Charge storage mechanisms of manganese oxide nanosheets and N-doped reduced graphene oxide aerogel for high-performance asymmetric supercapacitors

A Review of Green Aerogel- and Xerogel-Based Electrodes for Supercapacitors

The decline in fossil fuels on the earth has become a primary global concern which has urged mankind to explore other viable alternatives. The exorbitant use of fuels by an ever-increasing global population demands a huge production of energy from renewable sources. Renewable energy sources like the sun, wind, and tides have been established as promising substitutes for fossil fuels. However, the availability of these renewable energy sources is dependent on weather and climatic conditions. Thus, this goal can only be achieved if the rate of energy production from renewable sources is enhanced under favorable weather conditions and can be stored using high energy storing devices for future utilization. The energy from renewable sources is principally stored in hydropower plants, superconducting magnetic energy storage systems, and batteries.

In Situ Spectroelectrochemical Detection of Oxygen Evolution Reaction Intermediates with a Carboxylated Graphene-MnO2 Electrocatalyst

In electrocatalyst-assisted water splitting, the oxygen evolution reaction (OER) imposes a performance limit due to the formation of different catalyst-bound intermediates and the scaling relationship of their adsorption energies. To break this scaling relationship in OER, a bifunctional mechanism was proposed recently, in which the energetically demanding step of forming the *OOH intermediate, through the attack of a water molecule on the oxo unit (*O, with * representing a reactive metal center), is facilitated by proton transfer to the second catalytic site. This mechanism was supported theoretically but so far by only very few experiments with a proton-transfer agent in basic media. However, active metal-containing catalysts could be destroyed in alkaline media, raising questions on practical applications. To date, this mechanism still lacks a systematic spectroscopic support by observing the short-lived and limited amount of reactive intermediates. Here, we report an operando Raman spectroscopic observation of the OER intermediates in neutral media, for the first time, via a bifunctional mechanism using a carboxylated graphene-MnO₂ (represented by Gr-C-MnO₂) electrocatalyst. The formation of the Mn-OOH intermediate after the attack of a water molecule on the Mn═O complex is followed by a proton transfer from Mn-OOH to the functionalized carboxylates. The role of the functionalized carboxylates to improve the catalytic efficiency was further confirmed by both pH-dependent and isotope (H/D)-labeling experiments. Furthermore, with a unique strategy of using a hybrid aqueous/nonaqueous electrolyte, the OER was alleviated, allowing sufficient Mn-OH and Mn-OOH intermediates for in situ Raman spectroscopic observation.

Enhancement of V2O5 Li-ion cathode stability by Ni/Co doped Li-borate-based glass

In this research, we investigate the stability of a Li-ion cathode created by mixing a borate based glass which has been doped with Ni/Co and vanadium pentoxide (V2O5).

Reducing the Energy Band Gap of Cobalt Hydroxide Nanosheets with Silver Atoms and Enhancing Their Electrical Conductivity with Silver Nanoparticles

Although cobalt hydroxide (Co(OH)2) has been attracting attention in several applications, its photoelectrochemical property has not yet been fully investigated. In this work, tuning the energy band gap of Co(OH)2 nanosheets with silver atoms and enhancing their electrical conductivity with silver nanoparticles were then focused. A Ag-doped α-Co(OH)2 thin film was successfully synthesized via an electrodeposition method. The optical properties of the as-prepared materials were characterized by UV-vis and fluorescence lifetime spectroscopies and further confirmed by density functional theoretical calculation. It was found that Ag atoms between adjacent layers of Co(OH)2 can reduce its electronic band gap to 2.45 eV (α-Co(OH)2) as compared to 2.85 eV of β-Co(OH)2. In terms of electrochemical properties, silver nanoparticles (AgNPs) can enhance the electrical conductivity of Co(OH)2 nanosheets, leading to faster charge transfer reducing the internal resistance and significantly increasing the overall charge storage performance. Interestingly, under light illumination, Ag-doped α-Co(OH)2 exhibits ca. 0.8 times lower charge storage capacity as compared to that under the dark condition. This is because the photoelectrons can be recombined with the generated holes in the conduction band. The charge storage mechanisms of Ag-doped α-Co(OH)2 operated under dark conditions and light irradiation were further studied and confirmed using in situ electrochemical X-ray absorption spectroscopy (XAS). Overall, the in situ XAS supports the electrochemical result. This finding may pave a way to further develop photoactive advanced functional materials of metal hydroxides and oxides.

