Solar hydrogen: Photoanodes made of α-SnWO4 promise high efficiencies
TEM-Image of a α-SnWO4 film (pink) coated with 20 nm NiOx (green). At the interface of α-SnWO4 and NiOx an additional interfacial layer can be observed. © HZB
Photoanodes made of metal oxides are considered to be a viable solution for the production of hydrogen with sunlight. α-SnWO4 has optimal electronic properties for photoelectrochemical water splitting with sunlight, but corrodes easily. Protective layers of nickel oxide prevent corrosion, but reduce the photovoltage and limit the efficiency. Now a team at HZB has investigated at BESSY II what happens at the interface between the photoanode and the protective layer. Combined with theoretical methods, the measurement data reveal the presence of an oxide layer that impairs the efficiency of the photoanode.
Hydrogen is an important factor in a sustainable energy system. The gas stores energy in chemical form and can be used in many ways: as a fuel, a feedstock for other fuels and chemicals or even to generate electricity in fuel cells. One solution to produce hydrogen in a climate-neutral way is the electrochemical splitting of water with the help of sunlight. This requires photoelectrodes that provide a photovoltage and photocurrent when exposed to light and at the same time do not corrode in water. Metal oxide compounds have promising prerequisites for this. For example, solar water splitting devices using bismuth vanadate (BiVO4) photoelectrodes achieve already today ~8 % solar-to-hydrogen efficiency, which is close to the material’s theoretical maximum of 9 %.
Theoretical limit is 20 % in α-SnWO4
To achieve efficiencies beyond 9 %, new materials with a smaller band gap are needed. The metal oxide α-SnWO4 has a band gap of 1.9 eV, which is perfectly suited for photoelectrochemical water splitting. Theoretically, a photoanode made of this material could convert ~20 % of the irradiated sunlight into chemical energy (stored in the form of hydrogen). Unfortunately, the compound degrades very quickly in an aqueous environment.
Protection against corrosion comes with a price
Thin layers of nickel oxide (NiOx) can protect the α-SnWO4 photoanode from corrosion, but were found to also significantly reduce the photovoltage. To understand why this is the case, a team led by Dr. Fatwa Abdi at the HZB Institute for Solar Fuels has analysed the α-SnWO4/NiOx interface in detail at BESSY II.
Interface explored at BESSY II
"We studied samples with different thicknesses of NiOx with hard X-ray photoelectron spectroscopy (HAXPES) at BESSY II and interpreted the measured data with results from calculations and simulations," says Patrick Schnell, the first author of the study and a PhD student in the HI-SCORE International Research School at HZB. "These results indicate that a thin oxide layer forms at the interface, which reduces the photovoltage," explains Abdi.
Outlook: better protection layers
Overall, the study provides new, fundamental insights into the complex nature of interfaces in metal oxide-based photoelectrodes. "These insights are very helpful for the development of low-cost, scalable metal oxide photoelectrodes," says Abdi. α-SnWO4 is particularly promising in this regard. "We are currently working on an alternative deposition process for NiOx on α-SnWO4 that does not lead to the formation of an interfacial oxide layer, which is likely to be SnO2. If this is successful, we expect that the photoelectrochemical performance of α -SnWO4 will increase significantly."
- Freeze casting - a guide to creating hierarchically structured materialsFreeze casting is an elegant, cost-effective manufacturing technique to produce highly porous materials with custom-designed hierarchical architectures, well-defined pore orientation, and multifunctional surface structures. Freeze-cast materials are suitable for many applications, from biomedicine to environmental engineering and energy technologies. An article in "Nature Reviews Methods Primer" now provides a guide to freeze-casting methods that includes an overview on current and future applications and highlights characterization techniques with a focus on X-ray tomoscopy.
- IRIS beamline at BESSY II extended with nanomicroscopyThe IRIS infrared beamline at the BESSY II storage ring now offers a fourth option for characterising materials, cells and even molecules on different length scales. The team has extended the IRIS beamline with an end station for nanospectroscopy and nanoimaging that enables spatial resolutions down to below 30 nanometres. The instrument is also available to external user groups.
- A simpler way to inorganic perovskite solar cellsInorganic perovskite solar cells made of CsPbI3 are stable over the long term and achieve good efficiencies. A team led by Prof. Antonio Abate has now analysed surfaces and interfaces of CsPbI3 films, produced under different conditions, at BESSY II. The results show that annealing in ambient air does not have an adverse effect on the optoelectronic properties of the semiconductor film, but actually results in fewer defects. This could further simplify the mass production of inorganic perovskite solar cells.