The pseudocapacitive performance of MnO2 nanomaterials in negative potential window 1


The pseudocapacitive performance of MnO2 nanomaterials in negative potential window
1. Introduction
What are supercapacitors?
Super-or ultra-capacitors, also known as high-capacity capacitors, accumulate energy in an electrostatic manner, polarizing the electrolyte solution. Chemical reactions are not involved in the process of energy accumulation in the supercapacitor, although the supercapacitor is an electrochemical device. High-capacity or supercapacitors can be charged and discharged thousands of times due to the high reversibility of the energy storage mechanism. Supercapacitor is an electrochemical capacitor that has the ability to accumulate an extremely large amount of energy in relation to its size and in comparison with a traditional capacitor. This super capacitor property is of particular interest to the automotive industry in the production of hybrid vehicles, as well as in the production of transport on battery electric, where the super capacitor is used as an additional energy storage device.

Properties of supercapacitors:
Among the properties we note:
The highest capacity density
The lowest cost per 1 Farad
Reliable, long service life
High cycle efficiency (95% or more)
Uninterrupted operation
Ecological safety
A wide range of operating temperatures
High specific power and high specific energy
Very high charge/discharge rate
A large number (thousands) of cycles with a slight deterioration of the parameters
Good reversibility of the energy storage mechanism
Reduced toxicity of materials used
Low equivalent serial resistance (ESR))
Types of Supercapacitors:
Supercapacitors, another sort of electrochemical capacitor are likewise called ultracapacitors. The name is a non specific term for electric double-layer capacitors (EDLC), pseudocapacitors and half breed capacitors. Supercapacitors don’t have an ordinary strong dielectric. The capacitance estimation of an electrochemical capacitor is dictated by two stockpiling standards:
•Double-layer capacitance – Storage is accomplished by partition of charge in a Helmholtz double layer at the interface between the surface of a conductor and an electrolytic arrangement. The separation of detachment of charge in a double-layer is on the request of a couple of Angstroms (0.3– 0.8 nm). This stockpiling is electrostatic in starting point.
•Pseudocapacitance – Storage is accomplished by redox responses, electrosorbtion or intercalation on the surface of the cathode or by particularly adsorpted particles that outcomes in a reversible faradaic charge-exchange. The pseudocapacitance is faradaic in inception.
Double-layer capacitance and pseudocapacitance each add to the aggregate capacitance estimation of a supercapacitor. The proportion of the two relies upon the outline of the terminals and the creation of the electrolyte. Pseudocapacitance can expand capacitance by as much as a request of size.
Supercapacitors are partitioned into three families, as indicated by the relative measures of capacitance in the double layers versus pseudocapacitance:
•Double-layer capacitors – Static double-layer capacitance is significantly higher than faradaic pseudocapacitance
•Pseudocapacitors – faradaic pseudocapacitance significantly higher than static double-layer capacitance
•Hybrid capacitors – critical capacitance in both double-layer and pseudocapacitance. Lithium-particle is one such terminal material.
Supercapacitors have the most noteworthy capacitance esteems per unit volume and have the best vitality thickness of all capacitors, achieving 12,000 F/1.2 V, with capacitance esteems up to 10,000 times that of electrolytic capacitors. Supercapacitors are shutting the hole amongst capacitors and batteries. As far as particular vitality and particular power, this hole secured a few requests of extent. Supercapcitors have lessened this hole to a solitary request of size, offering around 10% of the limit of batteries. Supercapacitor control thickness is for the most part 10 to 100 times incredible than batteries. Power thickness consolidates the vitality thickness with the speed at which the vitality can be conveyed to the heap.
Not at all like batteries, in the faradaic redox responses, the particles stick to the nuclear structure of an anode, and no or insignificantly little compound changes are associated with charge/release. This vitality stockpiling with quick charge-exchange makes charging and releasing significantly quicker than batteries. Also, supercapacitors better endure rehashed quick charge and release cycles. This makes them appropriate for parallel association with batteries, and may enhance battery execution as far as power thickness,
The electrolyte associates the two cathodes. This recognizes electrochemical from electrolytic capacitors, in which the electrolyte is likewise the cathode, and consequently shapes the second terminal.
