| Transition metal oxides,which possess the advantages of good chemical stability,abundant reserves,low cost and environmental friendliness,have large application prospects in the field of energy storage and conversion,such as the fuel cells,lithium ion batteries,supercapacitors and all vanadium redox flow batteries.However,transition metal oxides still suffer from low specific surface area and poor conductivity,which hinders their further application in energy conversion and storage devices.In this paper,we adopted the biomass-derived nitrogen-doped hierarchical porous carbon(NHPC)with large specific surface area as a support to anchor trimanganese tetraoxide(Mn3O4)nanoparticles(NPs),generating the Mn3O4/NHPC hybrid material.The resultant composites were used as the electrode materials in all vanadium redox flow batteries and supercapacitors,respectively,and effects of composition and structure of composite on the energy storage performance were investigated.Firstly,the NHPC support was synthesized through the pre-carbonization and activation approach using cattle bone as precursor and potassium hydroxide as active agent.The obtained NHPC possess a high content of nitrogen(2.6 at%)and oxygen(15.9 at%),and an ultra-high specific surface area(SSA,2094 m2 g-1)with a micro-/meso-/macro-pores interconnected hierarchical porous structure.Then,the Mn3O4/NHPC composite was prepared through the hydrothermal approach by using ammonia water to control the nucleation process of Mn3O4.Results show that the abundant heteroatoms on NHPC can provide sufficient anchor sites for Mn3O4 NPs,while the ultra-high SSA of NHPC is beneficial to the uniform dispersion of Mn3O4 NPs.Optimization experiments indicate that Mn3O4 NPs with an average particle size of 13.3 nm are uniformly dispersed on the surface of NHPC as the hydrothermal temperature is 150℃ and the content of ammonia water is 1 mL.The obtained Mn3O4/NHPC has a large SSA of 774 m2 g-1,and inherits the hierarchical pore structure form NHPC with a nitrogen content of 3.4 at%and an oxygen content of 23.1 at%.Secondly,the as-prepared Mn3O4/NHPC composites were applied in the positive reaction of all-vanadium redox flow battery to catalyze V(Ⅳ)/V(Ⅴ)redox reaction.Results indicate that Mn3O4 in Mn3O4/NHPC can act as the major active sites for V(Ⅳ)/V(Ⅴ)redox reaction,and the inherited meso-and macro-porous structure is beneficial for the mass transfer of the electrolyte,thus further promoting the catalytic performance.Electrochemical tests show that the catalytic performance for V(Ⅳ)/V(Ⅴ)redox reaction is negatively correlated with the average particle size of Mn3O4 NPs in Mn3O4/NHPC.When the average particle size of Mn3O4 NPs is 13.3 nm,the peak current density of the oxidation peak was the highest(44.4 mA cm-2)among all simples.Then,the all-vanadium redox flow battery was assembled with Mn3O4/NHPC as the electrode material.The energy and voltage efficiency is determined as 81.5%and 87.2%at 40 mA cm-2,respectively,which is 13.8%and 14.5%higher than that of blank carbon paper,respectively.The voltage efficiency of the battery reaches as 73%at 100 mA cm-2.Finally,the Mn3O4/NHPC composite was uses as the supercapacitor electrode material and the effects of Mn3O4 loading and the surface chemical state of composite on the capacitance performance were investigated.Results shows that the abundant N(4.6 at%)and O(16.2 at%)heteroatoms of Mn3O4/NHPC are beneficial to the infiltration of electrolyte,and the large number of micropores can provide a large electric double layer capacitance.Furthermore,the inherited meso-and macro-pores pore facilitated the rapid transport of electrolytes that further improve the rate performance of composite.Electrochemical tests show that the specific capacitance of Mn3O4/NHPC increases with the increase of Mn3O4 loading.In a 1 M sodium sulfate electrolyte,the specific capacitance of the composite was determined as 270 F g-1 at a current density of 0.5 A g-1.By using Mn3O4/NHP as the positive electrode material and NHPC as the negative electrode material,an asymmetric supercapacitor was assembled.The energy density and power density were determined as 37.7 Wh kg-1 and 229 W kg-1 at a current density of 0.5 A g-1,respectively.And,the energy and power density reached as 31.6 Wh kg-1 and 5.2 kW kg-1 at a current density of 5 A g-1,respectively. |
