Keywords

1 Introduction

Due to the depletion and shortage of non-renewable fossil fuels-based energy sources, a serious challenge in the current century is the energy crisis [26]. Additionally, the environment, people, and human activities are negatively impacted by the growing emissions of pollutants from nonrenewable energy (oil, gas, and coal). According to the Environmental Protection Agency (EPA), Carbon dioxide (CO2) makes up 82% of emissions that are classified as greenhouse gases, followed by methane (CH4), 9%, nitrates (NOx), 6%, and fluorinated gases (perfluorocarbons, hydrofluorocarbons, and sulfur hexafluoride), which make up 3%. Climate change consequences have been made worse by greenhouse gas emissions. These outcomes have pushed research efforts towards the creation of renewable energy sources. Energy storage devices are necessary when harvesting energy from renewable sources like photovoltaic panels or wind farms because the captured energy (such as wind or solar energy) is not always available. For example, the sun doesn’t shine every day and night, and the wind doesn’t always blow when we need it to. Consequently, one of the key problems that will improve the use of sustainable energies in the future is energy storage, particularly electrical energy storage. Among various energy storage technologies, lithium-ion batteries (LIBs) and supercapacitors hold great promise in broad applications such as portable electronics, smart grids and electrical vehicles [6]. Since the Sony Company first began selling LIBs in 1991, this type of secondary battery has become increasingly common in people’s daily lives. Because LIBs have a high energy density of 150–200 Wh/kg, they may store electrical energy in portable devices. With a power density at least two orders of magnitude lower than that of fossil fuels, LIBs are very difficult to compare with them. Because of this, a significant number of LIBs are required to replace fuels, which sharply increases the mass of products. Different electrical energy storage devices and conversion technologies (Fig. 1) can be identified by many parameters, including energy storage mechanisms, charging and discharging processes, energy, and power densities, which define their uses. Because of their slow discharge process, batteries may be employed for long-term and steady energy storage density. On the other hand, Capacitors can be used in applications that demand quick energy transmission due to their fast charging and discharging capabilities. Between traditional capacitors and batteries, supercapacitors, also known as ultracapacitors or electrochemical capacitors ESs, are a new energy storage technology. Due to its many advantages over lithium-ion batteries, such as quick charging and discharging, enormous power density, wide working temperature ranges, and extended service life cycles, supercapacitors have gained increased attention in recent years. As a result, ESs have been found to be readily applicable in a wide variety of important applications such as hybrid and electrical vehicles, portable electronics, backup power supplies, aircraft, and smart grids [9]. Batteries, capacitors, and supercapacitors are compared in Table 1 in terms of their characteristics and performance.

Fig. 1
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(figure used by permission of AIP Publishing) [1]

Ragone plot for various electrical energy storage devices

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Table 1 Comparison of conventional capacitor, supercapacitor and battery (table used by permission of Elsevier) [63]
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2 Fundamentals of Supercapacitors

A basic supercapacitor consists of two plates (current collector) coated with porous materials and separated by a porous separator soaked in an electrolyte. Each component plays an important part in determining a supercapacitor’s overall performance. The supercapacitor electrode materials are deposited on conductive substrates called current collectors, it allows electrons to travel in the direction of the electrode’s thickness instead of along the electrode’s length vastly reducing the charge transport distance [86]. The porous separator allows charge transfer, and the electrolyte interacts with both electrodes. Because they determine the performance of supercapacitors, particularly the energy/power densities and life cycles of a supercapacitor, electrolytes (salt and solvent) used in supercapacitor cells are as important as the electrode materials. The electrolytes also influence the series resistance and the operating voltage of the cell. Finally, the electrode material is the most important component to achieve significant improvements in supercapacitor performance. Below is a list of the characteristics an electrode material must possess.

  • To create electrochemical double-layer capacitors, a high specific surface area is required, generally between 1000 and 2000 m2g−1.

  • Controlled distribution of pore size.

  • Reversible redox reactions to prevent stability loss.

  • Electrochemical stability beyond the limit of electrolyte decomposition.

  • Surface wettability of the electrolyte.

3 Types of Supercapacitors

Based on the mechanism of charge and discharge process, there are two main types of supercapacitors as shown in Fig. 2 namely: non-Faradic, electrochemical double-layer capacitors (EDLCs) and pseudocapacitors (or redox-based electrochemical capacitors, Faradic). However, there is a hybrid type that combines the mechanism of both types mentioned above, hybrid electrochemical capacitors.

Fig. 2
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(figure used by permission of Elsevier) [23]

Different types of supercapacitor electrodes

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3.1 Electrochemical Double Layer Capacitors

The EDLC supercapacitor type is the first family of supercapacitors that employ porous carbon-based materials with a large specific surface area as the electrode material. To be more specific, the charging-discharging of an EDLC is by the accumulating of electrolyte ions onto the surface of a porous electrode by polarization. The solvated ions flow towards the two carbon electrodes when DC voltage is applied, passing across the separator to create an electric double-layer that stores the electric charges as seen in Fig. 3. The two parallel charge layers that surround the plate are referred to as the double layer.

