Preparation And Oxygen Reduction Reaction Performance Of ZIF-Derived Iron-Based Nitrogen Doped Carbon Materials

In the face of more and more serious energy and environmental crisis,efficient and pollution-free energy conversion devices,such as fuel cells and metal-air batteries,have become the focus of current research.However,the hysteresis of cathodic oxygen reduction?ORR?kinetics is an important factor affecting its commercial application.Currently,the commercially used ORR catalyst is platinum?Pt?catalyst,which has limited reserves,high price and poor stability.Therefore,the research and development of high efficiency and low price non-noble metal ORR catalyst is the premise to realize the large-scale commercial application of the above energy conversion devices.Transition metal-nitrogen atom doped porous carbon materials?M-N-C?are favored by researchers due to their advantages of low cost,high performance and good stability.Metal organic frameworks?MOFs?have many advantages,such as stable structure,high specific surface area,adjustable pore structure,etc.,and the metal-nitrogen doped porous carbon materials obtained by high temperature pyrolysis have excellent performance in the field of electrochemical catalysis.In this paper,zinc-based zeolite imidazole framework?ZIFs?are used as the precursor to obtain two kinds of highly efficient iron-nitrogen doped porous carbon ORR catalysts,by means of controlled doping and dimensional regulation to improve the dispersibility of Fe active sites.The main research results are as follows:?1?An ORR electrocatalyst with regular dodecahedral shape was designed and constructed by means of valence state regulation.Copper foil was used to introduce Fe²⁺and Cu²⁺through a simple ion reaction(2Fe³⁺+Cu=Cu²⁺+2Fe²⁺),minimize the oxidation reaction of Fe²⁺to Fe³⁺species and lattice distortion in the synthesis process.At the same time,the agglomeration of Fe during carbonization is reduced and the dispersion of Fe is increased.The ZIF derived carbon skeleton-Cu@Fe-N-C,which was co-coordinated by Fe and Cu,was successfully synthesized.Fe was evenly distributed throughout the carbon skeleton,and Cu was mainly encapsulated in the carbon shell in the form of a single substance,playing an important role in improving ORR performance.The results showed that the half-wave potential?0.892 V?of the catalyst was 50 mV higher than that of commercial Pt/C,and it had excellent stability and anti-methanol poisoning ability,under the condition of 0.1 M KOH electrolyte.Under the condition of 0.5 M H₂SO₄ solution,its catalytic performance was close to that of Pt/C.When further applied to the cathode of zinc-air battery,Cu@Fe-N-C exhibited higher energy density(92 mW cm^-2)and excellent discharge stability compared with commercial Pt/C.?2?The dispersion of active sites and ORR catalytic performance of ZIF derivatives can be controlled by ZIF precursors with different dimensions.The Fe/N co-doped carbon nanotubes material?Fe-N-CNTs?was synthesized by one-step carbonization under Ar atmosphere using Fe doped two-dimensional leaf-like ZIF as precursor,by the effects of different solvents on the morphology of MOFs.The results showed that solvent plays an important role in the existence and distribution state of Fe active sites.In 0.1 M KOH,Fe-N-CNTs had an initial potential of 1.02 V,a half-wave potential of0.89 V and a limited current density of-5.52 mA cm^-2.Compared with commercial Pt/C,Fe-N-CNTs not only had better ORR performance,but also had better stability and anti-methanol toxicity ability than Pt/C.When applied to the cathode of zinc-air battery,Fe-N-CNTs exhibited a high energy density(136 mW cm^-2).And,under different constant current densities,after the discharge test as long as 50000 s,the voltage was basically unchanged,showing good discharge stability.