Microstructure-Controlled Synthesis And Electrocatalytic Water Splitting Of Vanadium Nitride

Hydrogen has a high energy density,and its combustion is environmentally friendly water,therefore utilizing the renewable energy to produce hydrogen is regarded as an important measure to solve the problems of energy shortage and environmental pollution.The energy conversion device based on the electrolytic water splitting has attracted much attention,but its energy conversion efficiency is restricted by the reaction of cathodic hydrogen evolution and anodic oxygen evolution.Therefore,highly active catalysts are required to reduce the overpotentials of the corresponding reactions and thus improve the efficiency of hydrogen production.At present,the high performances electrocatalysts are still these precious Pt/Pd-based electrocatalyst,but their large-scale commercial application is severely limited its high price.Inspired by the structure of hydrogenase,transition metal nitride is a kind of non-precious metal catalyst with platinum-like electronic structure and catalytic behavior,and is expected to replace precious metal electrocatalyst in electrolytic water splitting.Among transition metal nitrides,vanadium nitride（VN）has attracted much attention due to its high catalytic activity,excellent electronic conductivity,and corrosion resistance.However,because the synthesis process involves high temperature calcination process,which damages the structure of VN to limit its catalytic performances.Moreover,the d band center of V in VN is too high,which leads the VN with the too strong adsorption of HER reaction intermediates,further resulting in the sluggish reaction rate.Considering the above factors,we highlight the microstructure-controlled synthesis of vanadium nitride by nano-engineering technologies,such as constructing of novel nano-geometric structure,atomic-interface engineering,and single atom doping engineering,as well as advanced characterization techniques,and finally achieves the optimization of vanadium nitride geometric structure to improve the active areas/sites and mass transfer capacity,and meanwhile also optimization of their atomic and electronic structure and binding energies of intermediates to boost the electrocatalytic kinetics and activity.The details of this dissertation are summarized briefly as follow:（1）In view of the significant roles of active sites and mass transfer capacity of the electrocatalyst on the activity and kinetic rate of hydrogen evolution reaction,this chapter mainly focus that the VN with double-shell structure was prepared by ammoniation of the dopamine hydrochloride coated V-based nanosphere precursors.The experimental results show that the double-shell VN can effectively increase the effective active area to expose the catalytic sites and promote the mass transfer of electrolyte and target hydrogen gas in comparison with that of VN bulk nano spheres.As a result,the double-shell VN showed significantly improved electrocatalytic hydrogen evolution activity and kinetic in full pH range electrolytes.（2）By using of the ZIF-67 as self-template and NH₄VO₃ as V-anion source,the precursor of V-ZIF-67 was prepared by ion exchange under hydrothermal conditions.After subsequent high temperature ammonification,the VN/Co_5.47N heterojunction nanopolyhedron was prepared.Due to the different atomic arrangement and electron structure of VN and Co_5.47N,atomic disorder and electron transfer between interfaces were induced.Interfacial disordered atoms can increase the number of active sites,and the electron transfer between interfacial components can optimize the free energy of the intermediates.Benefiting from these above advantages,VN/Co_5.47N displayed significantly improved performances towards alkaline hydrogen evolution,oxygen evolution,and overall water splitting.（3）The high d-band center of VN leads to it with a strong adsorption between V and H species,which.is not conducive to the desorption of target H2 and fully limits the kinetics and activity of hydrogen evolution reaction.In this chapter,Co modified V-MIL-88B precursors（Co,V-MIL-88B）were firstly synthesized via a hydrothermal method,and then were ammoniated to obtain the VN stabilized Co single atom（Co-VN）.The results of experimental characterization and theoretical calculation indicated that Co atom replace the V site in VN,and can transfer electrons into the adjacent V atomic orbitals,which reduces the d-band center of adjacent V atoms to further optimize the Gibbs free energy of hydrogen intermediates,and finally improves the hydrogen evolution activity and kinetic of intrinsic VN.