Sandwich-like NiO/rGO nanoarchitectures for 4 V solid-state asymmetric-supercapacitors with high energy density

Layer-by-Layer Heterostructure of MnO2@Reduced Graphene Oxide Composites as High-Performance Electrodes for Supercapacitors

In this paper, δ-MnO₂ with layered structure was prepared by a facile liquid phase method, and exfoliated MnO₂ nanosheet (e-MnO₂) was obtained by ultrasonic exfoliation, whose surface was negatively charged. Then, positive charges were grafted on the surface of MnO₂ nanosheets with a polycation electrolyte of polydiallyl dimethylammonium chloride (PDDA) in different concentrations. A series of e-MnO₂@reduced graphene oxide (rGO) composites were obtained by electrostatic self-assembly combined with hydrothermal chemical reduction. When PDDA was adjusted to 0.75 g/L, the thickness of e-MnO₂ was ~1.2 nm, and the nanosheets were uniformly adsorbed on the surface of graphene, which shows layer-by-layer morphology with a specific surface area of ~154 m²/g. On account of the unique heterostructure, the composite exhibits good electrochemical performance as supercapacitor electrodes. The specific capacitance of e-MnO₂-0.75@rGO can reach 456 F/g at a current density of 1 A/g in KOH electrolyte, which still remains 201 F/g at 10 A/g. In addition, the capacitance retention is 98.7% after 10000 charge-discharge cycles at 20 A/g. Furthermore, an asymmetric supercapacitor (ASC) device of e-MnO₂-0.75@rGO//graphene hydrogel (GH) was assembled, of which the specific capacitance achieves 94 F/g (1 A/g) and the cycle stability is excellent, with a retention rate of 99.3% over 10000 cycles (20 A/g).

Synthesis of NiO/Nitrogen-Doped Carbon Nanowire Composite with Multi-Layered Network Structure and Its Enhanced Electrochemical Performance for Supercapacitor Application

Multi-layered NiO nanowires linked with a nitrogen-doped carbon backbone grown directly on flexible carbon cloth (NiO/NCBN/CC) was successfully fabricated with a facile synthetic strategy. The NiO/NCBN/CC was further used as a binding-free electrode for flexible energy storage devices, showing a boosted performance including a high capacitance of 1039.4 F g^-1 at 1 A g^-1 and an 83.4% capacitance retention ratio. More importantly, after 1500 cycles, the capacitance retention can achieve 72.5% at a current density of 20 A g^-1. The excellent electrochemical properties of the as-prepared NiO/NCBN/CC are not only attributed to the multi-layered structure that can help to tender unimpeded channels and accommodate the electrolyte ions around the electrode interface during the charge-discharge process, but is also due to the link between the NiO and N-doped carbon backbone and the nitrogen doping on the carbon substrate, which results in extra defects on the surface that could boost the interfacial electron transfer rate of the electrode.

Ionic Liquid-Based Gels for Applications in Electrochemical Energy Storage and Conversion Devices: A Review of Recent Progress and Future Prospects

Ionic liquids (ILs) are molten salts that are entirely composed of ions and have melting temperatures below 100 °C. When immobilized in polymeric matrices by sol-gel or chemical polymerization, they generate gels known as ion gels, ionogels, ionic gels, and so on, which may be used for a variety of electrochemical applications. One of the most significant research domains for IL-based gels is the energy industry, notably for energy storage and conversion devices, due to rising demand for clean, sustainable, and greener energy. Due to characteristics such as nonvolatility, high thermal stability, and strong ionic conductivity, IL-based gels appear to meet the stringent demands/criteria of these diverse application domains. This article focuses on the synthesis pathways of IL-based gel polymer electrolytes/organic gel electrolytes and their applications in batteries (Li-ion and beyond), fuel cells, and supercapacitors. Furthermore, the limitations and future possibilities of IL-based gels in the aforementioned application domains are discussed to support the speedy evolution of these materials in the appropriate applicable sectors.

Hierarchical Stratiform of a Fluorine-Doped NiO Prism as an Enhanced Anode for Lithium-Ion Storage

Doping is regarded as a prominent strategy to optimize the crystal structure and composition of battery materials to withstand the anisotropic expansion induced by the repeated insertion and extraction of guest ions. The well-known knowledge and experience obtained from doping engineering predominate in cathode materials but have not been fully explored for anodes yet. Here, we propose the practical doping of fluorine ions into the host lattice of nickel oxide to unveil the correlation between the crystal structure and electrochemical properties. Multiple ion transmission pathways are created by the orderly two-dimensional nanosheets, and thus the stress/strain can be significantly relieved with trace fluorine doping, ensuring the mechanical integrity of the active particle and superior electrochemical properties. Density functional theory calculations manifest that the F doping in NiO could improve crystal structural stability, modulate the charge distribution, and enhance the conductivity, which promotes the performance of lithium-ion storage.