Supercapacitors are enraptured, and might be worked just with the right extremity. The extremity is controlled by plan with awry cathodes, or by a voltage connected amid produce for those with symmetric terminals.
Supercapacitors bolster shifted applications, including:
•Long, little streams for static memory (SRAM) in electronic hardware
•Power hardware that require brief, high streams as in the KERS framework in Formula 1 autos, for electrical vitality stockpiling/conveyance
•Regenerative braking in vehicles, for example, transports and prepares
Supercapacitors are seldom tradable, particularly those with higher vitality densities. IEC standard 62391-1 Fixed electric double layer capacitors for use in electronic gear distinguishes four application classes:
•Class 1, Memory reinforcement, release current in mA = 1 • C (F)
•Class 2, Energy stockpiling, release current in mA = 0.4 • C (F) • (V)
•Class 3, Power, release current in mA = 4 • C (F) • (V)
•Class 4, Instantaneous power, release current in mA = 40 • C (F) • (V)
•Double-layer, Lithium-Ion and supercapacitors
•Double-layer capacitor with 1 F at 5.5 V for information buffering
•Radial (single finished) style of lithium particle capacitors for high vitality thickness
•Supercapacitor/Ultracapacitor cells and modules for high current burdens
2. Principles of Supercapacitor:
Electrochemical capacitors differ from conventional capacitors by storing charge using the double-layer concept in the absence of insulating dielectric material.17 The positive and negative ionic charges from electrolyte accumulate at the surface of conductive electrodes, compensating the opposite electronic charges at electrode surfaces.18 As a result, double-layer capacitance arises at the interface between electrodes and electrolyte, according to Eq. (1):
(1)
where ?? 0 is the space permittivity and ?? is the relative permittivity, A is the surface area of an electrode and ? is the effective thickness of the double layer. The term ? which represents the separation distance between the electrodes in the conventional capacitor is now referring to the effective thickness of the double layer at the solid/electrolyte interface. The separation of the charges at the interface typically is only in the order of few angstroms, depending on the electrolytes used.19 As a result of high internal surface area of the electrodes and nanometer scale thickness of the double layer, the capacitances obtained in electrochemical capacitors are several orders of magnitude higher than those of conventional capacitors of the same size.6 In terms of design and manufacturing, electrochemical capacitors are similar to batteries.11 Figure 2 shows the basic structure model of electrochemical capacitor. The active material is usually pressed or coated onto the current collector and immersed in the electrolyte. The separator prevents the two electrodes from short-circuiting each other, but is ion permeable allowing ionic charge transfer to take place.20 Electrochemical capacitors can be explained via two types of charge storage mechanism. The ¯rst category is the electrostatics storage of electric energy in which separation of charges occurs in a static double layer at the electrode–electrolyte interface without involving electron transfer process.21 In other words, there is no electrochemical reaction occurring on electrodes during charging/discharging processes. The double-layer capacitance is measured using Eq. (1), as mentioned earlier. In contrast, the second category mechanism utilizes the electrochemical storage of electric energy with fast and reversible redox reactions which result in pseudocapacitance. This is accomplished by electrosoprtion of specifically adsorbed electrolyte ions at the electrode–electrolyte interface and intercalation of electro-active species in the layer lattice.22 The pseudocapacitance can be estimated according to Eq. (2):
(2)
where q is the Faradaic charge required for adsorption/desorption of ions, d? is the change of surface coverage by adsorbed species and dV is the change of voltage.6 The real complete capacitor cell consists of two capacitors in series.23 Assuming that C?=C?=C in a symmetric device, the capacitance values of the two electrodes connected in series give rise to the total capacitance following the Eqs. (3) and (4):
229933547625
Subsequently, the specific capacitance of a complete cell can be obtained by dividing the capacitance (CT) by the combined mass of two electrode cell (2m), according to Eq. (5).24

It should be noted that the specific capacitance corresponding to a single electrode material using three-electrode cell configuration is four times larger than that of two-electrode cell as shown in Eq. (6), contributing significant difference between complete cell and single electrode.18

