Electrocatalytic And Photoassisted Electrocatalytic Properties Of Threedimensional Molybdenum-Nickel(Cobalt)-Based Nanostructures

Developing clean and effective energy conversion or storage systems is an effective way to solve the energy crisis and environmental pollution,such as electrolytic water splitting systems and alkaline zinc-air batteries.Hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）are involved in electrolytic water splitting systems,and OER and oxygen reduction reaction（ORR）are involved in zinc-air batteries.However,the kinetics of these reactions are sluggish,so efficient catalysts are required to facilitate these reactions.To date,precious metal Pt/C catalysts are mainly used for HER and ORR;Ru O₂ or Ir O₂ is considered as benchmark electrocatalysts for OER.Nevertheless,their large-scale application is severely limited due to the high cost,limited natural resources and poor stability of the above noble metals-based electrocatalysts.Transition group metal catalysts have become a research hotspot in the field of water electrolysis and zinc-air battery because of their high storage,low price and high catalytic activity.Among them,molybdenum-based catalysts have high catalytic activity,but poor stability and single catalytic performance.In order to solve the above problems,this thesis designed and synthesized three-dimensional（3D）molybdenum-nickel（Cobalt）based catalysts by introducing other transition metals,constructing heterogeneous structures,and morphologically controlling methods,so that they can be used as bifunctional or multifunctional catalysts for efficient operation of water splitting and zinc-air batteries.In addition,the study of photo-assisted electrocatalysis is also carried out in this thesis.The main research contents are as follows:1.Firstly,self-supported Ni MoO₄ nanowires were synthesized on the nickel foam substrate（Ni MoO₄/NF）,then the Ni MoO₄/NF was subjected to long-term electrochemical oxidation in alkaline medium to remove most of the Mo ions,being phosphated to prepare 3D self-supported Mo-doped Ni_xP/NF nanomaterials.It was found that due to its porous tripod-like structure and removal of unstable Mo elements,the Ni_xP/NF catalyst exhibited excellent HER catalytic activity,only a low overpotential of 71 m V was required to reach the current density of 10 m A cm^-2,and the reaction could be stably catalyzed for 40 h at high current density.Under the photo-assisted condition,the HER performance of the Ni_xP catalyst was improved,only 74 m V overpotential was required to reach the current density of 20 m A cm^-2,while 93m V was required under the darkroom condition.2.The 3D NiMoN/NF nanomaterial was prepared by directly pyrolyzing Ni MoO₄/NF in NH₃ atmosphere.Due to its excellent HER and OER bifunctional catalytic activities,as the cathode and anode of the water electrolytic device,only 1.507V voltage is required to drive the current density of 10 m A cm^-2,which is better than the water electrolyzer assembled with the commercial catalyst Pt/C as the cathode and Ir O₂ as the anode（1.540V）.The self-support and gap between nanowires are conducive to the diffusion of the electrolyte and the escape of bubbles,thus improving the catalytic activity.It was found that photo-assisted conditions could improve the HER and OER catalytic activities of NiMoN catalyst.3.In order to further improve the practical application value of catalysts,tri-functional catalysts were prepared.Co Zn-ZIF coated MoO₃ core-shell nanorods（MoO₃/Co Zn-ZIF）were synthesized by combining MoO₃ nanowire with Zeolitic imidazolate framework（ZIF）.Then,the 3D nanocomposites consisting of N-doped carbon coated Mo₂C and N-doped graphene carbon coated Co nanoparticles（Mo₂C@NC/Co@NG）were prepared through high-temperature annealing.Owing to the synergistic effect of Mo₂C,metal Co and N-doping,it showed excellent multi-functional catalytic activity and stability of HER,OER and ORR under alkaline conditions,and it’s applied to electrolytic water and zinc-air battery,which performance is better than Pt/C+Ir O₂ mixed catalyst.Under photo-assisted conditions,the HER and OER catalytic activities of Mo₂C@NC/Co@NG catalysts were more excellent than those of darkroom conditions.4.3D nanomaterial（MoCoZn/NCNTA）containing N-doped carbon framework and Co,Zn,N co-doped carbon nanotube array（CNTA）branching out from Mo C was prepared by thermal decomposition of MoO₃/Co Zn-ZIF nanomaterial and dicyandiamide.Due to the introduction of more CNT,the catalytic activity can be improved.Compared with Mo₂C@NC/Co@NG and benchmark Pt/C catalysts,it shows better ORR catalytic activity,and the half-wave potential is up to 0.914 V.Moreover,the HER and OER catalytic activities of MoCoZn/NCNTA catalyst were also improved under photo-assisted conditions.In summary,in order to obtain multifunctional catalytic materials with low cost and excellent catalytic activity,several effective methods were developed in this thesis to synthesize several 3D molybdenum-nickel（cobalt）-based nanomaterials,all of which show excellent catalytic activity under alkaline conditions.At the same time,it was found that their catalytic performance was improved under photo-assisted conditions.