Poonam 2019

Journal of Energy Storage 21 (2019) 801–825
Contents lists available at ScienceDirect
Journal of Energy Storage

journal homepage: www.elsevier.com/locate/est
Review of supercapacitors: Materials and devices T

a b a a,⁎
Poonam , Kriti Sharma , Anmol Arora , S.K. Tripathi
a
Department of Physics, Centre for Advanced Study in Physics, Panjab University, Chandigarh, 160014, India
b
Department of Physics, Goswami Ganesh Dutta Sanatan Dharma College, Sector 32-C, Chandigarh, 160014, India
A R T I C LE I N FO A B S T R A C T
Keywords: Supercapacitors have gained a lot of attention due to their unique features like high power, long cycle life and
Supercapacitor environment-friendly nature. They act as a link for energy-power difference between a traditional capacitor
Specific capacitance (having high power) and fuel cells/batteries (having high energy storage). In this perspective, a worldwide
Energy density research has been reported to address this and rapid progress has been achieved in the advancement of fun-
Power density
damental as well as the applied aspects of supercapacitors. Here, a concise description of technologies and
Novel electrode materials
New energy devices
working principles of different materials utilized for supercapacitors has been provided. The main focus has been
on materials like carbon-based nanomaterials, metal oxides, conducting polymers and their nanocomposites
along with some novel materials like metal-organic frameworks, MXenes, metal nitrides, covalent organic fra-
meworks and black phosphorus. The performance of nanocomposites has been analysed by parameters like
energy, capacitance, power, cyclic performance and rate capability. Some of the latest supercapacitors such as
electrochromic supercapacitor, battery-supercapacitor hybrid device, electrochemical flow capacitor, alternating
current line filtering capacitor, micro-supercapacitor, photo-supercapacitor, thermally chargeable super-
capacitor, self-healing supercapacitor, piezoelectric and shape memory supercapacitor have also been discussed.
This review covers the up-to-date progress achieved in novel materials for supercapacitor electrodes. The latest
fabricated symmetric/asymmetric supercapacitors have also been reported.
1. Introduction principle of electrochemical energy conversion. SCs have gained much

attention on account of high specific capacitance (Cs), long life cycle,
Energy is vital for human development. Energy consumption and high power density (Pd), being almost maintenance free, experiencing
production, which depend on combustion of fossil fuels, is going to no memory effect, safe and function as a bridge for power-energy dif-
affect the world economy and ecology severely. So, there has been an ference that exists between capacitor (high Pd) and fuel cells/batteries
increasing demand for environment-friendly, high-performance re- (large energy storage) [1–4]. These present a viable solution for pro-
newable energy storage devices. Electrochemical energy is an un- viding energy in rural areas, where no public grids are available or
avoidable part of the clean energy portfolio. Batteries, supercapacitors where a heavy cost of wiring and providing electricity is involved. SCs
(SCs) and fuel cells are unconventional energy devices working on the can also be utilized as power supplies for portable devices like mobile
Abbreviations: SC, supercapacitor; SCs, supercapacitors; ASCs, asymmetric supercapacitors; Cs, specific capacitance; Pd, power density; Ed, energy density; EDLCs,
electric double layer capacitors; ACs, activated carbons; CNTs, carbon nanotubes; CDC, carbide derived carbon; TMOs, transition metal oxides; MOs, metal oxides;
CPs, conducting polymers; PANI, polyaniline; PPy, polypyrrole; PVA, polyvinyl alcohol; PEDOT, poly34-ethylene-dioxythiophene; PSS, poly (4-styrene sulfonate);
PPV, poly-phenylene vinylene; PW, potential window; MOFs, metal organic frameworks; COFs, covalent organic frameworks; ACFM, activated carbon fibre material;
SSA, specific surface area; ILs, ionic liquids; CFC, carbon fibre cloth; MWCNTs, multi walled carbon nanotubes; PVDF, poly-vinylidene fluoride; PTFE, poly tetra-
fluoroethylene CVD chemical vapour deposition; GO, graphene oxide; SILAR, successive ionic layer adsorption and reaction; HPCNTs, hierarchical porous carbon
microtubes; GMAs, graphene macro assemblies; AQ, anthraquinone; CMG, chemically modified graphene; SWCNTs, single walled CNTs; Pind, polyindole; NWs,
nanowires; NS, nanosheets; ESR, equivalent series resistance; rGO, reduced graphene oxide; DMF, N N-dimethyl formamide; TGA, thermogravimetric analysis; SEM,
scanning electron microscopy; η, coulombic efficiency; CV, cyclic voltammetry; CoS2-rGO, cobalt disulphide-reduced graphene oxide; N-CNFs, nitrogen functiona-
lized carbon nanofibres; NFs, nanofibres; rGO@HTC, N-doped hydrothermal carbon coated graphene; PpPD, poly (phenylenediamine); HEG, hydrogen exfoliated
graphene; CQDs, Carbon Quantum Dots; +ve, positive; -ve, negative; photo-SCs, photosupercapacitors; DSSC, dye-sensitized solar cells; ATO, anodic titanium oxide;
PSSH, polystyrene sulfonic acid; PEO-NaOH, NaOH-treated polyethylene oxide; BC, biochar; SMSC, shape memory supercapacitor; FESEM, field effect scanning
electron microscopy; XRD, X-ray diffraction; TEM, transmission electron microscopy; CD, charge-discharge; NF, nickel foam
⁎
Corresponding author.
E-mail address: surya@pu.ac.in (S.K. Tripathi).
https://doi.org/10.1016/j.est.2019.01.010
Received 5 October 2018; Received in revised form 9 January 2019; Accepted 9 January 2019
Available online 29 January 2019
2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Poonam et al. Journal of Energy Storage 21 (2019) 801–825
materials had been investigated for pseudocapacitors (Faradaic charge

transfer).
During 1975–1980, B. E. Conway explored RuO2 pseudocapacitors
extensively. These capacitors store charge through electrosorption,
oxidation-reduction reactions and intercalation mechanism [6]. These
faradaic processes would let pseudocapacitors attain higher Cs and Ed
compared to EDLCs. Pseudocapacitance is linked to the electron charge-
transfer among electrolyte and electrode impending from de-solvated
and adsorbed ion. The adsorbed ions do not react with the atoms of the
material, but only the transfer of charge occurs. The capacity of elec-
trodes to achieve pseudocapacitance effect depends on the chemical
affinity of materials to the ions adsorbed on the surface of the electrode
along with the structure and the dimension of the electrode pores. The
charge storage increases linearly with the applied voltage. Materials
that exhibit redox behaviour and used in pseudocapacitors are transi-
tion-metal oxides (TMOs) eg. IrO2, RuO2, Fe3O4, MnO2, NiO, V2O5,
Co3O4 etc. transition metal sulphides and conducting polymers (CPs)
eg. polyaniline (PANI), polythiophene, polypyrrole (PPy), polyvinyl
Fig. 1. Ragone plot of different electrochemical energy conversion systems, alcohol (PVA), poly (3,4-ethylene dioxythiophene) (PEDOT), poly-
combustion engine, turbines and traditional capacitors [5]. (Reproduced with acetylene, poly (4-styrene sulfonate) (PSS), poly-phenylene-vinylene
permission from Ref. [5] Copyright American Chemical Society (2004)).
(PPV) etc.
Li-ion capacitors (hybrid capacitors) were explored by FDK in 2007.
phones, notebook computers, digital cameras etc. -being small, light- In such capacitors, a carbon electrode was combined with a Li-ion
weight and flexible. In electric and hybrid vehicles, SCs may be used to electrode which increased the capacitance, lowered the anode potential
offer high Pd required for short-term acceleration along with re- with enhanced cell voltage and hence increased the Ed. In such systems,
cuperation of energy during braking, hence saving energy and shielding the faradaic electrode, with high Cs, provides higher Ed and the non-
the batteries from the high frequency rapid charging-discharging pro- faradaic electrode provides higher Pd. Hence, research has been fo-
cess (dynamic operation). The Pd and energy density (Ed) are re- cussed on hybrid capacitors such as composites (coupling of carbon
presented by Ragone plot (Fig. 1). This plot explains that the fuel cells materials with either CPs or TMOs) and battery type (coupling a su-
are high-energy systems; whereas SCs are high-power systems. Batteries percapacitor (SC) electrode with a battery electrode) etc. Table 1 shows
have intermediary power and energy capabilities. There exists some the comparison of EDLC, pseudocapacitor and hybrid supercapacitor
overlap in Ed and Pd of fuel cells and SCs with batteries. Also, it is [7].
apparent from the figure that any single electrochemical device cannot The traditional capacitors have a rigid and massive structure and
compete with an internal combustion engine. So Ed and Pd of electro- hence are not suitable for future applications. Thinner, lighter, flexible,
chemical devices have to be increased to compete with the combustion transparent SCs with a number of novel features and functions are re-
engine [5]. quired for multifunctional consumer electronics. The ACs and TMOs are
The credit for the beginning of capacitor technology goes to the still the generally used electrode materials. The ACs possess non-regular
invention of the Leyden Jar (1745–1746) which was made up of a glass morphologies, feebly graphitized frameworks with wide pores, whereas
vessel with metal foils. The metal foils acted as electrodes and the jar TMOs exhibit low electronic conductivity, which is not able to keep
acted as a dielectric. In the charging process of the above mentioned pace with high rate energy storage environments [8]. So, new materials
device, positive (+ve) charges accumulated on one electrode and ne- for SC electrodes have been explored such as covalent organic frame-
gative (-ve) charges on the other. When these two charges were con- works (COFs), metal-organic frameworks (MOFs), MXenes, metal sul-
nected using a metal wire, a discharging process would take place. The phides, metal nitrides, mixed conductors, 2-D materials etc. [9–11].
first electrolytic capacitor came in the 1920s. In 1957, the first super- The key challenge for supercapacitor is the small Ed. As the Ed of
capacitor (electric double layer capacitors-EDLCs) was patented by capacitors is directly proportional to Cs and the square of the voltage
General Electric using activated charcoal as the plates. In EDLCs, charge (V), so to increase Ed, either Cs or potential or both of these quantities
storage takes place electrostatically (non-Faradaic) i.e. no shifting of should be increased. This can be accomplished by using electrode ma-
charge takes place between electrode and electrolyte (which makes terials having high Cs, electrolytes having broad potential windows and
them highly reversible along with high cycling stability). Carbon na- optimization of the structure of integrated systems. Even though the
nomaterials, like carbon aerogels, activated carbons (ACs), carbon na- fabrication of individual components (like electrode materials, elec-
notubes (CNTs), graphene, carbide-derived carbon (CDC) etc. are un- trolytes) of the SCs is relatively simple, but to promote their combined
ique structures for EDLCs with the huge specific surface area (SSA), effect, the compatibility between the pore size and structure of elec-
great mechanical and chemical stability and good electrical con- trode material with the electrolyte ion size is essential. Electrolytes/
ductivity. To increase the Cs of SCs, new electrochemically active solutions play an essential role in setting up important properties like
Table 1
Comparison of EDLC, pseudocapacitor and hybrid capacitor [7].
Electrochemical double layer capacitor (EDLC) Pseudocapacitor Hybrid capacitor
1. Carbon is used as electrode material. MOs and CPs are used as electrode A combination of carbon and MOs/CPs is used.
material.
2. Charge storage mechanism is through the electrochemical The charge is stored through the redox The charge is stored both by Faradaic and non-Faradaic processes.
double layer formation (non-Faradaic process). reactions (Faradaic process).
3. Low Ed, good rate capability, good cyclic stability, low Cs. High Cs, High Ed, high Pd, low rate High Ed, high Pd, good cyclability, polymer/carbon composite has
capability. moderate cost and moderate stability, Li/Carbon capacitors are of
high cost.
802
Fig. 2. (a) Schematic of the fabrication process

