Biomass-based Nanoporous Carbon Electrode Materials:Preparation And Supercapacitor Electrochemical Performance

Supercapacitors have been widely used due to their long cycle life,fast charging-discharging rate,high power output,and green safety.The performance of supercapacitors is determined by the physical and chemical properties of the used electrode materials.Among the various electrodes,carbon-based materials are always preferred owing to their large surface area,adjustable porous structures,high electric conductivity.Ameliorating the specific surface area and pore structure?porosity or pore size distribution?of the carbon materials has become an effective method to improve the supercapacitors performance.Porous carbon materials prepared by chemical activation usually have a large specific surface area.However,the activating agents?KOH,H₃PO₄,ZnCl₂,etc.?commonly used in chemical activation are highly corrosive.Furthermore,the incorporation of heteroatom?N,S,O,etc.?is beneficial to improve the carbon electrode performance.However,the process of incorporation of heteroatom is usually time-consuming and complicated.Thus,the biomass material with inherently abundant heteroatom and exceptional porous structure is a desirable carbon precursor.The innovations of this paper are as follows:Firstly,the beancurd is an ideal carbon source,due to its honeycomblike network structure which could yield a natural megaloporous structure in freeze-dried process.In addition,the nitrogen content in the beancurd makes it act as a cost-effective nitrogen source.The second is the relatively green,efficient CH₃COOK and K₂CO₃ were used as the activator to prepare nanoporous carbon.The details are as follows:1.Study on activation performance of potassium acetate:N-doped hierarchical porous carbons were synthesized via two steps method of pre-carbonization and CH₃COOK activation.Beancurd was applied as the carbon source and nitrogen source,and CH₃COOK was used as the activator,avoiding the use of corrosive activator?KOH,H₃PO₄,ZnCl₂,etc.?.The structural properties of as-prepared nanoporous carbon were characterized.The results showed that the specific surface area and pore structure of as-prepared nanoporous carbon could be adjusted by changing the dosage of CH₃COOK.The optimized carbon possesses a high BET specific surface area and nitrogen content are 1735 m² g^-1 and 1.33 at%,respectively.The electrochemical test was carried out in H₂SO₄ and KOH aqueous electrolytes,the sample possesses ultrahigh specific capacitance of 315 and 245 F g^-1 at 0.1 A g^-1,respectively.For comparison,carbon materials were prepared by KOH activation at the same conditions.The experiments have indicated that the carbon materials with CH₃COOK as an activating agent shows more optimized textural parameters and exhibits a higher specific capacitance.According to these features,CH₃COOK is an excellent activating agent,and this beancurd-based nanoporous carbon will have potential application in the supercapacitors.2.One-step preparation of frozen beancurd-based nanoporous carbon electrode material through of beancurd activation of potassium acetate:N-doped hierarchical porous carbons with large surface areas were prepared from beancurd through a one-step impregnation activation method at 750?with CH₃COOK as an activating agent.The results show that there are numerous macropores with a pore size of²00 nm observed by SEM and TEM as well as micropores probed by N₂-sorption analysis,indicating a hierarchical porous structure for these carbon materials.The effect of activating agent dosage on the porous structure of carbon was carefully investigated.The BET specific surface area and nitrogen content for the optimized carbon are 2180 m² g^-1 and 2.62 at%,respectively.As evidenced by electrochemical measurements,the as-prepared carbon has a good capacitive performance,and the specific capacitance is calculated to be 284 and 355 F g^-1 at a current density of 0.1 A g^-1 in KOH and H₂SO₄ electrolyte,respectively.More importantly,in KOH electrolyte,this carbon could deliver a high capacitance up to 118 F g^-1 at a very high current density of 30 A g^-1 due to its hierarchical porous structure,which will facilitate the ion transfer under high current density.The one-step impregnation activation method is simpler and more effective than the two-step method,and the as-prepared carbon material has better capacitance performance.3.One-step preparation of frozen beancurd-based nanoporous carbon electrode material through of beancurd activation of potassium carbonate:the N-doped hierarchical porous carbons materials derived from beancurd through a one-step impregnation activation method at750?with the activations of K₂CO₃.Characterizations reveal that the carbon materials have ultra-high surface areas(2514 m² g^-1),developed porosity with micro-meso-macro hierarchical pore structures and notable nitrogen content?1.85 at%?.The surface areas and hierarchical pore texture are related to the dosage of K₂CO₃.The electrochemical properties are monitored using conventional electrolytes H₂SO₄ and KOH.The optimized carbon displays high specific capacitance of up to 486 F g^-1(0.1 A g^-1,1 M H₂SO₄)and 404 F g^-1(0.1 A g^-1,6 M KOH)in a three-electrode system.According to these results,K₂CO₃ is also an excellent activating agent,which provides an inspired for the exploration of new green,environmentally friendly activating agents and the selection of biomass precursors for the synthesis of nitrogen-doped biomass-based nanoporous carbon.