Hydrogen Production from Water Based on Heterogeneous Photocatalysts

Abstract

Photocatalytic water splitting has attracted much attention as a means of renewable solar hydrogen production on a large scale [1,2]. A solar-to-hydrogen energy conversion efficiency (STH) of 5% or higher is considered to be necessary for practical operation of photocatalytic solar hydrogen plants. It is clearly necessary to harvest visible light to achieve a sufficient STH with reasonable quantum efficiencies. In addition, the scalability of water splitting systems is an important issue when solar hydrogen is used globally because of the low energy density of sunlight [3]. In this regard, development of particulate photocatalysts that are active in the water splitting reaction without an external power supply has a significant impact. A semiconductor photocatalyst can generate both hydrogen and oxygen when the band gap straddles the reduction and oxidation potentials of water. Alternatively, two different photocatalysts can be connected in series so that hydrogen and oxygen are generated on the respective photocatalysts. This reaction scheme based on two-step photoexcitation is often called Z-scheme water splitting. Photocatalytic materials can also be applied as photoelectrodes when being directly fabricated or embedded onto conductive substrates. In all cases, surface modifications to facilitate charge separation and surface redox reactions and to suppress backward reactions and self-decomposition are essential for activation of narrow band gap semiconductor photocatalysts in the water splitting reaction. Recently, the authors’ group developed photocatalyst sheets consisting of La- and Rh-codoped SrTiO3 as a hydrogen evolution photocatalyst and BiVO4 as an oxygen evolution photocatalyst embedded into a thin metal layer by the particle transfer method [4,5]. The photocatalyst sheet showed significantly higher activity than the corresponding powder suspension systems in the Z-scheme water splitting reaction, because the metal layer transferred photogenerated electrons between the photocatalyst particles effectively. Nevertheless, the oxide photocatalysts used for the photocatalyst sheet absorb visible light of up to approximately 540 nm, which may limit the potential of photocatalyst sheet based on the oxide semiconductors in the solar hydrogen production. Accordingly, application of photocatalysts with longer absorption edge wavelength should be envisioned. The authors’ group has studied various (oxy)nitride and (oxy)chalcogenide semiconductors as photocatalysts and photoelectrodes for visible-light-driven water splitting [1,6]. In these alternative semiconducting materials, the top of the valence band is composed of N 2p and S 3p (and Se 4p) orbitals that are located at more negative potential than O 2p orbitals, respectively. As a result, some (oxy)nitride and (oxy)chalcogenide semiconductors have band structures suitable for water splitting under visible light. Recently, we developed photoanodes and photocathodes based on particulate BaTaO2N [7] and La5Ti2Cu0.9Ag0.1S5O7 [8], respectively, by the particle transfer method. The BaTaO2N photoanode and the La5Ti2Cu0.9Ag0.1S5O7 photocathode generated photocurrents attributable to photoelectrochemical water oxidation and reduction, respectively, with relatively small applied voltages after surface modifications. Series-connected BaTaO2N photoanode and La5Ti2Cu0.9Ag0.1S5O7photocathode split water photoelectrochemically at the Faradaic efficiency of unity without an external power supply under irradiation up to 660 nm. In this talk, our recent advancement in the water splitting reaction on heterogeneous photocatalysts under visible light will be introduced. Future prospects toward application of photocatalytic water splitting on a large scale will also be presented.