(Invited) Photoelectrochemical Systems for Sunlight Driven Fuel Production
Authors: Minegishi Tsutomu , Domen Kazunari 
Published: February 2020

Photoelectrochemistry and photocatalysis have been attracted huge attentions as a promising mean to convert solar energy to storable and conveyable energy carriers such as hydrogen, formic acid, and methylcyclohexane (MCH). Hydrogen is the most promising candidate among the energy carriers because of its simple production through decomposition of abundant water and usability in combustion engines and fuel cells with no emissions except for water. In the present study, we investigated hydrogen evolution through overall water splitting, and MCH evolution through reduction of toluen in existence of water as a reductant without external bias voltage.
Photoelectrochemical (PEC) water splitting using the combination of photocathode and photoanode is one of the promising means to produce hydrogen via water splitting under sunlight. The configuration can relax the criteria for the photoelectrode used in the PEC cell and allows us to use photocatalytic materials with narrower bandgap resulting in utilization of larger wavelength region in sunlight spectrum. Considering the PEC cell composed of combined photocathode and photoanode, both photocathode and photoanode should show enough large photocurrent at around 0.6 V vs. reversible hydrogen electrode (RHE) under sunlight.
We found that the solid solution of ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) with optimal composition shows relatively large cathodic photocurrent with high onset potential under simulated sunlight of AM1.5G. As shown in figure 1, ZnSe poses proper band edge potentials. However, its bandgap is not enough narrow to capture sunlight efficiently. On the other hand, CIGS have enough narrow bandgap of 1.1 eV while its potential of valence band maximum is too shallow resulting in not enough high onset potential of cathodic photocurrent to construct PEC cell with photoanode as discussed above. To achieve photocathode material with preferable band edge potentials for water splitting and sunlight absorption, ZnSe:CIGS was investigated. The solid solution with optimal composition of ZnSe0.15:CIGS0.85 act as photocathode and its cathodic photocurrent is largely enhanced by surface modifications with CdS and Pt as the case of other chalcogenide photocathodes. The PEC cell composed of ZnSe:CIGS based photocathode and BiVO4 based photoanode in side-by-side configuration showed stoichiometric water splitting under simulated sunlight with solar-to-hydrogen conversion efficiency of ~1% without external bias voltage.
As discussed above, hydrogen is the promising energy carrier while the energy concentration of hydrogen gas is very thin in atmospheric pressure. Thus, pressure rising and/or liquefaction are indispensable to convey and to store. MCH is the one of the promising hydrogen carrier because of its outstanding high hydrogen concentration comparable to liquefied hydrogen. We investigated MCH production through an addition reaction of toluene with existence of reductant, water, using two-chamber PEC cell combined with membrane-electrode assembly. The PEC cell with SrTiO3 photoanode prepared by particle transfer method and MEA composed of Pt loaded carbon and anion exchange membrane showed MCH production spontaneously. We will discuss about PEC MCH production in detail in the presentation.

Figure 1