals, capable of directing light. Tok and his team create inverse opals—which have an improved photonic bandgap and thus are better at trapping light—by making artificial opals of polystyrene, using atomic layer deposition to surround them with titanium dioxide, then burning away the polystyrene, leaving the crystalline structure they want. They then connect the crystals to zinc oxide nanowires to create efficient photoelectrodes, which can absorb more light and improve how electrons flow.
One way to improve any kind of solar cell, not just one made to produce hydrogen, is to get it to capture a wider range of the wavelengths in sunlight. Thomas Fix, a photovoltaics researcher at the National Center for Scientific Research in Paris, is working on a scheme for “downshifting” incoming sunlight from ultraviolet wavelengths to the visible and near-infrared wavelengths that solar cells can absorb. He mixes nanoparticles of rare earths, such as terbium, ytterbium, and neodymium into the thin films encapsulating the cells. The method can increase the efficiency of solar cells by 7 or 8 percent, he says, and can be applied to not only silicon cells but also those made of cadmium telluride or copper indium gallium selenide.