Preparation Of Zeolite Imidazole Framework Derived Borides And Research The Performance Of Electrocatalytic Oxygen Evolution

Carbon dioxide emission from burning fossil fuels is the biggest risk factor to the greenhouse effect.The crisis of global energy and environmental pollution makes us focus on new energy.Hydrogen energy is regarded as the most potential secondary energy source because of the clean,efficient and zero-pollution.Water electrolysis is considered to be the most effective method for hydrogen production because of the advantages of sustainable,zero-pollution and high-purity.However,in its anodic oxygen evolution reaction（OER,4OH^-→O₂↑+2H₂O+4e^-）,the 4-electron transfer requires a high overpotential,resulting in a low energy conversion efficiency and too high reaction barrier.At present,the Ru O₂,Ir O₂ and other precious metals are regarded as the benchmark of OER catalyst,but the high cost,poor stability and other defects cannot be applied in a large scale.Therefore,commencing from low-cost transition metal-based electrocatalyst,improving the catalytic activity and resource utilization rate of the catalyst and effectively reducing the amount of precious metal is one of the ways to reduce the overall cost of hydrogen production by water cracking and improve the energy conversion efficiency.In this paper,ZIFs-derived transition metal boride electrocatalysts were synthesized via a one-step boronizing process,derived from zeolite imidazole frames（ZIFs）,toward enhanced electrochemical alkaline oxygen evolution reaction.This thesis designs and constructs Ni-B-Co nanoparticles grown in situ on ZIF-67,bimetallic cobalt-iron borides grown on B,N-doped carbon frame and the pomegranate-like Ni-doped cobalt boride nanoparticles were implanted into B,N-doped carbon nanocage composites.The chemical composition and OER catalytic performance of these materials are studied,which are as follows:1.The Ni-B-Co nanoparticles grown on the surface of ZIF-67 precursor（ZIF@Ni-B-Co）was synthesized by electrostatic adsorption and in-situ growth strategies.The OER catalytic performance of ZIF@Ni-B-Co catalyst was investigated systematically by adjusting the doping amount of Ni²⁺and the boronizing degree of Na BH₄.At a current density of 10 m A cm^-2,the overpotential reaches 300 m V and the Tafel slope is 77 m V dec^-1 of ZIF@Ni-B-Co catalyst,which is better than the precious metal Ru O₂.ZIF-67 as a precursor provides a large surface area and abundant cobalt source for ZIF@Ni-B-Co composite.Ni-B-Co nanoparticles are uniformly grown on ZIFs,maintaining the original dodecahedral morphology of ZIF-67 and increasing the contact area of electrolyte-catalyst,providing rich active sites for OER process.2.The Fe-doped bimetallic Co Fe-ZIF precursor with structural defects was constructed,which exposed a higher number of active sites.Subsequently,the bimetallic cobalt iron boride grown on the B,N-doped carbon frame（noted as Co Fe B@BNC）was successfully synthesized by one-step pyrolysis boronizing using Co Fe-ZIF as the precursor and H₃BO₃ as the boron source.Its overpotential is 297 m V at 10 m A cm^-2 and the Tafel slope is only 41 m V dec^-1,showing faster OER kinetics.The excellent OER activity of Co Fe B@BNC composite is attributed to the synergy of metal boride sites and additional B,N-doped carbon sites.The introduction of appropriate amount of Fe brings up high conductivity of Co Fe B@BNC composite and significantly accelerates OER kinetics.The Co Fe-ZIF precursor has a large number of surface structure defects,which provide abundant mesoporous and metal active sites for the Co Fe B@BNC composite.The B,N-doped carbon frame improves the stability of material.3.Pomegranate-like Ni-doped cobalt boride implanted in B,N-doped carbon nanocage was synthesized by one-step pyrolytic boronizing strategy（denoted as Ni-Co_xB@BNC）,derived from bimetallic Ni Co-ZIF precursor.The high-temperature boronizing reduces the porosity of Ni Co-ZIF,forming mesoporous Ni-Co_xB@BNC nanocage with abundant nitrogen species and active sites.When the current density is 10m A cm^-2,the overpotential of Ni-Co_xB@BNC is only 274 m V,and the Tafel slope is 80m V dec^-1,which has better OER catalytic performance than noble metals and other MOFs-derived transition metal electrocatalysts.The superior OER catalytic activity of Ni-Co_xB@BNC catalyst is ascribed to the following reasons:（i）The introduction of trace Ni not only causes structural defects,but also changes the electronic structure and optimal coordination state of the original ZIFs,which is conducive to exposing more active sites.（ii）High-temperature boronizing produces B,N-doped carbon layer,which not only enhances the intrinsic catalytic activity of Ni-Co_xB nanoparticles,but also provides additional active sites for OER process.These two systems synergistically accelerate the reaction rate of OER;（iii）B,N-doped carbon layer improves the conductivity of Ni-Co_xB@BNC catalyst while protecting the internal borides to improve the stability.