Research Progress of Polyimide as Electrode Materials for Supercapacitors

Research progress of polyimide as electrode materials for supercapacitors

DING Chengcheng ,YU Juan ,HUANG Pei ,WANG Xiaodong

Research Progress of Polyimide as Electrode Materials for Supercapacitors

（State Key Laboratory of Materials-Oriented Chemical Engineering，College of Chemical Engineering，Nanjing Tech University，Nanjing 211800，China）

DOI：10.3969/j.issn.1001-3539.2024.01.025

Fund Information： National Natural Science Foundation Project (22035007)

Research Progress of Polyimide as Electrode Materials for Supercapacitors

Abstract: The application advantages of supercapacitors and the development and characteristics of its electrode were briefly described，and the research progress of polyimide and its derived carbon materials in the field of supercapacitors in recent years were reviewed. In addition，the current research status of polyimide-based porous carbon，carbon nanosheets，carbon aerogel and carbon nanofibers as electrode active material was introduced，and the advantages and disadvantages of application of these materials and the improvement methods were analyzed. Finally，the future research direction of promoting the further development of polyimide and its derived carbon materials in the field of supercapacitors was discussed，which still need to be studied，including polyimide carbon aerogel，porous carbon fiber and self-supporting film electrode.

Keywords: polyimide；supercapacitor；electrode material；porous carbon electrode

Research Progress of Polyimide as Electrode Materials for Supercapacitors

1 Electrode Materials Research Progress of Polyimide as Electrode Materials for Supercapacitors

Electrode materials for supercapacitors are key factors affecting device performance, and common electrodes include transition metal oxides, conductive polymers, and carbon-based materials. All three types of electrode materials have their own advantages and disadvantages. To improve the electrochemical performance of capacitors, constructing multi-component composite electrode materials is currently a research hotspot in the field of electrode materials.

1.1 Transition Metal Oxides

Transition metal oxides have high theoretical specific capacitance and energy density, and their charge storage mechanism is based on rapid redox reactions on the electrode surface, making them common electrode materials for pseudocapacitors. Transition metal oxides include ruthenium oxide, manganese oxide, zinc oxide, nickel oxide, and cobalt oxide. Theoretical studies indicate that transition metal oxides exhibit significant pseudocapacitance behavior and high theoretical capacitance, but they are limited by conductivity, and their electrode materials typically have large ionic diffusion resistance, leading to actual capacitance being much lower than theoretical values. With in-depth research on transition metal oxides, recent years have seen widespread attention on binary transition metal oxides such as nickel cobaltate, cobalt molybdate, magnesium cobaltate, and nickel manganate. Compared to monomer transition metal oxides, binary transition metal oxides exhibit multiple redox processes and demonstrate superior electrochemical performance.

1.2 Conductive Polymers

As materials with significant pseudocapacitance, conductive polymers have the advantages of high capacitance, low preparation costs, and controllable material structures, and they are widely studied as electrode materials in supercapacitors. Commonly researched conductive polymers include polyaniline, polypyrrole, and polythiophene. The charge storage mechanism of conductive polymers involves rapid doping/de-doping between the electrolyte and the polymer backbone, facilitating ion exchange for charge storage. However, conductive polymer electrode materials generally suffer from reduced mechanical properties and unstable conductivity. To improve the conductivity stability of conductive polymers, one effective method is to composite them with carbon-based materials, which can provide a good conductive network within the conductive polymer.

1.3 Carbon Materials

Carbon materials have advantages such as abundant raw materials, high specific surface area, good conductivity, and excellent chemical stability. Commonly used carbon materials include activated carbon, carbon fibers, graphene, carbon aerogels, and biomass-derived carbon. To develop high-energy supercapacitors, research on carbon-based electrode materials with pseudocapacitance characteristics is an important direction for enhancing supercapacitor performance. Pseudocapacitance mainly originates from the redox reactions on the surface of carbon networks due to heteroatom doping or active materials such as transition metal oxides and conductive polymers. Therefore, constructing composite materials with pseudocapacitive materials to enhance the conductivity of carbon materials and the utilization of pseudocapacitive materials is a crucial approach to advancing capacitor development.