Intensified Energy Storage in High-Voltage Nanohybrid Supercapacitors via the Efficient Coupling between TiNb2O7/Holey-rGO Nanoarchitectures and Ionic Liquid-Based Electrolytes

Obtaining a comprehensive understanding of the energy storage mechanisms, interface compatibility, electrode-electrolyte coupling, and synergistic effects in carefully programmed nanoarchitectural electrodes and complicated electrolyte systems will provide a shortcut for designing better supercapacitors. Here, we report the intrinsic relationships between the electrochemical performances and microstructures or composition of complex nanoarchitectures and formulated electrolytes. We observed that isolated TiNb₂O₇ nanoparticles provided both a Faradaic intercalation contribution and a surface pseudocapacitance. The holey graphenes partitioned by nanoparticles not only fostered the fast transport of both electrons and ions but also provided additional electrical double-layer capacitance. The charge contributions from the diffusion-controlled intercalation process and capacitive behaviors, double-layer charging, and pseudocapacitance, were quantitatively distinguished in different electrolytes including a formulated ionic-liquid mixture, various nanocomposite ionogel electrolytes, and an organic LiPF₆ electrolyte. A steered molecular dynamics simulation method was used to unveil the underlying principles governing the high-rate capability of holey nanoarchitectures. High energy density and high rate capability in solid-state supercapacitors were achieved using the Faradaic contributions from the lithium-ion insertion process and its surface charge-transfer process in combination with the non-Faradaic contribution from the double-layer effects. The work suggests that practical high-voltage supercapacitors with programmed performances and high safety can be realized via the efficient coupling between emerging nanoarchitectural electrodes and formulated high-voltage electrolytes.

Alpha-MnO2 nanofibers/nitrogen and sulfur-co-doped reduced graphene oxide for 4.5 V quasi-solid state supercapacitors using ionic liquid-based polymer electrolyte

α-MnO₂ nanofibers combined with nitrogen and sulfur co-doped reduced graphene oxide (α-MnO₂/N&S-rGO) were prepared through simple hydrothermal and ball milling processes. Structural characterization results by X-ray diffraction, X-ray photoemission spectroscopy, electron microscopy and Raman spectroscopy demonstrated that α-MnO₂ nanofibers with the average diameter of ~40 nm were well dispersed on N&S-rGO nanoflakes. The synthesized material was incorporated into supercapacitor (SC) electrodes and assembled with the quasi-solid-state electrolyte comprising N,N-Diethyl-N-methyl-N-(2-methoxy-ethyl)ammonium bis (trifluoromethyl-sulfonyl)amide [DEME][TFSA]/polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP) to produce coin-cell SCs. Electrochemical performances of SCs were measured by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. From the electrochemical data, SC using α-MnO₂/N&S-rGO exhibited a good specific capacitance of 165F g^-1 at 0.25 A g^-1 with a wide potential window of 0-4.5 V, corresponding to a high energy density of 110 Wh kg^-1 and a power density of 550 W kg^-1. In addition, it exhibited good electrochemical stability with a capacitance retention of 75% after 10,000 cycles at 1 A g^-1 and a low self-discharge loss. The attained energy-storage performances indicated that the α-MnO₂/N&S-rGO composite could be highly promising for high-performance ionic liquid-based quasi solid-state supercapacitors.

Highly Efficient Quasi-Solid-State Asymmetric Supercapacitors Based on MoS2/MWCNT and PANI/MWCNT Composite Electrodes

Molybdenum disulfide (MoS₂) and polyaniline (PANI) electrodes were decorated with multi-walled carbon nanotubes (MWCNTs) on the basis of a facial hydrothermal and in situ polymerization methods and served in the asymmetric supercapacitor (ASC). The MoS₂ and MWCNTs with a mole ratio of 1:1 in MoS₂|MWCNTs electrode exhibited better electrochemical properties through extensive electrochemical studies, in terms of the highest specific capacitance of 255.8 F/g at 1 A/g, low internal resistance, and notable electrochemical stability with retention of the initial specific capacitance at 91.6% after 1000 cycles. The as-prepared PANI|MWCNTs electrode also exhibited good specific capacitance of 267.5 F/g at 1 A/g and remained 97.9% capacitance retention after 1000 cycles. Then, the ASC with MoS₂|MWCNTs and PANI|MWCNTs composite electrodes were assembled with polyvinyl alcohol (PVA)-Na₂SO₄ gel electrolyte, which displayed good electrochemical performance with the specific capacitance of 138.1 F/g at 1 A/g, and remained the energy density of 15.09 Wh/kg at a high power density of 2217.95 W/kg. This result shows that this ASC device possesses excellent electrochemical properties of high energy density and power output and thus showing a potential application prospect.