r03 battery.Research progress on application of supercapacitor composite electrode materials

　　Supercapacitor has the characteristics of high energy density, high power density, long cycle life and wide operating temperature range. Supercapacitors have extremely important application value in various fast and high-power starting systems, unattended and mobile energy systems and backup power systems in the fields of electricity, railways, green energy, special and special aerospace fields. From a structural point of view, supercapacitors are mainly composed of polarizing electrodes, electrolytes, current collectors, diaphragms and corresponding auxiliary components. The wide application prospects and potentially huge commercial value of supercapacitors have attracted the attention of many researchers. Research on supercapacitors mainly focuses on the preparation of high-performance electrode materials. Currently, commonly used electrode materials mainly include carbon materials, metal oxides and conductive polymers (Electrically Conducting Polymer, ECP). Composite materials such as carbon/oxide, carbon/ECP, oxide/ECP and other composites. Since the synergistic effect between components can be used to improve the overall performance, composite electrode materials have become a hot spot in current research.

　　1 carbon/metal oxide

　　The oxides involved in supercapacitor carbon/oxide composite materials include ruthenium oxide (RuO2) [1-9], manganese oxide (MnO2) [10-14] and other oxides.

　　1.1Ruthenium oxide/carbon

　　There are many methods for preparing ruthenium oxide/carbon composite materials. One of the methods is to first introduce ruthenium into the carbon material, and then convert the ruthenium into ruthenium oxide through other methods. Yan et al. [1] developed an efficient method to modify multi-walled carbon nanotubes (MWCNT) with Ru. Ru was fixed on MWCNT by water-in-oil inverse microemulsion method. Cyclic voltammetry tests showed that the ruthenium oxide electrode treated in the same electrolyte solution was significantly higher than the pristine MWCNT.

　　Fang et al. [2] improved the performance of supercapacitors through new RuO2 nanocomposites. The RuO2 nanocomposite is made by directly sputtering Ru on the MWC-NT array. X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) tests show that the prepared nanoparticles are made of Crystalline Ru is the core and RuO2 is the outer shell. RuO2-CNT composite material has a specific capacitance of 1380F/g, a charge and discharge rate of up to 600mV/s, and a cycle life of 5000 times.

　　The impregnation method has also been reported in the preparation of supercapacitor composite materials. He et al. [3] used chemical impregnation method to prepare hydrated ruthenium oxide/activated carbon black (ACB) composite for the first time. Tests show that as RuOx increases, the equivalent series resistance increases. Li et al. [4] prepared the ruthenium oxide/ordered mesoporous carbon composite by impregnating the ordered mesoporous carbon CMK-3 with RuCl3·xH2O solution, using NaOH as the precipitant, and then calcining it in a N2 atmosphere at 80-400°C. The composition content (mass fraction, the same below) of ruthenium oxide is 10.0% to 30.7%. As the RuO2 content increases, the specific capacitance increases, and when the content is the highest, the specific capacitance reaches 633F/g. The rate performance of the composite electrode becomes worse as the RuO2 content increases, which is due to the increase in the equivalent series resistance (ESR). Pico et al. [5] impregnated carbon nanotubes (CNT) with RuCl3·0.5H2O solution, filtered, treated with NaOH, and then heat-treated at 150°C for 2 hours to obtain a composite material.

　　When the RuO2 · OH) into coconut shell activated carbon to prepare composite materials. The specific capacitance of the composite material in 1M H2SO4 is 250F/g.

　　Other methods for preparing composite materials have also been reported. Such as electrodeposition method, thermal decomposition method, etc. Kim et al. [7] used electrodeposition method to obtain carbon/ruthenium oxide composite materials. Nanoscale ruthenium oxide with a three-dimensional porous structure is electrodeposited on a CNT film substrate. For comparison, ruthenium oxide was prepared on Pt sheets and carbon paper substrates. The results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that a ruthenium oxide layer with a thickness of 3 nm was electrodeposited on MWCNT. Compared with the ruthenium oxide deposited on Pt sheets and carbon paper substrates, the ruthenium oxide deposited on the CNT film not only has a higher specific capacitance of 1170F/g, but also has better rate performance due to its electrode components It contains a thin layer of electroactive material on a CNT substrate with three-dimensional nanopores. Lee et al. [8] used thermal decomposition method to prepare RuO2·xH2O and VGCF (nano ultra-long carbon fiber)/RuO2·xH2O nanocomposites.

