High-Performance Carbon-based Oxygen Reduction Electrocatalysts For Zinc-Air Batteries

Oxygen reduction reaction(ORR)occupies a pivotal position in a series of conversion devices,including metal-air batteries and fuel cells.Nevertheless,owing to the multistep proton-coupled electron transfer,the cathodic reaction is notorious for its sluggish kinetics.Therefore,suitable ORR electrocatalysts are required to expedite the reaction rate and efficiency.Even though platinum and its alloys are the best-known ORR electrocatalysts,their limited natural abudance and high cost have throttled their large-scale application.The search for low-cost substitutions with outstanding ORR activity and durability remains a daunting challenge.In our paper,we explored three kinds of carbon-based electrocatalysts to catalyze ORR and applied them in metal-air batteries.Three works are introduced as followed:1)Through regulating the compositon,morphology and structure of the precursor of the cobalt analog of Prussian blue,followed by high-temperature treatment under the inert atmosphere,we successfully obtained the metallic cobalt nanoparticles encapsulated in nitrogen-enriched graphene shells(Co@NG).We adopted the most optimized carbonization temperature by investigating the structure and catalytic performance under different temperatures.Upon the acid treatment to remove the metallic cobalt cores,the ORR activity of Co@NG-acid was not declined but improved,suggesting that most of the cobalt cores did not directly participate in the ORR electrocatalysis.Co@NG-acid exhibited a great ORR activity in acid or alkaline,indicating a four-electron reduction process of oxygen.We considered that the active center of Co@NG and Co@NG-acid was Co-N-C moieties with the assistance of the CN-poisoning test.Encouraged by the promising electrochemical results of Co@NG-acid,we went on to apply it to the primary Zn-air battery as the air cathode catalyst.Our device displayed a high open circuit voltage comparable to Pt/C,large discharge current density,high peak power density and excellent operation stability.2)We develped a three-step method to prepare cobalt-nitrogen codoped multiwalled carbon nanotubes(Co-N/CNT),using dopamine as the starting precursor.This biomolecule spontaneously selfpolymerized under weakly alkaline conditions and uniformly coated CNTs.Via the metal-catechol interaction,adequate cobalt cations were absorbed by the polymeric coating layer,resulting in the introduction of cobalt into the system.Followed by pyrolyzed under NH3,the final product revealed a core/shell structure.The highly defective carbonaceous shell was converted from the decomposition of nongraphitic polydopamine,which facilitated the formation of Co-N/C active sites during high-temperature treatment.On the other hand,the crystalline carbon nanotube core provided extraordinary electrical conductivity and desirable corrosion resistance.These two components corporately endowed the hybrid material with remarkable ORR performance.In 1 M KOH,the half-wave potential of Co-N/CNT is ~0.91 V with the peroxide yield of less than 7%.Furthermore,Co-N/CNT could be employed as the air catalyst of primary Al–air or Zn–air batteries.The formed primary battery delivered high discharge current density,large peak power density and prolonged operation durability at low or high current density.3)We successfully synthesized the MOF-5 nanoplates by the self-assembly between zinc acetate and terephthalic acid with polyvinylpyrrolidone acting as the surfactant.After pyrolysis in inert atmosphere to remove metal Zn,more pores were formed,which created a favorable condition for the future nitrogen-doping behavior in NH3.As a result,the metal-free carbon-based materals with nitrogen doping were prepared.The surface of the catalyst was rough and porous by the evaporation of Zn,but the nanoplate structure was remained.The catalyst possessed a good ORR activity and stability with a positive half-wave potential and a low peroxide yield in alkaline.The self-assembled Zn-air battery enables large discharge current density,power density and prolonged stability when the catalyst was used as the air catalyst in the Zn-air battery.