Controlled growth of nanostructured MnO2 on carbon nanotubes for high-performance electrochemical capacitors

Formation of MnO2-coated ITO electrodes with high catalytic activity for enzymatic glucose detection

We present the formation of a cheap and environmentally friendly working electrode material for glucose biosensors with good catalytic properties. The classic electrode in such devices consists of a conductive material modified with the enzyme glucose oxidase. The working principle is the electrochemical detection of hydrogen peroxide as a product of the enzymatic transformation of glucose. As a base material, we offer manganese dioxide; it is a natural highly selective catalyst for the decomposition of H2O2 and is electrochemically deposited onto the surface of ITO. We approached the formation of MnO2 films systematically. By changing parameters such as the deposition method, pH of the electrolyte, and the drying temperature of the precipitate, a series of electrodes were formed. These electrodes were characterized by SEM, electrochemical impedance spectroscopy, and XPS and their electrocatalytic activity was studied. Significant differences in the sensitivity of the electrodes were detected. The manganese dioxide film with the best catalytic characteristics is formed in the electrolyte with pH 1 by cyclic voltammetry and then drying at 60 °C. The surface of the electrode was then modified with a solution of GOx enzyme with a concentration of 2 mg ml-1 (100-250 units per mg solid). The sensitivity of such an electrode is 117.8 μA mmol-1 cm-2. The range of determined concentrations of glucose is from 0.1 mM to 3 mM. The sensitivity is comparable to that of electrodes based on expensive materials such as graphene and noble metals.

Flexible synthesis of high-performance electrode materials of N-doped carbon coating MnO nanowires for supercapacitors

The MnO/C composites were obtained by co-precipitation method, which used Mn₃O₄nanomaterials as precursors and dopamine solution after ultrasonic mixing and calcination under N₂atmosphere at different temperatures. By studying the difference of MnO/C nanomaterials formed at different temperatures, it was found that with the increase of calcination temperature, the materials appear obvious agglomeration. The optimal calcination temperature is 400 °C, and the resulting MnO/C is a uniformly dispersed slender nanowire structure. The specific capacitance of MnO/C nanowires can reach 356 F g^-1at 1 A g^-1. In the meantime, the initial capacitance of MnO/C nanowires remains 106% after 5000 cycles. Moreover, the asymmetric supercapacitor was installed, which displays a tremendous energy density of 30.944 Wh kg^-1along with a high power density of 10 kW kg^-1. The composite material reveals a promising prospect in the application of supercapacitors.

Electrochemically Exfoliated Graphene/Manganese Dioxide Nanowire Composites as Electrode Materials for Flexible Supercapacitors

Flexible supercapacitors are of great significance for the development of intelligent electronic products and wearable devices. Herein, through reasonable design, self-supporting flexible film composites that can be used as supercapacitor electrodes, are synthesised by vacuum filtration. The composites are composed of electrochemically exfoliated graphene nanosheets and MnO2 nanowires, in which the graphene nanosheets mainly play the role of skeleton support, enhance conductivity, and provide electric double-layer capacitance, while the MnO2 nanowires mainly provide pseudocapacitance. Results show that the sample with 20% MnO2 possesses the best electrochemical performance due to the mass ratio which can give full play to the pseudocapacitive properties of MnO2 and the conductivity of graphene. The maximum mass specific capacitance reaches 106.2F g−1 at 0.5A g−1, and the areal specific capacitance is 767.0mF cm−2 at 1mA cm−2. The electrode also maintains 86.7% of the initial capacitance after 10000 cycles, indicating good cyclic stability. Furthermore, an asymmetric solid supercapacitor based on flexible thin films is assembled. The energy density is 20.7Wh kg−1, the power density is 1000W kg−1, and the capacitance remains 84.2% after 3000 cycles at 5.0A g−1. These results suggest that the as-prepared self-supporting material has the potential to be used to construct flexible supercapacitors for wearable equipment.

Controllable synthesis of aluminum doped peony-like α-Ni(OH)2 with ultrahigh rate capability for asymmetric supercapacitors

Ion substitution and micromorphology control are two efficient strategies to ameliorate the electrochemical performance of supercapacitors electrode materials. Here, Al3+ doped α-Ni(OH)2 with peony-like morphology and porous structure has been successfully synthesized through a facile one-pot hydrothermal process. The Al3+ doped α-Ni(OH)2 electrode shows an ultrahigh specific capacitance of 1750 F g-1 at 1 A g-1, and an outstanding electrochemical stability of 72% after running 2000 cycles. In addition, the Al3+ doped α-Ni(OH)2 electrode demonstrates an excellent rate capability (92% retention at 10 A g-1). Furthermore, by using this unique Al3+ doped α-Ni(OH)2 as the positive electrode and a hierarchical porous carbon (HPC) as the negative electrode, the assembled asymmetric supercapacitor can demonstrate a high energy/power density (49.6 W h kg-1 and 14 kW kg-1). This work proves that synthesizing an Al3+ doped structure is an effective means to improve the electrochemical properties of α-Ni(OH)2. This scheme could be extended to other transition metal hydroxides to enhance their electrochemical performance.

