Synthesis and electrochemical properties of spin-capable carbon nanotube sheet/MnO(x) composites for high-performance energy storage devices

Multicore-shell MnO2@Ppy@N-doped porous carbon nanofiber ternary composites as electrode materials for high-performance supercapacitors

In this study, a multicore-shell ternary composite electrode material (MnO₂@Ppy@NPCNFs) with excellent electrochemical performances was prepared by using surface modification, in which core-shell Ppy@N-doped porous carbon nanofibers (Ppy@NPCNFs) with large specific surface area and high conductivity were used as the substrate (a multicore layer), and MnO₂ was loaded on the substrate by hydrothermal synthesis to form a shell layer, further improving the SC of electrode material. The parameters of hydrothermal growth of MnO₂ on Ppy@NPCNFs were explored by means of the control variable method and response surface methodology, and the optimal parameters were predicted and verified. Electrochemical test results showed that the SC of MnO₂@Ppy@NPCNFs prepared under the optimal reaction parameters was as high as 595.77 F g^-1, and its capacitance retention was 96.2 % after 1000 cycles. Moreover, a symmetric supercapacitor prepared with the optimal multicore-shell electrode showed an energy density of 9.36 Wh kg^-1 at a power density of 1000 W kg^-1 and a retention rate of 92.46 % after 1000 cycles, indicating the promising application of multicore-shell ternary composite electrode material in high-performance supercapacitors.

Nanoporous Conducting Polymer Nanowire Network-Encapsulated MnO2-Based Flexible Supercapacitor with Enhanced Rate Capability and Cycling Stability

Transition-metal-oxide-based electrochemical electrodes usually suffer from poor electron and ion transport, leading to deteriorated rate performance and cycling stability. Herein, we address these issues by developing a facile "conducting encapsulation" strategy toward a nanoporous PEDOT nanowire/MnO₂ nanoparticle/PEDOT nanowire composite electrode. Through encapsulation of the PEDOT nanowire network, the overall electrochemical performance of the resultant composite electrode is substantially enhanced. Specifically, the rate capability and capacitance retention are improved by ∼48.2 and ∼33%, respectively, which are 89.8% at 0.8-40 mA/cm² and 93% after 3000 charge/discharge cycles at 2.0 mA/cm², respectively. Moreover, the specific capacitance is increased by ∼6 times of that of the MnO₂@PEDOT NW electrode at ∼200 mA/cm². We find that a nanoporous conducting nanowire network that encapsulates a MnO₂ nanoparticle layer can provide efficient electron and ion transport paths and stabilize the structure of MnO₂ from collapse during charge/discharge cycling and mechanical deformation. This strategy can be applied to other pseudocapacitive material-based electrochemical electrodes, such as transition-metal oxides and conducting polymers.

Three-dimensional carbon foam-metal oxide-based asymmetric electrodes for high-performance solid-state micro-supercapacitors

A three-dimensional carbon foam (CF)-based asymmetric planar micro-supercapacitor is fabricated by the direct spray patterning of active materials on an array of interdigital electrodes. The solid-state asymmetric micro-supercapacitor comprises the CF network with pseudocapacitive metal oxides (manganese oxide (MnO), iron oxide (Fe₂O₃)), where CF-MnO composite as a positive electrode, and CF-Fe₂O₃ as negative electrode for superior electrochemical performance. The micro-supercapacitor, CF-MnO//CF-Fe₂O₃, attains an ultrahigh supercapacitance of 18.4 mF cm^-2 (2326.8 mF cm^-3) at a scan rate of 5 mV s^-1. A wider potential window of 1.4 V is achieved with a high energy density of 5 μW h cm^-2. The excellent cyclic stability is confirmed by 86.1% capacitance retention after 10 000 electrochemical cycles.

