| With the rapid development of technology and the progress of society,our demand for energy is constantly expanding.Many researchers pay more attention to the renewable energy sources in order to relieve the energy shortage problems.However,the drawbacks of unstable and intermittent nature of renewable energy sources(such as wind,solar and tidal),have greatly urgent us to develop green and efficient energy conversion and storage strategies.Fuel cells and rechargeable metal-air batteries,as the competent alternatives,are intensively investigated in the past decades.The bifunctional electrode for oxygen reduction/evolution reactions plays a paramount role in determining the performance of fuel cells and rechargeable metal-air batteries.But,due to the sluggish kinetics of the oxygen electrode reaction especially in the ORR,the power density and energy efficiency of these devices are severely constrained.Recently,some hierarchically porous carbon materials(HPCs)with well-defined pore structure were successfully constructed by using sodium chlorine micro-crystallites(NaCl MCs)as the template.NaCl MCs,which are generated under rapid freeze,are regarded as a facile template due to their environmental-friendly feature,easy to remove by water and recycle by recrystallization.In this paper,honeycomb-like hierarchically porous carbon material was synthesized by NaCl assisted template method and secondary calcination.As the same time,the electrochemical properties of the porous carbon material were studied in depth.On the one hand,the optimal oxygen reduction reaction catalyst was synthesized by screening different organic precursors,transition metal salts and calcination temperature.On the other hand,the comprehensive physical characterization and electrochemical tests of FeN-HPC-900(M)and FeN-HPC GDX were carried out to explore the morphology and material composition of the catalysts,so as to explain the electrochemical performance of their materials.Finally,the best ORR catalyst was assembled to a zinc-air battery to evaluate its actual performance of charging and discharging and cycle stability.In summary,we have synthesized 3D honeycomb-like FeN-doped hierarchical porous carbon(FeN-HPC GD2)materials with excellent ORR performance,which use the mixture of glucose and DCDA as organic precursors and a small amount of FeC13.Those experiments verified that the introduction of glucose to DCDA increased the carbonization yield of nitrogen doped carbon materials significantly.Due to the abundant macro-meso-micro pores,fast electron conductivity and high specific surface area(618.2 m2 g-1)with extremely high percentage(93.7%)of graphitic N&pyridinic N,FeN-HPC GD2 exhibits excellent ORR activity,which surpasses commercial Pt/C with higher half-wave potential(0.888 V vs.0.863 V)and kinetic current density(23.2 mA cm-2 vs.3.2 mA cm-2@0.85 V).The primary zinc-air battery using FeN-HPC GD2 electrocatalyst exhibits a maximum power density of 241 mW cm-2 at high current density of 344 mA cm-2.Moreover,after coupling with non-precious NiFe-hydroxides nanosheets as a bifunctional electrode,the rechargeable Zn-air battery exhibits a small potential gap(0.61～064 V)between charging and discharging with a high voltage efficiency(66.7～68.6%).Significantly,the battery exhibits excellent durability with discharging potential maintaining as high as 1.3 V after 1000 cycles.Even in the ultralong durability test of 5000 charge and discharge cycles,the discharge voltage of the FeN-HPC GD2&NiFe-LDH battery only decayed to 1.19 V.The charge-discharge voltage gap is 0.79 V,and the voltage efficiency is still 60.1%,which are much better than that of previously reported Zn-air batteries.Our work provides a promising way for building highly efficient electrocatalyst and high performance rechargeable zinc-air batteries. |
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