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."
- Fascinating archaeological find becomes a source of knowledgeThe Bavarian State Office for the Preservation of Historical Monuments (BLfD) has sent a rare artefact from the Middle Bronze Age to Berlin for examination using cutting-edge, non-destructive methods. It is a 3,400-year-old bronze sword, unearthed during archaeological excavations in Nördlingen, Swabia, in 2023. Experts have been able to determine how the hilt and blade are connected, as well as how the rare and well-preserved decorations on the pommel were made. This has provided valuable insight into the craft techniques employed in southern Germany during the Bronze Age. The BLfD used 3D computed tomography and X-ray diffraction to analyse internal stresses at the Helmholtz-Zentrum Berlin (HZB), as well as X-ray fluorescence spectroscopy at a BESSY II beamline supervised by the Bundesanstalt für Materialforschung und -prüfung (BAM).
- Element cobalt exhibits surprising propertiesThe element cobalt is considered a typical ferromagnet with no further secrets. However, an international team led by HZB researcher Dr. Jaime Sánchez-Barriga has now uncovered complex topological features in its electronic structure. Spin-resolved measurements of the band structure (spin-ARPES) at BESSY II revealed entangled energy bands that cross each other along extended paths in specific crystallographic directions, even at room temperature. As a result, cobalt can be considered as a highly tunable and unexpectedly rich topological platform, opening new perspectives for exploiting magnetic topological states in future information technologies.
- MXene for energy storage: More versatile than expectedMXene materials are promising candidates for a new energy storage technology. However, the processes by which the charge storage takes place were not yet fully understood. A team at HZB has examined, for the first time, individual MXene flakes to explore these processes in detail. Using the in situ Scanning transmission X-ray microscope 'MYSTIIC' at BESSY II, the scientists mapped the chemical states of Titanium atoms on the MXene flake surfaces. The results revealed two distinct redox reactions, depending on the electrolyte. This lays the groundwork for understanding charge transfer processes at the nanoscale and provides a basis for future research aimed at optimising pseudocapacitive energy storage devices.