Water splitting driven by solar energy has attracted much attention as a means of renewable solar hydrogen production. Photocatalytic water splitting is a representative approach utilizing particulate semiconductors.^1,2 When a particulate semiconductor photocatalyst absorbs a photon, an electron is excited from the valence band to the conduction band, and a positive hole is left in the valence band. These excited electrons and holes can drive reduction and oxidation reactions on each photocatalyst particle, respectively. A 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); however, to drive the reaction efficiently, it is generally necessary to load nanoparticulate metals and metal oxides, denoted as cocatalysts, on the photocatalyst as active sites for the hydrogen evolution reaction and the oxygen evolution reaction. Note that it is also possible to split water using two kinds of photocatalysts for hydrogen evolution and oxygen evolution, respectively. Such a reaction scheme based on two-step excitation is called Z-scheme. A solar-to-hydrogen energy conversion efficiency (STH) of 5% or higher is considered to be necessary for practical operation of photocatalytic solar hydrogen production. To attain such a high STH at reasonable quantum efficiencies, it is necessary to activate and stabilize narrow band gap photocatalysts.² Scalability of the system is also a critical issue.³  Water splitting systems based on particulate photocatalysts are promising in terms of scalability because they do not need any secure electric circuits and thus can be spread over a wide area easily.

Conventionally, photocatalytic water splitting has been studied using suspension of particulate photocatalysts. However, this approach is not feasible in practice because it is challenging to design a cheap photocatalytic reactor that can maintain a large amount of water over a wide area. It is probably a more feasible to use photocatalysts processed into panels and construct modules similar to photovoltaic applications.⁴ This approach, fixing particulate photocatalysts onto substrate, is also expected to be advantageous over the conventional powder suspension approach in operation and maintenance, because power input to stir a large amount of photocatalyst suspension is unneeded and used photocatalysts can be readily replaced with fresh ones.

Recently, the authors’ group developed photocatalyst sheets based on particulate hydrogen evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP) embedded into conductive layers by particle transfer for efficient and scalable water splitting (Figure).^5–8 The STH of water splitting using photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ (SrTiO₃:La,Rh) as a HEP and BiVO₄ as an OEP was 0.2%, superior to those for the corresponding powder suspension systems.⁶ The photocatalyst sheet maintained the high water splitting activity over a wide range of pH values and even in pure water. Moreover, the STH was improved to 1.1% through improvements in the preparation process of the photocatalyst sheet and the reaction conditions.⁷ The high activity of the photocatalyst sheet is due to the efficient electron transfer between HEP and OEP particles via the underlying conductive layer. In addition, evolution of hydrogen and oxygen in close proximity allows to prevent generation of pH gradient during the water splitting reaction.⁸ Therefore, the photocatalyst sheet is scalable directly without sacrificing the high activity. However, the absorption edge wavelengths of SrTiO₃:La,Rh and BiVO₄ are 540 nm at most. It is still important to develop photocatalysts with longer absorption edge wavelengths.

In this talk, recent progress and future challenges in photocatalytic water splitting and system development will be presented.

References

1. Hisatomi et al., Chem. Soc. Rev. 2014, 43, 7520.

2. Hisatomi et al., Catal. Lett. 2015, 145, 95.

3. Maeda et al., J. Phys. Chem. Lett. 2010, 1, 2655.

4. Xiong et al., Catal. Sci. Technol. 2014, 4, 325.

5. Minegishi et al., Chem. Sci. 2013, 4, 1120.

6. Wang et al., J. Catal. 2015, 328, 308.

7. Wang et al., Nat. Mater. 2016, 15, 611.

8. Wang et al., Faraday Discuss., in press.