Fig. 3
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(figure used by permission of Elsevier) [41]

Charged and discharged mechanism of an electric double layer capacitor

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The principle of EDLCs is like that in conventional capacitor, however EDLCs show high capacitance due to their maximum effective surface area and very small charge separation distances [63]. EDLCs exhibit excellent cycling stability due to the absence of chemical or composition changes associated with non-faradaic process and electrostatic charge transfer in this device is completely reversible. In EDLCs, the capacitance is related to the accumulation and separation of net electrostatic charge at the electrolyte– electrode interface. In this case, the net negative charges (mainly electrons) are accumulated at the surface of the electrode, while positive charges with an equal number (mainly cations) are accumulated near surface of the electrode facing the electrolyte solution, forming electric double-layers. Therefore, the level of the capacitance is thus determined by surface characteristics of the electrode materials. However, there is always a limitation in the magnitude of capacitance.

3.2 Electrochemical Pseudocapacitors

Unlike EDLCs, some electrochemically active materials called pseudocapacitive materials could present much higher capacitance. Pseudocapacitive materials are the second type of materials used in supercapacitors where the charging-discharging of pseudocapacitors happens at the electrode surface or close to it through fast and reversible redox reactions between electrolyte ions and electroactive species [34]. The common materials that exhibit pseudocapacitive behavior are conducting polymers and transition metal oxides/hydroxides [77]. The redox process can be described as:

$${O}_{X}+n{e}^{-}\iff {R}_{d}$$
(1)

where OX and Rd are the oxidant and the reductant which are insoluble in the electrolyte solution, n is the number of transferred electrons (e−) in the redox reaction. Due to the redox reactions, the change in quantity of the produced charge (∆q) is a continuous function of the change in applied potential (∆V), while the ratio ∆q/∆V is defined as the pseudocapacitance. On the one hand, pseudocapacitance materials such as conducting polymers and metal oxides can remarkably enhance the capacitance and energy stored of supercapacitors. On the other hand, because to the inadequate reversibility of the redox reaction, which affects the shape of the electrode, it displays limited mechanical stability and a short cycle life [61]. The comparison between two types of supercapacitors are summarized in Table 2.

Table 2 Comparison between electrical double layer capacitor and pseudocapacitor
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3.3 Hybrid Electrochemical Capacitors

The hybrid type supercapacitors are an energy storage device which exploits the advantages of EDLC and the pseudocapacitors using composites or two asymmetric electrodes. The high power density of the EDLC and higher capacitance and energy density of the pseudocapacitors is combined to form a device with better performance [47]. A single electrode in a hybrid configuration system that combines carbon-based materials with conducting polymers or metal oxides provides the advantages of both physical and chemical charge storage mechanisms. Asymmetric supercapacitor denotes to the combination of two separate electrode materials that use two distinct storage mechanisms. Ideally, a pseudocapacitive material electrode provides high energy density while the EDLC material electrode furnishes the system with high power capability [71].

4 Electrochemical Evaluation

The electrochemical assessment is based on their specific capacitance and capacity, as well as their energy and power densities. Different approaches, including as cyclic voltammetry (CV), and galvanometric charge discharge technique (GCD) can be used to calculate the above-mentioned parameters. These parameters are measured using techniques such as three electrode and two electrode configurations.

Because it is simple to determine the cycle life, CV has emerged as a key technique for assessing the performance of SCs. Additionally, the information on the impacts of internal resistance and the ensuing dissipative losses may be discovered by analyzing the shapes of the voltammograms as a function of scan rate s. The form of the cyclic voltammogram for EDLCs should resemble a rectangle, confirming no chemical reaction as in Fig. 4a, the rectangular shape is distorted with reversible oxidation and reduction peaks for the pseudocapacitive material, indicating a Faradaic redox process, as shown in Fig. 4b. CV data can be utilized to determine the specific capacitance Cs of an electrode material by calculating the area enclosed in cyclic voltammogram \(\int {\varvec{I}}\left({\varvec{V}}\right)\boldsymbol{ }\boldsymbol{*}{\varvec{d}}{\varvec{V}}\) using the following formula [60]:

Fig. 4
figure 4

(figure used by permission of Elsevier) [66]

Cyclic voltammograms of a EDL capacitor and b pseudo-capacitor

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$${\varvec{C}}{\varvec{s}}=\frac{\int {\varvec{I}}\left({\varvec{V}}\right)\boldsymbol{ }\boldsymbol{*}{\varvec{d}}{\varvec{V}}}{2{\varvec{m}}\boldsymbol{*}{\varvec{s}}\boldsymbol{ }\boldsymbol{*}\Delta {\varvec{V}}}$$
(2)

where m (g) is the mass loading of active material, \(\Delta {\varvec{V}}(\mathrm{V})\) is the potential window, and s (mV/s) is the scan rate

The galvanostatic charge and discharge method is a more precise approach than the CV method for determining the specific capacitance of active materials. The working electrode is exposed to positive and negative continuous currents that charge and discharge the electrode within a predetermined voltage range while timing the process. Figure 5 displays a typical GCD curve that plots the voltage as a function of time. The specific capacitance Cs by F/g related to the discharge current density (I) by A/g through the relation [61, 73].

