Manganese oxides (MnO2) have particularly received increasing attention owing to their high theoretical specific capacitance of 1370 F/g, low-cost, natural abundance, and environmental benignity. However, MnO2 suffers from low electrical conductivity (10−5 to 10−6 S/cm), low ionic diffusion constant (~10−13 cm2/V s), and low structural stability, which results in low electrochemical utilization and poor cycling life. It is therefore important to explore new strategies to improve the electrochemical performance of MnO2. The effective methods to maximize the performance involve (i) reducing MnO2 structures to a nanoscale range and (ii) compositing MnO2 with highly conductive materials. In this chapter, we will first introduce the rapid development of MnO2 nanostructures for supercapacitors. Then the fundamental charge storage mechanism of MnO2 will be specifically clarified. The preparation methods of MnO2 nanostructures and their composites will be subsequently summarized. Then, we will pay great attention to the most recent development of MnO2-based nanostructures for supercapacitors, which is the main body of this chapter. The practical application of MnO2 nanostructures for symmetric and asymmetric supercapacitors will be discussed. Finally, we will present a brief perspective regarding the rational design and synthesis of MnO2-based nanostructures.
- high performance
According to the charge storage mechanism, supercapacitors can be categorized as electrochemical double layer capacitors (EDLCs) and pseudocapacitors . The typical electroactive electrode materials for pseudocapacitors include transition-metal oxides and conducting polymers. To this end, massive efforts have been dedicated to developing low-cost and eco-friendly alternatives, such as MnO2, Co3O4, and NiO. Among the alternative candidates, MnO2 has received intensive attention in the past decades. Lee and Goodenough reported the first use of MnO2 for supercapacitors in 1999 . This finding paved a new avenue for exploring a huge variety of electroactive MnO2-based electrode materials for high performance supercapacitors. This is because MnO2 shows advantages of (i) a high theoretical specific capacitance of 1370 F/g; (ii) a wide operating potential window of about 1.0 V; and (iii) the ability to enable mild aqueous electrolytes with a much less chemical corrosion to current collectors or packages [3, 4]. In addition, MnO2 has additional merits of natural abundance, low-cost, and environmentally benignity . These unique characteristics will enable MnO2-based electrodes to act as a high-performance, safe, and a low-cost replacement.
However, the specific capacitance of MnO2 is almost one magnitude lower than the theoretical value [4–6]. This is because MnO2 shows poor electronic conductivity (10−5 to 10−6 S/cm), low ionic diffusion constant (~10−13 cm2/V s), and structure flexibility [7, 8]. Moreover, owing to a wide diversity of textural feature, crystal forms (e.g.,
In recent years, extensive endeavors have been dedicated to synthesizing MnO2 nanostructures or nanocomposites with different morphologies and crystal forms. This chapter will introduce the state-of-the-art MnO2 electrodes for high performance supercapacitors. The content includes a brief discussion of the charge storage mechanisms of MnO2, a summary of the synthetic methodologies, a main focus on the MnO2-based electrodes and their application in supercapacitors.
2. Charge storage mechanism of MnO2
The charge storage mechanism of MnO2 was first imitated from RuO2 and Zn/MnO2 cells in acidic electrolytes , which involve a reversible insertion/extraction of protons into/from MnO2 with a concomitant Mn3+–Mn4+ transition. However, the specific capacitance of MnO2 is found to be related to the species and concentrations of the alkali metal cations, such as Li+, Na+, and K+, regardless of the pH value [10, 11]. Consequently, a more rational mechanism based on the chemisorption of alkali metal cations on the surface of MnO2 was proposed and can be written as [3, 11]:
where M+ represents alkali metal cations, including Li+, Na+, and K+. Later, Belanger and co-workers developed a new complementary storage mechanism, that is, the alkali metal cations can insert/extract into/from MnO2, as shown in Eq. (2) :
It should be noted that MnO2 can crystallize in various forms. Can the above charge storage mechanisms be suitable for all MnO2 structures? Brousse et al. investigated a systematic comparison on the supercapacitive properties of MnO2 with five types of phase structures . It is revealed that the capacitance is closely associated with the tunnel size of crystalline MnO2, i.e., the larger the tunnel size, the larger the specific capacitance. More specifically, the one-dimensional (1D)
Recently, mixed-valent chemistry of metal oxides charge compensation mechanisms at oxygen centers and surfaces were proposed as a new charge storage mechanism. The mixed-valent MnOx nanostructures exhibited a high-specific capacitance of 2530 F/g in a two-electrode configuration, about double of the theoretical value (1370 F/g) . It is revealed that a large portion of charge compensation might be originated from the hole state redistribution toward the O sites, rather than merely reduction of Mn ions. In addition, the ionic defects (vacancies and misplaced ions), electronic defects (electron and holes), and structural defects (cavities, stacking faults, etc.) in the mixed-valent MnOx can also boost the kinetics of the surface redox reactions and the transport of charged species. These findings may offer critical insights into the fundamental understanding of the relationship between mixed-valent structures and electrochemical properties and to rational design of a new-generation supercapacitors.
