Carbon Nitride Derived Co/Fe-Based Carbon Nanotubes As Oxygen Electrodes For Metal-Air Batteries

Designing and preparing oxygen electrode materials with both electrocatalytic oxygen reduction reaction（ORR）and electrocatalytic oxygen evolution reaction（OER）ability is the key to improving the performance of metal-air batteries.Herein,we focus on the preparation of highly efficient metal（Co/Fe）-based carbon nanotubes materials by high-temperature conversion of carbon nitride,and their electrocatalytic OER,ORR,and metal-air batteries performances.The specific research contents as follow:Firstly,uniform cobalt-based carbon nanotubes（Co-NCNTs）were obtained by the high-temperature solid-phase reaction between cobalt nitrate（Co（NO₃）₂）and carbon nitride（C₃N₄）.The sample Co-NCNTs-N was treated by 2 M HNO₃,in which the metallic Co and Co O_xparticles on the top of the nanotubes were cleaned partially,with the sample exhibits excellent perfomances both in ORR（onset potential of 0.86 V and half-wave potential of 0.80 V）and OER(only need a potential of 1.58 V to reach current density of 10 m A cm^-2).Then,we use Co-NCNTs-N as oxygen electrode of Zn-air batteries,the maximum power density is 210 m W cm^-2,the cycle life is longer than 130 h,which is even better than the commercial Pt/C-Ru O₂ones.Secondly,the high-temperature solid-phase reaction products from three iron sources（ferric chloride,ferrous sulfate,and iron acetylacetonate）and carbon nitride（C₃N₄）were disscused.The results showed that C₃N₄could be converted into one-dimensional iron-containing carbon nanotubes combined with two-dimensional graphene material（Fe-NCNTs@RGO）when use iron acetylacetonate as the iron source.Then,the growth mechanism of carbon nanotubes was disscused,we found that the iron and iron cementite（Fe₃C）nanoclusters was generated at 700°C acting a key role for the growth of carbon nanotubes.In addition,Fe-NCNTs@RGO-N based Zn-air batteries and Al-air batteries showed great discharge performances(open circuit voltage,1.552 V and 1.602 V;maximum power density,85 m W cm^-2and 120 m W cm^-2),which are better than the commercial Pt/C ones.Thirdly,we successfully synthesis in situ growth of Co/Fe-containing carbon nanotubes（Fe-CNTs@CP and Co-CNTs@CP）,the CNTs was grown on the surface of carbon paper through a simple high-temperature solid-phase method.The specific surface areas of the prepared samples are 32.38 m²/g and 104.05 m²/g,which are 11.6times and 37.3 times of clean carbon paper,respectively.These two self-supporting electrodes were used as oxygen electrodes for Zn-air batteries directly,which exihibited excellent charge-discharge performances.For Fe-CNTs@CP,a voltage of0.64 V is need to reach a current density of 50 m A/cm².For Co-CNTs@CP,a voltage of 0.95 V is need to reach a current density of 50 m A/cm².In addition,they both show excellent stable charge-discharge performances longer than 40 h.Two in situ oxygen electrodes are better than the noble metal Pt/RuO₂ ones.