High Volumetric Energy Density Hybrid Supercapacitors Based on Reduced Graphene Oxide Scrolls

Oxygen-Vacancy Abundant Nanoporous Ni/NiMnO3/MnO2@NiMn Electrodes with Ultrahigh Capacitance and Energy Density for Supercapacitors

High-performance energy storage devices (HPEDs) play a critical role in the realization of clean energy and thus enable the overarching pursuit of nonpolluting, green technologies. Supercapacitors are one class of such lucrative HPEDs; however, a serious limiting factor of supercapacitor technology is its sub-par energy density. This report presents hitherto unchartered pathway of physical deformation, chemical dealloying, and microstructure engineering to produce ultrahigh-capacitance, energy-dense NiMn alloy electrodes. The activated electrode delivered an ultrahigh specific-capacitance of 2700 F/cm³ at 0.5 A/cm³. The symmetric device showcased an excellent energy density of 96.94 Wh/L and a remarkable cycle life of 95% retention after 10,000 cycles. Transmission electron microscopy and atom probe tomography studies revealed the evolution of a unique hierarchical microstructure comprising fine Ni/NiMnO₃ nanoligaments within MnO₂-rich nanoflakes. Theoretical analysis using density functional theory showed semimetallic nature of the nanoscaled oxygen-vacancy-rich NiMnO₃ structure, highlighting enhanced carrier concentration and electronic conductivity of the active region. Furthermore, the geometrical model of NiMnO₃ crystals revealed relatively large voids, likely providing channels for the ion intercalation/de-intercalation. The current processing approach is highly adaptable and can be applied to a wide range of material systems for designing highly efficient electrodes for energy-storage devices.

All-Carbon Monolithic Composites from Carbon Foam and Hierarchical Porous Carbon for Energy Storage

We designed high-volumetric-energy-density supercapacitors from monolithic composites composed of self-standing carbon foam (CF) as the conducting matrix and embedded hierarchically organized porous carbon (PICK) as the active material. Using multiprobe scanning tunneling microscopy at selected areas, we were able to disentangle morphology-dependent contributions of the heterogeneous composite to the overall conductivity. Adding PICK is found to enhance the conductivity of the monoliths by providing additional links for the CF network, enabling high and stable performance. The resulting all-carbon CF-PICK composites were used as self-standing electrodes for symmetric supercapacitors without the need for a binder, additional conducting additive, metals as a current collector, or casting/drying steps. Supercapacitors achieved a capacitance of 181 F g^-1 based on the entire mass of the monolithic electrode as well as an outstanding rate capability. Our symmetrical supercapacitors also delivered a record volumetric energy density of 19.4 mW h cm^-3 when using aqueous electrolytes. Excellent cycling stability with almost quantitative retention of capacitance was found after 10,000 cycles in 6.0 M KOH as the electrolyte. Furthermore, charge-discharge testing at different currents demonstrated the fast charge-discharge capability of this material system that meets the requirements for practical applications.

A Mechanically Flexible Necklace-Like Architecture for Achieving Fast Charging and High Capacity in Advanced Lithium-Ion Capacitors

Integration of fast charging, high capacity, and mechanical flexibility into one electrode is highly desired for portable energy-storage devices. However, a high charging rate is always accompanied by capacity decay and cycling instability. Here, a necklace-structured composite membrane consisting of micron-sized FeSe₂ cubes uniformly threaded by carbon nanofibers (CNF) is reported. This unique electrode configuration can not only accommodate the volumetric expansion of FeSe₂ during the lithiation/delithiation processes for structural robustness but also guarantee ultrafast kinetics for Li⁺ entry. At a high mass loading of 6.2 mg cm^-2 , the necklace-like FeSe₂ @CNF electrode exhibits exceptional rate capability (80.7% capacity retention from 0.1 to 10 A g^-1 ) and long-term cycling stability (no capacity decay after 1100 charge-discharge cycles at 2 A g^-1 ). The flexible lithium-ion capacitor (LIC) fabricated by coupling a pre-lithiated FeSe₂ @CNF anode with a porous carbon cathode delivers impressive volumetric energy//power densities (98.4 Wh L^-1 at 157.1 W L^-1 , and 58.9 Wh L^-1 at 15714.3 W L^-1 ). The top performance, long-term cycling stability, low self-discharge rate, and high mechanical flexibility make it among the best LICs ever reported.

