MnO₂ nanomaterials for flexible supercapacitors: performance enhancement via intrinsic and extrinsic modification

Abstract

Increasing power and energy demands for next-generation portable and flexible electronics have raised critical requirements (flexibility, stretch-ability, environmental friendliness, lightweight, etc.) for the energy storage devices. Flexible supercapacitors (SCs), as one of the most promising next-generation energy storage devices, have stimulated intensive interest owing to their outstanding features including small size, low weight, ease of handling, excellent reliability, and high power density. Manganese oxide (MnO₂), has attracted much interest in the development of flexible SCs with high electrochemical performance. Yet, the poor electronic and ionic transport in MnO₂ electrodes still limits its promotion in practical applications. This review aims to describe the recent progress in the application of MnO₂ materials in the development of flexible SCs and summarizes the intrinsic modification of MnO₂via crystallinity, crystal structure, and oxygen vacancy introduction and the extrinsic modification of MnO₂via non-three-dimensional (3D) and 3D flexible conductive scaffolds for high performance flexible SCs. Moreover, we also discuss briefly on the current challenges, future directions, and opportunities for the development of high-performance MnO₂ based flexible SCs.

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Teng Zhai

Teng Zhai received his PhD degree in Physical Chemistry from Sun Yat-sen University in 2015. He is now an Assistant Professor in Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology. His research interests include the synthesis of metal oxide nanomaterials and its application in energy storage.

Xihong Lu

Xihong Lu received his BS degree (2008) and his PhD degree (2013) in Physical Chemistry from Sun Yat-Sen University, P. R. China. Then he joined the Sun Yat-Sen University as an associate professor in the school of chemistry and chemical engineering at Sun Yat-Sen University. His research interests focus on the development of functionally nanostructured materials for applications in supercapacitors, Li ion batteries, and photocatalysts.

Hui Xia

Hui Xia received his PhD degree in Advanced Materials for Micro- and Nano- Systems from Singapore-MIT Alliance, National University of Singapore in 2008. He is now a Professor in School of Materials Science and Engineering, Nanjing University of Science and Technology. His research interests include electrode materials for lithium-ion batteries and supercapacitors, fabrication of all-solid-state thin film microbatteries, and gas sensors.

Yexiang Tong

Yexiang Tong is currently a professor in School of Chemistry and Chemical Engineering at Sun Yat-Sen University, P. R. China. Prof. Yexiang Tong received his BS in General Chemistry in 1985, MS in Physical Chemistry in 1988, and PhD in Organic Chemistry in 1999 from Sun Yat-Sen University. He joined Sun Yat-Sen University as an Assistant Professor of Chemistry in 1988. His current research focuses on the electrochemical synthesis of alloys, intermetallic compounds and metal oxide nanomaterials, and investigation of their applications for energy conversion and storage.

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1. Introduction

There has been an increasing demand for fossil fuel originated energy in the past thirty years. However, it may not be possible to provide sufficient energy as the world population continues to grow and the limited amount of fossil fuels has begun to diminish.¹ Sustainable energy sources including solar energy, wind, and tidal energy have emerged as promising candidates to replace the conventional fossil fuel.² Since the supply is intermittent and strongly dependent on the natural environment, the development of these sustainable energy sources has been greatly limited by their storage. Moreover, the highly efficient, low cost energy storage devices for these kinds of intermittent renewable energies are still one of the greatest challenges. Supercapacitors (SCs), also known as electrochemical capacitors (ECs), have been recognized for more than fifty years and considered as one of the energy storage systems with the most potential.^3–5 With the rapid growth of electronics, there is increasing demand for flexible or space-saving electronic devices such as wearable electronics, mobile phones, and flexible displays.^6,7 To catch up with the rapid growth of the demand for portable flexible electronics, it is essential to develop high performance and reliable flexible energy storage devices. In this regard, flexible supercapacitors with high energy density, high power density (high-rate current output/input), and high cycling stability must facilitate practical use of portable flexible electronics. However, it remains a major concern to achieve improved electrochemical performance and ultrahigh flexibility simultaneously.

According to the charge storage mechanism of electrode materials in SCs, they can be generally divided into two categories: electrical double-layer capacitors (EDLCs) and pseudocapacitors.⁸Fig. 1 compares the specific capacitances of various types of electrode materials based on these two mechanisms for SCs. The EDLC mechanism based carbon materials store the electrical charge via a physical charge separation process, which is strongly dependent on the pore size distribution and high surface area of the carbon materials. In this regard, highly conductive and chemically stable carbon materials with large surface area, such as graphene,^9–11 carbon nanotubes (CNTs),^12,13 and activated carbon,¹⁴ have been widely used as electrode materials for EDLCs and achieve specific capacitances of around 50–350 F g⁻¹. Due to their conductive electrochemical properties and physical storage process, carbon material based EDLCs can achieve high power (>10 kW kg⁻¹) and long term cycle stability (>10⁵ cycles). However, they suffer from low capacitance derived low energy density (1–10 W h kg⁻¹). In contrast, conducting polymer or metal oxide based pseudocapacitors can deliver much higher specific capacitance due to the fact that charges are stored via fast and reversible faradic reactions at the surface and bulk of the electrode materials. This will certainly lead to higher energy density (20–50 W h kg⁻¹) in comparison with EDLCs. It should be noted that, due to the slow diffusion of ions within the bulk of the electrode and a relatively slower charge storage mechanism compared with simple adsorption/desorption, the pseudocapacitors usually possess a relatively lower power density (0.5–2 kW kg⁻¹). In addition, they often begin to degrade under less than one or two thousand cycles due to changes in their physical structure and dissolution problems. Therefore, significant efforts have been devoted in the past years to the concerns of pseudocapacitors.

Fig. 1 Reported specific capacitances for electrode materials. Reprinted from ref. 15 with the permission from interface, Copyright 2008 – The Electrochemical Society.