Regarding this significant importance, attention must be given to the specifications of an electrochemical capacitor whether the values are calculated for a complete two-electrode cell or correspond to single electrode measurement. Unless otherwise specifically stated, the capacitance values discussed throughout this review should be based on single electrode material in order to avoid misleading. A voltage (V) is built up across the two electrodes when the electrochemical capacitor is charged.11 The maximum energy (E) stored by this capacitor is directly proportional to its capacitance (C) and the square of applied voltage (V²) according to Eq. (7)25:

Devices with aqueous electrolytes have the maximum operating voltage limited at about 1.2 V due to decomposition of water while organic electrolytes and ionic liquid allow wider voltages window up to 3.5 V and 4.5 V, respectively.26 The power density (P) of electrochemical capacitors is proportional to the square of applied voltage (V²) and is limited by the resistance R, following the Eq. (8)17:

where R is the equivalent series resistance (ESR) which is comprised of the electrode resistance, electrolyte resistance and resistance due to the diffusion of ions in the electrode porosity. In general, the inner resistance of electrochemical capacitors is much smaller than that of batteries as a result of fast electron/ion transfer as well as rapid combination of charges.11 Therefore, electrochemical capacitors can discharge the electrical energy stored rapidly in order to produce higher power density than in batteries.

From both Eqs. (7) and (8), it is evident that V, C and R are the three important parameters affecting the electrochemical performance of electrochemical capacitors. Choice of electrode materials and type of electrolyte determine the operating voltage of the cell which in turn contributes different magnitudes of energy density26 while electrolyte conductivity, the size of electrolyte ions and the porosity of electrode materials are among the key factors affecting on the ESR of the cell, which in turn determine its power output.14 It should be mentioned that both energy and power densities are proportional to the square of voltage, thus, a great number of researches have been conducted and focused on studying highly conductive and stable electrolytes with a wide operating voltage for a variety of applications.25 On top of increasing operating voltage, enhancing capacitances considered as one of the key approaches in electrochemical capacitor research and development. This can be attained by improving specific capacitance of electrode materials via optimization of electrode structures. In addition to improving voltage and capacitance, reducing ESR of electrochemical capacitors could also bring advantageous effects to performance enhancement. Therefore, contact resistance between electrode particles, a sum of electrode-current collector resistance, resistance due to the diffusion of ions in the electrode porosity and electrolyte resistance should become the major focus to optimize the performance of electrochemical capacitors.

3. Electrolyte
The choice of an electrolyte is particularly important because the amount of energy stored in the cell and how quickly this energy can be released are determined by the thermodynamic stability of the electrolyte employed.8 As discussed earlier, energy density of electrochemical capacitor is proportional to the squared voltage. Numerous research efforts have been aimed at exploring highly conducting, stable electrolytes with a wider voltage.25 Generally, three types of electrolyte are currently employed in electrochemical capacitor applications: (i) aqueous electrolyte, (ii) organic electrolyte and (iii) ionic liquid. In aqueous electrolyte, concentrated electrolytes such as H?SO? and KOH are able to minimize internal resistance and maximize power capability due to excellent ionic conductivity (0.5–1 S/cm) or much lower electrolyte resistivity (1–2 ?cm). The great number of proton (H?) and hydroxide (OH?) involved in proton hopping or proton transport explains the higher ionic conductivity in strong acid and alkali, respectively.15 However, the use of concentrated electrolyte limits the cycle life of electrochemical capacitor and restricts the range of possible electrode materials because most of metal oxides degrade dramatically in concentrated solutions. As a result, neutral and mild aqueous electrolytes including KCl, Na?SO?, Na?SO? and Li?SO? have therefore been considered for use with metal oxides.27 Compared with organic electrolytes, aqueous electrolytes exhibit higher capacitance due to relatively smaller ionic size (5 A to 10 A) and lower internal resistance. The resistance of electrochemical capacitor is strongly dependent on the conductivity of electrolyte and the size of electrolyte ions which penetrate into and out from the pores of electrode particles.15 With the same reasons, aqueous electrolytes also tend to produce faster rates of charges and discharge. Additionally, preparation of aqueous electrolytes undergoes less stringent