(Keynote) Large Scale Solar Hydrogen Production with Water Splitting Panel
Authors: Domen Kazunari 
Published: May 2020

Sunlight-driven water splitting has been studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier [1]. Both the efficiency and the scalability of water-splitting systems are essential factors for practical utilization of renewable solar hydrogen because of the low areal density of solar energy. Particulate photocatalyst systems do not involve any secure electric circuit and can be spread over wide areas by inexpensive processes potentially. Therefore, it is highly impactful to develop particulate photocatalysts and their reaction systems that efficiently split water. In my talk, the latest progress in photocatalytic materials and reactors and concepts toward large-scale operation will be presented.

A semiconductor photocatalyst can split water into hydrogen and oxygen thermodynamically when the band gap straddles the potentials of the hydrogen evolution reaction (0 V vs. RHE) and the oxygen evolution reaction (+1.23 V vs. RHE). SrTiO3 is a photocatalyst known to be active in overall water splitting under UV light irradiation after loading of proper cocatalysts since 1980s. The author's group has found that doping Al into SrTiO3 boosts the water splitting activity by two orders of magnitude [2]. Through tuning of the preparation and modification methods, the apparent quantum yield of photocatalytic water splitting using Al-doped SrTiO3 has reached 95% at wavelengths in the range of 350–360 nm, equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported, and confirms that a particulate photocatalyst can drive the greatly uphill overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively. Nevertheless, it is essential to develop photocatalysts active under visible light irradiation.

Various oxides, (oxy)nitrides, and (oxy)chalcogenides have been investigated [3]. Ta3N5 has a band structure suitable for water splitting under visible irradiation of up to 600 nm; however, overall water splitting had not been achieved because of high densities of trap states until recently. The author’s group has developed a Ta3N5 photocatalyst active in overall water splitting via one-step excitation under visible light [4]. Ta3N5 single crystal nanorods are directly evolved on KTaO3 particles by short-time nitridation accompanied by gradual evaporation of K contents. Ta3N5 single crystal nanorods have low defect densities and can utilize photoexcited carriers in the water splitting reaction when being loaded with appropriate cocatalysts. It was also found that Y2Ti2O5S2, an oxysulfide photocatalyst, could be activated in overall water splitting by coloading cocatalysts for hydrogen evolution and oxygen evolution and finely tuning the reaction conditions [5].

Two different photocatalysts can also be combined so that hydrogen and oxygen are generated on the respective photocatalysts. The author’s group has developed particulate photocatalyst sheets consisting of the hydrogen evolution photocatalyst (HEP) and the oxygen evolution photocatalyst (OEP) embedded into conductive layers by particle transfer [6,7]. A photocatalyst sheet consisting of La- and Rh-codoped SrTiO3 as a HEP and Mo-doped BiVO4 as an OEP embedded into a carbon conductor exhibits a solar-to-hydrogen energy conversion efficiency (STH) of 1.0% at ambient pressure. Some (oxy)chalcogenides and (oxy)nitrides with longer absorption edge wavelengths have also been shown to be applicable as the HEP and the OEP of particulate photocatalyst sheets recently.

The author's group has been developing panel reactors that accommodate photocatalyst sheets in view of large-scale application [2]. A prototype panel reactor containing Al-doped SrTiO3 photocatalyst sheets splits water and releases product hydrogen and oxygen gas bubbles at a rate corresponding to a STH of 10% under intense UV illumination even when the water depth is merely 1 mm, and it can maintain the activity over several months under sunlight illumination [8]. A 1-m2-sized photocatalyst panel reactor splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. Panel reactors can accommodate various kinds of photocatalyst sheets and are expected to be built using light and inexpensive materials, being suitable for large-scale solar hydrogen production from water. A prototype solar hydrogen production system with a greater size is currently under construction.

[1] Hisatomi et al.
Nat. Catal.
2019, 2, 387.

[2] Goto et al.
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[3] Chen et al.
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2017, 2, 17050.

[4] Wang et al.
Nat. Catal.
2018, 1, 756.

[5] Wang et al.
Nat. Mater.
2019, 18, 827.

[6] Wang et al.
Nat. Mater.
2016, 15, 611.

[7] Wang et al.
J. Am. Chem. Soc.
2017, 139, 1675.

[8] Lyu et al.
Chem. Sci., 2019, 10, 3196.