Among various pseudocapacitive materials, manganese oxide (MnO₂) is characterized by a low-cost (compared with highly expensive RuO₂) material with a high theoretical capacitance (∼1370 F g⁻¹) in a wider working potential window (∼0.8 to 1 V vs. Ag/AgCl in aqueous electrolytes). Over the past decades, MnO₂ has become one of the most widely investigated pseudocapacitive electrode materials. Latest developments in MnO₂ based supercapacitors have been recently reviewed.^4,16 However, the practical capacitance is around 200–600 F g⁻¹ at a mass loading of less than 1 mg cm⁻²,¹⁷ so the full potential of MnO₂ for SCs still needs to be further exploited. Intensive explorations have shown that one major issue crucial for MnO₂ based SCs to achieve fascinating capacitive performance is fast electron and ion transportation in materials or at interface. To solve this problem, considerable efforts have been devoted to the crystallinity,^18,19 crystal structure,^20–24 morphology, nano or micro structural design,^25–31 and vacancy introduction^32–36 of MnO₂ to pursue high-performance MnO₂ based SCs. Fortunately, progress has been made within recent years. Herein, we review the recent progress in the application of MnO₂ materials in the development of flexible supercapacitors and discuss briefly the current challenges, future directions and opportunities for the development of high-performance MnO₂ based flexible SCs.

2. Evaluation of electrodes and SCs

Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements are two main techniques to evaluate the electrochemical properties of SCs such as specific or areal/volumetric capacitance, rate capability, cycling stability, etc. Generally, the specific or areal/volumetric capacitance can be calculated viaeqn (1) and (2) as follows:

CV curves:

GCD curves:

where C_s (F g⁻¹) is the specific capacitance, Q (C) is the average charge during the charging and discharging process, m (g) is the mass loading of the active materials, ΔV (V) is the potential window, S (A V) is the integrated area of the CV curve, v (V s⁻¹) is the scan rate, I (A) is the constant discharging current, and Δt (s) is the discharging time. For areal (C_a) and volumetric (C_v) capacitance, the m in the calculations should be replaced by the area (cm²) and volume (cm³), respectively.

The average gravimetric power density (P_s) and energy density (E_s) were calculated by using the following eqn (3) and (4):

The corresponding volumetric average power density and volumetric energy density can be obtained by replacing C_s with C_v.

Large values of specific capacitance are always favoured for pursuing the corresponding high energy densities. It should be noted that the mass loading and charge/discharge rate are crucial for the evaluation of specific capacitance. High specific capacitance at higher mass loading and charge/discharge rate, which means high areal capacitance and output power, is more meaningful for SCs at a practical device level. With respect to high energy density, the unquestioning pursuit of a large voltage window is also meaningless. High coulombic efficiency and highly stable and reversible reactions in the selected voltage window are key parameters for evaluations.

3. High-performance MnO₂ based flexible electrodes

Since the first report about the supercapacitor behavior of MnO₂ by Lee and Goodenough in 1999,³⁷ it has attracted intensive attention and is considered as one of the most promising active materials for SCs due to its high theoretical capacitance (∼1300 F g⁻¹), low cost, environmentally friendly features.^38,39 The high capacitance of MnO₂ mainly originates from its pseudocapacitance, which can be attributed to the faradic reactions occurring at the surface and subsurface in the bulk of the MnO₂ phase. Previous reports^40,41 have indicated that the adsorption/desorption (at surface) or intercalation/deintercalation (at subsurface) of protons and cations are involved in the charge storage process, as well as the transition between Mn(III) and Mn(IV):

Mn^III_(x+y)Mn^IV_1−(x+y)OOC_xH_y ↔ Mn^IVO₂ + xC⁺ + yH⁺ + (x + y)e⁻ (5)

where C⁺ = Li⁺, Na⁺, K⁺. Since both the electrons and ions (protons, and/or cations) are involved in the charge storage process, it is crucial to achieve high ionic and electronic conductivity in MnO₂ electrodes.

Fig. 2a presents the schematic of cyclic voltammetry (CV) for a single MnO₂ electrode in a mild aqueous electrolyte (0.1 M K₂SO₄).⁴² The dotted lines highlight the successive multiple surface redox reactions occurring during the charge/discharge process in the potential window 0–1 V vs. Ag/AgCl. The oxidation from Mn(III) to Mn(IV) accounts for the upper (red) part, while the reverse reduction relates to the lower (blue) part. Clearly, the fast, reversible, and successive multiple reactions take place throughout the potential range 0–1 V, leading to a well-defined rectangular shaped CV curve (analogous to EDLC behavior). It has been reported that the reversible redox transition of Mn(II)/Mn(III) may also be involved in the charge storage process.^4,43 Very recently, Yu-Ting Weng et al. investigated in detail the transitions between Mn(III)/Mn(II), Mn(IV)/Mn(III) within the electrode potential window −1–1 V vs. Ag/AgCl.⁴³Fig. 2b shows the schematic expression of various charge-storage mechanisms for MnO₂ as a function of potential. At a low scan rate of 2 mV s⁻¹, the battery behavior of transition between Mn(II)/Mn(III) accounts for ∼1/2 of the charge storage. However, the attribution of Mn(II)/Mn(III) decreases sharply when the scan rate increases to 100 mV s⁻¹, indicating that achieving high ionic and electronic conductivities is the major issue for MnO₂ to achieve fascinating capacitive performance. Unfortunately, the MnO₂ always suffers from poor electronic (10⁻⁶ to 10⁻³ S cm⁻¹) and ionic conductivities, resulting in a poor specific capacitance of around 300–400 F g⁻¹ (∼1 mg cm⁻²), which is far from its theoretical capacitance of ∼1300 F g⁻¹.^44,45

Fig. 2 (a) Schematic of cyclic voltammetry for a MnO₂ electrode in a mild aqueous electrolyte (0.1 M K₂SO₄). The dotted lines highlight the successive multiple surface redox reactions occurring during the charge/discharge process: oxidation from Mn(III) to Mn(IV) accounts for the upper (red) part, while the reverse reduction relates to the lower (blue) part. Adapted with permission from ref. 42, Copyright Nature Publishing Group 2008. (b) A schematic expression of various charge-storage mechanisms as a function of potential; B: battery behavior; PC: pseudocapacitance; (IV), (III), and (II): Mn ions having valences of 4, 3, and 2, respectively. Adapted with permission from ref. 43, Copyright Wiley-VCH 2015.

