Study On Preparation And Electrochemical Performance Of Biomass- Derived Porous Carbon Materials

Electrode materials and electrolytes are key factors which determine the electrochemical performance and price of supercapacitors.Carbon materials are ideal electrode materials for electric double-layer supercapacitors due to their high specific surface area,controllable morphology,good electrical conductivity,low cost,and environmental friendliness.Various types of biomass are widely used in the preparation of carbon-based energy storage materials on account of their merits of abundant resources,diverse types,renewable,low-cost and eco-friendly.In this dissertation,high-performance porous carbon electrode materials were prepared using low-cost biomass materials as start materials.The effect of morphology,pore distribution,and surface elements on their electrochemical properties were studied.At the same time,this dissertation tries to simplify the preparation process of porous carbon materials through a self-activation strategy for further reducing the cost of porous carbon electrode materials.Furthermore,the rate performance of biomass-based porous carbon is improved to meet the requirement of ultra-fast charge/discharge of supercapacitors by improving the the degree of graphitization electrode material.Finally,the energy density of electric double layer supercapacitors is imporved by developing a new type of alkaline aqueous electrolyte with large stable working voltage.?1?A nitrogen doped microporous carbon is obtained by a one-step self-activation method by using celery as raw materials.The prepared nitrogen-doped microporous carbon inherits the developed water transport system in biological tissue of celery,and exhibits developed hierarchic porous structure with opened micropores.Its specific surface area is up to 1186 m² g^-1,and the average micropore diameter is 0.94nm.The nitrogen heteroatom doping enhances the wettability,electrical conductivity,and surface activity of the prepared material,improves the accessibility of the electrolyte solution in the micropores,and thus produces excellent capacitance performance.Under the three-electrode system,the specific capacitance of the prepared nitrogen-doped microporous carbon electrode material is as high as 245 F g^-1(0.5 A g^-1),and capacitance retention rate reached 68.6%at a current density of20 A g^-1.When assembled as a coin-type symmetrical supercapacitor device,the specific capacitance of the assembled device reaches 54.0 F g^-1,and when the current density increases from 0.5 A g^-1 to 20 A g^-1,the capacitance retention rate is as high as 93.1%and no significant capacitance attenuation is observed after 5000 charge/discharge cycles.The prepared nitrogen-doped microporous carbon material with excellent charge storage ability,perfect rate capability,and ideal cycle performance is considered as an ideal low-cost electrode material for supercapacitors.One-step conversion of biomass into porous carbon electrode material through self-activation strategy would avoid the using of additional activators which are always potentially harmful to operators and the environment,thereby greatly simplify the preparation process of porous carbon materials and reduce the harm of activators to operators and the environment.Therefore,the reported self-activation strategy provides a simple,convenient,environmentally friendly and low-cost preparation approach for the high-performance porous carbon electrode materials.?2?Amorphous,semi-graphitized,and graphitized porous carbons with tailored pore size distribution were facilely prepared via controlling the activation temperature of pyrolytic carbon derived from phoenix fallen leaves when employing potassium ferrate?K₂FeO₄?as catalyst and activation agent.The obtained semi-graphitized microporous carbon exhibits a large specific surface area(2208 m² g^-1),high proportion of large micropores?more than71.8%?and high conductivity(2.38 S cm^-1).The specific capacitance of the obtained semi-graphitized microporous carbon reaches 254 and 273 F g^-11 at a current density of 0.5A g^-1 under three-electrode configuration in KOH and H₂SO₄ aqueous electrolyte,respectively.More than 84.6%and 76.1%of initial capacitance is retained under a current density of 20 A g^-1 in KOH and H₂SO₄ aqueous electrolyte.A capacitance retention of 86.9%is obtained under an ultra-large current density of 100 A g^-1 when employed as electrode material for symmetrical supercapacitor,and no apparent attenuation was observed after a long time charge/diacharge cycling?10,000 cycles?.Furthermore,energy density of the symmetrical supercapacitor based on the semi-graphitized microporous carbons reaches 7.4 Wh kg^-1 at a power density of 151.4 W kg^-1,and more than 87.8%of energy density is kept even under an ultra-large power density of 29.1 kW kg^-1.By improving the graphitization degree of biomass-derived porous carbon,the electrical conductivity of porous carbon materials can be significantly improved,and the ohmic impedance of the device can be reduced even under an ultra-large current density,thereby a high specific capacitance can be retained at an ultra-high current density.?3?A semi-graphitized microporous carbon was obtained via one-step activation method by using pineapple skin as start material,potassium ferrate?K₂FeO₄?as catalyst and activated agent.Meanwhile,a novel mixed aqueous electrolyte of KOH and choline was successfully developed to extend the working voltage window of alkaline aqueous electrolyte to 1.4 V.The electrochemical behavior of the prepared semi-graphitized microporous carbon in the developed mixed aqueous electrolyte was further studied.The pineapple skin-derived semi-graphitized microporous carbon exhibits a high specific surface area of up to 1413 m²g^-1 with a large total pore volume of 0.81 cm³ g^-1.More than 50.7%of micropore volume was contributed by the large micropores with a pore size in the range of 1-2 nm.The conductivity of the obtained semi-graphitized microporous carbon reaches 3.46 S cm^-1.The specific capacitance of the obtained semi-graphitized microporous carbon in the mixed electrolyte?0.1M choline+1 M KOH?reaches 310 F g^-1 at current density of 0.5 A g^-1 within a voltage window of-1.0⁰.4 V,which value is slightly higher than the specific capacitance in a conventional alkaline aqueous electrolyte of 6M KOH within a voltage window of-1.0⁰ V(300 F g^-1).At a current density of 20 A g^-1,the capacitance retention rate of the semi-graphitized microporous carbon in the mixed alkaline electrolyte is as high as 51.6%.The assembled symmetric supercapacitor using the mixed alkaline electrolyte exhibits a specific capacitance of 43.1 F g^-1 within a working voltage of 1.4 V,and a capacitance retention rate of 80.1%at current density of 20 A g^-1.Under the power density of 210.9 W kg^-1,the energy density of the device in the mixed alkaline electrolyte reaches 11.72 Wh kg^-1,which is 32.2%higher than the traditional KOH electrolyte.The capacitance behavior of the prepared semi-graphitized microporous carbon in the mixed alkaline electrolyte is dominated by electric double layer capacitors.While greatly increasing the energy density,the excellent rate performance and high power density of electric double layer supercapacitors is maintained.