Preparation Of Bismuth Vanadate-based Electrode Materials And Study On Their Photoelectrochemical Water Splitting Performance

Due to the increasing energy shortage and global warming,the development of clean and efficient new energy sources has become an urgent task.Hydrogen energy is considered as one of the ideal new energy sources due to its combustion product of water with no carbon emission and high combustion energy density.One of the most promising ways to produce hydrogen is to use solar energy for photoelectrochemical（PEC）decomposition of aquatic hydrogen.BiVO₄ is composed of inexpensive elements,simple to prepare,highly stable in aqueous electrolytes,suitable band gap structure（2.4～2.6 e V）,and high light absorption,which make it widely used as photoelectrodes in photoelectrochemical hydrolysis.However,the PEC activity of BiVO₄ in practical applications is weakened due to the serious carrier recombination rate,low electron mobility,and slow reaction kinetics.In this paper,nanoporous BiVO₄photoelectrodes were prepared by electrodeposition method and heating treatment.Based on this,the BiVO₄ photoanode was modified by a series of improved processes,thus improving the PEC performance of BiVO₄;as well as the BiVO₄-based photoanode and photocathode were prepared by modulating the experimental preparation method,and the bias-free solar water decomposition device was achieved.The main contents are as follows:（1）The introduction of oxygen vacancies（O_V）into photoanodes has been considered an effective method to enhance the PEC water splitting performance.We present the fabrication of BiVO₄ photoanodes with bulk and surface O_V,and their respective roles in the PEC performance have been studied.The bulk O_V in as-prepared photoanode could increase carrier density and improve photogenerated electrons and holes separation efficiency.The surface O_V provide abundant surface active sites,and enhance the charge injection efficiency.Charge separation efficiency of the nitrogen-treated BiVO₄（N:BiVO₄）（69.1%at 0.75 V vs.RHE and 85.1%at 1.23 V vs.RHE）has a noticeable increase compared with that of BiVO₄（51.2%at 0.75 V vs.RHE and 64.6%at 1.23 V vs.RHE）,nevertheless,only a minor enhancement of charge injection efficiency（from 49.1%to 56.5%at 1.23 V vs.RHE）.After the deposition of Ni Fe OOH,the photoanodes present superior charges injection efficiency in the whole range of applied potential.The as-synthesized N:BiVO₄/U-Ni Fe OOH photoanode exhibits a photocurrent density of 5.52 m A·cm^-2 at 1.23 V vs.RHE with a 97%Faradaic efficiency for O₂/H₂ evolution.Thus,there is a synergistic effect between the bulk O_V and surface O_V on the BiVO₄ photoanode,exhibiting highly promoted PEC water splitting activity relative to the individual O_V decorated BiVO₄ for oxygen evolution reaction.（2）The BiOI nano-sheet was prepared by electrodeposition,and the nano-porous BiVO₄ was prepared by heat treatment,then Cu₂O was compounded by the second electrodeposition method.Due to the different voltage and the different growth position of Cu₂O,the forward and reverse p-n heterojunction of BiVO₄/Cu₂O is formed,and BiVO₄/Cu₂O photoanode and BiVO₄/Cu₂O-C photocathode were successfully prepared.Under the combined action of the built-in electric field of the p-n heterojunction and the Co-Pi co-catalyst layer,the photocurrent density of the optimized BVO/Cu₂O/Co-Pi photoanode reaches 1.97 m A·cm^-2 at 1.23 V vs.RHE.At the same time,the deposition of Cu₂O in BiVO₄ reversed the arrangement of BiVO₄/Cu₂O heterostructures,promoted the carrier separation under the action of the reverse internal electric field.The optimized BiVO₄/Cu₂O-C photocathode photocurrent density reached-1.54 m A·cm^-2 at 0 V vs.RHE.This chapter provides a new idea for the preparation of photoanode and photocathode.（3）BiVO₄ films are firstly synthesized,followed by electrodepositing of Cu₂O to construct BiVO₄/Cu₂O heterojunction electrodes for photoelectrochemical water splitting.By regulating the electrodeposition potential,it is possible to control that Cu₂O is deposited on the surface or inside of BiVO₄,which enables them n-type or p-type semiconductivity behavior for oxygen and hydrogen evolution reactions,respectively.BiVO₄/Cu₂O photoanode and BiVO₄/Cu₂O-C photocathode were successfully prepared.On the premise of preparing BiVO₄/Cu₂O photoanode and BiVO₄/Cu₂O-C photocathode,the performance of photoanode and photocathode were optimized.The photocurrent density of the optimized BiVO₄/Cu₂O/Ni Fe-LDH photocathode reaches 5.01 m A·cm^-2 at 1.23 V vs.RHE,and the photocurrent density of the optimized BiVO₄/Cu₂O-C/Ti O₂ photocathode reaches-2.35 m A·cm^-2（0 V vs.RHE）.Finally,the photocathode and photoanode are connected in series to form the BiVO₄/Cu₂O/Ni Fe-LDH—BiVO₄/Cu₂O-C/Ti O₂ series unbiased decomposition water system.The J_op of the series system is 0.98 m A·cm^-2 without applying external voltage,and the STH conversion efficiency is about 1.18%.It can be widely used in the field of solar water oxidation.