One of the most promising forms of artificial photosynthesis involves using solar energy to split liquid water to produce oxygen and hydrogen gas, which can be stored and used as a clean fuel. And one of the most promising semiconductor materials for such a task is gallium phosphide (GaP), which can convert sunlight into an electrical charge and also split water. Unfortunately, the material is expensive, but researchers have now used a processed form of gallium phosphide to create a prototype solar fuel cell that not only requires 10,000 times less of the precious material, but also boosts the hydrogen yield by a factor of 10.
While conversion efficiencies of around 15 percent have been achieved by connecting an existing silicon solar cell to a battery to split water through electrolysis, this is an expensive option. GaP offers the potential of an all in one "solar fuel cell" and now researchers at the Eindhoven University of Technology (TU/e) and the FOM Foundation have demonstrated how nanowires made of GaP are effective for photoelectrochemical (PEC) conversion of solar energy to fuel. Although not as efficient as silicon cells hooked up to a battery, these tests with GaP nanowires achieved an immediate boost in hydrogen yield to 2.9 percent, which was an improvement of a factor of 10 when compared to solar cells using GaP as a large flat surface.
Steps are taken to grow the nanowires in ideally-structured, ordered arrays measuring 500 nanometers long and 90 nanometers thick. This optimized growth geometry increases the surface area for light absorption across all wavelengths, while also decreasing light loss due to reflection. The advantage of using GaP nanowire arrays is that the cost is a fraction of what a comparable semiconductor film would be due to the huge reduction in the amount of the GaP material required. On top of that, these nanowire arrays can be transferred to a flexible polymer in order to create flexible devices with minimal material.
"For the nanowires we needed ten thousand [times] less precious GaP material than in cells with a flat surface," says research leader and TU/e professor Erik Bakkers. "That makes these kinds of cells potentially a great deal cheaper. In addition, GaP is also able to extract oxygen from the water – so you then actually have a fuel cell in which you can temporarily store your solar energy. In short, for a solar fuels future we cannot ignore gallium phosphide any longer."
The researchers acknowledge that there is room for improvement, stating that higher efficiency rates can be achieved by further studying the effects of introducing a doping profile and an electric field.
The team's research appears in the journal Nature Communications.