A High Energy Density 2D Microsupercapacitor Based on an Interconnected Network of a Horizontally Aligned Carbon Nanotube Sheet

Highly aligned carbon nanotubes (HACNT sheets) have recently attracted great attention in developing high-performing ultrathin supercapacitors, which take advantage of the long-range alignment to improve electrochemical performance. While there are investigations into sandwich electrode CNT sheet devices, there are no known reports on interdigitated electrode (IDE) HACNT sheet microsupercapacitors (MSCs). This paper reports a facile method for rapidly fabricating high energy density ultrathin HACNT sheet-based MSCs with IDE planar configuration. Increasing the electrode thickness from 32 nm (5 layers) to 300 nm (50 layers) results in an approximately three times factor in performance. The 50 layer devices (MSC-50L) yield a top energy density of 10.52 mWhcm^-3 and power density of 19.33 Wcm^-3, making its performance comparable to those of microbatteries with potential for further improvement. Additionally, incorporation of MnO₂ nanoparticles (NPs) within the MSCs-50L improves specific capacitance (242 Fcm^-3), energy density (33.7 mWhcm^-3), and power density (31 Wcm^-3), outperforming current thin-film MSCs and matching the performance of 3D MSCs. MSCs also demonstrate a long cycle life (7000 charge-discharge cycles) with less than 5% capacitance fade. These findings suggest that HACNT sheets have substantial potential as active electrode materials for ultrathin high energy density microscale power sources.

One-step synthesis of MnOx/PPy nanocomposite as a high-performance cathode for a rechargeable zinc-ion battery and insight into its energy storage mechanism

Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted significant attention in the energy storage field. Manganese-based materials are the most promising cathode materials for ZIBs but they suffer from low electronic conductivity. Herein, a high-performance cathode for ZIBs based on nanocomposites consisting of mixed-valence manganese dioxide (Mn III and IV) and polypyrrole (MnO_x/PPy) is prepared through an efficient one-step organic/inorganic interface redox reaction. The role of polypyrrole (PPy) in the MnO_x/PPy cathode is elaborated. It not only provides an effective conductive network for MnO_x but also contributes to the capacity of the composite. By optimizing the amount of PPy, the MnO_x/PPy composite with 12 wt% PPy exhibits the highest capacity. As a result, the corresponding Zn-MnO_x/PPy battery delivers a high capacity (302.0 mA h g^-1 at 0.15 A g^-1), excellent rate performance (159.9 mA h g^-1 at 3 A g^-1) and superior cycling stability. Furthermore, the results of ex situ characterization analysis reveal that H⁺ and Zn²⁺ insertion/extraction both occur in MnO_x/PPy particles during the discharging/charging process, while only Zn²⁺ insertion/extraction occurs in the PPy electrode. This work develops an efficient one-step synthesis method for large scale production of manganese-based materials/conducting polymers as the cathode for ZIB application, and provides an insight into its energy storage mechanism.

Rapid in situ growth of β-Ni(OH)2 nanosheet arrays on nickel foam as an integrated electrode for supercapacitors exhibiting high energy density

Ni(OH)2 has been widely investigated as a prospective electrode material because of its high theoretical capacitance and relatively low cost. However its synthesis usually needs a complex and lengthy process, and a binder is generally used for fabricating Ni(OH)2 based electrodes. In this work, a self-supporting binder-free β-Ni(OH)2@nickel foam (NF) integrated electrode was prepared by the in situ growth of β-Ni(OH)2 on NF using a rapid and facile approach. This approach consists of two processing steps: (1) the pre-treatment of NF with an acid and (2) the quick in situ electrochemical synthesis of β-Ni(OH)2 on the NF in the KOH electrolyte within half a minute under an applied voltage. The β-Ni(OH)2@NF integrated electrode possesses a three-dimensional network structure of nanosheet arrays and exhibits excellent electrochemical performance. Its areal capacity is 3.68 mA h cm-2 at a current density of 2 mA cm-2, and the capacity can retain 115.8% of its initial value even after 2000 cycles at a current density of 15 mA cm-2. Moreover, the as-assembled β-Ni(OH)2@NF//activated carbon (AC) asymmetric supercapacitor (ASC) exhibits a high energy density of 74.2 W h kg-1 with a power density of 776.9 W kg-1 and excellent cycling stability (89.9% retained after 10 000 cycles). This work provides an efficient, facile and economic method for fabricating Ni(OH)2 based integrated electrodes for high-performance supercapacitors.

