Biomass-derived Hierarchically Porous Carbon Electrode Material And Its Supercapacitive Performance

As an electrical energy storage device,supercapacitor plays an important role in the energy storage field with its ultra-fast charge and discharge rate,high power density and long cycle life.However,low energy density is still a major challenge for supercapacitors compared to secondary batteries like lithium ions,which greatly limits its practical application.In resent years,the energy density of supercapacitor has been improved by using new electrode materials to increase the specific capacitance or an organic or ionic liquid electrolyte with a high potential window.However,its high toxicity,flammability and high cost severely limit the widespread use of organic or ionic liquid electrolytes compared to aqueous electrolytes.Therefore,considerable efforts have been dedicated to developing innovative electrode materials with higher specific capacitance as well as long cycle life,low cost,and the ability to be produced on a large scale.The biomass carbon-based materials are the green and renewable resources,which can be derived from low-cost,easy preparation.Therefore,it has opened up a path for the development of new high-performance electrode materials.In present paper,we designed and synthesized a novel hierarchical porous carbon electrode material from natural biomass.Then,the morphology,structure,composition and electrochemical properties of the material are discussed in detailed.At the same time,we assembled a symmetrical supercapacitor with the same electrode material as the positive and negative electrodes respectively to evaluate the practical application performance of the material.The specific research contents are as follows:1.A facile one-step strategy is established to prepare nitrogen doped three-dimensional?3D?hierarchical porous carbon from buckwheat husk,in which the synchronous activation and nitrogen-doping is fulfilled via using KOH and urea.The synergistic effect of the built-in activator potassium hydroxide and urea penetrates into plant fiber tissue to form a special structure of BSCN-700.The obtained sample?BSCN-700?possesses an ultrahigh specific surface area of 2794.5m² g^-11 with a unique hierarchical carbon frame.In the three-electrode system,the BSCN-700 exhibits a high specific capacitance of 326 F g^-1 at current density of 1 A g^-1,with good rate capability(78.4%capacitance retention at 100 A g^-1)in 6 M KOH electrolyte.Furthermore,the assembled symmetric supercapacitor based on BSCN-700 and filled with 6 M KOH electrolyte delivers an outstanding energy storage performance(energy density of 20.4 Wh kg^-11 at power density 699 W kg^-1)with good cycling stability(95%retention after 5000 cycles at 5 A g^-1).These results indicate that the eco-friendly carbon materials derived from biomass have a huge potential in the applications of high-performance electrode materials for supercapacitors.2.We report a simple two-step process to fabricate hierarchical porous carbon from biomass.The so-called two-step refers to hydrothermal carbonization of cypress coats followed by activation with KOH.The morphology and porous parameters of the resulting porous carbon can be controlled by adjusting KOH/carbon mass ratio during activating stage.The optimal sample?SHPC-2?has a hierarchically porous structure containing micropores and meso-/macropores,high specific surface areas(1325.9 m² g^-1)with appropriate pore size and hydrophilous surface properties due to rich nitrogen and oxygen co-doping.The electrochemical measurements show the as-prepared SHPC-2 exhibits a high specific capacitance(345 and 330 F g^-1 at 1.0 A g^-1)and better rate capability in 1 M H₂SO₄ and 6 M KOH,respectively.The symmetrical supercapacitor assembled with SHPC-2 can achieve an operating voltage of 1.8 V and an energy density of 30.5 Wh kg^-1(power density of 900 W kg^-1)in 1 M Na₂SO₄ electrolyte.Even when the power density is increased to 1800 W kg^-1,the energy density can still maintain at 22.5 Wh kg^-1.3.A three-dimensional?3D?honeycomb-like hierarchical porous carbon?HRPC-800?which are derived from reed marshes has been prepared by carbonization and subsequent KOH activation process.The as-obtained materials not only inherited the natural rod-kike structure of reed marshes,but also developed different morphological characteristics through different activation temperatures.The electrode material could obtain a high specific capacitance of 308 F g^-1 at 1 A g^-1because of the specific porous structure facilitates the diffusion of electrolyte ions.Furthermore,the assembled symmetric supercapacitor based on HRPC-800 delivers high energy density of 19.71 Wh kg^-1 at power density of 890 W kg^-11 and good cycling stability(86.6%of the initial capacitance at 5 A g^-1 after 5000 cycles).