As reported in the literature in recent years, both the intrinsic modification (crystal structure and crystallinity, microstructural construction, and valence introduction) and extrinsic modification (non-3D and 3D carbon and metal oxide based composites) were used to synthesize MnO₂ with improved ionic and electronic properties for high performance flexible SCs.

3.1 Intrinsically modified MnO₂ electrodes for flexible SCs

3.1.1 Crystallinity and crystal structure. Similar to other transition metal oxides, crystallinity is one of the important factors to intrinsically tune the capacitive performance of MnO₂.^18,46,47 MnO₂ with lower crystallinity possesses a porous microstructure (higher surface area) and fast ion transportation. However, it will also result in a lower electrical conductivity. Thus, one optimized point with appropriate ion transportation and electrical conductivity can be found via tuning the crystallinity of MnO₂ electrodes. With respect to the crystallinity, the annealing temperature is a common effective approach to achieve optimal electrochemical properties. Wei et al.¹⁸ annealed the as prepared MnO₂ at 200 °C and found that the calcination lead to the reduced porosity and improved crystallinity. For crystallized MnO₂, a series of crystalline structures, including α-, β-, δ-MnO₂ (birnessite), etc., have been investigated in detail.^20–24,48 Among them, α- and β-MnO₂ possess a tunnel structure: a mixture of 2 × 2 octahedral and 1 × 1 octahedral units for α-MnO₂ (Fig. 3a); 1 × 1 octahedral units for β-MnO₂ (Fig. 3a). Fig. 3b shows the typical XRD patterns of α-MnO₂ and β-MnO₂. α-MnO₂ shows a pure tetragonal phase [space group: I4/m (87)] with lattice constants a = 9.7847 Å and c = 2.8630 Å (JCPDS 44-0141), while the β-MnO₂ presents a pure tetragonal phase [space group: P42/mnm (136)] with lattice constants a = 4.3999 Å and c = 2.8740 Å (JCPDS 24-0735). It has been reported that the crystalline α-MnO₂ achieved a bulk specific capacitance of ∼200 F g⁻¹,⁴¹ while the crystalline β-MnO₂ phase exhibited a bulk capacitance of ∼10 F g⁻¹.²³ The tunnel sizes of these two crystal structures are proposed as the explanation for this phenomenon.^49,50 Until very recently, Matthias J. Young and coworkers⁴¹ have presented for the first time the detailed charge storage mechanism of α-MnO₂ and explained the capacity differences between α- and β-MnO₂ using a combined theoretical electrochemical and band structure analysis. Fig. 3c shows the absolute band edge energies of α-MnO₂ and β-MnO₂ and the electrochemical scanned potential window (SPW) for MnO₂ in aqueous electrolyte at a pH of 7.4. The charge-switching states were induced by interstitial cations in α-MnO₂ through stabilization of Mn–O antibonding orbitals from the α-MnO₂ conduction band. Moreover, the cations stabilize high energy dangling O 2p bonds resulting from Mn vacancies. δ-MnO₂ (birnessite), as a 2D material with an open layered structure (inset of Fig. 3d), has received intensive attention in recent years.^51,52 Especially for planar supercapacitors, which enable the entire device to be much thinner and flexible, quasi-2D graphene like materials are more favorable.^53–55 Lele Peng and coworkers²² reported for the first time a high-performance in-plane SC based on hybrid nanostructures of quasi-2D ultrathin δ-MnO₂/graphene nanosheets. Fig. 3d shows the XRD pattern of the monoclinic potassium birnessite (JCPDS 80-1098). The peaks of (002) and (006) facets observed in the pattern indicate a well-defined c-orientation, which can facilitate the fabrication of c-oriented thin films. Fig. 3e shows the schematic description of the 2D planar ion transport favored within the 2D δ-MnO₂/graphene hybrid structures. The δ-MnO₂ nanosheets integrated on graphene can introduce more electrochemically active surfaces for absorption/desorption of electrolyte ions. Besides, it can also bring additional interfaces at the hybridized interlayer areas to facilitate charge transport during the charging/discharging process. The specific capacitance of the as prepared planar supercapacitors based on the 2D δ-MnO₂/graphene hybrid at different scan rates (from 50 to 400 mV s⁻¹), is shown in Fig. 3f. The δ-MnO₂/graphene based planar supercapacitor exhibits enhanced specific capacitance (254 F g⁻¹ at 0.5 A g⁻¹) in comparison with the one based on graphene (∼140 F g⁻¹ at 0.5 A g⁻¹). Furthermore, the former achieved a better rate capability (∼78%, 0.5 to 10 A g⁻¹), which is substantially higher than that of graphene-based electrodes (∼44%, 0.5 to 10 A g⁻¹), confirming the enhanced charge transportation.

Fig. 3 (a) Schematic crystal structure of α-MnO₂ and β-MnO₂. (b) XRD patterns of α-MnO₂ and β-MnO₂. Reprinted from ref. 104 with permission from Copyright© 2002, American Chemical Society. (c) Absolute band edge energies of α-MnO₂ (left) and β-MnO₂ (center) and the electrochemical scanned potential window (SPW) for MnO₂ in aqueous electrolyte at pH = 7.4 (right). Reprinted from ref. 41 with permission from Copyright© 2015, American Chemical Society. (d) The XRD pattern of the bulk birnessite. The inset is a schematic crystal structure of δ-MnO₂ (birnessite). (e) A schematic description of the 2D planar ion transport favored within the 2D δ-MnO₂ /graphene hybrid structures. (f) Specific capacitance of planar supercapacitors based on the 2D δ-MnO₂/graphene hybrid at different scan rates (from 0.5–10 A g⁻¹). The insets show the different bending states (folded and rolled) of planar supercapacitors. Reprinted from ref. 22 with permission from Copyright© 2013, American Chemical Society.

