Nickel Hydride No. 5.Research progress on organic conductive polymer electrode materials for supercapacitors

Chen Guanghua, Xu Jianhua, Yang Yajie, Jiang Yadong, Ge Meng (School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610051) Abstract: Organic conductive polymers are an important type of supercapacitor electrode materials. In the doped state of organic polymers, due to... Keywords: organic conductive polymer, supercapacitor, electrode material.

Chen Guanghua, Xu Jianhua, Yang Yajie, Jiang Yadong, Ge Meng

(School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610051)

Abstract: Organic conductive polymers are an important class of supercapacitor electrode materials. When the organic polymer is doped, it has a conjugated structure, which improves the delocalization of electrons and can conduct electricity externally. Supercapacitor organic polymer electrodes can be divided into 3 basic types based on doping types and combinations. The conductive principles and classification of organic polymer electrodes are expounded, and the research status and development trends of organic polymer electrodes are introduced.

0 Preface

Supercapacitor is a new energy storage device whose performance is between that of batteries and traditional capacitors. It has the advantages of high power density, fast charge and discharge speed, and long service life. It has broad application prospects, such as being used in portable instruments and equipment, data Memory storage system, electric vehicle power supply and emergency backup power supply, etc. Especially in electric vehicles, the combination of supercapacitors and batteries can provide high power and high energy respectively, which not only reduces the size of the power supply but also extends the life of the battery. Electrode materials are the most critical part of supercapacitors and the main factor that determines their performance. Therefore, developing electrode materials with excellent properties is the core topic in supercapacitor research. Conductive polymers are an important type of supercapacitor electrode material, and their capacitance mainly comes from Faraday quasi-capacitance. The conductive polymers currently used in supercapacitors mainly include polyaniline, polythiophene, polypyrrole, etc.

1 Working principle and characteristics of supercapacitor conductive polymer electrodes

Most polymer molecules are mainly composed of covalent bonds connecting various atoms with localized electrons or limited delocalized electrons (valence electrons). Among them, the bond and independent bond valence electrons are typical localized electrons or limited delocalized electrons. According to current research results, although bonds in organic compounds can provide limited delocalization, electrons are still not conductive free electrons.

When organic compounds have a conjugated structure, the electronic system increases, the delocalization of electrons increases, and the movable range expands. When the conjugated structure reaches a large enough size, the compound can provide free electrons. The larger the conjugated system, the greater the delocalization. Therefore, the condition for an organic polymer to become a conductor is that it has a conjugated structure that enables certain electrons or holes inside it to have the ability to move across bonds and delocalize. Figure 1 shows the cationic chain polymerization reaction of PEDOT (polyethylenedioxythiophene).

In supercapacitors with conductive polymers as electrodes, part of their capacitance comes from the double electric layer at the electrode/solution interface, and a more important part is provided by Faraday quasi (pseudo) capacitance. Its mechanism of action is: through rapid reversible N-type, P-type doping and de-doping redox reactions in the polymer film on the electrode, the polymer reaches a very high storage charge density and generates a very high Faraday quasi-( Fake) capacitor to store electrical energy.

The P-type doping process of conductive polymers refers to the external circuit absorbing electrons from the polymer skeleton, thereby distributing positive charges on the polymer molecular chain. The anions in the solution are located near the polymer skeleton to maintain charge balance (such as polyaniline, polyethylene). Pyrrole and its derivatives); when the N-type doping process occurs, the electrons transferred from the external circuit are distributed on the polymer molecular chain, and the cations in the solution are located near the polymer skeleton to maintain charge balance (such as polyacetylene, poly Thiophene and its derivatives).

Figure 2 is a schematic diagram of the molecular structure of PEDOT after P-type doping and doping processes. The electrochemical capacitive behavior of PEDOT is achieved through doping/dedoping reactions. The conductive polymer electrode stores charges within the three-dimensional structure, making it superior to high-surface-area activated carbon as a supercapacitor electrode material. The charge and discharge process of the latter only occurs in the double electric layer at the interface between the electrode material and the electrolyte. For the majority of P-type conductive polymers, a P-type doping reaction occurs during the anode electrochemical charging process. Electrons flow from the conductive polymer to the external circuit through the current collector, and the conductive polymer exhibits positive electricity. To maintain electrical neutrality, the negatively charged anions in the electrolyte migrate toward the electrode surface and enter the network structure gaps of the polymer to maintain overall electrical neutrality.

During the discharge process, the conductive polymer undergoes a dedoping reaction, electrons flow from the external circuit to the polymer electrode, the positively charged conductive polymer is neutralized, and the excess anions between the polymer network structure diffuse toward the electrolyte through concentration difference. Migrate in the liquid to maintain overall electrical neutrality. The charge and discharge process of N-type polymer electrode is opposite to that of P-type polymer. In a supercapacitor using conductive polymer as the electrode material, during charging and discharging, anions enter and exit the positive electrode, and cations enter and exit the negative electrode. The anions used in most organic electrolytes are larger diameter ions with a diameter of about 0.5nm. For example, the diameter of BF4- is 0.46nm, the diameter of PF4- is 0.50nm, and the diameter of ClO-4 is 0.48nm. Therefore, the amount of anions entering and exiting the positive electrode is one of the key factors that determine the capacity of polymer capacitors.

