Metal oxides are an intriguing class of materials that can potentially enable large-scale solar fuels production via photoelectrochemical (PEC) water splitting. Binary metal oxides, consisting of a single type of metal combined with oxygen, have been studied as photoelectrode materials for decades. Unfortunately, these materials have not yet enabled efficient and stable PEC water splitting due to their inherent limitations in light absorption, stability, and carrier transport. Recently, more complex, multinary metal oxides, composed of at least two metals and oxygen, have shown promise as photoelectrode materials. In many cases the multinary metal oxides have shown fewer material limitations and higher photoelectrochemical efficiencies than their binary counterparts. The number of available material combinations is much greater for multinary metal oxides and many combinations have not yet been explored. In our working group, we examine and develop novel complex metal oxides that absorb visible light, such as BiVO4, Fe2WO6, FeVO4, etc. We emphasize on the determination of performance limiting factors, and the correlation between defects and performance.
SOLAR FUELS DEVICE
Tandem Device for Solar Water Splitting
In order to relax the requirement for photoelectrode materials for solar water splitting, we employ a tandem configuration where two semiconductor materials with different bandgaps are used to absorb the sunlight. The large bandgap semiconductor absorbs the short wavelength part (high photon energy) of the solar spectrum, while the long wavelength part (low photon energy) are transmitted. The small bandgap semiconductor can then utilize this transmitted spectrum. This way, not only that we open up the possibilities for more materials to be employed as photoelectrodes, we also utilize the solar spectrum more efficiently. In this research area, we are looking into combining multiple cheap, abundant materials as large- and small-bandgap semiconductors. To this end, using multiple combinations (e.g. BiVO4 and Si), we have achieved solar-to-hydrogen (STH) efficiencies up to ~8%.
REACTOR ENGINEERING AND SCALE-UP
Modeling and Validation Experiments
With solar fuel devices demonstrating increasing efficiency, scalable pathways need to be prepared in order to allow a wide-scale implementation. Significant efforts therefore have to already be placed in identifying potential problems and challenges when moving from the laboratory scale to the large scale operation. Indeed, devices with larger area have been reported to show lower efficiency as the small area equivalent. In our group, we combine multiphysics modeling (finite element analysis) and validation experiments (e.g., fluorescence, particle image velocimetry) in order to identify any scale-up related performance loss mechanism. Based on these insights, optimum cell engineering and design can be proposed.