Electrode Materials, Structural Design, and Storage Mechanisms in Hybrid Supercapacitors.

Currently, energy storage systems are of great importance in daily life due to our dependence on portable electronic devices and hybrid electric vehicles. Among these energy storage systems, hybrid supercapacitor devices, constructed from a battery-type positive electrode and a capacitor-type negative electrode, have attracted widespread interest due to their potential applications. In general, they have a high energy density, a long cycling life, high safety, and environmental friendliness. This review first addresses the recent developments in state-of-the-art electrode materials, the structural design of electrodes, and the optimization of electrode performance. Then we summarize the possible classification of hybrid supercapacitor devices, and their potential applications. Finally, the fundamental theoretical aspects, charge-storage mechanism, and future developing trends are discussed. This review is intended to provide future research directions for the next generation of high-performance energy storage devices.

Robust VS4@rGO nanocomposite as a high-capacity and long-life cathode material for aqueous zinc-ion batteries.

Although vanadium (V)-based sulfides have been investigated as cathodes for aqueous zinc-ion batteries (ZIBs), the performance improvement and the intrinsic zinc-ion (Zn2+) storage mechanism revelation is still challenging. Here, VS4@rGO composite with optimized morphology is designed and exhibits ultrahigh specific capacity (450 mA h g-1 at 0.5 A g-1) and high-rate capability (313.8 mA h g-1 at 10 A g-1) when applied as cathode material for aqueous ZIBs. Furthermore, the VS4@rGO cathode presents long-life cycling stability with capacity retention of ∼82% after 3500 cycles at 10 A g-1. The structural evolution, redox, and degradation mechanisms of VS4 during (dis)charge processes are further probed by in situ XRD/Raman techniques and TEM analysis. Our results indicate that the main energy storage mechanism is derived from the intercalation/deintercalation reactions in the open channels of VS4. Notably, an irreversible phase transition of VS4 into Zn3(OH)2V2O7·2H2O (ZVO) during the charging process and the further transition from ZVO to ZnV3O8 during long-term cycles are also observed, which might be the main reason leading to the capacity degradation of VS4@rGO. Our study further improves the electrochemical performance of VS4 in aqueous ZIBs through morphology design and provides new insights into the energy storage and performance degradation mechanisms of Zn2+ storage in VS4, and thus may endow the large-scale application of V-based sulfides for energy storage systems.

Rational construction of K0.5V2O5 nanobelts/CNTs flexible cathode for multi-functional potassium-ion batteries.

Potassium-ion battery (KIB) is one of the emerging electrochemical energy storage technologies due to the abundance, low cost, and low redox potential of K. One of the most promising cathodes of KIBs is a layered vanadium-based compound, but it often suffers from fast capacity decay during repeated cycling. Herein, a K0.5V2O5/CNTs hybrid film composed of K0.5V2O5 nanobelt and carbon nanotube (CNT) network was synthesized by an electrostatic self-assembly and vacuum filtration process, and further used as the cathode in KIBs. The K0.5V2O5/CNTs cathode possessed a flexible and interconnected network structure, which not only offered fast kinetics for electron transfer and ion transportation, but also provided an elastic medium to buffer the large volume change of the K0.5V2O5 nanobelts during cycling. As a cathode for KIBs, the K0.5V2O5/CNTs electrode showed a reversible discharge capacity of ∼90 mA h g-1 at 50 mA g-1 and exhibited good cycling stability (88.8% capacity retention for 100 cycles at 50 mA g-1, 82.2% capacity retention for 300 cycles at 500 mA g-1) and excellent rate performance of ∼62 mA h g-1 at 500 mA g-1. K-Ion full battery testing further confirmed its good electrochemical performance by presenting a high reversible discharge capacity (68 mA h g-1 at 50 mA g-1) and long-term retention (>80% after 80 cycles). Interestingly, a cable-shaped KIB with the flexible K0.5V2O5/CNTs film as the cathode electrode was assembled and showed its further application potential as a power source for wearable electronics.

