Materials

Gallium phosphide nanowires boost hydrogen yield in prototype solar fuel cell

Gallium phosphide nanowires bo...
A close-up look of gallium phosphide nanowires at the heart of the prototype solar fuel cell
A close-up look of gallium phosphide nanowires at the heart of the prototype solar fuel cell
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Optimizing the gallium phosphide nanowire geometry is a key element to success
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Optimizing the gallium phosphide nanowire geometry is a key element to success
A close-up look of gallium phosphide nanowires at the heart of the prototype solar fuel cell
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A close-up look of gallium phosphide nanowires at the heart of the prototype solar fuel cell

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.

Source: Eindhoven University of Technology

3 comments
nickyhansard
Or - call me crazy - use the solar energy collected to power electric vehicles... Don't get me wrong, I know electric vehicles are still a young technology and have awhile to go before it's benefits out way those of internal combustion BUT we all know solar energy and electric vehicles are the end game. Let's reach that point as quick as possible and focus our minds and resources on electric motor efficiency, battery capacity and solar panel efficiency instead of on technology that will become obsolete at some point, probably in the next few decades...
Jacob Shepley
nickyhansard, not necessarily. hydrogen fuel-cell vehicles are 'electric vehicles', too. They just store energy in hydrogen. hydrogen is more energy dense than batteries and can 'recharge' a car in a few minutes rather than 15+ minutes for fast charging electric vehicles. it is still an emerging and evolving technology, and currently the vast vast majority of hydrogen is sourced as a by-product to the fossil fuel industry. if hydrogen can be produced from renewable sources it would be a useful energy source for longer range vehicles than current batteries can provide.
Dirk Scott
If our vehicular future follows the route of batteries and electric cars, we will have to at least double electrical supply grid size to charge them and work out what to do with all the waste batteries. If we crack the problem of producing cheap hydrogen, then we can utilise the existing internal combustion engine technology and the existing fuel supply infrastructure. Many less problems to solve this way.