CNT/High Mass Loading MnO2/Graphene-Grafted Carbon Cloth Electrodes for High-Energy Asymmetric Supercapacitors

Flexible supercapacitor electrodes with high mass loading are crucial for obtaining favorable electrochemical performance but still challenging due to sluggish electron and ion transport. Herein, rationally designed CNT/MnO₂/graphene-grafted carbon cloth electrodes are prepared by a "graft-deposit-coat" strategy. Due to the large surface area and good conductivity, graphene grafted on carbon cloth offers additional surface areas for the uniform deposition of MnO₂ (9.1 mg cm^-2) and facilitates charge transfer. Meanwhile, the nanostructured MnO₂ provides abundant electroactive sites and short ion transport distance, and CNT coated on MnO₂ acts as interconnected conductive "highways" to accelerate the electron transport, significantly improving redox reaction kinetics. Benefiting from high mass loading of electroactive materials, favorable conductivity, and a porous structure, the electrode achieves large areal capacitances without compromising rate capability. The assembled asymmetric supercapacitor demonstrates a wide working voltage (2.2 V) and high energy density of 10.18 mWh cm^-3.

Facile One-Pot Synthesis of Bimetallic Co/Mn-MOFs@Rice Husks, and its Carbonization for Supercapacitor Electrodes

Novel hybrid nanomaterials comprising metal-organic framework compounds carbonised in the presence of biomass material derived from rice husk have been investigated as a new class of sustainable supercapacitor materials for electrochemical energy storage. Specifically, two synthetic routes were employed to grow Co/Mn metal-organic framework compounds in the channels of rice husks, which had been activated previously by heat treatment in air at 400 °C to produce a highly porous network. Pyrolysis of these hybrid materials under nitrogen at 700 °C for 6 h produced metal-containing phases within the nanocarbon, comprising intimate mixtures of Co, MnO and CoMn2O4. The materials thus produced are characterized in detail using a range of physical methods including XRD, electron microscopy and X-ray photoelectron spectroscopy. The synthetic pathway to the metal-organic framework compound is shown to influence significantly the physical properties of the resulting material. Electrochemical evaluation of the materials fabricated revealed that higher specific capacitances were obtained when smaller crystallite sized bimetallic Co/Mn-MOFs were grown inside the rice husks channels compared to larger crystallite sizes. This was in-part due to increased metal oxide loading into the rice husk owing to the smaller crystallite size as well as the increased pseudocapacitance exhibited by the smaller crystallite sizes and increased porosity.

Molten-Salt-Assisted Synthesis of Hierarchical Porous MnO@Biocarbon Composites as Promising Electrode Materials for Supercapacitors and Lithium-Ion Batteries

Biomass-derived carbon composites (e.g., metal oxide/biocarbon) have been used as promising electrode materials for energy storage devices owing to their natural abundance and simple preparation process. However, low loading content/inhomogeneous distribution of metal oxides and inefficient cracking of biocarbon (BC) are intractable obstacles that impede the efficient utilization of biomass. In this work, hierarchical porous MnO/BC composites were prepared by a facile molten-salt-assisted strategy based on the superior salt-water absorption ability of agaric. The addition of NaCl induces a liquid reaction medium by formation of a molten salt mixture at high temperature to effectively realize the activation and cracking of the bulk carbon, and it also acts as a recyclable sacrificial template to form mesopores and macropores in the as-prepared hierarchical porous MnO/BC composites. The highly porous and uniform BC framework effectively enhances ion diffusion and electron-transfer ability, serves as a protective layer to prevent fracturing and agglomeration of MnO, and thus enables superior rate performance and cycling stability of the MnO/BC composite for both supercapacitor electrodes (94 % capacity retention at 20 mA cm^-2 after 5000 cycles) and lithium-ion battery anodes (783 mA h g^-1 after 1000 cycles). Notably, considering the simple and low-cost preparation process, this work opens a promising avenue for the large-scale production of advanced metal oxide/BC hybrid electrode materials for electrochemical energy storage.