2 Polyimide-based Electrodes

Traditional carbon-based electrode materials store energy based on the “double layer” theory at the interface, usually exhibiting low specific capacitance and energy density. According to literature reports, polymer-based carbon materials containing heteroatoms such as nitrogen and oxygen have a high doping rate of heteroatoms after high-temperature carbonization, causing different types of Faradaic reactions during charge storage, granting carbon-based electrodes both double-layer capacitance and pseudocapacitance behavior, significantly enhancing the capacity of carbon capacitors. PI, as a polymer containing nitrogen and oxygen in its main chain, also possesses a high doping rate of heteroatoms after high-temperature carbonization, making it a material with significant application potential among nitrogen-doped carbon electrode materials. Based on different electrode forms, polyimide-based carbon materials mainly include polyimide-derived porous carbon electrodes and polyimide-based carbon fiber self-supporting electrodes.

2.1 PI Porous Carbon Electrode Materials

Although double-layer capacitors exhibit high power density and long cycle stability, they are limited by double-layer capacitance, leading to many deficiencies in specific capacitance and energy density for carbon-based electrode materials. Therefore, enhancing the energy density of carbon-based materials while maintaining their traditional advantageous performance is crucial for promoting the rapid development of double-layer capacitors. Currently, polyimide-derived carbon materials for supercapacitors mainly include porous carbon, carbon nanosheets, and carbon aerogels.

2.1.1 Porous Carbon

PI porous carbon has advantages such as simple preparation, wide availability, and high conductivity. The main preparation methods are direct high-temperature carbonization and direct laser writing carbonization (DLWC). During carbonization, pyrolysis generates porous carbon materials with micro-mesoporous structures, typically containing a significant number of nitrogen and oxygen atoms in the carbon skeleton. Among them, the DLWC process is a means of creating mesoporous nanostructures by irradiating a laser on the surface of the target substrate, where gaseous nitrogen released during DLWC induces the formation of high specific surface area random mesoporous nanostructures in PI films. Currently, carbon-based micro-supercapacitors (MSC) prepared through DLWC are widely applied in flexible, portable, and wearable electronic devices. Wang et al. utilized DLWC to laser carbonize PI films and assembled MSCs with planar and multilayer 3D structures. Focused laser pulses rapidly convert insulating PI into highly conductive porous structures with hierarchical porosity and appropriate doping of heteroatoms. The specific capacitance of the assembled single-layer supercapacitor reached 22.40 mF/cm² at a current density of 0.1 mA/cm², while the stacked 2-layer and 3-layer MSCs achieved specific capacitances of 37.2 mF/cm² and 42.6 mF/cm², respectively, at a current density of 0.1 mA/cm². Kim et al. obtained PI-based porous carbon materials through direct laser writing carbonization and assembled MSCs. The focused laser beam directly carbonizes the PI substrate, creating porous carbon structures on its surface. The fluorine in the main chain structure of PI generates a large number of micropores during carbonization, enhancing the specific surface area and conductivity of the carbon electrode. Therefore, compared to conventional PI-based MSCs, MSCs assembled based on this electrode exhibit higher specific capacitance, better transmittance (44.9%), and excellent cycling performance and capacitance retention.

