Complex Metal Oxide Photoelectrodes
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.
Charge Carrier Dynamics at the Semiconductor-Electrolyte and Semiconductor-Catalyst Interface
A major limitation of metal oxide semiconductors is poor water oxidation kinetics, caused by either extensive charge carrier recombination at their surface or slow charge transfer from the semiconductor to the electrolyte. This has led to photocurrent losses of as high as one order of magnitude. Determining a way to recover these losses in a cheap metal oxide will lead to highly-efficient photoelectrodes. Deposition of a co-catalyst helps to alleviate this problem, but the main mechanism behind the improvements is not well-understood. As a consequence, progress in the discovery of effective metal oxide semiconductor/co-catalyst combinations has involved a lot of trial-and-error rather than rational material design. In our working group, we utilize multiple frequency-range in-situ photoelectrochemical techniques to probe the carrier dynamics at the interface (e.g. IMPS, PEIS, LMAS, LMMR), which allow us to monitor processes in a timescale of microseconds to seconds. By doing so, we are able to monitor the charge transfer and recombination at this interface, and pinpoint to the true nature of co-catalyst enhancement. We aim to fill the void of understanding in the metal oxide semiconductor/co-catalyst interaction, and shift the paradigm for PEC water splitting materials development from trial-and-error to materials by design.
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 of lareger than 5%.