3.1.2 Nanostructural and microstructural construction. Due to the high surface area, the nanostructured MnO₂ electrodes can achieve substantially improved electrochemical performance compared with the bulk MnO₂ electrodes. Thus, the construction of nanostructural MnO₂ electrodes is an effective approach to achieve fast ion transport for MnO₂. Up to now, MnO₂ with various morphologies, such as nanorods,^56–58 nanoflower arrays,⁵⁹ nanosheets,¹⁹ nanowires,⁶⁰etc., have been intensively reported. Accordingly, the specific surface area of nanostructured MnO₂ electrodes ranges from 20 to 150 m² g⁻¹. Lai and his coworkers²⁸ synthesized MnO₂ with nanosheet (NS) and nanorod (NRs) morphologies on highly conductive NiCo₂O₄-doped carbon nanofibers (NCCNFs). Fig. 4a and b present the illustration of the electron and ion transport pathways in the NiCo₂O₄-NCCNF@MnO₂ NS and NCCNF@MnO₂ NRs hybrid membranes. The construction of the nanostructured MnO₂ on the surface of NCCNFs effectively prevents the MnO₂ nanoparticles from aggregation, which will not only increase their specific surface area but also provide more active sites for the ionic adsorption. Furthermore, Xu et al.²⁵ reported a flexible asymmetric supercapacitor (ASC) based on the MnO₂ nanoflower@carbon nano fiber (CNF) cathode and CNF anode (Fig. 4c and d).²⁵ The flower-like MnO₂ material with a porous structure enables fast ion transportation via significantly shortening the diffusion paths, and consequently ensures a high electrochemical performance. A good rate capability, with about 54% retention of the specific capacitance as the current density increases from 3 to 30 mA cm⁻², was achieved by the flower-like MnO₂ on CNFs. It should be noted that this value is achieved at a high mass loading of 3.1 mg cm⁻², which is more persuasive. Significantly, the as assembled ASC device also shows outstanding mechanical flexiblity. Fig. 4e illustrates that the bending states exert a negligible effect on its electochemical properties. Finally, the MnO₂ nanoflower/CNF based ASC device delivers a maximum energy density of 11.3 W h kg⁻¹ at a power density of 352.6 W kg⁻¹, which is comparable and higher than other reported values.

Fig. 4 Illustration of the electron and ion transport pathways in the (a) NiCo₂O₄-doped carbon nanofiber (NCCNF)@MnO₂ nanosheet hybrid membrane and (b) NCCNF@MnO₂ nanorod hybrid membrane. Reprinted from ref. 28 with permission from Copyright© 2015, American Chemical Society. (c) The SEM image of MnO₂ nanoflowers on carbon nanofibers. (d) A schematic illustration of the as-assembled MnO₂ nanoflower based flexible SCs. The digital images present its flexibility. (e) CV curves (collected at 200 mV s⁻¹) of the as-assembled flexible SCs under flat and bending states. (f) Ragone plots of the MnO₂ nanoflower based device. The values reported for other ASCs are added for comparison. Adapted with permission from ref. 25, Copyright Wiley-VCH 2015.

Besides the nano-structured construction of MnO₂ materials, the micro-structured construction of MnO₂ electrodes is another effective method for MnO₂ to achieve fast ion transport during the charge storage process. The fiber or wire based SCs have been considered to possess distinct advantages over the planar counterparts in the development of reconfigurable, lightweight, and portable electronics.^30,61–63 Significantly, a wire-shaped device can be co-woven using the well-developed textile technology.⁶⁴ Xu et al.³¹ fabricated an all-solid-state fiber-shaped ASC via wrapping a conducting carbon paper on a MnO₂ nanoflower coated nanoporous gold wire. The MnO₂ nanoflower coated nanoporous gold wire electrode exhibits an outstanding capacitance retention of ∼88.2% as the current density increases from 0.5 to 8 mA cm⁻², indicating its high electronic and ionic conductivities. Fig. 5a shows the CV curves of the as fabricated fiber-shaped ASC device under different bending states collected at a scan rate of 100 mV s⁻¹. Negligible electrochemical performance changes can be observed for the fiber-shaped device under different bending states (0°, 90°, and 180°). Moreover, the devices are woven into textile structures to show the great potential for future flexible electronic devices (Fig. 5b).

Fig. 5 (a) CV curves of the fiber-shaped ASC device under different bending states collected at a scan rate of 100 mV s⁻¹. The insets are the photo images of the device under different bending states (0°, 90°, 180°). (b) The photograph of the fiber-shaped device integrated with conventional cotton yarns. Reprinted from ref. 31 with the permission from the Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014.

Aiming at applications in wearable electronics, the stretch ability has become an important factor for flexible fiber shaped SCs. However, the stretchable fiber shaped SCs are still rarely reported or still suffer from low capacitance, low working potential (usually 0.8 V).^65–67 Until very recently, Xu and his co-workers³⁰ assembled a stretchable fiber-shaped ASC constituted of an MnO₂/CNT hybrid fiber positive electrode, a CNT fiber negative electrode and a KOH–PVA electrolyte (Fig. 6a). By taking advantage of the stable working potential of the MnO₂/CNT fiber positive electrode (0–0.5 V vs. Ag/AgCl) and CNT fiber negative electrode (−0.4 to −1 V vs. Ag/AgCl), the voltage window of the as assembled ASC device was extended to 1.5 V. A high specific capacitance of ∼157.53 μF cm⁻¹ at 50 mV s⁻¹ was delivered in an extended voltage window of 0–1.5 V. Remarkably, a cyclic tensile strain of up to 100% presents negligible effects on the electrochemical performance of the stretchable ASC device. As shown in Fig. 6b, the redox peaks are still well maintained at different tensile strains (0%, 40%, 70%, and 100%) and even after 20 mechanical stretching–releasing cycles (MSRCs), indicating the negligible effects of stretch states to the capacitive performance. The minor capacitance change for the stretched ASC device at various scan rates again confirms its high stretch ability (Fig. 6c). Moreover, the specific capacitance still retains more than 99% after 10 000 galvanostatic charge/discharge cycles, indicating the long cycling stability.

