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.<br/><br/>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.<br/><br/><b>References</b>:<br/>1) Hisatomi <i>et al.</i> <i>Nat. Catal.</i> <b>2019</b>, <i>2</i>, 387. 2) Nishiyama <i>et al.</i> <i>Nature</i> <b>2021</b>, <i>598</i>, 304. 3) Yamada <i>et al.</i> <i>ACS Engineering Au</i> <b>2023</b>. DOI: 10.1021/acsengineeringau.3c00034. 4) Hisatomi <i>et al.</i> <i>Next Energy</i> <b>2023</b>, <i>1</i>, 100006. 5) Chen <i>et al.</i> <i>Nat. Rev. Mater.</i> <b>2017</b>, <i>1</i>, 17050. 6) Takata <i>et al.</i> <i>Nature</i>, <b>2020</b>, 581, 411. 7) Wang <i>et al.</i> <i>Nat. Catal.</i> <b>2018</b>, <i>1</i>, 756. 8) Wang <i>et al.</i> <i>Nat. Mater.</i>, <b>2019</b>, <i>18</i>, 827. 9) Xiao <i>et al.</i> <i>Angew. Chem. Int. Ed.</i> <b>2022</b>, <i>134</i>, e202116573. 10) Li <i>et al.</i> <i>ACS Catal.</i><b> 2022</b>, <i>12</i>, 10179. 11) Chen <i>et al.</i> <i>J. Am. Chem. Soc.</i> <b>2023</b>, <i>145</i>, 3839. 12) Wang <i>et al.</i> <i>Nat. Mater.</i> <b>2016</b>, <i>15</i>, 611. 13) Wang <i>et al.</i> <i>J. Am. Chem. Soc.</i> <b>2017</b>, <i>139</i>, 1675.