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Materials and Fabrication Methods for Electrochemical Supercapacitors: Overview

  • Prasad Eknath LokhandeEmail author
  • Umesh S. Chavan
  • Abhishek Pandey
Review article
  • 11 Downloads

Abstract

The rapid economic development and immense growth in the portable electronic market create tremendous demand for clean energy sources and energy storage and conversion technologies. To meet this demand, supercapacitors have emerged as a promising technology to store renewable energy resources. Based on this, this review will provide a detailed and current overview of the various materials explored as potential electrodes and electrolytes in the development of efficient supercapacitors along with corresponding synthesis routes and electrochemical properties. In addition, this review will provide introductions into the various types of supercapacitors as well as fundamental parameters that affect supercapacitor performance. Finally, this review will conclude with presentations on the role of electrolytes in supercapacitors and corresponding materials along with challenges and perspectives to guide future development.

Graphic Abstract

Keywords

Supercapacitor Metal oxide EDLC Electrolyte 

1 Introduction

Currently, fossil fuels are the primary energy source for continued population growth. However, augmented consumption rates have increased demand for efficient energy sources with advanced storage technologies [1, 2, 3, 4], and in recent years, alternative energy sources such as wind, tide, and solar have been extensively exploited to meet these increasing energy demands. In this context, technologies such as batteries and supercapacitors have been recognized as prominent methodologies with supercapacitors attracting increasing attention due to high power densities, excellent cycle lifespans (> 1,000,000 cycles), high rate capabilities, rapid charge/discharge rates and low maintenance costs [5, 6, 7, 8, 9, 10, 11]. And although different names (supercapacitor by Nippon Electric Company, ultracapacitor by Pinnacle Research Institute) have been coined for supercapacitors, primary functions remain the same in which energy is stored between a solid electrode and an electrolyte. General Electric was the first to demonstrate and patent supercapacitor technologies in 1957, and since then, it has remained the focus of academia and industry [9]. Supercapacitors contain electrodes with high surface areas and a thin dielectric separator that can provide significantly higher capacitances and greater energy densities than that of conventional capacitors [12, 13]. In addition, supercapacitors possess higher power densities than batteries and fuel cells but the amount of energy stored is less [14, 15]. To address this, researcher have focused on the enhancement of energy densities in supercapacitors to match rechargeable batteries [16, 17] so as to bridge the power and energy gap between batteries and conventional capacitors as illustrated in the Ragone plot in which energy and power densities are represented on the horizontal and vertical axes for various storage technologies (Fig. 1) [18].
Fig. 1

Ragone plot for various electrical energy storage devices (specific power against specific energy)

Overall, the shorter charging and discharging times and the extended life cycles as compared with conventional batteries enable supercapacitors to act as excellent recovery systems that can sustain millions of cycles of the charge storage mechanism in which charge is stored physically at the surface of the electrode. In addition, supercapacitors are designed to achieve higher power densities but suffer from limited energy densities and low potential windows [18]. Table 1 provides a comparison between battery technologies and supercapacitor technologies [19] in which batteries are electrochemical cells (one or more) connected together to provide electric energy, whereas supercapacitors are devices that store electric energy in electric double layers that form in between electrolytes and electrodes. Here, electrodes of supercapacitors do not undergo volume change after faradaic reversible reactions, unlike batteries that experience swelling. Supercapacitors also do not experience the rate limitations faced by battery technologies due to electrochemical kinematics through polarization resistances [18, 20]. And because of properties such as high power densities, excellent cycle lifespans, and stability, supercapacitors can be applied in a variety of applications including hybrid electric vehicles, power systems, memory storage systems, energy backup systems, and portable electronics devices [21]. Based on this, this review will provide extensive details on the various aspects of supercapacitors, including the fundamentals of supercapacitors, corresponding working mechanisms, factors affecting performance, electrode materials and properties, as well as fabrication methodologies. In addition, this review will provide discussions of various electrolytes and their working mechanisms [22].
Table 1

Performance comparison between supercapacitors and batteries

Parameters

Electrochemical capacitor

Battery

Energy density

Low

High

Power density

High (1–10 W g−1)

Low (150 W kg−1)

Charge–discharge cycles

105–106

500–104

Self-discharge times at room temperature

Days-weeks

Months

Lifetime at room temperature (years)

5–10

3–5

Cell voltage (V)

1.2–3.8

2.5–4.2

Mechanisma

Open image in new window

Open image in new window

aCopyright (2004) American Chemical Society

2 Fundamentals of Supercapacitors

Supercapacitors are also known as electrochemical capacitors or ultracapacitors and possess high surface area electrodes and thin dielectric separators to achieve higher capacitances than conventional capacitors. Similar to conventional capacitors, supercapacitors possess two electrodes separated by an insulating dielectric material.

2.1 Types of Supercapacitors

Supercapacitors can be categorized into three types based on the charge storage mechanism, including electrical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors [23, 24]. Here, EDLCs utilize double-layer electrodes to store charge electrostatically [25],pseudocapacitors store charge faradaically through redox reactions by transferring charge between electrodes and electrolytes [26], and hybrid supercapacitors store charge through a combination of faradaic and non-faradaic materials [27] (Fig. 2).
Fig. 2

Classification of electrochemical supercapacitors [23]

2.1.1 Electric Double-Layer Capacitors

EDLCs store charge non-faradaically, involve no chemical oxidation–reduction reactions (redox), and do not transfer charge between electrodes and electrolytes [28, 29]. Here, pure electrostatic charge accumulates at the electrode/electrolyte interface as voltage is applied and is stored through ion diffusion from the electrolyte into the pores of the electrode of the opposite charge across the separator by following the natural attraction of opposite charges [21]. And because of the high reversibility of the charge storage, EDLCs can operate for many charge–discharge cycles without experiencing volumetric or morphological change [30] in which a double layer is formed due to the electrode material, allowing excess or deficit charge to accumulate on the electrode surface along with oppositely charged ions from the electrolyte to balance electroneutrality (Fig. 3) [31]. In EDLCs, charging involves electrons moving from the negative electrode to the positive electrode through an external load in which cations in the electrolyte move toward the negative electrode and anions toward the positive electrode, with discharging involving the opposite process [32, 33]. And throughout the overall process, charge transfer does not occur across the electrode/electrolyte interface, the concentration of the electrolyte remains constant and therefore energy is stored at the double-layer interface. The specific capacitance for EDLCs can be measured by using following equation [34]:
$$C = \frac{{\varepsilon_{\text{r}} \varepsilon_{\text{o}} }}{d}A$$
(1)
in which εr is the relative permittivity of the medium in the EDL, εo is the permittivity of the vacuum, A is the specific area of the electrode, and d is the thickness of the EDL. And because of the lack of physical charge transfer, EDLCs possess higher cycle lifespans [35]. Furthermore, carbon-based materials ranging from activated carbon to graphene have been widely used as electrode materials in which EDLC performances can be enhanced by maintaining three factors, including the specific surface area, proper pore sizes for the diffusion of electrolytes, and an appropriate electrolyte solution.
Fig. 3

Schematic of supercapacitors: a EDLC; b Pseudocapacitor; c Hybrid capacitor. Reprinted with permission from Ref. [31]. Copyright (2012) American Society of Civil Engineers

2.1.2 Pseudocapacitors

The mechanisms for pseudocapacitors are different from that of EDLCs and are faradaic in nature, involving faster reversible redox reactions between the electrode active materials and electrolytes at the electrode surface and the near-surface [36]. Here, redox reactions occur to generate charge, which is transferred across the double layer (Fig. 3b). This faradaic process can be divided into three types, including reversible adsorption, redox reaction occurring in metal oxides, and reversible doping/de-doping electrochemical processes in conductive polymer-based electrodes [37]. Furthermore, the pseudocapacitance of pseudocapacitors occurs due to thermodynamic reasons as well as changes in potential and charge acceptance [38], and the specific capacitance and energy density of pseudocapacitors are higher than that of EDLCs because faradaic reactions can occur not only at the surface of electrodes, but also near the surface of electrodes [32, 39]. And in general, the theoretical capacitance of pseudocapacitors can be calculated with the following equation:
$$C = {\raise0.7ex\hbox{${n \times F}$} \!\mathord{\left/ {\vphantom {{n \times F} {M \times V}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${M \times V}$}}$$
(2)
in which n is the number of electrons transferred in the faradaic reaction, F is the Faraday constant, M is the molar mass of the active material and V is the voltage window. Moreover, pseudocapacitors require high-energy electrode materials based on conducting polymers and metal oxides/hydroxides such as polyaniline (PANI) [40], polypyrrole (Ppy) [41], RuO2 [42], MnO2 [43], Ni(OH)2 [44], Co(OH)2 [45], Co3O4 [28], and NiO [25]. And in terms of challenges, pseudocapacitors suffer from relatively low power densities as compared with EDLCs because faradaic processes are slower than non-faradaic processes. In addition, the electrodes of pseudocapacitors are more prone to volume change during charge/discharge, which can lower cycling lifespans and reduce mechanical stability [46].

2.1.3 Hybrid Supercapacitor

Pseudocapacitors possess high specific capacitances but suffer from poor power density and cyclic stability. Alternatively, EDLCs exhibit high power density and excellent cyclic stability but possess low specific capacitance [47]. Therefore, to obtain high power densities and mitigate the disadvantageous low cyclability in both EDLCs and pseudocapacitors, both faradaic and non-faradaic processes can be utilized together to store charge [48], in which correct electrode combinations can increase cell voltage and capacitance (Table 2 provides the merits and demerits of EDLCs and pseudocapacitors). And as a result, these hybrid supercapacitors can achieve higher energy and power densities and maintain longer cycle lifespans and stability [20]. Overall, hybrid supercapacitors can be classified based on their configuration, including composite, asymmetric and battery types [32]. Here, composite electrodes are fabricated by combining carbon-based materials with pseudocapacitive materials [e.g., Ni(OH)2 grown on graphene sheets] [49]. As for asymmetric supercapacitors, redox reactions occur at one electrode and non-faradaic reactions occur at the other, and battery-type supercapacitors combine supercapacitor electrodes with battery electrodes [32].
Table 2

Comparison between EDLCs, pseudocapacitors and hybrid capacitors [48]

Parameters

EDLC

Pseudocapacitor

Hybrid capacitor

Storage mechanism

Non-faradic/electrostatic, electrical charge stored at the metal/electrolyte interface

Faradic, reversible redox reaction

Both faradic and non-faradic

Specific capacitance

Lower

Higher

Higher

Energy density

Low

High

High

Cycle Life/stability

High

Low

High

Material

Carbon-based materials, e.g, activated carbon, carbon nanotubes

Metal oxides, conducting polymers, e.g., NiO, MgO, PANI

Metal oxide/carbon-based materials, conducting polymer/carbon-based materials, e.g., Ni(OH)2/rGO, PANI/rGO

