An Effective Way to Improve the Structural Stability and Photoelectrochemical Performance of BiVO4 Photoanodes in Basic Media: Surface Passivation with Zinc Ferrite
Authors: Kim Tae Woo , Choi Kyoung-Shin 
Published: February 2020

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. H2) 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 BiVO4, has recently been identified as a promising metal oxide photoanode for O2 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 BiVO4 studies for water-splitting PECs have mainly been investigated for use under neutral conditions (pH ∼7) because BiVO4 is chemically unstable and gradually dissolves in strong basic and acidic solutions. When the operating conditions of BiVO4 can be extended to basic or acidic media, BiVO4 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 (ZnFe2O4) as a protection layer to use BiVO4 photoanode in basic condition.
A thin layer of ZnFe2O4 was placed on the surface of a nanoporous BiVO4 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 ZnFe2O4 layer formed a uniform and conformal coating layer on a BiVO4 particles, and the thickness of the layer was 10 -15 nm. The ZnFe2O4 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 ZnFe2O4 was spinel structure.
The effect of the ZnFe2O4 layer on the prevention of chemical dissolution of BiVO4 in basic media in the dark was first tested by immersing BiVO4 and BiVO4/ZnFe2O4 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 ZnFe2O4-free BiVO4 electrode was considerably dissolved, whereas the ZnFe2O4-coated BiVO4 electrode did not show any detectable sign of dissolution. The effect of the ZnFe2O4 layer on the photoelectrochemical properties and photostabilities of BiVO4 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/cm2), using a three-electrode configuration. The obtained BiVO4/ZnFe2O4 electrode generated a photocurrent density of 2.76 mA/cm2 at 1.23 V vs. RHE with a significantly improved stability compared to the pristine BiVO4 electrode (ca. 1.04 mA/cm2 at 1.23 V vs. RHE). The incident and absorbed photon-to-current conversion efficiencies along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron-hole separation of the BiVO4 layer. In addition, when the surface of the ZnFe2O4 layer was modified with Co2+ ions as oxygen evolution reaction catalyst, the resulting BiVO4/ZnFe2O4/Co2+ electrode generates a more improved photocurrent density (ca. 2.83 mA/cm2 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 BiVO4-based photoanodes in basic media.