Fig. 5
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GCD curves of a EDL capacitor and b pseudo-capacitor

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$${\varvec{C}}{\varvec{s}}=\boldsymbol{ }\frac{{\varvec{I}}\boldsymbol{ }\boldsymbol{*}\boldsymbol{ }\boldsymbol{ }{{\varvec{t}}}_{{\varvec{d}}{\varvec{i}}{\varvec{s}}}}{\boldsymbol{ }{\varvec{m}}\boldsymbol{*}\Delta {\varvec{V}}}$$
(3)

where \({{\varvec{t}}}_{{\varvec{d}}{\varvec{i}}{\varvec{s}}}\) (s) is the time of discharge and \(\Delta {\varvec{V}}(\mathrm{V})\) is the working potential.

The characteristics of the active material are determined by the type of the GCD profiles, much like with the CV curves. Due to their consistent charge distributions during the charge–discharge process, an ideal EDLC profile has a linear GCD (Fig. 5a). The oxidation and reduction processes that occur throughout the charge–discharge process cause the pseudo-/Faradaic supercapacitors to have a non-linear GCD profile (Fig. 5b). To learn more about a supercapacitor electrode’s electrochemical performance, the GCD measurement is required. It is used to do additional calculations about electrochemical characteristics including energy, power densities, and capacitance retention. The energy density (E) and power density (P) can be calculated using the following equations [91]:

$${\varvec{E}}\boldsymbol{ }({\varvec{W}}\boldsymbol{ }{\varvec{h}}/{\varvec{K}}{\varvec{g}})=\frac{1}{2}\boldsymbol{*}{\varvec{C}}{\varvec{s}}\boldsymbol{*}\boldsymbol{ }{{\varvec{V}}}^{2}\boldsymbol{*}\frac{1000}{3600}$$
(4)
$${\varvec{P}}({\varvec{W}}/{\varvec{K}}{\varvec{g}})=\boldsymbol{ }\frac{{\varvec{E}}}{{{\varvec{t}}}_{{\varvec{d}}{\varvec{i}}{\varvec{s}}}}\boldsymbol{*}3600$$
(5)

where Cs (F/g) is the specific capacitance, V (V) is the potential window and \({{\varvec{t}}}_{{\varvec{d}}{\varvec{i}}{\varvec{s}}}\) (s) is the discharge time corresponding to Cs calculated from GCD curve.

5 Polyaniline PANI

The most studied conducting polymer among the family of conducing polymers is polyaniline (PANI) due to its interesting characteristics such as ease of synthesis, doping, dedoping, low cost, mechanical flexibility, chemical properties, and environmental compatibility. There are numerous ways to synthesize polyaniline, including chemical and electrochemical polymerization. Using chemical polymerization as an example, aniline monomer is placed in an aqueous acid solution and chemical oxidants such as ammonium persulfate or ferric chloride. The polyaniline precipitates out of the chemical reaction solution. When preparing PANI nanocomposites, chemical synthesis of polyaniline provides much greater flexibility for managing the nucleation and growth mechanisms throughout the polymerization process [14]. While electrochemical polymerization is typically performed in a three-electrode electrochemical cell with an electrolyte that supports the electrochemical reaction and dissolves the aniline monomer. The monomers oxidized by repeating cycles between a potential window where the oxidation takes place and the polymer will then be electrodeposited directly onto the substrate which is a worthwhile advantage for the fabrication of binder-free supercapacitors [53].

5.1 PANI-Based Materials for Pseudocapacitors

Morphology is one of the primary factors affecting PANI’s supercapacitive behavior. Varying the PANI morphology during the preparation can lead to better performance of supercapacitor [14]. Guan and coworkers [21] reported that adding a small amount of para-phenylenediamine (PPD) as additive during the polymerization of aniline can lead to the formation of longer, less entangled PANI nanofibers, which significantly enhances the electrochemical performance of PANI. Even in two electrode cells where the electroactive material is not immersed in the electrolyte to expand the electrode/electrolyte contact, the function of PANI morphology is crucially important. Also, changing the chemical polymerization temperature can produce different morphologies of PANI which could provide different cyclability. Porous PANI can also considerably enhance the performance of supercapacitor due to increasing the electrochemically accessible surface area. Sharma and coworkers [64] synthesized a nanoporous hypercrosslinked PANI with a SBET of 1059 m2/g which could deliver a specific capacitance of 410 F/g with outstanding cycling stability with 100% of the capacity retention after 1000 cycles. The electrochemical performance based on different morphologies of PANI reported in the literature are listed in Table 3.

Table 3 Capacitance performance of different morphologies of PANI
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The type of the dopant ions, electrolyte, pH of the solution, the substrate, and the deposition conditions all have an impact on the electrochemical and mechanical characteristics of the PANI supercapacitor electrode, which in turn has an impact on the capacitance of the PANI electrode [18]. The main drawbacks of PANI are the poor mechanical stability and solubility. As a result of repeated redox reactions, PANI suffer several physical/chemical changes, including swelling, shrinking, cracking or breaking, which over time reduces the material’s performance. Therefore, the synthesis of composite materials may be a promising choice to improve the mechanical performance of PANI.

