Template synthesis of hierarchical mesoporous δ-MnO 2 hollow microspheres as electrode material for high-performance symmetric supercapacitor

NiCo2S4nanosheets decorated on nitrogen-doped hollow carbon nanospheres as advanced electrodes for high-performance asymmetric supercapacitors

Taking advantage of both Faradaic and carbonaceous materials is an efficient way to synthesize composite electrodes with enhanced performance for supercapacitors. In this study, NiCo₂S₄nanoflakes were grown on the surface of nitrogen-doped hollow carbon nanospheres (NHCSs), forming a NiCo₂S₄/NHCS composite with a core-shell structure. This three-dimensionally confined growth of NiCo₂S₄can effectively inhibit its aggregation and facilitate mass transport and charge transfer. Accordingly, the NiCo₂S₄/NHCS composite exhibited high cycling stability with only 9.2% capacitance fading after 10 000 cycles, outstanding specific capacitance of 902 F g^-1at 1 A g^-1, and it retained 90.6% of the capacitance at 20 A g^-1. Moreover, an asymmetric supercapacitor composed of NiCo₂S₄/NHCS and activated carbon electrodes delivered remarkable energy density (31.25 Wh kg^-1at 750 W kg^-1), excellent power density (15003 W kg^-1at 21.88 Wh kg^-1), and satisfactory cycling stability (13.4% capacitance fading after 5000 cycles). The outstanding overall performance is attributed to the synergistic effect of the NiCo₂S₄shell and NHSC core, which endows the composite with a stable structure, high electrical conductivity, abundant active reaction sites, and short ion-transport pathways. The synthesized NiCo₂S₄/NHCS composite is a competitive candidate for the electrodes of high-performance supercapacitors.

Engineering Co3O4/MnO2 nanocomposite materials for oxygen reduction electrocatalysis

Stable and active electrocatalysts preparation for the oxygen reduction reaction (ORR) is essential for an energy storage and conversion materials (e.g. metal-air batteries). Herein, we prepared a highly-active MnO₂ and Co₃O₄/MnO₂ nanocomposite electrocatalysts using a facial co-precipitation approach. The electrocatalytic activity was examined in alkaline media with LSV and CV. Additionally, the physicochemical characteristics of the MnO₂ and Co₃O₄/MnO₂ composite materials were studied via SEM, XRD, BET, UV-Vis, TGA/DTA, ICP-OES and FTIR. Morphological studies indicated that a pure MnO₂ has a spherical flower-like architecture, whereas Co₃O₄/MnO₂ nanocomposites have an aggregated needle-like structure. Moreover, from the XRD investigation parameters such as the dislocation density, micro-strain, and crystallite size were analyzed. The calculated energy bandgaps for the MnO₂, Co₃O₄/MnO₂-1-5, and Co₃O₄/MnO₂-1-1 nanocomposites were 3.07, 2.6, and 2.3 eV, correspondingly. The FTIR spectroscopy was also employed to study the presence of M-O bonds (M = Mn, Co). The thermal gravimetric investigation showed that the Co₃O₄/MnO₂ nanocomposite materials exhibited improved thermal stability, confirming an enhanced catalytic activity of ORR for MnO₂/Co₃O₄-1-1 composite materials for ORR. These results confirm that the prepared Co₃O₄/MnO₂ composite materials are promising air electrode candidates for the energy storage and conversion technologies.

Application of multi-active center organic quinone molecular functionalized graphene in fully pseudocapacitive asymmetric supercapacitors

5, 7, 12, 14-pentacenetetrone (PT), polycyclic quinone derivatives, are rich in carbonyl, which were investigated as a novel organic electrode material for supercapacitors. PT with aπconjugated system, is a flat molecule, generating strongπ-πinteractions between molecules. PT molecules were uniformly fixed on conductive reduced graphene oxide (rGO) throughπ-πinteraction by one-step solvothermal method, forming a three-dimensional cross-linked PT@rGO hydrogel. This composite structure was conducive to reducing the charge transfer resistance and promoting the Faraday reaction of electrode, which achieved the superposition of electric double-layer capacitance and pseudocapacitance. Appropriate organic molecular loading can effectively improve electrochemical performance. The optimal PT@rGO electrode material displayed the specific capacitance of 433.2 F g^-1at 5 mV s^-1with an excellent rate capability in 1 mol l^-1H₂SO₄electrolyte. Finally, the fully pseudocapacitive asymmetric supercapacitor has been assembled by using PT@rGO as positive electrode and benz[a]anthracene-7,12-quinone (BAQ) modified rGO(BAQ/rGO)as negative electrode, which exhibited the good energy storage performance in a cell voltage of 1.8 V.

Effects of electrode thickness and crystal water on pseudocapacitive performance of layered birnessite MnO2

Manganese dioxide (MnO₂) nanomaterials with two-dimensional (2D) layered birnessite structures are promising pseudocapacitive electrode materials. However, the effects of structural factors on their electrochemical performance is not fully understood. We synthesize alkali-free crystal water containing 2D layered birnessite MnO₂ electrodes with controllable mass loading from 0.1 to 19.3 mg cm^-2 to investigate the effects of electrode thickness and crystal water functions on crystal structure and pseudocapacitive behavior, to promote its industrialization. We find that the crystal water enlarges the interlayer space of birnessite MnO₂ with electrolyte ions transported much more easily, resulting in higher specific capacitance of 702 F g^-1 (70.2 mF cm^-2) and excellent cycling stability of 20 000 charge-discharge cycles even at a mass loading of up to 10.8 mg cm^-2. Such gains in specific capacitance are weakened significantly with raised mass loading. Thus, compared to a carbon cloth substrate, a carbon nanotube film with enhanced electron space transport capability presents better performance, indicating an effective strategy for higher mass loading cases. The present work sheds light on an efficient method for achieving high capacitance and mass loading together, for practical applications.