Fig. 6 (a) Photo images of the as assembled stretchable ASC device. (b) CV curves at different stretching states. (c) Specific capacitance variations with scan rates. (d) Cycling performance of the asymmetric supercapacitor at a current density of 2.8 mA cm⁻² after 20 MSRCs. Reprinted from ref. 30 with permission from the Copyright© 2015, American Chemical Society.

3.1.3 Oxygen vacancy introduction. The introduction of oxygen vacancies into MnO₂ is believed to be helpful for improving the conductivity and charge storage, and thus enhancing its electrochemical performances.^68–70 The presence of oxygen vacancies in MnO₂ would lead to the charge compensation by changes of oxidation states of Mn ions (Mn²⁺, Mn³⁺, and Mn⁴⁺), which will further result in variation of charge carrier density and make the deficient MnO₂ more conductive.^105,106 Moreover, Song et al.⁶⁹ performed in situ X-ray absorption near edge spectroscopy (XANES) and density functional theory (DFT) calculations to gain insights into charge storage mechanisms of mixed-valent MnO_x. Due to the Mn³⁺ 3d and the O 2p orbitals, excess negative charges can be spilled onto the neighboring O atoms, which means that more charges can be stored in deficient MnO₂. In recent years, the researchers have developed two major strategies to introduce oxygen vacancies into MnO₂. The first one is atomic doping with lower valence state impurity.^71–74 Chen et al.⁷⁵ proposed a strategy to facilitate the formation of oxygen vacancies in ramsdellite-MnO₂ (R-MnO₂) via the introduction of lower valence-state doping, which is effective to enhance the conductivity and activity of R-MnO₂. Fig. 7a shows the charge density difference of (010) and (110) surfaces of Zn-doped ramsdellite-MnO₂ with oxygen vacancies, presenting that the oxygen vacancies are positively charged, and the electron transfer to near Mn or Zn atoms. Consequently, the charged surface Mn atoms will serve as active sites with better chemical activities. Moreover, the dangling bond density by coordinately unsaturated Mn will increase as the coordination number of Mn atoms near the vacancies decreases, which will enhance the surface activity.⁷⁶ In other words, as shown in Fig. 7b, enhanced electron diffusion to the surface can be achieved by the bulk Zn dopants. The surface oxygen vacancies will draw the electrons to the reaction sites, where the oxygen vacancies and reduced Mn ions will improve the activity of the electrode reactions in SCs. Kang et al.⁷⁷ also reported the remarkably improved conductivity of MnO₂via atomic doping of Au atoms (Fig. 7c) and the Au-doped MnO₂ film exhibited substantially improved capacitive performance.

Fig. 7 (a) The charge density difference of (010) and (110) surfaces of Zn-doped ramsdellite-MnO₂ with the oxygen vacancies of site I. The blue and yellow areas highlight the electron loss and gain, respectively. (b) A schematic illustration of the effects on the MnO₂ materials from Zn²⁺ doping. Reprinted from ref. 75 with the permission from Royal Society of Chemistry 2015. (c) A sketch of the fabrication process of the Au-doped MnO₂ electrodes. Reprinted from ref. 77 with the permission from 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Annealing in de-oxygen or reducing in ambient atmosphere is the second effective approach for the introduction of oxygen vacancies into MnO₂.^36,70 Zhai et al. demonstrated an effective strategy, annealed MnO₂ in hydrogen atmosphere (Fig. 8a), to intrinsically improve the conductivity and capacitive performance of MnO₂ by inducing oxygen vacancies.³⁶Fig. 8b shows the areal capacitance and surface Mn oxidation state of hydrogenated MnO₂ (H-MnO₂) as a function of the hydrogenated temperature. It could be observed that the H-MnO₂ samples exhibit a higher areal capacitance in comparison with the untreated MnO₂ sample. Moreover, H-MnO₂ hydrogenated at 250 °C with moderate oxygen vacancies exhibits the highest areal capacitance of 0.22 F cm⁻² (449 F g⁻¹) at 0.75 mA cm⁻². Significantly, a solid-state ASC based on the H-MnO₂ and reduced graphene oxide was assembled and exhibited a higher energy density and power density (0.25 mW h cm⁻³ at 1.01 W cm⁻³) over other reported SC devices (Fig. 8c). Finally, to test the feasibility of the H-MnO₂//RGO ASC device as an energy storage device in wearable electronics, the as-fabricated flexible ASC was knitted into laboratory clothing as a demonstration (Fig. 8d).

Fig. 8 (a) Schematic diagram illustrating the growth process for preparing hydrogenated MnO₂ (H-MnO₂) NRs on carbon cloth substrate. (b) Areal capacitance and surface Mn oxidation state of H-MnO₂ as a function of hydrogenated temperature. (c) Ragone plots of the H-MnO₂//RGO ASC device measured in the gel electrolyte. The values reported for other SC devices are added for comparison. (d) Photo images of the cloth model and the zoom-in image of wearable ASCs sewing on the cloth model. Reproduced from the ref. 36 with the permission from 2014 Elsevier Ltd.

3.2 Extrinsically modified MnO₂ electrodes for flexible SCs

In addition to intrinsic modification of MnO₂, the extrinsic combination of MnO₂ with highly conductive materials, such as carbon based materials,^78–82 metal oxides or conductive polymer based materials,^83–85 has also been investigated extensively for flexible SCs. As the flexible SCs raised critical requirements for these highly conductive supports, the supports for MnO₂ are required to have high mechanical integrity upon bending or folding and lightweight property and excellent electrochemical properties.⁸⁶ Herein, we review in detail the MnO₂ materials grown on non-3D and 3D conductive and flexible supports for flexible SCs.

3.2.1 Non-3D scaffold supported MnO₂ for flexible supercapacitors. To meet the demands of flexible supercapacitors, several kinds of non-3D supports including CNTs or graphene paper,^78,80,87,88 carbon fabrics,^84,85,89 polyethylene terephthalate (PET), transition metal oxide or conductive polymer nanowires,^38,85 are possible choices for flexible supercapacitor electrodes. Table 1 summarized the typical results of capacitive performance of electrodes and SCs based on different platform supports. Though good capacitive performance as well as outstanding flexibility has been achieved, the low surface area of these flat supports limited the further improvements of power and energy density for flexible SCs.

