Fabrication Of Hierarchical Porous Nitrogen-doped Carbon And Their Study On Zinc-air Battery Performace

As fossil energy crisis and environmental problems getting worse,it is vital to develop green and sustainable new energy.The clean energy conversion device represented by zinc-air battery has attracted extensive attention from researchers.However,the charging（OER）and discharging（ORR）reactions on the air electrode are strugle and complex.Therefore,the efficient electrocatalysts are urgently needed to be developed.However,the precious metal catalysts are difficult to be widely used due to factors such as cost and stability.Therefore,it is necessary to develop highly efficient,stable and inexpensive bifunctional electrocatalysts.Nitrogen-doped carbon catalysts have the advantages of cheap raw materials,simple structure and easy regulation.However,their intrinsic catalytic activity is not so good that they have not received extensive attention.Hierarchical porous structure has a rich pore structure,which can dramatically enhance the catalytic activities,but its stability is often not satisfactory.Therefore,in this paper,hierarchical porous nitrogen-doped carbon bifunctional electrocatalysts were prepared by using the ligand exchange strategy and the soft-hard dual template method,respectively.It was applied to the zinc-air battery,and the reasons for the excellent stability of the catalyst were speculated based on the Wenzel-Cassie coexisting state model.Nitrogen-doped carbon nanoflowers with specific mesoporous size were prepared by ligand exchange strategy through step-by-step pyrolysis.The catalyst exhibits a specific surface area of 965 m²/g and abundant 3.8 nm mesopores.The catalyst exhibits comparable electrocatalytic performance to that of commercial Pt/C and Ru O₂ catalysts,with a total overreaction potentialΔE of 0.7 V.The peak power density of zinc-air battery is 176 m W cm^-2.The specific capacity and energy density are 652 m Ah g^-1 and 767 Wh g^-1,respectively.Benifiting from the specific size mesopores of 3.8 nm,hp-NF-2g demonstrated better stability than commercial catalysts with no significant change in voltage range after 500 cycles in discharge-charge cycles.Hierarchical porous nitrogen-doped carbon bifunctional catalysts were prepared by using the soft-hard dual template method combined with the"sugar-blowing"method and the zinc hard template method.The catalyst exhibited bifunctional catalytic properties withΔE of 0.58 V,merely.In addition,hp-NC-3 has excellent catalytic stability,maintaining 97%and 92%current densities after 24 h ORR and OER cycles,respectively.When applied to zinc-air batteries,it shows better performance than the zinc-air batteries assembled by commercial composite catalysts.Its specific capacity and energy density are up to 789 m Ah g^-1 and 992 Wh kg^-1,respectively,and it also shows a peak power density of 213 m W cm^-2.The hp-NC-3assembled zinc-air battery is very stable,which can maintain the constant current charge-discharge stability for 20,000 minutes and has a good performance of rate capability.Combining with the Wenzel-Cassie coexisting state model,the excellent stability of hp-NC-3 is due to the abundant mesopores with 3.8 nm,which is conducive to the formation of such a Wenzel-Cassie coexisting state,improving the stability of the three-phase interface.