High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors

Metal oxides have attracted renewed interest as promising electrode materials for high energy density supercapacitors. However, the electrochemical performance of metal oxide materials deteriorates significantly with the increase of mass loading due to their moderate electronic and ionic conductivities. This limits their practical energy. Herein, we perform a morphology and phase-controlled electrodeposition of MnO₂ with ultrahigh mass loading of 10 mg cm^-2 on a carbon cloth substrate to achieve high overall capacitance without sacrificing the electrochemical performance. Under optimum conditions, a hierarchical nanostructured architecture was constructed by interconnection of primary two-dimensional ε-MnO₂ nanosheets and secondary one-dimensional α-MnO₂ nanorod arrays. The specific hetero-nanostructures ensure facile ionic and electric transport in the entire electrode and maintain the structure stability during cycling. The hierarchically structured MnO₂ electrode with high mass loading yields an outstanding areal capacitance of 3.04 F cm^-2 (or a specific capacitance of 304 F g^-1) at 3 mA cm^-2 and an excellent rate capability comparable to those of low mass loading MnO₂ electrodes. Finally, the aqueous and all-solid asymmetric supercapacitors (ASCs) assembled with our MnO₂ cathode exhibit extremely high volumetric energy densities (8.3 mWh cm^-3 at the power density of 0.28 W cm^-3 for aqueous ASC and 8.0 mWh cm^-3 at 0.65 W cm^-3 for all-solid ASC), superior to most state-of-the-art supercapacitors.

Synthesis of ultra-small gold nanoparticles decorated onto NiO nanobelts and their high electrochemical performance

Ultra-small gold nanoparticles are decorated onto NiO nanobelts using HAuCl₄ and Ni(OH)₂ nanobelts as precursors via a one-step thermal treatment; the effect of the gold content on the structure and electrochemical performance was investigated.

Asymmetric Supercapacitor Electrodes and Devices

The world is recently witnessing an explosive development of novel electronic and optoelectronic devices that demand more-reliable power sources that combine higher energy density and longer-term durability. Supercapacitors have become one of the most promising energy-storage systems, as they present multifold advantages of high power density, fast charging-discharging, and long cyclic stability. However, the intrinsically low energy density inherent to traditional supercapacitors severely limits their widespread applications, triggering researchers to explore new types of supercapacitors with improved performance. Asymmetric supercapacitors (ASCs) assembled using two dissimilar electrode materials offer a distinct advantage of wide operational voltage window, and thereby significantly enhance the energy density. Recent progress made in the field of ASCs is critically reviewed, with the main focus on an extensive survey of the materials developed for ASC electrodes, as well as covering the progress made in the fabrication of ASC devices over the last few decades. Current challenges and a future outlook of the field of ASCs are also discussed.

Self-Assembled Array of Tethered Manganese Oxide Nanoparticles for the Next Generation of Energy Storage

Many challenges must be overcome in order to create reliable electrochemical energy storage devices with not only high energy but also high power densities. Gaps exist in both battery and supercapacitor technologies, with neither one satisfying the need for both large power and energy densities in a single device. To begin addressing these challenges (and others), we report a process to create a self-assembled array of electrochemically active nanoparticles bound directly to a current collector using extremely short (2 nm or less) conductive tethers. The tethered array of nanoparticles, MnO in this case, bound directly to a gold current collector via short conducting linkages eliminates the need for fillers, resulting in a material which achieves 99.9% active material by mass (excluding the current collector). This strategy is expected to be both scalable as well as effective for alternative tethers and metal oxide nanoparticles.

Cheap, High-Performance, and Wearable Mn Oxide Supercapacitors with Urea-LiClO4 Based Gel Electrolytes

Here we report a simple, scalable, and low-cost method to enhance the electrochemical properties of Mn oxide electrodes for highly efficient and flexible symmetrical supercapacitors. The method involving printing on a printer, pencil-drawing, and electrodeposition is established to fabricate Mn oxide/Ni-nanotube/graphite/paper hybrid electrodes operating with a low-cost, novel urea-LiClO₄/PVA as gel electrolyte for flexible solid-state supercapacitor (FSSC) devices. The Mn oxide nanofiber/Ni-nanotube/graphite/paper (MNNGP) electrodes in urea-LiClO₄/PVA gel electrolyte show specific capacitance (C_sp) 960 F/g in voltage region 0.8 V at 5 mV/s and exhibit excellent rates of capacitance retention more than 85% after 5000 cycles. Moreover, the electrochemical behavior of the MNNGP electrodes in urea-LiClO₄/PVA at operating temperatures 27-110 °C was investigated; the results show that the MNNGP electrodes in urea-LiClO₄/PVA exhibit outstanding performance (1100 F/g), even at 90 °C. The assembled FSSC devices based on the MNNGP electrodes in urea-LiClO₄/PVA exhibit great C_sp (380 F/g in potential region of 2.0 V at 5 mV/s, exhibiting superior energy density 211.1 W h/kg) and great cycle stability (less than 15% loss after 5000 cycles at 25 mV/s). The oxidation-state change was examined by in situ X-ray absorption spectroscopy. FSSC devices would open new opportunities in developing novel portable, wearable, and roll-up electric devices owing to the cheap, high-performance, wide range of operating temperature, and simple procedures for large-area fabrication.

