Photoelectrochemical Materials For Sunlight-driven Water Splitting Devices | 101307
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Photocatalysis or photoelectrochemistry are attractive developing fields of engineering for building free-running sunlightdriven
water splitting to generate H2 and O2. We are surveying solar-spectrum-responding semiconductive materials
as the candidates for the visible light absorbers in the H2+O2 harvesting devices. We have been fabricating and testing
water photo-splitting devices composed of a pair of photocathode (p-type, for H2) and photoanode (n-type, for O2) both
decorated with catalysts for evolving those gases. As for photocathode, we developed H2 evolving flat layered sheets based on
chalcopyrite Cu(In, Ga)Se2 (CIGS, the cutoff wavelength of absorption ~ 1100 nm) and its doped versions with Zn, S, etc. The
photocurrent obtained by the solar simulator (AM 1.5G) can afford more than 10% of solar hydrogen conversion efficiency.
The photoanode material is the remaining problem to solve. BiVO4 (~540 nm), paired with CIGS, realized a stable operation
for the stoichiometric faradaic evolution of H2 and O2, however, the maximum solar-to-H2 efficiency has been below 4 %.
Obviously, we need n-type light absorbers with longer cutoff wavelength. We are also developing transition metal nitrides and
oxynitrides for the sunlight absorbers. Ta3N5 (~600 nm) has been the most intensively investigated, as particles embedded
on metal layers (particle transferred sheets) and flat layered thin films, both of which can serve as photoanodes. Foreign
materials can be assembled as the background layer or capping layer for the Ta3N5 layer to improve the electronic properties
and robustness as an electrode immersed in the electrolytic solution. We will discuss the best performance for Ta3N5 and
oxynitrides as O2-evolving photoelectrodes energized by solar irradiation.
Hiroshi Nishiyama completed his Ph.D. at Nagaoka University of Technology, Japan in 2005. In 1998–2013, he was an assistant professor at the Analysis and Instrumentation Center at Nagaoka University of Technology. He is currently a principal project researcher in the R&D Laboratory of Artificial Photosynthetic Chemical Process (ARPChem) at The University of Tokyo. His research focuses on the development of high-performance photoanode electrodes and high-performance PEC systems.