3 Performance of Supercapacitors

The performance of optimal supercapacitors can be evaluated based on several factors, including: (1) high specific capacitances, (2) high power densities and maintained high energy densities (> 10 Wh kg−1), (3) higher voltage windows, (4) excellent cyclic lifespans, (5) higher mechanical stability, (6) faster charge/discharge rates, (7) lower self-discharge rates, (8) safer operations, and (9) lower basic and maintenance costs [50]. Supercapacitors consist of a positive electrode, a negative electrode, and a separator, and the properties of each component can greatly affect performance. Here, major performance affecting properties include specific surface area, pore size distribution, electroactive material morphology, and electrical conductivity as well as the intrinsic properties of the electrolyte and the separator [51] in which specific surface area plays a vital role in electrode materials or active masses and can provide larger active sites to electrolytes and enhance capacitance [27]. In addition, the porous structure of active masses can increase surface areas, provide easy access to electrolytes, and reduce electrochemical polarization. Desirable pore sizes are another key factor to enhance capacitance in which macropores (> 50 nm) acting as ion buffering reservoirs do not contribute much to increased surface areas and micropores (> 2 nm) can restrict ion movement. Therefore, maximum specific capacitances can be obtained with pore sizes that are equivalent to or near the size of ions (mesoporous structure) [52]. The specific capacitance (SC) of electrode materials can be calculated using two methods [53], including cyclic voltammetry (CV) curves and galvanostatic charge/discharge (GCD) curves in which the equation for SC by using CV curves is as follows:
$$C_{\text{s}} = 1/\left( {mv\left( {V_{\text{f}} - V_{\text{i}} } \right)} \right) \int_{{V_{\text{i}} }}^{{V_{\text{f}} }} {I\left( V \right){\text{d}}V}$$
(3)
And the equation for SC by using GDC curves is as follows:
$$C_{\text{s}} = \frac{{I\Delta t}}{{m\Delta V}}$$
(4)
in which Cs is the specific capacitance (F g−1), m is the active mass of the electrode material (g), v is the potential scan rate (V s−1), ∆V = (Vf − Vi) is the voltage window (V), I is the instantaneous current (A) and ∆t is the discharge time (s).
Supercapacitors also suffer from lower energy densities as compared with batteries and the enhancement of energy densities up to battery levels is challenging [54]. Here, two important parameters to evaluate supercapacitor performances are energy density and power density and can be calculated by using the following equations [9]:
$$E = {\raise0.7ex\hbox{${CV^{2} }$} \!\mathord{\left/ {\vphantom {{CV^{2} } 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}} = {\raise0.7ex\hbox{${QV}$} \!\mathord{\left/ {\vphantom {{QV} 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}$$
(5)
$$P = {\raise0.7ex\hbox{${V^{2} }$} \!\mathord{\left/ {\vphantom {{V^{2} } {4R}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${4R}$}}$$
(6)
in which C is the total SC (F), V is the voltage of the cell (V), m is the total active mass of the supercapacitor and E is the equivalent series resistance (ESR in Ohms). And to achieve optimal supercapacitor cell performances in terms of energy density, SC and voltage need to increase and ESR needs to be minimized. And based on Eqs. (5) and (6), the energy density and power density of supercapacitors are proportional to the square of the voltage, suggesting that improvements in cell voltage can drastically enhance the electrochemical performance of supercapacitors. Here, cell voltages of supercapacitors are mainly influenced by intrinsic properties such as specific surface area, pore size, electrical conductivity, and additional redox capacitance as well as the electrolyte used (e.g., use of organic or ionic electrolytes instead of aqueous electrolytes);therefore, the optimization of these properties may enhance voltage [55]. And overall, higher specific capacitance values can be achieved by synthesizing nanostructured materials with higher surface areas, optimized pore sizes, and maintained electric conductivities, allowing for large active sites to be available for electrolyte ions and due to optimized porosity, their participation in faradaic reactions at enhanced rates [56]. As for ESR, this is generated from a combination of material intrinsic resistance, contact resistance between electroactive materials and current collectors, diffusion resistance of ions in electrode materials and ionic resistance of electrolytes [37]. Here, researchers report that the direct deposition of electroactive materials on conductive substrates such as Ni foam or Cu foam can minimize ESR values and lead to higher power densities [27].

4 Electrode Materials

Electrode materials in supercapacitors play a vital role in capacitance performance and can be categorized into three subsections, including carbon-based materials, transition metal oxides, and conducting polymers. Here, the proper selection and design of electrode materials are vital to enhanced supercapacitor performances [31].

4.1 Carbon-Based Materials

Carbon-based material possesses higher surface areas, more sophisticated synthesis techniques and lower cost as compared with other materials and can completely satisfy supercapacitor electrode requirements. These carbon-based materials, including activated carbon, carbon nanotubes, and graphene, possess desirable chemical and physical properties such as high surface areas, high porosity, low costs and varied forms (e.g., powders, nanotubes, aerogels).

4.1.1 Activated Carbon Materials (ACs)

Activated carbon materials possess large surface areas, complex porous structures, and good electrical conductivities and are relatively inexpensive, making them attractive electrode materials for supercapacitors [51, 57]. These materials are generally synthesized in two steps and involve the carbonization and activation of carbon-rich organic precursors such as coconut shells, wood, fossil fuels, coke, coal, or synthetic polymers [50]. In the carbonization process, carbon-rich precursors are treated at high temperatures (700 to 1200 °C) in inert atmosphere to form amorphous carbon and in the activation process, lower temperatures are applied (400 to 700 °C) in the presence of activating agents such as alkalis, carbonates, chlorides, or acids (e.g., KOH, NaOH, ZnCl), which can provide porous networks in the bulk of the carbon particles. And overall, these two processes can lead to tremendous increases in specific surface area and porosity in which the physiochemical property and specific surface area of ACs depend on precursor materials and activation methods. Here, a maximum achievable surface area of 3000 m2 g−1 and a usable surface area in the range of 1000–2000 m2 g−1 can be obtained [58] along with the formation of porous structures in various sizes ranging from nanoporous (< 2 nm) to mesoporous (2 to 50 nm) to microporous (> 50 nm) based on the different activation processes used [50]. Despite these enhancements, however, researchers also report that increased specific surface areas can also lead to possible risks of electrolyte decomposition and dangling bond positions [59]. In addition to specific surface area and pore shapes, size and distribution, electrical conductivity and surface functionality are also important factors influencing the performance of activated carbon [60]. Here, the specific capacitance of AC as an electrode material is higher in aqueous electrolytes than in organic electrolytes due to the larger size of electrolyte ions in organic solutions [50]. And to address the major disadvantage of low density in AC for practical applications, Moreno-Fernandez et al. [57] reported that the synthesis of AC fiber monolith can result in three times higher densities in which a maximum specific capacitance of 200 F g−1 can be achieved. In another study, Li et al. [61] reported that AC derived from lignite at various activation temperatures as an electrode material for SCs can exhibit a specific capacitance of 207.5 F g−1 and cyclic stability up to 3000 cycles at a current density of 0.5 A g−1 for an activation temperature of 650 °C. Despite these performances, however, due to poor mechanical properties and the need for metal current collectors, activated carbon-based supercapacitors are generally only available in button cell or spiral-wound configurations [62].

4.1.2 Carbon Nanotubes (CNT)

CNTs have attracted significant attention as electrode materials for supercapacitors due to excellent characteristics, unique internal structures, fully accessible external high surface areas, low mass densities, excellent electrical conductivities, and exceptional mechanical, thermal, and chemical stability [63, 64, 65, 67, 68]. In addition, the high mechanical resilience and open tubular structure of CNTs can provide a port to active materials [66, 50], and the mesoporous structure of CNT networks can allow for faster and easier diffusion of electrolytes, which can decease ESR and maximize usable power [18, 69, 70, 71, 72]. Carbon nanotubes are generally synthesized through the decomposition of certain hydrocarbons and by manipulating certain parameters in which crystalline orders can change to obtain different nanostructures such as single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs). Here, the specific capacitance electrode is influenced by the purity and morphology of the obtained CNT material [51]. For example, Niu et al. [71] prepared MWCNT sheet electrodes from catalytically grown carbon nanotubes (Fig. 4)and reported that the synthesized CNT possessed a diameter of 80 Ǻ and a specific surface area of 430 m2 g−1, which is higher than that of CNT materials obtained from different methods in which a maximum specific capacitance of 104 F g−1 was obtained for a single-cell test device by using a H2SO4 electrolyte solution. Researchers have also reported that CNT materials directly deposited onto conductive substrates can decrease contact resistance between active materials and current collectors. Based on this, Shi et al. [72] directly grew CNT onto Ni foam using chemical vapor deposition (CVD) and reported a specific capacitance was 127% higher than the CNT material obtained using the transfer method. Despite these excellent results, however, low specific areas hinder the application of CNT in high-performance supercapacitors.
Fig. 4

a TEM micrograph and b SEM image of highly dispersed catalytically grown carbon nanotubes. Reprinted with permission from Ref. [71]. Copyright (1997) AIP Publishing

4.1.3 Graphene

Graphene, a one-atom-thick planar sheet of sp2 bonded carbon atoms that are arranged in a closely packed honeycomb crystal lattice [73, 74, 75, 76, 77], exhibited excellent electrical, mechanical, and morphological properties that are suitable for high-performance SCs such as high carrier mobility and extensive surface area [76]. In addition, the layered structure of graphene can reduce electrode thickness and provide good flexibility, thermal and chemical stability, and large potential windows [77]. As an example, Zhang et al. [78] synthesized flexible carbon thin films and reported that the resulting graphene material provided a high specific surface area of 2400 m2 g−1 and an electrical conductivity of 5880 S m−1 in which the energy density and specific capacitance of the carbon thin film reached 26 Wh kg−1 and 120 F g−1, respectively. Furthermore, Wang et al. [79] reported that their graphene material as a supercapacitor electrode can provide a specific capacitance of 205 F g−1 with a power density of 10 W g−1 and an energy density of 28.5 Wh kg−1. Li et al. [80] also synthesized graphene oxide nanostructures with various morphology including single layers (SL), few layers (FL), and 3D porous networks (3DPN) (Fig. 5) using a one-step hydrothermal method in which the morphology was controlled through the addition of varying amounts of Ni2+. Here, the researchers reported that among the various morphology obtained, the 3D network showed an optimal specific capacitance of 352 F g−1 at a scan rate of 5 mV s−1. Moreover, to address the issues of water-based electrolytes being unable to operate at higher voltage windows and the synthesis of pore accessible and surface compatible graphene for ionic liquid electrolytes, Liu et al. [83] synthesized a mesoporous graphene structure that was accessible to ionic liquids and can operate up to 4 V (Fig. 6) and reported that the synthesized structure exhibited an energy density of 90 Wh kg−1 at a current density of 1 A g−1 at room temperature and 136 Wh kg−1 at 80 °C.
Fig. 5

a SEM image of 3DPN-GO and b SEM images of GO hydrogels obtained with r being 0.025, SL-GO. c 0.150, FL-GO. Reprinted with permission from Ref. [80]. Copyright (2014) Springer Nature

