|Hydrogen Solar’s Tandem Cell produces hydrogen directly from sunlight and water.|
The BOC Foundation has awarded a grant to Hydrogen Solar Ltd for the development and demonstration of scalable photo-catalytic hydrogen production using a Tandem Cell array.
In the first phase of the project, Hydrogen Solar will design, build and install a 100 m2 array of Tandem Cells to generate high-purity hydrogen from sunlight and water. The second phase will be to demonstrate the Tandem Cell array over a six month period at the Beacon Energy Ltd site at West Beacon Farm, Leicestershire. The system will provide hydrogen for fuel cell applications, to provide electricity for recharging electric vehicles, and for heat and power for various buildings.
|Schematic of the Tandem Cell|
The Tandem Cell consists of two photo-catalytic cells in series. The front cell has glass walls and contains a water-based electrolyte. The photocatalytic nanocrystalline film is on the rear wall of the cell on conducting glass.
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. There is not quite enough voltage difference in the first type of cell to spilt water, so the Tandem Cell uses a second cell, a Grätzel cell.
The longer wavelength light in the green-to-red region passes through the front cell and is absorbed by the Grätzel Cell, which produces electrical potential under nearly all light conditions. This boosts the energy of the electrons which then come back to a hydrogen electrode in the front cell.
A Grätzel cell, as described in a paper in Nature in 1991 by Brian O’Reagan and Michael Grätzel, is a dye-sensitized photoelectrochemical cells that uses photo-sensitization of wide-band-gap mesoporous oxide semiconductors.
The cells have a simple structure that consists of two electrodes and an iodide-containing electrolyte. One electrode is dye-absorbed highly porous nanocrystalline titanium oxide (nc-TiO2) deposited on a transparent electrically conducting substrate. The other is a transparent electrically conducting substrate only.
As reported in the Nature article, the overall light-to-electric energy conversion yield of a Grätzel cell is 7.1–7.9% in simulated solar light and 12% in diffuse daylight.
According to Hydrogen Solar, 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, although other elements need to be optimized, including the counter-electrodes, the electrolytes and the mechanical design to maximize light gathering and hydrogen collection.
Hydrogen Solar Presentation to National Hydrogen Association, 2004
“A low-cost, high-efficiency solar cell based on dye-sensitive colloidal TiO2 films,” O’Reagan, Brian and Grätzel, Michael; Nature 353.n6346 (Oct 24, 1991): 737(4)