Design,Preparation And Electrocatalytic Performance Of Carbon Fiber-based Transition Metal Catalysts

Clean and efficient metal-air batteries are considered as one of the ideal candidates for a new generation of green energy.The lack of low-cost and high-activity bifunctional catalysts for oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)is an important factor restricting the large-scale application of rechargeable metal air batteries.Metal-organic framework materials(MOFs)have high specific surface area,unique pore structure and abundant active sites.Three-dimensional network carbon fibers(CNFs)have the potential to promote the orderly conduction of electrons,which show significant advantages as cathode catalyst materials for rechargeable metal air batteries.Therefore,the perfect combination between MOFs and CNFs is more important to achieve efficient and stable catalysis of catalyst.In this paper,based on electrospinning and in-situ growth,a carbon based cobalt-manganese alloy electrocatalyst with uniform,dense and stable core-shell structure was prepared by the design of morphology and elemental composition,and used in rechargeable zinc-air battery.The details are as follows:(1)In order to achieve the perfect combination between ZIFs and CNF materials,polyacrylonitrile(PAN),polyvinylpyrrolidone(PVP),cobalt acetate and zinc acetate were firstly blended to prepare electrospun composite nanofibers.Subsequently,ZIF coating was grown in situ on the fiber surface by the coordination of cobalt nitrate,zinc nitrate and 2-methylimidazole.Finally,carbon-based metal electrocatalysts with stable core-shell structure(M@CFs)were obtained by two-step pyrolysis activation at low temperature and then high temperature.The influencing factors of forming uniform,dense and stable ZIF coating on the fiber surface were explored,mainly including the addition sequence of metal salts and ligands,the addition of PVP and in-situ growth time.At the same time,the effect of catalyst structure on catalytic performance was analyzed.The research showed that when the fiber contained metal salts,the first addition of ligand solution was conducive to the massive nucleation of ZIFs on the fiber surface,and then the addition of metal salts could promote the further growth of ZIF grains along the nucleus,and finally formed dense ZIF crystals.As a surfactant,PVP could promote the uniform distribution of ZIFs on the fiber surface,and the dissolution of some PVP provided a favorable space for the nucleation and growth of ZIFs inside the fiber,ensuring the stable combination between the final ZIF crystal and the fiber.In addition,the growth time of one hour could ensure that ZIF crystal was completely coated on the fiber surface,avoided the excessive deposition of ZIFs particles,and promoted the exposure of more core-shell fibers.The design of this one-dimensional core-shell porous structure made the optimal M@CFs electrocatalyst(Co@PCFs)expose a large number of accessible active sites(specific surface area was 320.438 m2/g).And the carbon matrix of core layer not only acted as a stable support carrier,but also provided effective electron conduction,so that the catalyst showed good ORR catalytic activity(half wave potential E1/2 was 0.82 V)and stability(△E1/2 was 32 mV after 5000 cycles).(2)In order to increase the number of active sites,enrich the structure of active sites and improve the probability of coupling catalysis,carbon-based alloy catalysts were prepared on the basis of core-shell composite fibers.Firstly,Co/Mn/Zn ZIFs uniformly wrapped core-shell composite fibers were prepared by in-situ growth of ZIFs on the surface of electrospun nanofibers using cobalt acetylacetonate,manganese acetylacetonate and zinc acetylacetonate as metal sources and melamine as nitrogen source.Then,nitrogen-doped CNFs loaded with Co/Co2Mn3O8 alloy nanoparticles were obtained by step carbonization in nitrogen environment,which were used as bifunctional oxygen electrocatalysts for rechargeable zinc air batteries.The influence mechanism of alloy nanoparticles on the activity and stability of catalyst was explored by testing and analyzing the structure and properties of various samples.The results showed that the electron transition between different valence metals in Co2Mn3O8 was conducive to regulating the charge distribution of catalytic active centers,improving the adsorption activity of oxygen and promoting the improvement of ORR catalytic activity.Nitrogen doping would introduce a large number of C-N and Co-Nx structure sites and increase the number of active sites of the catalyst.The one-dimensional core-shell structure promoted the exposure of active sites,and the hierarchical porous characteristics accelerated the mass transfer,promoting the efficient and rapid progress of catalytic reaction.The graphite carbon layer on the surface of alloy particles and the fast 4electron transfer path reduced the possibility of corrosion degradation of the catalyst material,and improved the stability of the catalyst.Moreover,the high lattice oxygen content in Co/CO2Mn3O8 also promoted the improvement of OER performance.Based on the above advantages,Co/Co2Mn3O8@NCFs catalyst showed excellent ORR activity(E1/2 was 0.826 V)and stability(△E1/2 was 25 mV after 5000 cycles)as well as good OER activity(overpotential was 382 mV at 10 mA/cm2)and OER/ORR bifunctional activity(AE was 0.786 V).Using Co/Co2Mn3O8@NCFs as the cathode electrocatalyst for rechargeable zinc-air batteries,the assembled battery exhibited high power density(118.19 mW/cm2),high energy density(968.10 Wh/kg)and excellent cycle stability(the voltage hardly attenuates after 300 cycles),reflecting the great potential of Co/Co2Mn3O8@NCFs catalyst in practical energy application.