Photocatalytic Water Splitting for Solar Hydrogen Production Using the Carbonate Effect and the Z‐Scheme Reaction

Abstract

for the direct utilization of solar energy have been very limited. Technologies for photovoltaic, solar thermal, and biomass energies have been realized commercially, but it is unclear whether the global energy problem will be solved in future by these simple extensional technologies. Thus, it is essential to develop new and simpler technologies with lower costs per area than photovoltaics to use low-energydensity solar energy. Photovoltaics have a serious disadvantage, as the electricity cannot be accumulated without an expensive battery. The cost of H 2 production using a simple combination of conventional photovoltaics and an electrolyzer will be highly expensive. As a promising candidate for the direct conversion of solar energy into chemical energy and solar energy accumulation, the photocatalytic splitting of water into H 2 and O 2 has been intensively investigated in the research fields of artificial photosynthesis and solar hydrogen technologies.Since the Honda-Fujishima effect was first reported, [2] TiO 2 semiconductor photo anodes have been used for water splitting to achieve low-cost H 2 production, and various photoanodes and powder photocatalysts have been widely studied. [3][4][5][6][7][8][9][10] In particular, oxide semiconductors have been widely investigated because they can be simply prepared by calcination in air and are stable for the O 2 evolution reaction. Figure 1 shows the reaction mechanisms for water splitting into H 2 and O 2 in photocatalyst and photoanode systems. In the case of n-type semiconductor photoanodes, O 2 is evolved on the photoanode and H 2 is evolved on the metal cathode, and an external bias is generally applied between the electrodes to supply a surplus potential for the reaction. In the case of powdered photocatalysts, a cocatalyst is often loaded on the semiconductor surface to accelerate the reaction. As both the oxidation and reduction reactions are completed on the small semiconductor particles, photocatalytic reactors can be simplified and easily enlarged for wide-scale use. The reaction mechanisms shown in Figure 1 are similar to the photoexcitation mechanism of semiconductors in photovoltaics; thus, the theoretical and maximum solar energy conversion efficiencies are very high (up to 30%). [11] It has been evaluated that the photocatalytic reactor cost for water splitting, where an oxide photocatalyst powder is suspended within an The development of innovative technologies for solar energy conversion and storage is important for solving the global warming problem and for establishing a sustainable society. The photocatalytic water-splitting reaction using semiconductor powders has been intensively studied as a promising technology for direct and simple solar energy conversion. However, the evolution of H 2 and O 2 gases in a stoichiometric ratio (H 2 /O 2 = 2) is very difficult owing to various issues, such as an unfavorable backward reaction and mismatched band potentials. Two important findings have widened the variety of photocatalysts available for stoichio...

Comprehensive review and future perspectives on the photocatalytic hydrogen production

“…However, the energy consumption associated with electrolysis must be significantly reduced before this process becomes competitive at large scales. Photocatalytic generation of H 2 from organic solutions or H 2 O, although considered a minor source in terms of produced H 2 volume, appears to be a complementary and renewable way for simultaneous H 2 generation and wastewater remediation …”

Bilayer MoSe2/HfS2 Nanocomposite as a Potential Visible-Light-Driven Z-Scheme Photocatalyst

“…In order to set up a sustainable society, photocatalytic water splitting for hydrogen production has been deemed as an effective route to solve the problems of environmental pollution and energy shortage [1]. Owing to the groundbreaking work by Honda and Fujishima in 1972 [2], a variety of semiconductor materials have been extensively investigated to explore high-performance photocatalysts for water decomposition [3][4][5][6][7].…”

Photocatalytic and Photoelectrochemical Systems: Similarities and Differences

“…Coupling two or more components into a photoactive composite may address some of these drawbacks by utilizing the strengths of individual semiconductors. The secondary, or additional, component can be another photoactive semiconductor (e.g., to form a plasmonic composite, Type I, Type II, or Z‐scheme configuration), cocatalyst (to promote surface kinetics or to serve as a passive layer), or a metallic/nonmetallic conductor (to boost electron transfer) . In brief, the multicomponent photoactive composites offer photocatalytic benefits one or more of the following aspects: 1) extending the light absorption spectrum; 2) promoting charge separation; 3) minimizing charge losses through recombination; and 4) passivating photocorrosion.…”