Direct Synthesis of MoS2 Nanosheets in Reduced Graphene Oxide Nanoscroll for Enhanced Photodetection

Due to their unique tubular and spiral structure, graphene and graphene oxide nanoscrolls (GONS) have shown extensive applications in various fields. However, it is still a challenge to improve the optoelectronic application of graphene and GONS because of the zero bandgap of graphene. Herein, ammonium tetrathiomolybdate ((NH₄)₂MoS₄) was firstly wrapped into the ((NH₄)₂MoS₄@GONS) by molecular combing the mixture of (NH₄)₂MoS₄ and GO solution on hydrophobic substrate. After thermal annealing, the (NH₄)₂MoS₄ and GO were converted to MoS₂ nanosheets and reduced GO (RGO) simultaneously, and, thus, the MoS₂@RGONS was obtained. Raman spectroscopy and high-resolution transmission electron microscopy were used to confirm the formation of MoS₂ nanosheets among the RGONS. The amount of MoS₂ wrapped in RGONS increased with the increasing height of GONS, which is confirmed by the atomic force microscopy and Raman spectroscopy. The as-prepared MoS₂@RGONS showed much better photoresponse than the RGONS under visible light. The photocurrent-to-dark current ratios of photodetectors based on MoS₂@RGONS are ~570, 360 and 140 under blue, red and green lasers, respectively, which are 81, 144 and 35 times of the photodetectors based on RGONS. Moreover, the MoS₂@RGONS-based photodetector exhibited good power-dependent photoresponse. Our work indicates that the MoS₂@RGONS is expected to be a promising material in the fields of optoelectronic devices and flexible electronics.

A Low-Temperature Dehydration Carbon-Fixation Strategy for Lignocellulose-Based Hierarchical Porous Carbon for Supercapacitors

Lignocellulose-based hierarchical porous carbon is a very promising electrode material for supercapacitors, but lower volumetric energy density and yield have hindered its practical applications. Herein, a low-temperature dehydration carbon-fixation method using NH₄ Cl as modification reagent was developed to prepare rice husk-based hierarchical porous carbon (RHPC) with high volumetric performance and yield. The RHPC-N electrode exhibited a higher volumetric specific capacitance of 134.4 F cm^-3 than that of the RHPC electrode (98.4 F cm^-3 ) in 1 m Et₄ NBF₄ /propylene carbonate electrolyte. The volumetric energy density (28.8 Wh L^-1 ) of the RHPC-N electrode was 37.1 % higher than that of the RHPC electrode (21.0 Wh L^-1 ), which greatly enhanced the practical application potential of RHPC in supercapacitors. Moreover, the yield of RHPC increased 1.2 times by this method, which greatly improved the production capacity and reduced the cost. This research establishes a simple and highly efficient method to improve the volumetric energy density and the yield of lignocellulose-based hierarchical porous carbon.

Ultra-High Energy Density Hybrid Supercapacitors Using MnO2/Reduced Graphene Oxide Hybrid Nanoscrolls