6 PANI/Carbon Based Material Composites

The aim of developing composite material electrodes is to incorporate the desirable characteristics of the various materials into a single electrode in order to improve stability and capacitance. Additionally, asymmetric type supercapacitors employ nanocomposite electrodes to boost operating voltage to enhance energy density. The electrodes in the PANI-carbon composites have not only efficient pseudocapacitive reactions due to synergistic effects, but also better rate capability due to the substrate’s high conductivity and super cycle stability due to the excellent mechanical properties of the supporting carbon materials. Due to synergistic effects, the electrodes in PANI-carbon composites not only have effective pseudocapacitive reactions but also better rate capability and super cycle stability due to the substrate’s high conductivity and the superior mechanical properties of the supporting carbon materials, respectively [45].

6.1 PANI/Activated Carbon

Activated carbons ACs are one of the cheapest materials which generated by either physical or chemical activations of natural precursors (e.g. wood, coal, nutshell, etc.) [89] The specific capacitance values of supercapacitor electrode based on activated carbon are typically between 100 and 300 F/g in both aqueous and organic electrolyte operating in potential windows between 1 and 3 V. The main drawback of ACs for supercapacitor application is the wide distribution of pore size, including micropores (<2 nm), mesopores (2–50 nm) and macropores (>50 nm) [56]. Since not all these pores are efficient for supercapacitor energy storage, Micropores with a diameter around 1 nm are not accessible to most of the organic electrolyte ions. These unnecessary micropores not only increase the volume that not contributing to the charge storage, but also hinder the electrical conductivity, limit their energy density (5–8 Wh/kg) and power density [88]. To solve this issue, nanocomposite of PANI and ACs have been investigated to enhance electrochemical performance. PANI nanorods uniformly polymerized onto the cellulous-derived highly porous activated carbons (C-ACs) framework by a chemical polymerization process have been investigated as a supercapacitor electrode material by Zhang and coworkers [93]. Due to their great electron conductivity, rapid ion transit, quick and stable surface redox reactions, PANI/C-AC composites with a 3D and hierarchically porous structure were able to achieve good capacitive performance. The PANI/C-AC composites displayed a specific capacitance of 765 F/g at a current density of 1 A/g, a specific power of 14 kW/kg, and a specific energy of 22.3 Wh/kg at a scan rate of 10 mV/s. Additionally, PANI/C-ACs composites shown outstanding cycling stability with 91% capacitance retention after 5000 cycles in a symmetric two-electrode system, in contrast to PANI electrodes, which display 82% capacitance retention. Furthermore, PANI/C-ACs have a higher rate capability because the 3D C-ACs skeletons provide enough mechanical support to avoid PANI volumetric changes during the charge/discharge process. As a result, PANI/C-ACs are potential electrode materials for high-performance SCs.

To overcome the low conductivity and hydrophobicity of ACs, nitrogen doped ACs been investigated for the purpose of increasing specific capacitance. The purpose of nitrogen doping is to increase pseudo-capacitance as a result of the redox reaction that occurs in the nitrogen-contained functional groups. Furthermore, the functional groups improve the electrode’s wettability, resulting in an increase in capacity.

Yu and coworkers [88] developed hierarchical nitrogen-doped porous carbon (HPC)/polyaniline (PANI) using wheat flour as carbon precursor, as shown in Fig. 6. The PANI nanowire arrays were well-ordered on both the interior and external surfaces of the hierarchical porous HPC, facilitates the electrolyte ion into the whole electrode and ensures the highly efficient charge storage, resulting in a high specific capacitance of 1080 F/g. The 3D interconnected honey-comb-like porous structure with nitrogen doping can further increase the electrode’s surface wettability and provide mechanical support for PANI nanowires.

Fig. 6
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(figure used by permission of John Wiley and Sons) [88]

Schematic illustration for the preparation of hierarchical nitrogen-doped porous carbon HPC/PANI composites

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6.2 PANI/Carbon Nanotubes

Carbon nanotubes (CNTs) have gained popularity as promising materials in an energy storage device as a result of their unique properties. These properties such as large surface area (SBET = 1600 m2 g−1), different spaces for storage the electrolyte ions based on their structure, light weight, high electrical conductivity (=105 S cm−1) and mechanical properties [63]. SWCNT, MWCNT, and functionalized CNT are examples of CNT layers that may be used to achieve different characteristics in supercapacitors. Surface functionalization of hydrophilic groups increased the wettability of CNTs in solvents [75]. In recent years, PANI/CNT composites have received a lot of interest in the field of energy storage based on the PANI or CNTs alone. CNT facilities the electrolyte ions to diffuse into the PANI/CNT nanocomposite electrode materials and can also improve the cycle stability of the composite during charging/discharging. Furthermore, the greater mechanical strength of PANI/CNTs electrodes allows us to construct lightweight, flexible, and foldable supercapacitors with thinner separators, eliminating the need for a binder or heavy metal foil current collectors [45]. To enhance capacitive performance, smaller size of CNT arrays with hierarchically porous structures should be aligned, for example, Zhang and coworkers [90] prepared core–shell structured PANI/CNTs arrays composite Using an electrodeposition approach. PANI/CNTs electrode material showed specific capacitances as high as 1030 F/g, excellent rate capability (95% capacity retention at 118 A/g) and great stability (5.5% capacity loss after 5000 cycles).