In recent years, with the growth in production and demand for PI films, a large amount of waste film scraps has been generated. These waste materials also pose high-temperature processing challenges, and direct disposal can severely pollute the environment. To promote the recycling of waste PI films, Liu et al. prepared sheet-like porous carbon materials from discarded PI film scraps using a KOH physical activation method, which exhibit hierarchical pore structures and are rich in nitrogen and oxygen heteroatoms. The sheet-like structure provides the porous carbon material with lower impedance than commercial activated carbon, and the single electrode demonstrates a specific capacitance of 482 F/g with excellent capacitance retention. The assembled symmetrical supercapacitor electrode exhibits high energy density and excellent cycling stability. To further increase the specific surface area of the waste PI-based porous carbon materials and enhance the energy density of the electrode materials, Liu et al. selected Na₂SO₄ and 1,1-sodium bis(4-fluorobenzene) tetrafluoroborate (SBPBF₄) as electrolytes, utilizing H₃PO₄-assisted KOH activation to prepare porous carbon. The pore size distribution of the porous carbon ranges from 0.7 to 1 nm and >2 nm, with numerous crack structures along the cross-section, effectively increasing the active sites for energy storage and promoting the ion diffusion rate of the electrolyte. Under the highly synergistic interaction between the electrolyte and electrode structure, the assembled symmetrical capacitor can withstand a working voltage of up to 1.8 V in Na₂SO₄ electrolyte, achieving an energy density of 23.2 (W·h)/kg at a power density of 450 W/kg. In SBPBF₄ electrolyte, the capacitor’s tolerance voltage reaches 3.2 V, with an energy density of 70.2 (W·h)/kg. This work establishes a synergistic system of porous carbon electrodes and electrolytes, enhancing the energy density of supercapacitors while providing a sustainable strategy for the resource utilization of waste PI films.

Besides specific surface area, pore structure is also an important factor affecting the electrochemical performance of porous carbon materials. The synergistic optimization effect between composite materials can enable carbonized materials to possess network pore structures and interwoven porous structures, further enhancing the electrochemical performance of carbon materials for supercapacitors. To optimize the pore structure of PI-based porous carbon materials, Yu et al. employed a pyrolysis method to prepare nitrogen-doped fiber-reinforced PI composite materials with multi-scale and porous structures. The prepared nitrogen-doped derived carbon materials exhibit a three-dimensional conductive network structure, high nitrogen content, and high specific surface area, providing interconnected channels for the rapid diffusion of ions and electrons in the PI-derived porous layer. The energy density and power density of the supercapacitor assembled based on this electrode are 24.5 Wh/kg and 209.8 W/kg, respectively. Li et al. prepared a three-dimensional structure of carbonized PI/cellulose (CPC) composite material using a simple “one-step” carbonization method, where CPC acts as a crosslinking agent to adjust the molecular structure of polyamic acid (PAA), ensuring the formation of an interwoven porous network carbon skeleton after carbonization. The carbonized composite material exhibits high specific surface area and uniformly distributed pores, with the CPC-5 electrode demonstrating an excellent specific capacitance of 300 F/g in 6 M KOH electrolyte. The symmetrical supercapacitor assembled based on this electrode achieves an energy density of 22.6 (W·h)/kg at a power density of 800 W/kg, with a capacitance retention rate of 91.4% after 10,000 cycles.

