Study On Electrochemical Properties Of Dandelion Stem Porous Carbon In Redox Electrolyte

Supercapacitors are also known as electrochemical capacitors.It has received extensive research attention due to its high power density,long cycle life,and high safety compared with rechargeable batteries.However,the low energy density of supercapacitors,especially carbon-based materials,directly affects its practical application,so how to improve the energy density has become a key concern in this field.In general,electrode materials and electrolytes,as components of supercapacitors,jointly determine the energy density of supercapacitors.From the perspective of electrode materials,low-cost natural waste and biomass-derived porous carbon electrode materials are beneficial to increase energy density due to their renewability,wide availability,and rich pore structure.From the perspective of the electrolyte,the introduction of redox active substances into the conventional aqueous electrolyte can provide additional pseudocapacitance,which is also a good boost.In this paper,the porous carbon DS-2 prepared from dandelion rods was used as the electrode material,and the redox active substances naphthol green B（NGB）and p-phenylenediamine（PPD）were introduced into In 1M H₂SO₄ and 6 M KOH electrolytes,the improvement degree of redox active substances on the electrochemical performance of DS-2 was systematically studied,and the similarities and differences of the two addition methods were compared.The specific research contents and conclusions are as follows:（1）Using corn stalks and dandelion stalks as carbon sources,the corn stalk biomass porous carbon CS-X was prepared through two steps of pre-carbonization and KOH high-temperature activation（X represents the mass ratio of KOH to precursor,X=2,3,4）and dandelion stem biomass porous carbon DS-X（X=1,2,3,4）.The electrochemical performance test results of the two carbon materials show that:when the current density is 1 A/g,in 1 M H₂SO₄ electrolyte,the specific capacitance of CS-3 and DS-2 are 121 F/g and 249 F/g,respectively;In 6 M KOH electrolyte,the specific capacitances of the two are 139 F/g and 286 F/g,respectively.From this point of view,the specific capacitance of DS-2 is higher than that of CS-3 and DS-2.The results for surface areas of 1765 m²/g and 2705 m²/g are consistent.Symmetrical supercapacitors（device 1 and device 2）were assembled with DS-2 as the electrode and 1 M H₂SO₄ and 6 M KOH as the electrolyte,respectively.The test results show that at a current density of 1 A/g,the specific capacitance of device 1 is 50.6 F/g,the energy density is 7.03 Wh/kg,and the power density is 500 W/kg;the specific capacitance of device 2 is 62.4 F/g,8.11 Wh/kg,468 W/Kg,the energy density of both devices is lower than 10 Wh/kg.After 10,000 cycles,the capacitance retention of device 2 was 87.4%,and that of device 1 could reach 93.2%.The comparison shows that the supercapacitor composed of DS-2 has better cycle life in acidic electrolyte,and higher energy density in alkaline electrolyte.（2）The redox active substance NGB was introduced,and DS-2 was used as the electrode material,test the electrochemical performance in 1 M H₂SO₄+5 m M NGB,when the current density is 1 A/g,the specific capacitance is 450 F/g,Higher than the result in 1 M H₂SO₄electrolyte（249 F/g）,the rate at 20 A/g is 66.7%.Then in 1 M H₂SO₄+5 m M NGB electrolyte,use cyclic voltammetry to scan 50 cycles,confine NGB to DS-2 to get DS-2/NGB₅₀-5 electrode,and test the results in 1 M H₂SO₄ electrolyte It shows that when the current density is 1 A/g,the specific capacitance is as high as 700 F/g,and the rate performance can reach 86.4%at 20A/g,which are better than the results of adding NGB directly to the electrolyte.The mechanism study found that:in the 1M H₂SO₄ electrolyte,the Fe-O bond in NGB was broken,the complex structure was destroyed,and Sodium 1-nitroso-2-naphthol-6-sulfonate was generated.Through ex-situ XPS analysis,it can be seen that the nitroso group in the ligand undergoes a redox reaction under the action of an electric field and transforms into an amino group,and the active molecules confined in the small micropores of DS-2 make its specific surface area from 2705m²/g is reduced to 751 m²/g.Two symmetrical capacitors were assembled.Device 1（DS-2electrode material,1 M H₂SO₄+5 m M NGB as electrolyte）and Device 2(DS-2/NGB₅₀-5 as electrode material,1 M H₂SO₄ as electrolyte).The test results show that at a current density of1 A/g,the specific capacitance of device 1 is 69 F/g,the energy density is 16 Wh/kg,and the power density is 650 W/kg;the specific capacitance of device 2 is 156 F/g g,the energy density is 36.6 Wh/kg,and the power density is 655 W/kg.Obviously,the energy density of device 2 is much higher than that of device 1,indicating that the NGB confined in the nanopore can better play the role of increasing the energy density.After 10,000 charge-discharge cycles,the capacitance retention of device 2 is 78.4%,which is better than that of device 1（72.7%）.After the two devices 2 are connected in series,the voltage doubles,and seven LED lights can be successfully lit,which proves that the DS-2/NGB₅₀-5 electrode material has certain practicability.（3）The redox active substance PPD was introduced,and DS-2 was used as the electrode material to study its electrochemical performance in 6 M KOH+18.6 m M PPD electrolyte.When the current density was 1 A/g,the specific capacitance was 507 F/g,higher than the result in 6 M KOH electrolyte（286 F/g）,the capacitance retention at 20 A/g is 90%.Then in 6 M KOH+18.6 m M electrolyte,cyclic voltammetry was used to scan 30 cycles,and the PPD was confined to DS-2 to obtain DS-2-30 electrode.The test results in 6 M KOH electrolyte showed that:when the current density is 1 A/g,the specific capacitance is 610 F/g,and the capacitance retention rate is 82.5%at 20A/g.Then,put the DS-2-30 electrode back into the 6 M KOH+18.6 m M PPD electrolyte for electrochemical test,the result is:the specific capacitance reaches1106 F/g,and the capacitance retention rate of 20 A/g is 86.3%,proving that PPD in the electrolyte and in the nanopores can work synergistically to enhance the performance of porous carbon electrodes.Two symmetrical capacitors-Device 1（DS-2-30 electrode material,6 M KOH as electrolyte）and Device 2（DS-2 as electrode material,6 M KOH+18.6 m M electrolyte）were assembled,and the test results showed:At a current density of 1 A/g,the specific capacitance of device 1 is 120 F/g,the energy density is 32.6 Wh/kg,and the power density is698 W/kg;the data of device 2 are 76 F/g,23.75 Wh/kg and 750 W/kg.Obviously,the energy density of device 1 is higher than that of device 2,indicating that the PPD confined in the nanopore can better improve the performance of electrode materials.However,the stability of the two devices is not rational.After 10,000 charge-discharge cycles,the capacitance retention rate of device 1 is 66.3%,and that of device 2 is 62.4%.