Spruce Bark Derived Hierarchical Porous Carbons For High-performance Electrochemical Supercapacitors

The rapid depletion of non-renewable fossil fuels and increasingly worsening environmental pollution have stimulated intensive research on high-performance and low-cost electrochemical energy storage to meet urgent future energy requirements worldwide.Among various electrochemical energy storage devices,supercapacitors have attracted tremendous attentions in the past decade due to their superior power density,long cycle life,fast charge/discharge processes and higher reliability than batteries.Generally,the performance of the supercapacitor is largely determined by the intrinsic properties of the electrode materials,such as high electrical conductivity for instantaneous electron transport,large ion-accessible surface area for efficient charge accumulation,and high electrochemical stability for long cycle life.Accordingly,considerable research efforts have been put forth to design and synthesis of novel electrode materials for supercapacitors.Among various electrode materials,carbon-based nanostructured materials are considered as the most attractive electrode materials because of their large specific surface area,low cost,excellent electrical and mechanical properties.Up to now,number of carbonaceous materials,such as activated carbons?ACs?,carbon nanotubes,graphene and carbon nanofibers,have been developed as electrode materials for supercapacitors.However,there are also several drawbacks in practical applications owning to the multiple and delicate synthesis processes and the relatively expensive templates or carbon precursors as applied.Therefore,it is still highly desirable to develop a simple,eco-friendly and effective synthesis strategy for novel carbon materials from low-cost carbon precursors,especially from renewable waste biomass.As an important renewable energy,biomass has been attracting more and more attentions due to their advantages of low-cost,ease of processability,renewable and lower pollution.In addition,the biomass derived-carbon materials can maintain the unique structure of the biomass with high specific surface area and desired pore size.During the past few years,many kinds of biomass have been widely used to prepare carbon materials.However,the typical top-down approaches are incapable of controlling the material features,including morphology,porosity and surface chemistry,which goes against the electrode materials for specified applications.To address these questions,new strategies must be developed to adjust the physical structures and surface properties in the resulting carbon.Based on this,we have carried out a series of studies using biomass for the preparation of active carbon materials.The main research content are as follows:1.3D vertically aligned graphene nanosheet arrays?3D VAGNAs?have been synthesized successfully through a facile,low-cost,catalyst-free and reliable method from biomass waste of spruce bark,which is abundant,renewable,and widely available.As the electrode material for supercapacitor,the 3D VAGNAs exhibit a high capacitance of 393 F g^-1 at a current density of 0.5 A g^-1 and an outstanding cycling stability?96.3%capacitance retention after 10,000 cycles?in 6.0 M KOH electrolyte.Moreover,in TEABF₄/AN electrolyte,the VAGNA-900 based symmetric supercapacitor exhibits an outstanding capacitance of 239 F g^-1 at a current density 1 A g^-1,and a high energy density of 74.4 W h kg^-1 could be obtained at power density of 743.7W kg^-1.Such a remarkable electrochemical performance is highly desirable for supercapacitors to compete with other energy storage devices for real applications.2.Novel three-dimensional interconnected hierarchical porous carbons?3D HPCs?are fabricated from biomass waste,i.e.pine bark using a one-step combined method of simultaneous carbonization-activation process.Due to the unique 3D interconnected hierarchical porosity,narrow pore size distribution and high specific surface area(ca.1491m² g^-1),the as-prepared 3D HPCs exhibits a high specific capacitance of 386 F g^-1,excellent cycling stability with 92.4%of capacitance retention after 10 000charge/discharge cycles in 6 M KOH in the three-electrode system.In addition,the assembled HPC-2//HPC-2 symmetric supercapacitor in 6 M KOH delivers a high en-ergy density of 9.7 W h kg^-1 at power density of 253 W k g^-1.More importantly,in TEABF₄/AN electrolyte,HPC-2 based symmetric supercapacitor exhibits an out-standing energy density of 52.4 W h kg^-1 at power density of 75 W kg^-1.The present work not only provides a simple method towards low-cost and large-scale production of 3D HPCs for high-performance energy storage devices,but also offers a reliable strategy for high-value utilization of biomass waste and universal resource acquisition from nature.3.A new kind of hierarchically porous carbon nanosheets?PCSs?derived from spruce bark has been prepared by a facile one-step pyrolysis route.The as-prepared PCSs possesses unique porous nanosheet morphology with high specific surface area of ca.2293m² g^-1.The PCS-based supercapacitors are tested in three aqueous electrolytes including 1.0M Na₂SO₄,1.0 M H₂SO₄,and 6.0 M KOH.In 1.0 M Na₂SO₄ electrolyte,the as-assembled PCS-based symmetric supercapacitor gives a high energy density of 27.3 W h kg^-1 and a remarkable long-term cycling stability with 98%retention of intinal capacitance,indicating the promising of the as-prepared PCSs for electrochemical energy storage and conversion.