of MnO2-PEDOT: PSS nanostructured compo-
site by co-electrodeposition on the stainless
steel mesh and its fabrication as an asymme-
trical supercapacitor (b) SEM images of MnO2-
PEDOT: PSS at diverse magnifications [23].
(Reproduced with permission from Ref. [23]
Copyright Royal Society of Chemistry (2013)).
Pd, temperature range and conductivity. Other requirements of elec- synthesis methods are described here briefly:
trolytes for SC are: a wide potential window (PW), high ionic con-
centration, good electrochemical stability, low equivalent series re-
sistance (ESR), less volatility, less viscosity, non-toxicity, small solvated 2.1. Sol-gel method
ion radius and low cost [12].
Aqueous, organic, redox-type, solid or semi-solid electrolytes and Sol-gel is a facile method to prepare materials with greater purity
ionic liquids (ILs) have been investigated extensively for SCs. Aqueous and homogeneity. The sol-gel method is so named, as in it micro-par-
electrolytes (like KOH, Na2SO4, H2SO4 and NH4Cl aqueous solution ticles in the solution (sol) agglomerate and link together in regulated
etc.) provide higher ionic concentration, lower resistance, smaller ionic conditions to form an integrated network (gel). Two basic variations of
radius, higher Cs and higher Pd than the organic electrolyte. Moreover, the sol-gel method are the colloidal method and the polymeric or the
in the case of aqueous electrolytes, there is no strict need of controlling alkoxide method, which are different from each other on the type of
the parameters during their preparation process, whereas in an organic precursors used. In both methods, the precursor is mixed in a liquid
electrolyte, there are strict processes and conditions to get ultra-pure (usually water is used for the colloidal method and alcohol for poly-
electrolytes. The conductivity of the aqueous electrolyte is ˜1 Scm−1 meric method) and is then activated with the addition of an acid or a
and it also has a minimum pore size requirement in comparison to the base. Then, as obtained activated precursor reacts forming a network,
organic electrolyte [13]. which it develops with temperature and time maximally up to the
The main shortcoming of the aqueous electrolyte is its small PW container size [18]. Many TMOs have been prepared by this method.
(approximately 1.2 V) due to water decomposition at 1.23 V. Organic This process provides the advantage of preparing materials of different
electrolytes have smaller electrical conductivity (10 to 60 mScm−1), so morphologies. The electrode material prepared by this process pos-
lower Pd, but have higher Ed (due to wide PW of 2.5–2.7 V). ILs are sesses high SSA with better electrochemical behaviour which can also
appropriate for making SCs electrolyte because of their properties like be controlled by temperature, change of surfactants, solvents and re-
high thermal and chemical stability [14–16], low vapor pressure, wide action time [19].
potential window, low flammability and conductivity around 10 Yusin et al. [20] have reported this method for the production of
mScm−1. Ion size in ILs is well-identified because solvation shell is not activated carbon fibre material (ACFM)-Ni(OH)2 composite which ex-
there due to the solvent-free nature of ILs [13]. With the advancements hibits the Cs of ˜370–380 Fg−1. Also, the dependence of shape, structure
in ILs, PW of the SCs can be extended up to 4 V, though they have small and volume of material on the composition and concentration of the
ionic conductivity and high viscosity. Furthermore, the study of semi- solution was established. Liu et al. [21] have deposited NiCo2O4 films
solid electrolytes has led to the growth of flexible or solid-state SCs by a sol-gel method which exhibit the Cs of 2157 Fg−1 at a 0.133
which have no potential leakage issues. Of late, redox-type electrolytes mAcm-2 current density and good cycling stability (96.5% Cs retained
are introduced because of their additional pseudocapacitance from the after 10,000 cycles). NiO/LaNiO3 electrode fabricated by spin-coating
redox reactions at the electrode/electrolyte interface [17]. on Pt/Ti/SiO2/Si (100) substrate by Liu et al. [22] showed a Cs of 2030
Fg−1 at a 0.5 Ag−1 and high stability (83% of the Cs retention after
1000 cycles). This superior electrochemical response can be related to
2. Synthesis approach for electrode materials
high porosity, well-connected network structures with reduced mass-
transfer resistance between electrolyte and ion which facilitates the
The method of synthesis of electrode materials plays an important
electron hopping in nanoparticles.
role in controlling the structures and properties of the materials. Some
803
2.2. Electro-polymerization/Electrodeposition the slurry formed by adding active material with acetylene black and
PTFE onto Ni foam.
This is a common synthesis technique which provides precise reg-
ulation over the thickness of films and on the rate of polymerization. By 2.5. Chemical vapour deposition (CVD)
suitable choice of deposition solution, nanostructured films with dif-
ferent mass loading and morphologies can be prepared by this method. CVD technique is generally used where the porosity is very im-
This technique involves simple processing conditions and not much portant. This process is performed under vapour phase, where the in-
toxic chemicals are used in it. It is generally used for preparing CPs such itial material is prepared in vapour form, flowed and subjected to a high
as PANI, PEDOT, PPy etc. Su et al. [23] have prepared MnO2-PEDOT: temperature (800–1000 °C). The as-prepared structures have even
PSS composite by co-electrodeposition strategy which exhibits an areal morphology [33]. Among various synthesis methods of graphene for
Cs of 1670 m F cm−2 at 0.5 mAcm−2 and excellent mechanical ro- instance, mechanical cleavage of graphite, chemical exfoliation of
bustness. Fig. 2 depicts the fabrication process of MnO2-PEDOT: PSS graphite (in organic solvents), manufacturing of multi-layered gra-
composite and its scanning electron microscopy (SEM) images. Also, an phene by arc discharge, reduction of graphene oxide (GO) synthesized
ultra thin (< 200 μm) asymmetric supercapacitor (ASC) is fabricated from the oxidation of graphite, graphene synthesized by CVD provides
with high Ed, Pd and rate capability. Nanosized MnO2 electrodes on Au better results owing to their large crystal domains, monolayered
nanowire stems are grown electrochemically by Chen et al. [24] which structure and fewer defects in the sheets, which are helpful for en-
exhibit high Cs (1130 Fg−1 at 2 mVs−1), high Ed (15 Whkg−1 at 50 hancing carrier mobility [34]. Kalam et al. [35] demonstrated that
Ag−1), high Pd (20 kW kg−1 at 50 Ag−1) and long-term stability (90% high-efficiency SCs with improved electrochemical characteristics can
of Cs left after 5000 cycles). ZnO@Ni3S2 core-shell nanorods are formed be fabricated by CVD grown graphene hybridized with MWCNTs. Lo-
by the electrodeposition method by Xing et al. [25] which exhibit a Cs biak et al. [36] prepared hybrid carbon materials consisting of
of 1529 Fg-1 at 2 Ag−1 and retain 42% of initial Cs after 2000 cycles. MWCNTs and graphitic layers, produced by CVD, over MgO assisted
Stretchable CNT-PPy films are deposited by electrochemical deposition metal catalyst, as depicted in Fig. 3. Such materials provide fast charge
by Guo et al. [26]. transport in the cell.
2.3. In-situ polymerization 2.6. Vacuum filtration technique
In this process, monomers are dispersed into an aqueous solution This quick and proficient technique uses the simple concept of va-
using the sonication process. Then an oxidizing agent is mixed to in- cuum filtration to prepare nanocomposites from a physical combination
itialize the polymerization in the aqueous solution and the sample is of different materials. Generally, a mixture of materials is prepared
obtained by filtering the solution. Earlier this method yielded only ir- followed by simple vacuum filtration and drying the filtrate. In this
regular aggregates with a little portion of nanofibres, but with slight method, the composition can be simply altered by varying the con-
modification, nanoparticles, nanorods, and nanofibres were reported centration or the weight percentage of each constituent in the mixture.
with better solution processability and better physical and chemical Graphene suspension, developed by vacuum filtration deposition by
properties. A simple strategy for growth of PEDOT structures on carbon Zhang et al. [37] for fabricating graphene-based Ni foam electrode,
fibre cloth (CFC) by in situ polymerization is reported [27]. When a shows a higher Ed and Pd along with good cycling performance. Xu et al.
supercapacitor device is fabricated with these nanostructures, it ex- [38] have synthesized a nanocomposite of graphene/AC/PPy by va-
hibits a Cs of 203 Fg−1 at 5 mVs−1, an Ed of 4.4 Whkg−1 and Pd of cuum filtration method. As prepared electrode exhibits the Cs of
40.25 kW kg−1 in 1 M H2SO4 electrolyte. Also, it possesses 86% Cs re- 178 Fg−1 at 0.5 mAcm−2 and retains 64.4% of Cs after 5000 charge/
tention after 12,000 cycles. Wang et al. [28] have deposited PANI na- discharge cycles. Y. Gao [39] has used this technique to prepare gra-
nowires within the multi-walled carbon nanotubes (MWCNTs) by in phene/polymer electrode on Ni foam in which the vacuum pressure and
situ electro-polymerization. The aligned MWCNTs provide support to its duration controls the distribution of graphene.
the organic polymers along with providing a pathway for the transfer of
charge. Also, confined MWCNT channels limit the structural changes in 2.7. Hydrothermal/solvothermal method
PANI chains while charging-discharging and enhance the lifetime of the
structure. The films made with CPs encapsulated in MWCNTs showed a The hydrothermal process can be ascribed as environment-friendly
Cs of 296 Fg−1 at 1.6 Ag−1. Different Π-conjugated sulfonate templates superheated aqueous solution dispensation. In addition, this provides
and additional assistance of graphene and MWCNTs are employed to controlled diffusivity within a closed system. The process has super-
enquire the polymerization behaviour of PEDOT by Zhou et al. [29]. As iority over other techniques as it is ideal for preparing designer parti-
prepared PEDOT: MWCNT composite reveals interconnected network culates (particles with high purity, crystallinity, quality and controlled
due to the Π-Π interaction of PEDOT with non-covalent functionalized chemical and physical characteristics). Also, this is a low-temperature
MWCNT and exhibits a Cs of 199 Fg−1 at 0.5 Ag−1. sintering process with a small energy requirement which is simple to
implement and scale up [40]. However, this process has a lesser control
2.4. Direct coating over nanoparticle aggregation. The solvent properties (e.g. dielectric
constant, solubility) change radically in the supercritical phase. Thus,
This technique is employed for the fabrication of those SC electrodes supercritical phase gives a favourable condition for particle formation
in which active material, in the form of slurry, is applied directly on the owing to increased reaction rate and great supersaturation. If some
substrate. Often, additives such as carbon black, polyvinylidene other solvent is used instead of water, then the method is called sol-
fluoride (PVDF), acetylene black, polytetrafluoroethylene (PTFE) are vothermal synthesis. A lot of SC electrodes have been fabricated using
introduced as binders to provide maximum adhesion along with re- this process such as rod-like hollow CoWO4/Co1-xS [41], Cobalt dis-
taining electrical conductivity. The working electrode is fabricated with ulfide-reduced graphene oxide (CoS2-rGO) [42], hexagonal NiCo2O4
90 wt% electrode materials (NiO) and 10 wt% PVA in millipore water as nanoparticles [43] etc.
a solvent and the slurry obtained is pasted on the Pt disc. [30]. Jana
et al. [31] prepared supercapacitor electrode slurry by mixing nitric 2.8. Co-precipitation method
acid treated carbon cloth with 10% PVDF and DMF (N,N-dimethyl
formamide) and the prepared slurry is coated on a stainless-steel sub- This is a facile method for large-scale production of powder sam-
strate. Du et al. [32] synthesized supercapacitor electrode by coating ples. For precipitation to take place, the concentration of one solute
804
Fig. 3. Schematic representation of the preparation of nitrogen-doped porous MWCNTs hybrid by using Ni or Co polyoxomolybdate clusters [36].
should be more than the solubility limit and temperature should be high eliminate a component with high vapour pressure, for producing 3-di-
enough for fast separation into precipitates. Here, it is difficult to reg- mensional bicontinuous open nanoporosity. With this technique, ex-
ulate the morphology of prepared samples due to the fast rate of pre- tensive elements can be fabricated with tunable pore sizes along with
cipitation. Various supercapacitor structures have been reported using full recovery of the evaporated component. Flexible electrodes of Co3O4
this method such as CoFe2O4-magnetic nanoparticles with different flakes and γ-Fe2O3 nanoparticles have been prepared by oxidation as-
precursors [44], Ni3(PO4)2@GO composite [45] which exhibits a Cs of sisted dealloying method for the first time by Wang et al. [54].
1329.59 Fg−1 at a 0.5 Ag−1 and 88% of the Cs retention after 1000
cycles.
2.10. Other synthesis methods
Several other synthesis methods have been reported for SC elec-

