A Study On The Preparation And Electrochemical Performance Of Gasified Rice Husk Carbon-based Metal Sulfide Composite Materials

Energy and environmental crises have become global issues that restrict sustainable development.Considered as a green and renewable energy,biomass energy plays a vital role in combating climate change,imbalance between energy supply and unsustainable development.Biomass gasification polygeneration is one of the important technologies for biomass energy utilization by employing pyrolysis,oxidation and reduction reactions under high temperature and oxygen-limited conditions to produce flammable gas,biomass carbon,and biomass liquid.The generated flammable gas from biomass gasification polygeneration can be used for power generation?or heat supply?,while the biomass carbon can be utilized as activated carbon,industrial carbon material,soil improver,etc.,according to the properties of different raw materials,in addition,the biomass liquid can be also used as liquid fertilizer.This paper elaborates a novel and integrated method to achieve the high-value utilization of gasification biomass carbon.The gasified rice husk carbon as a by-product of biomass gasification power generation was prepared into supercapacitor electrode material.Activated carbon with 3D net pore structurewas first prepared via carbon-silicon separation by lye and activation using CO₂,and then used as supercapacitor electrode materials.In order to enhance the energy density of carbon materials,the synthesis of porous carbon-based metal sulfide composites and the performance of supercapacitor were further studied.The main research contents are summarized as follows:?1?Using gasified rice husk carbon as raw material,the porous rice husk activated carbon?RHAC?was prepared via boiling in lye which enables the removal of the silica template in the gasified rice husk carbon and the combination of lye and carbon dioxide for one-step activation.The developed pore structure with micropores,mesopores and macropores in activated carbon not only provides sufficient space for ion transport and charge storage,but also benefits the penetration and diffusion of electrolyte ion.Experimental results show that the porous carbon obtained by 30%KOH solution boiling and 850°C physicochemical one-step activation has a specific surface area of 1383 m²/g,a mesoporosity of 61.2%,and oxygen-enriched functional group?oxygen content is14.07 at%?.In 6.0 mol/L KOH electrolyte,the specific capacitance of RHAC was 168 F/g at the current density of 0.25 A/g.The specific capacitance of RHAC can be maintained at 75.5%whencurrent density increased from 1 A/g to 6 A/g.After 2000 charge and discharge cycles at 0.25A/g,88.1%of the peak specific capacity?initial value?of the RHAC could still be maintained.Overall,porous RHAC has excellent rate performance,voltampere characteristics,cycle stability and impedance characteristics,thus can be used as an ideal electrode material for supercapacitors.?2?The porous carbon/sulfur-doped tin oxide composite was synthesized in situ by solvothermal method.The synthesized hetero-nanostructure composite features a highly conductive porous matrix and a short ion/electron transport path,which could enhance the pseudocapacitance dynamics.SnO₂ with high theoretical specific capacity tends to form nanostructures in the composite and provide more reactive sites.The introduction of electron-rich S atoms into the carbon material increased its polarization and electronic conductivity.Pseudocapacitance was produced via redox reaction where S combines with C and O to form sulfur-functional groups during charging and discharging.The Faraday reaction enables these reactive sites to store more charge,thereby increasing the specific capacity of the material.The specific capacitances of the S doped RHAC/SnO₂ composite electrode was 283 F/g,228 F/g,186F/g and 176 F/g at the current density of 0.4 A/g,1 A/g,2 A/g,and 4 A/g,respectively.Compared to the untreated porous carbon,the specific capacitance of S-doped RHAC/SnO₂ composite electrode increased by 59.4%at 1 A/g.When the current density increased to 6 A/g,the specific capacitance of the S-doped RHAC/SnO₂ composite electrode can reach 76.8%of that under 1 A/g,suggesting good rate performance.At 0.4 A/g,the specific capacitance of S doped RHAC/SnO₂composite electrode was maintained at 78.5%after 5000 cycles of charging and discharging,indicating the long cycle life of the electrode material.?3?A three-dimensional porous gasified rice husk activated carbon composite Ni₃S₂nanosheet electrode directly grown on nickel foam was prepared by a simple one-step hydrothermal method,and the RHAC/Ni₃S₂-16//RHAC asymmetric supercapacitor was assembled.The absence of binder in the electrode reduced the internal resistance and improved the efficiency of the electrode.The porous RHAC as a substrate exposed more structural units of Ni₃S₂ to the electrolyte leading more electrolyte flowing into the interior of the electrode material and facilitating adsorption-desorption of electrolyte ions and Faraday reaction.Furthermore,RHAC stabilized the volume structure of metal sulfide during charging and discharging and improved the conductivity of the sulfide.When the current density was 20 A/g,RHAC/Ni₃S₂-16 showed 600F/g of high specific capacitance.When the current density was increased from 1 A/g to 6 A/g,the specific capacitance was maintained at 80%,and the RHAC/Ni₃S₂-16 electrode had excellent rate performance.After 5,000 charge and discharge cycles at 10 A/g,88.8%of the maximum?imital?specific capacitance of the electrode was maintained.The assembled RHAC/Ni₃S₂-16//RHAC asymmetric supercapacitor achieved a specific capacitance of 114.3 F/g at 0.2 A/g,a power density of 150 W/kg,and an energy density of up to 35.7 Wh/kg,the energy density was remained at 27.9Wh/kg with a high power density at 1500 W/kg.Developed pore structure of gasified rice husk porous carbon provide sufficient space for the contact between the electrode material and the electrolyte,therefore the potentially high electrochemical activity,specific capacitance and energy density of the transition metal sulfide were fully exhibited.?4?The flower-shape RHAC/NiCo₂S₄ composite nanomaterials with good dispersibility were synthesized in situ by adjusting the content of porous carbon in the composite.The NiCo₂S₄nanosheets in the composite material can be evenly dispersed on the skeleton structure of porous carbon.The developed pore structure provided sufficient space for the adsorption/desorption and the rapid transfer of electrolyte ions,thus enabling the quick penetration of the electrolyte into the electrode material to reach more electrochemically active sites and increasing the efficiency of NiCo₂S₄.At the current density of 1 A/g,the specific capacitance of the RHAC/NiCo₂S₄-120composite electrode was as high as 1680 F/g.When the current density was increased to 6 A/g,the specific capacitance was 1500 F/g with a retention of 89%,indicating the excellent rate performance of the composite electrode.The assembled RHAC/NiCo₂S₄-120//RHAC asymmetric supercapacitor had a specific capacitance of 133.3 F/g?current density of 0.2 A/g?,a power density of 150 W/kg,and an energy density as high as 41.6 Wh/kg.At a high power density of 1500 W/kg,the energy density was maintained at 29.2 Wh/kg.After 5000 cycles charging and discharging at 1A/g,86%of the initial specific capacitance of the RHAC/NiCo₂S₄//RHAC asymmetric supercapacitor can still be maintained,showing excellent cycle stability.Developed pore structure of gasified rice husk porous carbon RHAC used as the substrate could effectively disperse NiCo₂S₄nanosheets and reduce the agglomeration of NiCo₂S₄.The combination of nickel-cobalt bimetallic sulfides and the synergistic effect between metal cations of different valence states can effectively improve the electrochemical reactivity of the electrode materials and induce more redox reactions.Furthermore,the multi-components of the RHAC/NiCo₂S₄//RHAC asymmetric supercapacitor can significantly reduce the band gap of the material and improve the conductivity of the electrode material.The synergistic effect and high conductivity eventually improve the electrochemical properties of electrode materials.