Taro Yamada1 2 Kazumari Domen1 2 3

1, University of Tokyo, Tokyo, , Japan
2, ARPChem, Tokyo, , Japan
3, Shinshu University, Nagano, , Japan

Today's human-race-scale desire for sustainable and renewable sources of energy lets us consider it rational and natural to pursue the sunlight, which have served as the invariant resource since living organisms started on the earth. A number of basic studies have been conducted for recovery, storage, and transport of solar energy towards development for mass-scale industry. Photocatalytic splitting of water into H2 and O2 is one of the promising technologies for solar energy harvesting. Both of the gases are valuable materials in chemical industry, and moreover, utilization of H2 is strategically involved in CO2 fixation to realize artificial photosynthesis on the industrial basis.
Design of future mass production plants for solar water splitting will involve solar catalytic panels, delivery system of reactant water, transportation system of product H2 and O2, separation and purification system of these gases. There may be two main categories for water photo-splitting devices. One is the photoelectrochemical design, in which photocathode and photoanode separately evolve H2 and O2 by the aid of gas separation/ion transmitting membrane. This scheme is rational for the product delivery to the following processes, and a high solar-to-H2 energetic conversion efficiency is anticipated by optimal choice of the two electrodes. The other is application of a single powder photocatalyst, which evolve 2H2+O2 mixture. In the following process, first 2H2+O2 mixture must be separated into H2 gas and O2 gas by an efficient separation membrane, which might be additional sophistication. Nevertheless, the single power photocatalysis system is anticipated to be low cost, and therefore suitable for mass-scale plants.
This time we will discuss the latter system. So far the most developed and successful powder photocatalyst is SrTiO3. The original study of stoichiometric 2H2+O2 evolution by UV light was simply achieved by NiO catalyzed SrTiO3 [1]. Later, doped with various additives, SrTiO3 exhibited p-type or n-type characteristics as a semiconductor, extending the range of light absorption wavelength. Recently, Al-doped SrTiO3 was introduced as an UV-active stoichiometric water splitting photocatalyst that realizes a quantum efficiency of 69% at 320 nm by the aid of CrOx+Rh cocatalyst [2]. Due to the small fraction of UV light within the sunlight on the earth, the solar-to-H2 energetic conversion efficiency is approximately 0.6 ~ 0.7 %. Nonetheless this simplicity of water photo-splitting mechanism is a preferable feature in designing facile and low-cost solar H2 panels.
In this talk, we will further discuss some novel visible-light active photocatalysts. Also, the following processes for safe transportation of explosive 2H2+O2 mixture and operation of H2/O2 separation membrane will be generalized to enumerate tasks to be tackled for this development.

[1] Domen et al., J. Chem. Soc. Chem. Commn. 1980, 12, 543.
[2] Ham et al., Chem. Commn, 2016, 52, 5011.