Significant progress has been recently reported in solar water splitting, but two major challenges remain. First, although solar-to-hydrogen (STH) efficiency as high as 30% has been demonstrated, the use of expensive and non-scalable III-V semiconductors is required. On the other hand, low-cost metal-oxide based devices, using mainly BiVO4 as the absorber, have only achieved STH efficiency of < 10%, due to the limited absorption of BiVO4. Novel metal oxide semiconductors with a bandgap of 1.7-1.9 eV, which are stable and efficient, are therefore desired. The second challenge is related to the scale-up of the devices, from laboratory experiments to large-area device demonstrations. Several efforts exist, but the performance of the large-area devices is still significantly smaller (up to a factor of 5) as compared to the small-area equivalent. In this talk, I will outline our efforts in overcoming the two challenges above. The first is on the development of  SnWO4 (Eg = 1.9 eV) as a novel photoanode, which suffers from poor stability. This could be overcome by depositing NiOx as a protection layer, but it is also accompanied by the reduction of the photovoltage. We performed a thorough α-SnWO4/NiOx interface investigation by means of synchrotron-based hard X-ray photoelectron spectroscopy,[1] which reveals the presence of an interfacial oxide layer (likely to be SnO2). Control experiments show that such a layer can indeed be responsible for the reduction of photovoltage; its formation must therefore be avoided to overcome the photovoltage limitation. Concerning the scale-up of devices, I will discuss our approach in quantifying the different loss mechanisms associated with scaling-up photoelectrodes using a combination of multiphase multiphysics simulations and in-situ quantitative analysis of the fluid dynamics as well as pH of the electrolyte.[2-4] Based on these insights, electrochemical engineering strategies to overcome the losses associated with scale-up are offered.

1. Schnell et al. Adv. Energy Mater. 11 2021 2003183
2. Abdi et al. Sust. Energy Fuels 4 2020 2734
3. Obata et al. Energy Environ. Sci. 13 2020 5104
4. Obata et al. Cell Rep. Phys. Sci. 2 2021 100358

Fatwa Abdi

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