2.9. Dealloying method
trodes. The microwave assisted method has been used for rapid
synthesis of tin selenide [55]. Nitrogen functionalised carbon nano-
Dealloying method, also known as selective dissolution, is an easy,
fibres (N-CNFs) are prepared by carbonizing PPy-coated nanofibres
flexible and economical technique to produce nanoporous metallic
(NFs), which in turn are obtained by ‘electrospinning’ and deacetylation
materials (NPMs) with structures like core-shell, hollow core-shell and
of electrospun cellulose acetate NFs and PPy polymerization [56]. An
porous nanoparticles [46]. In this method, more active material is re-
additive free, cost-effective and scalable ‘successive ionic layer ad-
moved from a solution of binary metallic solid by electrolytic dissolu-
sorption and reaction (SILAR) method’ has been quoted to prepare Ni-
tion thus producing an interconnected porous structure. Such structures
Co binary hydroxide on rGO [31] shown in Fig. 4. The pulsed layer
possess higher surface area, good mechanical and compression strength
deposition method is used to fabricate NiO on graphene foam [57].
along with size-scale dependent elastic modulus [46–48]. Much atten-
Free-standing 3D porous rGO and PANI hybrid foam has been fabri-
tion has been given to NPMs prepared by this method since the im-
cated by ‘dipping and dry method’ [58]. Hierarchical porous carbon
portant work of Erlebacher et al. [46] and has become a very important
microtubes (HPCNTs) have been synthesized by carbonization along
method to produce NPMs in the last decade. Li et al. [49] examined the
with KOH activation [59].
fixed voltage dealloying of AgAu alloy particles in the size range of
2–6 nm and 20–55 nm. They demonstrated that only the core-shell
structures (2–6 nm in diameter) evolved above the potential corre- 3. Electrode materials
sponding to Ag+/Ag equilibrium. CuS nanowire on nanoplate network
with improved electrochemical performance has been prepared by Electrodes of SCs must have high conductivity, temperature stabi-
Wang et al. [50] using an improved dealloying method at two con- lity, good chemical stability (inertness), high SSA, corrosion resistance,
trasting reaction temperatures. Cu2O has been synthesized by oxidation should be environment-friendly and have lower cost. Also, the cap-
assisted dealloying method [51]. Free-dealloying method has been used ability of the material to carry out faradaic charge transfer increases the
for the synthesis of Cu-based metallic glasses in HF and HCl solutions total Cs. In general, the smaller the pores, the greater is the Cs and hence
[52]. Lu et al. [53] reported a green and universal technique (vapour- the Ed. But, smaller pore enhances ESR and hence decrease Pd. So,
phase dealloying) for fabricating porous materials by using vapour applications which require more peak currents should have SC elec-
pressure among constituent elements in an alloy, to selectively trodes with larger pores, whereas electrode materials having smaller
805
Fig. 4. a) Schematic illustration of Ni-Co binary hydroxide (BH) on a rGO surface by SILAR method (b) SEM images of the prepared composite (c) digital photograph
of nickel foam (NF) and (d) NF coated with Ni-Co-BH-G [31]. (Reproduced with permission from Ref. [31] Copyright Royal Society of Chemistry (2016)).
pores are useful in applications which need higher Ed [59,60]. they can give mechanical and vibrational constancy for SCs for their use
in the high-vibration environment. Carbon aerogel electrodes with the
3.1. Nanostructured carbon-based materials Cs of 104 Fcm−3, yielding an Ed of 90 Whkg-1 and Pd of 20 Wkg-1 have
been obtained [68]. Roldan et al. [69] increased the Cs of many carbon
Carbon nanomaterial with high SSA is the suitable electrode mate- electrodes by adding hydroquinone (HQ) to the electrolyte, but their
rial. It provides high electrical conductivity, chemical and electro- stability remained a challenge (65% of the Cs retention after 4000 cy-
chemical stability with less cost. Also, the good rectangular shape of cles). Wang et al. [70] used rGO electrodes treated with hydrophobic
Cyclic Voltammetry curves and symmetrical galvanostatic charge-dis- tBu-hydroquinone and achieved good Cs by retaining 94% capacitance
charge profile of carbon materials suggest that carbon based materials after 800 cycles. Anjos et al. [71,72] have studied the capacitive per-
are the suitable capacitive materials. The factors which influence the formance of many PAH-quinones adsorbed on carbon and shows their
electrochemical performance are their SSA, structure and shape of the superior cycling stability (97% of the initial Cs retention after 10,000
pore, pore-size distribution, electrical conductivity and functionality of cycles). Wang et al. [73] have reported an enhancement in capacitance
the surface [60–63]. by adding anthraquinone with porous CNT.
ACs are the first material selected for EDLC electrodes. These are Campbell et al. [74] have reported a method to improve the Ed of
porous carbon materials with high SSA. Although their electrical con- graphene macro assemblies (GMAs) through non-covalent functionali-
ductivity is less (1250–2500 Sm−1), still it is enough for SCs. Their zation with anthraquinone (AQ) and the resulting AQ-GMAs hybrid
porous structure consists of micropores (< 20 Å), mesopores (20–500 electrodes possess 2.9 times (up to 23Whkg−1) Ed in comparison to
Å) and macropores (> 500 Å) to attain high SSA [60,61]. For ACs, untreated GMA electrodes. Fig. 5 shows the synthesis procedure of GMA
whole SSA is not useful for the capacitance because electrolyte ions that electrodes by non-covalent functionalization and Thermo-gravimetric
are very big to enter into smaller micropores do not contribute in analysis (TGA) curves of bulk AQ and AQ-GMA disk.
charge storage. Research is going to estimate the most suitable pore size CDC possesses high SSA with tunable pore diameter to increase ion
for a given ion size and getting better methods to adjust the pore size confinement and hence increasing pseudocapacitance. CDC electrodes
distribution in the fabrication process. A few studies show that pore size with designed pore distribution can give approximately 75% greater Ed
of either 0.4 or 0.7 nm can be suitable for the aqueous electrolyte, while than ACs [75]. The theoretical SSA of graphene is 2630 m2 g−1 which
pore size of 0.8 nm may be helpful for organic electrolytes [62,63]. In can theoretically provide the Cs of 550 Fg−1. El-Kady et al. [76] utilized
some papers, the coordination among the pore size and the ion size was graphene sheets as electrodes. A graphene-based SC have used curved
confirmed by getting a maximum Cs [17,64]. Also, the functional graphene sheets which do not stack, forming mesopores which were
groups attached to the surface of carbon materials may enhance far- wetted by ionic electrolytes (up to voltages of 4 V). Also, the SC exhibits
adaic redox reactions resulting in about 5–10 % increase in Cs [65]. An the Ed of 85.6 Whkg−1 (equal to nickel metal hydride battery) with
electrode having an approximate SSA of 1000 m2 g-1 results in Cs of ˜10 larger Pd greater than that of batteries [77]. The 2-D structure of gra-
μFcm-2 (100 Fg-1). Many commercial SCs make use of AC obtained from phene enhances the charging-discharging process and charge carriers
coconut shells. ACs obtained from coconut shells possess more micro- can quickly enter into and out from the deep pores of the electrodes,
pores than AC made from charcoal. ACF obtained from activated carbon thus increasing power. Such SCs may be employed for 100/120 Hz filter
(surface area ˜2500 m2 g-1) can have micropores with a very narrow applications [78]. Chemically modified graphene (CMG) materials
pore size distribution which can be conveniently controlled. The ad- prepared from one-atom-thick carbon sheets, functionalized according
vantage of AFC electrode is its small electrical resistance and good to our need, exhibits a Cs of 135 Fg−1 in aqueous electrolyte and 99
contact with the collector [66]. These electrodes possess mainly double- Fg−1 in organic electrolyte [79]. Xu et al. [80] showed that flexible SCs
layer capacitance. A little pseudocapacitance arises due to micropores. with a 120 μm thick graphene film could show good capacitive beha-
Carbon Aerogel (frozen smoke) is a very porous, ultra light, syn- viour with the high Cs of 186 Fg−1 (up to 196 Fg−1 for a 42 μm thick
thetic material made up of a continuous network of carbon nano- electrode), small leakage current (10.6 μA), good cycling stability and
particles with mixed mesopores. It does not require a binding agent as it remarkable mechanical flexibility.
can itself bond chemically with the current collector. Thus, it has low The Cs of Graphene electrodes (in the form of rGO) is only 100-150
ESR which provides high Pd [67]. Aerogel electrodes prepared by Fg−1 in organic electrolytes [81,82] and 150–230 Fg−1 in inorganic
pyrolysis of resorcinol-formaldehyde aerogels are better conductors electrolytes [83,84] which is less than theoretical capacitance (550
than activated carbons. They provide thin and firm electrodes so that Fg−1). The lower capacitances are essentially due to the irreversible
806
Fig. 5. (a) Schematic of synthesis of non-covalent AQ functionalization of GMA electrodes (b) TGA curve for AQ-GMA disk (c) TGA curve of AQ. Dashed lines show
their derivative mass loss curves [74]. (Reproduced with permission from Ref. [74] Copyright Royal Society of Chemistry (2014)).
Fig. 6. (a) SEM image of rGO (b) & (c) SEM images of rGO@HTC (d) & (e) TEM and HRTEM images of rGO@HTC (f) XRD pattern of Glu/EDA, rGO@HTC, GO and
rGO [86]. (Reproduced with permission from Ref. [86] Copyright Royal Society of Chemistry (2015)).
restacking of individual rGO sheets in the reduction and drying pro- decay over 2000 cycles) in 6 mol L−1 KOH electrolyte solution [86].
cesses [81], which make the major surface of rGO not available for Fig. 6 shows the schematic of formation rGO@HTC, SEM, transmission
storing charge. Huang et al. [85] explained that a small quantity of GO electron microscopy (TEM), high-resolution TEM (HRTEM) and X-ray
addition might improve the electrochemical performance of biomass- diffraction (XRD) pattern of the as-prepared composite.
derived carbon. The rGO@HTC (N-doped hydrothermal carbon coated Electrodes of CNTs have been developed as an intertwined mat of
graphene) composites exhibited high Cs (340 Fg−1 at 0.1 Ag−1), high carbon nanotubes, with an open and available network of mesopores.
rate capability (203 Fg−1 at 50 Ag−1) and good cycling stability (no Cs The mesopores in CNTs are interconnected, forming a continuous
807
distribution, thus utilizing the SSA more efficiently [87]. CNT electrode only conducting polymer which can be converted into highly ordered
has lower ESR than ACs, as the ions of the electrolyte can easily pe- films is PPV. It has a small band gap and doping can be done to form an
netrate into the mesoporous network [88]. Li et al. [89] deduced that electrically conductive polymer with the maximum conductivity of
the ion size and the electrode/electrolyte accessibility are the promi- 10−3 Scm−1. Its properties can be altered by linking functional side
nent factors which affect the performance of flexible single-walled groups [15]. Polyindole (Pind) has gathered attention due to the mixed
CNTs (SWCNTs) SCs. SWCNTs have a theoretical SSA of 1315 m2 g−1, properties of both poly (p-phenylene) and PPy, such as high redox
whereas MWCNTs has lower SSA compared to that of ACs. MWCNTs property, good thermal stability, meager degradation, and better air
have mesopores that permit easy flow of ions across the electrode- stability [99] in comparison to PPy and PANI [100]. PANI-Sol nano-
electrolyte interface. When the pore size becomes comparable to the crystal, PANI-Eml nanopetal, and PANI-Int nanosphere (depending on
size of the ion, the molecules of the solvent are partially exposed which the pathway of polymerization: solution (Sol), emulsion (Eml), inter-
results in high ionic packing density. However, their mechanical sta- facial (Int)) are reported [101] which exhibit the Cs of 460, 424 and 300
bility decreases due to significant volume change due to repetitive in- Fg−1, the Ed of 23, 21.2, and 15 Whkg−1, respectively at Pd of 200
tercalation and depletion [90]. Peng et al. [91] reported a Cs of 50 Fg−1 Wkg−1. Symmetric SCs based on alternate layers of different CPs have
for MWCNTs. Xie et al. [59] have prepared HPNCTs with willow catkins been studied by Aradilla et al. [102]. The capacitive properties of as-
by an easy carbonization process and exhibit SSA of 1775.7 m2 g−1, Cs prepared multilayered system are better than individual CP which may
of 292 Fg−1 at 1 Ag-1 and good rate capability with 83.5% of the Cs be related to the better porosity of multilayered material. Flexible
retention at 10 Ag-1 for HPNCT-800. Ogata et al. [92] have proposed a worm-like SC electrodes are fabricated using cellulose nanofibres
rGO/GO/rGO device which operates as an SC till 1.2 V and as a battery (CNFs) and graphite nanoplatelets (GNP), doped with PANI by in situ
for voltages greater than 1.5 V A high Cs of 185 Fg-1 at 0.5 Ag-1 for a polymerizations [103]. The Cs of 421.5 Fg−1 has been obtained for
