Preparation Of High-performance Bismuth Vanadate Photoanodes And Their Application In Photoelectrochemical Wastewater Treatment Coupled With Hydrogen Production

As the energy crisis and environmental pollution become increasingly prominent,developing renewable and green energy production technology has become a common concern of humankind.With the advantages of high energy density and no secondary pollution after combustion,hydrogen has been considered as one of the most promising energy sources in the future.Photoelectrochemical（PEC）hydrogen production technology can convert unlimited solar energy into hydrogen energy,which has been proved a promising hydrogen energy production strategy.The photoanode plays a key role in PEC hydrogen production system,therefore,developing efficient photoanode has important significance to promote the industrial application of PEC hydrogen production system.An ideal PEC photoanode should possess strong light harvesting ability,especially the utilization of visible light,fast separation and transfer of photo-induced electon-hole pairs,high photo-to-current conversion efficiency and good stability of long-term operation.Among many kinds of semiconductors being studied,BiVO₄ has been recognized as one of the most promising photoanode semiconductors with a narrow band gap of 2.4 e V for visible light capture.However,due to its short carrier diffusion length,the recombination of photo-induced charge carriers is serious,which limits its oxidation ability.In addition,the photo-corrosion phenomenon of BiVO₄ photoanode is easy to occur in aqueous medium,causing inactivation.Therefore,the design and preparation of stable and efficient BiVO₄ photoanode has become an urgent problem to be solved in the field of PEC research.In addition,in order to realize a more green and sustainable PEC hydrogen production system,combining PEC hydrogen production with wastewater treatment is recognized as a reasonable strategy to solve the environment-energy issues.In this paper,morphology and facet control and novel oxygen evolution catalyst loading strategies are employed to enhance the PEC performance of BiVO₄ photoanode.And the mechanism of prepared photoanodes in the application of PEC antibiotic wastewater degradation coupled hydrogen production and PEC water splitting were explored in detail.The main research contents are as follows:（1）Morphology and facet engineering are effective strategies to prepare high-performance PEC photoelectrode.WO₃/BiVO₄ heterojunction photoanodes with different morphologies were prepared by controlling the charge amount of electrodeposition.Among them,the coral-like WO₃/BiVO₄ photoanode exhibited the optimal PEC performance due to the orientation growth of the（110）and（011）active facets of BiVO₄ top layer.The exposure of（110）and（011）active facets significantly improves the separation and transfer of photoinduced electron-hole pairs,while the exposure of the（-121）facet of BiVO₄ top layer will have a negative effect on the PEC performance of the photoanode.In addition,the coral-like structure of prepared photoanode also boosts the light-harvesting ability and affords numerous active sites on the photoanode surface.The photocurrent densities of the coral-like WO₃/BiVO₄photoanode for sulfite oxidation and water oxidation are 4.71 mA cm^-2 and 2.9 mA cm^-2,respectively.With the deposition of FeOOH thin layer,the photocurrent density of coral-like WO₃/BiVO₄@FeOOH photoanode for water oxidation is further increased to 3.7 mA cm^-2.（2）The prepared coral-like WO₃/BiVO₄@FeOOH photoanode was applied to construct a dual-functional PEC-Cl system for sulfamethoxazole（SMX）wastewater degradation concurrent with clean hydrogen production.The results demonstrated that the constructed PEC-Cl system can efficiently remove the SMX and reduce its biological toxicity.Various influencing factors for SMX removal were investigated,including different systems,external bias,solution p H and Cl^-concentration,and the repeatability of the system was also tested.The SMX removal was enhanced with higher external bias voltage,Cl^-concentration and lower solution p H,but the best conditions should be selected according to the actual situation.In addition,the active oxidation species for SMX removal were determined using different radical scavengers.The results showed that active chlorine species（RCS）,especially Cl₂·^-and Cl O·,were the main active oxidants for SMX removal in the system.The bioluminescence inhibition test was conducted to assess the toxicity of SMX wastewater with different degradation time.The decomposition intermediates and degradation pathway of SMX using the PEC-Cl system was also analyzed by LC-MS.Finally,the mechanism of the dual-functional system dominated by the coral-like WO₃/BiVO₄@FeOOH photoanode was proposed.（3）The effective removal of low concentration antibiotic wastewater has always been a challenge in the field of wastewater treatment.CoNi（metal-organic framework）MOF nanosheets modified W-BiVO₄ photoanode（W-BiVO₄@CoNi-MOFN photoanode）was prepared for the first time.The decoration of CoNi MOF nanosheets effectively promoted the separation and transfer of charge carriers of W-BiVO₄photoanode.The photocurrent of W-BiVO₄@CoNi-MOFN photoanode is 3.92 mA cm^-2 at 1.23 V_RHE,which is about 2.3 times higher than that of W-BiVO₄ photoanode(1.74mA cm^-2).Compared with the PEC system constructed using W-BiVO₄ photoanode,the PEC system constructed by W-BiVO₄@CoNi-MOFN photoanode exhibited significantly improved degradation capacity of low-concentration tetracycline wastewater.On the one hand,the decoration of CoNi MOFs optimized the PEC performance of W-BiVO₄ photoanode.Therefore,more holes and active oxidants can be generated for tetracycline decomposition.On the other hand,due to the theπ-πstacking and hydrogen-bond interaction,the W-BiVO₄@CoNi-MOFN photoanode displays significantly enhanced capature capacity for tetracycline,which further improves the removal of tetracycline.In addition,the W-BiVO₄@CoNi-MOFN photoanode also shows excellent stability.The results of TOC and fluorescence inhibition tests demonstrate that the constructed PEC system can effectively mineralize tetracycline and reduce its biotoxicity,and also has a high hydrogen evolution efficiency.（4）Recently,MOF has been proved a very effective oxygen evolution catalyst（OEC）for improving the water oxidation performance of photoanodes.However,the types of organic ligands used for the preparation of MOF-based OEC are relatively single,and terephthalic acid（TPA）is the most used one,which is not conducive to the diversified development of MOF-based OECs.On the other hand,in order to ensure the stability of MOF-based OECs,cobalt and nickel ions are usually employed as coordinated metal precursors,which are harmful to the ecosystem and human health.For the first time,2D layered FcZn MOFs modified W-BiVO₄ photoanode（W-BiVO₄@FcZn-MOF）was prepared via a solvothermal process using 1,1’-ferrocenedicarboxylic acid（FCDA）as organic ligand.The synergy of Fc layer and FcZn MOFs can efficiently promote the separation and transfer of photogenerated electron-hole pairs,and also improve the light harvesting ability,thus improving the PEC performance of W-BiVO₄ photoanode.4.24 mA cm^-2 of photocurrent was achieved at 1.23 V_RHE in 0.5 M Na₂SO₄ under illumination of 100 m W cm^-2,which is almost 3.5 times as high as that of bare BiVO₄(1.22 mA cm^-2).Especially,the stability of W-BiVO₄@FcZn-MOF photoanode for water oxidation was obviously improved which can operate for 20 hours without no attenuation.