SOLAR cells which can be submerged in water, and produce clean, renewable fuel by splitting water into its chemical components may not be far off.
Researchers from the RENEW project (Research into Emerging Nanostructured Electrodes for the Splitting of Water), hailing from the Tyndall National Institute and Stanford University published a paper in Nature Materials, detailing the design of a water-resistant solar cell which is submerged, and uses the electricity from the photovoltaic cell to split water elements into pure oxygen and hydrogen.
Test systems have produced a record-breaking voltage, indicating with further improvements, it will be possible to use sunlight-generated electricity to drive the “artificial photosynthesis” process.
The produced hydrogen is considered a clean fuel because it only produces water when it is burned. It can also be used to produce methane (natural gas) and methanol from reactions involving hydrogen and carbon dioxide.
With sunlight as a sole source of energy, water-splitting reactors could provide a renewable source of hydrogen, methane and methanol, and potentially funnel carbon dioxide into reactors for transformation into fuel instead of releasing it into the atmosphere.
The greatest challenge for the researchers was developing solar cells that do not corrode under water, and produce enough voltage, while submerged, to split water molecules without requiring an additional source of electricity.
A breakthrough in the corrosion issue was achieved by Professor Paul McIntyre and colleagues at Stanford University in 2011, who added an extremely thin layer of titanium dioxide to the anode part of the solar cell to protect its surface from water corrosion.
This was a double-edged solution, however, as the thickened protective layer would reduce the voltage generated by the silicon-based cell beneath.
Dr Paul Hurley of Tyndall National Institute in Cork suggested adding a new layer of silicon doped with an excess positive charge between the silicon cell and the protective layer. This created a little more photovoltage than achieved with just one type of silicon. Testing at Stanford found that the new hybrid cell performed better than expected, breaking the record for the voltage produced by this type of anode.
This allowed the thicker protective layer without the penalty of reduced voltage, allowing the anode to be improved for long-term.
The researchers are now examining different materials that can protect silicon, conduct electricity and be transparent enough to transmit light.
While the voltage produced from the new design, at 0.6V, is not yet enough to split water, this record-breaking voltage opens the way for cells to be tested that use the new design principles.
These could combine the silicon anode with an amorphous silicon absorbing layer into what is called a tandem cell structure, with the potential to produce sufficient voltage to split water with no externally applied voltage.