2.1.2 Carbon Nanosheets

Porous carbon nanosheets possess unique in-plane porosity and rich interlayer channels, facilitating ion penetration and transfer, and have gained significant application in electrochemical energy storage in recent years. Compared to sheet-like and bulk porous carbon materials, carbon nanosheets exhibit higher surface utilization efficiency, with ample storage space internally to shorten the transfer distance of electrons/ions. The precursor for PI-based carbon nanosheets is spherical flower-like nanosheets, obtained by adjusting the chemical composition and microstructure of the polymer to achieve a suitable hydrothermal precursor solution, where the polymer chains fold back and forth during the hydrothermal process to form a 2D layered structure, subsequently growing into a 3D spherulite structure. PI-based derived carbon nanosheets primarily operate based on double-layer capacitance, with specific surface area being one of the important parameters. To prepare PI carbon nanosheets with higher specific surface area, common methods involve activation agents’ activation and pore-expanding effects. Lee et al. adopted a nonsolvent-induced phase separation method with melamine-assisted carbonization of PI sheet materials to produce nitrogen-rich porous carbon sheet electrode materials. The symmetrical supercapacitor assembled with the above electrodes achieves a specific capacitance of 176 F/g at 1 A/g, retaining an impressive capacitance retention rate of 99.2% after 10,000 cycles. The energy density of this capacitor is 6.3 (W·h)/kg, significantly improved compared to conventional electrodes. Peng et al. synthesized PI nanosheets with nanosheet entanglement using low-cost aromatic diamines and anhydrides as monomers via a hydrothermal self-assembly method. Using FeCl₃ as an activator and catalyst, they simultaneously activated and catalyzed the carbonization to prepare hierarchical PI-based porous carbon nanosheets (PI-CNFs). Due to their unique entangled porous nanosheet structure, high specific surface area, and high nitrogen doping content, PI-CNFs exhibit high specific capacitance, high capacity retention, and ultra-long cycle life. The newly assembled symmetrical aqueous supercapacitor based on PI-CNFs has a large operating voltage of 2 V, achieving a high specific energy of 18.3 (W·h)/kg at a power density of 500 W/kg, along with excellent cycling stability. Building on this, Peng et al. utilized self-assembled PI nanosheets as precursors for nitrogen-doped carbon, selecting sodium zinc ethylenediaminetetraacetate (EDTA-Na₂Zn) as an activator to prepare nitrogen-doped porous carbon nanosheets via one-step activation and self-generating template-assisted carbonization. The assembled aqueous supercapacitor based on nitrogen-doped porous carbon nanosheets demonstrates excellent cycling stability, with a working voltage of 2 V in Na₂SO₄ electrolyte, achieving an energy density of 17.92 (W·h)/kg at a power density of 500 W/kg.

KOH, as one of the commonly used physical activators, can significantly enhance the micropore ratio of carbon materials, thus increasing their specific surface area. Wang et al. prepared flower-like PI nanosheets using 4,4′-oxydianiline (ODA) and cyclobutane-1,2,3,4-tetracarboxylic anhydride (CBDA) as raw materials via a solvothermal method. To improve electrochemical performance, KOH was selected as the activator, achieving high specific surface area of 1,498 m²/g for the active flower-like carbon particles, with a specific capacitance of 313.4 F/g at 0.5 A/g, maintaining high rate performance (73.4%) at 10 A/g. The assembled symmetrical supercapacitor achieves an energy density of 28.08 (W·h)/kg at a power density of 500 W/kg. Wu et al. prepared nitrogen-sulfur co-doped porous carbon materials (S,N-PIC) using PI as the carbon source, urea as the nitrogen source, and KOH as the activator through a one-step activation carbonization method. The electrodes based on S,N-PIC-1 achieve an ultra-high specific capacitance of 235.3 F/g at 0.5 A/g, retaining 96% capacitance after 10,000 cycles. In the electrolyte of 1-butyl-3-methylimidazolium tetrafluoroborate, the assembled symmetrical supercapacitor exhibits a wide working voltage of 3 V and a high energy density of 40.95 (W·h)/kg at 749.2 W/kg power density. The combination of carbon materials with pseudocapacitive materials such as metal oxides can significantly enhance the specific capacitance and energy density of electrode materials. Liu et al. prepared flower-like carbon nanosheets derived from PI through solvothermal and carbonization methods, loading MnO₂ nanosheets on the surface of flower-like carbon nanosheets via hydrothermal methods. They investigated the effects of the mass ratio of carbon nanosheets to potassium permanganate, hydrothermal time, and temperature on the electrochemical behavior of the composite materials. The optimized electrode material exhibits a high specific capacitance of 523.43 F/g at 0.5 A/g, with the assembled symmetrical supercapacitor achieving an energy density of 16.7 (W·h)/kg at a power density of 193 W/kg.

