Combining the best of both worlds in a “hybrid photoelectrode” concept, researchers at the Helmholtz Centre Berlin for Materials and Energy (HZB), Germany, and the Technical University of Delft, The Netherlands, have married a simple silicon-based thin-film solar cell with a chemically stable, low-cost metal oxide photoanode to create a solar-to-fuel device that can perform artificial photosynthesis, splitting water into hydrogen and oxygen and chemically storing 5% of the solar energy in the form of hydrogen. Ultimately, the technology could enable both the harvesting and storage of solar energy on a large scale at affordable prices.
“We invented a new trick to reduce recombination in highly doped semiconductors, such as the BiVO4 that we studied here,” says Roel van de Krol, head of the HZB Institute for Solar Fuels. We did this by introducing a gradient in the concentration of the tungsten (W) dopant throughout the film. The film is highly doped near the back contact, and undoped near the surface. This dopant gradient improves the distribution of the electric field in the material, which helps to separate the charges and thus to decrease recombination.”
To build the solar-to-fuel device, the optimized, gradient-doped BiVO4 photoanode was combined with a double-junction amorphous silicon cell. “The BiVO4 absorbs the high-energy photons (short wavelengths), while the low-energy photons are transmitted through the BiVO4 and absorbed by the amorphous silicon cell behind it,” the materials scientist explains. “The silicon cell generates the extra bit of voltage that the BiVO4 needs in order to split water into hydrogen and oxygen.”
Comparing his approach to other solutions for solar water splitting, which are based on coupled PV/electrolysis systems or on silicon solar cells modified with electrocatalysts for water reduction and oxidation, van de Krol says, “Our concept shows better chemical stability, a simpler device structure and potentially higher efficiencies at lower cost.”
So far, van de Krol and his collaborators have achieved 5% solar-to-hydrogen conversion efficiency (10% is generally accepted as the threshold for commercialization). “Higher efficiencies of 15% or even 20% are perhaps possible, but at higher cost,” van de Krol speculates. “In the end, the consumer price (in EUR/kg) of the produced hydrogen will determine where the optimum is.”
We need chemical fuels since this is “by far the best method known to store energy on a truly massive scale and in compact form,” says the expert. “Hydrogen is one of the most promising fuels because it is 100% clean (no CO2 or other harmful byproducts) and because it has a high gravimetric energy density,” he explains. The challenge, though, is the low volumetric energy density of hydrogen, which makes it difficult to store it in a compact form. According to van de Krol, liquid fuels, such as methanol, are the solution. “So whatever fuel we will use in the future, hydrogen is guaranteed to play a key role.”
This European research advance could represent a significant step towards autonomous devices that can produce chemical fuels in a truly sustainable manner from abundant resources; in this case, sunlight and water. “We foresee application of this technology on the scale of a single home, where the hydrogen can be stored and used to produce either electricity on demand (also at night) or be used to fuel your car,” van de Krol says. “But it can also be used in large solar fuel farms, where hydrogen is produced as end-product or used for on-site processing into liquid fuels.”
Before this vision can become reality, van de Krol’s team is tasked with further improving the solar-to-hydrogen conversion efficiency, to closer to 10%. “We can achieve about 9% with BiVO4 if we do everything right,” he says. “But doing everything right is actually very hard to achieve.” They are therefore also exploring new materials that absorb a large part of the solar spectrum. “Such materials have theoretical efficiencies of 15% or more, which would make it a bit easier to reach the 10% efficiency goal. The challenge of course is to find such new materials,” he says in conclusion.
Written by Sandra Henderson, Research Editor, Solar Novus Today
Image: Copyright TU Delft