symmetrical supercapacitor of HSG (hierarchically porous nanocarbon hybrid PANI electrodes at 1 Ag−1 with 20 wt% CNFs loading along with
and graphene) has been obtained accompanying an Ed of 78 Whkg-1 at excellent electrochemical properties and Cs retention over 1000 cycles
Pd of 875 Wkg-1 [93]. of repeated bending. Also, an all-solid-state symmetric SC has been
fabricated using PANI/CNF (20% loading)/ GNP electrodes which
3.2. CPs based materials shows good Cs retention at various bending angles as shown in Fig. 7.
Self-doped PANI nanofibres are fabricated on the Pt electrode by re-
CPs have attained considerable attention as they supply high Cs (due verse pulse voltammetry which exhibits the Cs of 400 Fg−1, an Ed of 9.4
to their redox behaviour), rapid charge-discharge process, lesser cost Whkg−1 and Pd of 436 Wkg−1 at 5 mAcm-2 current density [104].
than carbon-based material and a low ESR value. Particularly, the n/p Stable PPy films with high doping degree have been obtained by pulse
type polymer configuration has huge potential for high Ed and Pd [15] polymerization by Sharma et al. [105]. Pulse on time controls the chain
but, the lack of proficient n-doped conducting material and less cycling size along with chain defects and pulse off time controls orientation and
stability has delayed the progress of CPs pseudocapacitor. PANI is conjugation of the polymer chain. In these films, a Cs of 400 Fg−1 has
lightweight, highly conductive, mechanically flexible, low cost, en- been obtained with Ed of 250 Whkg−1 at 5 mAcm-2 current density
vironment-friendly and possesses high theoretical capacitance. The along with long cycle life.
problem is that, because of ion doping/dedoping, PANI shrinks and
swells during the charge/discharge process. To surmount this problem, 3.3. MOs-based materials
the PANI layer is coated on MOs/carbon materials forming PANI/MOs/
nanocarbon ternary hybrid which possesses good cyclic stability and Cs TMOs have been explored a lot as a material for SCs electrodes
[94]. Also, PANI exhibits a wide range of colours due to their many because they possess high conductivity [6]. B.E. Conway described
protonation and oxidation forms. These electrochromic properties can TMOs such as RuO2, IrO2, Fe3O4, MnO2, NiO, Co3O4 etc. which pos-
be used for fabrication of electrochromic SCs. Polyacetylene is the most sessed high pseudocapacitance [6]. The ESR of RuO2 is much less than
crystalline CP, but it is easily oxidized in air. Polypyrrole and poly- other electrode materials. Thus, it has higher Ed and Pd than EDLCs and
thiophene can be synthesized directly in doped form and are very stable CPs supercapacitors, but it is very costly and shows poor performance at
[15]. PPy has greater density and higher flexibility than other CPs. It high current densities [106]. TMOs are a suitable material for SCs
has a high electrical conductivity (10–500 Scm−1) and it itself can electrode due to their chemical stability and variable valence. Co oxide
undergo a rapid redox reaction for charge storage [95]. Lignin-PPy has been investigated much due to high theoretical Cs (3560 Fg−1),
composite has been prepared by coating lignin with PPy by the poly- reversibility [107] and better electrochemical performance. Several Co
merization of PPy with and without the presence of methyl orange, oxide nanostructures are prepared. For instance, ultra layered Co3O4
which leads to the formation of PPy films of globular and nanotubular structure synthesized by Rao et al. [108] exhibits a Cs of 548 Fg−1 at 8
morphology. Thereafter the composites are converted to carbon mate- Ag−1, Wang et al. [109] have reported 3D hollow Co3O4 with a Cs of
rials rich in nitrogen atoms by pyrolysis in N2 atmosphere. The SSA of 820 Fg−1 at 5 mVs−1, nanoporous Co3O4 prepared using solvothermal
the prepared materials has been increased up to 10 times than that of method exhibits Cs, Ed, and Pd of 476 Fg−1, 42.3Whkg−1 and
carbon materials [96]. PEDOT is an intrinsically CP (ICP). Although the 1.56 kW kg−1 respectively [110]. Reddy et al. [111] reported a sym-
conductivity of ICP is much less than metals, but still it is useful due to metric MnO2//MnO2 supercapacitor, Dubal et al. [112] fabricated a
its other properties such as flexibility, easy processing and drying at low symmetric Mn3O4//Mn3O4 supercapacitor and Lu et al. [113] designed
temperatures [15]. a symmetric SC, based on Ni-Co oxide electrodes. Xia et al. [114]
Polythiophenes (PTs) have been prepared by chemical oxidative prepared a RuO2//RuO2 supercapacitor with a PW of 1.6 V. Juodkazis
polymerization using FeCl3 as an oxidant in the presence and absence of et al. [115] suggested a high theoretical Cs of Ru (3800 Fg−1)
different surfactants. It is observed that surfactants change the mor- Das et al. [116] reported Cs of 1715 Fg−1 (very close to its predicted
phology of PTs which is clear from the results as PTs prepared with theoretical Cs of 2000 Fg−1) for RuO2 based SCs in which RuO2 is
TritronX-100 shows a Cs of 117 Fg−1 whereas the Cs for surfactant-free electrodeposited on the SWCNTs film electrode. RuO2 deposited on
PTs is 78 Fg−1 [97]. PVA is a low cost, environment-friendly, water- graphene foam exhibits the Cs of 502.78 Fg−1, the Ed of 39.28 Whkg−1
soluble, colourless and odourless synthetic polymer. It possesses high and Pd of 128.01 kW kg−1 for greater than 8000 cycles with stable
tensile strength, an excellent capability of film formation, emulsifying performance [117]. Hu et al. [118] have utilized AAO membrane-
and bonding properties along with flexibility. However, the properties templates to deposit hydrous RuO2 arrayed nanotubes onto graphite
described above are humidity dependent. More humidity reduces its and obtained a Cs of 1300 Fg−1. Zhang et al. [119] have made a na-
tensile strength, but increases its elongation and tear strength [98]. The notubular hydrous RuO2 based electrode from manganite nanorods and
808
Fig. 7. Electrochemical behavior of all solid state symmetric SC assembled using PANI/CNF (20% loading)/GNP electrodes: (a) Optical image showing bending of SC
(b) CV curves at 5 mVs−1 scan rate (c) Cs and GCD curves at various bending angles (d)Cs retention at bending angle of 90° [103]. (Reproduced with permission from
Ref. [103] Copyright Royal Society of Chemistry (2017)).
a Cs of 861 Fg−1 is obtained. Also, low-cost oxides of vanadium, iron, nanostructures exhibited a Cs of 450 Fg−1 at 0.5 Ag−1 and retains 89%
cobalt and nickel have been checked in aqueous electrolytes. NiO na- of Cs after 500 cycles [128]. An ASC developed using MnO2 doped V2O5
nostructures (Eg = 3.6 to 4.2 eV) may increase Cs due to improved and AC as electrodes exhibits a Cs of 61 Fg−1 with an Ed of 8.5 Whkg−1.
faradic redox reactions [57]. Several methods have been used to syn- Compared to monometallic compounds, bimetallic compounds re-
thesize nanostructured NiO by both chemical as well as physical veal improved performance. NiCo2O4 shows more electronic con-
methods, for instance, sol-gel method, wet-chemical solution method, ductivity and better electrochemical performance than NiO and Co3O4
evaporation and oxidation, electrochemical deposition, coprecipitation [129–131]. The Ni-Co hydroxide shows higher Cs along with the im-
and reactive pulsed laser ablation. Dutt et al. [30] synthesized porous proved rate capability than Ni(OH)2 and Co(OH)2 [132–134]. Wang
NiO nanostructures (particle size ˜70 nm) by the hydrothermal method et al. [135] have synthesized GeSe2 nanostructures and obtained a Cs of
which showed a Cs value of 132 Fg−1 at 10 mVs−1. 300 Fg−1 at 1 Ag−1. Zhang et al. [136] synthesized SnSe and obtained a
Wang et al. [120] have reported Ni-Zn system materials (Nix Zn1-x Cs of 228 Fg−1 at 0.5 Ag−1. Chen et al. [137] synthesized bimetallic Ni-
OH, NiO-ZnO and NixZn1-x). In this system, Nix Zn1-x S porous nano- Co selenides with different Ni-Co ratios and obtained Cs of 535 Fg−1 at
particles (diameters ˜30 nm) with an SSA of 148.4 m2 g−1 exhibit the Cs 1 Ag−1 and Cs retention ˜82% after 2000 cycles higher than Ni-Co
of 1867 Fg−1 at 1 Ag−1 along with excellent rate capability. Cd(OH)2 oxides and Ni-Co sulphides. An ASC synthesized using Ni0.67 Co0.33 Se
nanowires (NWs) have been fabricated on a stainless-steel substrate by and rGO as electrodes exhibited a Cs of 176 Fg−1 at 1 Ag−1 and Ed of
Patil et al. [121]. Cd(OH)2 NWs electrode exhibits the Cs of 267 Fg−1 at 36.7 Whkg−1 at a Pd of 750 Wkg−1. Wang et al. [138] have prepared
5 mVs−1 with good cycling life. A symmetric device fabricated using hierarchical NiCo2O4 electrode material by the hydrothermal method
this electrode exhibited an Ed of 11.09 Whkg−1 and Pd of 799 Wkg−1 at which displays a Cs of 1393 Fg−1 at 0.5 Ag−1, a high Ed (21.4 Whkg−1)
0.84 Ag−1. Rui et al. [122] have prepared the hydrated V2O5 na- at a Pd of 350 Wkg−1 with remarkable stability. Ma et al. [139] have
nosheets by sol-gel technique and used them in organic electrolyte ul- developed ZnCo2O4@MnO2 core-shell nanotube arrays as shown in
tracapacitors. Zhu et al. [123] have prepared 3-D nanostructures of Fig. 8, which possesses Cs of 1981 Fg−1 at 5 Ag−1. Also, an ASC with
V2O5 NS by the freeze-drying process and studied their symmetrical ZnCo2O4@MnO2 nanotubes on Ni foam as anode and porous Fe2O3 on
device behaviour. Nagaraju et al. [124] have reported the synthesis of Fe foil as cathode is fabricated which acquired a Cs of 161 Fg−1, Ed of
2D V2O5 nanosheets (NS) and rGO composite. The V2O5 and rGO/V2O5 37.8 Whkg−1 at 2.5 mAcm-2 and superior stability with 91% of the Cs
NS delivered a Cs of 253 Fg−1 and 635 Fg−1 and the corresponding Ed retention after 5000 cycles, in voltage window of 1.3 V. H2Ti3O7 na-
of 39 Whkg−1 and 79.5 Whkg−1 at a Pd of 900 Wkg−1 in an ASC. The notubes prepared by Yang et al. [140] when employed as an electrode
Ed is higher than reported for Ppy@V2O5 nanoribbon composite (32 in SC material with non-aqueous electrolyte, possessed the Cs of 414
Whkg−1, 900 Wkg−1) [125], V2O5 and PANI nanofibers (26.7 Whkg−1, Fg−1 at 0.5 Ag−1. The theoretical values of Cs and electrical con-
222 Wkg−1) [126], graphene composites of V2O5 nanowires and MnO2 ductivity of some TMOs are reported such as NiO (2584 Fg−1 [141],
nanorods (15.4 Whkg−1, 436 Wkg−1) [127] etc. MnO2 doped V2O5 0.01–0.32 Scm−1 [142]), MnO2 (1380 Fg−1 [143], 10−5–10−6 Scm−1
809
Fig. 8. (a) XRD of ZnCo2O4@MnO2 nanotube arrays (b) TEM images of ZnCo2O4@MnO2 nanotube arrays (c) EDS (electron diffraction spectroscopy) mapping of
individual ZnCo2O4@MnO2 nanotube arrays [139]. (Reproduced with permission from Ref. [139] Copyright Royal Society of Chemistry (2015)).
[144], V2O5 (2120 Fg−1, 10-4–10−2 Scm−1 [145]), Co3O4 (3560 Fg−1 retention at 50 mVs-1. Mao et al. [158] also reported the synthesis of
[146,147], 10−4–10−2 Scm−1 [148] and RuO2 (1200–2200 Fg−1 graphene/PANI nanofiber composites with Cs of 526 Fg-1 at 0.2 Ag-1.
[149], 103-1 Scm−1 [150]). Freestanding PEDOT-PSS/SWCNTs have been reported which exhibit
the Cs of 104 Fg-1 at 0.2 Ag-1, Ed of 7 Whkg-1, Pd of 825 Wkg-1 and 90%
of the Cs retention after 1000 cycles [159]. Snook et al. [15] prepared
3.4. Nanocomposite materials
PEDOT/PSS and CNTs composite that could reach Cs varying from 85
Fg-1 to 150 Fg-1; while the Ed could exceed 0.92 Whkg-1 and Pd could
Generally, the composites contain two or more materials in which
each individual component possesses its own properties (physical, range from 100 Wkg-1 to 3000 Wkg-1. Frackowiak et al. [160] reported
PEDOT/PSS and MWNT composites with a Cs value of 100 Fg-1. Han
chemical and mechanical). Nanocomposite electrodes incorporate
carbon materials into MOs or CPs and put together a non-faradaic et al. [161] have reported electrodes of PEDOT/PSS and GO in 1 M
(physical) and a faradaic (chemical) charge storage mechanism in a H2SO4 which yield capacitance of 108 Fg-1 with Cs retention of 78%
single electrode. High SSA is provided by the carbon materials and over 1200 cycles. Symmetric (PPy//PPy) and asymmetric (PPy//AC)
pseudocapacitive materials further increase the capacitance. CNTs act SCs have been prepared using Cladophora algae-derived cellulose as a
as a backbone for uniform distribution of MOs or CPs, producing high binder [162]. These SCs exhibit capacitance values ranging from 0.45 F
pseudocapacitance and electric double-layer capacitance. Such type of to 3.8 F. The rGO aerogels generally suffer from low Ed, small life cycle
electrodes attains higher Cs than individual carbon, MOs or CPs elec- and poor flexibility. Yang et al. [163] have prepared rGO aerogel-PANI
trodes [151]. composite by electro-deposition of PANI arrays on rGO aerogel which
possesses the usefulness of rich open pore and high conductivity of
cross-linked framework of 3D aerogel and high capacitance of PANI.
3.4.1. Carbon materials with CPs The prepared composite exhibits a specific capacitance of 432 Fg-1 at a
The AC cathode coupled with a CPs anode provides higher Ed and Pd current density of 1 Ag-1, the energy density of 25 Whkg-1, 85% capa-
than EDLCs and improved cycling performance than pseudocapacitors citance retention after 10,000 cycles with outstanding stability in dif-
[151]. A CNT hydrogel with PANI had a Cs of 680 mFcm−2 at 1 ferent bending conditions.
mAcm−2 [152]. Jaidev and S. Ramaprabhu have prepared poly (phe-
nylenediamine) (PpPD) and hydrogen exfoliated graphene (HEG) sheets
