Preparation And Electrochemical Performance Of Macroporous Carbon Derived From Luffa Sponge And Its Composites

Facing the exhaustion of fossil energy resources and the environmental problems due to their application, the development and utilization of renewable clean energy become increasingly urgent. Among numerous energy devices supercapacitor and fuel cell have received great attention for their outstanding advantages. Supercapacitors have the advantages of fast charge/discharge rate, long cycling life, and low cost. The current main problem of supercapacitor as the energy storage devices is its low energy density. The main way to increase the energy density of supercapacitors is to prepare high performance electrode material. Fuel cell has the advantages of high conversion efficiency, low emissions, abundance of fuels, and so on. One of the main factors hindering the application of fuel cells currently is the high cost of platinum catalyst it uses. Therefore, the research and development of low-cost non-noble metal catalysts with high performance for fuel cell applications is an urgent issue. In this dissertation for the existing problems of the application of the supercapacitor and fuel cell, macroporous carbon were prepared from luffa sponge and MnO₂ nanosheets, metal ions-doped MnO₂ nanosheets, and polyaniline nanowires were grown on the internal surface of macroporous carbon, obtaining the composite materials with the active materials growing on the internal surface of the macroporous carbon. The performance of the macroporous carbon and its composite materials as the electrode materials of supercapacitors and electrocatalysts for oxygen reduction reaction were investigated.The main research contents and results are as follows:The macroporous carbon was prepared by carbonizing luffa sponge fibers in NH₃ at high temperature and subsequent activation in N2. The macroporous carbon has densely packed and parallel channels with diameter of 4-10μm and wall thickess of 0.3-1μm, which was inherited from the natural structure of the luffa sponge fibers.Micro- and mesopores were generated on the internal walls after activation, forming a hierarchically porous structure with specific surface area of 1678.1 m2/g. As the electrolyte can flow into the inner part of the macroporous carbon through the macropores, the utilization efficiency of the electrode were greatly enhanced. When used as the supercapacitor electrode materials the specific capacitances of the macroporous carbon at current density of 1A/g are 167, 196, and 249F/g in Na2SO4,KOH, and H2SO4 electrolyte solutions, respectively. The macroporous carbon also has good cycling stability in Na2SO4, KOH, and H2SO4 solutions with the capacitance retention of 95.7%, 95.0%, and 87.6% after 5000 cycles respectively.When used as the electrocatalysts for oxygen reduction reaction the macroporouscarbon exhibit excellent electrocatalytic activity with the peak potential at-0.10 and0.32 V, the peak current density at 0.27 and 0.26 m A/cm², and the onset potential at0.00 and 0.51 V in alkaline and acidic media, respectively. As the active sites are located on the inner surface of the macroporous carbon, the performance degradation caused by aggregation can be avoided during the application. The macroporous carbon has prominent stability, retaining 81.3% and 88.5% of the initial current density after 12 hours in KOH and H2SO4 media, respectively.Especially, the stability of the MC is prominent in acidic media.MnO₂ nanosheets were grown on the internal surface of the macroporous carbon by reaction with potassium permanganate solution. The full coverage growth was realized due the large pore size. Because the large pore size allows the electrolyte solution to access easily, the MnO₂ nanosheet grown on the internal surface of the macroporous carbon are fully exposed towards the electrolyte,resulting in high mass specific capacitance at different loadings such as 1332F/g at0.15mg/cm², 567F/g at 2.15mg/cm², and 354F/g at 5.69mg/cm². Moreover, the active materials also exhibit high areal specific capacitance at high mass loading such as 2.9F/cm² at the MnO₂ loading of 5.69mg/cm². Symmetric supercapacitors assembled using the composites of MnO₂ and macroporous carbon show a high areal energy density of 194mWh/cm² at the power density of 4.5m W/cm². Cycling stability test indicates that 80% of the initial capacitance can be retained after 5000 cycles at the current density of 5m A/cm².In order to further improve the supercapcitive property and electrocatalytic property for oxygen reduction reaction of MnO₂ nanosheets, transition metal ions-doped MnO₂ nanosheets were grown on the internal surface of macroporous carbon by adding the corresponding salts into the reaction solutions at room temperature. The effects of the type of metal ions on the supercapacitor performance and oxygen reduction catalytic activity of MnO₂ nanosheets were investigated systematically. It is manifested that the electrochemical performances of MnO₂ were enhanced by doping with appropriate metal ions. When used as the electrode materials for supercapacitors, the Ni-doped MnO₂ nanosheets exhibit the best performance with a high specific capacitance of 445F/g at 1A/g（based on the total mass of the MnO₂ and the macroporous carbon） and high capacitance retention of90% after 1000 cycles. When used as the catalyst for oxygen reduction reaction the Co-doped MnO₂ nanosheets show the highest activity with redox peak potential,current density, onset potential, and electron transfer number at-0.13 V, 0.37 m A/cm²,-0.04 V, and 3.64, respectively. Besides, the Co-doped MnO₂ nanosheets possess high operation stability with the current retention at 86.9% after 12 h, higher than that of the undoped MnO₂ nanosheets.Aligned polyaniline nanowires were grown on the internal surface ofmacroporous carbon by in situ polymerization of aniline monomers. It is found that the concentration of the aniline has an important influence on the morphology of the polyaniline. The aligned polyaniline nanowires perpendicular to the wall surface of the macroporous carbon were obtained by adjusting the concentration of aniline.Due to full exposure towards electrolyte the polyaniline nanowires exhibit high utilization efficiency, leading high specific capacitances up to 1500F/g at the current density of 1A/g. As the macropores allow easy penetration of the electrolyte into the electrode the polyaniline nanowires show high rate capability with the capacitance retention up to 70% with increasing the current density from 1 to 10A/g. The supercapacitors assembled using the composite of polyaniline and macroporous carbon show a high energy density of 19Wh/kg at power density of 0.9k W/kg. The cycling stability test indicates that 83% of the initial capacitance can be retained after 7000 cycles at the current density of 2A/g, showing high cycling stability.