3. Synthesis methods
3.1. Chemical precipitation
MnO2 nanostructures can be easily prepared
Amorphous hydrated MnO2 powders are the resulting products at room temperature. Increasing the reaction temperature not only leads to an increase in the degree of MnO2 crystallinity, but also generates different morphologies, such as nanorods, nanoflowers, and nanotubes . Because of the strong oxidation of MnO4−, a large number of reducing species, including acid, polymer monomers, carbon, etc., can be used to obtaining MnO2 nanostructures . In addition, MnO2 can also be gained by changing other oxidants, such as Na2S2O8, KClO3, etc. These flexible derived reactions facilitate the formation of MnO2-carbon and MnO2-conducting polymer nanocomposites .
3.2.1. Anodic electrodeposition
Anodic electrodeposition of MnO2 nanostructures involves an oriented diffusion of Mn2+ species to the anode surface under an applied electric field and a subsequent electro-oxidation of the species into MnO2. The anodic deposition reaction can be expressed as :
Three different deposition techniques have been developed: i.e., potentiostatic deposition, potentiodynamic deposition, and galvanostatic deposition. The morphology can be well controlled by modifying the deposition parameters (such as applied potentials, applied currents, scan rates, electrolyte ingredients/concentrations, and pH value, etc.) and by tuning the physiochemical nature of substrates (such as a porous structure, a hydrophilic surface, etc.).
In addition to pure MnO2, conducting polymer-MnO2 nanocomposites can also be fabricated using solution precursors containing both Mn2+ species and monomers . One-step or two-step electrochemical co-deposition has been explored to effectively prepare a nanocomposite. The co-electrodeposition process primarily involves the anodic deposition of MnO2 and the simultaneous electropolymerization of conducting polymers. The morphology of the nanostructured hybrids can be controlled by modifying the deposition parameters.
3.2.2. Cathodic electrodeposition
Cathodic electrodeposition of MnO2 nanostructures can take place at the cathode surface
3.3. Electrostatic interaction assembly
Electrostatic interaction assembly is an effective technique for fabricating nanocomposites
4. Electrochemical performance of pure MnO2 nanostructures
4.1. Amorphous MnO2 powder electrodes
The MnO2 powders prepared at low temperatures are generally amorphous or poorly crystalline in nature . In addition, the as-prepared samples often contain a certain amount of hydrated content, which exists in the form of residual structural water and/or hydrated trivalent MnOOH. The hydrated MnO2 powders have large surface area because of ultrafine nanoparticle morphology. Although the amorphous structure can be maintained up to 300°C, the hydrated amount and the surface area decrease significantly with increasing annealing temperature and dwelling time [8, 24].
The electrochemical properties of the amorphous MnO2 powder electrode are closely related to their surface area and hydrated amount. According to the Eq. (1), an increase in the surface area is rather favorable for enhancing specific capacitance. The hydrated species can also modify the MnO2 surface affinity, which facilitates the electrolyte access and ion transport for enhanced performance. It was revealed that the amorphous MnO2 had the largest surface area and also possessed the best electrochemical performance. A dramatic decrease in the surface area and water content were observed after annealing treatments, which, in turn, degraded the specific capacitance.
