Materials and Fabrication Methods for Electrochemical Supercapacitors: Overview
- 11 Downloads
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.
KeywordsSupercapacitor Metal oxide EDLC Electrolyte
Performance comparison between supercapacitors and batteries
High (1–10 W g−1)
Low (150 W kg−1)
Self-discharge times at room temperature
Lifetime at room temperature (years)
Cell voltage (V)
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
2.1.1 Electric Double-Layer Capacitors
2.1.3 Hybrid Supercapacitor
Comparison between EDLCs, pseudocapacitors and hybrid capacitors 
Non-faradic/electrostatic, electrical charge stored at the metal/electrolyte interface
Faradic, reversible redox reaction
Both faradic and non-faradic
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
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 .
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 . 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  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 . 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 . 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 . 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 . And to address the major disadvantage of low density in AC for practical applications, Moreno-Fernandez et al.  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.  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 .
4.1.2 Carbon Nanotubes (CNT)
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)
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 . 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 . 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 .
RuO2-based electrodes used in supercapacitors and corresponding specific capacitances
190 F g−1
863 F g−1
720 F g−1
Incipient wetness method
710 F g−1
1340 F g−1
Non-ionic surfactant templating method
58 F g−1
1263 F g−1
740 F g−1
650 F g−1
440 F g−1
Co precipitation method
720 F g−1
Metal vapor CVD
5.21 F g−1
Non-aqueous electrolyte containing EMIBF4
6 μF cm−2
52.66 F g−1
RuO2 on AISI 317 SS
520 F g−1
1485 F cm−2
480 F g−1
Ultrasonic spray pyrolysis
2192 F g−1
1561 F g−1
510 F g−1
Wet ball-milling precipitation
571 F g−1
1297 F g−1
BeSO4 + Al2(SO4)3
397 F g−1
1724 F g−1
4.2.2 Manganese Oxide (MnO2)
The electrochemical performances of MnO2 and corresponding composite-based electrodes for supercapacitors
698 F g−1
166 F g−1
320 F g−1
Plate-like + nanorods
168 F g−1
353 F g−1
297 F g−1
Hydrothermal (180 min)
315 F g−1
392 F g−1
309 F g−1
0.35 F cm−2
487 F g−1
1260.9 F g−1
838 F g−1
662 F g−1
352.8 F g−1
620 F g−1
471 F g−1
725 F g−1
1065 F g−1
793 F g−1
540 F g−1
210 F g−1
467 F g−1
401 F g−1
220 F g−1
2242 mF cm−2
784 F g−1
The electrochemical performances of NiO and corresponding composite-based electrodes for supercapacitors
Specific capacitance (F g−1, if not specified)
Retention % (cycles)
Facile ammonia evaporation
401.1 mF cm−2
The electrochemical performances of Ni(OH)2 and corresponding composite-based electrodes for supercapacitors
Specific capacitance (F g−1, if not specified)
Retention % (cycles)
Carbon fiber paper
2.03 F cm−2
4.6 F cm−2
618 C g−1
CQD decorated Ni(OH)2
Chemical solution deposition
Less time required, morphology can be controlled through the control of synthesis parameters such as time, temperature etc
Unsuitable for large-scale production
Nanostructured film and powder
Large-scale production, easy control of morphology
High-temperature and time-consuming operations
Chemical bath deposition
Faster than hydrothermal method, large-scale production, easy control of morphology
Only some metal oxide can be possible to synthesis
Nanostructured film and powder
Difficult to produce porous films
Powders, colloidal nanostructures
Large-scale production, fast process
Difficult to control morphology
Large-scale production, controlled morphology, fast process
Thin-film formations not possible, limited to only nanoparticles
4.2.3 Nickel Oxide (NiO)
4.2.4 Nickel Hydroxide (Ni(OH)2)
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 . For example, Lang et al.  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 . 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 . For example, Chai et al.  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  in which during the oxidation process, ions transfer to the polymer backbone and are released back into the electrolyte during reduction . 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 . However, despite all the advantages of CPs, low power densities due to slow ion diffusion rates in bulk materials hinder performance .
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 . 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 . 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
5.2 Hydrothermal Method
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 . Nagayama et al.  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 . For example, Dubal et al.  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.  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.  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.
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 . 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 . 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 . For example, Lin et al.  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.  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.  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.  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 . For example, Zhang et al.  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.  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.  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.  prepared nanoparticles of Mn2O3 through chemical precipitation for pseudocapacitors and also reported excellent electrochemical performances.
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 , in which due to continuous impact, chemical reactions can occur at the interface of nanostructured grains , allowing for the synthesis of various transition metal oxide/hydroxide nanoparticles . 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 . For example, Sun et al.  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.  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.  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.
6.1 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 . 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 . 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 .
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 . 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 . 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 . 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  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 . In general, ionic liquid electrolytes can be classified asaprotic, protic, and zwitterionic  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 . 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 . 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 .
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 , 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 . 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 . 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 . 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 . 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 . Furthermore, due to the lower ionic conductivity of polymers, potential power outputs are also limited in organic solid-state electrolytes . 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 .
6.3 Redox Active Electrolytes
Furthermore, Roldan et al.  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 , 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 . 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 , 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
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.
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.
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.
The development of synthesis methods with precise control over morphology, pore distribution, and pore size needs more focus.
The development of electrolyte materials for supercapacitors to enhance the voltage range will ultimately lead to the enhancement of energy density.