Fig. 6

a Cyclic voltammograms for graphene electrodes at different scan rates using theEMIMBF4 ionic liquid electrolyte. b Galvanostatic charge–discharge curve of a curved graphene electrode (6.6 mg each) at a constant current density of 1 A g−1 using the EMIMBF4 ionic liquid electrolyte. Reprinted with permission from Ref. [259]. Copyright (2010) American Chemical Society

4.2 Metal Oxides/Hydroxides

Carbon-based electrode materials suffer from low specific capacitances; therefore, researchers are shifting their focus to alternative materials. And because the pseudocapacitance of pseudocapacitors is 3 to 10 times greater than that of EDLCs due to charge being stored in the double layer and fast and reversible redox reactions, pseudocapacitive materials are the most promising candidates in modern supercapacitors. Here, conducting polymers and transition metal oxides are the most common materials for pseudocapacitors and have attracted major attention from researchers due to high specific capacitances that originate from fast reversible redox reactions and long operation lifespans. Currently, several metal oxides/hydroxides are being investigated, including RuO2, IrO2, MnO2, NiO, Co3O4, SnO2, V2O5, CuO, Ni(OH)2, and Co(OH)2, in which the most studied materials are RuO2, MnO2, NiO and Ni(OH)2 due to their higher theoretical specific capacitances.

4.2.1 Ruthenium Oxide (RuO2)

Ruthenium oxide is one of the most explored TMOs for supercapacitor electrodes because of its high theoretical capacitance (1200–2200 F g−1), high electric conductivity, reversible redox reaction, wide potential window (1.2 V), long cycle lifespan, good rate capability, and excellent stability [81]. In addition, ruthenium oxide possesses three oxidation states (Ru4+, Ru3+, and Ru2+) [82, 84, 85] and the fast faradaic redox reaction of RuO2 can be represented as follows [83]:
$${\text{RuO}}_{x} \left( {\text{OH}} \right)_{y} + \delta {\text{H}}^{ + } + \delta {\text{e}}^{ - } \to {\text{RuO}}_{x - \delta } \left( {\text{OH}} \right)_{y + \delta }\quad \left( {0 < \delta > 2} \right)$$
(7)

The presence of structural water can enhance proton conductivity but can also reduce electric conductivity. And in the case of RuO2, supercapacitive performance can be enhanced by optimizing the combined water (x = 0.5) content to maintain a balance in proton conductivity and electric conductivity [86, 87, 88]. Furthermore, crystallinity, annealing temperature, and particle size can also affect the pseudocapacitive performance of RuO2 [89, 90, 91] in which the high crystallinity of RuO2 can cause low specific capacitances due to the compact nature of RuO2, which can restrict the insertion/extraction of ions/electrons, leading to increased electrochemical impedance [88]. Because of this, many researchers have shifted their focus to amorphous RuO2, which can deliver higher specific capacitances due to redox reactions on both the surface and back of the material [88]. As for the charge storage mechanism of RuO2, four stages are involved, including the hopping of electrons within RuOx·nH2O, the hopping of electrons between particles, the hopping of electrons between electrodes and current collectors, and proton diffusion without RuOx·nH2O particles [47].

As for the synthesis of RuO2 for supercapacitor electrode materials, various synthesis methods have been used. For example, Zheng et al. [92] prepared hydrous ruthenium oxide using a sol–gel method at an annealing temperature of 150 °C and reported specific capacitance of 720 F g−1 for the prepared material. In another example, Ramani et al. [89] prepared RuO2–carbon composites using electroless deposition and studied the effects of electrochemical oxidation and temperature treatment on the performance of the prepared material and suggested that increased oxidation temperatures can reduce electrochemical performance. Kim et al. [87] also fabricated an electrode by varying the RuO2 loading on carbon using a colloidal method and reported that a maximum specific capacitance of 407 F g−1 can be achieved at a 40% RuO2 loading. In addition, specific surface area has a major influence on the electrochemical performance of active materials and the preparation of mesoporous materials is a viable strategy to enhance surface area. For example, Kuratani et al. [90] studied the influence of mesoporous structure on supercapacitive performance and found that the introduction of mesoporous structures can improve specific capacitances. Furthermore, Subramanian et al. [24] synthesized anhydrous mesoporous RuO2 using a non-ionic surfactant templating method and reported a specific capacitance of 58 F g−1, demonstrating that hydrous RuO2 performs better than anhydrous RuO2. The direct deposition of RuO2 on structurally tailored conductive surfaces is another viable strategy to enhance specific capacitance in which as in the above case, nickel-flashed stainless steel can be used as the substrate for deposition. Here, Arunachalam et al. [94] fabricated high-performance RuO2 electrodes using simple pulse electrodeposition with two precursors (ruthenium nitrosyl sulfate and ruthenium trichloride) and reported that the active material deposited using ruthenium nitrosyl sulfate achieved an optimal specific capacitance of 1724 F g−1 at a current density of 5 A g−1. These researchers subsequently synthesized thin-film hydrous ruthenium oxide on nickel-flashed AISI317 stainless steel and reported a nanospherical material (Fig. 7a) that provided a maximum specific capacitance of 520 F g−1 at a current density of 1 mV s−1 [85].
Fig. 7

a Uniform deposition of RuO2 on stainless steel. b Specific capacitance as a function of scan rate and CV curve at various scan rates in small window. Reprinted with permission from Ref. [85]. Copyright (2015) Elsevier

The CV curve for RuO2 is nearly rectangular (Fig. 7b) and is in agreement with the capacitive behavior. However, the high cost of RuO2 limits practical application and efforts have been made to reduce the loading of RuO2 through doping with other materials. For example, Li et al. [95] synthesized TiO2–SnO2-doped RuO2 composites using ball milling and precipitation and reported that the 35 wt% TiO2–SnO2-doped RuO2 can deliver a maximum specific capacitance of 571 F g−1. In addition, Mahajernia et al. [91] used core–shell TiO2 nanotubes as a highly conductive scaffold for the decoration of RuO2 nanoparticles to obtain a unique and well-dispersed structure and achieved a specific capacitance of 0.048 mg cm−2 for the prepared material with 1297 F g−1 being delivered. Although nanomaterials can show high electrochemical performances at low mass loading levels, this performance is drastically reduced in practical applications in which high mass loading levels are required due to reduced accessible areas as a result from the densely packed materials. Therefore, electrode material properties such as surface area, pore structure, packing density and electrode thickness must be carefully considered from a practical application point of view to enhance volumetric capacitances [92]. In addition, the practical application of RuO2 in supercapacitors is restricted due to serious nanoparticle agglomeration, intrinsically low electron–proton transport and weak conductivity between nanoparticles, all of which can lead to low specific capacitances [93]. Furthermore, RuO2 is expensive and rare, further hindering practical application. Because of these factors, attention needs to focus on the development of hybrid electrodes that combine RuO2 with carbon-based materials with high specific surface areas [94]. For example, Kong et al. [95] synthesized RuO2/GC nanocomposites through a two-step electrochemical route and reported high capacitances up to 480.3 F g−1 and 89.4% capacitance retention after 10,000 cycles. Co et al. [96] also prepared RuO2/graphene thin films using cathodic electrodeposition in which graphene insertion layers enhanced structural and electrochemical properties and the prepared nanocomposite exhibited a specific capacitance of 1561 F g−1 at a scan rate of 5 mV s−1. Moreover, Zhang et al. [97] fabricated 3D hetero-RuO2/CAC and reported that the material possessed high conductivity, efficient electron transport and abundant active sites, resulting in high specific capacitances (510 F g−1) and good stability (87.05% after 3000 cycles). Table 3 summarizes recent results reported in the literature for RuO2.
Table 3

RuO2-based electrodes used in supercapacitors and corresponding specific capacitances

Material

Method

Electrolyte

Specific capacitance

References

RuO2-carbon

Electroless deposition

H2SO4

190 F g−1

[89]

RuO2

Colloidal method

H2SO4

863 F g−1

[87]

RuO2

Sol–gel

H2SO4

720 F g−1

[88]

RuO2/SnO2 xerogel

Incipient wetness method

KOH

710 F g−1

[98]

AC/RuO2

Sol–gel

H2SO4

1340 F g−1

[99]

RuO2

Non-ionic surfactant templating method

H2SO4

58 F g−1

[24]

RuO2//TiO2

PAV–H3PO4–H2O

1263 F g−1

[100]

RuO2

Anodic deposition

H2SO4

740 F g−1

[101]

RuO2

Electrostatic deposition

H2SO4

650 F g−1

[102]

RuO2/AC

Sol–gel

KOH

440 F g−1

[103]

RunMn1-nOx

Co precipitation method

Na2SO4

720 F g−1

[104]

RuO2-CNT

Metal vapor CVD

KOH

5.21 F g−1

[105]

RuO2/AC

Sol–gel

Non-aqueous electrolyte containing EMIBF4

6 μF cm−2

[106]

RuO2//RuO2

Hydrothermal

Na2SO4

52.66 F g−1

[107]

RuO2 on AISI 317 SS

Electrodeposition

non-aqueous NaOH

520 F g−1

[85]

RuO2/Graphene

Disassembly-reassembly strategy

H2SO4

1485 F cm−2

[92]

RuO2/GC

Electrophoretic electrodeposition

H2SO4

480 F g−1

[95]

RuO2

Ultrasonic spray pyrolysis

H2SO4

2192 F g−1

[108]

RuO2/Graphene

Cathodic electroplating

H2SO4

1561 F g−1

[96]

RuO2/CAC

Hydrothermal

KOH

510 F g−1

[97]

RuO2/TiO2-SnO2

Wet ball-milling precipitation

H2SO4

571 F g−1

[109]

RuO2/TMTA

Anodization–precipitation

Na2SO4

1297 F g−1

[91]

RuO2

BeSO4 + Al2(SO4)3

397 F g−1

[23]

RuO2

Pulse electrodeposition

H2SO4

1724 F g−1

[110]

