Preparation And Properties Of BiVO₄ Based Composite Photoanode

Photoelectrochemical（PEC）water splitting is a process that converts solar energy into chemical energy and stores it,providing a promising strategy for renewable hydrogen fuel production.The key of PEC water decomposition technology lies in the design of efficient and stable photoelectrode,which not only has enough light absorption,but also has high efficiency of charge separation and transmission,as well as certain stability.BiVO₄is one of the most promising photoelectrode materials due to its inherent advantages such as low starting potential,narrow band gap（about 2.4 e V）,favorable band edge position and good stability.However,the hole diffusion length of BiVO₄is short,and the carrier mobility and surface charge transfer efficiency are poor,which leads to most of the photogenerated holes recombination with electrons in the BiVO₄bulk phase and are consumed at the electrode/electrolyte solution interface before participating in the water oxidation reaction.In order to solve the above problems,BiVO₄based composite photoanodes were prepared by doping the transition metal elements Mo,W and Co,doping the non-metallic element B,and supporting layered bimetallic hydroxide（LDH）as co-catalyst.The composition and structure of the composite photoanode were characterized in detail.The photochemical properties of the composite photoanode were measured and the mechanism of its performance improvement was discussed.The main research conclusions are as follows:The crystal structure and light absorption characteristics of BiVO₄were not significantly changed by elemental doping.W⁶⁺and Mo⁶⁺entered BiVO₄structure by doping instead of V⁵⁺site,and oxygen defects were generated as shallow donor to enhance the carrier concentration and n-type conductivity,and the photocurrent density was increased by 32%and 76%compared with that before modification.Co²⁺and B³⁺enter BiVO₄crystal structure by replacing Bi³⁺site and interstice doping.Co²⁺doping also increases the concentration of carrier,and the photocurrent density of Co:BiVO₄reaches 4.88 m A/cm²at 1.23 V_RHEand 98%of bulk phase charge separation efficiency.After B doping,it acts as a ligand and passivation site in the bulk phase,inhibits bulk charge recombination and induces secondary micro-nano structure nano flowers on the surface,increasing the electrochemically active surface area of the photoanode.Bulk and ultra-thin CoAl-LDH nanosheets were prepared by hydrothermal method and co-precipitation method.Mo:BiVO₄/CoAl-LDH composite photoanode was prepared by impregnation method.The systematic characterization and research showed that Mo doping increased the bulk phase charge separation efficiency of BiVO₄,while CoAl-LDH and Mo:BiVO₄formed an n-n heterojunction with energy band matching,which improved the surface charge transfer efficiency.In addition,the change of valence state from Co²⁺to Co³⁺in CoAl-LDH can quickly capture the photogenerated holes,accelerate the proton transfer step in the water oxidation process and partially change the OER mechanism to lattice oxygen mechanism,thus speeding up the kinetic rate of water oxidation.Under the synergistic effect of the above two,the optimal composite photoanode can achieve a low starting potential of 0.26V_RHE,a photocurrent density of 5.8 m A/cm²at 1.23 V_RHEand a surface charge transfer efficiency of 98.4%.