Carbon Nanotubes In-situ Encapsulation Of Cobalt-based Nanoparticles For High-performance Zinc-air Batteries

As fossil fuels continue to burn,environmental concerns have become increasingly important.How to realize sustainable clean energy technology becomes the key to solve the energy problem.Among them,zinc-air batteries have attracted much attention of researchers due to their low cost,stable discharge platform,high safety and high energy density.Oxygen evolution（OER）and oxygen reduction（ORR）reactions occur during the charge-discharge process of Zn-air batteries,so it is urgent to develop high-performance ORR and OER bifunctional catalysts.Although noble metal catalysts have excellent electrocatalytic performance,problems such as low stability,scarcity,and the inability of a single noble metal to simultaneously have excellent dual functions have seriously hindered the development and application of zinc-air batteries.The study of relatively inexpensive non-precious metal catalysts is the key to large-scale application of zinc-air batteries.This paper mainly studies the co-encapsulation of metal cobalt and other metals in carbon nanotubes,and achieves high-performance catalysis by regulating the morphology and structure and improving the electronic structure of the carbon layer.（1）FeCo alloys were successfully synthesized by a simple one-step pyrolysis method,and Fe Co and Co nanoparticles were encapsulated in N-doped carbon nanotubes.Fe Co-Co/NC has multiple active species（Fe Co,Co,NC）and the tight binding between these active species（Fe Co,Co,NC）,and this ingenious structure can rapidly catalyze OER and ORR processes.Electrochemical studies show that the Fe Co-Co/NC has an onset potential of 0.93 V,a half-wave potential of 0.808 V and a limiting current density of 5.01 m A cm^-2during the ORR process.During OER,at an anodic current density of 10 m A cm^-2,an overpotential of only 353 m V was exhibited,and a low Tafel slope and good durability were also observed.The assembled Fe Co-Co/NC zinc-air battery can be cycled up to 2000times in charge-discharge tests.This provides a new strategy for the preparation of efficient bifunctional electrocatalysts for OER and ORR.（2）To further enhance the performance of ORR,we prepared a Co/Cu_0.2@NC catalyst with carbon nanotubes encapsulating metallic Co and Cu nanoparticles by introducing metallic copper.The as-prepared Co/Cu_0.2@NC catalyst exhibits remarkable ORR（half-wave potential of 0.838 V）and OER(322 m V overpotential at 10 m A cm^-2current density)electrochemical activity.The introduction of copper nanoparticles improves the ORR activity of the material,and cobalt nanoparticles can induce the formation of carbon nanotubes and have an efficient catalytic effect on OER.The assembled Co/Cu_0.2@NC rechargeable Zn-air battery exhibits a high power density(170 m W cm^-2)and a high open circuit voltage of 1.485 V,more importantly,Co/Cu_0.2@NC exhibits Ultra-long stability,not only the assembled liquid zinc-air battery can be cycled stably for 400 h,but also the assembled flexible zinc-air battery can be cycled stably for 480 cycles（80 h）.This excellent bifunctional catalyst will have broad application prospects in flexible electronics.（3）In order to prepare binder-free air electrodes,carbon nanotube-coated Fe Co and Co metal nanoparticles were in-situ generated on iron foam by a simple high-temperature pyrolysis method.The experimental results show that the as-prepared Fe Co@NC/FF can efficiently catalyze HER,OER,and ORR processes,and thus can be directly used as electrodes for rechargeable zinc-air batteries and for total water splitting.The total water splitting device composed of Fe Co@NC/FF catalyst as both cathode and anode requires only 1.623 V at 10 m A cm^-2.The prepared liquid zinc-air battery can be cycled for 500 h and has a high specific discharge capacity of 817.31 m Ah g^-1.In addition,the Fe Co@NC/FF solid-state Zn-air battery exhibits a high peak power density of 83.19 m W cm^-2,and the initial charge-discharge voltage difference during charge-discharge cycling is only 0.57 V,which exceeds that of noble metal catalysts.Due to the good performance of the assembled liquid Zn-air battery,solid-state Zn-air battery and total water splitting device,both liquid and solid Zn-air batteries prepared with Fe Co@NC/FF can drive the FeCo@NC/FF as bi-electrode.Full dehydration device.