Most conductive polymers have an amorphous structure with gaps between molecules that can accommodate large-diameter anions. The more developed the network structure of the conductive polymer is, the higher the internal nanoporosity is, and the more anions will enter the molecular gap, and the more obvious its high power and high capacity characteristics will be. This kind of intermolecular gap in conductive polymers may exist on the surface of the material or inside the material, which is of great significance for supercapacitors that require high capacity and high power.

2 Classification of conductive polymer electrode materials

Due to the different doping forms of conductive polymers and the types of conductive polymers that can be doped, conductive polymers can be used in different combinations when used as supercapacitor electrode materials. Currently, there are three main combinations of conductive polymer electrodes in supercapacitors.

The two electrodes of type I polymer supercapacitor are composed of identical P-type doped conductive polymers. When the capacitor is fully charged, the polymer on the cathode is in an undoped state and the polymer on the anode is in a fully doped state. During discharge, the polymer in the undoped state is oxidatively doped, and the polymer in the doped state is reduced. When the discharge reaches both electrodes in a semi-doped state, the voltage difference between the two electrodes is zero. It can be seen that the number of charges released during the discharge process of the Type I capacitor is only 1/2 of the full doping charge, and the potential difference between the two poles is small (about 1V).

The two electrodes of type II polymer supercapacitors are composed of different types of conductive polymers, and both can be P-type doped. Due to different conductive polymer electrode materials selected, the potential range in which doping occurs is different, so that the capacitor can have a higher voltage difference (generally 1.5V) in a fully charged state. During the discharge process, when the voltage difference between the cathode and anode is zero, the dedoping rate of the P-type doped conductive polymer at the anode is greater than 50%, so the electrode has a greater discharge capacity.

The electrode materials of the two electrodes of type III polymer supercapacitors are composed of conductive polymers that can be doped with both N-type and P-type doping. In the fully charged state, the cathode of the capacitor is in a completely N-type doped state, and the anode is in a completely P-type doped state, thus making the voltage difference between the two electrodes larger (3~3.2V). The doped charges can all be released during the discharge process, greatly increasing the capacitance of the capacitor. This type of capacitor can make full use of the anions and cations in the solution during charging and discharging, and has discharge characteristics similar to batteries, so it is considered to be the most promising supercapacitor. The main advantages of this type of structural capacitor are that the capacitor voltage is high, the charge is completely released, both electrodes are doped during charging, and the charge storage capacity is large. In addition, since both electrodes are doped at the same time, the conductivity of the electrode material is high, the internal resistance of the capacitor is small, and the output power is high.

3 Research progress on conductive polymer electrodes for supercapacitors

3.1 Pure organic polymer electrode

Since MacDiamid discovered the conductive properties of polyaniline in 1984, after more than 20 years of development, polyaniline has attracted widespread attention due to its good chemical stability and rapid dedoping ability. Kwang Sun Ryu et al. studied LiPF6-doped polyaniline electrode materials and obtained a specific capacitance of 107F/g.

D.Krishna Bhat et al. used N-type and P-type doped PEDOT as electrode materials and obtained a specific capacitance of 121F/g. Yang Hongsheng et al. used (NH4)4S2O8 as the oxidant and used chemical oxidation method to synthesize polyaniline particles with a multi-layered structure. Through scanning electron microscopy analysis, it was found that the secondary particles of polyaniline are composed of primary particles, and the particle size of the primary particles is 1 μm. The following; and assembled two pieces of polyaniline electrodes into a supercapacitor for testing. The specific capacitance of a single electrode can reach 324F/g.

The preparation of organic polymer electrode materials is generally in-situ chemical polymerization and electrochemical polymerization. S.Ivanov et al. compared in detail the differences in surface structure and electrochemical behavior of polyaniline electrode materials prepared by chemical polymerization and electrochemical polymerization under protonic acid doping conditions. Polyaniline electrodes prepared by chemical polymerization have a rough and uniform surface structure, while polyaniline electrodes prepared by electrochemical polymerization have a compact and granular surface structure. Generally, polyaniline electrode materials prepared by electrochemical polymerization have better capacitance and cycle performance. This is because polyaniline prepared by chemical polymerization has a faster nucleation rate, and the formed polymer chains usually have a winding structure, which is not conducive to ion diffusion in the electrolyte; while polyaniline prepared by electrochemical polymerization, especially the potentiodynamic Scanning the prepared polyaniline shows that the oxidation potential gradually increases, which is more conducive to the directional polymerization and growth of polyaniline during the synthesis process.

For organic polymer electrode materials, since the electrolyte ions repeatedly enter and exit the electrode material during charging and discharging of the capacitor, the molecular structure of the electrode material is damaged to a certain extent, the conjugated system is reduced, and the electron delocalization is reduced. As a result, the conductivity of the electrode decreases and the mechanical properties are destroyed. When a supercapacitor is repeatedly charged and discharged, the specific capacitance and energy density will be significantly reduced, and the cycleability is poor.