A plasma polymerization method was developed by Hussain and coworkers [29] to synthesize a PANI/CNT composite that is green, simple, fast, oxidizers and binder free. It produces a pinhole-free ultra-thin film with controlled thickness. Low pressure plasma accelerated chemical vapour deposition of PANI on CNT ensures a regular morphology of PANI in contrast to simple electrochemical deposition, which results in random morphology and narrows the conducting pathways. Vertically aligned CNTs and PANI/CNT electrodes have specific capacitances of 12 F/g and 1225 F/g at 5 mV/s, respectively. In a three-electrode system, the PANI/CNT electrodes demonstrated high cycling stability (65% at 15 A/g after 5800 cycles).

6.3 PANI/Carbon Fiber

Carbon nanofibers (CNFs) are an excellent alternative to CNTs for energy storage devices due to their superior mechanical, electrical, and thermal conductivity properties. Furthermore, they are less expensive, and are simpler to manufacture. There is a lot of interest in developing PANI-CNF composite with superior supercapacitor performance since PANI materials have high specific capacitance and CNFs have a long cycle life. CNFs not only offer shorter electron pathways [45] but also allow ions to enter the fiber from directions perpendicular to its longitudinal axis, which makes them ideal for CNF-based supercapacitors.

Yanilmaz and coworkers [84] used the sol–gel and electrospinning methods to fabricate binder-free flexible PANI/carbon nanofiber electrodes. The hybrid electrode outperformed an individual carbon nanofiber electrode in terms of specific capacitance (234 F/g) and cycling stability (90% after 1000 cycles). In addition, it has a high energy density of 32 Wh/kg at a power density of 500 W/kg. The remarkable pseudocapacitive characteristics of the PANI coating on the carbon nanofiber are responsible for the high performance. The possible reason for poor rate capability of PANI/CNF nanocomposites was the high internal resistance of CNFs carbonized below 1000 °C [45], which led to a negative effect on electron transport and decrease the capacitive behavior of the nanocomposite. The electrical conductivity and hydrophilicity of CNFs should be considered in order to increase the rate capability, graphitized electrospun carbon fibers GECF which carbonized at 2200 °C were immersed in concentrated sulfuric acid for a short time [98]. Then Hou and coworkers [25] used in situ polymerization to grow long, ordered and needle-like PANI nanowires on GECF surface. 3D flexible composite of PANI/GECF electrodes without using any binders and conductive additives presented a high specific capacitance of 976.5 F/g at 0.4 A/g, an energy density approached 86.8 Wh/kg, and a capacitance retention ratio of 89.2% after 1000 cycles at 10 A/g. Furthermore, at a high current density of 50 A/g, PANI/GECF electrodes kept a specific capacitance of about 500 F/g and a coulombic efficiency of 95%.

To achieve the best possible supercapacitive performance and to push the energy density of supercapacitor electrode to a new limit; the active material, electrolyte, and substrate must all be engineered to operate together in a supercapacitor. Hashemi and coworkers [24] fabricated tubular form with rectangular pores of PANI as an active material on chemically functionalized carbon cloth FCC as a substrate as shown in Fig. 7a–b. Then, they added 1,4-napthoquinone (NQ) to the electrolyte as a redox additive, which not only offers pseudo- capacitance through direct redox processes on the electrode surface, but also forms the basis for a regeneration pathway to long-term usage of electrode active materials. After that, asymmetric configuration device used electrolyte of an acidic polymer hydrogel containing the redox additive and made up of two electrodes PANI/FCC as the positive electrode and negative electrode of AC-FCC as shown in Fig. 7a. In the presence of NQ and at current density of 1.4 A/g, this device exhibited an outstanding specific capacitance of roughly 4007 F/g, which is more than 14.5 times greater than when NQ wasn’t present. (Fig. 7c). Furthermore, the device shows extremely high energy density of 1091 Wh/kg, high-power density up to 196 kW/kg, and 84% cycling stability under 35 A/g current density during 7000 cycles Fig. 7d–e. A clock, a red LED, and a rotor were all successfully operated for 1 h and 17 min, 47 min, and more than 20 s, respectively, by two asymmetric devices connected in series. (Fig. 7f).

Fig. 7
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(figure used by permission of Elsevier) [24]

a Schematic design of (PANI-FCC) device. b Field emission scanning electron microscope (FE-SEM) images of PANI. c estimated capacitance for an AC-FCC//PANI-FCC device from 1.4 to 105 A/g in the presence of napthoquinone NQ as a function of current density. d Ragone plots for an asymmetric device at different current densities compared to active mass normalized commercial devices. e Cyclability under 35 A/g current density during 7000 cycles; and f photographs of running a clock, lighting a red LED and turning a rotor with two asymmetric supercapacitors in series

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6.4 PANI/Graphene Electrode Materials

Graphene, a one-atomic thick layer of 2-D with unique morphology, was discovered in 2004 by Novoselov and Geim [55] to be a potential electrode material. Graphene has exceptional properties, such as [69]:

  • High electrical conductivity (~200 Sm−1),

  • Great thermal conductivity (5000 W m−1 K−1),

  • Large surface area (> 2600 m2g−1),

  • Great charge mobility (> 200,000 cm2 V−1 s−1),

  • Excellent chemical and thermal stability,

  • Strong Young’s modulus (1 TPa); and

Based on the advantages of PANI and graphene that mentioned previously, PANI/graphene nanocomposites have become crucial electrodes for supercapacitors due to the high mechanical stability with a wide thermal operating range and the synergistic effects. On the one hand, homogeneous PANI coating on graphene sheets acts as a spacer to separate nearby graphene sheets, prevent their irreversible aggregation from occurring, and raise the usage ratio, adding more EDL capacitance to the overall capacitance [45]. On the other hand, graphene sheets offer outstanding chemical stability, high surface area, significant increase in electrical conductivity, fast electron/ion transport and a wide working potential in electrochemical devices. These advantages render it as an attractive support material to restrict the volumetric changes (expansion/contraction) and improve the cycle stability of PANI [45]. Depositing a PANI on a graphene electrode without a binder by electrochemical polymerization can overcome the drawbacks of poor stability and high resistance of the powder sample. Ye and coworkers [85] synthesized an ordered PANI nanowire array by electrochemical method on partial exfoliated graphite substrate to produce a hierarchical nanostructure. It effectively reduced the disadvantage of “dead volume” caused by the PANI stacking. Moreover, a pathway for the quick transport of electrons and ions is created by the flawless bonding of the graphene nanosheets and orderly PANI nanowire arrays. Finally, high cycle stability is ensured by the presence of graphene, which reduces the volumetric changes of PANI. A high capacitance of 3.57 F/cm2 (607 F/g at 1 A/g) and outstanding cycling stability (80.4% after 10,000 GCD cycles) were displayed by the ordered PANI nanowire and the graphene network [85]. Adding functional groups and vacancy defects to the graphene surface provide a great beneficial in energy storage mechanism. By enriching the active sites of graphene, the specific capacitance and cycling stability of PANI/graphene composites may be greatly enhanced. Zheng and coworkers [96] developed a route for the preparation of a high electrochemical performance electrode based on three-dimensional multi-growth site graphene MSG/PANI nanocomposite. Active sites on the surface of graphene nanosheets, such as oxygen functional groups and carbon vacancy defects, were produced using chemical treatment (70% HNO3 and 30%H2O2). MSG/PANI nanocomposites were found to have a specific capacitance of 912 F/g at 1 A/g, which is much greater than GO/PANI without acid treatment (432 F/g), maximum specific energy and maximum specific power of 30 Wh/kg and 3200 W/kg, respectively. The capacitance retention rate of the nanocomposite achieved 86.4% at a high current density of 20 A/g, and after 10,000 GCD cycles at 10 A/g, it attained 89.5% cycle stability. Tabrizi and coworkers [68] found that an acid-treated GO/PANI nanocomposite with a porous structure and high specific surface area revealed a maximum capacitance of 727 F/g which attributed to acid functionalization and well-defined PANI nanoarrays on GO sheets. The symmetrical device designed by combining these electrodes has a maximum energy density of 40 Wh/kg and a power density of up to 15.3 kW/kg with an outstanding stability (95.7% retention after 5000 cycles). Li and coworkers [39] used a supra-molecular in-situ assembly approach to decorate carbon nanodots on graphene-PANI (rGO@CN/PANI). The nanocomposites reveal high electrochemical performance with specific capacitance of 871.8 F/g at 0.2A/g and acceptable cycling performance (72% after 10,000 CV cycles at 30 mV/s). Mangisetti and coworkers [52] prepared nitrogen doped 3D porous carbon-graphene/PANI (3D PC-g/PANI) by a simple in-situ polymerization process. They also synthesized N-doped porous carbon/gC3N4 from bio- waste seeds to use as a negative electrode for ASC device. The 3D porous carbon prevents graphene nanosheets from aggregating and produces a 3D PC-g composite with a well-connected structure, that provides high conductivity, quick ion and charge transport, and high surface area (2418 m2/g). Also, more space can be provided when using 3D PC-g as a template for PANI dispersion, which minimized the volumetric changes of PANI through charging and discharging cycles. In 0.5 M Na2SO4 electrolyte, symmetric SC device of 3D PC-g/PANI shows a high energy density of 117 Wh/kg and retained about 94% of initial capacitance. Also, The ASC device offered a high electrochemical performance with energy density of 97.5 Wh/kg and cyclic stability of up to 91% after 10,000 cycles in 0.5 M Na2SO4 as shown in Fig. 8. Table 4 lists some recent studies on graphene/PANI composites.

Fig. 8
figure 8

(figure used by permission of Elsevier) [52]

Ragone plot of 3D N-doped porous carbon-graphene/Polyaniline nanohybrid as a function of current densities in different electrolytes for SSC and ASC

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Table 4 Various supercapacitor electrode materials based on graphene and PANI nanocomposites
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6.5 PANI/Graphene Quantum Dots Electrode Materials

The chemical interaction can introduce graphene quantum dots as a molecule dopant into PANI chains. The graphene quantum dots GQDs did not only offer the double-layer properties to the nanocomposites, but it also improved the charge transfer to the surface of PANI. Malik and coworkers [54] prepared GQDs from graphene oxide flakes, followed by synthesis GQD-PANI by the chemical oxidation method of aniline. The synthesized GQD-PANI composites show a specific capacitance of 1044 F/g at 1 A/g with 80.1% cyclic stability after 3000 cycles and this high specific capacitance is attributed to higher surface areas in nanotube morphology which provide better conductive paths for fast electron transportation. In previous studies our group showed that the Sulfur and nitrogen co-doped GQDs has a positive impact on the electrochemical properties of PANI. PANI/S,N:GQDs nanocomposite exhibited maximum specific capacitance of 2524 F/g at 2 A/g with an excellent cyclic stability of 100% after 1000 cycles [60].