4.2. Crystalline MnO2 powder electrodes
As mentioned earlier, MnO2 can exist in various crystal forms. The arrangement of [MnO6] basic building unit enables the construction of one-dimensional, two-dimensional, and three-dimensional tunnel structures. The tunnel space, depending on the tunnel sizes, can accommodate cations, such as K+, Na+, Li+, Ca+, Mg+, etc., which favors the energy storage according to Eq. (2). Recently, Ghodbane et al. has carried out a systematic study dealing with the microstructural effects on electrochemical properties in crystalline MnO2 . As shown in Figure 1, the three-dimensional spinel
Hydrothermal or solvothermal methods are appropriate techniques to prepare MnO2 nanostructures, including nanorods, nanotubes, and nanowires, [4, 15]. The as-prepared products showed distinct morphologies, but the electrochemical results are highly scattered. There is no consistent relationship among the synthesis conditions, microstructures, and specific capacitance. In general, MnO2 with high crystallinity shows a low-specific surface area, and the cations in the tunnel structure are difficult to be extracted for TSM, thereby resulting in limited specific capacitance.
4.3. Thin-film MnO2 electrodes
Thin-film MnO2 electrodes have been massively explored for deepening fundamental studies and finding potential applications as microscale energy storage devices. In this regard, a number of MnO2 thin-film electrodes have been directly prepared on a current collector, such as metal foils, carbon cloth, etc., through anodic/cathodic electrodeposition. In terms of the charge storage mechanism, the most desirable morphology should be of three-dimensional and porous nanoarchitectures with good connection to a highly conductive substrate. Therefore, much effort has focused on the morphology-controlled growth of porous MnO2 nanostructures, with a purpose of obtaining more accessible electroactive sites and short ion diffusion pathways. This can be achieved by controlling the deposition conditions and using porous templates. For example, galvanostatic, or potentiostatic techniques were employed to fabricate porous fibrous network, nanorods, and nanosheets [25–27].
As for anodic electrodeposition, chemically stable metal (e.g., Ti or Ta foils, etc.) and carbon (graphites, carbon fabric, etc.) substrates should be employed to address the problem of anodic oxidation and dissolution of metal substrates (e.g., stainless steel). In sharp contrast, cathodic deposition is a good method to break this limit. Electrodeposition from a dilute NaMnO4− solution (0.02 M) produced a typical surface morphology of fibrous film with a birnessite-type crystal structure . Compared to the pure MnO2 powders in Sections 4.1 and 4.2, the MnO2 thin films can deliver much higher specific capacitance.
Pure MnO2 nanostructures with different crystal phases and morphologies can be easily prepared using solution-based methods. Unfortunately, the overall specific capacitances of the amorphous and crystalline MnO2 powders are mostly ranging from 100 to 250 F/g, which are far from satisfaction. According to the charge storage mechanisms, the Faradaic reactions of MnO2 only occur at the surface or subsurface within tens of nanometers. Thus, to maximize the high electrochemical utilization of MnO2, the thickness of MnO2 should be kept in a nanoscale range. This is confirmed by a high-specific capacitance of 300–800 F/g when MnO2 thin films were deposited on the conductive substrates. The specific capacitance can even reach the theoretical value when thickness of MnO2 film is only several nanometers. The reason can be rationalized by the nanostructured MnO2 shortening both ionic and electronic transport/diffusion distances for faster reaction kinetics. However, the thin films are limited by their low mass loading, which cannot meet large scale practical applications.
5. Electrochemical performance of MnO2-based nanocomposites
5.1. MnO2-carbon nanocomposites
Over the past decades, carbon nanostructures have received intensive attention in various communities of material science and chemistry. First, carbon nanostructures are the most effective electrode material candidates for EDLCs owing to the versatile size dimensionality, high surface area, good electrical conductivity, and strong structural stability. Nevertheless, carbon-based electrodes are limited by their low specific capacitance. To break this limitation, carbon structures often serve as favorable supports for MnO2 by making complementary functions. In this section, we emphasize on the composites with one-dimensional carbon nanotubes (CNTs) two-dimensional, graphene, and three-dimensional mesoporous carbon .
