Although metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells, their performance is often limited by poor charge carrier transport. Researchers from UC Berkeley and colleagues have now addressed this issue by using separate materials for light absorption and carrier transport.
The team reports on their host-guest system of Ta:TiO2|BiVO4 as a photoanode for use in solar water splitting cells in an open-access paper in the journal ACS Central Science. BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. The host–guest nanowire architecture allows for simultaneously high light absorption and carrier collection efficiency for efficient solar water oxidation.
Harnessing energy from sunlight is a means of meeting the large global energy demand in a cost-effective and environmentally benign manner. However, to provide constant and stable power on demand, it is necessary to convert sunlight into an energy storage medium. An example of such a method is the production of hydrogen by photoelectrochemical (PEC) water splitting.
The direct splitting of water can be achieved using a single semiconductor; however, due to the voltage requirement of the water splitting reaction and the associated kinetic overpotentials, only wide-band-gap materials can perform overall water splitting, limiting the efficiency due to insufficient light absorption. To address this issue, a dual-band-gap z-scheme system can be utilized, with a semiconductor photoanode and photocathode to perform the respective oxidation and reduction reactions. This approach allows for the use of lower-band-gap materials that can absorb complementary portions of the solar spectrum and yield higher solar-to-fuel efficiencies. In this integrated system, the charge flux is matched in both light absorbers of the photoelectrochemical cell. Therefore, the overall performance is determined by the limiting component. In most photoelectrosynthetic cells, this limiting component is the semiconductor photoanode.—Resasco et al.
Many materials that can perform the reaction exist, but most of these candidates suffer from issues, ranging from efficiency to stability and cost. The Berkeley system uses TiO2 nanowires as the host for guest nanoparticles from BiVO4.
BiVO4 is a newly introduced material that is among the best ones for absorbing light and performing the water splitting reaction, but does not carry charge well while TiO2 is stable, cheap and an efficient charge carrier but does not absorb light well.
Together with a unique studded nanowire architecture, the new system works better than either material alone. The authors state their approach can be used to improve the efficiencies of other photoconversion materials.
The authors acknowledge funding from the Department of Energy, the National Science Foundation and the University of California, Berkeley.
Joaquin Resasco, Hao Zhang, Nikolay Kornienko, Nigel Becknell, Hyunbok Lee, Jinghua Guo, Alejandro L. Briseno, and Peidong Yang (2016) “TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment” ACS Central Science doi: 10.1021/acscentsci.5b00402