Researches Of Electrochemical Capacitor Electrodes On The Basis Of Novel Carbon/Carbon Composite Material

Under dual pressures of fossil energy crisis and environmental protection,governments have attached great importance to the development of clean and renewableenergy.And energy storage is the key link of renewable energydevelopment, such assolar, wind and tidal energy. Electrochemical capacitor (EC) is a novel energy storagedevice with many advantages such as fast charge and discharge rate, high power density,good temperature characteristics, excellent safety performance, environmentalfriendliness and long cycle life.It has been considered to be one of the most promisinggreen novel energy storage devices in this century. ECs, with wide potential applicationsin back-up power, electric vehicles, power systems, space technology and defenseindustry, have received extensiveattentions and have become a research hotspot.Electrode materials are the key to the manufacture of high-performance electrochemicalcapacitors.This article is focused on the preparation of high-performance electrode materialsfor electrochemical capacitors. We have fabricated a novel carbon nanotubes (CNTs)/carbon fiber paper (CFP) composite by depositing CNTs on the surface of CFP using asimple chemical vapor deposition (CVD) method. On the base of the novel composite,we have synthesized manganese oxide/CNTs/CFP (MMC) and nickel-manganeseoxide/CNTs/CFP (NMC) composite. The microstructures of all products are studiedusing X-ray diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Teller (BET)measurement, scanning electron microscope (SEM), transmission electron microscope(TEM) and EnergydispersiveX-rayspectroscopy (EDS). And the electrochemicalproperties are also studied using galvanostaticcharge-dischargemethod,cyclicvoltammetry and electrochemicalimpedancespectroscopy. The detail researchcontents and main conclusions are shown as follow:1. CNTs/CFP composite material is synthesized by depositing CNTs on the surfaceof CFP using CVD. The catalyst precursors (Fe(NO3)3, Cu(NO3)2and NiCl2) are loadedonto the CFP substrate using anincipientwetnessimpregnationand reduced to metalcatalysts using reducing atmosphere during the preparation process. The self-dispersionmechanism of Ni catalyst and the synthetic mechanism of MWCNTs are studied.Moreover, the microstructure and electrochemical performance of MWCNTs/CFPcomposite are also investigated. The results are shown as follow.(1) MWCNTs cannot be catalytic synthesized using Fe or Cu catalyst under our experiment condition, only Nicatalyst is capable to catalytic synthesize MWCNTs.(2) The NiCl2precursor is reducedto Ni metal by hydrogen, and then reacts with ethyleneto form uniformly dispersedcatalyst nanoparticles on the surface of CFP. Carbon source gaswill be catalyticpyrolyzed by the as-prepared particles and thusthe MWCNTs/CFP composite is formed.(3) Compare to general synthetic method of catalyst particles, the method we used toprepare uniformly dispersed catalyst nanoparticles in this article is much simpler andrequired less efforts.(4) MWCNTs are uniformly dispersed on the surface of CFP,which are randomly oriented and forming a3D conductive frame. The as-preparedMWCNT is composed of23graphite layers, the wall thickness of MWCNTs is about7nm and the diameter of MWCNTs is in the range of10to30nm.(5) The specificsurface area (SSA) and the graphitizationdegree of MWCNTs/CFP composite areobviously higher than CFP’s. The SSA of MWCNTs/CFP is14.33m2/g.(6)MWCNTs/CFP composite possesses high electrical conductivity, low equivalent seriesresistance (ESR) and pretty good capacitive property. The specific capacitance ofMWCNTs/CFP is23.17mF/cm2at0.5mA/cm2.2. Manganese oxide is deposited on the surface of MWCNTs/CFP composite usingelectrochemical oxidation method and thus MMC composite is produced. Themicrostructure and electrochemical properties of MMC and the influence of theMWCNTs on them are investigated. The comparison with manganese oxide/CFP (MC)composite is also carried out.The results are shown as follow.(1) The manganese oxideparticles obtained on the surface of MWCNTs/CFP composite using electrochemicaloxidation have flower-like structures and every petal of these“flowers” is a manganeseoxide nanosheet. The diameter of these particles is about300nm. These manganeseoxide particles evenly cover the surface of every single MWCNT and together producea uniform3D porous structure on the surface of CFP. This unique microstructure willallow ions and electrons to be transported directly to the surface of nano-scalemanganese oxide, which will shorten the transport distance of ions and electrons, andthus improve the utilization rate of the active material and the power density of thecomposite.(2) MC composite is also synthesized using electrochemical oxidation, andthe as-prepared manganese oxide has formed a dense layer on the surface of CFP, whichwill hinder the transport of ions and electrons. Meanwhile, the dense layer shatterseasily, which will reduce the utilization rate of active material and will causeelectrochemical property deteriorations.(3) MMC composite electrode has excellent capacitive property that its capacitance is much higher than it of MC owing tothesynergistic effect of MWCNTs. The specific capacitance of the electroactive materialreaches480F/g at a current density of1mA/cm2.(4) The electrical conductivity ofMMC is better than it of MC that the ESR of MMC is much smaller than it of MC.(5)MMC composite possesses good rate capability, the CV curves can remain rectangularshape under100mV/s.(6) MMC composite material has a relatively high cycle stability,the capacitance retention rate reaches83.3%after1000cycles.3. Nickel-manganese oxide (NMO) is deposited on the surface of MWCNTs/CFPcomposite using cyclicvoltammetry and thus the NMC compositeis formed. Themicrostructure and electrochemical properties of NMC are studied and the comparisonswith MMC are also made. The results are shown as follow.(1) NMO nanoparticles areobtained on the surface of MWCNTs/CFP composite using cyclicvoltammetry. Theaverage diameter of these particles is about20nm.(2) The structure stability of NMO ispretty high that its tiny structure can effectively reduce the impact of volume expansiontowards the composite’sstructure during the charge-discharge process. Moreover, theseparticles deposited directly on the flexible MWCNTs, volume change will not affect thebinding properties.(3) NMO particles are evenly distributed on the surface of everysingle MWCNT, which take full advantage of the3D porous conductive frame of theMWCNTs/CFP composite, and thus the transport distance of ions and electrons isshorten and the utilization rate of the active materialis improved.(4) Owing to thedoping of Ni element, the working voltage of NMC electrode is0.1V smaller than it ofMMC.(5) NMC electrode possesses wonderful capacitive property that its CV curve ishighly symmetrical and has a rectangle-like shape.(6) NMC electrode also possesseswonderful rate capability, its CV curve only changes slightly even at125mV/s.(7)Compare to MMC composite electrode, the electrical conductivity of NMC electrode isbetter, the specific capacitance is larger and the cycle stability is higher. The ESR ofNMC composite electrode is6.8, the specific capacitance of NMC is as high as961.5F/g and the capacitance retention rate reaches89.32%after1000cycles.MWCNTs/CFP composite has a unique structure and a good electrical conductivity,which is a wonderful substrate for oxide/carbon composite electrode. The preparation oftransition metal oxide/MWCNTs/CFP composite materials and the applications in thefields of electrochemical capacitors, water electrolysis, etc. have wide researchprospects.