4.2.2 Manganese Oxide (MnO2)

The high cost and rarity of RuO2-based materials hinder application in supercapacitors; therefore, alternative materials are needed to replace RuO2-based materials as electrode materials [111]. Here, MnO2 has been reported to be a promising alternative due to its exceptional properties such as high natural abundance, a wide electrochemical potential window, high theoretical specific capacitance (1370 F g−1) as well as its low cost, low toxicity, and low environmental impact [43, 45, 114]. In addition, MnO2 possesses various crystal structures (α, β, δ, ɤ, λ) that can influence supercapacitive values through the size of tunnels and the control of cation intercalation in which α and δ crystal structures have been reported to exhibit the highest specific capacitances among other crystal structures [113]. Furthermore, the pseudocapacitance energy storage mechanism of MnO2 can be attributed to the reversible redox reaction and its multi-oxidation states in which the charge storage mechanism can be described with the following equation:
$${\text{MnO}}_{2} + {\text{C}}^{ + } + {\text{e}}^{ - } \to {\text{MnOOC}}$$
(8)
Here C represents protons and alkali metal cations (K+, Na+, H+, and Li+) in the electrolyte and two mechanisms are responsible for charge storage including the insertion/reinsertion of cations into the bulk of the electrode and the second adsorption/desorption of electrolyte cations onto the electrode surface in which in both mechanisms, the reversible transition between III and IV oxidation states occurs [114]. As for limitations, MnO2 suffers from low conductivity and slow proton and cation diffusion, leading to bulk redox reactions near the subsurface [43]. Furthermore, researchers report that chemical and physical factors have major influences on the electrochemical performance of MnO2, such as cyclic stability being mainly dictated by the morphology of the prepared material and the specific capacitance being dependent on the chemical hydration state of MnO2 [115]. And in terms of electrochemical applications, the current focus is on amorphous and nanocrystalline MnO2 compounds due to the porous nature that allows for easy accessibility to ion and cation diffusion [116]. For example, Sun et al. [117] investigated the proportional relationship between specific capacitance and MnO2 as well as the percentage of effective Mn centers acting as active sites in the overall energy storage process. Here, the researchers reported that increases in the percentage of effective Mn centers through reduction in crystal size can allow for higher specific capacitances. The morphology of prepared MnO2 is also closely related to electrochemical performance in which morphological surface to volume ratios can affect specific capacitance. For example, Godbane et al. [118] studied microstructural effects on the charge storage properties of MnO2 in which the researchers prepared a series of MnO2 allotropic phases (1D channels, 2D layers, and 3D interconnected tunnels). Here, electrochemical results demonstrated an optimal specific capacitance for the 3D interconnected tunnels (241 F g−1) and a lowest specific capacitance for the 1D channels (28 F g−1). And contrary to conventional assumptions, the results in this study demonstrated that the electrochemical performance of MnO2-based supercapacitors is not dependent on specific surface area and is little affected by electric conductivity. In addition, nanofiber MnO2 films deposited onto stainless steel by using potentiodynamic techniques also reportedly delivered a maximum capacitance of 392 F g−1 [119]. Zhu et al. [114] also synthesized 1D hierarchical tubular MnO2 nanostructures (Fig. 8) using a hydrothermal method and annealing in air in which various microstructures were obtained based on reaction times (180 min, 45 min, and 10 min) (Fig. 9). Here, the researchers reported that the tubular MnO2 nanostructure exhibited optimal electrochemical performances among the different structures, providing an energy density of 21.1 Wh kg−1 and a power density of 13.33 W g−1. Despite these high theoretical specific capacitances, however, MnO2-based supercapacitors suffer from insufficient experimental specific capacitance values and low specific capacitances at high charge/discharge rates as well as poor cycling stability due to MnO2 dissolution, wide band gaps, low ionic diffusion constants, and low conductivity [120, 121]. To resolve these issues and achieve higher supercapacitive properties, researchers have proposed the use of synergetic effects arising from carbon-based materials and MnO2. Based on this, Lu et al. [122] synthesized MnO2/nitrogen-doped ultra-microporous carbon nanospheres and reported that the composite with 57% MnO2 doping content delivered a specific capacitance of 401 F g−1 at a current density of 1 A g−1 and excellent cyclic stability. Dong et al. [3] further prepared flower-like MnO2 nanocomposites embedded into nitrogen-doped graphene using a hydrothermal method and reported an average specific capacitance of 220 F g−1 at a current density of 0.5 A g−1 with 98.3% of the original specific capacitance retained after 3000 cycles (Tables 4, 5, 6, 7).
Fig. 8

FESEM and TEM images of MnO2 tube nanostructures obtained under different reaction times ac 10 min, df 45 min. Reprinted with permission from Ref. [114]. Copyright (2012) American Chemical Society

Fig. 9

a Representative cyclic voltammetry (CV) curves of MnO2-10, MnO2-45, MnO2-180, porous MnO2 nanobelts and sponge-like MnO2 nanowires at a scan rate of 5 mV s−1. b Comparison of the specific capacitances of MnO2-10, MnO2-45, MnO2-180, porous MnO2 nanobelts and sponge-like MnO2 nanowires at different scan rates. Reprinted with permission from Ref. [114]. Copyright (2012) American Chemical Society

Table 4

The electrochemical performances of MnO2 and corresponding composite-based electrodes for supercapacitors

Material

Method

Morphology

Electrolyte

Specific capacitance

Retention

References

MnO2

Electrodeposition

Na2SO4

698 F g−1

90% (1500)

[123]

α-MnO2

Coprecipitation

Platelets

Na2SO4

166 F g−1

100% (1000)

[112]

α-MnO2

Anodic deposition

Na2SO4

320 F g−1

[124]

α-MnO2

Hydrothermal

Plate-like + nanorods

Na2SO4

168 F g−1

[116]

MnO2

Electrodeposition

3D network

Na2SO4

353 F g−1

[120]

α-MnO2

Microemulsion

Spherical particles

Na2SO4

297 F g−1

70% (500)

[113]

MnO2

Hydrothermal (180 min)

Tubular

Na2SO4

315 F g−1

99.99% (3000)

[114]

MnO2

Potentiodynamic

Nanofiber

Na2SO4

392 F g−1

[119]

MnO2

Electrochemical conversion

Nanoflower

Na2SO4

309 F g−1

93% (1650)

[125]

MnO2-NiO

Hydrothermal

Nanoflakes

LiOH

0.35 F cm−2

96.4% (1500)

[126]

Ni(OH)2/MnO2

Core–shell

Na2SO4

487 F g−1

97.1% (3000)

[127]

ZnO/MnO2

Wet chemical

Core–shell

Na2SO4

1260.9 F g−1

87.5% (10,000)

[128]

α-Fe2O3/MnO2

Wet chemical

Core–shell nanowire

KOH

838 F g−1

98.5% (1000)

[129]

MnO2/TiN

Facile deposition

Nanotubes

LiClO4

662 F g−1

[130]

PPy/MnO2

Chemical Oxidation

Pellets

Na2SO4

352.8 F g−1

91.2% (100)

[131]

MnO2/PPy

Co-deposition

Nodular grain

Na2SO4

620 F g−1

90% (4000)

[132]

MnO2/CNT

Electrodeposition

Nanotubes

KCl

471 F g−1

91% (100)

[133]

PANI/MnO2

Potentiodynamic

Corn-like

Na2SO4

725 F g−1

96.5% (5000)

[134]

CF-ACNT/MnO2/PEDOT

Petal-like nanosheet

Na2SO4

1065 F g−1

95% (1000)

[135]

C/MnO2 DNTAs

Electrodeposition

Nanotubes

Na2SO4

793 F g−1

97% (5000)

[136]

MnO2/CNT/paper

Electrodeposition

Dendrite-like nanoflakes

Na2SO4

540 F g−1

[137]

MnO2/PEDOT

Co-electrodeposition

Nanowires

Na2SO4

210 F g−1

[138]

MnO2/CFF

Hydrothermal

Coral-like

Na2SO4

467 F g−1

99.3% (5000)

[139]

MnO2/N-UCNs

Spherical

Na2SO4

401 F g−1

86.3% (10,000)

[122]

NG-MnO2

Hydrothermal

Nanoflower

Na2SO4

220 F g−1

98.3% (1000)

[3]

ACFC/PANI/CNTs/MnO2

 

H2SO4

2242 mF cm−2

75% (10,000)

[140]

γ-MnO2/ACNT

Induced deposition

Nanotubes

Na2SO4

784 F g−1

99.5% (800)

[141]

Table 5

The electrochemical performances of NiO and corresponding composite-based electrodes for supercapacitors

Material

Substrate

Method

Morphology

Specific capacitance (F g−1, if not specified)

Retention % (cycles)

References

NiO

Glass

CBD

Honeycomb

167

[148]

NiO

Hydrothermal

Nanocolumns

390

[156]

NiO

Si-MCP

Electroless plating

Macroporous

586.4

95.2 (500)

[149]

NiO

Ni foam

Hydrothermal

Nanorod

2018

92 (400)

[157]

NiO

Nickel foil

CBD

Nanoflakes

309

89 (4000)

[145]

NiO

Ni foam

Facile ammonia evaporation

Nanoflakes

232

150 (4000)

[144]

NiO

Stainless steel

Potentiodynamic electrodeposition

Nanoflakes

222

94 (1000)

[158]

NiO

Sol–gel

Flower like

480

[159]

NiO

Ni foam

Thermal oxidation

Nanoflakes

1784.2

75.8 (20,000)

[160]

NiO

Ni foam

Solvothermal

Granules

1386

78.5 (5000)

[161]

NiO

Stainless steel

SILAR

Nanoflakes

674

72.5 (2000)

[147]

NiO-CuO

Hydrothermal

Flower like

280

91.4 (3000)

[162]

Co3O4@NiO

Ni foam

Hydrothermal

Nanowire

1236.67

91.3 (5000)

[152]

NiO@CuO@Cu

Cu foil

Electrodeposition

Spheroidal

58.14

97.42 (100)

[151]

α-Fe2O3@NiO

Carbon cloth

Hydrothermal

Nanosheet–nanorod

361.2

96.2 (3000)

[163]

ZnO@C@NiO

Carbon cloth

CBD, Hydrothermal

Nanorod arrays

677

71 (5000)

[155]

NiO/Ni(OH)2/PEDOT

 

Electrodeposition

Flower like

401.1 mF cm−2

82.2 (1000)

[143]

NiO/Graphene

Ni foam

SILAR

Porous

783

84 (1000)

[22]

NiO/MWCNTs

Stainless steel

SILAR

Nanoflakes

1727

91 (2000)

[164]

