(Invited) Direct Synthesis of Hydrogen Carrier Using Membrane Integrated-Photocatalyst Sheets Under Sunlight

Abstract

Photocatalytic and Photoelectrochemical (PEC) reaction are desirable technologies to construct a sustainable social system. They can convert solar energy of renewable energy to storable and transportable energy carriers such as hydrogen, hydrocarbons formic acid, and methylcyclohexane (MCH), directly. In the present study, we investigated sunlight driven MCH evolution through reduction of toluene in existence of water as a hydrogen source. Chalcopyrite materials have intensively studied as photocathode material for hydrogen evolution from water under sunlight because of its tunable bandgap, high optical absorption coefficients, durability in PEC reduction reactions and so on. Cu(In, Ga)Se2 (CIGS) -based photocathode with appropriate surface modifications and optimal composition shows high photocurrent under simulated sunlight, ca 35 mA cm-2, and the highest half-cell solar-to-hydrogen conversion efficiency (HC-STH) of 12.5% among the non-single crystalline photocathodes.[1] However, its driving force of PEC reaction is limited because of the existence of Cu 2p orbitals in valence band. Forming solid solution of ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) could diminish the influence of the orbitals resulting in largely increased driving force of CIGS. ZnSe:CIGS with optimal composition shows combined long absorption edge wavelength of ~900 nm and large driving force of PEC reactions, indicating large photocurrent and high onset potential of cathodic photocurrent under simulated AM 1.5G light. The solid solution with optimal composition of (ZnSe)0.15(CIGS)0.85 worked as photocathode and its cathodic photocurrent was largely enhanced by surface modifications. [2] The CIGS-based and ZnSe:CIGS-based photocathodes were also applied for PEC production of hydrogen carrier, MCH, through a hydrogenation of toluene with existence of water as a hydrogen source using two-chamber PEC cell combined with membrane-photoelectrode assembly. Further surface modifications by very thin compact protective layers and Pt particles loaded on transparent porous layers onto the photocathodes allows us to obtain MCH directly from only water and toluene under simulated sunlight with Faradaic efficiency of ~100%. [3] In the presentation, we will discuss about the novel photocathodes and MCH evolution by overall water splitting reactions using these PEC cells in detail.