which shows a Cs of 248 Fg−1 at 2 Ag−1 [153]. Also, an ASC is fabri- 3.4.2. Carbon-based nanomaterials with MOs
cated which exhibits an Ed of 8.6 Whkg−1 at a Pd of 0.5 kW kg−1. TMOs have low electronic conductivity, poor Cs and low electro-
Li et al. [154] have synthesized PANI nanorods on graphite NS chemical stability. To improve its performance, nanostructured TMOs
which exhibit a Cs of 1665 Fg−1 at 1 Ag-1. Zhao et al. [155] synthesized are mixed with the carbon material to make composites. This combi-
graphene-based PVA composites with 150% better tensile strength and nation of MOs and carbon is useful for high-performance SCs. Various
approximately 10 times increase in Young’s modulus with graphene hybrid materials such as Co3O4/graphene [164,165], Co3O4/CNTs
loading of 1.8 vol %. Yu et al. [156] prepared PANI/eCFC (etched [166], and Co3O4/CNFs [167] with improved electrical conductivity
carbon fibre cloth) composite with Cs of 1035 Fg-1 at 1 Ag-1, 88% ca- and a huge surface area have been prepared. Wang et al. [57] have
pacity retention. Wang et al. [157] have investigated the GNS/PANI reported a NiO/GF hybrid electrode which shows a Cs (1225 Fg−1 at 2
composite which shows a Cs of 532.3 Fg-1 at 2 mVs-1, 99.6% Cs Ag−1). An ASC has been synthesized using NiO/GF as anode and
810
Fig. 9. (a) CD curves of electrodes based on CNS@5%GR nanocomposite (b) Cs comparison of CNS/GR nanocomposite, CNS nanoparticles and GR at diverse current
densities (c) Cyclic performance and (d) CD curves for the last 20 cycles at 4 Ag−1 current density of CND@5%GR nanocomposite [32]. (Reproduced with permission
from Ref. [32] Copyright Royal Society of Chemistry (2014)).
HPNCNTs as a cathode in the KOH solution which exhibits an Ed of 32 by a hydrothermal method which show the Cs ˜1400 Fg−1, higher Ed
Whkg-1 at a Pd of 700 Wkg−1 with 94% Cs retention after 2000 cycles. (30 Whkg−1) and Pd (39 kW kg−1) at 100 mAcm-2 with no loss of Cs
Li et al. [168] presented CNTs@NCS@MnO2 composites which had Cs after 3000 cycles. Ratha et al. [182] have reported patronite hybrid,
of 312.5 Fg-1 at 1 Ag-1 with good cycling stability (92.7% Cs retention VS4/rGO, which shows Cs of ̴ 877 Fg−1 at 0.5 Ag−1, an Ed of 117
after 4000 cycles). An ASC was designed with CNTs@NCS@MnO2 (+ve Whkg−1 and Pd of 20.65 kW kg−1. Lu et al. [183] constructed an
electrode) and ACs (-ve electrode) which possessed a high Cs with a aqueous sodium-ion ASC by using Mn hexacyanoferrate (HCF) as –ve
stable PW of 1.8 V, Ed of 27.3 Whkg-1 at Pd of 4500 Wkg-1. Perera et al. electrode and Fe3O4/rGO nanocomposites as + ve electrode with an
[169] have assembled a coin cell type ASC using V2O5-CNT as an anode extended PW of 1.8 V which exhibited Cs of 96 Fg−1, higher Pd (2183.5
and carbon fibre as a cathode which delivers an Ed of around 46.3 Wkg−1) and Ed (27.9 Whkg−1). Guan et al. [184] have presented
Whkg-1 and a Pd of 5.26 kW kg-1. Du et al. [32] constructed the needle-like Co3O4 deposited on graphene as the supercapacitor material
CoNi2S4@graphene (CNS@GR) nanocomposite showing Cs 2009.1 Fg-1 with Cs of 157 Fg−1 at 0.1 Ag−1.
at 1 Ag-1 and the Cs could be retained at 755.4 Fg-1 (4 Ag-1) after 2000 Liu et al. [185] prepared cobalt-based nanoparticle on mesoporous
cycles. Fig. 9 shows the CD curves and cycle performance of CNS@GR carbon nanospheres. The synthesis technique used was the ‘colloidal
nanocomposites. amphiphile template oxidative polymerization of dopamine’. Poly-
Many single-phase materials or nanocomposites of NiXCo3-XO4 have dopamine possesses enough binding sites to coordinate metal ions. As
been prepared with superior energy storage properties e.g. nanosheets prepared composite exhibited very fine size, porous structure, complete
[170], nanoneedles [171] and nanowires [172] of NiCo2O4, usage of conductive carbons and manageable chemical compositions
Ni0.3Co2.7O4 hierarchical structures [173], NiCo2O4/(CNTs) [174] and which provided high rate capability with long-term cyclic stability. Wu
NiCo2O4/graphene oxide (GO) [175] etc. Yuan et al. [176] reported et al. [186] have prepared hybrid structure consisting of graphene-en-
NiCo2O4 nanosheets with Cs of 1450 Fg−1 at 20 Ag−1. Ternary NiCo2S4 capsulated carbon and Ni-Al layered double hydroxide which shows a
also offer richer redox reactions, as ternary NiCo2S4 have a higher high Cs (1710.5 Fg−1 at 1 Ag−1) and Ed of 35.5 Whkg−1 at a Pd of 670.7
electronic conductivity than NiXCo3-XO4 which can reduce the charge Wkg−1 at 1 Ag−1. Sahoo et al. [187] have prepared ZnCo2O4/rGO/NiO
transfer resistance, resulting in a smaller interior resistance (IR) loss at composite on Ni foam which possesses the Cs of 1256 Fg−1 at 3 Ag−1,
higher current density. Hence, a greater rate capability and Pd can be higher Ed of 62.8 Whkg−1, Pd of 7492.5 Wkg−1 and low ESR (0.58 Ω).
achieved [177]. The synthesis of NiCo2S4 nanotubes through sacrificial The composite retained 80% of Cs after 3000 charge-discharge cycles.
templates with Cs of 933 Fg−1 at 1 Ag−1 has been reported [178]. The effect of concentration of electrolyte on the electrochemical be-
Urchin-like NiCo2S4, prepared by a precursor transformation method, haviour has also been examined.
with Cs of 1050 Fg−1 at 2 Ag−1 has been reported [179]. However, the Table 2 compares supercapacitor electrodes of carbon and its
two-step transformation method increased the preparation cost. Hence, composites in terms of various parameters such as pore size, surface
composite of NiCo2S4 nanosheets/graphene is again produced by hy- area, Cs, rate capability, stability and cost. It is clear from the table that
drothermal method with a Cs of 760 Fg−1 at 20 Ag−1 [180]. for obtaining a good performance supercapacitor, composites of carbon
Xiong et al. [181] have prepared Ni-Co-Mn hydroxide nanoneedles materials and MOs is a suitable candidate with high Cs, high rate
811
Table 2
Comparison of carbon and its composite as SC electrodes [177].
Electrode material Pore size SSA Cs Rate capability Stability Cost
Carbon Pore size can be designed High Low High Good Low
MOs Difficult to tailor Low High Low Poor High
Carbon-MOs composite Pore size can be tailored Controlled by the carbon material High Good Good Moderate
capability and stability with moderate cost. Fe3C/Fe as the + ve and -ve electrodes which possesses the Ed of 89
Whkg−1 at Pd of 1.1 kW kg−1 and retained 91% of Cs after 2500 cycles.
3.4.3. MOs and CPs composites Xie et al. [245] have designed C-LiFePO4 over titanium nitride substrate
The composite of MOs and CPs may give an improved electro- as the electrode for a Li-ion supercapacitor which exhibits the Cs of 972
chemical performance in SCs due to compatibility between them Fg−1 at 1.0 Ag−1, presenting a Cs improvement of 210% in comparison
[188,189]. Therefore, many efforts have been done to develop MOs to 314 Fg−1 for LiFePO4/TiN. The C-LiFePO4/TiN nanowires exhibit
embedded with CPs that increase the conductivity of electrodes very excellent cyclic stability with a 3.7% loss of Cs after 400 cycles. Table 5
much by enhancing the Cs, rate capability and cyclic stability. Liu et al. shows the summary of some of the reported asymmetric/symmetric
[190] fabricated PPy coated MoO3 by in-situ polymerization which SCs.
showed Cs of 110 Fg−1 at 100 mAg−1 and Ed of 20 Whkg−1 at Pd of 75
Wkg−1. The ASC designed with PPy@ MoO3 as the + ve electrode and 5. New materials
AC as the –ve electrode in 0.5 M K2SO4 aqueous solution exhibited an Ed
of 12 Whkg−1 at 3 kW kg−1. Raj et al. [191] prepared Co3O4-Pind 5.1. MOFs
which achieved Cs of 1805 Fg−1 at 2 Ag−1 and Cs of 1625 Fg−1 at 25
Ag−1. Table 3 shows some of the latest fabricated SC electrodes (non- TMOs, despite exhibiting attractive properties as good electrical
flexible) with their various parameters such as Cs, Ed, Pd, capacitance conductivity, superior electrochemical response, less manufacturing
retention etc. Table 4 shows the summary of flexible planar SC elec- cost and simple processability, have limited practical use due to decline
trodes/devices. in their capacitive response after some time during continuous Faradaic
reactions. Hence, there is a requirement of new synthesis techniques,
4. Asymmetric supercapacitors (ASCs) which could provide stable porous structures and control over phase
with dimensions of MOs for getting better capacitive performance.
ASCs have superiority over symmetrical SCs as these SCs use the Nowadays, MOFs have gathered much attention as templates for the
faradaic active electrode, which significantly contributes to the pseu- synthesis of porous MOs and nanocomposites of porous carbons and
docapacitance, besides increasing the PW, along with EDLC electrode metal/metal oxides [264] first developed by Yaghi et al. in 1995 [265].
which supplies high power. Moreover, the longer discharge times and MOFs have been prepared by joining inorganic and organic units via
dissimilar discharge profiles provide higher Ed and Pd to ASCs [237]. As strong chemical bonds. The polyvalent organic carboxylates, when as-
ASCs combine the advantages of both pseudocapacitive electrode and sociated with metal-containing units, can yield three-dimensional
the capacitive electrode, so ASCs can provide higher Ed than symmetric structures which have well-defined pore size distributions and large
SCs while maintaining cyclic and rate performance [238,239]. Wang SSA (1000-10,000 m2 g−1). Transition metals (eg Zn, Co, Cu, Fe, Ni),
et al. [240] fabricated a non-aqueous ASC from two spherical materials: alkaline earth elements (eg Ba, Sr), p-block elements (eg In, Ga) and
an activated mesocarbon microbead-AMCMB (-ve electrode) and MnO2 mixed metals are used for the development of MOFs [266]. The an-
nanowire-sphere (+ve electrode) over a voltage range (0.0–3.0 V) nealing time and temperature variation can control the composition,
using 1 M Et4 NBF4 in acetonitrile as electrolytes. The AMCMB/MnO2 pore size and SSA of MOF derived oxides. MOF based oxides can be
supercapacitor explored a Cs of 228 Fg−1 and Ed of 128 WhKg−1 at 10 combined with different carbon-based materials such as rGO, CNTs,
mVs−1. An asymmetric high voltage SC (1.9 V) had been produced graphene etc. to enhance their electrochemical performance. Few of the
[241] using AC as the -ve electrode and a silicon carbide-MnO2 (SiC-N- porous TMOs obtained from MOFs are cupric oxide, zinc oxide, iron
MnO2) composite as the + ve electrode in Na2SO4 electrolyte solution oxide, nickel oxide, cerium oxide, cobalt oxide, titanium dioxide,
having Cs of 59.9 Fg−1 at 2 mVs−1 and Ed of 30.06 Whkg−1 and Pd of manganese oxide, magnesium oxide. Also, MOFs may be favourable for
113.92 Wkg−1 with an approximate 3.1% Cs loss after 1000 charge- the synthesis of mixed MOs and their composites, such as Co3O4
discharge cycles. /NiCo2O4, Co3O4/ZnFe2O4, CuO/Cu2O, Cu/Cu2O@TiO2, CuO@NiO,
An ASC using Cu2O as + ve electrode and AC as –ve electrode ex- Fe2O3@TiO2, Fe2O3/NiCo2O4, NiFe2O4/Fe2O3, ZnO@ Co3O4, ZnO/
hibited an Ed of 20.04 Whkg−1 with an extended PW of 1.65 V and ZnFe2O4 [267]. Thus, these materials can be employed for manu-
retained 93.3% capacitance after 5000 cycles [51]. As-prepared flexible facturing SCs electrodes because of the multiple functions and high SSA,
all-solid-state ASC illuminated 52 red coloured LEDs using four charged but still face some major hurdles, such as low electrical conductivity at
devices in series. A flexible solid-state ASC has been fabricated by Wang higher charge-discharge rates, short cycle life at higher rates, short-
et al. [54] using Co3O4 flakes and γ-Fe2O3 nanoparticles as electrodes comings in the diffusion distance of the electrolyte within porous MOs
which delivers a high Ed of 38.1 Whkg−1 along with extended PW of owing to high crystallinity [268].
1.7 V. Also, the fabricated device illuminated 52 LEDs, for at least The capacitive performance of MOF-derived MOs can be enhanced
7 min, along with good charge-discharge behaviour under different by: (a) mixing the MOF-derived MOs with conductive carbon materials
bending conditions. Wu et al. [242] reported an Ed of 30.4 Whkg−1 and as rGO, CNTs, graphene etc. along with a secondary metal oxide,
Pd of 5 kW kg−1 using graphene/MnO2/graphene hybrid cells. An ASC thereby improving the electrical conductivity (b) increasing the SSA of
with Carbon Quantum Dots (CQDs)/NiCo2O4 composite as + ve elec- MOF-derived MOs by heating the MOF precursors under N2 atmosphere
trode and the AC as –ve negative electrode has been designed which prior to heating them in air, which is beneficial for stopping the fast
possesses a Cs of 88.9 Fg−1, Ed of 27.8 Whkg−1, Pd of 128 Wkg−1, great release of volatile gases which would have resulted in the collapse of
cycling stability (101.9% of Cs retention over 5000 cycles) and high the frameworks (c) optimizing the pore size of MOF-derived materials
coulombic efficiency (η) of almost 100% during the cycling process with size of electrolyte ion by matching the heating conditions so that
[243]. Hadi et al. [244] have fabricated an ASC with Ni4.5Co4.5S8 and g- the ions can enter in to the pores of MOs up to larger distances which
812
Poonam et al.
Table 3
Summary of the latest fabricated SC electrodes.
Electrode material Electrolyte Preparation method Cs (Fg−1) Ed (Whkg−1) Pd (kWkg−1) Retention (cycles) Ref
Porous Au/MnO2 – – 1145 at 50 mVs−1 – – 80%(500) [192]