CNTs exhibit fascinating physicochemical properties of high electrical conductivity, high surface area, good chemical stability, and strong mechanical strength . CNTs are increasingly employed as substrates for anchoring pseudocapacitive MnO2, to form nanocomposites. The nanocomposites can utilize the large pseudocapacitance of MnO2 and the conducting and mechanical merits from CNTs. Moreover, the nanocomposites could inherit the well-developed CNTs networks with open mesopores. Consequently, the MnO2-CNTs nanocomposites are expected to exhibit superior electrochemical properties. The key issue to obtain high-performance MnO2/CNT nanocomposites is how to uniformly deposit nanostructured MnO2 onto CNTs. To ensure the uniformity, the CNT powders should be homogeneously dispersed in a solution by means of surface modification and/or ultrasonic treatments. The three-dimensional CNT network in the suspension can combine MnO2 nanostructures (e.g., nanorods or nanoclusters) using various chemical or electrochemical deposition methods. The most favorable method is based on a spontaneously redox reaction of MnO4− and carbon, enabling
In addition to the surface deposition, MnO2 nanoparticles can also be embedded into the well-defined CNT nanochannels, as shown in Figure 2b . The confined MnO2 are likely to exist in a more reduced state, and accordingly, the pseudocapacitive performance of MnO2 can be improved by modifying the redox couples of Mn3+/Mn4+. The specific capacitance normalized by MnO2 is almost close to the theoretical value (1250 F/g). These intriguing results may provide new insights into the configuration of MnO2 with nanocarbon for developing high-performance electrode materials.
Graphene is a hottest star material in recent years due to its attractive characteristics of high electrical conductivity, good mechanical flexibility, high theoretical surface area (2600 m2/g), and high thermal and chemical stability . To exploit the potential of graphene-MnO2 nanomaterials for supercapacitors, the main challenge is to prevent graphene nanosheets from restacking during material synthesis and cycling operation. MnO2 nanostructures can act as interlayer spacers to effectively suppress the graphene restacking, thus facilitating fast ion diffusion/transport within the electrode materials. Moreover, the electrical intimate interaction of MnO2 and graphene can boost interfacial charge transfer to ensure rapid redox reactions of MnO2. In addition, graphene is capable of playing a "flexible confinement" role to enwrap MnO2 nanostructures through preventing inner MnO2 from electrochemical dissolution. Therefore, a strong synergetic effects benefiting from the enhanced conductivity, increased interfacial area as well as reinforced structural stability can be yielded in MnO2-graphene nanocomposites. To date, a great number of studies are devoting to (i) addressing the synthesis complexity and scalability, (ii) tailoring MnO2 nanostructures with a desired morphology and mass loadings in between graphene nanosheets, and (iii) improving electrical and mechanical connections between graphene and MnO2.
There are two favorable approaches for the controlled fabrication of the target MnO2/graphene nanocomposites. The first one is based on an
5.1.3. MnO2/carbon nanotube/graphene
CNTs could serve as a spacer to prevent adjacent graphene nanosheets from restacking and as conductive networks to accelerate electron transport of hybrid carbon composites. Recently, a few studies have exploited MnO2/CNT/graphene nanocomposites for high performance supercapacitors. The highly porous and conductive CNT/graphene composites can offer fast electronic and ionic channels for reversible Faradic redox reaction of MnO2 nanostructures. MnO2-coated CNTs were intercalated in between graphene nanosheets to form hierarchical nanocomposites . The core step is the creation of positively charged MnO2/CNT functionalized with PDDA, which drives an electrostatical self-assembly with highly negatively charged graphene nanosheets. In this architecture, MnO2/CNT is able to effectively prevent graphene nanosheets from severe agglomeration. In addition to solution-based chemical assembly, chemical vapor deposition (CVD) method was recently utilized to grow CNT spacers in between graphene nanosheets . The highly conductive network and the porous architecture enable the nanocomposites to exhibit an improvement in the specific capacitance.
5.1.4. MnO2-mesoporous carbon
Compared to activated carbon, ordered mesoporous carbons (OMC) possess uniform mesopores of several nanometers in diameter. The presence of the ordered mesopores is beneficial for electrolyte wetting and rapid ionic motion, which could address the rate-limiting issue of supercapacitors. The favorable mesostructure also enables OMC to be an ideal host material for compositing MnO2 nanostructures. However, the encapsulated MnO2 may result in disappearance of uniform mesopores and generate micropores between the nanoparticles, which prevent the mass transfer of electrolyte ions and also the formation of a double-layer required for high performance supercapacitors . Hence, controlled growth of the nanoparticles homogeneously within the mesopores is crucial.
