Solar Hydrogen: Photoanodes manufactured with alpha-SnWO4 promise high efficiency

Hydrogen is an important element in a sustainable energy system. The gas stores energy in chemical form and can be used in many ways: as fuel, as food for other fuels and chemicals or even to generate electricity in fuel cells. One solution to extracting hydrogen in a climate-neutral way is to scavenge hydroelectricity with the help of sunlight. This requires photoelectrodes that provide photovoltage and photocurrent when exposed to light and at the same time do not erode in water. Metal oxide fertilizers have promising precursors for this. For example, solar water heaters using bismuth vanadate (BiVO4) photoelectrodes already achieve today’s solar-to-hydrogen efficiency of 8%, which is nearly 9% at most.

The theoretical limit is 20% in α-SnWO4

To achieve efficiencies in excess of 9%, new products with a smaller band gap are required. The α-SnWO4 metal oxide has a band gap of 1.9 eV, which is well suited for photoelectrochemical water sputtering. Theoretically, a photonode made of this material could convert ~ 20% of irradiated sunlight into chemical energy (stored in the form of hydrogen). Unfortunately, the fertilizer decomposes very quickly in an aqueous environment.

Protection against corruption comes with a price

Thin layers of nickel oxide (NiOx) can protect the α-SnWO4 photoanode from corrosion, but have been found to significantly reduce photovoltage. To understand why this is so, a team led by Dr. Fatwa Abdi at the HZB Institute for Solar Fuel has analyzed the α-SnWO4 / NiOx interface in detail at BESSY II.

Interface tested at BESSY II

“We studied samples of different thickness of NiOx with a rigid X-ray photoelectron spectroscope (HAXPES) at BESSY II and interpreted the measured data with results from calculations and simulations,” said Patrick Schnell, the study’s first author and a PhD student in the HI-SCORE School of International Studies at HZB. “These results show that a thin coating of oxide forms at the interface, which reduces the photovoltage,” Abdi explains.

Preview: better protective layers

Overall, the study introduces new, fundamental insights into the complex nature of interfacing in metal oxide-based photoelectrodes. “These insights are very helpful for developing low-cost metal oxide photoelectrodes,” says Abdi. Α-SnWO4 is particularly promising in this regard. “We are currently working on another investment process for NiOx on α-SnWO4 that does not lead to the formation of an oxide interface coating, which is likely to be SnO2. If this is successful, we expect performance photoelectrochemical α – SnWO4 will increase significantly. ”

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