Transition Metals And Nitrogen Doped Carbon Materials As Efficient Oxygen Electrocatalysts For Zinc-air Batteries Applications

Rechargeable zinc-air batteries are the most promising future candidate of energy storage devices to replace the commercially available Lithium-ion batteries,which are now the subjects of many controversies,ranging from overcharge to explosion and scarcity of the intercalation materials.The rechargeable zinc-air batteries use natural abundant,available materials and possesses high theoretical specific energy(1370 Wh kg-1)among various electrochemical energy storage technologies.However,the oxygen reduction and evolution reactions(ORR/OER)on electrode materials are recognized as kinetically limiting component which consequently leads to power loss and induces large overpotential in rechargeable zinc-air batteries.The transition metals(Fe,Co,Ni)embedded in nitrogen(N)-doped carbon nanomaterials(NCNMs)including N-doped carbon nanotubes(NCNTs)and N-doped hierarchical porous carbon(NHPC)synthesized through facile and lowcost approaches,having excellent conductivity,high specific surface area and exposed active sites,can catalyze both ORR and OER with low overpotential for high performance rechargeable zinc-air batteries.The in-situ grown nitrogen doped carbon nanotubes(NCNTs)-embedded two phases of CoFe alloy and CoFe2O4 spinel oxide CoFe2O4-CoFe@NCNTs is synthesized via programed pyrolysis temperature using inexpensive materials of glucose,urea and cobalt/iron acetates,without any template or surfactant.The assynthesized CoFe2O4-CoFe@NCNTs benefits the synergistic effect among CoFe alloy,CoFe2O4 spinel oxide and NCNTs,displays the high content of N-doped(10.12%)and high density of active N species(pyridinic-N,graphitic-N and M-Nx),as well as the high specific surface and efficient mass transport channel for both electrolyte and reactants.Therefore,CoFe2O4-CoFe@NCNTs exhibits more positive half-wave potential of 0.88 V for ORR and low potential of 1.58 V at 10 mA cm-2 for OER,resulting in a low potential gap ?E value of 0.70 V.Moreover,CoFe2O4CoFe@NCNTs applied as air electrode in rechargeable zinc-air battery manifests auspicious rechargeability performance with high discharge and low charge potentials and stable potential gap under 550 cycles,outperforming those of commercial precious metals Pt-Ru/C catalyst.This work provides a promising high performance bifunctional non-precious metal-nitrogen doped carbon catalysts for metal-air batteries.The in-situ grown nitrogen doped carbon nanotube encapsulated Co nanoparticles(Co Nps)(Co@NCNTs)on MOF-derived carbon materials is fabricated by a facile uric acid(UA)assisted pyrolysis of ZIF-67.In this strategy,the uric acid(UA)sharply decomposes at 440? to carbonaceous gases,which facilitate the nucleation of Co nanoparticles for the catalytic growth of NCNTs microspheres structural morphology from the intermediate ZIF-67 polyhedrons.The as-synthesized Co@NCNTs exhibits high N content,abundant well-confined Co@NC active species located at the tip of each one dimensional(NCNT),high electron conductivity and large specific area on hierarchical micro-mesoporous structure.Consequently,Co@NCNTs surpass commercial precious metals Pt-Ru/C catalyst by exhibiting more positive ORR half wave potential of 0.85 V and limiting current density of 5.50 mA cm-2 and lower OER potential at 10 mA cm-2 of 1.66 V.In terms of bifunctional performance,Co@NCNTs displays low ?E value of 0.81 V between ORR and OER.Furthermore,rechargeable zinc-air battery using Co@NCNTs as air electrode material shows outstanding performance in terms of discharge/charge voltage and cycling stability,better than those of Pt-Ru/C catalyzed rechargeable zinc-air battery.This strategy shows great potential in facile synthesis of NCNTs assembled carbon composites as active low-cost bifunctional catalysts for the new generation energy storage devises.Iron and nitrogen doped hierarchically ultra-thin porous carbon(Fe-N-HPC)derived from MOF is synthesized by simple bottom-up gas diffusion approach using ZIF-8 in upper position and iron acetate in the bottom of ceramic boat separated from each other by nickel foam.The as-constructed Fe-N-HPC exhibits interconnected three-dimensional porous structure,like honeycomb structure with excellent electron conductivity,large specific surface area and high N content(9,76%)and total percentage of pyridinic-N and graphitic-N(92.32%),Hence,for oxygen reduction reaction(ORR),Fe-N-HPC compared to Pt/C commercial precious catalyst,shows higher onset potential(0.97 V vs.0.96 V),half-wave potential(0.86 V vs.0.83 V)and limiting current density(5.50 mA cm-2 vs.5.20 mA cm-2).Additionally,the isolated Fe-N-HPC and NiFe-LDH electrode materials are applied for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)in rechargeable zinc-air battery,respectively.Fe-N-HPC&NiFe-LDH battery displays better cycle stability performance than Pt/C&NiFe-LDH battery.The discharge voltage and charge-discharge voltage gap of Fe-N-HPC&NiFe-LDH battery are 1.29 V and 0.69 V,after 1200 charge-discharge cycles.These results proved that the FeN-HPC&NiFe-LDH battery has higher discharge voltage and excellent cycling stability.This special designed composite electrode of Fe-N-HPC&NiFe-LDH greatly improved the cycling stability of rechargeable zinc-air batteries and displays promising potency for efficient energy storage applications.