Effects of cathode coating materials and operational time on the mercury removal performance of electrokinetic remediation system for marine sediment

Electrokinetic remediation (EK) is a promising in-situ technique for removing mercury (Hg) from contaminated sites; yet it demands long operational periods when conventional electrodes are used. Herein, we investigate the effectiveness of lab-prepared cathodes (Cu foam coated with reduced graphene oxide (rGO) or manganese oxide (MnO₂)) to enhance Hg removal rates from sediment by EK. Although short term (2 h) Hg removal rates were insignificantly different (p-value > 0.05) when using the uncoated and coated Cu foam cathodes, long term (60 h) operations saw greater Hg removal by coated Cu foam cathodes over pure Cu foam, probably owing to the time required for Hg to migrate towards the electrodes from sediment. The highest Hg removal at the cathode was achieved when an αMnO₂-coated Cu foam cathode was used with a strong-base anion exchange membrane (AEM) in the system. Using H₃PO₄, as a cathodic electrolyte resulted in a higher Hg removal efficiency than using NaCl and HCl electrolytes. Electromigration was found to be the dominant Hg-ions (e.g. HHgO₂^-, Hg²⁺) transport mechanism in the marine sediment during remediation. Overall, this research demonstrates that employing enhanced electrodes and AEMs can enhance Hg removal by EK processes in relatively shorter operating times than conventional EK processes.

Synthesis of V-shaped MnO2 nanostructure and its composites with reduced graphene oxide for supercapacitor application

A unique V-shaped MnO₂ nanostructure is synthesized with a weak acid (acetic acid) using the microwave-assisted hydrothermal technique. To improve the performance of MnO₂ in supercapacitor applications, its composite was prepared with reduced graphene oxide (rGO), i.e., MnO₂/rGO, with different weight ratios of MnO₂ and rGO. The specific capacitance values of the as-synthesized V-shaped MnO₂ nanostructure and MnO₂/rGO nanocomposite were calculated to be 64.75 and 88.95 F g^-1 at a current density of 0.5 A g^-1, respectively, in 1 M Na₂SO₄ electrolyte. Furthermore, a two-electrode asymmetric supercapacitor device was fabricated using the MnO₂/rGO nanocomposite as a positive electrode and activated carbon as a negative electrode. The device has shown energy densities of 25.14 and 17.95 W h kg^-1 at 0.25 and 1 kW kg^-1 power densities, respectively. These values suggest that the MnO₂/rGO nanocomposite is a promising material for supercapacitor devices.

Ultra-long cycle life and binder-free manganese-cobalt oxide supercapacitor electrodes through photonic nanostructuring

We report a novel photonic processing technique as a next-generation cost-effective technique to instantaneously synthesize nanostructured manganese-cobalt mixed oxide reduced graphitic oxide (Mn-Co-rGO) for supercapacitor electrodes in energy storage applications. The active material was prepared directly on highly conductive Pt-Si substrate, eliminating the need for a binder. Surface morphological analysis showed that the as-prepared electrodes have a highly porous and resilient nanostructure that facilitates the ion/electron movement during faradaic redox reactions and buffers the volume changes during charge-discharge processes, leading to the improved structural integrity of the electrode. The presence of distinct redox peaks, due to faradaic redox reactions, at all scan rates in the cyclic voltammetry (CV) curves and non-linear nature of the charge-discharge curves suggest the pseudocapacitive charge storage mechanism of the electrode. The electrochemical stability and the life cycle were examined by carrying out galvanostatic charge-discharge (GCD) measurements at 0.40 mA cm^-2 constant areal current density for 80 000 cycles, and the electrode showed 95% specific capacitance retention, exhibiting excellent electrochemical stability and an ultra-long cycle life. Such remarkable electrochemical performance could be attributed to the enhanced conductivity of the electrode, the synergistic effect of metal ions with rGO, and the highly porous morphology, which provides large specific surface area for electrode/electrolyte interaction and facilitates the ion transfer.

