Preparation Of High Performance BiVO₄-based Composite Photoanodes And Study On The Photoelectrochemical Water Splitting Performance

With the emergence of energy shortage and environmental pollution,photoelectrochemical(PEC)water splitting is regarded as the most attractive technology to produce hydrogen and oxygen in a clean and sustainable manner.Among various metal semiconductors,BiVO4 has attracted much attention due to its ideal bandgap(～ 2.4 eV),long carrier lifetime(～ 40 ns),suitable band position as well as chemical stability in aqueous media.However,low carrier mobility(～4 × 10-2 cm2 V-1 s-1),poor charge separation/transport,serious surface charge recombination as well as sluggish water oxidation kinetics greatly restrict its the photoconversion efficiency,resulting in its photocurrent densities are far below its theoretical value(7.50 mA cm-2)under AM 1.5G(100 mW cm-2)irradiation.To settle these issues,this thesis focuses on designing and preparing two kinds of novel high-performance and steady BiVO4 based composite photoanodes and then researches their applications in PEC water splitting to produce hydrogen and oxygen.This work points to the effective utilization of cheap borate and cocatalysts on photoanodes and provides a strategy for designing high performance photoanodes in PEC water splitting.Specific research results are as follows:(1)The β-FeOOH-B-BiVO4 composite photoanode was prepared via a two-step simple solution immersion method.It is found that the as-preparedβ-FeOOH-B-BiVO4 photoanode shows an excellent photocurrent density of 4.96 mA cm-2 at 1.23 V vs.RHE,over 3-fold larger than that of pure BiVO4(1.45 mA cm-2)photoanode and also superior to that of B-BiVO4 photoanode(3.80 mA cm-2).Furthermore,the applied bias photon-to-current efficiency and the surface charge transfer efficiency of β-FeOOH-B-BiVO4 photoanode are determined as 1.94%(ac.0.62 V vs.RHE)and 90.2%(1.23 V vs.RHE),respectively.The results show that the co-modification of borate andβ-FeOOH cocatalyst is beneficial to inhibit charge recombination,improve charge transport ability,reduce onset potential and accelerate water oxidation kinetics to boost the photoconversion efficiency of BiVO4 photoanode.Additionally,the morphology and photocurrent density of theβ-FeOOH-B-BiVO4 photoanode remains stable during the 20 h continuous illumination by tuning the composition of the electrolyte solution,indicating the good stability of as-prepared photoanode.(2)The B-BiVO4/FexCo1-xOOH composite photoanode was prepared by loading the FexCo1-xOOH cocatalyst on the borate-treated BiVO4 photoanode via a simple pH control solution immersion method.It is found that the optimized B-BiVO4/FexCo1-xOOH photoanode displays an outstanding photocurrent density than that of BiVO4(1.14 mA cm-2)and B-BiVO4(3.26 mA cm-2)photoanodes,yielding an extremely high photocurrent density of 5.21 mA cm-2(1.23 V vs.RHE)with an excellent photostability of over 10 h,as well as the applied bias photon-to-current efficiency is 2.05(～0.60 V vs.RHE).Moreover,the surface charge transfer efficiency and the incident photon-current efficiency of B-BiVO4/FexCo1-xOOH photoanode are determined as 90.7%(1.23 V vs.RHE)and 82.2%(420 nm),respectively.The experimental results show that the borate species as a passivator to inhibit charge recombination and thus provide an unobstructed transfer pathway for holes.Meanwhile,the synergy of Fe and Co species in FexCo1-xOOH cocatalyst is demonstrated to be beneficial to activity enhancement of the oxygen evolution and the BiVO4 photostability.