Power/Performance Bits: June 3

Caltech researchers have devised a method to protect the materials in a solar-fuel generator; Vanderbilt engineers have created structural energy storage materials.


Stabilizing common semiconductors for solar fuels generation
Researchers around the world are trying to develop solar-driven generators that can split water, yielding hydrogen gas that could be used as clean fuel. These devices would require efficient light-absorbing materials that attract and hold sunlight to drive the chemical reactions involved in water splitting. As semiconductors like silicon and gallium arsenide are excellent light absorbers—as is clear from their widespread use in solar panels – it seems like these would be good candidates but they rust when submerged in the type of water solutions found in such systems.

However, paving the way for the use of these materials in solar-fuel geneators, Caltech researchers at the Joint Center for Artificial Photosynthesis (JCAP) have develped a method for protecting these common semiconductors from corrosion even as the materials continue to absorb light efficiently.

In the type of integrated solar-fuel generator that they are trying to produce, the researchers explained, two half-reactions must take place—one involving the oxidation of water to produce oxygen gas; the other involving the reduction of water, yielding hydrogen gas. Each half-reaction requires both a light-absorbing material to serve as the photoelectrode and a catalyst to drive the chemistry. In addition, the two reactions must be physically separated by a barrier to avoid producing an explosive mixture of their products.

Atomic layer deposition was used to form a layer of titanium dioxide (TiO2)—a material found in white paint and many toothpastes and sunscreens—on single crystals of silicon, gallium arsenide, or gallium phosphide. The key was that they used a form of TiO2 known as “leaky TiO2″—because it leaks electricity. First made in the 1990s as a material that might be useful for building computer chips, leaky oxides were rejected as undesirable because of their charge-leaking behavior. However, leaky TiO2 seems to be just what was needed for this solar-fuel generator application. Deposited as a film, ranging in thickness between 4 and 143 nanometers, the TiO2 remained optically transparent on the semiconductor crystals—allowing them to absorb light—and protected them from corrosion but allowed electrons to pass through with minimal resistance.

On top of the TiO2, the researchers deposited 100-nanometer-thick “islands” of an abundant, inexpensive nickel oxide material that successfully catalyzed the oxidation of water to form molecular oxygen.

The work appears to now make a slew of choices available as possible light-absorbing materials for the oxidation side of the water-splitting equation. However, the researchers emphasize, it is not yet known whether the protective coating would work as well if applied using an inexpensive, less-controlled application technique, such as painting or spraying the TiO2 onto a semiconductor. Also, thus far, the Caltech team has only tested the coated semiconductors for a few hundred hours of continuous illumination.

Structural energy storage materials
Imagine a future in which our electrical gadgets are no longer limited by plugs and external power sources. This intriguing prospect is one of the reasons for the current interest in building the capacity to store electrical energy directly into a wide range of products, such as a laptop whose casing serves as its battery, or an electric car powered by energy stored in its chassis, or a home where the dry wall and siding store the electricity that runs the lights and appliances.

It also makes the small, dull grey wafers that Vanderbilt researchers have made far more important than their nondescript appearance suggests.

The devices demonstrate, believed to be for the first time, that it is possible to create materials that can store and discharge significant amounts of electricity while they are subject to realistic static loads and dynamic forces, such as vibrations or impacts,.

Close-up of structural supercapacitor. (Source: Joe Howell / Vanderbilt)

Close-up of structural supercapacitor.
(Source: Joe Howell / Vanderbilt)

The researchers stress this is important because structural energy storage will change the way in which a wide variety of technologies are developed in the future because when energy can be integrated into the components used to build systems, it opens the door to a whole new world of technological possibilities. All of a sudden, the ability to design technologies at the basis of health, entertainment, travel and social communication will not be limited by plugs and external power sources.

The new device that that the Vanderbilt team has developed is a supercapacitor that stores electricity by assembling electrically charged ions on the surface of a porous material, instead of storing it in chemical reactions the way batteries do. As a result, supercaps can charge and discharge in minutes, instead of hours, and operate for millions of cycles, instead of thousands of cycles like batteries.

They report that their new structural supercapacitor operates flawlessly in storing and releasing electrical charge while subject to stresses or pressures up to 44 psi and vibrational accelerations over 80 g (significantly greater than those acting on turbine blades in a jet engine).


Side view of a structural supercapacitor shows the blue polymer electrolyte that glues the silicon electrodes together. (Source: Joe Howell / Vanderbilt)

Side view of a structural supercapacitor shows the blue polymer electrolyte that glues the silicon electrodes together. (Source: Joe Howell / Vanderbilt)