Table 6

The electrochemical performances of Ni(OH)2 and corresponding composite-based electrodes for supercapacitors

Material

Substrate

Method

Morphology

Specific capacitance (F g−1, if not specified)

Retention % (cycles)

References

α-Ni(OH)2

Microwave assisted

Flower like

277

98 (100)

[150]

Ni(OH)2

Ni foam

CBD

Flower like

1065

[176]

β-Ni(OH)2

Ni foam

Hydrothermal

Nanowalls

2675

96 (500)

[178]

α-Ni(OH)2

Solvent free

Nanosheet

2080.8

82 (2000)

[179]

α-Ni(OH)2

Ti plate

Electrodeposition

Cabbage-like

1903

90.13 (3000)

[180]

α-Ni(OH)2

Solvent deficient

Granules

2338

91.08 (5000)

[181]

Ni(OH)2/Ni

Carbon cloth

Porous

1760

88 (3000)

[182]

CoNi(OH)2

Chemical Precipitation

Nanoplates

708

[183]

MnO2-Ni(OH)2

Ni foam

Hydrothermal

3D ridge

1015

86 (10,000)

[184]

Co3O4@Ni(OH)2

Ni foam

Hydrothermal

Nanosheet

1306

90.5 (5000)

[185]

Co-doped Ni(OH)2/Ni3S2

Ni foam

Hydrothermal

Cactus

3023.4

75.7 (1500)

[186]

Ni(OH)2-MnOx

Carbon fiber paper

Electrodeposition

Nanosheet

344

92.5 (10,000)

[187]

ZnS/ZnO/Ni(OH)2

Ni foam

Hydrothermal

Nanosheet

1773

72 (1000)

[188]

TiO2@Ni(OH)2//N–C NWs

Carbon cloth

Core–shell

150.6

90 (5000)

[189]

Ni0.33Co0.67(OH)2

CNFP

Electrodeposition

Nanosheet

2.03 F cm−2

77 (1000)

[190]

ZnCo2O4@Ni(OH)2

Ni foam

Electrodeposition

Microsheet

4.6 F cm−2

70 (2200)

[191]

FeOF/Ni(OH)2//AC

Carbon cloth

Solvothermal-chemical precipitation

Core–shell

100.6

84.4 (5000)

[192]

g-C3N4@Ni(OH)2

Ni foam

Honeycomb

1768.7

84 (4000)

[193]

CNRG/Ni(OH)2

CNRG

Precipitation

Nanosheet

1785

71.3 (5000)

[194]

Ni(OH)2- RGO

Ni wire

Situ transformation

Nanosheet

1326

83.5 (5000)

[195]

β-Ni(OH)2-rGO

Hydrothermal

Flower like

618 C g−1

90 (2000)

[196]

Ni(OH)2- rGO

Solvothermal

Nanosheet

1886

70 (1000)

[49]

Ni(OH)2/ECF

ECF

Electrodeposited

Nanofiber

277.5

100 (1000)

[197]

CQD decorated Ni(OH)2

Hydrothermal

Nanoplates

2900

[198]

rGo/Ni(OH)2/PANI

Hydrothermal

Flower like

514

94.4 (1000)

[142]

Ni(OH)2-POV

Stainless steel

Chemical solution deposition

nanoplates

1440

85 (2000)

[2]

Table 7

Advantages and disadvantages of synthesis methods for electrode materials of supercapacitors [253, 254]

Method

Morphology

Advantages

Disadvantages

Electrochemical deposition

Nanostructured film

Less time required, morphology can be controlled through the control of synthesis parameters such as time, temperature etc

Unsuitable for large-scale production

Hydrothermal method

Nanostructured film and powder

Large-scale production, easy control of morphology

High-temperature and time-consuming operations

Chemical bath deposition

Nanostructured film

Faster than hydrothermal method, large-scale production, easy control of morphology

Only some metal oxide can be possible to synthesis

Sol–gel

Nanostructured film and powder

Large-scale production

Difficult to produce porous films

Chemical precipitation

Powders, colloidal nanostructures

Large-scale production, fast process

Difficult to control morphology

Mechanochemical method

Nanostructured powder

Large-scale production, controlled morphology, fast process

Thin-film formations not possible, limited to only nanoparticles

4.2.3 Nickel Oxide (NiO)

NiO has recently attracted attention from researchers as a promising alternative to RuO2 due to its high theoretical capacitance (2584 F g−1), environmentally friendly nature, high chemical stability, and its low cost and ease of availability [142, 143]. As for the energy storage process of NiO, there are two main theories involving a single-step conversion between NiO and NiOOH (Eqs. 9, 10) and a two-step conversion with an initial conversion between NiO and Ni(OH)2 and a subsequent conversion between Ni(OH)2 and NiOOH (Eqs. 11, 12) [31, 146, 147, 148, 149]. Here, both supercapacitive reactions are a result of the transformation of Ni2+ to NiOOH through the loss of electrons and can be represented in the following equations:
$${\text{NiO}} + {\text{OH}}^{ - } \leftrightarrow {\text{NiOOH}} + {\text{e}}^{ - }$$
(9)
$${\text{NiO}} + {\text{H}}_{2} {\text{O}} \leftrightarrow {\text{NiOOH}} + {\text{H}}^{ + } + {\text{e}}^{ - }$$
(10)
Or
$${\text{Ni}}\left( {\text{OH}} \right)_{2} \leftrightarrow {\text{NiOOH}} + {\text{H}}^{ + } + {\text{e}}^{ - }$$
(11)
$${\text{Ni}}\left( {\text{OH}} \right)_{2} + {\text{OH}}^{ - } \leftrightarrow {\text{NiOOH}} + {\text{H}}_{2} {\text{O}} + {\text{e}}^{ - }$$
(12)
And among these two theories, the first theory is widely accepted whereas the second theory is also practical because NiO can combine with OH to produce Ni(OH)2 in alkaline electrolytes and contribute to capacitance generation. As for challenges, NiO possesses low electrical conductivities, which can result in lower specific capacitances and poorer cyclability, restricting practical application as a supercapacitor electrode material. To address this issue, researchers have proposed solutions such as the direct deposition of NiO onto conductive materials or the formation of nanocomposites with conductive carbon-based materials to enhance conductivity [37]. In addition, the electrochemical performance of NiO also relies on specific surface area and porosity, which can be achieved through effective synthesis methods and precise morphology formations [47]. For example, Gund et al. [147] synthesized hierarchical nanoflake structured NiO films on SS 304 through the use of a successive ionic layer adsorption and reaction method and reported that the prepared material delivered a specific capacitance of 674 F g−1 and good cyclic stability in which 72.5 of the specific capacitance remained after 2000 cycle. Here, the researchers attributed these excellent performances to the mesoporous structure and large accessible area. In another example, Patil et al. [148] prepared binder-free electrodes by depositing NiO onto glass substrates and reported a specific capacitance of 167 F g−1. Furthermore researchers report that the structural property of NiO can influence electrochemical properties in which ordered mesoporous structures can enhance ionic motion, increase active mass and boost charge/discharge rates [149]. Based on this, Ren et al. [150] synthesized a hierarchical and porous NiO nanoflower-like structure that presented a specific capacitance of 277 F g−1 and good cyclic stability. Apart from direct deposition, great efforts have also been devoted to the enhancement of specific capacitance and structural stability through the use of hybridization techniques in which hybridized heterostructures (e.g., NiO@CuO@Cu [151], Co3O4@NiO [152], NiO/Ni(OH)2/PEDOT [153]) can present the advantages of individual components, such as the presence of mixed valance cations providing higher conductivity and electrochemical activity in comparison with single metal oxides [154]. And among the various heterostructures, core–shell structures based on excellent conductive substrates can achieve high electrochemical performances. For example, Ouyang et al. [155] developed ZnO@C@NiO core–shell nanorod arrays (CSNAs) grown on a carbon cloth (CC) conductive substrate using a three-step method involving hydrothermal and chemical baths (Fig. 10) and reported that the synthesized electrode exhibited a specific capacity of 377 C g−1 with 71% capacitance retention after 5000 cycles.
Fig. 10

Schematic of the formation of CC/ZnO@C@NiO. Reprinted with permission from Ref. [155]. Copyright (2018) American Chemical Society

4.2.4 Nickel Hydroxide (Ni(OH)2)

Among transition metal hydroxides, nickel hydroxide has attracted major attention due to its high specific capacitance (2082 F g−1), excellent stability in strong alkaline electrolytes, good rate capability, lower costs, and ease of availability [165] in which previous studies have demonstrated that the electrochemical performance of Ni(OH)2 is mainly dependent on phase structure, morphology, porosity and surface area [166]. In addition, two pseudo-polymorphs of α and β exist for Ni(OH)2 that possess special layered structures similar to layered double hydroxides in which the interplanar spacing for α-Ni(OH)2 is larger due the intercalation of water molecules and anions [167, 168]. As a result, α-Ni(OH)2 can deliver higher specific capacitances than β-Ni(OH)2 but is not as stable in alkaline electrolytes and will transform to the β phase [170, 171]. In the late 1960s, Bode et al. [169] revealed a scheme for the redox behavior of nickel hydroxides involving the oxidation of nickel hydroxide to nickel(III) oxyhydroxide and the subsequent reduction back to nickel hydroxide (Fig. 11). This scheme also involves two phases (α and β) for nickel hydroxide and two phases (β and ɤ) for the oxidized material.
Fig. 11

Schematic of the chemical and electrochemical reaction occurring at the nickel hydroxide electrode [169, 260]

Parameters such as precipitation conditions, structural defects, microstructures, and crystallinity have influences on the performance of Ni(OH)2 [89, 90, 91, 92] and large specific surface areas and increased depth of the electrochemical process can enhance specific capacitance [172, 173]. In addition, the crystal structure and crystallinity of Ni(OH)2 can decide H+ mobility and govern the charge/discharge rate [174]. For example, Lang et al. [175] synthesized loosely packed Ni(OH)2 using a facile chemical precipitation method in which low crystallinity nanoflakes were formed and reported a maximum specific capacitance of 2055 F g−1 for the prepared sample. Here, the researchers suggested that such high specific capacitances were due to the novel structure of the prepared Ni(OH)2, which allowed for the easy access of electrolyte OH ions to the Ni(OH)2 nanoflakes along with enhanced diffusion rates. In another study, we prepared a nanoflower-like Ni(OH)2 and deposited it onto a Ni foam as an electrode for supercapacitor applications and obtained a specific capacitance of 1065 F g−1 with excellent rate capabilities [176]. Alternatively, the semiconductor nature of Ni(OH)2 can limit rate capabilities and the aggregation of Ni(OH)2 particles can decrease electrochemical performances. To overcome these issues, researchers have proposed the formation of nanocomposites with highly conductive materials such as activated carbon and graphene [49]. For example, Chai et al. [177] synthesized Ni(OH)2/graphene composites through a chemical precipitation route in which the graphene sheet can act as a highly conductive medium and provide larger surface areas to enhanced electrochemical performance. Here, the researchers also reported that the 3D loose structure can provide more active sites for redox reactions and more interfacial contact with electrolytes to allow for faster electron transfer rates and diffusion rates. As a result, a maximum specific capacitance of 2053 F g−1 was obtained for the GNS/Ni(OH)2 composite with 97% capacitance retention after 1000 cycle.