Phyto-inspired and scalable approach for the synthesis of PdO-2Mn2O3: a nano-material for application in water splitting electro-catalysis

A modified co-precipitation method has been used for the synthesis of a PdO-2Mn₂O₃ nanocomposite as an efficient electrode material for the electro-catalytic oxygen evolution (OER) and hydrogen evolution reaction (HER). Palladium acetate and manganese acetate in molar ratio 1 : 4 were dissolved in water, and 10 ml of an aqueous solution of phyto-compounds was slowly added until completion of precipitation. The filtered and dried precipitates were then calcined at 450 °C to obtain a blackish brown colored mixture of PdO-2Mn₂O₃ nanocomposite. These particles were analyzed by ultra violet visible spectrophotometry (UV-vis), infrared spectroscopy (FTIR), powder X-ray diffractometry (XRD), scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) for crystallinity, optical properties, and compositional and morphological makeup. Using Tauc's plot, the direct band gap (3.18 eV) was calculated from the absorption spectra. The average crystallite sizes, as calculated from the XRD, were found to be 15 and 14.55 nm for PdO and Mn₂O₃, respectively. A slurry of the phyto-fabricated PdO-2Mn₂O₃ powder was deposited on Ni-foam and tested for electro-catalytic water splitting studies in 1 M KOH solution. The electrode showed excellent OER and HER performance with low over-potential (0.35 V and 121 mV) and Tafel slopes of 115 mV dec^-1 and 219 mV dec^-1, respectively. The outcomes obtained from this study provide a direction for the fabrication of a cost-effective mixed metal oxide based electro-catalyst via an environmentally benign synthesis approach for the generation of clean energy.

Nanostructured manganese oxides electrode with ultra-long lifetime for electrochemical capacitors

We describe the instantaneous fabrication of a highly porous three-dimensional (3D) nanostructured manganese oxides-reduced graphitic oxide (MnO_x-rGO) electrode by using a pulse-photonic processing technique. Such nanostructures facilitate the movement of ions/electrons and offer an extremely high surface area for the electrode/electrolyte interaction. The electrochemical performance was investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) with 1 M KOH as the electrolyte. The as-prepared thin film electrode exhibits excellent electrochemical performance and an ultra-long lifetime by retaining 90% of the initial capacitance even after 100 000 GCD cycles at constant areal current density of 0.4 mA cm⁻². We attribute this excellent lifetime performance to the conductive reduced graphitic oxide, synergistic effects of carbon composite and the metal oxides, and the unique porous nanostructure. Such highly porous morphology also enhances the structural stability of the electrode by buffering the volume changes during the redox processes.

We describe the instantaneous fabrication of a highly porous three-dimensional (3D) nanostructured manganese oxides-reduced graphitic oxide (MnO_x-rGO) electrode by using a pulse-photonic processing technique.

Electrochemical Adsorption of Cs+ Ions on H-Todorokite Nanorods

In this work, manganese oxide with a 3 × 3 tunnel structure was synthesized. Mg-todorokite was treated with HNO₃ to obtain H-todorokite, which was used as an electrode for adsorption of Cs⁺ from aqueous solution by an electrochemical reaction. H-todorokite was electrochemically reduced to lower oxidation state to enhance its negative charge and simultaneously increase the amount of cations because of the charge compensation. The Cs⁺ adsorption on H-todorokite was completed by a coupled electrochemical reaction (the redox reaction between Mn³⁺ and Mn⁴⁺) and an ion-exchange reaction between Cs⁺ and H⁺ ions. Cyclic voltammetry measurements at different pHs and Cs⁺ concentrations were performed. H-todorokite revealed high electrochemical adsorption capacity for Cs⁺ because of the high crystallinity and stability of the materials, which reached 6.0 mmol g^-1 in 0.1 mol·L^-1 Na₂SO₄ solution.

A polypyrrole-adorned, self-supported, pseudocapacitive zinc vanadium oxide nanoflower and nitrogen-doped reduced graphene oxide-based asymmetric supercapacitor device for power density applications

Herein, a distinctive approach has been implemented for exploiting a typical battery material zinc vanadium oxide (ZV) as a supercapacitor electrode material.

Metal Oxide and Hydroxide-Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need-Tailored Devices

Energy storage devices that efficiently use energy, in particular renewable energy, are being actively pursued. Aqueous redox supercapacitors, which operate in high ionic conductivity and environmentally friendly aqueous electrolytes, storing and releasing high amounts of charge with rapid response rate and long cycling life, are emerging as a solution for energy storage applications. At the core of these devices, electrode materials and their assembling into rational configurations are the main factors governing the charge storage properties of supercapacitors. Redox-active metal compounds, particularly oxides and hydroxides that store charge via reversible valence change redox reactions with electrolyte ions, are prospective candidates to optimize the electrochemical performance of supercapacitors. To address this target, collaborative investigations, addressing different streams, from fundamental charge storage mechanisms and electrode materials engineering to need-tailored device assemblies, are the key. Over the last few years, significant achievements in metal oxide and hydroxide-based aqueous supercapacitors have been reported. This work discusses the most recent achievements and trends in this field and brings into the spotlight the authors' viewpoints.

