Pseudocapacitance multiporous vanadyl phosphate/graphene thin film electrode for high performance electrochemical capacitors

Diffusion of ferrocene through vanadyl phosphate by density functional theory

Here, we employed the nudged elastic band (NEB) method to simulate the diffusion of ferrocene through vanadyl phosphate (VOPO4), with a focus on understanding the diffusion pathways arising from the complex structure of ferrocene. We systematically evaluated a total of 36 potential diffusion paths, categorizing them into three groups based on their directional orientation: 15 paths between V sites along the [110] direction, 15 paths from V to P sites along the [100] direction, and 6 paths between P sites also along the [110] direction. Our analysis revealed that the energy barriers for diffusion along the [110] direction typically ranged between 0.25 and 0.35 eV, which are notably higher than those observed for pathways along the [100] direction, where the energy barriers ranged from 0.11 to 0.20 eV. To further elucidate the complex deformation of ferrocene during diffusion, we established four key measures to characterize the structural conformation: the angle of the axis of the ferrocene molecule relative to the [010] direction within the (001) plane, the dihedral angle between the two cyclopentadienyl rings, the orientation angle of the -CH bonds with respect to the [001] direction, and the angle between two -CH bonds from the two cyclopentadienyl rings.

Multifunctional Carbon Modification Enhancement for Vanadium-Based Phosphates as an Advanced Cathode of Zinc-Ion Batteries

In recent years, rechargeable aqueous zinc-ion batteries (ZIBs) have shown extraordinary potential due to their safety, nontoxicity, sustainable zinc resources, and low price. However, the lack of suitable cathode materials hinders the development of ZIBs. Recently, layered phosphates have been widely used as cathode materials. As one typical phosphate cathode, vanadium oxyphosphate (VOPO₄) has inherently low electronic conductivity and structural dissolution in electrochemical reactions, limiting its development. To solve these problems, VOPO₄/C is prepared by combining multifunctional carbon material with a VOPO₄ interlayer and an external surface, which not only improves the electronic conductivity of the composite material but also effectively inhibits the dissolution of VOPO₄ in the electrolyte. As a result, the prepared VOPO₄/C could deliver a reversible capacity of 140 mA h g^-1 at a current density of 100 mA g^-1. Furthermore, the rate performance of the VOPO₄/C composite has also been improved significantly. In the process of charging and discharging, zinc ions in the composite show perfect intercalate and deintercalate performance.

Design of novel self-assembled MXene and ZIF67 derivative composites as efficient electroactive material of energy storage device

High surface area and tunable pore size are beneficial for metal organic frameworks (MOFs) as electroactive material of energy storage devices. Novel ZIF67 derivative proposed in our previous work, nickel cobalt fluoride coupled with ammonia ions (NCNF), is synthesized using ammonia fluoride to solve poor electrical conductivity of MOFs. MXene is commonly incorporated in pseudo-capacitive materials to enhance electrical conductivity and energy storage ability. In this study, it is the first time to design MXene and NCNF composites (MXene/NCNF) with different MXene amounts via incorporating MXene in growing process of NCNF. MXene and NCNF are combined via self-assembly in a simple room temperature solution process. The optimized MXene/NCNF electrode shows a higher specific capacitance of 1020.0 F g^-1 (170.0 mAh g^-1) than that of NCNF electrode (574.2 F g^-1 and 95.7 mAh g^-1) at 20 mV s^-1, due to excellent surface properties of MXene/NCNF with conductive network of MXene and high electrocapacitive performance of NCNF. A symmetric energy storage device composed of the optimized MXene/NCNF electrodes presents outstanding cycling stability with Coulombic efficiency of 100% during whole cycling process and a high capacitance retention of 99% after 6000 cycles. Excellent electrochemical performance and simple synthesis of MXene/NCNF open new blueprints for designing novel electrocapacitive materials for electrochemical applications.

Free-Standing rGO-CNT Nanocomposites with Excellent Rate Capability and Cycling Stability for Na2SO4 Aqueous Electrolyte Supercapacitors

Facing the increasing demand for various renewable energy storage devices and wearable and portable energy storage systems, the research on electrode materials with low costs and high energy densities has attracted great attention. Herein, free-standing rGO-CNT nanocomposites have been successfully synthesized by a facile hydrothermal method, in which the hierarchical porous network nanostructure is synergistically assembled by rGO nanosheets and CNT with interlaced network distribution. The rGO-CNT composite electrodes with synergistic enhancement of rGO and CNT exhibit high specific capacitance, excellent rate capability, exceptional conductivity and outstanding long-term cycling stability, especially for the optimal rGO-CNT₃₀ electrode. Applied to a symmetric supercapacitor systems (SSS) assembled with an rGO-CNT₃₀ electrode and with 1 M Na₂SO₄ aqueous solution as the electrolyte, the SSS possesses a high energy density of 12.29 W h kg⁻¹ and an outstanding cycling stability, with 91.42% of initial specific capacitance after 18,000 cycles. Results from these electrochemical properties suggest that the rGO-CNT₃₀ nanocomposite electrode is a promising candidate for the development of flexible and lightweight high-performance supercapacitors.

Flexible Supercapacitors Based on Graphene/Boron Nitride Nanosheets Electrodes and PVA/PEI Gel Electrolytes

All-solid-state supercapacitors have gained increasing attention as wearable energy storage devices, partially due to their flexible, safe, and lightweight natures. However, their electrochemical performances are largely hampered by the low flexibility and durability of current polyvinyl alcohol (PVA) based electrolytes. Herein, a novel polyvinyl alcohol-polyethyleneimine (PVA-PEI) based, conductive and elastic hydrogel was devised as an all-in-one electrolyte platform for wearable supercapacitor (WSC). For proof-of-concept, we assembled all-solid-state supercapacitors based on boron nitride nanosheets (BNNS) intercalated graphene electrodes and PVA-PEI based gel electrolyte. Furthermore, by varying the electrolyte ions, we observed synergistic effects between the hydrogel and the electrode materials when KOH was used as electrolyte ions, as the Graphene/BNNS@PVA-PEI-KOH WSCs exhibited a significantly improved areal capacitance of 0.35 F/cm2 and a smaller ESR of 6.02 ohm/cm2. Moreover, due to the high flexibility and durability of the PVA-PEI hydrogel electrolyte, the developed WSCs behave excellent flexibility and cycling stability under different bending states and after 5000 cycles. Therefore, the conductive, yet elastic, PVA-PEI hydrogel represents an attractive electrolyte platform for WSC, and the Graphene/BNNS@PVA-PEI-KOH WSCs shows broad potentials in powering wearable electronic devices.