3.2 Organic composite electrodes

In order to improve the mechanical properties and conductivity of organic electrodes, many people at home and abroad have studied composite electrode materials based on conductive polymers, among which CNT (carbon nanotube) and ECP (electronically conductive organic polymer) composite materials are the representative ones.

Jie Wang et al. comparatively studied the supercapacitors of PEDOT electrodes and PEDOT/SWNTs composite electrodes, and found that when the scan rate was 10mV/s, the specific capacitances of the supercapacitors of PEDOT and PEDOT/SWNTs electrode materials were 120F/g and 210F/g respectively. .

Ling-Bin Kong et al. used an in-situ chemical polymerization method to prepare a composite electrode of multi-walled nanotubes and polyaniline (MWNTs/PANI), and measured its specific capacitance to reach 224F/g. Deng Meigen et al. used chemical in-situ polymerization to coat polyaniline on the surface of carbon nanotubes to prepare CNT-PANI nanocomposites. They found that when the current density was 10mA/cm2, the specific capacitances of CNT and CNT/PANI nanocomposites were respectively are 52F/g and 201F/g, and the energy density is 6.97W·h/kg. Ying-ke Zhou et al. examined the electrochemical properties of CNT/PANI composite electrode materials and prepared CNT/Pani using an in-situ chemical polymerization method. Through AC impedance and HRTEM characterization, they found that CNT and Pani formed a tight charge transport mixture instead of Simple weak molecular connections.

This electron transport hybrid significantly reduces ion diffusion resistance, which is beneficial to charge transport, and effectively improves the phenomenon of specific capacitance attenuation under high power density. As can be seen from Figure 3, carbon nanotubes can provide a three-dimensional spatial network structure due to their good mechanical properties. Organic polymers are coated on carbon nanotubes through in-situ chemical polymerization or electrochemical polymerization, which greatly increases the specific surface area. In addition, carbon nanotubes have good electrical conductivity, which improves the phenomenon of reduced electrical conductivity of polymers during charge and discharge. There are also many people studying the composite of polymers and other materials. Mao Dingwen used ammonium persulfate (APS) as the oxidant to prepare polyaniline/activated carbon composite electrode materials. Research shows that when m (activated carbon): m (aniline): m (ammonium persulfate) = 7:1:1, the shrinkage rate of aniline is more than 95%, and the specific capacitance reaches 409F/g; the pore size distribution of activated carbon has a great influence on the performance of composite electrodes.

Although organic composite electrode materials can improve electrode conductivity and mechanical properties, the stable operating voltage of composite electrode material supercapacitors is not high, generally less than 1.5V, which affects the energy storage density and power density of supercapacitors and limits supercapacitors. Applications.

3.3 Organic hybrid electrode

In order to improve the energy density and power density of supercapacitors, many people have conducted research on hybrid supercapacitors. Hybrid supercapacitors prepared by using activated carbon and polyaniline can significantly increase the stable potential window of the electrode, thereby effectively increasing the energy density of the supercapacitor. Wang Xiaofeng and others used chemical oxidation polymerization to prepare pure polyaniline electrode materials, and also used secondary activation methods to prepare high-specific surface active carbon materials. Using polyaniline as the positive electrode, activated carbon as the negative electrode, and 38% sulfuric acid solution as the electrolyte, the supercapacitor has a stable potential window of up to 114V, and its maximum specific energy and specific power can reach 1515W·h/kg and 214W/kg. Zhang Qingwu and others used C/Pani as the positive electrode and activated carbon electrode material as the negative electrode. They also increased the stable potential window of the hybrid capacitor to 1.4V, and its energy density reached 814W·h/kg.

E. Frackowiak conducted a comparative study on hybrid supercapacitors with a variety of conductive polymers as electrodes. The study showed that when the mass fraction of CNT is 20%, the composite electrode shows the best performance. Observed under an electron microscope, both PPy and PANI can coat uniform CNTs, but PEDOT shows an obvious tendency to self-aggregate. The porosity of electrochemically polymerized composites is significantly higher than that of in-situ chemically polymerized composites.

As can be seen from Table 1, when PANI and PPy are used as the positive and negative electrodes respectively, the specific capacity reaches the maximum (320F/g). When PEDOT and activated carbon are used as positive and negative electrodes respectively, the working voltage reaches the maximum (1.8V). In addition, they believe that replacing CNT with acetylene black will achieve better results because ECP has better dispersion and higher conductivity.

3.4 All-solid-state supercapacitor electrodes

Traditional supercapacitors use liquid electrolytes. However, all-solid supercapacitors using solid electrolytes have the advantages of easy assembly, no leakage, and small self-discharge. Therefore, all-solid supercapacitors have become a hot spot in research. Many people have also studied supercapacitors composed of electrode materials and solid electrolytes. The two focuses of all-solid-state supercapacitor research are the study of solid electrolyte properties and the relationship between electrode materials and