Manganese oxide (MnO2) is a promising material for supercapacitor applications, with a theoretical ultra-high energy density of 308 Wh/kg. However, such ultra-high energy density has not been achieved experimentally in MnO2-based supercapacitors because of several practical issues, such as low electrical conductivity of MnO2, incomplete utilization of MnO2, and dissolution of MnO2. The present study investigates the potential of MnO2/reduced graphene oxide (rGO) hybrid nanoscroll (GMS) structures as electrode material for overcoming the difficulties and for developing ultra-high-energy storage systems. A hybrid supercapacitor, comprising MnO2/rGO nanoscrolls as anode material and activated carbon (AC) as a cathode, is fabricated. The GMS/AC hybrid supercapacitor exhibited enhanced energy density, superior rate performance, and promising Li storage capability that bridged the energy-density gap between conventional Li-ion batteries (LIBs) and supercapacitors. The fabricated GMS/AC hybrid supercapacitor demonstrates an ultra-high lithium discharge capacity of 2040 mAh/g. The GMS/AC cell delivered a maximum energy density of 105.3 Wh/kg and a corresponding power density of 308.1 W/kg. It also delivered an energy density of 42.77 Wh/kg at a power density as high as 30,800 W/kg. Our GMS/AC cell's energy density values are very high compared with those of other reported values of graphene-based hybrid structures. The GMS structures offer significant potential as an electrode material for energy-storage systems and can also enhance the performance of the other electrode materials for LIBs and hybrid supercapacitors.

3D CNTs/graphene network conductive substrate supported MOFs-derived CoZnNiS nanosheet arrays for ultra-high volumetric/gravimetric energy density hybrid supercapacitor

With the increasing demand for miniaturization and portable energy storage system, it is an urgent necessary that developing high volumetric energy density supercapacitors with small volumes. Herein, an integrated self-supporting CoZnNiS@CNTs/rGO composite film electrode with the thickness of about 6 μm was designed. In the unique structure, porous CNTs/rGO film is served as conductive substrate to support the CoZn-MOFs derived vertically oriented two-dimensional CoZnNiS nanoarrays. The self-supporting film endows the electrode a high volumetric mass density of 1.28 g cm^-3 and superior electron-ion transport channel, which displays a maximum specific capacitance of 1349.2 F g^-1 as well as high volumetric capacity of 1727.0 F cm^-3 at 1 A g^-1. Besides, a porous film of pure carbon materials (carbon spheres integrated graphene) was designed and used as the negative electrode in supercapacitor. When assembled a hybrid supercapacitor based on the above two self-supporting electrodes, the device delivers up an ultra-high volumetric/gravimetric energy density of 65.2 W h L^-1 (60.4 W h kg^-1) at a power density of 1308 W L^-1 (1200 W kg^-1). Moreover, the asymmetric supercapacitor also displays an ultra-long lifetime with 90.6% retention after 10,000 cycles. These outstanding performances make the CoZnNiS@CNTs/rGO electrode could be a promising candidate for next-generation high volumetric/gravimetric energy density supercapacitors, especially in the limited space.

Graphite Felt Modified by Atomic Layer Deposition with TiO2 Nanocoating Exhibits Super-Hydrophilicity, Low Charge-Transform Resistance, and High Electrochemical Activity

Graphite felt (GF) is a multi-functional material and is widely used as electrodes of electrochemical devices for energy and environmental applications. However, due to the inherent hydrophobicity of graphite felt, it must be hydrophilically pretreated to obtain good electrochemical activity. Metal oxides coating is one of the feasible methods to modify the surface of GF, and in order to ensure that the metal oxides have a better conductivity for obtaining higher electrochemical activity, a subsequent H₂ heat-treatment process is usually adopted. In this study, atomic layer deposition (ALD) is used to deposit TiO₂ nanocoating on graphite felt (GF) for surface modification without any H₂ thermal post-treatment. The results show that the ALD-TiO₂-modified GF (ALD-TiO₂/GF) owns excellent hydrophilicity. Moreover, the ALD-TiO₂/GF exhibits excellent electrochemical properties of low equivalent series resistance (R_s), low charge-transfer resistance (R_ct), and high electrochemical activity. It demonstrates that ALD is an applicable technique for modifying the GF surface. In addition, it can be reasonably imagined that not only TiO₂ film can effectively modify the GF surface, but also other metal oxides grown by ALD with nanoscale-thickness can also obtain the same benefits. We anticipate this work to be a starting point for modifying GF surface by using ALD with metal oxides nanocoating.