Table 1 Typical results of capacitive performance of electrodes and SCs based on different platform supports

Active materials	Conductive scaffold	Capacitance	Mass loading	Flexibility (capacitance retention %)	Energy density retention of SCs	Energy density (E)	Power density (P)	Cycle stability	Ref.
MnO2	Graphene/CNT paper	486.6 F g^−1 at 2 mV s^−1	—	Negligible variation of electrical conductivity (1000 bending cycles)	∼40% (power density form 150 to 2250 W kg^−1)	24.8 W h kg^−1	∼150 W kg^−1	—	80
MnO2	3D hollow structured graphene	∼280 F g^−1 at 2 mV s^−1	0.42 mg cm^−2	92% (200 bending cycles)	∼55% (P form 62 to 2500 W kg^−1)	6.8 W h kg^−1	62 W kg^−1	81.2% after 5000 cycles	78
MnO2	3D graphene/CNT	343.1 F g^−1 at 2 mV s^−1	0.21 mg cm^−2	—	∼56.8% (P form 170.5 to 22727 W kg^−1)	33.71 W h kg^−1	170.5 W kg^−1	95.3% after 1000 cycles	90
MnO2	Graphene foam	422.5 F g^−1 at 1 A g^−1	—	∼97% after bending 180° for 100 times.	∼57.2% (P form 453.6 to 9188.1 W kg^−1)	31.8 W h kg^−1	453.6 W kg^−1	84.4% after 10 000 cycles	82
MnO2	H-TiO2 nanowires@ carbon cloth	449.6 F g^−1 at 10 mV s^−1	0.23 mg cm^−2	Negligible variation at twisted and bending states	∼56.7% (P form 35 000 to 45 000 W kg^−1)	0.30 mW h cm^−3 (59 W h kg^−1)	0.23 W cm^−3 (45 kW kg^−1)	91.2% after 5000 cycles	85
MnO2 nanorod	Graphene film	209 F g^−1 at 1 mV s	—	∼97.2% at the bent states	∼24% (P form 101.5 to 24 500 W kg^−1)	50.8 W h kg^−1	101.5 W kg^−1	81% after 1000 cycles	87
MnO2	Few walled CNT paper	203 F g^−1 at 2 mV s^−1	—	—	89% (P form 130 to 7800 W kg^−1)	23.9 W h kg^−1	7.8 kW kg^−1	95% after 2000 cycles	88
Ppy/MnO2	Carbon fiber	—	—	99.76% when it was rolled up	∼26% (P form 0.05 to 2 W cm^−3)	6.16 mW h cm^−3	0.05 W cm^−3	86.7% after 1000 cycles	84
MnO2	CNF paper	525 mF cm^−2 (110 F g^−1) at 3 mA cm^2	3.1 mg cm^−2	Yes	80.5% (P form 352.6 to 3370 W kg^−1)	43.4 μW h cm^−2 (11.3 W h kg^−1)	1.35 mW cm^−2 (352.6 W kg^−1)	85% after 4000 cycles	25
CuCo2O4@MnO2 nanowire	Carbon fabrics	327 F g^−1 at 1.25 A g^−1	0.5 mg cm^−2	No significant deviations at 0, 30, 60, 90°	70% (P form 0.4745 to 3120 mW cm^−2)	94.3 W h cm^−2	0.4757 mW cm^−2	Long-term cycling life over 3000 cycles in different bent states	89
MnO2	Polyethylene terephthalate	4.72 mF cm^−2 at 5 μA cm^−2	—	SC No obvious change bent at 0, 90°	—	—	—	85.5% capacitance retention after 500 cycles	91
Ppy/MnO2	CNT textile	461.0 F g^−1 at 0.2 A g^−1	—	SC 96.2% of initial value after 750 000 bending cycles	—	31.1 W h kg^−1	22.1 kW kg^−1	93.8% capacitance retention after 10 000 cycles	92
MnO2	Stainless steel mesh	667 F g^−1 at 5 mV s^−1	—	—				—	93

---

Qiu and his co-workers⁹¹ recently developed for the first time a novel Au@MnO₂ core–shell nanomesh structure on a flexible polymeric substrate. Fig. 9a shows the illustration of the fabrication of flexible SCs based on the flexible Au@MnO₂ nanomesh electrode. The average diameter of the applied polystyrene (PS) particle was around 700 nm and the as prepared Au nanomesh film with a thickness of 50 nm achieved a sheet resistance of 13–18 Ω sq⁻¹, indicating a higher surface area (compared with platform supports) and good conductivities. Moreover, a flexible SC (Fig. 9c) based on the Au@MnO₂ nanomesh electrode was assembled to explore the advantages of this novel design. A stable coulombic efficiency of greater than 95% over 1000 cycles was obtained at flat and bent states, indicating the stable Au nanomesh supports and strong coupling between the Au nanomesh and MnO₂ nanosheets. However, the surface area of Au nanomesh flexible supports still cannot be compared with the one-dimensional (1D) nanostructured supports. Furthermore, the use of Au may hinder the wide range of applications of the Au nanomesh. Lu et al.⁸⁵ developed a high-performance and flexible solid-state ASC device based on 1D core–shell nanowire (NW) electrodes (Fig. 10a). The hydrogen-treated TiO₂ (denoted as H-TiO₂) NWs were adopted as the core (conducting scaffold) to support electrochemically active MnO₂. Fig. 10b presents the CV curves collected for H-TiO₂, MnO₂, TiO₂@MnO₂ and H-TiO₂@MnO₂ electrodes at a scan rate of 100 mV s⁻¹. Obviously, the H-TiO₂@MnO₂ electrode exhibits a substantially higher current density than the values obtained for the MnO₂ and TiO₂@MnO₂ electrodes, which can be attributed to the increased surface area through the H-TiO₂ nanostructured supports and the enhanced charge transport for TiO₂ after hydrogenation. As the scan rate increased from 10 to 200 mV s⁻¹, the H-TiO₂@MnO₂ electrode retained 54.6% of its capacitance, which is also substantially higher than that of the MnO₂ (29.4%) and TiO₂@MnO₂ (43.4%) electrodes, again confirming the enhanced charge transport during the charge/discharge process. Moreover, the electrode materials directly grown on carbon cloth endow the ASC device with outstanding flexibility (Fig. 10d). The as assembled flexible ASC device delivered a maximum energy density of 0.30 mW h cm⁻³ (59 W h kg⁻¹) (Fig. 10e), which is higher than most of the reported SSCs.^11,79,94,95