Hierarchical Branched Vanadium Oxide Nanorod@Si Nanowire Architecture for High Performance Supercapacitors

Vanadium oxide (VO_x ) nanorods are uniformly synthesized on dense Si nanowire arrays. This 3D hierarchical nanoarchitecture offers a novel high-performance supercapacitor electrode design with significantly improved specific capacitance and high-rate capability.

Electrochemical Fabrication of Monolithic Electrodes with Core/Shell Sandwiched Transition Metal Oxide/Oxyhydroxide for High-Performance Energy Storage

Transition metal oxides/oxyhydroxides (TMOs) are promising high-capacity materials for electrochemical energy storage. However, the low rate and poor cyclability hinder practical applications. In this work, we developed a general electrochemical route to fabricate monolithic core/shell sandwiched structures, which are able to significantly improve the electrochemical properties of TMO electrodes by electrically wiring the insulating active materials and alleviating the adverse effects caused by volume changes using engineered porous structures. As an example, a lithium ion battery anode of porous MnO sandwiched between CNT and carbon demonstrates a high capacity of 554 mAh g^-1 even after 1000 cycles at 2 A g^-1. An all-solid-state symmetric pseudocapacitor consisting of CNT@MnOOH@polypyrrole exhibits a high specific capacitance of 148 F g^-1 and excellent capacitance retention (92% after 10000 cycles at 2 A g^-1). Several other examples and applications have further confirmed the effectiveness of improving the electrochemical properties by core/shell sandwiched structures.

Electrochemical and in situ X-ray spectroscopic studies of MnO2/reduced graphene oxide nanocomposites as a supercapacitor

Electrochemical and in situ X-ray absorption spectroscopy (XAS) measurements of various MnO2-coated carbon materials (MnO2/acid-functionalized carbon nanotubes (C-CNT), MnO2/reduced graphene oxide (RGO), and MnO2/RGO-Au electrodes) were conducted to evaluate the supercapacitive performances and electronic structures. MnO2 was deposited on the surface of C-CNT, RGO, and RGO-Au via a spontaneous redox reaction to facilitate the growth of the bulk form of MnO2/C-CNT and the surface forms of MnO2/RGO-based materials. Various forms of MnO2 on the carbon materials exhibited different charge/discharge behaviors. The specific capacitances of the MnO2/RGO and MnO2/RGO-Au electrodes at a current density of 1 A g(-1) were about 433 and 469 F g(-1), respectively; these values are about 1.5 times that of the MnO2/C-CNT (259 F g(-1)) electrode. Specific capacitances of 220 and 281 F g(-1) with retention rates of about 50-60% were obtained from MnO2/RGO and MnO2/RGO-Au, respectively, even at a high current density of 80 A g(-1). Experimental results revealed that the long-term electrochemical stability of the MnO2/RGO-based electrodes (with ∼90% retention) exceeded that of the MnO2/C-CNT electrode (with ∼60% retention) after 1000 cycles at a high scan rate of 80 A g(-1). This finding indicates that MnO2/RGO-based electrodes feature excellent cycling stability and rate capacity retention performance. To elucidate the atomic/electronic structures of the MnO2/C-CNT, MnO2/RGO, and MnO2/RGO-Au electrodes during the charge/discharge process, in situ XAS of the Mn K-edge was performed. The MnO2/RGO-based electrodes exhibited the least variations in the pre-peak intensity of the Mn K-edge during the charge/discharge process because a nano-network of MnO2 is homogeneously decorated on the outer surfaces of RGO-based electrodes to facilitate the growth of surface forms of MnO2/RGO and MnO2/RGO-Au. Analytical results further revealed suppression of changes in tunnel size and promotion of insertion/extraction behavior. This work, particularly the combination of cyclic voltammetry with in situ XAS measurements, will be of general value in the fields of nanomaterials and nanotechnology, and in their use in energy storage.

