Hydrogen is an important factor in a sustainable energy system. The gas stores energy in chemical form and can be used in a variety of ways: as fuel, as a raw material for other fuels and chemicals or to generate electricity in fuel cells.
Hydrogen can be produced in a climate-neutral way by electrochemically splitting water with sunlight. The necessary photovoltage and photocurrent supply suitable photoelectrodes when exposed to light, which remain stable in the water . Some metal oxide compounds meet these requirements. This is how solar water splitters with bismuth vanadate (BiVO 4 ) photo electrodes already achieve efficiency (solar-to-hydrogen)of about 8%, which is close to the theoretical maximum of the material (9%). To achieve efficiencies beyond 9%, new materials with a smaller band gap are required.
α-SnWO 4 : Theoretically up to 20% efficiency possible
The metal oxide α-SnWO 4 has a band gap of 1.9 eV, which is perfect for photoelectrochemical water splitting. Theoretically, a photoanode made from this material could convert around 20% of the incident sunlight into chemical energy, stored in the form of hydrogen. Unfortunately, the compound decomposes very quickly in an aqueous environment.
Protective layer reduces the photo voltage
Thin layers of nickel oxide (NiO x ) can protect the α-SnWO 4 photo anode from corrosion. However, it was also found that they significantly reduce the photo voltage. To understand why this is the case, a team led by Dr. Fatwa Abdi at the HZB Institute for Solar Fuels analyzed the α-SnWO 4 / NiO x interface at BESSY II in detail.
HAXPES measurement on BESSY II
“We examined samples with different NiO x thicknesses using hard X-ray photoelectron spectroscopy (HAXPES) at BESSY II and interpreted the measurement data with results from calculations and simulations,” says Patrick Schnell, first author of the study and doctoral student at HI-SCORE International Research School at the HZB. “These results indicate that a thin oxide layer forms at the interface, which reduces the photovoltage,” explains Dr. Fatwa Abdi.
Outlook: protective layer without disadvantages
Overall, the study provides fundamental new insights into the complex nature of interfaces in metal oxide-based photoelectrodes. “These insights are very useful in developing inexpensive, scalable metal oxide photoelectrodes,” says Abdi. α-SnWO 4 is particularly promising in this regard. “We are currently working on an alternative deposition process for NiO x on α-SnWO 4 that does not lead to the formation of an interface oxide layer . If this succeeds, we expect that the photoelectrochemical performance of α-SnWO 4 will increase significantly.”