Manganese-Oxide-Based Electrode Materials for Energy Storage Applications: How Close Are We to the Theoretical Capacitance?

Of the transition metals, Mn has the greatest number of different oxides, most of which have a special tunnel structure that enables bulk redox reactions. The high theoretical capacitance and capacity results from a greater number of accessible oxidation states than other transition metals, wide potential window, and the high natural abundance make MnO_x species promising electrode materials for energy storage applications. Although MnO_x electrode materials have been intensely studied over the past decade, their electrochemical performance is still insufficient for practical applications. Currently, there is a trade-off between specific capacitance and loading mass. MnO_x species have intrinsically poor electrical conductivity, and current structural designs are not sophisticated enough to accommodate enough redox-active sites. Recent studies have certainly made progress in increasing capacitance through making use of electrically conductive components and controlling the morphology of the MnO_x species to expose more surface area. To increase the capacitance of MnO_x electrodes to the largest extent without limiting loading mass, further structural design at the nanoscale and manipulation of the electrically conductive component are required. An ideal nanostructure is proposed to guide future research toward closing the gap between achieved and theoretical capacitance, without limiting the loading mass.

High-performance asymmetric supercapacitor based on hierarchical nanocomposites of polyaniline nanoarrays on graphene oxide and its derived N-doped carbon nanoarrays grown on graphene sheets

Activated carbon (AC), as a material for asymmetric supercapacitor (ASC), is the most widely used as negative electrode. However, AC has some electrode kinetic problems which are corresponded to inner-pore ion transport that restrict the maximum specific energy and power that can be attained in an energy storage system. Therefore, it is an important topic for researchers to extend the carbonaceous material with qualified structure for negative electrode supercapacitor. In this work, novel promoted ASC have been fabricated using nanoarrays of polyaniline grown on graphene oxide sheets (PANI-GO) as positive electrode and also, carbonized nitrogen-doped carbon nanoarrays grown on the surface of graphene (CPANI-G) as negative electrode. The porous structure of the as-synthesized CPANI-G can enlarge the specific surface area and progress ion transport into the interior of the electrode materials. From the other point of view, nitrogen doping can impressively improve the wettability of the carbon surface in the electrolyte and upgrade the specific capacitance by a pseudocapacitive effect. Because of the high specific capacitance and distinguished rate performance of PANI-GO and CPANI-G and moreover, the synergistic effects of the two electrodes with the optimum potential window, the ASC display excellent electrochemical performances. In comparison with the symmetric cell based on PANI-GO (40 Wh kg^-1), the fabricated PANI-GO//CPANI-G ASC exhibits a remarkably enhanced maximum energy density of 52 Wh kg^-1. Furthermore, ASC electrode exhibits excellent cycling durability, with 90.3% specific capacitance preserving even after 5000 cycles. These admirable results show great possibilities in developing energy storage devices with high energy and power densities for practical applications.

Nitrogen self-doped carbon aerogels derived from trifunctional benzoxazine monomers as ultralight supercapacitor electrodes

Ultralight benzoxazine-derived porous nitrogen self-doped carbon aerogels with good yield can be prepared by direct polymerization of trifunctional benzoxazine monomers under acid catalysis using concentrated hydrochloric acid. This allows for a significantly widened density range (0.8-4.5 mg cm-3) and avoids any sacrificial etching. When serving as electrode materials for supercapacitors, the resulting hierarchical porous carbon aerogels show ultrahigh specific capacitance, excellent rate performance and good cycling stability (retention of 97.3% even after 10 000 continuous charge-discharge cycles). Besides energy storage devices, the interconnected nanoporous carbon aerogels can also find applications in oil/water separation, heavy metal removal, catalyst supports, and so forth.

High-Performance Asymmetric Supercapacitors Based on the Surfactant/Ionic Liquid Complex Intercalated Reduced Graphene Oxide Composites

In this paper, ionic surfactants are employed to intercalate thermally-reduced graphene oxide (TRG). The ionic interaction between the intercalated surfactant and the ionic liquid could lead to the formation of large-sized ionic aggregates and, hence, enlarge the interlayer distance between the TRG sheets. The morphology and vibration modes of these composites were systematically characterized using XRD (X-ray diffraction), SAXS (small-angle X-ray scattering), and FTIR (Fourier transform infrared spectroscopy). An asymmetric supercapacitor, which consisted of a cationic surfactant-intercalated electrode on one side and an anionic surfactant-intercalated electrode on the other, was examined. It was found that, with the increased interlayer distance, the energy density and capacitance of the cells were improved. It seems that the cell with a cationic surfactant as the cathode had the best energy density of 67.8 Wh/kg, which is 4.4-fold higher than that of the TRG cell.

