Redox-Active Two-Dimensional Tetrathiafulvalene-Copper Metal-Organic Framework with Boosted Electrochemical Performances for Supercapatteries

Metal-organic frameworks (MOFs) have attracted noticeable attention as promising candidates for electrochemical energy storage. However, the lack of electrical conductivity and the weak stability of most MOFs result in poor electrochemical performances. Here, a tetrathiafulvalene (TTF)-based complex, formulated as [(CuCN)₂(TTF(py)₄)] (1) (TTF-(py)₄ = tetra(4-pyridyl)-TTF), is assembled by in situ generation of coordinated CN^- from a nontoxic source. Single-crystal X-ray diffraction analysis reveals that compound 1 possesses a two-dimensional layered planar structure, which is further stacked in parallel to form a three-dimensional supramolecular framework. The planar coordination environment of 1 is the first example of a TTF-based MOF. Attributed to the unique structure and redox TTF ligand, the electrical conductivity of 1 is significantly increased by 5 orders of magnitude upon iodine treatment. The iodine-treated 1 (1-ox) electrode displays typical battery-type behavior through electrochemical characterizations. The supercapattery based on the 1-ox positrode and AC negatrode presents a high specific capacity of 266.5 C g^-1 at a specific current of 1 A g^-1 with a remarkable specific energy of 62.9 Wh kg^-1 at a specific power of 1.1 kW kg^-1. The excellent electrochemical performance of 1-ox is one of the best among those reported supercapatteries, demonstrating a new strategy for developing MOF-based electrode materials.

Synthesis of manganese molybdate/MWCNT nanostructure composite with a simple approach for supercapacitor applications

Recently, magnesium molybdate materials have attracted scientific attention for application in supercapacitor devices due to advantages like low synthesis cost and good redox reactions. Nevertheless, these materials endure low electrical conductivity leading to inferior electrochemical performance. To eliminate this drawback, we prepare a composite powder containing magnesium molybdate and functionalized carbon nanotubes (MMO/C) using a simple process to improve the supercapacitive properties. The results proved an electrostatic interaction between the two components of the composite powder, which contains 18-30 nm magnesium molybdate nanoparticles. A crystal model related to magnesium molybdate powder (MMO) was simulated, illustrating that MnO₆ octahedra are formed next to MoO₄ tetrahedra. The mesoporous structure of both powders was confirmed whereas the specific surface area of the MMO was enhanced by 69.9% to 36.86 m² g^-1 in the MMO/C powder with more electroactive sites. The higher electrical conductivity of the MMO/C electrode was proved using electrochemical impedance spectroscopy (EIS) results, with the MMO/C electrode achieving a specific capacitance of 571 F g^-1 at 1 A g^-1 current density, improved by more than 4.5 times that of the MMO. Furthermore, the rate performance and cycling stability of the MMO/C electrode reached 87% and 85.2%, respectively. Finally, a two-electrode energy storage device (MMO/C//AC) was assembled. It reveals a specific capacitance of 94.7 F g^-1, a maximum energy density of 29.6 W h kg^-1 at a power density of 660.1 W kg^-1, and cycling performance of 84.3% after 2000 cycles. As a result, the resulting data demonstrate that the MMO/C electroactive material has promising abilities in capacitive energy storage systems.

Formation of NiMoO4 Anisotropic Nanostructures under Hydrothermal Conditions

Abstract—The synthesis of NiMoO4 hierarchical nanostructures using the hydrothermal method has been studied. The decomposition of NiMoO4·xH2O crystalline hydrate formed during the synthesis has been studied using synchronous thermal analysis upon heating in a stream of air and argon. According to X-ray diffraction as well as scanning and transmission electron microscopies, the proposed conditions allow one to synthesize single-phase nanosized (average CSR size of about 25 ± 2 nm) nickel(II) molybdate, which has a spinel-type monoclinic structure (space group C2/m) without impurity inclusions.

One-Pot Hydrothermal-Derived NiS2 -CoMo2 S4 with Vertically Aligned Nanorods as a Binder-Free Electrode for Coin-Cell-Type Hybrid Supercapacitor

Transition bimetallic sulfides are exploited as high-capacity electrode materials in energy storage devices owing to their abundant electroactive sites and relatively high electrical conductivity compared with metal oxides. Here, an in situ conversion of metal ions into NiS₂ -CoMo₂ S₄ vertically aligned nanorod arrays on nickel foam (NS-CMS NRAs@NF) using a one-step hydrothermal technique to address the "dead-mass" limitation and multi-step preparation methods is reported. An in situ-converted NS-CMS NRAs obtained for 12 h of reaction time (NS-CMS NRAs-12 h@NF) delivers a superior areal capacity of 780 μAh cm^-2 to the other NS-CMS electrodes synthesized for 6 h (543.1 μAh cm^-2 ) and 18 h (636.7 μAh cm^-2 ) at 7 mA cm^-2 . A coin-cell-type hybrid supercapacitor (HSC) is also fabricated to unveil the practical adaptability of NS-CMS NRAs-12 h@NF electrode. Utilizing its structural and active material intriguing features, assembled coin-cell-type HSC achieves a high areal capacitance of 246.2 mF cm^-2 (5 mA cm^-2 ) along with maximum areal energy density (147 μWh cm^-2 ) and power density (21.3 mW cm^-2 ), respectively. Furthermore, the capability of coin-cell-type HSC in real-time applications is also inspected. This work promotes in situ deposition strategy to fabricate metal sulfide-based nanostructures for high-performance electrochemical capacitors.

