Preparation And Oxygen Reduction Performance Of Conductive Metal-organic Framework Materials Based On Hexaiminotriphenylene Ligand

The large-scale application of fuel cell and metal-air battery technology is limited by expensive and scarce cathode noble metal oxygen reduction（ORR）catalysts.Therefore,it is necessary to develop high-performance,low-cost non-noble metal ORR catalysts.Metal-organic frameworks（MOFs）are ideal precursors for high-performance non-noble metal carbon-based catalysts due to their high specific surface area,tunable pores,and diverse compositions.However,high temperature pyrolysis tends to destroy unstable coordination bonds in MOFs,resulting in a decrease in effective metal active sites and limiting the further improvement of electrocatalytic activity.Based on this,in this paper,a series of highly conductive non-carbonized MOFs materials were prepared by selecting hexamine-rich hexaiminotriphenylene（HITP）withπconjugated structure as ligand and forming a high charge delocalization plane with metal coordination.By creating active sites and using surface engineering strategies,the ORR performance of conductive MOFs is significantly improved.Influence of the composition and structure of conductive MOFs based on HITP ligands on ORR performance is elucidated.The main results are as follows:Firstly,Ni Cl₂ was used as the metal source,HITP as the ligand,and sodium acetate as the deprotonating agent.A uniform one-dimensional rod-shaped Ni-based conductive MOFs,Ni₃HITP₂,was prepared by solvothermal method.The conductivity of Ni₃HITP₂ was 11.86 S cm^-1,and the specific surface area was550.8 m²·g^-1.The results show that Ni₃HITP₂ has the best crystallinity when the temperature is 70 ^oC and the concentration of sodium acetate is 3.5 mol L^-1.In addition,the volume ratio of H₂O/DMF in the mixed solvent significantly affects the morphology of Ni₃HITP₂.When the volume ratio of H₂O/DMF is3/1,the rod morphology is the most uniform,with an average diameter of～43.8nm and length of 100-200 nm.Electrocatalytic tests show that the ORR performance of Ni₃HITP₂ is much lower than that of Pt/C,and the half-wave potential in alkaline electrolyte is 0.70V.In order to further improve the catalytic activity of Ni₃HITP₂,a high performance Ni/Co bimetallic conductive MOFs nanorods decorated with highly dispersed Co₃O₄ nanoparticles electrocatalyst（Co₃O₄/Ni Co-HITP）was prepared by introducing the second metal Co to create the active site.The results show that Co₃O₄/Ni Co-HITP retains the morphology of Ni₃HITP₂ nanorods with an average diameter of～55.6 nm,and a remarkably meso/macropore structure due to the nanorod structure that is favorable for mass transport.When the Ni²⁺/Co²⁺molar ratio is 1/2,Co₃O₄ NPs are highly dispersed on Ni Co-HITP nanorods with the average particle size of～17.7 nm,conductivity of 7.23 S cm^-1 and specific surface area of 614.6 m² g^-1.Optimized syntheses show a promising ORR activity with a high half-wave potential（0.77 V）compared with pure Ni₃HITP₂ in alkaline electrolyte.Furthermore,a rechargeable Zn–air battery using the as-prepared material as air-cathode also shows a high power density.XANES and electrochemical testing indicate that both the Co₃O₄ and Co-N₄ sites formed by the unsaturated N coordination on the surface of Co and Ni₃HITP₂ cooperatively catalyze ORR reactions,leading to superior ORR activity.Finally,surface engineering strategy is adopted to further improve ORR performance.A highly conductive core-shell catalyst（ZIF-67@HITP）with ZIF-67 as the core and HITP as the shell was constructed by using HITP as ligand to modify ZIF-67.The results show that the conductivity of ZIF-67@HITP is 2.16*10^-4S cm^-1,which is 8 orders of magnitude higher than that of pure ZIF-67.Electrochemical tests showed that when the mass ratio of HITP/ZIF67 was 0.5,the catalyst showed excellent ORR electrocatalytic activity in alkaline medium.The ORR electrocatalytic activity of ZIF-67@HITP with a half-wave potential of 0.82 V,even comparable to a commercial Pt/C.In addition,ZIF-67@HITP shows similar performance to Pt/C catalyst in primary zinc-empty cell applications with a peak power density of 102 m W cm^-2.DFT calculation shows that N in HITP can not only coordinate with unsaturated Co sites on ZIF-67 surface,but also have thermodynamic ligand exchange with dimethylimidazole ligands in ZIF-67.