Energy of Hydrogen

Scientists suggest using photolysis as a method to turn sunlight into useful fuel. If this could be realized, water photolysis would provide a pathway for efficient solar energy. Penn State researchers have a proof-of-concept device that can split water and produce recoverable hydrogen. “This is a proof-of-concept system that is very inefficient. But ultimately, catalytic systems with 10 to 15 percent solar conversion efficiency might be achievable,” says Thomas E. Mallouk, the DuPont Professor of Materials Chemistry and Physics.

Although solar cells can now produce electricity from visible light at efficiencies of greater than 10 percent, solar hydrogen cells have been limited by the poor spectral response of the semiconductors used. In principle, molecular light absorbers can use more of the visible spectrum in a process that is mimetic of natural photosynthesis. Photosynthesis uses chlorophyll and other dye molecules to absorb visible light

Mallouk and W. Justin Youngblood, postdoctoral fellow in chemistry, together with collaborators at Arizona State University developed a catalyst system that, combined with a dye, can mimic the electron transfer and water oxidation processes that occur in plants during photosynthesis.

The key to their process is a tiny complex of molecules with a center catalyst molecules surrounded by dye molecules which absorbs sunlight. When visible light strikes the dye, the energy excites electrons in the dye, which, with the help of the catalyst, can split the water molecule, creating free oxygen. These clusters are about 2 nanometers in diameter can cycle through the water oxidation reaction about 50 times per second. That is comparable to the turnover rate of Photosystem II in green plant photosynthesis. Photosystem II is the protein complex in plants that oxidizes water and starts the photosynthetic process. The water splitting requires 1.23 volts, and the current experimental configuration cannot quite achieve that level so the researchers add about 0.3 volts from an outside source. Their current system achieves an efficiency of about 0.3 percent.

Hydrogen Solar Ltd, the UK-based hydrogen production research and development company has developed a break-through Tandem Cell technology for producing high purity hydrogen by photolysis.

The Tandem Cell™ consists of two photo-catalytic cells in series: the front cell absorbs the high energy ultraviolet and blue light in sunlight, using nano-crystalline metal oxide thin films to generate electron-hole pairs. This cell does not generate enough voltage to split the water, so the electrons are connected to the back cell. The longer wavelength green and red light passes through the front cell and is absorbed in the back dye-solar cell producing electrical potential under nearly all light conditions. The two cells are connected electrically and together provide the potential required to split the water molecules in the electrolyte. This is not the arrangement used in real modules, but illustrates the principle, if this arrangement were used, a transparent membrane would have to be placed in the water cell to separate the hydrogen and oxygen.  No external electricity is required.

The key to the Tandem Cell™ is the performance of the metal oxides in reacting to the photons of the incident light. The metal oxides are expected to be the limiting feature of Tandem Cell™ efficiency. One of the benefits of Hydrogen Solar’s Tandem Cells is that they are inexpensive. The Cell is fabricated from widely-available and cheap materials, and as a result, the hydrogen production is competitive. On the small scale, producing hydrogen at one third the cost than from PV solar panel-electrolysis systems.  On the large scale it is about twice the cost of steam reforming with natural gas.

Tandem Cell array is capable of charging a domestic refueling station for hydrogen vehicles.  An 7m x 7m Tandem Cell unit, with 10% efficiency, covering double garage will produce enough hydrogen to fuel a production hydrogen vehicle for 11,000 miles over a year in Los Angeles light conditions.  Arrays placed on domestic rooftops or incorporated into industrial buildings will eliminate the transportation costs for the hydrogen.

The above narrative represents the companies views on their process. The researchers have a variety of approaches to improve the technologies oh photolysis. They can amend photolysis processes if investigate improving the efficiency of the dye, improving the catalyst and adjusting the general geometry of the system.

Resources:

EurekAlert

Hydrogen Solar Ltd, Guildford, UK