Insight into the PEC and interfacial charge transfer kinetics at the Mo doped BiVO4photoanodes

Sriram Kumar,Satyaprakash Ahirwar,Ashis Kumar Satpati

doi:10.1039/c9ra08743e

Abstract

BiVO4 is a promising photoanode material for the photoelectrochemical (PEC) oxidation of water; however, its poor charge transfer, transport, and slow surface catalytic activity limit the expected theoretical efficiency. Herein, we have investigated the effect of Mo doping on SnO2 buffer layer coated BiVO4 for PEC water splitting. SnO2 and Mo doped BiVO4 layers are coated with layer by layer deposition through a precursor solution based spin coating technique followed by annealing. At 5% doping of Mo, the sample (SBM5) shows a maximum current density of 1.65 mA cm−2 at 1.64 V vs. RHEl in 0.1 M phosphate buffer solution under AM 1.5 G solar simulator, which is about 154% improvement over the sample without Mo (SBM0). The significant improvement in the photocurrent upon Mo doping is due to the improvement of various bulk and interfacial properties in the materials as measured by UV-vis spectroscopy, electrochemical impedance spectroscopy (EIS), Mott–Schottky analysis, and open-circuit photovoltage (OCPV). The charge transfer kinetics at the BiVO4/electrolyte interface are investigated to simulate the oxygen evolution process in photoelectrochemical water oxidation in the feedback mode of scanning electrochemical microscopy (SECM) using 2 mM [Fe(CN)6]3− as the redox couple. SECM investigation reveals a significant improvement in effective hole transfer rate constant from 2.18 cm s−1 to 7.56 cm s−1 for the hole transfer reaction from the valence band of BiVO4 to [Fe(CN)6]4− to oxidize into [Fe(CN)6]3− with the Mo doping in BiVO4. Results suggest that Mo6+ doping facilitates the hole transfer and suppresses the back reaction. The synergistic effect of fast forward and backward conversion of Mo6+ to Mo5+ expected to facilitate the V5+ to V4+ which has an important step to improve the photocurrent.

Highlights

Photoelectrochemical (PEC) splitting of water is one of the most promising methods for simultaneous conversion of hydrogen from water using solar energy as a sustainable and clean energy source and zero carbon footprint; the process has inherently high power and energy densities.[1,2,3,4,5] PEC water splitting consists of a photoanode and photocathode on which an oxygen evolution reaction (OER) and a hydrogen evolution reaction (HER) respectively are taking place
The increase in the at band potential and open circuit photovoltage (OCPV) suggests the improvements in the charge separation upon the Mo doping which resulted in the enhancement in PEC efficiency.[63,64,65,66]
Strong correlation among the optical property of the material, the open circuit photovoltage (OCPV), and onset potential was observed in relation to the improvement in the PEC efficiency on Mo doping

Summary

Introduction

The SnO2 underneath of BiVO4 blocks the surface state of the ITO/FTO. These modi cations improve the injection of the photogenerated holes to the electrode–electrolyte interface. Photoelectrochemical measurements were performed using the CH Instrument (920 D model) using a three-electrode cell with an Ag/AgCl (3.0 M KCl) reference electrode, glassy carbon rod as counter and modi ed ITO coated with the catalyst material as the working electrode. Photoanode materials were used as the substrate, Ag/AgCl (3 M KCl) as a reference and glassy carbon rod as a counter electrode and results are reported in terms of RHE potential. Probe Approach Curve (PAC) technique was used to record the approach curve to the substrate in dark and under the illumination of light to measure the kinetic parameter using four electrodes system at different polarization potentials, from the tting of the probe approach plots the interfacial charge transfer kinetics were obtained. X-ray photoelectron spectroscopy (XPS, MULTILAB, VG Scienti c, Al Ka radiation as monochromator) was used to investigate the binding energy of the components of Mo doped BiVO4 photoanodes

Findings

Result and discussion

Conclusion