Oxygen Evolution Assisted Fabrication of Highly Loaded Carbon Nanotube/MnO2 Hybrid Films for High-Performance Flexible Pseudosupercapacitors

To date, it has been a great challenge to design high-performance flexible energy storage devices for sufficient loading of redox species in the electrode assemblies, with well-maintained mechanical robustness and enhanced electron/ionic transport during charge/discharge cycles. An electrochemical activation strategy is demonstrated for the facile regeneration of carbon nanotube (CNT) film prepared via floating catalyst chemical vapor deposition strategy into a flexible, robust, and highly conductive hydrogel-like film, which is promising as electrode matrix for efficient loading of redox species and the fabrication of high-performance flexible pseudosupercapacitors. The strong and conductive CNT films can be effectively expanded and activated by electrochemical anodic oxygen evolution reaction, presenting greatly enhanced internal space and surface wettability with well-maintained strength, flexibility, and conductivity. The as-formed hydrogel-like film is quite favorable for electrochemical deposition of manganese dioxide (MnO2 ) with loading mass up to 93 wt% and electrode capacitance kept around 300 F g(-1) (areal capacitance of 1.2 F cm(-2) ). This hybrid film was further used to assemble a flexible symmetric pseudosupercapacitor without using any other current collectors and conductive additives. The assembled flexible supercapacitors exhibited good rate performance, with the areal capacitance of more than 300 mF cm(-2) , much superior to other reported MnO2 based flexible thin-film supercapacitors.

Reconstruction of Mini-Hollow Polyhedron Mn2O3 Derived from MOFs as a High-Performance Lithium Anode Material

A mini-hollow polyhedron Mn2O3is used as the anode material for lithium-ion batteries. Benefiting from the small interior cavity and intrinsic nanosize effect, a stable reconstructed hierarchical nanostructure is formed. It has excellent energy storage properties, exhibiting a capacity of 760 mAh g-1 at 2 A g-1 after 1000 cycles. This finding offers a new perspective for the design of electrodes with large energy storage.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

One-dimensional metal oxide-carbon hybrid nanostructures for electrochemical energy storage

Numerous metal oxides (MOs) have been considered as promising electrode materials for electrochemical energy storage devices, including lithium-ion batteries (LIBs) and electrochemical capacitors (ECs), because of their outstanding features such as high capacity/capacitance, low cost, as well as environmental friendliness. However, one major challenge for MO-based electrodes is the poor cycling stability derived from the large volume variation and intense mechanic strain, which are inevitably generated during repeated charge/discharge processes. Nanostructure engineering has proven to be one of the most effective strategies to improve the electrochemical performance of MO-based electrode materials. Among various nanostructures, one-dimensional (1D) metal oxide-carbon hybrid nanostructures might offer some solution for the challenging issues involved in bulk MO-based electrode materials for energy storage devices. Herein, we give an overview of the rational design, synthesis strategies and electrochemical properties of such 1D MO-carbon structures and highlight some of the latest advances in this niche area. It starts with a brief introduction to the development of nanostructured MO-based electrodes. We will then focus on the advanced synthesis and improved electrochemical performance of 1D MO-carbon nanostructures with different configurations, including MO-carbon composite nanowires, core-shell nanowires and hierarchical nanostructures. Lastly, we give some perspective on the current challenges and possible future research directions in this area.