Generic Strategy to Synthesize High-Tap Density Anode and Cathode Structures with Stratified Graphene Pliable Pockets via Monomeric Polymerization and Evaporation, and Their Utilization to Enable Ultrahigh Performance in Hybrid Energy Storages

Hybrid energy storage systems have shown great promise for many applications; however, achieving high energy and power densities with long cycle stability remains a major challenge. Here, a strategy to synthesize high-tap density anode and cathode structures that yield ultrahigh performance in hybrid energy storage is reported. First, vinyl acetate monomers are polymerized into molecular sizes via chain reactions controlled by the surface free radicals of graphene and metals. Subsequently, molecular-size polymers are thermally evaporated to construct battery-type anode structures with encapsulated tin metals for high-capacity and stratified graphene pliable pockets (GPPs) for fast charge transfer. Similarly, sulfur particles are attached to GPPs via monomeric polymerization, and capacitor-type hollow GPP (H@GPP) cathode structures are produced by evaporating sulfur, where sublimated S particles yield mesopores for rapid anion movement and micropores for high capacity. Moreover, hybrid full-cell devices with high-tap density anodes and cathodes show high gravimetric energy densities of up to 206.9 Wh kg^-1 , exceeding those of capacitors by ≈16-fold, and excellent volumetric energy densities of up to 92.7 Wh L^-1 . Additionally, they attain high power densities of up to 23 678 W kg^-1 , outperforming conventional devices by a factor of ≈100, and long cycle stability over 10 000 cycles.

A novel RGO/N-RGO supercapacitor architecture for a wide voltage window, high energy density and long-life via voltage holding tests

Here, we demonstrated a unique symmetric supercapacitor (SSC) device architecture based on reduced graphene oxide (RGO) and nitrogen-doped RGO (N-RGO) electrodes.

An Ultra-High-Energy Density Supercapacitor; Fabrication Based on Thiol-functionalized Graphene Oxide Scrolls

Present state-of-the-art graphene-based electrodes for supercapacitors remain far from commercial requirements in terms of high energy density. The realization of high energy supercapacitor electrodes remains challenging, because graphene-based electrode materials are synthesized by the chemical modification of graphene. The modified graphene electrodes have lower electrical conductivity than ideal graphene, and limited electrochemically active surface areas due to restacking, which hinders the access of electrolyte ions, resulting in a low energy density. In order to solve the issue of restacking and low electrical conductivity, we introduce thiol-functionalized, nitrogen-doped, reduced graphene oxide scrolls as the electrode materials for an electric double-layer supercapacitor. The fabricated supercapacitor exhibits a very high energy/power density of 206 Wh/kg (59.74 Wh/L)/496 W/kg at a current density of 0.25 A/g, and a high power/energy density of 32 kW/kg (9.8 kW/L)/9.58 Wh/kg at a current density of 50 A/g; it also operates in a voltage range of 0~4 V with excellent cyclic stability of more than 20,000 cycles. By suitably combining the scroll-based electrode and electrolyte material, this study presents a strategy for electrode design for next-generation energy storage devices with high energy density without compromising the power density.

Electrochemical Performance of Iron Oxide Nanoflakes on Carbon Cloth under an External Magnetic Field

In this work, the iron oxide (Fe2O3) nanoflakes on carbon cloth (Fe2O3@CC) were triumphantly prepared and served as the electrode of supercapacitors. By applying an external magnetic field, we first find that the magnetic field could suppress the polarization phenomenon of electrochemical performance. Then, the influences of the mono-/bi-valent cations on the electrochemical properties of the Fe2O3@CC were investigated under a large external magnetic field (1 T) in this work. The chemical valences of the cations in the aqueous electrolytes (LiNO3 and Ca(NO3)2) have almost no influences on the specific capacitance at different scan rates. As one of important parameters to describe the electrochemical properties, the working potential window of the Fe2O3@CC electrode was also investigated in this work. The broad potential window in room-temperature molten salt (LiTFSI + LiBETI (LiN(SO2CF3)2 + LiN(SO2C2F5)2)) has been obtained and reached 1.2 V, which is higher than that of the traditional aqueous electrolyte (~0.9 V).