7 PANI/Metal Compounds

Because of their potential for pseudocapacitance over a large range of potential and excellent stability, metal compounds are interesting candidates for supercapacitors. However, they typically have limited electrical conductivity. In recent years, the nanocomposites of PANI with high conductivity and metal compounds such as Metal oxides/hydroxide, metal sulfides, metal chlorides, and metal nitrides have all been employed as supercapacitor electrodes to improve electrochemical performance.

Metal oxides/hydroxides (MOx) electrode materials are characterized as Faradaic pseudocapacitive materials. In general, they offer higher energy density than carbon-based materials and better electrochemical stability than conducting polymers for supercapacitors [66]. The following general conditions must be satisfied before using metal oxide in supercapacitors.

  1. (i)

    High specific surface area.

  2. (ii)

    Electrical conductivity.

  3. (iii)

    Possibility of existing in two or more oxidation states across continuous range without phase transitions; and

Among the metal oxides, RuO2, MnO2, V2O5, cobalt oxide/hydroxide, nickel oxide/hydroxide, etc. have been investigated as electrode materials for supercapacitors applications. Due to its wide working potential window, highly reversible redox processes, and three different oxidation states, RuO2 is the most researched metal oxide. Additionally, RuO2 has a high specific capacitance (900 F/g), metallic conductivity, good thermal stability, and extended cycle life. However, the poor performance at high current densities, toxicity, and high cost limit practical applications of RuO2-based supercapacitors.

MnO2, on the other hand, has a large theoretical specific capacitance (1370 F/g), is inexpensive, and is environmentally friendly, but its poor conductivity restricts its applicability [3, 79] Considering these facts, constructing a MnO2/PANI composite is a suitable way to increase the electrochemical use of MnO2 and PANI in supercapacitors. PANI/MnO2 composites have been synthesized electrochemically via pulse electrodeposition by Liu and coworkers [46]. The prepared composite has rod-like structure and MnO2 particles were uniformly distributed on PANI nanorod. MnO2 and PANI work together synergistically to produce MnO2/PANI composite with higher specific capacitance (810 F/g) than pure PANI (662 F/g) at 0.5 A/g. Also, after 1000 cycles, it maintained 86.3% of its original capacitance. The electrochemical properties of PANI/MnO2 composite were studied extensively in recent years. However, it suffers from poor contact between MnO2 and PANI membrane restricts the complete interface of electrolyte and MnO2 resulting the loss in energy density. A super bridge between the PANI membrane and the MnO2 nanostructure is required. The MnO2/PANI composite with silver nanoparticle decoration has a higher specific capacitance and superior conductivity. Silver nanoparticle not only facilitate the electron transfer but also reduce the internal resistance of metal oxide pseudocapacitive materials and increase the proton diffusion throughout the electrodes [57]. Among metal oxides, double metal oxides have attracted attention for electrochemical energy storage due to their ability to create multiple oxidation states and electrical conductivity in ways that single metal oxides and carbon-based materials cannot [5]. Yu and coworkers [87] showed that the core of NiCo2O4 nanowires has strong electrical conductivity and may be used as a backbone and electron highway for charge storage, overcoming MnO2’s low electrical conductivity. In the core–shell structure, the NiCo2O4 core was also employed to improve the structural instability of PANI. Jabeen and coworkers [30] provided a new method for fabricating a core–shell NiCo2O4/PANI nanorod arrays for high stability PANI based-electrode material for SCs as illustrated in Fig. 9a. Highly porous conductive core NiCo2O4 not only acts as a strain buffer support for PANI layer but also offers rapid electron transport pathways between PANI and current collector, resulting in small electrode polarization and great power capability. The heterostructure achieves a high specific capacitance of 901 F/g at 1 A/g, exceptional cycling stability of ∼91% after 3000 GCD cycles at 10 A/g and good coulombic efficiency shown in Fig. 9b.

Fig. 9
figure 9

Reprinted with permission from [30], copyright (2016) American Chemical Society

a Schematic diagram of the fabrication process for NiCo2O4@PANI nanorod arrays. b Cycle performance of the NiCo2O4@PANI nanorod arrays electrode for 3000 successive charge–discharge cycles at a large current density of 10 A/g and corresponding Coulombic efficiency (inset is the typical charge–discharge curves).

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Transition metal molybdates, such as MnMoO4, CoMoO4, and NiMoO4, with superior pseudocapacitive characteristics have developed as promising electrode materials in recent years [50]. For example, Liu and coworkers [44] reported a facile method for fabricating CoMoO4-NiMoO4. xH2O bundles with a high specific capacitance but poor cycle life (only 75.1% of the initial specific capacitance remained after 1000 GCD cycles). The structural collapse of NiMoO4 during charge and discharge operations might be prevented by PANI with high mechanical stability. As a result of the synergistic effect between them, PANI/NiMoO4 nanocomposite exhibited high capacitance retention of 80.7% after 2000 cycles at 5 A/g, and high specific capacitance of 1214 F/g at 1 A/g, revealing their good electrochemical stability [15].