purification and drying processes due to the absence of flammable and toxic solvent. Consequently, the fabrication and material cost of aqueous electrolytes are much lower in comparison to organic electrolytes. However, the main drawback of aqueous electrolytes is their relatively smaller voltage window, which is limited by the electrolysis of water to 1.23 V at 25°C. 26 As can be seen from Eqs. (7) and (8), aqueous electrolytes have limitation in term of enhancing both energy and power densities because of their narrow voltage window. This is the reason why organic electrolytes are often recommended. Moving from aqueous to organic electrolyte, the voltage window is increased from 1.2 V to 3.5 V.15 By all means, this is a great advantage of organic over aqueous electrolytes. Typically, the operating voltage is set to 2.5 V to prevent decomposition of the electrolyte through over-charging. In order to ensure that organic electrolytes can operate at higher voltages, an inert atmosphere which is free of water and oxygen is needed to handle these electrolytes. This is because the evolution of H? and O? gases occurred at the potential difference above 1.23 V.26 Among organic electrolytes, acetonitrile is the most commonly used solvent.11 However, its toxicity and flammability require more stringent and costly preparation processes. Organic electrolytes have low conductivity (0.01–0.05 S/cm) which leads to high internal resistance or ESR (20–60?cm). The increased viscosity in organic electrolytes also further increases the ESR. The organic ions with relatively larger size (15–20 A) are more difficult to diffuse into or out of the pores of electrode materials. This results in poorer specific capacitance in organic-based electrochemical capacitors. Ionic liquids (ILs) are room-temperature liquid solvent-free electrolyte. ILs are essentially molten salts with melting temperatures at or below room temperature.26 Imidazolium, pyrrolidinium and quaternary ammonium slats are among the most widely studied ILs for electrochemical capacitor applications. The main advantages of using ILs include wide electrochemical stability window up to 6 V, high thermal and chemical stability, low flammability and low vapor pressure.11 Their voltage window stability is thus only driven by the electrochemical stability of the ions. The ionic conductivity of these liquids at room temperature is very low, so they are mainly used at higher temperature.25 Unfortunately, ILs have relatively high viscosities and much lower ionic conductivities than aqueous electrolytes. In addition, ILs is hygroscopic which require to be handled in stringent and controlled processes. Another major disadvantage of using ILs is their high cost, restricting the larger scale production.

4. Electrode Material
Several significant factors for capacitors have been improved, which include energy storage density, cyclic stability, and rate capability. Currently, supercapacitors that utilize carbon as the electrode have energy densities lower than 10 W h kg?¹. Electrochemical batteries, however, have larger energy densities of 20-35 W h kg?¹ for a lead acid battery and 120-170 W h kg?¹ for a lithium-ion battery 22.

In order to improve different parameters for different applications of supercapacitors, a large number of materials have been investigated. The many different allotropes of carbon allow for varying properties of the supercapacitor electrodes which enable manipulation of the carbon composition for different applications 8. Some examples of these allotropes are graphene, diamond, boron doped diamond, graphite, glassy carbon, onion-like carbon, activated carbon, fullerenes, and carbon nanotubes (single-walled and multi-walled). See figure 3 for drawings of some of the allotropes. Applications that require a high power density utilize the good conductivity supported by graphene, while applications that require a hard and strong electrode utilize diamond. In addition, glassy carbon has been shown to be an exceptional electrode material due to the high density of electronic states caused by the disordered structure of graphitic regions ?. In order to obtain high performance for the supercapacitors, the electrodes should have high electrical conductivity, be electrochemically inert, and have a highly porous structure and high specific surface area (SSA). The carbon is electrochemically inert, since the carbon is present even after releasing of an electron when a pseudocapacitance layer is formed. Porosity partially determines the amount of SSA, but also reduces conductivity. This fact leads to the necessity of achieving an optimum between electrical conductivity and porosity. In addition, the pore size of the electrode material must be adequate for electrolyte penetration, or else the effective surface area is drastically reduced. The electrochemical reactivity of the electrode will depend on the atomic structure obtained via the allotrope of carbon used.

