Photoelectrochemical Water Splitting By Using a Porous ZnRh2O4 Photocathode Under Visible Light Irradiation and Significant Effect of Necking Treatment
Authors: Kamimura Sunao , Ohno Teruhisa 
Published: February 2020

Photoelectrochemical (PEC) water splitting utilizing semiconductor electrodes has attracted considerable attention as a potential means of converting solar energy into chemical energy in the form of usable hydrogen, which is a clean and renewable energy. To date, there have been many studies on the oxide-based photoanodes such as WO3,Fe2O3,and BiVO4,but there have been few reports on the oxide-based photocathodes. Examples of the latter include Cu2O, Rh-doped SrTiO3, and CaFe2O4 photocathodes, which are p−type oxide semiconductors with visible light responses.
We have successfully developed a porous ZnRh2O4 electrode by an electrophoretic deposition method on a fluorine−doped tin oxide (FTO) substrate. In addition, we have investigated the PEC water splitting performance in a three−electrode configuration using a silver−silver chloride (Ag/AgCl) reference electrode and a Pt coil counter electrode under visible light irradiation (λ>420 nm). The electrolyte was 0.1 M Na2SO4 solution (pH 6). The electrolyte was stirred and purged with Ar gas for 30 min before measurement. The ZnRh2O4 electrode exhibited a cathodic photocurrent in response to irradiation of visible light, indicating that ZnRh2O4 is a p−type oxide semiconductor. The cathodic photocurrent of ZnRh2O4 was increased with an increase in the applied potential, and its photocurrent density reached –110 mA/cm2 at 0 V vs. RHE. The onset potential of ZnRh2O4 was ca. +1.20 V vs. RHE, which compares favorably with any p−type semiconductor oxide such as Cu2O (ca. +0.55 V vs. RHE), CaFe2O4 (+1.24 V vs. RHE), and CuBi2O4 (+1.05~1.1 V vs. RHE). For further improvement of the photocurrent, we attempted to do necking treatment over the ZnRh2O4 electrode by using a zinc oxide (ZnO) precursor solution. Since p−n heterojunction diodes composed of p–type ZnRh2O4 and n–type ZnO have been reported,

the following two significant effects was anticipated by ZnO necking treatment: (1) bridging ZnRh2O4 particles with ZnO for smooth carrier conduction, and (2) a depletion layer formed at the ZnRh2O4/ZnO p–n junction could also assist in extracting photo–generated electrons from ZnRh2O4. Actually, the photocurrent of ZnO–treated ZnRh2O4 photocathode was enhanced to ca. two−fold higher than that of the bare electrode. The incident photon to charge carrier efficiencies of the ZnRh2O4 and ZnO–treated ZnRh2O4 photocathodes were calculated to be ca. 8% and ca. 13% at 400 nm, respectively, at 0 V vs. RHE in aqueous Na2SO4 solution. Evolved H2 gas was detected by an on−line gas chromatograph (GC) with a thermal conductivity detector (Agilent Technology Co. MicroGC) equipped with MS−5A column. The hydrogen evolution from ZnRh2O4 and ZnO–treated ZnRh2O4 photocathodes was confirmed with the faradaic efficiency of ca. 77% and ca. 85%, respectively.
We will describe the PEC performance of ZnRh2O4 and ZnO–treated ZnRh2O4 photocathodes in poster session, and discuss the mechanism of photocurrent improvement by ZnO necking treatment.