Tungsten oxide (WO3) has gained popularity as a viable SC electrode material in recent years due to its large specific surface area, electrochemical stability, and environmental friendliness [95]. However, low capacitance of WO3 limit its application in high performance pseudocapacitor. WO3 modified with PANI has attracted a lot of attention for enhancing specific capacitance and cycle stability due to the synergistic effects of each component. Yang and coworkers [83] fabricated the inner/outer coating structural hexagonal WO3/PANI through the hydrothermal-electrodeposition route. The hexagonal WO3 nanowires were grown-up on the titanium Ti surface, and the outer PANI layer decorates the inner WO3, resulting in WO3-PANI hybrid nanostructures as shown in Fig. 10a. The specific capacitance (278 F/g at 1 A/g) and good cycle stability (91.9% after 1500 cycles) of WO3/PANI electrode Fig. 10b should be attributed to its innovative nanostructure design and the interaction between the outer PANI layer with the inner WO3 layer. On the one hand, PANI could dramatically increase the capacitance of hybrid electrodes, which is due to two factors: high specific capacitance and excellent electrical properties. On the other hand, the free spaces within the porous hexagonal-WO3 interior helps the volumetric expansion of PANI during charge and discharge cycling.

Fig. 10
figure 10

(figure used by permission of Elsevier) [83]

a Schematic illustration of h-WO3/PANI hybrid nanostructures. b Cycling performance of the PANI, WO3, and WO3/PANI at a current density of 3 A/g, respectively

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Recently, Metal tungstates such as CoWO4 [7], ZnWO4 [20], MnWO4 [58], FeWO4 [19], and CuWO4 [36] are regarded the most attractive transition-metal oxides due to low toxicity, abundance, rich polymorphism, and stable multifunctional properties. CoWO4/PANI electrode exhibited high specific capacitance of 653 F/g and outstanding cycling stability of 93.3% even after 5000 GCD cycles [59].

Supercapacitor electrode materials can also be made of other metal compounds, such as metal sulfides, metal chlorides, and metal nitrides. They are more stable in acidic electrolytes than metal oxides, but they lack the electrical conductivity and electrochemical performance of metal oxides. Majhi and coworkers [51] synthesized PANI/CoCl2 composites through in situ polymerization process with different doping levels of CoCl2 (10, 15 and 20 wt%). The CV test revealed that the PANI–10% CoCl2 composite had a significantly higher specific capacitance value (918 F/g) than pure PANI (382 F/g). This shows that the electrochemical performance of PANI–10% CoCl2 is significantly influenced by the quantity of CoCl2 present in the composite.

Because of its unique structural features and larger theoretical capacitance (than graphite 2D molybdenum disulfide MoS2 nanosheets have increasingly attracted a lot of attention in the fields of energy storage. MoS2 monolayers are made up of three atom layers (S-Mo-S), and their adaptability can be related to the analogous structure of graphene, which can provide a large surface area. Unfortunately, 2D MoS2 nanosheets are easy to agglomerate due to the strong interlayer van der Waals forces [62], resulting in a decrease in active surface area and poor specific capacitance. The combination of conductive PANI with molybdenum disulfide MoS2 not only prevents 2D MoS2 nanosheet agglomeration, but also improves PANI cycle stability, which is beneficial to their electrochemical abilities [28]. Zhao and coworkers [94] have created a highly conductive metallic MoS2 and PANI monolayers with unique alternating heterostructure, which will help to enhance electron/ion transfer across the electrode material while also providing great structural stability (91% capacitance retention after 2000 cycles). Zhang and coworkers [92] investigate the MoS2/PANI core–shell structure (Fig. 11) as a supercapacitor electrode. This pompon- shaped MoS2/PANI composites with a high specific surface area and a more mesoporous pores presented a specific capacitance of 633 F/g with cycle stability of 86% for 1000 cycles.

Fig. 11
figure 11

(figure used by permission of Elsevier) [92]

Schematic illustration of the formation of MoS2/PANI core/shell microsphere

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Among the metal nitrides, Titanium nitrides (TiN) has an excellent candidate as an electrode material for SCs in highly corrosive electrolytes [12] due to its corrosion resistance, low-cost, thermal stability, mechanical properties and good electrical conductivity [82, 97]. Xia and coworkers [80] prepared a PANI/TiN core–shell nanowire arrays (NWAs) by electrodepositing PANI onto TiN NWAs. The TiN NWAs core was vital in improving the electrode’s rate performance by providing a large surface area and rapid electron transport. The PANI shell on TiN NWAs might improve cycle stability and help to achieve excellent pseudo-capacitance performance. The PANI/TiN core–shell NWAs electrode exhibited a very high specific capacitance of 1064 F/g at 1A/g and kept 95% capacity retention after 200 cycles.

8 Conclusion

This chapter summarized current developments in PANI as a supercapacitor electrode material, including their design, and synthesis process. Pure PANI is unable to fulfil the rising demand due to its poor cycling stability and inefficient capacitance contribution. As a result, PANI must be used in conjunction with other active materials such as carbon materials and metal compounds. Due to the synergistic effect, PANI works as a conductive layer in various PANI based composite structures, and the resulting PANI based composites have showed improved electrochemical performance in supercapacitors.