Study Of Metal Organic Framework Derived Carbon-Based Composite Materials As Electrocatalysts For Oxygen Reduction Reaction

Proton exchange membrane fuel cells（PEMFCs）is a green and efficient energy conversion device which can convert the chemical energy of fuel and oxidant into electric energy directly.However,the sluggish kinetics of cathodic oxygen reduction reaction（ORR）is the rate control step of fuel cells.Therefore,it will be of great significance to design and modification ORR catalyst for further improve the performance of fuel cells.At present,platinum-based catalyst is the most active catalyst for ORR,but its high price,poor stability and sensitivity to methanol and CO limit the large-scale application of fuel cells.In view of the above problems,the purpose of this paper is to study a new type of non-precious metal-based catalyst with low price,high stability,durability and catalytic activity.The metal-organic framework derived carbon-based composite materials,were prepared via a simple method.The ORR activity and mechanisation of the catalysts were investigated.The research contents and results are as follows:1)A facile direct precipitation method was used to prepare the zeolite imidazole framework-8（ZIF-8）precursor at room temperature,and the Nitrogen doped carbon（N/C）catalysts were synthesized by direct pyrolysis of ZIF-8 precursor at different temperatures.By means of XRD,SEM,TEM,XPS,BET and electrochemical performance test,the phase,crystal structure,morphology,particle size,valence bond and catalytic activity of the products were studied.The effect of different pyrolysis temperature on the catalytic performance of ORR were investigated.The results show that with the increase of pyrolysis temperature,the catalytic activity of N/C increases first and then decreases.N/C-910 catalyst shows the best catalytic activity among the catalysts pyrolyzed at different temperatures（700-1000℃）.The onset potential（E₀）and half-wave potentials(E_1/2)are 0.841 V and 0.683 V,respectively and the limiting current density is 3.52 mA cm^-22 at 0.4 V in 0.1 M KOH solution.The ORR performance of N/C-910 catalyst can be attributed to the pyridine N.2)A facile direct precipitation method was used to prepare the Fe_x/ZIF-8precursor with different Fe contents,and the N/C supported Austenite（γ-Fe）nanocatalysts were prepared by one-step heat treatment,and the effect of Fe contents on the catalytic performance of ORR were studied.The activity mechanism was analyzed by combining experimental results and theoretical calculation.The results showed that Fe_0.05-N/C catalyst shows superior ORR activity(E₀=0.990 V and E_1/2=0.851)comparable to 20 wt%Pt/C(E₀=0.990 V,E_1/2=0.851).The current density of 6.0 mA cm^-22 for Fe_0.05-N/C is better than 20 wt%Pt/C(5.7 mA cm^-2).The calculation of RRDE and K-L shows that the Fe_0.05-N/C catalyst follows an efficient four-electron transfer path.The test of Methanol resistance and current-time（i-t）shows that the Fe_0.05-N/C catalyst exhibit excellent methanol resistance and high stability.Besides the Fe-N_x sites,the high ORR activity for Fe_0.05-N/C should be attributed to the synergetic effect between the N-doped carbon and metallic Fe nanoparticles.3)The carbon supported iron（Fe/C-910）catalyst were prepared by pyrolysis of Fe（BTC）precursor.Fe/C-910 and N/C-910 were physically mixed at room temperature to form Fe/C-N/C-Ph catalyst and chemically mixed via repyrolysis to form Fe/C-N/C-Ch catalyst.The results show that the ORR performance of Fe/C-N/C-Ph catalyst is between Fe/C and N/C,while the Fe/C-N/C-Ch catalyst shows superior ORR activity with E₀=0.967 V,E_1/2=0.825 V,and a limit current density of 6.04 mA cm^-22 at 0.4 V,which is comparable to 20 wt%Pt/C.The rotating ring disk electrode（RRDE）and K-L results show that the ORR of Fe/C-N/C-Ch occurs mainly through a four-electron transfer pathway.The methanol resistance test shows that the catalyst has good methanol resistance.The high ORR activity for Fe/C-N/C-Ch should be attributed to the synergetic effect between the N-doped carbon and metallic Fe nanoparticles.