Figure 4a exhibits a MnO2/OMC composite prepared using an
5.2. MnO2-conducting polymer nanocomposites
Conducting polymers are another class of pseudocapacitive materials due to their fast and reversible doping/undoping kinetics, ease in preparation, environmental stability, and anticorrosion purposes . The conducting polymers could provide good electronic conductivity for MnO2, and in turn, MnO2 offers a solid support and a percolated electrical conducting pathway by interlinking the polymer chains, thus improving charge exchange efficiency and stability during redox cycling. In combination of high pseudocapacitance of both components, a strong synergistic effect is expected to integrate into a composite of MnO2/conducting polymers.
5.2.1. MnO2-polyainiline (PANi)
Polyaniline (PANi) is one of the most used conducting polymers due to its high doping level, good electrical conductivity, and environmental stability. Theoretical specific capacitance of PANi reaches 750 F/g, which makes it being widely used to combine various MnO2 . KMnO4 is the common oxidant to polymerize aniline monomers into PANi accompanying with an instantaneous formation of MnO2:
This method leads to a good contact at an inter-molecule level between each component. It is demonstrated mesoporous MnO2/PANi hollow spheres using an interfacial synthesis technique (Figure 5a) . The coupling reaction was carried out at the organic/water interfacial region, favoring the self-assembly of the composites with high surface area, uniform pore-size distribution, and hierarchical architecture. In addition to the aniline monomers, PANi nanostructures were used as reactive templates to reduce KMnO4, with MnO2 nanostructures depositing onto the PANi surface. Figure 5b shows ultrathin MnO2 nanorods grown on surfaces of conducting polymer nanofibers (PANi, polypyrrole (PPy), and PEDOT (poly(3,4-ethylenedioxythiophene)) by simply soaking the nanofibers in a KMnO4 aqueous solution . Furthermore, owing to the high chemical oxidation potential of MnO2 (1.23 V) in an acidic condition, the aniline monomers can be polymerized with a simultaneous reduction of MnO2 into soluble Mn2+ ions, which provides a rational chemical method for
5.2.2. MnO2-polypyrrole (PPy)
PPy is another low-cost conducting polymer that can be used as promising electrode materials for supercapacitors. It is noteworthy that, unlike PANi showing good supercapacitive behavior in an acidic electrolyte, PPy holds the capability to possess excellent electrochemical properties in neutral electrolytes, which are highly compatible with MnO2. KMnO4 is a proper oxidant to chemically polymerize pyrrole monomers into PPy in relation to the formation of MnO2 by reducing MnO4− . The dispersed MnO2 nanoparticles adhered to PPy chains increase the surface area and retard the structural deterioration of PPy backbones. As mentioned earlier, MnO2 nanostructures could serve as solid reactive templates for
5.2.3. MnO2-polythiophone (PTh)
Lu and Zhou reported a one-pot interfacial synthesis to fabricate mesoporous MnO2/PTh nanocomposite having uniform submicron-sphere/nanosheet hierarchical structures . The significant roles of PTh in the nanocomposite are emphasized in terms of their functions on enhancing the electrical conductivity and constraining the dissolution of MnO2 component.
5.3. MnO2-carbon-conducting polymer ternary nanocomposites
In order to fully exploit the advantages of different carbon and/or conducting polymers, MnO2 can be combined with the dual-supports to make a maximum use of its utilization. Carbon nanostructures (e.g., CNTs) can facilitate easy electrolyte accessibility and fast electron transport into the bulk electrode materials, meanwhile the conducting polymers can contribute to more charge storage and better inter-particle connectivity to MnO2. This ternary design approach is expected to show improved performance, since each component in the ternary nanocomposite serves its unique and desired functions to collectively optimize their electrochemical properties.
