Crystal Facet Exposure Of Monoclinic Photoanodes And Study On Their Photoelectrochemical Water Oxidation To H₂O₂

As a widely used environmentally friendly oxidant,the demand for hydrogen peroxide（H₂O₂）is increasing.Compared with the traditional anthraquinone oxidation method with high energy consumption and high transportation cost,electrochemical reduction of O₂ or water oxidation provides a cheaper option for the preparation of small-scale H₂O₂.Among them,water oxidation to H₂O₂ is a process using two electrons to catalyze water oxidation using water as raw material,which allows us to further utilize photocatalytic reactions to achieve the preparation of H₂O₂.However,currently reported photocatalysts,such as bismuth vanadate,tungsten trioxide,titanium dioxide and other metal oxides,have shown lower catalytic activity and selectivity,and the constraints that caused low efficiency are unclear.Because the catalytic activity of semiconductors is greatly affected by crystal facets,hence,this work takes monoclinic bismuth vanadate and tungsten trioxide as the research object,focusing on crystal fecet control strategies,improving the selectivity of 2-electron water oxidation（2e-WOR）.In this work,by adding crystal growth directing agents to the hydrothermal reaction,BiVO₄ photoanodes with different exposure degree of（040）crystal facets were successfully prepared,and polycrystalline BiVO₄ photoanode was also prepared as a reference.And the above photoanodes were applied to 2e-WOR.It was found that exposure of（040）crystal facet can improve the 2e-WOR selectivity of BiVO₄ photoanodes.Characterization techniques such as SEM and XRD proved that the exposure ratio of（040）crystal facets of the prepared polycrystalline BiVO₄（P-BiVO₄）,decagonal BiVO₄（（040）BiVO₄-D）,and plate BiVO₄（（040）BiVO₄-P） photoanodes were increased in order.The tests of photoelectrochemical performance showed that the Faraday efficiencies of P-BiVO₄,（040）BiVO₄-D,and（040）BiVO₄-P photoanodes for 2e-WOR are 30%,60%,and 85%,respectively.Cyclic voltammetry test showed that the electrochemically active areas of（040）BiVO₄-D and（040）BiVO₄-P were basically the same,thereby showing the exposure extent of（040）crystal facet had a direct relationship with the 2e-WOR selectivity of BiVO₄ photoanode.In this work,to improve the photocurrent density of（040）BiVO₄,BiVO₄ structure with a vertical（040）crystal facet（（040）BiVO₄-VP）was constructed.The vertical（040）crystal facets were interlaced to form a three-dimensional porous structure,which can increase the contacted area of the electrolyte with the（040）crystal facet,thereby improving the photo-generated charge transport at the solid-liquid interface.The results showed that the photocurrent density of（040）BiVO₄-VP（3.8 m A/cm²）was much higher than（040）BiVO₄-P（1 m A/cm²）and P-BiVO₄（2.8 m A/cm²）,and the Faraday efficiency remained at a high level（85%）.Experimental results showed that the vertical structure can increase the photocurrent density while maintaining high faraday efficiency.In this work,by applying（002）WO₃and polycrystalline WO₃ to photoeletrocatalytic 2e-WOR for demonstrating the universality of crystal facet engineering.The photocurrent density of（002）WO₃ photoanode reached 1.8 m A/cm² at 1.76 V vs RHE,which was about 1.8-fold higher than that of the polycrystalline WO₃photoanode（1 m A/cm²）.At the same time,the Faraday efficiency of the（002）WO₃ photoanode was 65%,which was much higher than that of polycrystalline WO₃（40%）.The above experimental results showed that it is also suitable for monoclinic WO₃ to improve the 2e-WOR selectivity by controlling the exposure of the （002） facet.