Inhibition of Redox Behaviors in Hierarchically Structured Manganese Cobalt Phosphate Supercapacitor Performance by Surface Trivalent Cations

The stability and performance of supercapacitor devices are limited by the diffusion-controlled redox process occurring at materials' surfaces. Phosphate-based metal oxides could be effectively used as pseudocapacitors because of their polar nature. However, electrochemical energy storage applications of Mn-Co-based phosphate materials and their related kinetics studies have been rarely reported. In this work, we have reported a morphology-tuned Mn _x Co_3-x (PO₄)₂·8H₂O (MCP) spinel compound synthesized by a one-step hydrothermal method. Detailed physical and chemical insights of the active material coated on the nickel substrate are examined by X-ray diffraction, field-emission scanning electron microscopy, field-emission transmission electron microscopy, and high-resolution X-ray photoelectron spectroscopy analyses. Physiochemical studies reveal that the well-defined redox behavior usually observed in Co²⁺/Ni²⁺ surface-terminated compounds is suppressed by reducing the divalent cation density with an increased Co³⁺ and Mn³⁺ surface states. A uniform and dense leaflike morphology observed in the MnCo₂ phosphate compound with an increased surface area enhances the electrochemical energy storage performance. The high polar nature of P-O bonding formed at the surface leads to a higher rate of polarization and a very low relaxation time, resulting in a perfect square-shaped cyclic voltagram and triangular-shaped galvanostatic charge and discharge curve. We have achieved a highly pseudocapacitive MCP, and it can be used as a vital candidate in supercapacitor energy storage applications.

Fabrication of graphene/copper–nickel foam composite for high performance supercapacitors

An rGO/PDA/CNF composite electrode is fabricated by an immersing and annealing process and exhibits superior electrochemical performance.

Graphene/Polyaniline Aerogel with Superelasticity and High Capacitance as Highly Compression-Tolerant Supercapacitor Electrode

Superelastic graphene aerogel with ultra-high compressibility shows promising potential for compression-tolerant supercapacitor electrode. However, its specific capacitance is too low to meet the practical application. Herein, we deposited polyaniline (PANI) into the superelastic graphene aerogel to improve the capacitance while maintaining the superelasticity. Graphene/PANI aerogel with optimized PANI mass content of 63 wt% shows the improved specific capacitance of 713 F g^-1 in the three-electrode system. And the graphene/PANI aerogel presents a high recoverable compressive strain of 90% due to the strong interaction between PANI and graphene. The all-solid-state supercapacitors were assembled to demonstrate the compression-tolerant ability of graphene/PANI electrodes. The gravimetric capacitance of graphene/PANI electrodes reaches 424 F g^-1 and retains 96% even at 90% compressive strain. And a volumetric capacitance of 65.5 F cm^-3 is achieved, which is much higher than that of other compressible composite electrodes. Furthermore, several compressible supercapacitors can be integrated and connected in series to enhance the overall output voltage, suggesting the potential to meet the practical application.

Three-Dimensional Bi-Continuous Nanoporous Gold/Nickel Foam Supported MnO2 for High Performance Supercapacitors

A three-dimensional bi-continuous nanoporous gold (NPG)/nickel foam is developed though the electrodeposition of a gold–tin alloy on Ni foam and subsequent chemical dealloying of tin. The newly-designed 3D metal structure is used to anchor MnO₂ nanosheets for high-performance supercapacitors. The formed ternary composite electrodes exhibit significantly-enhanced capacitance performance, rate capability, and excellent cycling stability. A specific capacitance of 442 Fg⁻¹ is achieved at a scan rate of 5 mV s⁻¹ and a relatively high mass loading of 865 μg cm⁻². After 2500 cycles, only a 1% decay is found at a scan rate of 50 mV s⁻¹. A high power density of 3513 W kg⁻¹ and an energy density of 25.73 Wh kg⁻¹ are realized for potential energy storage devices. The results demonstrate that the NPG/nickel foam hybrid structure significantly improves the dispersibility of MnO₂ and makes it promising for practical energy storage applications.