Fig. 9 (a) Schematic illustration for the fabrication of flexible SCs based on the Au@MnO₂ nanomesh. (b) A schematic diagram of ion diffusion pathways and charge transport channels of the Au@MnO₂ nanomesh electrode. (c) Photo images of the Au@MnO₂ nanomesh electrode based SCs sealed in a PDMS film, flat and bent. (d) Cyclic stability and coulombic efficiency of the device at flat and bending states. Reprinted from ref. 91 with the permission from 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 10 (a) Schematic diagram illustrating the growth processes for H-TiO₂@MnO₂ and H-TiO₂@C core–shell NWs on a carbon cloth substrate. (b) CV curves collected for H-TiO₂, MnO₂, TiO₂@MnO₂ and H-TiO₂@MnO₂ electrodes at a scan rate of 100 mV s⁻¹. (c) Specific capacitance of these electrodes as a function of the scan rate. (d) CV curves collected at a scan rate of 100 mV s⁻¹ for the ASC device under flat, bent, and twisted conditions. The insets are the device pictures under test conditions. (e) Ragone plots of the ASC devices measured in aqueous and gel electrolytes. The values reported for other SC devices are added for comparison. Reprinted from ref. 85 with the permission from 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Similar to other transition metal oxides, MnO₂ also suffers from the delamination problems caused by the large volume changes during charge/discharge cycles, and hence a decrease in electrochemical stability.^96,97 To prevent the delamination of nanostructured MnO₂ electrodes, researchers have tried to coat a thin conductive layer on the top of MnO₂.^92,98 Yun et al.⁹² used the polypyrrole (Ppy) conductive polymer to coat on the top of MnO₂ nanoparticles to prevent delamination (Fig. 11a). With the protected conductive polymer layer, the Ppy–MnO₂/CNT electrode achieved improved cycling stability, with 93.8% normalized capacitance after 10 000 cycles (Fig. 11b), which is higher than that of the MnO₂/CNT electrode. Significantly, the Ppy–MnO₂/CNT based SCs exhibited excellent flexibility (Fig. 11c). As shown in Fig. 11d, the Ppy–MnO₂/CNT based SCs still well retained the almost original capacitive performance even after 750 000 bending cycles, with capacity retention of 98.9, 98.4, and 96.2% of initial value after 250 000, 500 000, and 750 000 bending cycles, respectively.

Fig. 11 (a) Schematic illustration of a Ppy–MnO₂-coated textile SC. (b) Cycling stability of the Ppy–MnO₂-coated textile electrode. (c) A schematic of the bending test performed on the Ppy–MnO₂-coated textile SC. (d) CV curves of the supercapacitor under 13% bending strain collected at 0, 250 000, 500 000, and 750 000 bending cycles. Reprinted from ref. 92 with permission from Copyright© 2015, American Chemical Society.

3.2.2 3D scaffold supported MnO₂ for flexible supercapacitors. The design of 3D architectures for electrodes has been considered as a highly efficient approach because of is ability to reduce “dead surface” of active materials and porous channel facilitated ionic diffusion process. In recent years, 3D constructed MnO₂ electrodes have attracted particular interest compared to lower dimensional structures.⁹⁹ The 3D constructed MnO₂ electrodes usually consist of MnO₂ active materials and conductive 3D scaffolds assembled from low-dimensional units (including one or more types of 0D, 1D, and 2D nanomaterials), which can retain the intrinsic properties of the low-dimensional structure as well as faster and more efficient electronic and ionic transport. Graphene based 3D architectures are one of the most used flexible 3D scaffolds for MnO₂ based SCs owing to the graphene resulted high conductivity and surface area. In addition, its lightweight feature resulted high active mass ratio is quite meaningful for SCs at practical application. For example, Zhai et al.¹⁰⁰ developed an electrochemical capacitor using a highly conductive 3D graphene hollow (3DGH) structure (Fig. 12a) as a current collector to pursue a high active material ratio for the MnO₂ based cathode and V₃S₄ anode. The as prepared 3DGH exhibited an outstanding flexibility (Fig. 12b). Finally, V₃S₄@3DGH and MnO₂@3DGH were assembled into an ASC (Fig. 12c) with a high active material ratio of 24% and delivered a remarkable energy density of 7.4 W h kg⁻¹ (based on the weight of the entire device) at an average power density of 3000 W kg⁻¹. He and his co-workers⁷⁸ also fabricated a flexible symmetrical SC (of weight less than 10 mg and thickness ∼0.8 mm; Fig. 12d) consisting of a sandwich structure of two pieces of 3D graphene/MnO₂ composite network. The 3D graphene network facilitates the synthesis of electrodes with a high active material ratio (∼92.9% of the mass of the entire electrode). Moreover, the 3D graphene/MnO₂ composite network-based symmetrical SC delivered an acceptable cycling stability performance even at bending angle of 90° (Fig. 12e). It should be noted that the mass loading of active materials for the 3D graphene network to assemble into the flexible SC is usually limited at ∼1 mg cm⁻².^78,100 The electrode will be more rigid at a high active material loading level, which will severely limit its further progress. Xia et al.¹⁰¹ reported a new type of 3D porous and thin graphite foam (GF) as a light and conductive substrate for the growth of metal oxide core/shell nanowire arrays to form integrated electrodes. The porous GF possesses a porosity of ∼99.8% and a super high surface area of ∼980 m² g⁻¹, which result in its lightweight (∼4 mg cm⁻³) and excellent scaffold for active materials (Fig. 13a–d). In addition to its excellent conductivity, the GF based electrode presents high specific capacitance (based on the mass of the whole weight of the electrode), good rate capability, and further enhanced energy/power density (Fig. 13e).