　　The analysis and test results show that: the scan rate is 10mV/s, the specific capacitance of RuO2·xH2O is 410F/g; the specific capacitance of the VGCF/RuO2·xH2O composite is 1017F/g, the scan rate is 1000mV/s; the ratio of RuO2·xH2O The capacitance is 258F/g, and the specific capacitance of the VGCF/RuO2·xH2O composite is 824F/g. The specific capacitance values of RuO2·xH2O and VGCF/RuO2·xH2O remained at 90% and 97% of the initial values respectively after 10,000 cycles.

　　Lee et al. [9] reported the supercapacitive properties of composite films formed by MWCNT and ruthenium oxide. The RuO2-coated MWCNT three-dimensional nanopore structure promotes electron and ion transport in the MWCNT film. RuO2/MWCNT composites loaded with different RuO2 contents were tested. The maximum specific capacitance was 628F/g, which was about 3 times higher than the energy density of MWCNT.

　　1.2 Manganese oxide/carbon

　　Due to the high price of noble metal oxides, research on other oxides such as manganese oxide/carbon composites has also become a hot topic. For the preparation of manganese oxide/carbon composite materials, potassium permanganate (KMnO4) is mostly used as the manganese source in reports. Chen et al. [10] first immersed MWCNT in boiling sulfuric acid (H2SO4), dispersed it, and added KMnO4 powder while stirring. H2SO4 formed a precipitate in the aqueous solution, and single crystal α-MnO2 nanorods were grown on the MWCNT. Its average particle size is 15nm and it can adhere to MWCNT very densely.

　　Supercapacitors mechanically mixed with MWCNTs/α-MnO2 nanorods are better modified.

　　Subramanian et al. [11] first studied a composite composed of amorphous MnO2 and single-walled carbon nanotubes (SWCNT) that still has long cycle performance under high charge and discharge current (2A/g). Disperse SWCNT in a saturated solution of KMnO4 under magnetic stirring, mix evenly, and then add ethanol dropwise to form a precipitate. The composite with 20% MnO2 still has good Coulombic efficiency (75%) and specific capacitance (110F/g) after 750 cycles.

　　Li et al. [12] used chemical co-precipitation method, first added carbon aerogel to KMnO4, and then added Mn(CH3COO)2·4H2O to prepare MnO2·xH2O/CRF composite. Research results show that MnO2·xH2O/CRF composite materials have good electrochemical properties, high reversibility and good charge and discharge properties. When the loading of MnO2·xH2O is 60%, the specific capacitance of the composite material reaches 226.3F/g, while the specific capacitance of simple carbon aerogel is only 112F/g. Additionally, there are other methods for preparing manganese oxide/carbon composites.

　　Li et al. [13] used electrophoretic deposition method to prepare thin film electrodes formed of MnO2/MWC-NT composite materials. By changing the deposition time and voltage, films with a thickness of 1 to 20 μm were produced. The material pore size is 10~100nm. The electrolyte is 0.1M Na2SO4 solution, the voltage range is 0~1.0V, and the cyclic voltammetry curve measured when the standard calomel electrode is the standard electrode is ideal and has a large specific capacitance. The specific capacitance of the composite is higher than that of MWCNT-free, and the specific capacitance decreases with the increase of film thickness and scan rate.

　　Raymundo et al. [14] prepared amorphous MnO2 through chemical coprecipitation in aqueous media, which has a relatively high surface area.

　　CNTs are used as an alternative additive to carbon black to improve the conductivity of manganese oxide electrodes used in capacitors. The results show that CNT can effectively increase the capacitance and improve the electrochemical performance of α-MnO2·nH2O. The α-MnO2·nH2O electrode has better capacitive performance than using carbon black as an additive. The improvement of this performance is due to the high degree of entanglement of CNTs to form an open mesoporous network, making the bulk MnO2 easily accessible to ions. In terms of performance optimization, it is necessary to control the pH value of the electrolyte to avoid irreversible reactions, so that the negative electrode changes from Mn(Ⅳ) to Mn(Ⅱ), and the positive electrode changes from Mn(Ⅳ) to Mn(Ⅶ).

　　1.3 Other oxides/carbons

　　Other oxides such as nickel oxide and vanadium oxide and carbon composites have also been studied. Lee et al. [15] prepared supercapacitor nickel oxide NiO/CNT nanocomposites through a simple chemical precipitation method. CNT networks in NiO significantly improve:

　　1) The conductivity of the NiO body is improved by forming a CNT conductive network;

　　2) By increasing the specific surface area, the active sites for redox reactions are increased. A CNT content of 10% can increase the specific capacitance by 34%.