Solid-State Thin-Film Supercapacitors with Ultrafast Charge/Discharge Based on N-Doped-Carbon-Tubes/Au-Nanoparticles-Doped-MnO2 Nanocomposites

Although carbonaceous materials possess long cycle stability and high power density, their low-energy density greatly limits their applications. On the contrary, metal oxides are promising pseudocapacitive electrode materials for supercapacitors due to their high-energy density. Nevertheless, poor electrical conductivity of metal oxides constitutes a primary challenge that significantly limits their energy storage capacity. Here, an advanced integrated electrode for high-performance pseudocapacitors has been designed by growing N-doped-carbon-tubes/Au-nanoparticles-doped-MnO2 (NCTs/ANPDM) nanocomposite on carbon fabric. The excellent electrical conductivity and well-ordered tunnels of NCTs together with Au nanoparticles of the electrode cause low internal resistance, good ionic contact, and thus enhance redox reactions for high specific capacitance of pure MnO2 in aqueous electrolyte, even at high scan rates. A prototype solid-state thin-film symmetric supercapacitor (SSC) device based on NCTs/ANPDM exhibits large energy density (51 Wh/kg) and superior cycling performance (93% after 5000 cycles). In addition, the asymmetric supercapacitor (ASC) device assembled from NCTs/ANPDM and Fe2O3 nanorods demonstrates ultrafast charge/discharge (10 V/s), which is among the best reported for solid-state thin-film supercapacitors with both electrodes made of metal oxide electroactive materials. Moreover, its superior charge/discharge behavior is comparable to electrical double layer type supercapacitors. The ASC device also shows superior cycling performance (97% after 5000 cycles). The NCTs/ANPDM nanomaterial demonstrates great potential as a power source for energy storage devices.

Hierarchical structures composed of MnCo2O4@MnO2 core–shell nanowire arrays with enhanced supercapacitor properties

In this paper, hierarchical MnCo₂O₄@MnO₂ core–shell nanowire arrays (MnCo₂O₄@MnO₂ NWAs) with mesoporous and large surface area are synthesized on 3D nickel foam via a facile, two-step hydrothermal approach without any adscititious surfactant and binder.

Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations

We review here recent developments in unconventional supercapacitors from nanocarbon-based electrode materials to device configurations.

50–100 μm-thick pseudocapacitive electrodes of MnO2 nanoparticles uniformly electrodeposited in carbon nanotube papers

The 62 μm-thick, 1.09 g cm⁻³-dense hybrid electrode of 82 wt% MnO₂ and 18 wt% CNTs realized high total capacitances of 120 F g⁻¹, 131 F cm⁻³, and 0.81 F cm⁻² at 2 mV s⁻¹ scan rate.

Three dimensional manganese oxide on carbon nanotube hydrogels for asymmetric supercapacitors

Three dimensional manganese oxide on a CNT hydrogel has been developed with a satisfactory electrochemical performance.

Morphology-controlled syntheses of α-MnO2 for electrochemical energy storage

The morphological transformations of MnO₂ and morphology-dependent electrochemical performance were systematically investigated.

Microneedles for Transdermal Biosensing: Current Picture and Future Direction

A novel trend is rapidly emerging in the use of microneedles, which are a miniaturized replica of hypodermic needles with length-scales of hundreds of micrometers, aimed at the transdermal biosensing of analytes of clinical interest, e.g., glucose, biomarkers, and others. Transdermal biosensing via microneedles offers remarkable opportunities for moving biosensing technologies and biochips from research laboratories to real-field applications, and envisages easy-to-use point-of-care microdevices with pain-free, minimally invasive, and minimal-training features that are very attractive for both developed and emerging countries. In addition to this, microneedles for transdermal biosensing offer a unique possibility for the development of biochips provided with end-effectors for their interaction with the biological system under investigation. Direct and efficient collection of the biological sample to be analyzed will then become feasible in situ at the same length-scale of the other biochip components by minimally trained personnel and in a minimally invasive fashion. This would eliminate the need for blood extraction using hypodermic needles and reduce, in turn, related problems, such as patient infections, sample contaminations, analysis artifacts, etc. The aim here is to provide a thorough and critical analysis of state-of-the-art developments in this novel research trend, and to bridge the gap between microneedles and biosensors.