In addition to conventional monomer polymerization combined with solvothermal reactions, other methods such as pyrolysis rearrangement, high-temperature decomposition of other materials, or templating methods are also used to adjust the pore structure and specific surface area of carbon fiber membranes. Lee et al. prepared carbon fiber membrane electrode materials with high specific surface area by using a novel aromatic PI and polyvinylidene fluoride (PVDF) blend as precursors, employing electrospinning and thermal treatment. The thermal stability of PVDF is lower than that of PI, allowing for the formation of additional pores on the surface of carbon fibers during high-temperature carbonization, improving the specific surface area of the electrode materials and thus enhancing the performance of supercapacitors. The electrode materials prepared from mixed precursors exhibit a specific capacitance of 283 F/g, which is 37% higher than that obtained from pure PI carbonization. Hydroxyl-PI (HPI) undergoes thermal rearrangement during high-temperature carbonization, further increasing the specific surface area of carbon materials, thereby improving the electrochemical performance of carbon electrodes. Xu et al. prepared HPI nanofiber membranes via electrospinning using pyromellitic dianhydride and 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane as monomers. Through thermal rearrangement and pyrolysis, nitrogen and oxygen-doped HPI-based carbon nanofibers were obtained. Due to the rich microporous and mesoporous structures, the specific surface area of the material can reach up to 1,107.6 m²/g, exhibiting a specific capacitance of 263.9 F/g at a current density of 0.5 A/g. The assembled capacitor retains a capacitance retention rate of 99.82% after 10,000 charging and discharging cycles, indicating excellent long-cycle stability. The hard template method is also one of the effective and controllable structural methods. Among them, the hard template method using MgO as a template agent can prepare porous carbon materials with interconnected porous networks, but MgO also has the problem of forming pore sizes larger than micrometers due to its large particles. Therefore, to obtain carbon-based materials with hierarchical pore structures containing mesopores/micropores, Le et al. employed electrospinning technology combined with Mg(NO₃)₂ treatment, preparing PI porous carbon fiber membranes with a specific surface area of up to 1,836 m²/g after high-temperature carbonization. The symmetrical capacitor assembled in LiPF₆ organic electrolyte exhibits a specific capacitance of 140 F/g and an energy density of 30 (W·h)/kg.

3 Summary and Outlook

In recent years, the research on PI-based porous materials has promoted the application of PI-based porous carbon, carbon nanosheets, carbon aerogels, and carbon nanofiber materials in supercapacitor electrode materials. However, to continue advancing the development of PI-based electrode materials, further research is still needed in the following areas:

(1) As the most typical and high specific surface area PI aerogel materials, they are rarely used in the electrochemical field, which also requires large specific surface areas. The main issue is that the pore structure of aerogels is prone to collapse during high-temperature carbonization, causing the original specific surface area of the material to almost disappear after carbonization, limiting their application in the electrochemical field. Research should focus on modifying PI carbon aerogel materials.

(2) Specific surface area is one of the important indicators affecting capacitor performance. Traditional PI fibers often have dense structures; although numerous pores can form between the fibers, the materials generally have low specific surface areas after carbonization. Future research can delve into PI porous carbon fibers.

(3) Self-supporting film electrodes possess good mechanical properties, higher specific capacitance, and energy density, allowing them to serve as electrode materials without the need for any binders, conductive agents, or current collectors. Besides fiber membranes, the precursors of PI porous carbon materials are mostly membrane materials, but due to high-temperature shrinkage, the rigidity of the carbonized membranes is too large to be directly applied as film electrodes. Future directions should focus on modifying the flexibility of porous membrane materials to enable their direct application as film electrodes in supercapacitors.

Research Progress of Polyimide as Electrode Materials for Supercapacitors

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Research Progress of Polyimide as Electrode Materials for Supercapacitors

Research progress of polyimide as electrode materials for supercapacitors

1.1 Transition Metal Oxides

1.2 Conductive Polymers

1.3 Carbon Materials

2 Polyimide-based Electrodes

2.1 PI Porous Carbon Electrode Materials

3 Summary and Outlook

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