A Porous and Conductive Graphite Nanonetwork Forming on the Surface of KCu7S4 for Energy Storage.

A flexible all-solid-state supercapacitor is fabricated by building a layer of porous and conductive nanonetwork on the surface of KCu7S4 nanowires supported on the carbon fiber fabric, where the porous and conductive nanonetwork is assembled by graphite nanoparticles. This porous graphite layer plays a key role in providing ion diffusion channels to access the KCu7S4 through the pores for electrochemical reactions and forming electron transport pathways from the graphite network to the electronic collector of the carbon fiber fabric. This flexible supercapacitor exhibits excellent electrochemical performance with high specific capacitance of 408 F g-1 at a current density of 0.5 A g-1 and high energy density of 36 Wh kg-1 at a power density of 201 W kg-1. Moreover, it is cost-effective, easy to scale up and environmentally friendly with high flexibility. Our investigation demonstrates that such a porous and conductive nanonetwork could be used to improve the charge storage efficiency for a wide range of electrode materials.

Urchin-Like Ni2/3Co1/3(CO3)1/2(OH)·0.11H2O for High-Performance Supercapacitors.

Here, we report our finding in the fabrication of novel porous urchin-like Ni2/3Co1/3(CO3)1/2(OH)·0. 11H2O (denoted as NC) nanomaterial composed of numerous nanoneedles through an one-step hydrothermal method, which deliveres a high specific capacity of 318 C g-1 at a current density of 1 A g-1. Moreover, an architectural composite electrode consisting of the porous NC nanoneedles wrapped by reduced graphene oxide (rGO) nanosheets exhibits large specific capacity (431 C g-1 at 1 A g-1), high rate capability and long cycling life (94% capacity retention after 5,000 cycles at 20 A g-1). The presence of rGO in the composite electrode greatly improves the electronic conductivity, providing efficient current collection for fast energy storage.

"One-for-All" Strategy in Fast Energy Storage: Production of Pillared MOF Nanorod-Templated Positive/Negative Electrodes for the Application of High-Performance Hybrid Supercapacitor.

Currently, metal-organic frameworks (MOFs) are intensively studied as active materials for electrochemical energy storage applications due to their tunable structure and exceptional porosities. Among them, water stable pillared MOFs with dual ligands have been reported to exhibit high supercapacitor (SC) performance. Herein, the "One-for-All" strategy is applied to synthesize both positive and negative electrodes of a hybrid SC (HSC) from a single pillared MOF. Specifically, Ni-DMOF-TM ([Ni(TMBDC)(DABCO)0.5 ], TMBDC: 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid, DABCO: 1,4-diazabicyclo[2.2.2]-octane) nanorods are directly grown on carbon fiber paper (CFP) (denoted as CFP@TM-nanorods) with the help of triethylamine and function as the positive electrode of HSC under alkaline electrolyte. Meanwhile, calcinated N-doped hierarchical porous carbon nanorods (CFP@TM-NPCs) are produced and utilized as the negative counter-electrode from a one-step heat treatment of CFP@TM-nanorods. After assembling these two electrodes together to make a hybrid device, the TM-nanorods//TM-NPCs exhibit a wide voltage window of 1.5 V with a high sloping discharge plateau between 1-1.2 V, indicating its great potential for practical applications. This as-described "One-for-All" strategy is widely applicable and highly reproducible in producing MOF-based electrode materials for HSC applications, which shortens the gap between experimental synthesis and practical application of MOFs in fast energy storage.

High-Performance Energy Storage and Conversion Materials Derived from a Single Metal-Organic Framework/Graphene Aerogel Composite.

Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM)2 (mIM = 2-methylimidazole). The presence of cobalt oxide (CoOx) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoOx/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for the oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co was removed, the resulting nitrogen-rich porous carbon-graphene composite electrode (denoted as C/NG-A) displayed exceptional capacitance and rate capability in a supercapacitor. Further, this method is readily applicable to creation of functional metal oxide hollow nanoparticles on the surface of other carbon materials such as graphene and carbon nanotubes, providing a good opportunity to tune their physical or chemical activities.