4.3 Conducting Polymers (CPs)

Recently, CPs have also attracted attention from researchers due to their relatively high capacitance, high energy density, high voltage windows, adjustable redox activity through chemical modification and good conductivity in doped states as well as ease of fabrication, feasibility, low costs, and low environmental impact [199, 200]. In addition, specific capacitances can arise in CPs due to fast and reversible redox reactions related to the π-conjugated polymer chains [54] in which during the oxidation process, ions transfer to the polymer backbone and are released back into the electrolyte during reduction [27]. Furthermore, CPs can store charge in its bulk because no structural changes such as phase changes occur during the charging/discharging process. As a result of all of this, CPs can provide higher capacitances due to redox storage capabilities and larger surface areas. And currently, p-dopable polymers are mostly studied by researchers because of stable performances against degradation as compared with non-dopable polymers [201]. However, despite all the advantages of CPs, low power densities due to slow ion diffusion rates in bulk materials hinder performance [81].

Currently, the most commonly used CPs for supercapacitor electrodes include polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and various derivatives [202]. Here, various methods are available for the synthesis of CPs in which the oxidation of monomers through chemical or electrochemical methods is the most common approach [81]. Furthermore, CP-based supercapacitors possess three types of configurations [203, 204, 205, 206, 207, 208, 209, 210, 211], including p–p supercapacitors in which both electrode materials are the same p-doped polymer, n–p supercapacitors in which both electrode materials are the same polymers but one is n-doped and the other is p-doped and p–p′ supercapacitors in which two different p-doped CPs with different oxidation and reduction electroactivities are used. Here, PANI and PPy are only p-dopable with their n-doping potential being much lower than the reduction potential of common electrolyte solutions, whereas PTh can be p- and n-doped [206, 208]. For example, Wang et al. [210] fabricated PANi with a unique nanowire structure using an electrochemical polymerization method as the active material for a supercapacitor and reported a high specific capacitance of 950 F g−1 at a current density of 1 A g−1. Here, the researchers reported that due to the nanowire array (Fig. 12), diffusion paths are reduced along with charge transfer resistances, allowing this material to achieve high specific capacitances at high current densities. Furthermore, the need of emerging energy storage devices for high-performance electrode materials can be satisfied by structure-controlled conductive polymer hydrogels with tunable electrochemical properties and mechanical flexibility. For example, Shi et al. [211] prepared nanostructured conductive polypyrrole hydrogels using an interfacial polymerization method and reported that the 3D porous nanostructured PPy exhibited a high specific capacitance of 380 F g−1 and excellent rate capabilities. In another example, Gnanakan et al. [212] synthesized polythiophene nanoparticles using a cationic surfactant-assisted dilute polymerization method for supercapacitor electrodes and reported a specific capacitance of 134 F g−1 and an energy density of 8 Wh kg−1 with a power density of 396 W kg−1. Researchers have also reported that volume change in CPs during the insertion/removal of ions due to swelling and shrinkage in conventional electrolytes can lead to mechanical degradation and subsequent low cycle stability and capacitance decay for capacitors [213, 214, 215, 216, 217]. In addition, CPs also suffer from low cyclic stability due to the instability of redox sites in CP backbones for repeated redox cycles [216, 217]. To resolve this, researchers have proposed that the composition of CPs with carbon-based materials is an excellent solution [209, 216, 218]. For example, Zhu et al. [219] electrochemically polymerized polythiophene (PTh) onto multiwall carbon nanotubes (MWCNT) using a galvanostatic method to prepare a PTh/MWCNT composite electrode material for supercapacitors and reported a maximum specific capacitance of 216 F g−1 with excellent cycle stability.
Fig. 12

Schematic of the optimized ion diffusion path in nanowire arrays. Reprinted with permission from Ref. [210]. Copyright (2010) American Chemical Society

5 Synthesis Methods

Morphology plays an important role in the enhancement of electrochemical performance, and it is widely accepted that pseudocapacitance is an interfacial phenomenon that is closely related to the morphology of prepared electrode materials [144]. As a result, the dimension and morphology of optimal electrode materials need to be selectively tailored through the control of various parameters in the synthesis process, such as growth temperature and time and concentration of reactants [220]. In addition, researchers have demonstrated that various synthesis methods can be used to achieve desired characteristics in electrode materials, including electrodeposition, hydrothermal, chemical bath deposition, sol–gel, chemical precipitation, and mechanochemical, all of which have been extensively applied in the synthesis of electrode materials for supercapacitors.

5.1 Electrochemical Deposition

The most basic form of electrochemical deposition is electrode position, in which redox reactions occur and coatings or films are formed on the surface of substrates by using an electric current. And in typical material deposition on substrates, a negative charge is applied on the substrate and dipped in a salt solution of the deposition material in which positively charged salt ions become attracted toward the negatively charged substrate (cathode) and accept electrons to undergo a reduction process. Here, electrode position possesses two reaction mechanisms, including anodic electrodeposition and cathodic electrodeposition [47]. In addition, electrochemical deposition can allow for the control of electrode material structures and morphologies through the control of synthesis parameters; however, large-scale production is not possible using electrodeposition method [31]. Scientist reported the use of three-electrode electrodeposition methods to obtain desirable materials in which the substrate used for the deposition of films can act as an active electrode, with Pt foil being used as the counter electrode and a saturated calomel electrode being used as the reference electrode. For example, Dubal et al. [178] synthesized a cauliflower-like CuO nanostructure in which stainless steel plates were used as the substrate and graphite rods and a saturated calomel electrode was used as the counter and reference electrode, respectively (Fig. 13a) and reported that the synthesized material as a supercapacitor electrode can produce a specific capacitance of 179 F g−1 with 81% capacitance retention after 2000 cycles (Fig. 13b). In another study, Fu et al. [221] electrodeposited Ni(OH)2 onto Ni foam to achieve a particle-like morphology and a loosely packed structure in which the resulting material delivered excellent electrochemical performances and a specific capacitance of 2595 F g−1. In addition, researchers have reported that porous and 3D nanostructured Ni(OH)2 deposited onto Ni foam through electrodeposition exhibited a high specific capacitance of 3152 F g−1 [222]. Electroless deposition and electrophoretic deposition methods can also be used as alternatives to electrolytic deposition in which in the case of electroless deposition, material deposition can be performed without the application of electric fields, with electrons being supplied by a reducing agent in the solution. For example, Yu et al. [225] synthesized nanosheets of NiO@CNT using an electroless plating technique and reported a specific capacitance of 1177 F g−1 at 2 A g−1 current density. And in the case of electrophoretic deposition, charge particles suspended in liquid media can move toward electrodes under an electric field. For example, Du et al. [16] fabricated carbon nanotube thin films using electrophoretic deposition that demonstrated small ESR and high power density.
Fig. 13

a Schematic of the experimental setup used for the potentiodynamic mode of electrodeposition for the synthesis of CuO cauliflower onto a stainless steel substrate. b Variations of capacity retention with the number of cycles. Inset shows cyclic voltammograms (CVs) of the CuO cauliflower at the 1st, 500th, 1000th and 2000th cycle. Reprinted with permission from Ref. [261]. Copyright (2013) Elsevier

5.2 Hydrothermal Method

The hydrothermal technique is a widely accepted one-pot synthesis method for the synthesis of transition metal oxides. Here, the term “hydrothermal” refers to heterogeneous chemical reactions in the presence of solvents above room temperature and at high pressures in which required materials are prepared through a combination of reaction precursors and heated in a sealed Teflon-lined stainless steel autoclave. This autoclave is generally kept above 100 °C and pressure is automatically generated due to the closed system. And at high temperatures and pressures, solvents dissolve and recrystallize into desired materials in which factors such as reaction temperatures, reaction times and amount of solvents all have significant impacts on the morphology and structure of the final product. For example, Lu et al. [178] constructed a 3D mesoporous film of Ni(OH)2 on Ni foam using the hydrothermal method and reported an ultrahigh specific capacitance of 2675 F g−1 and excellent cyclic stabilities. In addition, Fu et al. [223] studied the effects of hydrothermal reaction temperature on the morphology of β-Ni(OH)2 and reported that low reaction temperatures can allow for the formation of a flower-like morphology whereas increased reaction times decreased nanoflake thickness. Wang et al. [228] were also able to synthesize different MnO2 morphologies (nanorods, nanotubes, and nanowires) using a one-step hydrothermal method. Furthermore, Zhu et al. [224] synthesize hierarchical and interconnected reduced graphene oxide/β-Ni(OH)2 nanobelts using the hydrothermal method and reported a specific capacitance as high as 362 F g−1 with excellent cyclic stability (96.3% capacitance retention after 10,000 cycles). Lastly, Gund et al. [41] in their study demonstrated the influence of hydrothermal reaction temperature on the morphology of Ni(OH)2 as well as the subsequent effect on electrochemical performance in which nanoplates, stacked nanoplates, nanobelts, and nanoribbons can be obtained at hydrothermal reaction temperatures of 60 °C, 80 °C, 100 °C and 120 °C, respectively (Fig. 14). These researchers also reported that among the various morphology, nanoplates displayed the highest specific capacitance of 357 F g−1 at a scan rate of 5 mV s−1.
Fig. 14

Schematic of the formation of different nanostructures of Ni(OH)2 at different temperatures. Reprinted with permission from Ref. [39]. Copyright (2013) Royal Society of Chemistry

5.3 Chemical Bath Deposition (CBD)

Chemical bath deposition is an easy, convenient and cost-effective low-temperature method for the formation of thin films of semiconducting materials; especially metal oxides/hydroxides, on large surface area substrates [225, 226], in which the direct deposition (controlled chemical precipitation of compounds from solution) of materials from solution onto substrates can occur at medium temperatures without an external electric source. In addition, CBD does not require sophisticated instruments and can operate at low temperatures in which the possibility of oxidation and corrosion of metallic substrates is significantly reduced [148]. Nagayama et al. [227] in 1988 were the first to propose CBD as a method for silica coating in which in typical experimental setups, suitable substrates were immersed in a solution containing a chalcogenide source, metal ions, and precipitation agents. And by controlling parameters such as the pH and temperature of the solution and the concentration of the regents, film thicknesses can be controlled. Here, the solid phase from the solution can form in two steps involving nucleation and particle growth. As for precipitation, a minimum number of ions or molecules can form clusters/nuclei to allow for nucleation in which the molecule clusters undergo rapid decomposition to allow particles to combine and grow to certain thicknesses. In addition, further growth can occur through ion condensation or the adsorption of colloidal particles from the solution onto the substrate [228]. For example, Dubal et al. [234] fabricated various morphologies of CuO nanostructures through CBD using various surfactants and reported that due to the various surfactants used as soft chemical templates, woolen clump, stacked nanosheet and nanobuds morphology can be obtained in which the nanobuds morphology provided a 3D conductive framework that led to desirable characteristics such as easy access to electrolytes, high surface areas and shortened ion pathways, allowing for a maximum specific capacitance of 396 F g−1 with excellent rate and cycling stability. In another study, Patil et al. [229] studied the influence of bath temperatures on supercapacitive properties and found that higher temperatures lead to porous nanoflakes, resulting in higher specific capacitances. Gurav et al. [230] also synthesized nanograined Co(OH)2 thin films on glass and stainless steel substrates through CBD and reported a specific capacitance of 120 F g−1.

