Preparation And Electrocatalytic Properties Of Vanadium Nitride-Based Electrocatalysts

Because of the excessive use of fossil fuels and the rising severity of environmental pollution,the development of renewable energy sources and highefficiency energy technologies is a pressing task.The production of hydrogen through water electrolysis is regarded as an effective method for obtaining green energy,and fuel cell as a power generation technology is very forward-looking.Slow chemical kinetics and high reaction barriers,however,limit their effectiveness and large-scale application.The use of commercial noble metal catalysts on a broad scale is severely limited due to their high cost and limited availability.As a result,the viability of the above two technologies still depends on the accessibility of earth-rich,non-noble metal-based electrocatalytic materials.Vanadium nitride,as a typical transition metal nitride,has similar properties to Pt,but its catalytic performance can not meet the industrial needs.As another candidate for forming non-noble metal-based catalysts,transition metal phosphates are characterized by insufficient catalytic active metal components and lack of appropriate charge carriers in the composition,it still has unsatisfactory catalytic activity and stability.Multiple heterostructures can be constructed,which can increase active sites,affect the material’s electrical structure or energy band structure,and optimize chemical kinetics.Therefore,we designed and constructed the heterogeneous structure electrocatalysts of vanadium nitride and transition metal phosphates reasonably,which can not only keep the excellent characteristics of single-phase,electron transfer or coupling will also occur at the heterogeneous interface to improve the catalytic performance.In this thesis,vanadium nitride is taken as the research object,and multiple heterostructures are constructed by combining with different transition metal phosphides through interface engineering.A reasonable combination can induce the redistribution of interface electrons and adjust the state of surface charge to promote the kinetics of water decomposition.At the same time,the intrinsic rule of structure modulation and functionality of vanadium nitride and transition metal phosphide composite phase are simultaneously investigated using structure characterization and performance tests.The following are the primary tasks:(1)VN/MoP@NC samples were created using a straightforward one-step solid-state sintering technique using sodium molybdate dihydrate,vanadium chloride,dicyandiamide,and sodium hypophosphite monohydrate as raw materials.The influences of annealing temperature and molybdenum concentration on VN/MoP@NC phase,morphology,and electrocatalytic characteristics were examined.The heterojunction constructed by interface engineering increased the active surface area and produced strong coupling effect,and its catalytic activity of HER was outstanding(114 mV@10 mA cm-2)in alkaline medium,and the catalyst was stable for 50 h.(2)VN/WP@NC samples were synthesized from sodium tungstate dihydrate,vanadium chloride,dicyandiamide and sodium hypophosphite monohydrate.The effects of sintering temperature and tungsten content on VN/WP@NC materials were investigated.Due to the higher instability of tungsten,it can bring higher activity,and its abundant high-energy flaws can improve the electrocatalytic hydrogen production performance of the composite phase.The prepared VN/WP@NC material showed excellent catalytic activity for HER(99 mV@10 mA cm-2)in alkaline environment and was stable for 120 h.(3)A straightforward one-step "Inside-out" gas-solid reaction procedure was used to create Co/Co2P/VN nanocomposites embedded in nitrogen-doped carbon framework using cobalt nitrate hexahydrate,vanadium chloride,dicyandiamide,and sodium hypophosphite monohydrate as raw materials.Co metal nanocrystals induce the formation of carbon nanotubes and improve the electrical conductivity of the materials.The rich interface heterostructure have a positive effect on the catalytic process of Co/Co2P/VN.Co/Co2P/VN has trifunctional catalytic activity(HER:η10=111 mV;OER:η10=379 mV;ORR:Eoneset=1.057 V,E1/2=0.89V)and long-term stability under alkaline conditions.