High-Performance Biscrolled MXene/Carbon Nanotube Yarn Supercapacitors

Yarn-shaped supercapacitors (YSCs) once integrated into fabrics provide promising energy storage solutions to the increasing demand of wearable and portable electronics. In such device format, however, it is a challenge to achieve outstanding electrochemical performance without compromising flexibility. Here, MXene-based YSCs that exhibit both flexibility and superior energy storage performance by employing a biscrolling approach to create flexible yarns from highly delaminated and pseudocapacitive MXene sheets that are trapped within helical yarn corridors are reported. With specific capacitance and energy and power densities values exceeding those reported for any YSCs, this work illustrates that biscrolled MXene yarns can potentially provide the conformal energy solution for powering electronics beyond just the form factor of flexible YSCs.

Rapidly Synthesized, Few-Layered Pseudocapacitive SnS2 Anode for High-Power Sodium Ion Batteries

The abundance of sodium resources has recently motivated the investigation of sodium ion batteries (SIBs) as an alternative to commercial lithium ion batteries. However, the low power and low capacity of conventional sodium anodes hinder their practical realization. Although most research has concentrated on the development of high-capacity sodium anodes, anodes with a combination of high power and high capacity have not been widely realized. Herein, we present a simple microwave irradiation technique for obtaining few-layered, ultrathin two-dimensional SnS₂ over graphene sheets in a few minutes. SnS₂ possesses a large number of active surface sites and exhibits high-capacity, rapid sodium ion storage kinetics induced by quick, nondestructive pseudocapacitance. Enhanced sodium ion storage at a high current density (12 A g^-1), accompanied by high reversibility and high stability, was demonstrated. Additionally, a rationally designed sodium ion full cell coupled with SnS₂//Na₃V₂(PO₄)₃ exhibited exceptional performance with high initial Coulombic efficiency (99%), high capacity, high stability, and a retention of ∼53% of the initial capacity even after the current density was increased by a factor of 140. In addition, a high specific energy of ∼140 Wh kg^-1 and an ultrahigh specific power of ∼8.3 kW kg^-1 (based on the mass of both the anode and cathode) were observed. Because of its outstanding performance and rapid synthesis, few-layered SnS₂ could be a promising candidate for practical realization of high-power SIBs.

Efficient Method of Designing Stable Layered Cathode Material for Sodium Ion Batteries Using Aluminum Doping

Despite their high specific capacity, sodium layered oxides suffer from severe capacity fading when cycled at higher voltages. This key issue must be addressed in order to develop high-performance cathodes for sodium ion batteries (SIBs). Herein, we present a comprehensive study on the influence of Al doping of Mn sites on the structural and electrochemical properties of a P2-Na_0.5Mn_0.5-xAl_xCo_0.5O₂ (x = 0, 0.02, or 0.05) cathode for SIBs. Detailed structural, morphological, and electrochemical investigations were carried out using X-ray diffraction, cyclic voltammetry, and galvanostatic charge-discharge measurements, and some new insights are proposed. Rietveld refinement confirmed that Al doping caused TMO₆ octahedra (TM = transition metal) shrinkage, resulting in wider interlayer spacing. After optimizing the aluminum concentration, the cathode exhibited remarkable electrochemical performance, with better stability and improved rate performance. Electrochemical impedance spectroscopy (EIS) measurements were performed at various states of charge to probe the surface and bulk effects of Al doping. The material presented here exhibits exceptional stability over 100 cycles within a 1.5-4.3 V window and outperforms several other Mn-Co-based cathodes for SIBs. This study presents a facile method for designing structurally stable cathodes for SIBs.

Controlled multimodal hierarchically porous electrode self-assembly of electrochemically exfoliated graphene for fully solid-state flexible supercapacitor

We present here, a concentration dependent freeze-dry technique to obtain 3D graphene architectures with predetermined micron sized macropores and multimodal hierarchical nanopores for electrodes in flexible energy storage devices.