Figure 3: Some of the different allotropes of carbon
5. Pseudocapacitors
RuO? in either a crystalline or amorphous hydrous form have been studied extensively in acidic solutions in the past 30 years because of their high specific capacitance and high conductivity characteristics.25 In 1971, Trasatti and Buzzanca firstly used RuO? as electrochemical capacitor electrodes in aqueous H?SO?. They recognized that the capacitive performance is attributed to a successive electron transfer at redox active sites (Ru²?, Ru³?, Ru??), balanced by the proton transfer which leads to the conversion of OH? to O²? sites in the oxide structure.44 Similarly, Lokhande and co-workers (2010) have identified that the electron transfer between and within the RuO? particles, electron hopping between the particles and current collectors and proton diffusion within RuO? particles are the primary factors affecting the capacitive performance of hydrous RuO?. 45 Although crystallization of RuO? can reduce the intraparticle electron hopping resistance of RuO??nH?O particles, Gujar and coworkers claimed that the crystalline nature of RuO? electrodes limits capacitive performance through an increase in the diffusion barrier of proton within the rigid lattice of crystalline structure.46 In fact, literature survey of electrochemical capacitors shows that recent research focuses on amorphous hydrous RuO??nH?O thin film-based electrochemical capacitor via various methods including sol–gel, cyclic voltammetric deposition, anodic deposition, spray deposition, hydrothermal synthesis, oxidative synthesis, etc.46,47 The advantages of amorphous hydrous ruthenium oxides (RuO??nH?O ) include high specific capacitance, high conductivity and good electrochemical reversibility.48 As reported in a literature,49 hydrous RuO? ? 0:5H?O exhibits high capacitance value (~ 900 F/g). However, the capacitance decreased to 29 F/g when the water content was reduced to RuO? ? 0:35H?O. Most of the review articles of RuO?-based electrochemical capacitors have focused on the capacitive performance of electrodes mainly in the concentrated sulfuric acid (H?SO?).45 Some researchers claimed that conducting metal oxides such as RuO? are only suitable for aqueous electrolytes, suggesting that high capacitance value and fast charging are the pseudocapacitance contribution from surface reaction between Ru ions and protons (H?). Hence, a strong acid is therefore necessary to provide good ionic conductivity.28 However, it should be noted that the cycling performance of metal oxide electrodes will be deteriorated because electrodes tend to dissolute rapidly in the highly concentrated acidic electrolyte.50 There has been an increasing interest in MnO?- based electrode for energy storage applications because of its cost effectiveness and low toxicity in comparison to other metal oxides such as RuO?, NiO and CoO?. 51 In general, the pseudocapacitance of MnO? is believed to be predominantly attributed to the redox transitions associated between Mn (III)/Mn(II), Mn(IV)/Mn(III) and Mn(VI)/Mn (IV).52 In comparison to the studies of RuO²-based electrochemical capacitor, milder aqueous solutions such as KCl and Na?SO? have therefore been adopted for MnO? electrode.45 The first study on Faradaic pseudocapacitive behavior of nano MnO? by Lee and Goodenough has shown that mild KCl aqueous solution can replace strong acid such as H?SO? as the electrolyte of electrochemical capacitor.53 Based on the experiment, the electrode materials containing amorphous MnO? ?nH?O and AB showed a specific capacitance of 200 F/g.53 Typically, thick films of MnO? yield the specific capacitance values range from 125 F/g to 250 F/g.51 Conversely, ultrathin MnO? film electrodes exhibit excellent specific capacitance which could be due to the fact that the thinner films possess considerable lower contact resistance between ¯lm matrix and current collector.54 On top of that, thin films could provide a shorter proton diffusion path length. Pang et al. has demonstrated a specific capacitance of 700 F/g for very thin MnO? films prepared by sol–gel techniques.54 Broughton and Brett also have observed a specific capacitance of 700 F/g in thin MnO? films prepared