Ni Co2O4 nanoneedle arrays – – 1118.6 – – 89.4%(2000) [193]
CuO@ AuPd@MnO2 core-shell Whiskers 1 M KOH Electrodeposition 1376 at 5 mVs−1 0.55 mWhcm−3 413 mWcm−3 99%(5000) [194]
Ni0.61Co0.39oxide on Ni foam – Electrodeposition 1523 at 2 Ag−1 36.46 0.142 95.3%(1000) [195]
CoO-PPY on 3D Ni foam – – 2223 at 1 mAcm−2 – – 99.8%(2000) [95]
Mn/MnO2 core-shell 3D porous structure – Electrodeposition 1200 at 5 mVs−1 – – 96%(2000) [196]
VA-CNT-graphene with Ni(OH)2 coating 2 M KOH – 1065 at 22.1 Ag−1 – – 96%(20,000) [197]
B-Ni(OH)2/GO /CNTs – Phase transformation method 1815 at 2 Ag−1 – – 97%(2000) [198]
Ni Co2O4 – Hydrothermal method with annealing at 1393 at 0.5 Ag−1 21.4 0.35 – [138]
300 °C
CoNi2S4/ grapheme – – 2009.1 at 1 Ag−1 – – – [32]
Ni-Co-Mn triple hydroxide (NCMTH)/(GPs) – Hydrothermal method 1400 30 39 ˜100% (10,000) [181]
graphitic petals
Ni(OH)2-MnO2-rGO – – 1985 54 – – [199]
Ni(OH)2/rGO on Ni foam – Hydrothermal method 3328.7 at 1.5 Ag−1 15.65 Fcm−2 – – 90.6%(5000) [200]
N-CNF/N-CNF and Ni(OH)2 6 M KOH Electro-spinning 1045 51 117 84%(5000) [56]
Co(OH)2 –NPG – – 1800 – – – [201]
Ni-CO-BH (binary hydroxide)/rGO – SILAR method 2130 at 2 Ag−1 92 7.0 80%(10,000) [31]
813
GF/Ni foam/Co(OH)2 0.08 M K3Fe(CN)6/ 1 M – 7514 at 16 Ag−1 – – – [202]
KOH
Co3O4 /NH2-GS – Hydrothermal method 2108.4 at 1 Ag−1 1356.7 at 15 Ag−1 59.3 .225 – [203]
Ni-Mn LDH/rGO – Co-precipitation method 1635 at 1 Ag−1 33.8 (at potential – – [204]
1.7 V)
nickel-based metal organic – – 1698 at 1 Ag−1 – – 94.8%(1000) [205]
frameworks (MOFs)
−1
Manganese molybdate nanosheet/Ni foam – Hydrothermal 1271 at 5mVs 31.6 .935 84.5%(2000) [206]
NiO/LaNiO3 – Spin coating 2030 at 0.5 Ag−1 – – 83%(1000) [22]
nanopore NiCo2O4 – Sol-gel 2157 (mass Cs) 40.6 mFcm−2 (areal – – 96.5%(10,000) [21]
capacitance)
Ni3S2/NiCo2O4 – – 4569.1 mFcm−2 at 1mAcm-2 – – 89.2%(1000) [207]
Co3O4 NCs – – 1913 at 8 Ag−1 – – – [208]
V2O5 nanosheets/rGO 1 M KCl – 635 at 1 Ag−1 – – – [124]
RuO2 decorated TiO2 nanotube – – 1263 – – – [209]
VS4/Rgo – Hydrothermal synthesis 877 at 0.5 Ag−1 117 20.65 – [182]
Co3O4/ polyindole 1 M KOH In situ cathodic electrodeposition 1805 at 2 Ag−1 – – 83%(1000) [191]
NiO/GF – Pulsed laser deposition 1225 at 2 Ag−1 – – 89%(1000) [57]
NixZn1-xS 3 M KOH Hydrothermal method 1867 at 1 Ag−1 – – 77.4%(1000) [120]
ZnO@Ni3S2 – Electrodeposition 1529 at 2 Ag−1 – – 42%(1000) [25]
ZnCo2O4/rGo/NiO 6 M KOH 1256 at 3 Ag−1 62.8 7.4925 – [187]
Ni(OH)2/CNS – – 2218 at 1.0 Ag−1 – – – [210]
Poonam et al.
Table 4
Summary of the fabricated flexible planar SC electrodes/devices.
Electrode material Electrolyte Preparation method Cs (Fg−1) Ed (Whkg−1) Pd (kWkg−1) Retention (cycles) Ref
−1
CNT-graphene films 1 M H2SO4 Drop casting, vacuum filtration, air brush ˜140 at 0.1 Ag – – – [211]
spraying
−1
rGO/carbon black PVA/H2SO4 – 79 at 1 Ag – – – [212]
Carbon black pillared graphene film 1 M H2SO4 vacuum filtration 215 at 0.1 Ag−1 – 414 97%(10,000) [213]
Functionalized rGO film Nafion vacuum filtration 118.5 at 0.1 Ag−1 – – 90%(1000) [214]
Macroporous graphene film – Hard template, vacuum filtration 92.7 at 3 mVs−1 – – – [215]
Graphene/PANI composite paper 1 M H2SO4 Vacuum filtration& electropolymerization 233 at 2 mVs−1 – – – [216]
Graphene/PANI hybrid paper 1 M H2SO4 Vacuum filtration& polymerization 489 at 0.4 Ag−1 – 33.9 96%(500) [217]
NiO-graphene 3D networks 3 M KOH CVD & sacrificial template ˜816 at 5 mVs−1 – – ˜100%(2000) [218]
Graphene-MnO2 -CNTs nanocomposite films 1 M Na2SO4 Co-precipitation & vacuum filtrarion 372 2.2 42 ˜95%(1000) [219]
Embossed rGO-MnO2 hybrid films 1 M Na2SO4 Vacuum filteration & sacrificial template 389 at 1 Ag−1 44 25 95%(1000) [220]
PPy/graphene KCl – 237 at 0.01 Vs−1 – – – [221]
Functionalized graphene hydrogel 1 M H2SO4 Hydrothermal treatment 441 at 1 Ag−1 – – 86%(10,000) [222]
3D N&B co-doped graphene hydrogel 1 M H2SO4 Hydrothermal treatment 239 at 1 mVs−1 8.7 (all solid state) 1.65(all solid state) 100%(1000) [223]
574 93%(10,000) [224]
814
PANI-oriented graphene hybrid film – Vacuum filtration & in situ polymerization – –
Graphene - PANI composite 1 M H2SO4 Coating & electropolymerization 763 at 1 Ag−1 – – 82%(1000) [225]
3D graphene -MnO2 composite networks 0.5 M Na2SO4 CVD & electrochemical deposition 465 at 2 mVs−1 6.8 2.5 81.2%(5000) [226]
Graphene-MnO2 nano structured textiles 0.5 M Na2SO4 Dip drying & electrochemical deposition ˜315 at 2 mVs−1 12.5 110 ˜95%(5000) [227]
Co-Al LDH/rGO films 1 M KOH Layer by layer assembly 1240 at 5 mVs−1 (90 – – 99%(2000) [228]
mFcm-2)
Graphene- MnO2 nanostructured sponges 1 M Na2SO4 Dip-drying ˜450 at 2 mVs−1 8.34 94 90%(10,000) [229]
PANI nanowire-carbon cloth – – 1079 at 1.73 Ag−1 – – 86%(2100) [230]
PANI nanowire arrays-Au coated PET films – – 588 F cm−3 – – – [231]
Ultra-thin MnO2 / Zn2SnO4 nanowire-carbon microfibers – – 621.6 at 2 mVs−1 – – 98.8%(1000) [232]
PANI nanoparticles-carbon nanofiber – – 638 at 2 Ag−1 – – 90%(1000) [233]
Ni(OH)2 nanosheet-graphene – – 660.8 F cm−3 – – 98.2%(2000) [234]
NiCo2O4@ polypyrrole core-shell nanowire on hemp-derived carbon – – 2055 17.5 0.5 90%(5000) [235]
(HDC) microfiber
3D rGO-F/ PANI – Dipping and drying method 790 17.6 98 80%(5000) [58]
NiAs-type Cobalt sulphide – Hydrothermal method 867 – – [236]
Cu2O 1 M KOH Dealloying method 210.9 at 0.5 Ag−1 94.5%(5000) [51]
Co3O4 flakes 1 M KOH Dealloying method 410 at 0.7 Ag−1 – – 80.5%(5000) [54]
γ – Fe2O3 nanoparticles 1 M KOH Dealloying method 187 at 0.7 Ag−1 – – 93.2%(5000) [54]
Table 5
Summary of some of the reported asymmetric/symmetric SCs.
Electrodes Cs (Fg−1) Ed (Whkg−1) Pd (kWkg−1) Capacitance retention Ref
CNT/PANI//CNT/MnO2/GR, by vacuum filtration method in 1 M – 24.8 at 1.6 V – – [246]