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.

Hierarchical assembly of manganese dioxide nanosheets on one-dimensional titanium nitride nanofibers for high-performance supercapacitors

MnO₂ has attracted considerable attention for use in supercapacitors. Nevertheless, its low electrical conductivity greatly hinders its potential application. Here, we demonstrate the fabrication of a high-performance electrode material via facile coating of hierarchical MnO₂ nanosheets onto highly electrically conductive one-dimensional (1D) TiN nanofibers for use in supercapacitors. The TiN nanofibers are prepared through nitridation electrospinning of TiO₂ nanofibers via ammonia annealing. The obtained TiN@MnO₂ composites exhibit enhanced electrochemical properties, such as high specific capacitance of 386 F/g at a current density of 1 A/g, and long cycle stability of ∼111.7% capacity retention after 4000 cycles at 6 A/g. The unique nanostructure and significant synergistic effect between TiN and MnO₂ are responsible for its good electrochemical performance.

Hierarchical Porous Carbon Derived from Sichuan Pepper for High-Performance Symmetric Supercapacitor with Decent Rate Capability and Cycling Stability

Hierarchical micro-mesoporous carbon (denoted as HPC-2 in this study) was synthesized by pre-carbonization of biomass Sichuan pepper followed by KOH activation. It possessed well-developed porosity with the specific surface area of 1823.1 m² g⁻¹ and pore volume of 0.906 cm³ g⁻¹, and exhibited impressive supercapacitive behaviors. For example, the largest specific capacitance of HPC-2 was tested to be ca. 171 F g⁻¹ in a three-electrode setup with outstanding rate capability and stable electrochemical property, whose capacitance retention was near 100% after cycling at rather a high current density of 40 A g⁻¹ for up to 10,000 cycles. Furthermore, a two-electrode symmetric supercapacitor cell of HPC-2//HPC-2 was constructed, which delivered the maximum specific capacitance and energy density of ca. 30 F g⁻¹ and 4.2 Wh kg⁻¹, respectively, had prominent rate performance and cycling stability with negligible capacitance decay after repetitive charge/discharge at a high current density of 10 A g⁻¹ for over 10,000 cycles. Such electrochemical properties of HPC-2 in both three- and two-electrode systems are superior or comparable to those of a great number of porous biomass carbon reported previously, hence making it a promising candidate for the development of high-performance energy storage devices.

Greatly Enhanced Faradic Capacities of 3D Porous Mn3O4/G Composites as Lithium-Ion Anodes and Supercapacitors by C-O-Mn Bonding

Through C-O-Mn bonding, graphene nanosheets are homogeneously dispersed in porous Mn₃O₄ to take full advantages of porous Mn₃O₄ and graphene nanosheets, making the as-formed three-dimensional porous Mn₃O₄/reduced graphene oxide (rGO) composite exhibit good electrochemical performance. Besides, C-O-Mn bonding is demonstrated to greatly promote the Faradic reactions of the composite, resulting in the enhancement of its real capacity in supercapacitor (SC) electrodes as well as lithium-ion battery (LIB) anodes. By simply fine-tuning the content of graphene (<7 wt %), the composite with 2.8 wt % of rGO delivers a high capacitance of 315 F g^-1 at 0.5 A g^-1 with a high rate capability of 64.7% at 30 A g^-1 and an excellent cycling stability of 105% (5 A g^-1, 5000 cycles) as an SC electrode. Also, the one with 6.9 wt % rGO can present a reversible capacity of more than 1500 mAh g^-1 at 0.05 A g^-1 as the LIB anode, the highest value reported to date, which remains 561 mAh g^-1 at 1 A g^-1.

Phase and morphology controlled polymorphic MnO2 nanostructures for electrochemical energy storage

In this work, a series of MnO₂ nanostructures with different crystallographic structures, including δ-MnO₂ nanotubes and α-MnO₂ nanorods, were prepared using polycarbonate membrane as a template.

Mixed-Phase MnO₂/N-Containing Graphene Composites Applied as Electrode Active Materials for Flexible Asymmetric Solid-State Supercapacitors

MnO₂/N-containing graphene composites with various contents of Mn were fabricated and used as active materials for the electrodes of flexible solid-state asymmetric supercapacitors. By scanning electron microscopes (SEM), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectrometer (XPS), fourier-transform infrared spectroscopy (FTIR) and Raman spectra, the presence of MnO₂ and N-containing graphene was verified. The MnO₂ nanostructures decorated on the N-containing graphene were of α- and γ-mixed phases. N-containing graphene was found to reduce the charge transfer impedance in the high-frequency region at the electrode/electrolyte interface (R_CT) due to its good conductivity. The co-existence of MnO₂ and N-containing graphene led to a more reduced R_CT and improved charge transfer. Both the mass loading and content of Mn in an active material electrode were crucial. Excess Mn caused reduced contacts between the electrode and electrolyte ions, leading to increased R_CT, and suppressed ionic diffusion. When the optimized mass loading and Mn content were used, the 3-NGM1 electrode exhibiting the smallest R_CT and a lower ionic diffusion impedance was obtained. It also showed a high specific capacitance of 638 F·g^-1 by calculation from the cyclic voltammetry (CV) curves. The corresponding energy and power densities were 372.7 Wh·kg^-1 and 4731.1 W·kg^-1, respectively. The superior capacitance property arising from the synergistic effect of mixed-phase MnO₂ and N-containing graphene had permitted the composites promising active materials for flexible solid-state asymmetric supercapacitors. Moreover, the increase of specific capacitance was found to be more significant by the pseudocapacitive MnO₂ than N-containing graphene.