An Effective Way to Improve the Structural Stability and Photoelectrochemical Performance of BiVO4 Photoanodes in Basic Media: Surface Passivation with Zinc Ferrite

A photoelectrochemical water splitting technology (PEC) using sustainable solar energy has been considered as one of the most promising methods to produce directly renewable energy (i.e. H 2 ) from water. This system has still lots of challenges in improving water-splitting PEC efficiency. In particularly, development of electrode materials to covert efficiently solar energy to hydrogen is one of the most challenges facing many scientists and engineers in this field. Among the electrode materials for use in a water-splitting PEC cell, n-type bismuth vanadate, or BiVO 4 , has recently been identified as a promising metal oxide photoanode for O 2 evolution, because of a narrow band gap (2.4-2.5 eV) for absorbing substantial position of visible spectrum and a favourable conduction band edge position which is very near the thermodynamic hydrogen reduction potential. Most of the BiVO 4 studies for water-splitting PECs have mainly been investigated for use under neutral conditions (pH ∼7) because BiVO 4 is chemically unstable and gradually dissolves in strong basic and acidic solutions. When the operating conditions of BiVO 4 can be extended to basic or acidic media, BiVO 4 can be coupled to more diverse catalysts or photocathodes, which perform optimally only under basic or acidic conditions. Additionally, using basic or acidic media may offer an advantage of achieving higher solution conductivities without using additional supporting electrolytes or buffers for PEC operation. In order to solve such weakness, we tried to add spinel zinc ferrite (ZnFe 2 O 4 ) as a protection layer to use BiVO 4 photoanode in basic condition. A thin layer of ZnFe 2 O 4 was placed on the surface of a nanoporous BiVO 4 electrode using following process: (1) photodeposition of iron oxyhydroxide (FeOOH), (2) a mild chemical and thermal treatment of FeOOH with Zn precursor. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that a ZnFe 2 O 4 layer formed a uniform and conformal coating layer on a BiVO 4 particles, and the thickness of the layer was 10 -15 nm. The ZnFe 2 O 4 coating layer was very thin and was x-ray amorphous structure as evidenced by a conventional powder x-ray diffractometer. But, the selected area electron diffraction (SAED) clearly indicated that ZnFe 2 O 4 was spinel structure. The effect of the ZnFe 2 O 4 layer on the prevention of chemical dissolution of BiVO 4 in basic media in the dark was first tested by immersing BiVO 4 and BiVO4/ZnFe 2 O 4 electrode in a 0.1 M KOH solution (pH 13) for 72 h. The SEM images taken after 72 h of immersion showed that the ZnFe 2 O 4 -free BiVO 4 electrode was considerably dissolved, whereas the ZnFe 2 O 4 -coated BiVO 4 electrode did not show any detectable sign of dissolution. The effect of the ZnFe 2 O 4 layer on the photoelectrochemical properties and photostabilities of BiVO 4 was tested by measuring J−V and J-t plots in 0.1 M KOH (pH 13) under simulated AM 1.5G irradiation (100 mW/cm 2 ), using a three-electrode configuration. The obtained BiVO 4 /ZnFe 2 O 4 electrode generated a photocurrent density of 2.76 mA/cm 2 at 1.23 V vs. RHE with a significantly improved stability compared to the pristine BiVO 4 electrode (ca. 1.04 mA/cm 2 at 1.23 V vs. RHE). The incident and absorbed photon-to-current conversion efficiencies along with absorption spectra suggested that the ZnFe 2 O 4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron-hole separation of the BiVO 4 layer. In addition, when the surface of the ZnFe 2 O 4 layer was modified with Co 2+ ions as oxygen evolution reaction catalyst, the resulting BiVO 4 /ZnFe 2 O 4 /Co 2+ electrode generates a more improved photocurrent density (ca. 2.83 mA/cm 2 at 1.23 V vs. RHE) with a more significantly improved stability. These results suggest that further investigation of protection and catalyst layers can enable more stable and efficient operation of BiVO 4 -based photoanodes in basic media.