Research On Preparation And Performance Of Transition Metal-Nitrogen-Carbon Based On ZIF-67 As Oxygen Reduction Catalyst

The fuel cell is a device that converts chemical energy stored in fuel and oxidants into electrical energy.It has become a research hotspot in the world due to its high energy conversion rate,zero-emission,fast start-up,and many other advantages.For the cathodic Oxygen reduction reaction（ORR）,even if the Pt load is very high,the ORR reaction rate is much slower than that of the anodic hydrogen oxidation,which not only results in great electrochemical kinetic loss,but also significantly increases the fuel cell cost.At the same time,the penetration of anode fuel can further poison the ORR catalyst and accelerate its life decay.Therefore,development of low cost,high activity,good stability,ORR catalysts is one of the main challenges for fuel cell research,even using anion exchange membrane alkali anion membrane fuel cell,the development of a new type of highly efficient,high stability,low price of fuel cell cathode oxygen reduction（ORR）catalyst still has important theoretical significance and commercial value.In this paper,using ZIF-67 as the precursor,a series of ORR catalysts were prepared by optimizing the preparation process,introducing metal ions,and doping P atoms.The main conclusions were as follows:（1）In the synthesis process of ZIF-67,the particle size of ZIF-67 could be controlled by controlling the ratio of Co（NO₃）₂·6H₂O to 2-methylimidazole.When the ratio of cobalt nitrate to 2-methylimidazole was 1:6（ZIF-67（1:6））,the uniform dispersion of ZIF-67 with complete regular dodecahedron structure could be obtained.The sample ZIF-67-700 obtained at 700℃had a positive onset potential（0.91 V）,a higher half-wave potential（0.82 V）,and a higher limiting current density（5.33 m A/cm²）in O₂-saturated 0.1 M KOH solution,which was close to the performance of commercial 20%Pt/C catalyst.After potentiostatic polarization of18000 s at 0.666 V,the current density only decayed by 11%,showing good ORR catalytic performance and stability.（2）Adding Fe²⁺and Ni²⁺in the process of synthesis of ZIF-67 could realize the synthesis of double-doped Fe²⁺and Ni²⁺catalysts.Among all the doped samples,Z-Fe_0.05Ni_0.5-H showed the best electrochemical performance,with onset potential,half-wave potential and limiting current density of 0.94 V,0.83 V and 5.84 m A/cm²,respectively.After potentiostatic polarization of the double-doped Z-Fe_0.05Ni_0.5-H at 0.666 V for 18000 s,the current density attenuation decreased to 9%,and the stability was further improved.The unique porous structure and the synergistic effect of abundant Co-N bond and Fe-N bond active sites were the main reasons for the improvement of ORR catalytic performance.（3）Phosphorus doped M-N-C catalyst(Z-Fe_0.05Ni_0.5P₁-H)was successfully prepared by phosphating the double-doped samples.In the O₂-saturated 0.1 M KOH solution,the onset potential,half-wave potential and limiting current density of Z-Fe_0.05Ni_0.5P₁-H were further increased to 0.97 V,0.85 V and 5.95 m A/cm²,respectively.The current density of the potentiostatic polarization at 0.666 V only decayed by 8%,showing good stability.Z-Fe_0.05Ni_0.5P₁-H showed good electrochemical performance,which provided more options for ORR catalysts for the cathode of basic anionic membrane fuel cells.