Preparation Of Non-noble Metal Electrocatalyst And Its Investigation On Oxygen Reduction Reaction

Oxygen reduction reaction(ORR)have attracted considerable attention due to its significant role in energy storage devices including fuel cells and metal-air batteries.So far noble metals(for example,Pt,Pb,Ag and its alloys)with superior ORR activity are regarded as the best catalysts to solve the sluggish kinetics of ORR.However,the rare resources,high price,inferior stability and poor anti-poisoning by methanol/carbon monoxide have seriously hindered the development and large-scale applications in ORR-based energy devices.By contrast,recent years numerous low-cost non-noble ORR catalysts have been exploited for replacing the noble metal ORR catalysts.Additionally,some non-noble metal ORR catalysts have been considered as a promising alternative for satisfying electrocatalytic ORR,but most of them still showed inferior ORR performance compared to Pt-based catalyst.Therefore,it is desirable and significant to develop efficient strategy for enhancing the ORR performance in M-Nx/C catalyst toward practical application.In this work,we have rationally designed a series of catalyst precursors by bimetal synergistic effect,host-guest confinement and hierarchical pore control strategy,and fabricated a series of ORR catalysts with superior ORR performance and stability.The main research contents are as follow:(1)The feature of tunable constituents in covalent organic polymers(COPs)via combination of predesigned building units is of great interest in exploring multi-functional catalysts.Here,we report constituent-tunable metalphthalocyanine-based covalent organic polymers as self-sacrificial precursors for fabrication of mono-(Fe or Ni)and bi-metallic(Fe-Ni)heteroatoms/carbon catalysts toward both oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).As a result,the as-prepared FeNi-COP-800 exhibits an efficient 4e-ORR performance with a half-potential of 0.80 V as well as an IrO2-like OER activity with a potential of 1.63 V at 10 mA cm-2,which surpass those of monometallic Fe-COP-800 and Ni-COP-800,suggesting that the enhanced bi-electrocatalytic activities of FeNi-COP-800 are closely associated with the coexistence of Fe and Ni elements in the FeNi-COP-800.Moreover,a zinc-air flow battery using FeNi-COP-800 catalyst not only shows a higher power density than the battery using Pt/C-IrO2/C but also displays superior stability in practical operation.On the other hand,the power density of the solid battery using FeNi-COP-800 catalyst reached 92.5 mW cm-2.Although the bimetal synergistic effect is able to enhance the ORR performance of the catalysts,avoiding the aggregation of metal species is also significant to maximumly use the active sites for enhancing the ORR performance.(2)In order to solve the aggregation of metal active sites,in this work porous ZIF-8 was used as the host platform to include the guest molecule with M-N4 active site for fabrication of catalyst precursor like Fe(Phen)2@ZIF-8.The Fe-N/C ORR catalysts were obtained by further pyrolysis of the catalyst precursors.The results showed that the ORR performance was enhanced by reducing the metal contents and confining the guest molecule into the cavity of ZIF-8 to reduce the aggregation of metal active sites into metal particle.The as-prepared 3%Fe-N/CFe(Phen)2 catalyst exhibited an outstanding ORR performance with a half potential of 0.938 V which is higher than commercial Pt/C.Moreover,the half potential of 3%Fe-N/CFe(Phen)2 only lost 18 mV after 5000 cycles CV test,and the ORR activity still kept 90%above after 25000 s IT test.The host-guest confinement strategy,in some way,have achieved the preparation of highly active ORR.However,the occupation of the pore of ZIF-8 by guest molecules induced more micropore in the catalyst,which is not beneficial for the mass transfer of the oxygen and water molecules during the ORR process.(3)To handle the inferior mass transfer induced by micropore structure and efficiently expose the active sites.We here reported a dual ligands strategy to prepare a series of Fe-N/C ORR catalyst with hierarchically porous structure.It was easy to control the porous structure and modulate the electrochemical active surface by changing the ratio of the two ligands.The superior 2%Fe-N/C80-20 ORR catalyst was prepared by optimizing the ligands ratio and the metal contents/species.In the alkaline condition,the as-obtained 2%Fe-N/C80-20 showed better ORR performance and stability than the commercial Pt/C.However,in 0.1 M HCIO4 acidic condition even if the ORR activity of 2%Fe-N/C80-20(half potential of 0.795 V)was not good enough compared to that of the commercial Pt/C(half potential of 0.815 V),the 2%Fe-N/C80-20 catalyst was more stable since the ORR activity was still above 90%after the 25000 s accelerated aging test.Furthermore,a H2-O2 fuel cell using 2%Fe-N/C80-20 catalyst showed a power density of 600 mW cm-2 and an open-circuit voltage of 0.917 V.Accordingly,the dual ligands strategy not only can achieve the modulation of the hierarchically porous structure for exposing the active site and further improving the active site density but expand the scope of another catalysts.