by anodic oxidation.55 Obviously, in comparison to thin-film electrodes, the capacitance of thick MnO? electrodes is ultimately limited by the poor electrical conductivity of MnO?. 56 Nickel oxide (NiO) is being considered as one of the promising potential electrode materials for electrochemical capacitors with an anticipation that it could serve as a low-cost alternative of noble oxide RuO? -based electrochemical capacitor, owing to its easy availability, environmental benign nature and cost effectiveness.57 Nickel becomes the primary material in alkaline batteries including nickel-metal hydride and nickel-cadmium batteries as a result of the formation of protective oxide/hydroxide surface in alkaline electrolytes.58 Wu et al. demonstrated the comparative studies on the deposited NiO film in 1 M KOH and 1 M Na?SO? solutions, respectively.59 The authors claimed that the redox between NiO and OH? results in high capacitance of NiO in alkaline solution. In contrast, no significant peaks were observed for the deposited nickel oxide film in 1 M Na?SO? solution, suggesting that the solution has a much lower concentration of OH in comparison to 1 M KOH solution. Consequently, the capacitive performance in Na?SO? solution mainly comes from the double-layer effect of the deposited nickel oxide film. Recently, Wu et al. have successfully synthesized the porous nickel oxide film with interconnected nanoflakes and open macropores by anodic electrodeposition.60 The nanoflakes are believed to be beneficial for the capacitance enhancement of NiO due to their high surface area and shortened diffusion path in solid phase. On top of the nanoflakes, a porous film with open macropores also provides a pathway to aid the electrolyte ion penetration into the interior oxide, facilitating the electrolyte penetration, leading to an increase in specific capacitance of NiO (351 F/g). Fe?O? has been identified as potential electrode material in view of the low-cost and environmentally friendly nature.61 The preliminary evaluations revealed that the relatively low electrical conductivity of Fe?O? requires the introduction of conductive additive such as CB and AB in order to increase its conductivity and boost its capacitance.62 Wu and his co-workers first compared the capacitive performance of Fe?O?-CB electrodes showing the specific capacitance of 38 F/g in sodium sulfate (Na?SO?).62 Similarly, Brousse and Belanger synthesized Fe3O4 nanopowders with high surface area (115 m2 /g) and reported a specific capacitance value of about 75 F/g in 0.1 M K?SO?. 27 Wang et al. investigated the capacitance mechanism of electroplated Fe?O? thin-film electrodes in Na?SO?, Na?SO ? and KOH aqueous solutions.63 Experimental results indicate that Fe?O? electrodes in Na?SO? exhibit highest specific capacitance value (170 F/g), followed by Na?SO ? (25 F/g) and KOH (3 F/g). The authors suggested that significant improvement on the capacitive performance in Na?SO? is attributed from the synergy effect of both EDLC and the pseudocapacitance which involves successive reduction of the adsorbed sulfate anions. In contrast, the capacitive current is attributed to EDLC only in Na2SO4. In recent years, cathodic electrodeposition method has been developed for the fabrication of iron oxide films, containing chitosan additive as a binder.64 The iron oxide films exhibited specific capacitance as high as 210 F/g in 0.25 M Na?SO? solution. It is claimed that the capacitive performance of Fe?O? is heavily dependent on the nature of anions and the surface area of electrodes.27,62,63 Conducting polymers can offer relatively cost-effective alternative to the conventional electrode materials as a result of their fast doping/dedoping capabilities, excellent electrochemical reversibility and high conductivity in a doped state.69 Nevertheless, the poor power density caused by the slow diffusion of electrolyte ions within the entire electrode becomes the main disadvantage of conducting polymers.69 In addition, conducting polymer-based electrochemical capacitors exhibit lower cycle life due to volume change or swelling as a result of the doping (intercalation)/de-doping (de-intercalation) of ions.67 The conductivity of conducting polymer was first reported in 1963 by McNeill and co-workers.65 They exhibit the pseudocapacitive and conductive behavior via electron delocalization in conjugated chemical bond system along the polymer backbone. Among various conducting polymers, polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly (3, 4-ethylenedioxythiophene) (PEDOT) and poly(styrene sulfonate) (PSS) are most commonly studied for use in electrochemical capacitors.17 The experimental specific capacitance values of PANI, PPy and PEDOT are 240, 530 and 92, respectively.69
