Applications Of Transition Metal Vanadium-based Co-catalysts In Photocatalytic Hydrogen Evolution

Environmental pollution and energy shortage caused by fossil fuel consumption make it urgent to explore efficient,environmentally friendly,and sustainable energy sources.Hydrogen energy,as a new energy source that can replace fossil fuel,has attracted extensive attention.Photocatalytic technology can be used to split water into H₂ and O₂ under sunlight by using semiconductor photocatalyst or dye-sensitized system,which is a promising technology to convert solar energy into hydrogen energy.Cocatalyst plays an important role in photocatalytic hydrogen evolution reaction.It can not only accelerate the separation and transfer of charge,delay the electron-hole pair recombination,but also provide more active sites and reduce the over-potential of hydrogen evolution reaction.Among many cocatalysts,precious metals,such as Pt,are widely used because of their high hydrogen evolution activity.However,considering the high cost and scarcity of precious metals,it is imperative to find an efficient and stable noble-metal-free cocatalyst to replace noble metals.Transition metal carbides,nitrides,and borides have good metallic electrical conductivity,which not only is advantageous to the fast electron transfer,but also can provide a kind of Pt-liked hydrogen evolution sites.In addition,they also have higher catalytic activity,selectivity,stability,and resistance to toxicity,as non-noble metal cocatalyst have been widely applied photocatalytic hydrogen evolution reaction.Based on the new transition metal vanadium-based cocatalyst（including vanadium carbide,vanadium nitride,and vanadium diboride）in the application of photocatalytic water splitting hydrogen evolution as the research content,the new and efficient dye-sensitization and semiconductor photocatalytic hydrogen evolution system were built,and the transition metal vanadium carbide,vanadium nitride,and vanadium diboride in the mechanism of photocatalytic water splitting hydrogen evolution system were illustrated,providing a train of thought for exploring new and highly efficient hydrogen evolution cocatalyst,and the following innovative results were obtained:（1）Under visible light irradiation（≥420 nm）,the three-component system of dye sensitization was constructed by using vanadium carbide（VC）as a hydrogen evolution cocatalyst,Er B dye as a photosensitizer,and TEOA as an electron donor.The results show that VC,as an effective hydrogen evolution cocatalyst,can catalyze hydrogen evolution reaction in molecular system under visible light irradiation.In addition,considering that organic dye photosensitizers can only absorb incident light of specific wavelength or narrow region,and are less stable than semiconductors,they will be rapidly degraded in the process of hydrogen production,the photocatalytic hydrogen evolution performance of VC in the semiconductor system under visible light irradiation was also studied.The results of photocatalytic hydrogen evolution experiment shows that when VC is coupled with semiconductor cadmium sulfide（CdS）,the hydrogen evolution activity of CdS/VC composite photocatalyst is nearly12 times higher than that of pristine CdS,and the quantum efficiency can reach up to8.7%.The results of electrochemical and photoelectrochemical tests show that VC is an efficient electrocatalyst,which can not only reduce the over-potential of H⁺/H₂O reduction to hydrogen,but also effectively promote the separation of photogenerated electron-hole pairs,thus effectively improving the photocatalytic hydrogen evolution performance of CdS/VC.（2）Vanadium nitride（VN）was combined with semiconductor CdS by simple solution mixing method to construct efficient CdS/VN composite photocatalyst,and the photocatalytic hydrogen evolution performance of the composite catalyst under visible light irradiation was studied with lactic acid as sacrifice reagent.The photocatalytic experimental results show that the efficient CdS/VN composite photocatalyst shows better photocatalytic hydrogen evolution activity than CdS,about6.7 times of the activity of CdS alone,and maintains good hydrogen production stability after 30 h stability test.Studies on the mechanism show that VN,as a cocatalyst,can not only promote the transfer of interface charge from CdS to VN,so as to enhance the separation efficiency of photogenerated charge,but also can effectively catalyze hydrogen evolution due to its low overpotential,which may be the main reason for the improvement of photocatalytic activity.（3）Using Vanadium diboride（VB₂）as a hydrogen evolution cocatalyst and CdS as a photosensitizer,a CdS/VB₂ photocatalyst was constructed and its photocatalytic hydrogen evolution performance under visible light irradiation was discussed.The results showed that the photocatalytic hydrogen evolution activity of CdS/VB₂composite photocatalyst in aqueous solution of 10 vol.%lactic acid（LA）was about11 times higher than that of pure CdS.Fluorescence analysis and photoelectrochemical test results show that the enhanced photocatalytic hydrogen evolution activity of CdS/VB₂ composite photocatalyst can be attributed to the good cocatalytic effect of VB₂,which can not only promote the charge separation of CdS,but also accelerate the kinetics of hydrogen evolution reaction,so as to achieve efficient photocatalytic hydrogen evolution reaction.