5.4 Sol–Gel

The sol–gel method, or solution gelling, is a method used to produce thin films that involves multiple steps with various physical and chemical approaches such as hydrolysis, polymerization, gelation, condensation, drying, and densification [231]. Here, sols are dispersions of solid particles (1 to 100 nm) in liquid and gels are interconnected, rigid networks with sub-micrometer dimensional pores and micrometer length polymeric chains [232]. A major advantage of the sol–gel method is the ability to gain control at the atomic level for various components through the mixing of precursors in a solution at low temperatures. In addition, the texture, composition, structure, and homogeneity of resulting films can also be controlled by varying parameters in the sol–gel method [233]. For example, Lin et al. [234] prepared manganese oxide electrodes using the sol–gel method and reported a specific capacitance of 189.9 F g−1 after a 400 °C heat treatment in which 78.6% of the capacitance was retained after 300 cycles. Liu et al. [241] also synthesized a NiO/NiCo2O4/Co3O4 composite as an electrode material using the sol–gel method with calcination at 250 °C and reported that the prepared sample exhibited excellent electronic conductivities and a mesoporous structure, resulting in improved electrochemical performances including specific capacitance of 1717 F g−1 with excellent rate capabilities and 94.9% capacitance retention after 1000 cycles. In addition, Jayalakshmi et al. [235] developed a vanadium pentoxide thin film with varying thicknesses (6–12 layer) on glass as a conductive substrate using the sol–gel method and reported that the vanadium pentoxide with a thickness 202 nm (8 layers)exhibited a maximum specific capacitance of 346 F g−1 at a scan rate 5 mV s−1. Furthermore, Zhang et al. [243] prepared monolith NiO aerogels using a facile citric acid-assisted sol–gel method that possessed a highly porous aerogel microstructure with a rigid nanoparticle backbone that lead to large numbers of active sites for redox reactions and the easy transformation of ions due to high pore volumes, resulting in improved pseudocapacitive performances.

5.5 Chemical Precipitation

Chemical precipitation is another simple and easy method to produce active materials in the powder form for supercapacitors and can be used in the large-scale synthesis of macro-/nanosized materials such as transition metal oxides/hydroxides. In chemical precipitation, solute concentrations in solution are kept above solubility limits to allow precipitation to occur due to supersaturation in which desired metal ions form into salts (nitrate) that are coprecipitated from alkaline/base media such hydroxides or carbonates. This precipitate is subsequently collected from the solution, washed with distilled water and dried at required temperatures in appropriate atmospheres to obtain the final product [244]. For example, Zhang et al. [236] synthesized flower-like CuO nanostructures using chemical precipitation as a supercapacitor electrode and reported a specific capacitance of 133.6 F g−1 and excellent cyclic performances. In addition, Wu et al. [245] synthesized nanoplatelet-like Ni(OH)2 that was further annealed to obtain NiO as a method to enhance electric conductivity in which a specific capacitance of 108 F g−1 was obtained. Lang et al. [237] also synthesized loosely packed flake-like NiO materials using chemical precipitation to achieve low crystallinity and a flake structure that resulted in a high specific capacitance of 942 F g−1 and good rate capabilities. Furthermore, Gnana Sundara Raj et al. [247] prepared nanoparticles of Mn2O3 through chemical precipitation for pseudocapacitors and also reported excellent electrochemical performances.

5.6 Mechanochemical

In the mechanochemical method of synthesis, mechanical energy is provided through means of either hand grinding or ball milling as activation steps for the synthesis of products in which the first step is the breakdown of particles to allow for size reduction and simultaneous increase in surface area and surface energy. Mechanochemical activation can also alter material structure and chemical composition [238], in which due to continuous impact, chemical reactions can occur at the interface of nanostructured grains [239], allowing for the synthesis of various transition metal oxide/hydroxide nanoparticles [239]. In addition, because the mechanochemical method does not require solvents, high-energy inputs or long reaction times and is pollution-free and high-yielding, it has attracted attention for the synthesis of electrode materials at the nanoscale [240]. For example, Sun et al. [241] used a mechanically assisted solid-state reaction method to prepare graphene/Ni(OH)2 nanocomposites as a supercapacitor electrode material in which an agate mortar was used to grind the precursors together for activation of the chemical reaction. As a result, a specific capacitance of 1568 F g−1 was reported for the graphene/Ni(OH)2 composite at a current density of 4 A g−1. In addition, Guo et al. [252] synthesized [Cu(tu)]Cl.1/2H2O nanobelts using a one-step facile mechanochemical method for pseudocapacitive applications and reported a high specific capacitance of 1145 F g−1 with good rate capabilities. Kore et al. [181] also prepared a hierarchical mesoporous network of α-Ni(OH)2 using a solvent-deficient approach and obtained a specific capacitance of 2338 F g−1 for the α-Ni(OH)2 obtained from ammonium bicarbonate.

6 Electrolytes

Electrolytes are electrically conducting solutions with solutes dissolved in a polar solvent such as water that can separate out cations and anions after dissolution [47]. And to achieve high electrochemical performances, optimal electrolytes are vital. This is because, electrolytes can act as a ion source, electric charge conductor, and electrode particle adhesive (Fig. 15) in which major properties required for optimal electrolytes include conductivity and temperature coefficients that decide the ESR of supercapacitors, wide voltage windows to improve energy density, high ionic concentrations to avoid depletion problems, high electrochemical stability, low volatility, low viscosity, low toxicity and low costs [242]. Here, both energy and power densities are directly proportional to the square of the cell voltage (Eqs. 5, 6) [54], and the electrolyte conductivity can be optimized through the selection of mixed solvents in which the interactions between ions and solvents, and between electrolytes and electrode materials determine the cycle lifespan and self-discharge of supercapacitors. In addition, electrolytes can be classified into two categories, including aqueous and solid/quasi-solid-state electrolytes (Chart [46], Fig. 16).
Fig. 15

Effects of the electrolyte on the electrochemical performance of supercapacitors. Reprinted with permission [242]. Copyright (2015) Royal Society of Chemistry

Fig. 16

Classification of electrolytes used in supercapacitor applications

6.1 Liquid Electrolytes

6.1.1 Aqueous Electrolytes

In general, electrolytes for supercapacitor applications are selected on the basis of the ionic conductivity and radii of hydrated cations and anions. Here, aqueous electrolytes possess higher ionic conductivities, higher ionic concentrations and smaller solvated ions as compared with organic electrolytes, allowing for lowered supercapacitor ESRs and higher power densities and specific capacitances as compared with organic electrolytes [27]. In addition, advantages such as low costs, low viscosity, non-flammability, non-corrosiveness, safety and excellent stability make aqueous electrolytes attractive. Furthermore, aqueous electrolytes can be produced large-scale without particular synthesis requirements. However, the narrow potential window of aqueous electrolytes can reduce energy densities for supercapacitors in which aqueous electrolyte-based supercapacitors can only deliver a maximum operating potential window of 1.2 V due to the thermal decomposition of water at 1.229 V and low overpotential for hydrogen [242]. In general, strong acidic, alkaline, or neutral solutions (H2SO4, KOH, Na2SO4, NH4Cl, etc.) are used as aqueous electrolytes in which strong acidic or alkaline solutions limit the potential window up to 1.23 V, whereas neutral solutions can extend the potential window up to 1.6 V regardless of the electrode material. Here, H2SO4 and KOH are commonly used strong acidic and alkaline solutions due to high ionic conductivities [47].

6.1.2 Organic Electrolytes

The low voltage windows of aqueous electrolytes have led researchers to search for alternative electrolytes that can deliver higher voltages and energy densities. Here, researchers report that organic electrolytes in supercapacitors can provide high voltage windows up to 2.5–3.0 V and can enhance energy densities by 6 to 9 times higher. Organic electrolytes are prepared by dissolving conducting salts into organic solvents and the performance of organic electrolytes are dependent on factors such as the nature of the dissolved salts and solvents as well as corresponding conductivity, purity and ionic size. As for commonly used solvents in organic electrolytes, acetonitrile (ACN) and propylene carbonate (PC) are popular in which ACN possesses the capability to dissolve various organic salts but is toxic, whereas PC possesses excellent properties such as higher flash points, lower toxicity, wider electrochemical windows and wider range of working temperatures [27]. Furthermore, organic electrolytes are typically prepared by dissolving organic salts such as tetraethylammonium tetrafluoroborate, tetraethylphosphonium tetrafluoroborate, and triethymethylammonium tetrafluoroborate (TEMABF4) in organic solvents. Alternatively, organic electrolytes also suffer from a series of drawbacks such as high costs, safety concerns due to toxicity and flammability as well as high viscosities and larger sizes of solvated ions. In addition, water- and oxygen-free atmospheres are required to produce organic electrolytes due to its hygroscopic and sensitive nature to moisture. Furthermore, because high resistivity and water content can limit working voltages and power densities in organic electrolytes, water needs to be extracted through numerous purification cycles, which further increases costs and leads to issues such as electrode corrosion.