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.
We would like to thank Dr. C. D. Lokhande and Dr. Ram Dayal for their valuable guidance during the preparation of this review.
- 31.Vangari, M., Pryor, T., Jiang, L.: Supercapacitors: review of materials and fabrication methods. J. Energy Eng. 139, 72–79 (2013). https://doi.org/10.1061/(ASCE)EY.1943-7897.0000102 CrossRefGoogle Scholar
- 33.Jayalakshmi, M., Balasubramanian, K.: Simple capacitors to supercapacitors—an overview. Int. J. Electrochem. Sci. 3, 1196–1217 (2008)Google Scholar
- 58.Simon, P., Burke, A.: Nanostructured carbons: double-layer capacitance and more. Electrochem. Soc. Interface 17, 38–43 (2008)Google Scholar
- 108.Fugare, B.Y., Lokhande, B.J.: Study on structural, morphological, electrochemical and corrosion properties of mesoporous RuO2 thin films prepared by ultrasonic spray pyrolysis for supercapacitor electrode application. Mater. Sci. Semicond. Process. 71, 121–127 (2017). https://doi.org/10.1016/j.mssp.2017.07.016 CrossRefGoogle Scholar
- 119.Dongale, T.D., Jadhav, P.R., Navathe, G.J., et al.: Development of nano fiber MnO2 thin film electrode and cyclic voltammetry behavior modeling using artificial neural network for supercapacitor application. Mater. Sci. Semicond. Process. 36, 43–48 (2015). https://doi.org/10.1016/j.mssp.2015.02.084 CrossRefGoogle Scholar
- 135.Lv, P., Feng, Y.Y., Li, Y., et al.: Carbon fabric-aligned carbon nanotube/MnO2/conducting polymers ternary composite electrodes with high utilization and mass loading of MnO2 for super-capacitors. J. Power Sources 220, 160–168 (2012). https://doi.org/10.1016/j.jpowsour.2012.07.073 CrossRefGoogle Scholar
- 139.Cakici, M., Reddy, K.R., Alonso-Marroquin, F.: Advanced electrochemical energy storage supercapacitors based on the flexible carbon fiber fabric-coated with uniform coral-like MnO2 structured electrodes. Chem. Eng. J. 309, 151–158 (2017). https://doi.org/10.1016/j.cej.2016.10.012 CrossRefGoogle Scholar
- 149.Miao, F., Tao, B., Ci, P., et al.: 3D ordered NiO/silicon MCP array electrode materials for electrochemical supercapacitors. Mater. Res. Bull. 44, 1920–1925 (2009). https://doi.org/10.1016/j.materresbull.2009.05.004 CrossRefGoogle Scholar
- 181.Kore, R.M., Lokhande, B.J.: Hierarchical mesoporous network of amorphous α-Ni(OH)2 for high performance supercapacitor electrode material synthesized from a novel solvent deficient approach. Electrochim. Acta 245, 780–790 (2017). https://doi.org/10.1016/j.electacta.2017.06.001 CrossRefGoogle Scholar
- 212.Gnanakan, S.R.P., Rajasekhar, M., Subramania, A.: Synthesis of polythiophene nanoparticles by surfactant—assisted dilute polymerization method for high performance redox supercapacitors. Int. J. Electrochem. Sci. 4, 1289–1301 (2009)Google Scholar
- 221.Fu, G.R., Hu, Z.A., Xie, L.J., et al.: Electrodeposition of nickel hydroxide films on nickel foil and its electrochemical performances for supercapacitor. Int. J. Electrochem. Sci. 4, 1052–1062 (2009)Google Scholar
- 223.Fu, X.M.: The Influence of the hydrothermal temperature on the morphologies of β-Ni(OH)2 nanospheres and nanoflakes. Appl. Mech. Mater. 159, 376–379 (2012). https://doi.org/10.4028/www.scientific.net/AMM.159.376 CrossRefGoogle Scholar
- 236.Zhang, H., Feng, J., Zhang, M.: Preparation of flower-like CuO by a simple chemical precipitation method and their application as electrode materials for capacitor. Mater. Res. Bull. 43, 3221–3226 (2008). https://doi.org/10.1016/j.materresbull.2008.03.003 CrossRefGoogle Scholar
- 238.Wieczorek-Ciurowa, K., Gamrat, K.: Some aspects of mechanochemical reactions. Paper Presented at the 1st Workshop on Synthesis and Analysis of Nanomaterials and Nanostructures/3rd Czech-Silesian Saxony Mechanics Colloquium. Wroclaw, Poland, 21–22 November 2005Google Scholar
- 239.Marx, W.: Mechanochemical synthesis of nanoparticles. J. Mater. Sci. 39, 5143–5146 (2004). https://doi.org/10.1023/B:JMSC.0000039199.56155.f9 CrossRefGoogle Scholar
- 245.Kowsari, E.: High-performance supercapacitors based on ionic liquids and a graphene nanostructure. In: Pesek, K. (ed.) Ionic Liquids—Current State of the Art, pp. 75–100. Rijeka, InTech (2015)Google Scholar
- 261.Dubal, D.P., Gund, G.S., Lokhande, C.D., et al.: CuO cauliflowers for supercapacitor application: novel potentiodynamic deposition. Mater. Res. Bull. 48, 923–928 (2013). https://doi.org/10.1016/j.materresbull.2012.11.081 CrossRefGoogle Scholar