As for the MnO2-CNT-conducting polymer nanocomposites, Figure 6a presents a typical synthesis route . The CNT surface was modified by wrapping negatively charged PSS, which electrostatically attract Mn2+ species and monomers (i.e., aniline, pyrrole, and EDOT). The large number of surface sites (–SO3−) have strong interactions of growing nucleus, which facilitate an ordered growth of nanostructured MnO2 and conducting polymers and further hinder the inter-particle agglomeration. The molecular level dispersion of MnO2 in the CNT networks and conducting polymer matrixes results in a strong synergistic interaction. Electrochemical polymerization can also be used to deposit nanometer-thick outer layer of PEDOT onto a coaxial MnO2/CNT composite . Moreover, sonochemical processing was employed to wrap a water-soluble conducting polymer of PEDOT-PSS onto a binary composite of hierarchical MnO2 nanospheres/CNTs, as shown in Figure 6b and c . PEDOT-PSS not only served as a dispersant to stabilize the composite suspension, thus facilitating the electrode fabrication without use of binders, but also offered a good inter-particle connectivity between MnO2 and CNTs.
Carbon nanostructures offer excellent substrates for enhancing the electrochemical performance of MnO2. The carbon can be dispersed homogeneously into a solution, facilitating a uniform deposition of MnO2 nanodeposits onto the carbon surface through a solution-based processing. The high surface area of carbon supports also provides a large number of anchoring sites for MnO2 formation and enlarges the contact interfaces between MnO2 and electrolyte. It is worthy to note that a porous carbon having well-ordered pore channels can permit fast electrolyte transport, meanwhile, a well-graphitic carbon with enhanced electrical conductivity can enable rapid charge collection/transfer under a high-rate operation. Conducting polymers can work in good synergy with MnO2 to capture a maximum electrochemical harvesting from the large pseudocapacitances. A rational design strategy is necessary to control the MnO2 dispersion into conducting polymers by tuning the synthesis conditions. For the
Ternary nanocomposites are capable of maximizing the desirable functions of each component. The synergistic contribution results in great capacitance enhancement, high-rate delivery and better cycling performance. Furthermore, it is important to bridge MnO2 and carbon to electrolytes with a large interaction area by controlling the thickness of conducting polymer layers and their spatial distribution in the ternary structure.
6. Asymmetric supercapacitors based on MnO2 nanostructures
6.1. Fabrication principles
To make an asymmetric supercapacitor that can operated stably in a wide potential range, the critical issue is to couple electrode materials with different high overpotentials for hydrogen or oxygen evolutions . It is known that MnO2 nanostructures normally work in the electrochemical potential window of 0.0–1.0 V (vs. SCE). Therefore, the coupled electrode materials should have a potential window in a range far beyond 0.0–1.0 V. Since the first report of MnO2|| activated carbon asymmetric supercapacitor by Hong et al. in 2002 , carbon materials have attracted massive attention as coupled electrodes, because they can operate in a wide potential window from −1.0 to 0.1 V vs. SCE (Saturated Calomel Electrode) . A wide operating potential window of 2.0 V is yielded in the assembled asymmetric supercapacitor in a Na2SO4 aqueous electrolyte. The asymmetric supercapacitor could even work in a working voltage as high as 2.3 V after elaborating the electrode materials (e.g., mass ratio) and cell assembly (e.g., oxygen expelling). Such a high-cell voltage is comparable to a commercially available symmetric supercapacitors using organic electrolyte (2.5 V) . Consequently, the energy density of asymmetric supercapacitors is exceptionally high, reaching over 20 Wh/kg, almost one order of magnitude higher than that of the aqueous-based symmetric supercapacitor and also greater than that of organic-based SSCs (17–18 Wh/kg).
In addition to the carbon materials, pseudocapacitive materials, such as metal oxides and conducting polymers (e.g., PANi, PPy, and PEDOT), can be used as coupled electrodes for asymmetric supercapacitors. It is noted that the electrochemical oxidation/reduction potential is essentially associated with the work function of a metal oxide . A large work function difference of two metal oxides provide an opportunity of enlarging operating voltage larger than the dissociation energy of an aqueous electrolyte because the water decomposition is kinetically limited by hydrogen and oxygen evolution reaction on the surface of metal oxides. Hence, a metal oxide with a largest work function difference from MnO2 is more appropriate for asymmetric supercapacitor assembly to maximizing cell voltage in neutral aqueous electrolytes.
Moreover, another important technological issue is to balance the charge (
6.2. Asymmetric supercapacitor cells
Carbon materials are of particular interest as negative electrodes for asymmetric supercapacitors due to their characteristics of good EDLC, a high-overpotential for hydrogen evolution, excellent electrical conductivity for high power delivery, and good chemical inertness for long cycle life. In this regard, various carbon materials have been explored including graphene, CNTs, activated carbon nanofibers (ACNFs), carbon sphere (CS), ordered mesoporous carbon, etc., to couple MnO2.