Folded Elastic Strip-Based Triboelectric Nanogenerator for Harvesting Human Motion Energy for Multiple Applications.

A folded elastic strip-based triboelectric nanogenerator (FS-TENG) made from two folded double-layer elastic strips of Al/PET and PTFE/PET can achieve multiple functions by low frequency mechanical motion. A single FS-TENG with strip width of 3 cm and length of 27 cm can generate a maximum output current, open-circuit voltage, and peak power of 55 µA, 840 V, and 7.33 mW at deformation frequency of 4 Hz with amplitude of 2.5 cm, respectively. This FS-TENG can work as a weight sensor due to its good elasticity. An integrated generator assembled by four FS-TENGs (IFS-TENG) can harvest the energy of human motion like flapping hands and walking steps. In addition, the IFS-TENG combined with electromagnetically induced electricity can achieve a completely self-driven doorbell with flashing lights. Moreover, a box-like generator integrated by four IFS-TENGs inside can work in horizontal or random motion modes and can be improved to harvest energy in all directions. This work promotes the research of completely self-driven systems and energy harvesting of human motion for applications in our daily life.

Faradic redox active material of Cu7S4 nanowires with a high conductance for flexible solid state supercapacitors.

The exploration of high Faradic redox active materials with the advantages of low cost and low toxicity has been attracting great attention for producing high energy storage supercapacitors. Here, the high Faradic redox active material of Cu7S4-NWs coated on a carbon fiber fabric (CFF) is directly used as a binder-free electrode for a high performance flexible solid state supercapacitor. The Cu7S4-NW-CFF supercapacitor exhibits excellent electrochemical performance such as a high specific capacitance of 400 F g(-1) at the scan rate of 10 mV s(-1) and a high energy density of 35 Wh kg(-1) at a power density of 200 W kg(-1), with the advantages of a light weight, high flexibility and long term cycling stability by retaining 95% after 5000 charge-discharge cycles at a constant current of 10 mA. The high Faradic redox activity and high conductance behavior of the Cu7S4-NWs result in a high pseudocapacitive performance with a relatively high specific energy and specific power. Such a new type of pseudocapacitive material of Cu7S4-NWs with its low cost is very promising for actual application in supercapacitors.

Nanorod-aggregated flower-like CuO grown on a carbon fiber fabric for a super high sensitive non-enzymatic glucose sensor.

A novel and exceptionally sensitive glucose biosensor based on nanorod-aggregated flower-like CuO grown on a carbon fiber fabric (CFF) is developed for glucose detection, which is prepared by a simple, fast and green hydrothermal method. The electron transfer resistance of the CuO/CFF electrode on the interface between the electrode and the electrolyte is as low as 12.79 Ω as evaluated by electrochemical impedance spectroscopy. A cyclic voltammetry study reveals that the CuO/CFF electrode displays an excellent electrocatalytic activity toward the direct oxidation of glucose. Besides, chronoamperometry demonstrates a high sensitivity of 6476.0 µA mM-1 cm-2 at an applied potential of 0.45 V (vs. Ag/AgCl), with a fast response time and a low detection limit of only 1.3 s and ∼0.27 µM, respectively. In addition, the glucose sensor has high reproducibility with a relative standard deviation (R.S.D.) of 1.53% over eight identically fabricated electrodes and long-term stability with a minimal sensitivity loss of ∼9.9% over a period of one month as well as excellent anti-interference ability. Importantly, the CuO-CFF composite has such good flexible characteristics and can be fabricated into flexible electrodes for application in various complicated circumstances. This work presents a new strategy to achieve highly sensitive glucose sensors with flexibility by growing glucose electroactive nanostructure materials directly on multichannels and highly conductive carbon fiber fabrics.