Design and fabrication of hydrotalcite-like ternary NiMgAl layered double hydroxide nanosheets as battery-type electrodes for high-performance supercapacitors

Hydrotalcite is an abundant mineral in nature and can be cost-effectively prepared in the laboratory, but there is almost no discussion about its application in the field of supercapacitors.

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.

One-pot hydrothermal synthesis of novel 3D starfish-like δ-MnO2 nanosheets on carbon fiber paper for high-performance supercapacitors

Novel 3D starfish-like δ-MnO₂ nanosheets with a hierarchical nanostructure supported on carbon fiber paper were synthesized through a facile hydrothermal method.

Electrochemical hydrogenated TiO2nanotube arrays decorated with 3D cotton-like porous MnO2enables superior supercapacitive performance

Highly conducting TiO₂nanotube arrays (EH-TNTAs) decorated with unique 3D cotton-like porous MnO₂enables superior supercapacitive performance.

In-situ growth of MnO2 crystals under nanopore-constraint in carbon nanofibers and their electrochemical performance

Growing MnO₂ nanocrystals in the bulk of porous carbon nanofibers is conducted in a KMnO₄ aqueous solution aimed to enhance the electrochemical performance of MnO₂. The rate of redox reaction between KMnO₄ and carbon was controlled by the concentration of KMnO₄ in a neutral solution. The MnO₂ nanoparticles grow along with (211) crystal faces when the redox reaction happens on the surface of fibers under 1D constraint, while the nanoparticles grow along with (200) crystal faces when the redox reaction happens in the bulk of fibers under 3D constraint. The composite, where MnO₂ nanoparticles are formed in the bulk under a constraint, yields an electrode material for supercapacitors showing good electron transport, rapid ion penetration, fast and reversible Faradaic reaction, and excellent rate performance. The capacitance of the composite electrode could be 1282 F g⁻¹ under a current density of 0.2 A g⁻¹ in 1 M Na₂SO₄ electrolyte. A symmetric supercapacitor delivers energy density of 36 Wh kg⁻¹ with power density of 39 W kg⁻¹, and can maintain 7.5 Wh kg⁻¹ at 10.3 kW kg⁻¹. It exhibits an excellent electrochemical cycling stability with 101% initial capacitance and 95% columbic efficiency even after 1000 cycles of charge/discharge.

Importance of polypyrrole in constructing 3D hierarchical carbon nanotube@MnO2 perfect core-shell nanostructures for high-performance flexible supercapacitors

This study reports the preparation of 3D hierarchical carbon nanotube (CNT) @MnO2 core-shell nanostructures under the assistance of polypyrrole (PPy). The as-prepared CNT@PPy@MnO2 core-shell structures show a perfect coating of MnO2 on each CNT and, more importantly, a robust bush-like pseudocapacitive shell to effectively increase the specific surface area and enhance the ion accessibility. As expected, a high specific capacity of 490-530 F g(-1) has been achieved from CNT@PPy@MnO2 single electrodes. And about 98.5% of the capacity is retained after 1000 charge/discharge cycles at a current density of 5 A g(-1). Furthermore, the assembled asymmetric CNT@PPy@MnO2//AC capacitors show the maximum energy density of 38.42 W h kg(-1) (2.24 mW h cm(-3)) at a power density of 100 W kg(-1) (5.83 mW cm(-3)), and they maintain 59.52% of the initial value at 10,000 W kg(-1) (0.583 W cm(-3)). In addition, the assembled devices show high cycling stabilities (89.7% after 2000 cycles for asymmetric and 87.2% for symmetric), and a high bending stability (64.74% after 200 bending tests). This ability to obtain high energy densities at high power rates while maintaining high cycling stability demonstrates that this well-designed structure could be a promising electrode material for high-performance supercapacitors.

Supercapacitor electrode with a homogeneously Co3O4-coated multiwalled carbon nanotube for a high capacitance

Cobalt oxide (Co3O4) was homogeneously coated on multiwalled carbon nanotube through a simple chemical deposition method and employed in supercapacitor electrodes. SEM image indicated the uniform distribution of Co3O4 nanoparticles on the surface of the multiwalled carbon nanotube. A maximum specific capacitance of 273 Fg(-1) was obtained at the charge-discharge current density of 0.5 Ag(-1). After 500 cycles of continuous charge-discharge process, about 88% of the initial capacity could be retained.

Dealloying of Cu-Based Metallic Glasses in Acidic Solutions: Products and Energy Storage Applications

Dealloying, a famous ancient etching technique, was used to produce nanoporous metals decades ago. With the development of dealloying techniques and theories, various interesting dealloying products including nanoporous metals/alloys, metal oxides and composites, which exhibit excellent catalytic, optical and sensing performance, have been developed in recent years. As a result, the research on dealloying products is of great importance for developing new materials with superior physical and chemical properties. In this paper, typical dealloying products from Cu-based metallic glasses after dealloying in hydrofluoric acid and hydrochloric acid solutions are summarized. Several potential application fields of these dealloying products are discussed. A promising application of nanoporous Cu (NPC) and NPC-contained composites related to the energy storage field is introduced. It is expected that more promising dealloying products could be developed for practical energy storage applications.

Facile one pot synthesis and Li-cycling properties of MnO2

MnO₂ compounds prepared by a molten salt method (MSM) using three different Mn-salts and studied for its electrochemical properties.