Graphene-based materials have been received intensive attention for asymmetric supercapacitors. First, graphene itself exhibits outstanding physiochemical and capacitive properties. Second, the ultrathin two-dimensional structure of graphene provides a large surface area for anchoring MnO2 nanostructures [54, 55]. In addition, graphene-based materials can also be compacted into free-standing electrodes, which endow asymmetric supercapacitor with flexibility and lightweight. The energy and power values would be more fascinating at a device level taking the additives (binders and conductors) and the current collectors into account. Figure 7 demonstrated porous hybrid thin-film structures of MnO2/graphene and graphene/Ag, which are highly flexible, mechanically robust, and ultra-lightweight using the ordinal filtration assembly method . The flexible device extended the operating potential window to 1.8 V and showed an energy density of 50.8 Wh/kg.
CNTs and carbon nanofiber fabrics were demonstrated to serve excellent one-dimensional negative materials for energy storage devices due to their high electrical conductivity, good mechanical stability, and flexibility [56–60]. On one hand, MnO2 nanostructures can be uniformly deposited surround the one-dimensional carbons and the resulting nanocomposites showed pronounced electrochemical properties. On the other hand, the porous one-dimensional carbons possess good electric double-layer capacitance, thus becoming highly promising negative candidates for asymmetric supercapacitors. The asymmetric supercapacitors assembled by one-dimensional carbons as the negative electrodes and MnO2/carbon nanocomposites as the positive electrode can be cycled reversibly in a high-voltage region of 0–2.0 V and exhibited a superior energy density of 30–35 Wh/kg in a neutral aqueous Na2SO4 electrolyte. In particular, the asymmetric supercapacitors based on electrospun carbon nanofibers are rather promising to meet the ever-increasing demands of high energy/power densities because of the scalability, easy-fabrication, and low-cost of the electrospinning technique .
In addition to the carbon materials, metal oxides and conducting polymers, such as SnO2, MoO3 PANi, PPy, and PEDOT, were also explored as negative materials to couple MnO2 nanostructures [51, 61]. MoO3 would be the most favorable coupled pair of MnO2 because of their largest work function difference . To this end, MoO3/graphene composite and MnO2/graphene nanocomposite were prepared to serve as the negative electrode and the positive electrode for the fabrication of an asymmetric supercapacitor. The as-assembled device can operated stably in a wide voltage window of 2.0 V, and more significantly, exhibited a high-specific capacitance of 307 F/g and a high-energy density of 42.6 Wh/kg.
7. Summaries and perspectives
MnO2-based electrode materials hold a particular prospect for future supercapacitor applications although there are still some obstacles that need to be conquered. To better utilize the electrochemical performance of MnO2, it is necessary to create more electrochemically active sites or reduce the ion/electron transport distance by modifying surface chemistry and structure of MnO2. One of the most effective approaches is incorporating nanoscaled MnO2 into a highly porous and electronically conductive framework, such as carbon scaffolds and conducting polymers, to form a hybrid material. Benefiting from the synergistic contribution from each component in the composite, the consistent progress in the research and development of MnO2-based nanocomposites have achieved many breakthroughs in terms of charge storage mechanism, smart design strategies, and technological innovations. It should be noted that more advanced characterization tools and new methodologies as well systematic studies are still required to deepen the fundamental understanding of material chemistry and electrode/electrolyte interface. The energy and power densities of supercapacitors can be significantly improved by one magnitude through a simple asymmetric configuration method. Although many limiting factors, such as optimal coupled electrode, high-cost issue, complex fabrication process, moderate cycle life, etc., are required to be conquered urgently, it is believed that the ongoing research would bring MnO2-based asymmetric supercapacitors to an acceptable level for practical applications.
The author acknowledge the financial supports of this work by the National Natural Science Foundation of China (51402236), the Natural Science Foundation of Shaanxi Province (2015JM5180), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.: 123-QZ-2015), and the Key Laboratory of New Ceramic and Fine Processing (Tsinghua University, KF201607).
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