Three-Dimensional Binder-Free Nanoarchitectures for Advanced Pseudocapacitors

The ever-increasing energy demands for electrification of transportation and powering of portable electronics are driving the pursuit of energy-storage technologies beyond the current horizon. Pseudocapacitors have emerged as one of the favored contenders to fill in this technology gap, owing to their potential to deliver both high power and energy densities. The high specific capacitance of pseudocapacitive materials is rooted in the various available oxidation states for fast surface or near-surface redox charge transfer. However, the practical implementation of pseudocapacitors is plagued by the insulating nature of most pseudocapacitive materials. The wealth of the research dedicated to addressing these critical issues has grown exponentially in the past decade. Here, we briefly survey the current progress in the development of pseudocapacitive electrodes with a focus on the discussion of the recent most exciting advances in the design of three-dimensional binder-free nanoarchitectures, including porous metal/graphene-based electrodes, as well as metal-atom/ion-doping-enhanced systems, for advanced supercapacitors with comparable energy density to batteries, and high power density.

Carbon Quantum Dot-Induced MnO2 Nanowire Formation and Construction of a Binder-Free Flexible Membrane with Excellent Superhydrophilicity and Enhanced Supercapacitor Performance

Manganese oxides (MnO₂) are regarded as typical and promising electrode materials for supercapacitors. However, the practical electrochemical performance of MnO₂ is far from its theoretical value. Nowadays, numerous efforts are being devoted to the design and preparation of nanostructured MnO₂ with the aim of improving its electrochemical properties. In this work, ultralong MnO₂ nanowires were fabricated in a process induced by carbon quantum dots (CQDs); subsequently, a binder-free flexible electrode membrane was easily obtained by vacuum filtration of the MnO₂ nanowires. The effects of the CQDs not only induced the formation of one-dimensional nanostructured MnO₂, but also significantly improved the wettability between electrode and electrolyte. In other words, the MnO₂ membrane demonstrated a superhydrophilic character in aqueous solution, indicating the sufficient and abundant contact probability between electrode and electrolyte. The binder-free flexible MnO₂ electrode exhibited a preeminent specific capacitance of 340 F g^-1 at 1 A g^-1; even when the current density reached 20 A g^-1, it still maintained 260 F g^-1 (76% retention rate compared to that at 1 A g^-1). Moreover, it also showed good cycling stability with 80.1% capacity retention over 10 000 cycles at 1 A g^-1. Furthermore, an asymmetric supercapacitor was constructed using the MnO₂ membrane and active carbon as the positive and negative electrodes, respectively, which exhibited a high energy density of 33.6 Wh kg^-1 at 1.0 kW kg^-1, and a high power density of 10 kW kg^-1 at 12.5 Wh kg^-1.

Charge storage performances and mechanisms of MnO2 nanospheres, nanorods, nanotubes and nanosheets

Manganese dioxide (MnO₂) has been widely used as an active material for high-performance supercapacitors due to its high theoretical capacitance, high cycling stability, low cost, and environmental friendliness. However, the effect of its crystallographic phase on charge storage performances and mechanisms is not yet clear. Herein, MnO₂-based supercapacitors with different structures including nanospheres, nanorods, nanotubes, and nanosheets have been fabricated and investigated. Among such structures, δ-MnO₂ nanosheets exhibit the highest specific capacitance of 194.3 F g^-1 at 1 A g^-1 when compared with other phases and shapes. The maximum specific energy of the δ-MnO₂ nanosheet supercapacitor is 23.4 W h kg^-1 at 971.6 W kg^-1 and the maximum specific power is 4009.2 W kg^-1 at 15.9 W h kg^-1 with a capacity retention of 97% over 15 000 cycles. The δ-MnO₂ nanosheet mainly stores charges via a diffusion-controlled mechanism at the scan rates of 10-100 mV s^-1, whilst the α-MnO₂ with different morphologies including nanospheres, nanorods, and nanotubes store charges via a non-faradaic or non-diffusion controlled process especially at fast scan rates (50-100 mV s^-1). Understanding the charge storage performance and mechanism of the MnO₂ nanostructures with different crystallographic phases and morphologies may lead to the further development of supercapacitors.

Charge storage mechanisms of electrospun Mn3O4 nanofibres for high-performance supercapacitors

Mixed oxidation states of manganese oxides are widely used as the electrodes in supercapacitors due to their high theoretical pseudocapacitances.

Hydrothermal fabrication of reduced graphene oxide/activated carbon/MnO2 hybrids with excellent electrochemical performance for supercapacitors

A graphene/activated carbon/MnO₂ (GAM) composite was synthesized by transferring to a hydrothermal synthesis reactor and maintained at 140 °C for 2 h.