Large Scale Solar Hydrogen and Fuels Production Systems Using Particulate Photocatalysts

Sunlight-driven water splitting has been studied as a means of producing renewable solar hydrogen. Overall water splitting using particulate photocatalysts is of growing interest as a means of producing renewable hydrogen, because systems based on particulate photocatalysts can be spread over large areas using potentially inexpensive processes.¹⁾ A solar hydrogen production system based on 100-m² arrayed photocatalytic water splitting panels and an oxyhydrogen gas separation module was recently built, and its performance and system characteristics, including safety issues, were reported.²⁾ In addition, the hydrogen produced can be used to convert carbon dioxide into chemical fuels.³⁾ However, it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH efficiency) of photocatalysts and to develop suitable reaction systems.⁴⁾ In my talk, recent advances in photocatalytic materials and reaction systems for hydrogen and fuel production will be presented.

The author’s group has studied various semiconductor oxides, (oxy)nitrides, and (oxy)chalcogenides as photocatalysts for water splitting.⁵⁾ SrTiO₃ is an oxide photocatalyst that has been known to be active in overall water splitting under UV irradiation since 1980. Recently, the apparent quantum yield of overall water splitting using SrTiO₃ has been improved to more than 90% at 365 nm, corresponding to an internal quantum efficiency of nearly unity, by refining the preparation conditions of the photocatalyst and the loading conditions of the cocatalysts.⁶⁾ This observation means that particulate photocatalysts can drive the endergonic overall water splitting reaction with almost no recombination loss as in photon-to-chemical conversion processes during photosynthesis. However, for practical solar hydrogen production, it is essential to develop photocatalysts that are active under visible light. Ta₃N₅,⁷⁾ Y₂Ti₂O₅S₂,⁸⁾ TaON,⁹⁾ BaTaO₂N¹⁰⁾, SrTaO₂N¹¹⁾ have been shown to be active in photocatalytic overall water splitting via one-step excitation under visible light. It is also possible to combine hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) to split water into hydrogen and oxygen via two-step excitation. Such a process is widely known as the Z-scheme. Particulate photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ as HEP and Mo-doped BiVO₄ as OEP immobilized on Au and C layers split water into hydrogen and oxygen with STH efficiencies exceeding 1.0%.^12,13) Some (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths are also applicable to Z-scheme photocatalyst sheets and hold the promise of realizing greater STH efficiencies.

References:
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