NiMoO4@NiMnCo2O4 Heterostructure: A Poly(3,4-propylenedioxythiophene) Composite-Based Supercapacitor Powers an Electrochromic Device

The hierarchical heterostructure of NiMoO₄@NiMnCo₂O₄ (NMO@NMCO) with furry structures of NMCO juxtaposed with NMO nanowires are endowed with multiple electrochemically active and accessible sites for ion storage, thus delivering an ultrahigh specific capacitance of 2706 F g^-1, nearly two-fold times greater than that of sole NMCO. Electrodeposition of an overlayer of a highly robust and electrically conducting polymer, poly(3,4-propylenedioxythiophene) (PProDOT), not only improves the energy storage performance but also assists the binary oxide cathode in retaining its structural integrity during redox cycling. Coupling with an anode of porous flaky carbon (FC) derived from groundnut shells results in an asymmetric supercapacitor of FC//PProDOT@NiMoO₄@NiMnCo₂O₄, which delivers an outstanding capacitance of 552 F g^-1, energy and power density ranges of 172-40 Wh kg^-1 and 0.75-10 kW kg^-1, respectively, and a remarkable cycle life of 50 000 cycles, with ∼97.8% capacitance retention, over an operational voltage window of 1.5 V. From an application perspective, the charged supercapacitor was connected to a complementary coloring reversible electrochromic device (ECD) of Prussian blue//PProDOT, and the ECD state transformed from a pale-blue to a deep blue hue, thus signaling the efficient utilization of energy stored in the supercapacitor. The energy-saving attribute of the ECD was realized in terms of an integrated visible-light modulation of 39% that accompanied the optical transition. Deployment of low-cost devices at homes and commercial spaces, capable of storing and saving energy, is the way forward, and this is one significant step in this direction.

A facile synthesis of a NiMoO4@metal-coated graphene-ink nanosheet structure towards the high energy density of a battery type-hybrid supercapacitor

Recently, reagent-based electrode materials (urea, ammonium fluoride, hexamethylenetetramine, l-cysteine, acetic acid, and triethylamine) have received great attention in numerous applications (e.g., solar cells, fuels cells, and battery-type hybrid supercapacitors (BHSCs)). Herein, we designed a wrinkle-type nanosheet structure with a mixture of NiMoO4 using triethylamine via a hydrothermal synthesis. The dual metal ions of Ni2+/Ni3+ and Mo2+/Mo3+ can significantly boost the energy storage performance by the support of triethylamine. NiMoO4 was deposited onto a nickel foam substrate without any binders, and the electrode exhibited a high specific capacitance/capacity of 415.66 F g-1 (249.4 mA h g-1) at 8 mA cm-2 with a good long-term cycling performance over 5000 cycles. Moreover, we assembled flexible HSCs (NiMoO4@MCG-ink) as the core, and the outer material delivered a high specific energy of 40.14 W h kg-1 at 1066.39 W kg-1. The HSC device demonstrated excellent long-term durability with a capacity retention of 99%, and various bending conditions were also tested for flexibility of the device after 5000 cycles. Furthermore, a series of two HSC devices were efficient to light up red and green light-emitting diodes (LEDs) for about 60 minutes, indicating its potential applications in energy storage devices. Hence, our work provides new insights into the preparation of new electrode materials for high energy storage performance of lithium-ion batteries (LIBs) and SCs.

Hierarchical NiMoO4@Co3V2O8 hybrid nanorod/nanosphere clusters as advanced electrodes for high-performance electrochemical energy storage

Herein, a synergistic strategy to construct hierarchical NiMoO₄@Co₃V₂O₈ (denoted as NMO@CVO) hybrid nanorod/nanosphere clusters is proposed for the first time, where Co₃V₂O₈ nanospheres (denoted as CVO) have been uniformly immobilized on the surface of the NiMoO₄ nanorods (denoted as NMO) via a facile two-step hydrothermal method. Due to the surface recombination effect between NMO and CVO, the as-prepared NMO@CVO effectively avoids the aggregation of CVO nanosphere clusters. The unique hybrid architecture can make the most of the large interfacial area and abundant active sites for storing charge, which is greatly beneficial for the rapid diffusion of electrolyte ions and fast electron transport. The optimized NMO@CVO-8 composite nanostructure displays battery-like behavior with a maximum specific capacity of 357 C g^-1, excellent rate capability (77.8% retention with the current density increasing by 10 times) and remarkable cycling stability. In addition, an aqueous asymmetric energy storage device is assembled based on the NMO@CVO-8 hybrid nanorod/nanosphere clusters and activated carbon. The device shows an ultrahigh energy density of 48.5 W h kg^-1 at a power density of 839.1 W kg^-1, good rate capability (20.9 W h kg^-1 even at 7833.7 W kg^-1) and excellent cycling stability (83.5% capacitance retention after 5000 cycles). More notably, two charged devices in series can light up a red light-emitting diode (LED) for 20 min, demonstrating its potential in future energy storage applications. This work indicates that the hierarchical NiMoO₄@Co₃V₂O₈-8 hybrid nanorod/nanosphere clusters are promising energy storage materials for future practical applications and also provides a rational strategy for fabricating novel nanostructured materials for high-performance energy storage.