Fig. 12 (a) Photo images of 3D graphene/Ni foam (3DG/NFs) and 3D hollow graphene (3DGH) samples. The insets are the mass of each sample (sample area, 6 cm²). (b) An illustration of the flexibility of the 3DGH sample. (c) A schematic illustration of the V₃S₄/3DGH//MnO₂/3DGH device. Reprinted from ref. 100 with permission from Copyright© 2015, American Chemical Society. (d) Digital photographs show the flexible SC at bent states. (e) Cycling stability of the as assembled flexible SC collected at a current density of 1.5 mA cm⁻² for 5000 cycles. The inset shows its cycling performance for bending cycles with a bending angle of 90°. Reprinted from ref. 78 with permission from Copyright© 2012, American Chemical Society.

Fig. 13 (a–d) Schematics of the fabrication process of thin 3D porous graphite foams (GF) and GF based Co₃O₄/PEDOT-MnO₂ core/shell nanowire arrays. (e) Specific capacitance of four positive electrodes as a function of current density (it should be noted that the capacitance of a positive electrode is calculated based on the mass of the whole electrode). (f) Ragone plot of four positive electrodes. (g) Photos of the as assembled flexible asymmetric supercapacitor device. Reprinted from ref. 101 with permission from Copyright© 2014, American Chemical Society.

4. Summary and prospect

Manganese oxide (MnO₂) has stimulated intensive interest due to its application in the development of flexible supercapacitors owing to its outstanding features such as low-cost, high theoretical capacitance, and wide working potential window. However, one of the crucial challenges for the use of MnO₂ as an electrode for flexible supercapacitors is its poor electronic and ionic conductivities, which have hindered its wider application in SCs. To overcome this challenge, intensive efforts have been devoted. In this review, recent advances in intrinsically and extrinsically modified MnO₂ have been summarized, including the approach to tune the crystallinity, synthesis of MnO₂ with certain targeted crystal structure, oxygen vacancy introduction, construction of flexible non-3D and 3D architecture composites. The integration of MnO₂ into low-dimensional supports with high surface areas, and highly conductive and flexible 3D scaffolds could achieve high specific capacitance close to the theoretical value and good rate capability. While, the mass loading of MnO₂ in most of the reports is limited below 1 mg cm⁻², which is meaningless to flexible SCs at a practical level since it will result in a super low volumetric capacitance. Moreover, the interfacial problems between the MnO₂ and the conductive support are still rarely investigated. Due to the introduction of oxygen vacancies into MnO₂, the electronic conductivity is enhanced significantly. However, the concentration of oxygen vacancies cannot be introduced controllably. In particular, there is still lack of a facile, efficient method to introduce oxygen vacancies into bulk MnO₂ instead of just the surface. Given these unresolved problems or challenges, the following aspects might be possible ways to develop really applicable MnO₂ electrodes with ultrahigh energy density and power density for flexible SCs:

1. The mass loading of commercial level (∼10 mg cm⁻²) for MnO₂ active materials has already been achieved via various approaches.^17,102 However, the specific capacitance is still too far from the theoretical value (∼1370 F g⁻¹), i.e., MnO₂/Graphene gel/NFs (234 F g⁻¹, 13.61 mg cm⁻², 10 mV s⁻¹),¹⁷ graphene–MnO₂ composites (147 F g⁻¹, 9.6 mg cm⁻², 2 mV s⁻¹),⁷⁸ MnO₂-CNT-textile (337 F g⁻¹, 8.3 mg cm⁻², 0.05 mV s⁻¹),¹⁰² MnO₂-PEDOT:PSS composites (196 F g⁻¹, 8.5 mg cm⁻², 0.5 mA cm⁻²),¹⁰³ even comparable with commercial activated carbon. The integration with high-surface area carbon or other conductive materials is a common approach for MnO₂ to achieve high capacitive performance at a high mass loading level. However, since the limitation of MnO₂ with high mass loading is the electronic transport in its phase, the intrinsic modification should be the efficient way. In this regard, further investigation might better be focused on the oxygen vacancy introduction to pursue fast electronic transport and high capacitance at a high mass loading level.

2. Introduction of oxygen vacancies, as one of the most efficient methods to intrinsically improve the electronic conductivities, has drawn intensive attention in recent years. Various approaches including annealing in de-oxygen atmosphere, atomic or ion doping are adopted to introduce oxygen vacancies into the MnO₂ active materials. Enhanced electrochemical properties of MnO₂ have been achieved by the oxygen vacancy introduction. However, these kinds of methods can only introduce the deficiency into the surface area of MnO₂. With respect to MnO₂ at a high mass loading, the contribution of the oxygen vacancies is limited to the electrochemical performance of the MnO₂ electrodes. Thus, further investigations of new methods to introduce the oxygen vacancies into the bulk or inactive phase of MnO₂ might be valuable for MnO₂ based flexible SCs.

3. The mechanical properties of 3D conductive scaffolds are still a major concern for their application in flexible SCs. The flexible, wearable electronics have raised critical requirements such as stretch ability, flexibility and twist ability for the flexible SCs and also the electrode. Nevertheless, the 3D scaffolds for MnO₂ active materials in the current study involve the construction of 3D structures. Their flexibility is usually realized via impregnation of elastic polymers like PMMA,⁷⁸ which will remarkably hinder the electron transport and prevent the porous structure from the electrolyte ions. Therefore, new modifications of existing 3D scaffolds such as 3D graphene foam or graphene hydrogel and/or construction of a new type of 3D scaffold need to be developed.

Author contributions

The manuscript was written through the contributions of all the authors. All the authors have given approval to the final version of the manuscript.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We acknowledge the financial support of this work received by the Natural Science Foundation of China (21403306 and 21273290), Guangdong Province Natural Science Foundation for Distinguished Young Project (2014A030306048), Foundation for Youth Innovative Talents in Higher Education of Guangdong (2014KQNCX003) and The Research Foundation of IARC-SYSU (201408).

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