Design,Synthesis And Electrochemical Performance Of Three-dimensional Nanostructured Catalyst

In this article,we mainly focus on problems such as poor catalytic kinetic performance,unstable catalyst structure,less active sites,small specific surface area,serious agglomeration of catalyst nanoparticles and slow migration rate of ions of cathode and anode catalysts in low-temperature proton exchange membrane fuel cell.Based on the catalytic mechanism of transition metals,non-metal and noble metal catalysts,a novel three-dimensional?3D?structural catalyst with high specific surface area interconnected pores and active center were constructed by simple,green and economical methods.The catalysts based on 3D carbon materials and MXenes,not only solved the agglomeration problem of catalysts but also provided ion migration.It also provides channels and additional active sites for ion migration and improves conductivity.The details are as follows:?1?Nano-porous NiCo₂O₄?NCO?catalyst with nano-morphology was prepared by simple hydrothermal and annealing method.Also its methanol oxidation and oxygen reduction activities were studied.?a?NCO catalyst was prepared at different annealing temperature.Without decomposition,it shows enhanced electrocatalytic activity after annealing at 400?.The redox pair(Ni²⁺/Ni³⁺=1.56,C_O²⁺/C_O³⁺=0.93)and the surface coverage of redox species(7.92×10^-7mol cm^-2)are more abundant.The material has high methanol catalysis(the peak current density of oxidation is 15.31 m A cm^-2 at concentration of 0.5 mol L^-1 methanol),oxygen reduction reaction?ORR?activity(the half wave potential and the limiting current corresponding to 1600 rmp are 0.77 V and3.25 mA cm^-2 respectively),and shows high stability.The interaction of nickel and cobalt ions on the surface of the material provides stable,abundant active sites and rapid ion dynamic migration in catalytic process,which evidences high catalytic activity and stability.?b?Nano-porous NCO catalyst with three-dimensional spiny spheres was prepared by annealing in different atmospheres.The surface coverage of the redox species(14.61×10^-7mol cm^-2)and the redox pair(Ni²⁺/Ni³⁺=1.142,C_O²⁺/C_O³⁺=0.835)of NCO were significantly increased under annealing.The catalyst showed strong methanol oxidation activity and its peak current density was 15.51 mA cm^-2 at concentration of 0.5 mol L^-1.At the same time,the half wave potential and the limiting current density of ORR were increased to 0.78 V and 3.45 m A cm^-2 at 1600 rmp.The annealing atmosphere provides conditions for optimizing the valence distribution on the surface of NCO spinel,which further increases the active sites of the material,thus obtaining higher catalytic activity and stability.?2?Three dimensional?3D?porous graphene aerogel?GA?was used as carrier to support nano-needle like porous NiCo₂O₄ array catalyst.After the appropriate NCO load is obtained,the high activity NiCo₂O₄-GA catalyst can be obtained under the synergistic action of large specific surface area and high conductivity graphene.Under the condition of oxygen annealing,the"synergism"of graphene optimizes the surface coverage of redox pair(Ni²⁺/Ni³⁺=1.06,Co²⁺/Co³⁺=0.96)and redox species(16.29×10^-7 mol cm^-2)in NCO structure;meanwhile,the ablation of graphene by oxygen makes it more conducive to electrolyte transport and ion conduction.It shows excellent catalytic activity.The peak current density of methanol oxidation reaches 31.98 m A cm^-2 at 0.5mol L^-1,and the half wave potential reaches 0.803 V and the limiting current density reaches 4.36 mA cm^-2 at 1600 rmp.Ni Co₂O₄-GA has excellent electrocatalytic activity,which is due to the"synergy"between nano-needle NiCo₂O₄ and RGO.NiCo₂O₄-GA is a promising electrocatalyst material in the development of high performance non platinum based alkaline fuel cells because of its high stability in methanol oxidation and ORR catalytic reaction.?3?A novel N-doped carbon nanotube aerogel has been synthesized by pyrolysis of three-dimensional tubular polypyrrole networks and evaluated as a metal-free electrocatalyst for ORR.The carbon aerogel catalysts are pyrolyzed at 500 ??PPy/C-500?,700 ??PPy/C-700?,900 ??PPy/C-900?and 1050 ??PPy/C-1050?and it displays pyrolyzed temperature-dependent activity and stability.The catalytic activity is increased with increasing pyrolysis temperature from 500 ? to 1050 ? ascribed to high electronically conductive graphitic phases,thin wall of the three-dimensional tubular carbon aerogel framework and the increased effective active sites.The nitrogen content is reduced with the increase of pyrolysis temperature,and which plays an important role in the stability of the pyrolyzed catalyst.After 1500 cycles,the PPy/C-1050 catalyst shows more positive half-wave potential than the commercial Pt/C catalyst and displays a quasi-four-electron pathway toward ORR in the acid solution.This work provides an effective strategy for preparing the low cost and metal-free N-doped carbon aerogel catalyst from the conjugated conducting polymer,to achieve high activity and stability.?4?Pt/Mxenes catalysts were prepared by using two-dimensional graphene like Mxenes as support materials.The widely used carbon supports in the commercial Pt/C catalyst of fuel cells usually suffer rapid corrosion and instability and developing alternative support with stable structure and high electric conductivity is highly required.In this study,the accordion-likeTi₃C₂T_x nanosheets are synthesized by a modified Li F/HCl solution based method,and the obtained MXenes demonstrate good hydration properties,high conductivity and abundant functional groups on the layer surface,which are conducive for the uniform loading of Pt particles.The Pt/MXenes catalyst demonstrates superior durability and activity to Pt/C?commercial?for oxygen reduction reactions in both acid and alkaline environments.The reaction exhibits a dominant four-electron oxygen reduction process.The facile,green and economical approach is promising to produce 2D MXenes based nanocomosite catalysts with well-developed three-phase reaction region to enhance the fuel cell performance.