Study On Graded Porous Non-noble Metal Oxygen Reduction Catalyst

Proton exchange membrane fuel cells（PEMFC）usually use noble metal catalysts to promote the slow oxygen reduction reaction of the cathode,and a graded porous structure with large specific surface area and proper pore distribution can give non-noble metal catalysts excellent performance.It is expected to significantly reduce the cost of proton exchange membrane fuel cells by replacing precious metal catalysts,which is a promising development direction.The hard template method can easily control the size and morphology of the pore structure by using different templates and adjustment procedures to obtain an ideal graded porous structure.In this paper,a hierarchical pore-making strategy using zinc oxide as a hard template is adopted to obtain a series of high-performance non-noble metal oxygen reduction catalysts in an acid environment.First,we applied this strategy to ZIF-8 with high micropore content,o-phenanthroline as an additional nitrogen source,and ferrous acetate as the iron source.After high temperature heat treatment,the hard template was removed in situ to introduce the mesopores.The Fe-N-C catalyst with hierarchical porous structure with both pores and micropores eliminates the negative effects of ZIF-8 single micropore structure on diffusion and mass transfer.As the zinc oxide nanoparticles with a small particle size（30 nm）,the optimal amount of iron added（the mass ratio of Fe,o-phenanthroline,and ZIF-8 is 1/20/80）,and the appropriate amount of hard template（ZIF-8/Zn O=1:1）,the prepared catalyst py-Fe-ZIF/Zn O not only has a large BET specific surface area（840 m²/g）,a suitable ratio of mesopores(S_Micro/S_Meso=0.209),high Nitrogen content（6.74at%）and pyridine nitrogen content（3.24at%）,but also showed excellent half-wave potential（0.797 V vs.RHE）and initial potential（0.933 V vs.RHE）in half-cell test.The yield of hydrogen oxide is less than 6.70%,the corresponding number of transferred electrons is about 3.90,and the reaction is close to the four-electron process.However,due to the adverse effect of the Fenton reaction on the iron-based catalyst,the half-wave potential of the catalyst was attenuated by 19 m V in the stability test of the catalyst circulating in nitrogen for10,000 cycles,and the stability was not good in an acidic environment.Later,in order to obtain a non-noble metal catalyst with both activity and stability,the natural cobalt-containing transition metal macrocyclic compound vitamin B12 was selected as the precursor.Continuing the hierarchical porous strategy with zinc oxide as the hard template,and introducing zinc nitrate hexahydrate as the second template into the micropores,the zinc oxide hard template was completely removed after heat treatment and dilute sulfuric acid pickling,and a group of metal cobalt particles were obtained.It is a Co-N-C catalyst with uniform distribution of cobalt,nitrogen and carbon elements and a meso-microporous graded porous structure.In an acidic environment,the half-wave potential can reach 0.747 V vs RHE,which is about 110 m V higher than the half-wave potential of direct pyrolysis carbon-loaded20wt.%VB12,the yield of hydrogen peroxide is less than 8%,and the number of transferred electrons is about 3.90.Furthermore,the relationship between the half-wave potential representing the catalytic activity of the catalyst and the pore size distribution of the sample was discussed.It was verified once again that the hierarchical porous strategy applied to the catalyst for oxygen reduction reaction can achieve better activity improvement.In addition,the cobalt-based catalyst had a half-wave potential drop of only 4 m V after 10,000 cycles of CV in nitrogen,and it had better stability than iron-based catalysts and commercial platinum-carbon catalysts.The reason for the increased stability compared to iron-based catalysts is the lower hydrogen peroxide yield in the high potential range and the absence of ionization of iron ions that can promote Fenton reaction.The exploration in this paper provides ideas for the follow-up research of high-performance non-noble metal catalysts.