Facile Synthesis of Coaxial CNTs/MnOx-Carbon Hybrid Nanofibers and Their Greatly Enhanced Lithium Storage Performance

Carbon nanotubes (CNTs)/MnO_x-Carbon hybrid nanofibers have been successfully synthesized by the combination of a liquid chemical redox reaction (LCRR) and a subsequent carbonization heat treatment. The nanostructures exhibit a unique one-dimensional core/shell architecture, with one-dimensional CNTs encapsulated inside and a MnO_x-carbon composite nanoparticle layer on the outside. The particular porous characteristics with many meso/micro holes/pores, the highly conductive one-dimensional CNT core, as well as the encapsulating carbon matrix on the outside of the MnO_x nanoparticles, lead to excellent electrochemical performance of the electrode. The CNTs/MnO_x-Carbon hybrid nanofibers exhibit a high initial reversible capacity of 762.9 mAhg⁻¹, a high reversible specific capacity of 560.5 mAhg⁻¹ after 100 cycles, and excellent cycling stability and rate capability, with specific capacity of 396.2 mAhg⁻¹ when cycled at the current density of 1000 mAg⁻¹, indicating that the CNTs/MnO_x-Carbon hybrid nanofibers are a promising anode candidate for Li-ion batteries.

Fast and stable redox reactions of MnO₂/CNT hybrid electrodes for dynamically stretchable pseudocapacitors

Pseudocapacitors, which are energy storage devices that take advantage of redox reactions to store electricity, have a different charge storage mechanism compared to lithium-ion batteries (LIBs) and electric double-layer capacitors (EDLCs), and they could realize further gains if they were used as stretchable power sources. The realization of dynamically stretchable pseudocapacitors and understanding of the underlying fundamentals of their mechanical-electrochemical relationship have become indispensable. We report herein the electrochemical performance of dynamically stretchable pseudocapacitors using buckled MnO2/CNT hybrid electrodes. The extremely small relaxation time constant of less than 0.15 s indicates a fast redox reaction at the MnO2/CNT hybrid electrodes, securing a stable electrochemical performance for the dynamically stretchable pseudocapacitors. This finding and the fundamental understanding gained from the pseudo-capacitive behavior coupled with mechanical deformation under a dynamic stretching mode would provide guidance to further improve their overall performance including a higher power density than LIBs, a higher energy density than EDLCs, and a long-life cycling stability. Most importantly, these results will potentially accelerate the applications of stretchable pseudocapacitors for flexible and biomedical electronics.

Nanoflaky MnO2/functionalized carbon nanotubes for supercapacitors: an in situ X-ray absorption spectroscopic investigation

The surfaces of acid- and amine-functionalized carbon nanotubes (C-CNT and N-CNT) were decorated with MnO2 nanoflakes as supercapacitors by a spontaneous redox reaction. C-CNT was found to have a lower edge plane structure and fewer defect sites than N-CNT. MnO2/C-CNT with a highly developed surface area exhibited favorable electrochemical performance. To determine the atomic/electronic structures of the MnO2/functionalized CNTs (MnO2/C-CNT and MnO/N-CNT) during the charge/discharge process, in situ X-ray absorption spectroscopy (XAS) measurements were made at the Mn K-edge. Both C-CNT and N-CNT are highly conductive. The effect of the scan rate on the capacitance behavior was also examined, revealing that the π* state of CNT and the size of the tunnels in pseudo-capacitor materials (which facilitate conduction and the transport of electrolyte ions) are critical for the capacitive performance, and their role depends on the scan rate. In the slow charge/discharge process, MnO2/N-CNT has a more symmetrical rectangular cyclic voltammetry (CV) curve. In the fast charge/discharge process, MnO2/C-CNT with a highly developed surface provides fast electronic and ionic channels that support a reversible faradaic redox reaction between MnO2 nanoflakes and the electrolyte, significantly enhancing its capacitive performance over that of MnO2/N-CNT. The MnO2/C-CNT architecture has great potential for supercapacitor applications. The information that was obtained herein helps to elucidate CNT surface modification and the design of the MnO2/functionalized CNT interface with a view for the further development of supercapacitors. This work, and especially the combination of CV with in situ XAS measurements, will be of value to readers with an interest in nanomaterial, nanotechnology and their applications in energy storage.

Fabrication of Mn2O3 nanorods: an efficient catalyst for selective transformation of alcohols to aldehydes

Self-assembled high surface area Mn₂O₃ nanorods have been fabricated through an effective polymer–surfactant interaction and their outstanding catalytic property for the selective transformation of alcohols to aldehydes has been discovered.