6. Manganese oxide (MnO2)-based composites
MnO2 is the second extensively studied electrode materials after RuO? because of its low cost and environmental friendly nature. However, the capacitive performance and power characteristics of MnO? electrode are limited by its poor conductivity.52 Introduction of conductive, porous and high surface area carbon-based materials is a feasible route to enhance the charge-storage capability by shortening electron transport distance.52 The research which focused on MnO?/CNT nanocomposites showed good capacitive behavior, with the highest specific capacitance of 356 F/g in 0.5 M Na?SO?. 73 The results indicated that the samples containing CNT showed significantly lower resistance compared to the sample without CNT, suggesting that the porous microstructure of CNTs has facilitated the access of electrolyte ions to the active material. Cheng and co-workers have successfully fabricated electrochemical capacitors using MnO2-coated graphene to provide a high specific capacitance of 328 F/g and energy density of 11.4 Wh/kg in 1 M KCl.45 The access of electrolyte ions to high-surface area graphenes are enhanced due to the physical adjustment of nanosheets between graphene materials. In addition to the ideal structure of graphene for ion adsorption, the MnO? nanoparticles grown on the graphenes also help increase the distance between nanosheets in order to accommodate more electrolyte ions.

The incorporation of other metal elements such as lead (Pb) and iron (Fe), onto the MnO?-based electrodes was found to further improve the electronic conductivity and charge-storage capability of the composite electrodes by introducing more defects and charge carriers via the doping process.52 It should be noted that the amount of metal additives has significant effect on the capacitive performance of the composite electrodes. The MnO?-based electrode with the addition of 20% Pb has increased the specific capacitance of MnO? electrode from 166 F/g to 185 F/g.74 The observed capacitance enhancement can be explained by the increased surface area of the composite electrodes due to the provided large surface area and short ion diffusion path that contribute to the high capacitive performance.85 On top of their poor electronic conductivity, the electrochemical cyclability is another important issue for MnO? electrodes.52 Zhang et al. achieved promising result from MnO?/PANI composites with the maximum specific capacitance of 320 F/g compared to that of pure MnO? (125 F/g). In addition, the specific capacitance retains approximately 84% of the initial value after 10 000 cycles, indicating the good cycle stability of the composite electrode.76 With excellent conductivity and mechanical stability, PANI polymers enable enhanced electrochemical and mechanical properties of the composites. Another study on MnO?-based electrode with the addition of PPy showed the improved specific capacitance of the MnO?-PPy nanocomposite (290 F/g) compared to that of pure MnO? electrode (221 F/g). The enhancement is attributed to a combination of the improved conductivity effect and the high specific capacitance of PPy.52 Mesoporous MnO?/ PANI composite with unique morphology via interfacial synthesis has been synthesized successfully by Wang et al. 77 This amorphous composite electrode exhibits higher surface area and more uniform pore-size distribution compared to MnO?/PANI composites prepared by chemical co-precipitation. These findings reveal that the well-defined mesoporous microstructure processes reduced diffusion length path, allowing faster diffusion of electrolyte ions into the uniform mesopores.