　　Kud et al. [16] studied high-rate vanadium pentoxide V2O5 gel/carbon composite intercalated electrode materials. V2O5 sol is obtained by reacting vanadium with hydrogen peroxide H2O2 solution. Acetylene black and acetone are added to the V2O5 sol to obtain a homogeneous precipitation. Amorphous V2O5 and carbon were supported on the porous nickel current collector and heated at 120°C to obtain an electrode. The electrical properties were tested in lithium perchlorate LiClO4/polycarbonate PC or lithium hexaoxyphosphate LiPF6/butyrolactone (γ-BL) electrolyte. When the ratio of V2O5 to carbon in the composite material is 0.7, 54% of the ideal capacitance can occur, that is, 360mAh/g (4.2~2.0V) based on V2O5, and the discharge rate reaches 150C or 54A/gV2O5. Using the diffusion model, assuming D=10-12cm2/s, simulating the discharge curve, the diffusion length of the host-guest system is estimated to be 30~50nm. At a rate of 20C, the reversibility is still very good after thousands of cycles, with no capacity loss.

　　Since metal oxides and their hydrates undergo reversible Faradaic reactions at the electrode/solution interface, they can produce a Faradaic quasi-capacitance that is much larger than the electric double layer capacitance of carbon materials, thus arousing the interest of researchers.

　　The current work focuses on the following four aspects:

　　1) Use various methods to prepare noble metal oxides and their hydrates with large specific surface areas;

　　2) Compound precious metal oxides and their hydrates with other materials to achieve the purpose of reducing the dosage and increasing the specific capacity of the material;

　　3) Find other cheap materials to replace precious metal oxides and their hydrates to reduce material costs;

　　4) Find suitable electrode materials to assemble hybrid supercapacitors.

　　Among them, the most critical thing is to synthesize new composite materials to improve the energy density of supercapacitors.

　　The energy storage mechanism of metal oxides and their hydrates to store energy through the Faraday quasi-capacitance generated by the redox reaction at the electrode/solution interface. Although it has a large specific capacity, due to the structure of this type of material (generally The crystal below is not conducive to the penetration of electrolyte. The electrode material has less contact with the electrolyte solution, so the conductivity is poor and the utilization rate of the material is not high. The specific surface area and pore capacity of the material need to be further improved to improve it. Metal oxide and its hydrate composite materials can not only make up for the shortcomings of simple metal oxides and their hydrates, but also reduce the amount of metal oxides and their hydrates, reduce material costs, and increase the specific capacity of the material.

　　2 Carbon/Conductive Polymer

　　Carbon/conductive polymer (ECP) composites combine the high specific capacitance of ECP with the rapid charge and discharge electric double layer capacitance and good mechanical properties of carbon. Compared with CNTs/metal oxide composite materials, CNTs/ECP composite materials can not only increase the specific capacitance of supercapacitors, but also reduce costs, and their Faraday quasi-capacitive effect is also relatively stable. The most studied ECP materials mainly include polyaniline (PAn), polythiophene (PTh), polyacene (PAS), polypyrrole (PPy) and polyethylene ferrocene (PVF).

　　2.1 Carbon/Polyaniline

　　Carbon/PAn composite materials can be prepared through microwave polymerization, in-situ chemical polymerization, interfacial polymerization, electrochemical polymerization, electrodeposition and in-situ deposition. MWCNT/PAn composites are more common. This composite material has better rate performance and better capacitance retention. Mi et al. [17] rapidly prepared MWC-NT/PAn composites through microwave-assisted polymerization. TEM shows that this composite material is a polyaniline layer (50-70nm) with a composite core-shell structure. When the energy density is 22W·h/kg, the specific capacitance is 322F/g, which is 12 times higher than pure MWCNTs. Dong et al. [18] prepared MWCNT/PAn composite through in-situ chemical oxidation polymerization method and used it as a new type of electrode material. The specific capacitance of the composite is as high as 328F/g. Sivakkumar et al. [19] prepared PAn nanofibers by interfacial polymerization. At a constant current of 1.0A/g, its initial specific capacitance reaches 554F/g. MWC-NT/PAn composites were prepared by in situ chemical polymerization. Its specific capacitance reaches 606F/g and its cycle stability is good.

　　Relevant research has also been done on SWCNT-based composite materials. Gupta et al. [20] obtained the composite PAn/SWCNT by electrochemically polymerizing polyaniline on SWCNT, and tested its electrical properties in 1M H2SO4 electrolyte. The specific capacitance, specific energy and specific power of the composite are higher than those of pure PAn and SWCNT. The composite prepared by depositing 73% PAn on SWC-NT has the highest specific capacitance, and its specific capacitance, specific energy and specific power are respectively

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