6.1.3 Ionics Liquids

Ionic liquids are composed of a special combination of organic nitrogen-containing heterocyclic cations and inorganic anions, leading to low melting points [256, 257]. In addition, ionic liquids are solvent-less electrolytes that possess excellent characteristics such as high thermal and chemical stability, negligible vapor pressure, broad electrochemical stability potential windows and wide space parameters for ion selection, making them potential candidates as electrolytes in energy storage devices [245]. And in terms of the wide potential window, ionic liquids are resistant to oxidation and reduction and can provide a cell voltage of ~ 4.5 V, with some achieving 6 V [246]. Furthermore, ionic liquids composed of ion-containing salts that are liquid at ambient temperatures [room-temperature ionic liquids (RTIL)] have attracted interest as electrolytes in supercapacitors due to desirable properties such as non-volatility, inflammability and heat resistance [247] in which through a variety of combinations of cations and anions, it is possible to design ionic liquid electrolytes with conductivities as high as 40 mS cm−1 and potential windows of over 4 V [52]. In general, ionic liquid electrolytes can be classified asaprotic, protic, and zwitterionic [248] and commonly used ionic liquids include imidazolium and pyrrolidinium as well as asymmetric, aliphatic quaternary ammonium salts with anions such as tetrafluoroborate, trifluoromethanesulfonate, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, or hexafluorophosphate [246]. Despite the excellent characteristics of ionic liquids, however, disadvantages such as high costs, high viscosity, and low ionic conductivity at room temperature limit ionic liquids as electrolytes in practical application [249]. In addition, because supercapacitors generally operate at a temperature range of − 30 to 60 °C, the low conductivity of ionic liquids at this temperature range prevents application [52].

6.2 Solid-State Electrolytes

Solid-state electrolytes are materials with ionic conductive cations or anions with negligible electric conductivity. These solid-state electrolytes can serve as ionic conductive media as well as separators in which the advantages of using solid-state electrolytes include the lack of liquid leakage, high performance, light weightiness, and simplifications to the packaging and fabrication process of supercapacitors. During the synthesis of solid-state electrolytes, various requirements need to be taken into consideration, such as high ionic conductivity, high chemical, electrochemical and thermal stability, and sufficient mechanical strength [242], and until now, the majority of solid-state electrolytes synthesized for supercapacitors are based on polymer electrolytes with little research being conducted on inorganic electrolytes. Polymer-based solid-state electrolytes can further be classified into three groups, including solid polymer electrolytes (SPEs), gel polymer electrolytes (GPE), and polyelectrolyte [250]. Here, SPEs contain polymers such as PVA and salts such as LiCl without the addition of any solvent in which the transportation of salt ions in the polymer provides ionic conductivity. And in the case of GPEs, these consist of polymers and an aqueous electrolyte or salts dissolved in a solvent and can be referred to as quasi-solid-state electrolytes due to the presence of liquid phases [251]. As for polyelectrolytes, charge polymer chains are responsible for ionic conductivity. And among these solid-state electrolytes, GPEs can deliver higher ionic conductivities as compared with the others due the presence of solvents. However, GPEs also possess issues such as low mechanical strength, internal short circuiting and narrow operating temperatures if water is used as the solvent [252]. In addition, the limited contact surface area between solid-state electrolytes and electrode materials can increase ESR values, leading to less utilization of active mass, reduced rate performances and lower specific capacitances [242]. Furthermore, the exposure to high-temperature operations can lead to the decomposition of organic solid-state electrolytes. Alternatively, inorganic solid-state electrolytes can allow for higher operating temperatures; however, it is difficult to ensure reliable contact between electrolytes and electrodes in inorganic electrolytes. Researchers have proposed, however, that this issue can be resolved through the use of mechanically and chemically compatible electrodes and electrolytes such as nanocomposites of 0.4LiClO4–0.6Al2O3 [253]. Furthermore, due to the lower ionic conductivity of polymers, potential power outputs are also limited in organic solid-state electrolytes [254]. Alternatively, inorganic solid-state electrolytes possess higher lithium ionic conductivities that are below their melting point, which can enhance electrochemical performances. In addition, inorganic electrolytes possess higher mechanical strengths and thermal stability. Despite this, inorganic solid-state electrolytes are not bendable and have no flexibility [242].

6.3 Redox Active Electrolytes

Recently, redox active electrolytes are becoming popular due to the contribution of additional pseudocapacitance to the overall capacitance from the active components present in the electrolyte. These active compounds, including hydroquinone, m-phenylenediamine, KI, and lignosulfonates, are commonly used in redox active electrolytes [255, 256] and among these halides, iodide ions possess excellent properties such as environmental benignity and rich oxidation states beneficial for redox active electrodes. For example, Lota et al. [257] prepared carbon electrodes in a KI electrolyte (simple bifunctional electrolytic medium based on an iodide aqueous solution) for a supercapacitor in which KI acted as the conducting electrolyte and supplied additional capacitances due to the intriguing faradaic reaction of iodide. Here, the researchers observed the intriguing effects of iodide on the positive electrode and a maximum specific capacitance of 1840 F g−1 based on the following equation:
$$3{\text{I}}^{ - 1} \leftrightarrow {\text{I}}_{3}^{ - 1} + 2{\text{e}}^{ - }$$
(13)
$$2{\text{I}}^{ - 1} \leftrightarrow {\text{I}}_{2} + 2{\text{e}}^{ - }$$
(14)
$$2{\text{I}}_{3}^{ - 1} \leftrightarrow 3{\text{I}}_{2} + 2{\text{e}}^{ - }$$
(15)
$${\text{I}}_{2} + 6{\text{H}}_{2} {\text{O}} \leftrightarrow 2{\text{IO}}_{3}^{ - 1} + 12{\text{H}}^{ + } + 10{\text{e}}^{ - }$$
(16)

Furthermore, Roldan et al. [258] added hydroquinone active compounds to a supporting electrolyte and obtained excellent improvements in the specific capacitance of a carbon-based supercapacitor in which for the chemically activated carbon-based material (AC-KOH), a specific capacitance of 901 F g−1 was achieved. Here, the researchers attributed the additional pseudocapacitance to the faradaic reactions of the added hydroquinone/quinine.

Supercapacitors possess higher power densities in the range of 1–10 W g−1 as compared with other modern energy storage devices (Ragone plot in Fig. 1) in which the electrical charge in supercapacitors is stored not only at the surface but also at the near-surface of bulk electrodes, allowing for rapid charge/discharge rates. In addition, the cyclability of supercapacitors is much higher than that of batteries due to the lack of chemical transfer reactions or phase changes in the storage of charge. Furthermore, long shelf lives, wide range of operating temperatures, and higher efficiencies are also advantages making supercapacitors attractive for practical application [27], including in electric vehicles, electric hybrid vehicles, pulse layer techniques, mobile phones, electrical tools, digital cameras, digital communication devices as well as for storage applications of energy generated from renewable sources. The use of supercapacitors is also a promising method to resolve issues in current battery technologies such as memory backups, the frequent replacement of batteries due to poor cycle lifespans and low power densities that cannot reach peak load requirements [35]. Despite the many advantages over batteries, however, supercapacitors also suffer from challenges in its current state, including low energy densities, high manufacturing costs, and high self-discharge rates [242], in which the energy density for commercially available supercapacitors is ~ 5 Wh kg−1, which is far less than batteries at 50 Wh kg−1. In addition, the high cost of electrode materials and manufacturing in supercapacitors significantly hinder commercialization.

7 Future Perspectives

The basic requirements of high-performance supercapacitors include high specific surface areas, high electric conductivities, and high stability at higher temperatures as well as matching pore structures with electrolyte ions and lower synthesis costs in which the performance of supercapacitors can be enhanced through the use of appropriate synthesis methods to achieve pore distributions with uniformly distributed pore sizes and specific surface areas. Based on this, there is significant room for the development of highly conductive, phase controlled, appropriate pore-sized and higher surface area electrode materials.
  1. 1.

    Specific surface area must be larger and more easily accessible to electrolytes to facilitate surface reactions. Here, nanocomposites of metal oxides, carbon-based materials and conductive foam used as substrates can enhance the surface area in which the design of high surface area electrode materials and the utilization of this area are major challenges that require attention.

     
  2. 2.

    The stability of materials depends on morphology and material morphology should last as long as the lifetime of supercapacitor. However, redox reactions can alter morphology due to strain generated during the charge/discharge process. Therefore, support structures that can maintain stability are needed.

     
  3. 3.

    The integrity between electrode materials and substrates is important to reduce internal resistances in which higher internal resistances can lead to lower power densities and capacitances. Therefore, highly conductive, binder-free electrodes that can maintain long-term integrity between composite electrode materials and substrates need to be developed.

     
  4. 4.

    The development of synthesis methods with precise control over morphology, pore distribution, and pore size needs more focus.

     
  5. 5.

    The development of electrolyte materials for supercapacitors to enhance the voltage range will ultimately lead to the enhancement of energy density.

     

8 Conclusions

A growing demand for clean and renewable energy sources has allowed supercapacitors to be noticed as a promising technology to provide high power densities, efficiencies, and long cycle lifespans. Based on this, this review provided a detailed summary of the influences of various parameters on the electrochemical performance of supercapacitors in which electrode materials and corresponding fabrication methodologies can widely alter performances. This review also included detailed discussions on various electrode and electrolyte materials, corresponding fabrication techniques and related advantages and disadvantages to provide improved accuracy in the selection of materials and methods. Overall, supercapacitors are emerging energy technologies that are hindered from effective utilization by the lack of maintenance in corresponding power density and energy density. Based on this, continuous efforts from industry and academia have been devoted to the investigation of different electrode materials such as carbon-based materials, metal oxides/hydroxides and conducting polymers to further enhance the efficacy of supercapacitors. And although carbon-based materials have demonstrated high specific surface areas and rational pore distributions in the recent years, the achievement of high energy density is far from realized. In addition, metal oxides/hydroxides also suffer from issues related to poor stability and short cycle lifespans. Furthermore, various technological shortfalls have also been noticed for conducting polymers, including shrinkage and swelling that can significantly reduce supercapacitor lifespans. And in order to address all of these issues, novel approaches, such as hybridizations of different materials (e.g., Ni(OH)2/graphene, PTh/MWCNT), various synthesis methods to synthesize different morphology (e.g., core–shell, nanoflower) and alterations to electrolyte materials (instead of aqueous solid-state or redox active) are needed.

Notes

Acknowledgements

We would like to thank Dr. C. D. Lokhande and Dr. Ram Dayal for their valuable guidance during the preparation of this review.

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Copyright information

© Shanghai University and Periodicals Agency of Shanghai University 2019

Authors and Affiliations

  1. 1.Department of Mechanical EngineeringVishwakarma Institute of TechnologyPuneIndia
  2. 2.Department of Mechanical EngineeringSinhgad Institute of TechnologyLonavalaIndia
  3. 3.ABES Engineering CollegeGhaziabadIndia

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