Rational Design of Porous MnO 2 Tubular Arrays via Facile and Templated Method for High Performance Supercapacitors

Neatly arranged mesoporous MnO2 nanotubes with oxygen vacancies for electrochemical energy storage

Intrinsically poor conductivity, sluggish ion transfer kinetics, and limited specific area are the three main obstacles that confine the electrochemical performance of manganese dioxide in supercapacitors. Herein, one-dimensional mesoporous MnO2 nanotubes were prepared using a polycarbonate film as a template and a large number of oxygen vacancies were introduced by calcination under a N2 atmosphere. The effects of calcination temperature on the crystal structure, micro-morphology and electrochemical performance of MnO2 nanotubes were studied. The presence of oxygen vacancies increases the redox capacity of ov-MnO2-300 nanotubes, and the unique one-dimensional mesoporous structure also provides an effective channel for ion transport. Therefore, the ov-MnO2-300 nanotube has an excellent specific capacitance of 459.0 F g-1 at a current density of 1 A g-1 and also has outstanding rate performance and cycle performance. An asymmetric supercapacitor assembled with ov-MnO2-300 nanotubes as the positive electrode and graphene@MoS2 as the negative electrode delivered an energy density of 40.2 W h kg-1 at a power density of 1024 W kg-1. The excellent capacitance performance is mostly attributed to the introduction of oxygen vacancies to increase the intrinsic conductivity of MnO2, and the unique one-dimensional mesoporous nanotube structure increases the active sites of redox reactions.

High-rate asymmetrical supercapacitors based on cobalt-doped birnessite nanotubes and Mn-FeOOH nanotubes

Co-Doped MnO2 nanotubes (Co-MnO2-5) were prepared as the positive electrode of supercapacitors via a simple one-step hydrothermal method. Co doping and one-dimensional tunneling of nanotubes result in low internal resistance and good ionic contact, enhancing the conductivity and electrochemical performance of the electrodes.

Birnessite based nanostructures for supercapacitors: challenges, strategies and prospects

In the past few years, intensive attention has been focused on birnessite based electrodes for supercapacitors. Much progress has been achieved in developing birnessite based nanostructures with high electrochemical performance. However, challenges still remain in taking full advantage of birnessite and building smart structures to overcome the gap between the obtained capacitance and its theoretical capacitance. In this review, the basic information on birnessite and its preparation strategies are summarized and the current challenges are put forward. Finally, some new strategies for preparing high electrochemical performance birnessite based nanostructures are highlighted.

One-Step Synthesis of Mesoporous Chlorine-Doped Carbonated Cobalt Hydroxide Nanowires for High-Performance Supercapacitors Electrode

Self-stabilized and well-defined chlorine-doped carbonated cobalt hydroxide nanowires have been obtained as a binder-free electrode via a facile method. The Co material has a unique well-defined needle-like structure, composed of highly aligned monomer with the diameter of about 3-10 nm and numerous surface pores, which makes it have potential for high-performance electrochemical capacitors. The test results show the directly acquired Co-ClNWs(NiE) electrode in three-electrode system can reach the specific capacity of more than 2150 F/g under the current density of 1 A/g, accompanied by a good cycling stability of 94.3% capacitance retention after 500 cycles, and exhibits a high energy density of 41.8 W h/kg at the power density of 1280.7 W/kg when using it as the positive electrode of an asymmetric supercapacitor. After making a comparison of the current material with the conventional electrodes, we can find that a better electrochemical performance can be achieved with a more convenient one-step method. Therefore, we, in this work, may provide a new type of manufacturing concept for future electrode treatment.

Controllable synthesis of MnO2 nanostructures anchored on graphite foam with different morphologies for a high-performance asymmetric supercapacitor

MnO₂ nanostructures with different morphologies (nanowires, nanowire bundles, flower-like nanosheet bundles) were synthesized via a simple and surfactant-free hydrothermal method.

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.

Reconstruction of TiO2/MnO2-C nanotube/nanoflake core/shell arrays as high-performance supercapacitor electrodes

Construction of electrodes with fast reaction kinetics is of great importance for achieving advanced supercapacitors. Herein we report a facile combined synthetic strategy with atomic layer deposition (ALD) and electrodeposition to rationally fabricate nanotube/nanoflake core/shell arrays. ALD-TiO₂ nanotubes are used as the skeleton core for assembly of electrodeposited MnO₂-C nanoflake shells forming a core/shell structure. Highly porous architecture and good electrical conductivity are combined in this unique core/shell structure, resulting in fast ion/electron transfer. In tests of electrochemical performance, the TiO₂/MnO₂-C core/shell arrays are characterized as cathode for asymmetric supecapacitors and exhibit high specific capacitance (880 F g^-1 at 2.5 A g^-1), excellent rate properties (735 F g^-1 at 30 A g^-1) and good long-term cycling stability (94.3% capacitance retention after 20 000 cycles). The proposed electrode construction strategy is favorable for fabrication of other advanced supercapacitor electrodes.

Fabrication of a snail shell-like structured MnO2@CoNiO2 composite electrode for high performance supercapacitors

MnO₂@CoNiO₂ snail shell-like structures exhibits superior specific capacitance and cyclic stability than the MnO₂ and CoNiO₂ electrodes.

Branched ultra-fine nickel oxide/manganese dioxide core–shell nanosheet arrays for electrochemical capacitors

Hierarchically core–shell structure composed of two dimensional NiO and MnO₂ nanosheets demonstrate good electrochemical performance in supercapacitor.