Na2SO4/PVP
e-CMG/MnO2//e-CMG in 1 M Na2SO4 – 44 11.2 95% after 1000 cycles [247]
MnO2/OCN/PVDF//MnO2/OCN/PVDF 363.28 64.39 3.87 [248]
Co2AlO4@MnO2 nanosheet//Fe3O4 nanoflakes 99.1 35.3 0.8001 92.4% after 5000 cycles [249]
24.11 8.033
MnO2 -V2O5//AC(activated carbon) in 0.5 M K2SO4 61 8.5 – – [128]
NiO-GF//HPN CNTs in 1 M KOH 116 at 1Ag−1 17 42 94% after 2000 cycles [57]
NiO//porous carbon – 11.6 0.028 – [250]
Ni(OH)2 nanosphere//AC 120 at 4.8 Ag−1 – – – [251]
Ni0.67 Co0.33 Se//RGO 176 at 1Ag−1 36.7 0.750 [142]
Ni Co2 O4 -RGO//AC – – – 83% after 2500 cycles [252]
Ni-Co oxide//AC – 7.4 1.9 85% after 2000 cycles [253]
V2O5 NS//RGO 95 at 1Ag−1 39 0.900 92% after 3000 cycles [124]
rGO - V2O5 NS//RGO 195 at 1 Ag−1 75.9 0.900 94% after 3000 cycles [124]
Ni Co2 O4//AC in 2 M KOH 135 at 1 Ag−1 21.4 0.350 95.6% after 1000 cycles [138]
Ni Co2 O4 - MnO2//activated graphene – 5.8 2.5 – [254]
Ni Co2 O4//AC – 6.8 2.8 – [255]
Ni-Zn-Co oxide/ hydroxide//AC – 16.62 2.9 – [256]
ZnCo2O4@MnO2// Fe2O3 in 1 M KOH by hydrothermal method 161 at 2.5 mAcm−2 37.8 0.648 91% after 5000 cycles at 20 [139]
Ag−1
NiO//AC 73.4 15 0.447 – [257]
Ni Co2 O4 @MnO2//AC – 35 0.163 – [258]
Co3O4@MnO2//AC – 17.7 0.600 – [259]
Ni-Co sulphide//AC – 25 0.447 – [260]
CNT@NCS@MnO2//AC in 1 M Na2SO4 – 27.3 4.5 at 1.8 V 78.8% after 2000 cycles [168]
H2Ti6O13//CMK-3 (mesoporous carbon) – 90 11 – [140]
PPy@MoO3//AC in 0.5 M K2SO4 – 12 3 – [190]
Mn HCF//Fe3O4@rGO 96 at 1 mAcm−1 27.9 2.183 at 1.8 V 82.2% after 1000 cycles [183]
CNT/graphene//Mn3O4/graphene 72.6 at 0.5 Ag−1 22.9 9 86% after 10,000 cycles [261]
MnO2// MnO2 26 mF cm−2 at 0.7 V – – – [262]
Ru//Ru 68 mF cm−2 at 1 – – – [115]
mAcm−2
Cd(OH)2//Cd(OH)2, by chemical bath deposition in 1 M NaOH 51 at 5mVs−1 11.09 0.799 at 0.84 Ag−1 – [121]
HPCNTs//HPCNTs in 1 M LiPF6 139 at 1 Ag−1 37.9 0.700 at 1 A g−1 90.6% after 4000 cycles [59]
RGO//RGO 6.8 49.8 – [263]
Ni-Mn LDH/rGO//AC 82.26 at 1Ag−1 33.8 at 1.7 V – – [204]
MnMoO4.nH2O//AC – 31.6 0.935 – [206]
CNG@NCH//rGO – 78.75 0.473 – [213]
RuO2-NPG//Co(OH)2-NPG 350 120 – – [21]
N-CNFs/Ni(OH)2//N-CNFs – 51 117 84% after 5000 cycles [22]
Ni-Co-BH-G//CCN 340 92 7 – [31]
Ni(OH)2-CNS//AC 198 56.7 4.0 93% after 10,000 cycles [217]
CoWO4/Co1-xS4//AC 103.1 22.5 4 87.27% after 5000 cycles [41]
Cu2O//AC by dealloying method 53 at 0.5 Ag−1 20.04 7.1 93.3% after 5000 cycles [51]
Co3O4 flakes//γ-Fe2O3 nanoparticles by dealloying method 94.7 at 0.7 Ag−1 38.1 – 80.1% after 5000 cycles [54]
can be made feasible by reducing the crystallinity of material [268]. was reported by Yaghi and co-workers in 2005, utilizing boronate ester
Wang et al. [264] reported MOF (polyhedral ZIF-8) for SC appli- with boroxine linkages. It resulted in porous materials with good
cation with enhanced values of Ed, Pd and cycling life in comparison to crystallinity [272]. The COFs are synthesized by using solvent condi-
current hybrid capacitors. Kaur et al. [269,270] assembled graphene- tions that result in a suspension or slurry [273]. By designing the sol-
MOF composite on TiO2/FTO substrate and QD-MOF (quantum dot- vent conditions, the COF formation rate can be obtained by turbidity
MOF) nanocomposite. The 1-D and 2-D materials can facilitate easy measurements. In this study, the COF formation rate is related to the
highways for ion intercalation and deintercalation, thus improving aromatic stacking capacity of the monomer. Another report explored
charge-discharge. Thus, higher Ed and Pd may be obtained by using 3-D the effect of dihedral angles among aromatic rings of the monomers on
materials which can supply many reaction sites in their 3-D networks. the crystallinity and porosity in COFs [274]. However, the lesser elec-
But, very few methods provide control over the dimensions of the trical conductivity and less stability of COFs put hurdles in their prac-
prepared metal oxides which illustrates the significance of MOF-derived tical application. A strategy has been planned to eradicate these lim-
materials. Hence an extensive understanding of the charge-discharge itations by confining conducting polymers within porous frameworks
process of MOF-derived MOs is needed to get better electrochemical which results in improved cycle stability along with maintaining elec-
performance. trical conductivity and mechanical stability.
Recently, Dichtel et al. [275] have developed this process by using
PEDOT-modified COF (DAAQ-TFP COF) to obtain very high-rate char-
5.2. COFs
ging with higher Ed. Xu et al. [276] have explained that the PEDOT-
modified DAAQ-TFP COF films possess a better current response in CV
COFs, novel microporous materials, which have molecularly or-
than pristine DAAQ-TFP COF film which can be ascribed to the wiring
dered structures formed by the covalent linking of organic building
effect of PEDOT chains. However, ion transport is improved by vertical
blocks, are gaining importance because of their well-defined structures,
pore channels by reducing the diffusion length. Chandra et al. [277]
high porosity, versatile molecular design and precise control over the
reported COFs [TpPa-(OH)2, TpBD-(OH)2] for SCs. Fig. 10 shows the
placement and character of redox-active groups [271]. The first COF
815
Fig. 10. Comparision of XRD pattern for (a) TaPa-(OH)2 (b)TpBD-(OH)2 and (c) TaPa-(OMe)2; Inset of (a), (b), and (c) shows the pore structure and ᴫ-ᴫ stacking
distance for COFs Comparision of N2 adsorption isotherms at 77 K for (d) TaPa series and (e) TpBD series (f) Pore size distribution and (g) TEM images for TaPa-
(OH)2, TpBD-(OH)2, TaPa-(OMe)2, and TpBD-(OM)2 [277]. (Reproduced with permission from Ref. [277] Copyright American Chemical Society (2017)).
XRD, N2 adsorption curves, pore size distribution and TEM images of MnO2on MXene nanosheets (ε-MnO2/Ti2CTx and ε-MnO2/Ti3C2Tx) by
COFs. TpPa-(OH)2 exhibited Cs of 416 Fg−1 at 0.5 Ag−1 with 66% Cs chemical synthesis. The ε-MnO2 nanocrystalline whiskers enhance the
retention after 10,000 cycles. High Cs is due to the exact molecular SSA of the nanocomposite electrode and hence improve the Cs by ap-
control of redox functionalities in the COF. Han et al. [278] fabricated proximately three times in comparison to that of pure MXene-based
nanocoatings of COFs on Ni NWs which exhibited Cs of 314 Fg−1 at 50 symmetric SCs. The fabricated ε-MnO2/MXene SCs possess good cycling
Ag−1 with 74% of the Cs retention at 2 Ag-1. The high current density stability (∼88% of the Cs retention after 10,000 cycles). Hu et al. [286]
made the charge-discharge phenomenon very rapid. Kim et al. [271] studied Ti3C2Tx MXenes obtained by etching Ti3AlC2 in HF aqueous
fabricated N-doped carbon by carbonization of COFs through an azine- solution (at different concentrations-6 M and 15 M) as shown in Fig. 11.
linked 2-D network (ACOF1). In ACOF1, micropores (diameters < 1 A higher Cs had been obtained in 6 M HF-etched MXene (Ti3C2Tx-6 M).
nm) have been formed with high SSA (1596 cm2 g-1). The Cs of car-
bonized ACOF1 is 234 Fg-1 at the 1.0 Ag-1 which is greater than the 5.4. Metal nitrides (MNs)
carbonized COF1 (191 Fg-1). Romero et al. [279] fabricated COFs of
polyimine with many metal ions (FeIII, CoII, and NiII). MNs have captured the interest as SC electrode material due to their
good electrochemical properties, high chemical stability and standard
5.3. MXenes technological approach [287]. Zhu et al. [288] grew two MNs: TiN
porous layers and Fe2N nanoparticles on vertical-aligned graphene
MXenes (2-D inorganic compounds) were first developed in 2011 by sheets and used as the electrodes for solid-state SCs. Das et al. [289]
Yury Gogotsi [280] and comprise few atom deep layers of transition prepared MNs (M = Co, Cr) nanoparticles (particle size approximately
metal nitrides, carbides or carbonitrides. MXenes provide an exclusive 20–30 nm) in NH3 + N2 atmosphere at small temperature. The Cr-urea
amalgamation of conductivity and hydrophilicity (because of their complex directly changes to CrN, however, CoN has been obtained from
hydroxyl surfaces) along with the superior mechanical properties. Co3O4. The ASC fabricated using MNs and AC as electrodes shows high
MXenes have the general formula Mn+1AXn, where M is a transition Cs of 37 and 75 Fg−1 for M = Co, Cr, respectively at 30 mA g−1.
metal, A is group 13 or 14 element, X is C and/or N and n = 1, 2, or 3.
These are produced by selective etching of element A from their 3D 5.5. Black phosphorus (BP)
layered MAX phase. So far, approximately 20 MXenes have been pro-
duced which have shown superior Cs for the reversible intercalation of BP (newest members in the 2D material family), has recently
metal cations (eg. Li+, Na+, K+, Mg2+, Al3+, etc.) [281]. gathered much attraction due to its higher theoretical Cs (2596
Fu et al. [282] prepared a flexible paper electrode using layered 2D mAhg−1), distinct structures with corrugated planes of P atoms which
Ti3C2Tx which achieved a high volumetric capacitance of 892 F cm−3 are linked by strong interlayer PeP bonding and weak interlayer
along with excellent cycling performance (no capacitance loss after Vander Waals forces [290]. The bulk BP may be converted into thin
10,000 cycles). Yarn SCs are fabricated using MXenes and PEDOT-PSS sheets (few layers to even a single layer) by breaking the weak inter-
which show excellent stability and device performance even during actions. Few layered BP (phosphorene) has a direct band gap of
bending and twisting [283]. Shah et al. [284] demonstrated the scrol- 0.3 eV–2.2 eV which can be controlled by the number of layers. Also, BP
ling, bending and folding of Ti3C2Tx nanosheets into 3D crumpled has an interlayer spacing of 5.3 Å, greater than graphite (3.6 Å) and
structures and also the change was found to be reversible on rehydra- comparable to that of 1 T MoS2 phase (6.15 Å) [291]. Hao et al. [292]
tion. Rakhi et al. [285] reported the formation of nanocrystalline ε- fabricated flexible SC using liquid-exfoliated BP nanoflakes which
816
Fig. 11. (a) Normalized weight of Ti3C2Tx-6 M and Ti3C2Tx-15 M dried at different temperatures, Inset shows the schematic of Ti3C2Tx-6 M and Ti3C2Tx-15 M dried at
room temperature (b) (0002) peaks of XRD pattern shows the shrinkage of interlayer spacing upon drying at 120 °C for Ti3C2Tx-6 M and Ti3C2Tx-15 M (c) 1H NMR
spectra of as-synthesized MXenes and those dried at 120 °C overnight [286]. (Reproduced with permission from Ref. [286] Copyright American Chemical Society
(2018)).
delivered Cs (volumetric) of 13.75 Fcm-3, Pd of 8.83 Wcm-3 and Ed of the color change of WO3 so e-WO3 can be integrated with other ma-
2.47 mWhcm-3 and a very long lifespan of over 30,000 cycles. terials to form a smart hybrid SC.
Yang et al. [293] fabricated all-solid-state SC using flexible BP na-
noflake/CNT composite paper as electrodes. CNTs increase electrolyte
shuttling and forbid the restacking of BP nanoflakes. The prepared SC 6.2. Battery-supercapacitor hybrid (BSH) device
with BP/CNTs (ratio 1:4) exhibited Cs (volumetric) of 41.1 Fcm−3 at
0.005 Vs-1, a high Pd of 821.62 Wcm−3, high Ed of 5.71 mWhcm−3, BSH devices are of immense interest due to their future applications
excellent mechanical flexibility and high cycle stability (91.5% Cs re- in smart electric grids, electric vehicles and miniaturized electronic-
tention after 10,000 cycles). Chen et al. [294] have synthesized a het- optoelectronic devices etc. Along with traditional Pb-acid, Ni-MH, Ni-
erostructure of BP and red phosphorus through the sonochemical pro- Cd, Li-ion batteries (LIBs), several advanced batteries such as Li-air, Li-
cess. The Cs of BP/Red Phosphorus hybrid is approximately 60.1 Fg-1 sulfur, Na-ion, Al-ion batteries and aqueous metal ion batteries are
retaining 83.3% of Cs after 2000 cycles. emerging. Energy storage using a high Cs battery-type electrode and
high Pd capacitive electrode called BSH provides a potential way to
fabricate a device with the qualities of both batteries and SCs [298].
6. New Devices/Applications for SCs In Li-ion BSH, the SC electrode materials are carbon materials (such
as ACs, CNTs, graphene etc.) and battery material are MOs, intercala-
6.1. Electrochromic SC tion compounds, and their composites. Zheng et al. [298] combined
EDLC type + ve electrode with battery type -ve electrode which
Along with improving electrochemical performance of SCs, research achieved an Ed of 147 Whkg−1 at a Pd of 150 Wkg−1 and also retained
has been focussed on the integration of SCs with multiple functions an Ed of 86 Whkg−1 at a Pd of 2587 Wkg−1. Peng et al. [299] have
such as flexibility, wearability for their use in portable devices and introduced a new SC/Li-ion battery (SC/BT) topology hybrid energy
adding smart functionalities so that people can easily determine the storage system (HESS) for electric vehicle (EV) using ADVISOR simu-
electrical energy storage (EES) [295]. The most striking change is the lator. In this braking regeneration energy is harvested by SC pack. The
visual change which can be easily identified. Among SC materials, WO3 constraint on Li resources has forced researchers to use other elements
has been found to be smart material due to its good contrast amid the abundant on earth. In this context, Na-ion SCs have been explored
bleached transparent state and blue coloured state. The change in which couples battery electrode with high Cs and surface adsorption
colour with potential or EES of the electrode demonstrates its smart based SC electrode with good rate capability. Lu et al. [183] con-
function [296]. Zhu et al. [297] synthesized WO3 by electro-deposition structed an advanced Na-ion SC using Mn hexacyanoferrate as the
method (e- WO3) as a smart electrode material. Optical density has been cathode and Fe3O4/rGO as an anode in the aqueous electrolyte with an
used for investigation of colour change of WO3 film based SCs. A linear extended PW of 1.8 V, Pd of 2183.5 Wkg−1, Ed of 27.9 Whkg−1 and
dependence between optical density and EES is discovered. Then hybrid good cycling stability (82.2% Cs retention after 1000 cycles). Along
