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A porous and thick photoelectrode of WO3 in the monoclinic phase was prepared to realize the recovery of H2 and high-value-added oxidation reagents with efficient solar energy conversion. The WO3 photoelectrode enabled the efficient production and accumulation of O2 , S2 O8 (2-) , Ce(4+) , and IO4 (-) as oxidation products. Most notably, S2 O8 (2-) , which possesses the highest oxidizability among all the peroxides, was generated with high applied bias photon-to-current efficiency (2.2 %) and faraday efficiency (≈100 %) upon irradiation from the back side of the photoelectrode. The design of a tandem photoelectrode system combining a dye-sensitized solar cell (DSSC) was also challenged for the realization of this photoelectrode system without external bias. A high solar energy conversion efficiency (5.2 %) was achieved in the tandem system comprising the WO3 photoelectrode connected to two DSSCs with a near-IR-utilizing dye in series for the production of H2 and S2 O8 (2-) .
Fuku et al. (Tue,) studied this question.