SC is fabricated to integrate the color-change based EES indicators to with Na-ion based BSH, potassium-ion based BSH is also attractive due
various high-performance SCs. As the EES of SCs is visually apparent by to its low cost and abundance of potassium (K) in nature.
817
Fig. 12. (a) Schematic explanation of nanoporous silver (NPS) electrode and current collector (b)Extinction efficiency of pre-baked organometallic solution and
procedure of a laser-sintering process (c) Change in structure of silver conductor based on the laser energy [313]. (Reproduced with permission from Ref. [313]
Copyright Royal Society of Chemistry (2017)).
Komaba et al. [300] proposed a graphite-polyacrylate electrode as a designing of carbon electrodes with appropriate pore structure is ne-
–ve electrode for 4 V K-ion SC. Multivalent metal ion (e.g. Al3+) based cessary. Yoo et al. [306] reported graphitic mesoporous carbon as an
BSH devices are also of concern along with monovalent metal ions, due electrode in SCs and fabricated a 2.5 V SC with a Cs (areal) of (□560
to their high Ed. Li et al. [301] reported a BSH system with an Ed of 13 mFcm−2) and rapid frequency response (ϕ ˜ -80°) at 120 Hz. Also,
Whkg−1 using Al0.2CuFe-PBA as the + ve electrode, AC as the -ve mixing a small amount of CNTs to the electrode material enhanced the
electrode. Another Al-ion BSH system using PPy@MoO3 as the -ve voltage to □40 V. Rangom et al. [307] fabricated superior-perfor-
electrode with AC as the + ve electrode has been reported with an Ed of mance, self-standing composite electrodes with SWCNTs. The 3-D me-
30 Whkg−1 and operating voltage up to 1.5 V in aqueous electrolyte soporous SWCNT-based electrodes permitted unimpaired ionic trans-
[302]. port in thick films and provided better results in an ac line frequency of
120 Hz. Measurements of 601 μFcm−2 with a -81° phase angle and a
6.3. Electrochemical flow capacitor (EFC) time constant of 199 μs had been obtained and as fabricated electrodes
were capable of cycling at higher than 200 Vs-1 showing a parallelepi-
In EFC, energy is stored in electric double layers formed by charging pedic CV shape at 1 kVs-1. Current densities were greater than 6400 Ag-
1
carbon particles. Here, a slurry type carbon-electrolyte mixture is de- and the electrodes preserved greater than 98% of Cs over 1 million
ployed as the active material for the charge storage. EFC consists of a cycles. Preparation of graphene-based ac line-filters on a large scale has
cell having two external reservoirs which possess a blend of electrolyte been reported by Wu et al. [308]. Here, GO reduced by patterned metal
and carbon material. The uncharged mixture is passed from reservoir interdigits has been employed as the electrode and the fabricated device
tanks to flow cell, where the energy is given to the carbon material. explored a phase angle of -75.4° at 120 Hz, a time constant of 0.35 ms, a
After being charged, the slurry can be kept in big tanks until the need Cs of 316 μFcm-2 and retains 97.2% of the Cs after 10,000 charge-dis-
for energy arises and at the time of need, the complete process is re- charge cycles. Kurra et al. [309] have reported PEDOT micro-super-
versed. EFCs can sustain a large number (hundreds of thousands) of capacitor with ultra-high scan rate capability of 500 Vs-1 and a cross
charge-discharge cycles [303,304]. A flowable electrode made up of over frequency of 400 Hz at a phase angle of -45° which exhibits Cs
HQ/carbon spheres yielded a Cs of 64 Fg−1 which was 50% more than (areal) of 9 m F cm-2 in 1 M H2SO4. These devices retain the Cs of 80%
that of only carbon-based flowable electrodes [305]. after 10,000 cycles, maintained efficiency (η) of 100% and exhibit an Ed
of 7.7 mWhcm-3.
6.4. Alternating current (ac) line-filtering SCs
6.5. Micro-supercapacitors (Micro-SCs)
A supercapacitor can replace bulky Al electrolytic capacitors (AECs)
used exhaustively in ac line filtering, resulting in the miniaturization of Thin film batteries and microsized batteries suffer from certain
the devices. However, the SCs developed for this purpose, have re- limitations, for instance, short lifetime, small Pd and complex archi-
stricted applied voltage range of ˜20 V. To enhance the voltage range, tecture, which restrict their integration in portable and miniaturized
818
devices. Micro-SCs with planar architecture have the advantage of easy provides self-healing capability along with mechanical flexibility. Bio-
fabrication into miniaturized electronics [310]. Shao et al. [311] re- char (BC), obtained from pyrolysis (low-temperature) of biological
ported a quasi-solid state micro-SC with cellular graphene films as an wastes along with the inclusion of the rGO, is a carbon material that
active material and PVA/H3PO4 as the gel electrolyte. The 3D graphene provides mechanical strength and electrical conductivity. The SC fab-
films served as high-performance SC electrodes as well as an ion re- ricated using BC-rGo electrodes, exhibit higher Ed of 30 Whkg−1 with
servoir for the electrolyte. Liu et al. [312] have reported a photo- ˜90% Cs retention after 5000 cycles at Pd of 50 Wkg−1 at room tem-
switchable micro-SC based on the diarylethene-graphene film which perature and an Ed of 10.5 Whkg−1 at a Pd of 500 Wkg−1 at −300 °C.
exhibits the Cs modulation up to 20% demonstrating a photoswitchable The low-temperature behaviour excellence may be linked to non-
micro-SC. Terahertz spectroscopy explored that the photoswitching is freezable water near hydrophilic polymer chains. This can further in-
related to charge transfer at the graphene-diarylethene interface on spire researchers to explore the phase performance of water close to
light modulation. A flexible micro-SC of the self-generated silver layer polyampholyte chains. Wang et al. [322] reported rGO based spring
has been designed by Lee et al. [313] using the laser-induced growth- electrodes for stretchable SCs synthesized by casing fibre springs in a
sintering technique. As designed SC exhibits a high Ed (volumetric) of self-healing polymer which show 82.4% Cs retention after almost 100%
16.3 mWhcm−3 and a Pd of 3.54 Wcm−3 as shown in Fig. 12. stretch, and 54.2% Cs retention after third healing.
6.6. Photo-supercapacitors (photo-SCs) 6.9. Shape memory SC (SMSC)
Photovoltaic energy generation is among the fastest growing power Flexible wearable SCs experience deformations which do not re-
sector. In photo-SCs, the fluctuating electricity generated from solar verse after long-term stress, leading to structural and functional fatigue.
cells is integrated with SCs. SCs with a photoactive layer along with a For solving this problem, an SMSC has been fabricated by Huang et al.
supercapacitive layer into a single device have been presented [314]. [323] which is flexible and gets easily deformed but when heated above
This device exhibits the capabilities of self-charging upon illumination a specific temperature, it recuperates its original shape by restoring all
with self-storage of charge. The photogenerated current is about 2 deformations. Also, a shape memory textile fabricated with these
mAg−1 along with the Cs of ˜140 Fg−1. Xu et al. [315] reported a stack- SMSCs can act as smart sleeves which can remember its previous shape
integrated photo-SC`, composed of DSSC (dye-sensitized solar cells) and and register automatic cooling when overheated.
a SC fabricated on anodic titanium oxide (ATO) nanotube array, where
an enhanced SC output was obtained by plasma-assisted hydrogenation 6.10. Piezoelectric SCs
process. The Cs (areal) of selectively hydrogenated ATO had been 1.0
mFcm-2 at 1mAcm-2. The optimized photo-SCs showed good photo- The advancement in the integration level along with minimizing
electric conversion efficiency with storage efficiency (approximately energy losses in power management circuits is the need of the hour.
1.64%) with quick response and better cycling capability. Generally, a full wave rectifier is employed among the piezoelectric
nanogenerator and the storage device which reduces integration den-
6.7. Thermally chargeable SCs sity and enhances energy loss. Recently, Xing et al. [324] demonstrated
a self-charging cell for energy conversion and storage by integrating
Low-grade thermal energy, which at present is wasted, can be stored piezoelectric separator and Li-ion battery. However, due to slow char-
for powering devices such as wearable electronics and sensors. Thermo- ging and reduced cyclability of Li-ion battery, the SCs have gained
electric energy conversion is a good method for waste heat manage- immense attention. The integration of pseudocapacitor and piezo-
ment, but impediments like low output voltage with no energy storing electric material in the energy storage device has been reported by
capability require other components (e.g. voltage boosters and capaci- Ramadoss et al. [325]. Song et al. [326] integrated a PVDF film in SC as
tors). Thermal self-charging SC uses the Seebeck effect, thermally ac- the energy harvester and separator. An SC has been formed by using
tivated ion diffusion and temperature dependent electrochemical redox PVDF film coated with H2SO4/PVA gel as anode and carbon cloth with
potential. It comprises two electrodes kept at dissimilar temperatures H2SO4/PVA electrolyte as a cathode. The piezoelectric PVDF film due to
[316]. An innovative process of producing a large voltage from the mechanical force showed the charging of the SC and possessed a Cs of
temperature gradient like conventional thermoelectronics is reported 357.6 Fm−2, an Ed of 400 mW m−2 and a Pd of 49.67 mWhm−2. Maitra
[317]. The PANI coated graphene and CNT electrodes sandwich the et al. [327] have fabricated a bio-piezoelectric run self-charging ASC
polystyrene sulphonic acid (PSSH) film in which thermally excited consisting of NiCoOH-CuO@Cu foil as a + ve electrode and rGO@Cu
electrochemical reactions result in charging without the requirement of foil as -ve electrode with a PVA-KOH gel electrolyte dipped porous fish
any external source for power supply. With little temperature difference swim bladder as a bio-piezoelectric separator. This SC can be charged
(5 K), the thermally chargeable supercapacitor produces a voltage of up to 281.3 mV in ˜80 s.
38 mV and a large Cs (areal) (1200 Fm−2). Al-zubaidi et al. [318] ex-
amined the thermally-induced phenomena of an ionic electrolyte and 7. Conclusions and challenges for SCs
solid-liquid interface and also reviewed the studies on thermally excited
self-charging in SCs. Wang et al. [319] explored a thermal charging Electrochemical SCs are developing as promising devices for energy
process to restore the energy wasted in SCs after electrical charging and storage. In this review, a detailed description of electrode materials
discharging. Zhao et al. [320] used an asymmetric polymer electrolyte based on carbon materials, CPs, MOs and their composites has been
prepared from NaOH-treated polyethylene oxide (PEO-NaOH) to gen- given. Further research is needed for high-performance SC electrodes
erate a thermally-induced voltage in SCs. They employed electrodes of which can simultaneously assure high capacitance, cyclic stability and
Au and of MWCNTs deposited on Au and obtained a thermopotential of excellent rate. The authors believe that more research should be fo-
10 mV K-1, the Cs (areal) of 1.03 mFcm−2 and Ed of 1.35 mJcm−2 at the cussed on different nanocomposite materials made up of carbon, MOs
temperature difference of 4.5 K. and CPs for fabricating high-performance SC electrodes. Also, the state-
of-the-art developments in SC electrode materials have been in-
6.8. Self-healing SCs corporated in this article along with some novel materials and new
devices for SCs. Continuous research efforts are needed to allow these
A self-healing SC has been fabricated by Li et al. [321] which ex- materials and novel devices to meet the growing energy demands. Also,
hibits higher Ed by employing biochar-based electrodes and poly- it is essential to improve synthesis parameters and material properties
ampholyte gel electrolyte. Polyampholyte is a tough hydrogel which for full capability exploration of the SC electrode materials. MOFs,
819
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Conflicts of interest mance PEDOT/PEDOT symmetric supercapacitor by facile in-situ hydrothermal
polymerization of PEDOT nanostructures on flexible carbon fibre cloth electrodes,
Mater. Today 6 (2017) 96–104.
There are no conflicts to declare. [28] R. Wang, Q. Wu, X. Zhang, Z. Yang, L. Gao, J. Ni, O.K.C. Tsui, Flexible super-
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Chem. A 4 (2016) 12602–12608.
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Ms. Poonam is thankful to UGC, New Delhi, India for providing Effective network formation of PEDOT by in-situ polymerization using novel or-
Teacher Fellowship under ‘Faculty Development Programme’ to com- ganic template and nanocomposite supercapacitor, Electrochim. Acta 247 (2017)
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plete Ph.D. at Panjab University, Chandigarh.
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Journal of Energy Storage 21 (2019) 801–825

Contents lists available at ScienceDirect

Journal of Energy Storage

Review of supercapacitors: Materials and devices T

1. Introduction principle of electrochemical energy conversion. SCs have gained much

materials had been investigated for pseudocapacitors (Faradaic charge

Fig. 2. (a) Schematic of the fabrication process

2.3. In-situ polymerization 2.6. Vacuum ﬁltration technique

Several other synthesis methods have been reported for SC elec-

Porous Au/MnO2 – – 1145 at 50 mVs−1 – – 80%(500) [192]

CNT/PANI//CNT/MnO2/GR, by vacuum ﬁltration method in 1 M – 24.8 at 1.6 V – – [246]

6.6. Photo-supercapacitors (photo-SCs) 6.9. Shape memory SC (SMSC)

• More novel manufacturing techniques should be developed to pre-

• Moreover, to improve the Ed, along with electrode material, elec-

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