Novel mesoporous electrode materials for symmetric, asymmetric and hybrid supercapacitors

Binder-free all-carbon composite supercapacitors

Carbon-based electrode materials have widely been used in supercapacitors. Unfortunately, the fabrication of the supercapacitors includes a polymeric binding material that leads to an undesirable addition of weight along with an increased charge transfer resistance. Herein, binder-free and lightweight electrodes were fabricated using powder processing of carbon nanofibers (CNFs) and graphene nanoplatelets (GNPs) resulting in a hybrid all-carbon composite material. The structural, morphological, and electrochemical properties of the composite electrodes were studied at different concentrations of GNPs. The specific capacitance (Cs) of the CNFs/GNPs composite was improved by increasing the concentration of GNPs. A maximum Cs of around 120 F g-1was achieved at 90 wt% GNPs which is around 5-fold higher in value than the pristine CNFs in 1 M potassium hydroxides (KOH), which then further increased to 189 F g-1in 6 M KOH electrolyte. The energy density of around 20 Wh kg-1with the corresponding power density of 340 W kg-1was achieved in the supercapacitor containing 90 wt% GNPs. The enhanced electrochemical performance of the composite is related to the presence of a synergistic effect and the CNFs establishing conductive/percolating networks. Such binder-free all-carbon electrodes can be a potential candidate for next-generation energy applications.

Vanadate-based Fe-MOFs as promising negative electrode for hybrid supercapacitor device

In the supercapacitor field, negative electrodes are mainly concentrated in carbon-based materials, such as activated carbon, carbon nanotubes, graphene, and so forth. However, materials based on metal-organic frameworks (MOFs) as negative active components are relatively rare. Herein, a series of composite materials based on graphene oxide (GO) and vanadate-based Fe-organic frameworks have been prepared by hydrothermal method namely GO/Fe-VO4-BIPY. The deposition amount of polyoxometalate-based metal-organic frameworks (POMOFs) on the surface of graphene is adjusted by changing the content of POMOFs. Through the deposition, it can effectively reduce the accumulation between graphene, and increase the dispersion of POMOFs. As a result, the charge storage performance of the as-obtained materials is greatly improved. Among these materials, GO/Fe-VO4-BIPY-1 has the most prominent performance, with a specific capacitance of 190 F g-1at 0.5 A g-1, which is attributed to the excellent synergistic effect between the Faraday chemical reaction and electric double-layer capacitance. In comparison with pristine Fe-VO4-BIPY, GO/Fe-VO4-BIPY-1 delivers more excellent surface area and therefore exhibits abundant redox reaction sites, achieving better electrochemical performance the best. After assembly with the positive Ni(OH)2electrode, the maximum energy density of 46.84 W h kg-1at a power density of 850 W kg-1is achieved.

Concave Ni(OH)2 Nanocube Synthesis and Its Application in High-Performance Hybrid Capacitors

The controlled synthesis of hollow structure transition metal compounds has long been a very interesting and significant research topic in the energy storage and conversion fields. Herein, an ultrasound-assisted chemical etching strategy is proposed for fabricating concave Ni(OH)2 nanocubes. The morphology and composition evolution of the concave Ni(OH)2 nanocubes suggest a possible formation mechanism. The as-synthesized Ni(OH)2 nanostructures used as supercapacitor electrode materials exhibit high specific capacitance (1624 F g-1 at 2 A g-1) and excellent cycling stability (77% retention after 4000 cycles) due to their large specific surface area and open pathway. In addition, the corresponding hybrid capacitor (Ni(OH)2//graphene) demonstrates high energy density (42.9 Wh kg-1 at a power density of 800 W kg-1) and long cycle life (78% retention after 4000 cycles at 5 A g-1). This work offers a simple and economic approach for obtaining concave Ni(OH)2 nanocubes for energy storage and conversion.

Intrinsically Conducting Polymer Composites as Active Masses in Supercapacitors

Intrinsically conducting polymers ICPs can be combined with further electrochemically active materials into composites for use as active masses in supercapacitor electrodes. Typical examples are inspected with particular attention to the various roles played by the constituents of the composites and to conceivable synergistic effects. Stability of composite electrode materials, as an essential property for practical application, is addressed, taking into account the observed causes and effects of materials degradation.

Multimetallic transition metal phosphide nanostructures for supercapacitors and electrochemical water splitting

Transition metal phosphides (TMPs) have recently emerged as an important class of functional materials and been demonstrated to be outstanding supercapacitor electrode materials and catalysts for electrochemical water splitting. While extensive investigations have been devoted to monometallic TMPs, multimetallic TMPs have lately proved to show enhanced electrochemical performance compared to their monometallic counterparts, thanks to the synergistic effect between different transition metal species. This topical review summarizes recent advance in the synthesis of new multimetallic TMP nanostructures, with particular focus on their applications in supercapacitors and electrochemical water splitting. Both experimental reports and theoretical understanding of the synergy between transition metal species are comprehensively reviewed, and perspectives of future research on TMP-based materials for these specific applications are outlined.

Core-shell NiSe/Ni(OH)2with NiSe nanorods and Ni(OH)2nanosheets as battery-type electrode for hybrid supercapacitors

Novel core-shell nanostructure electrodes benefit from the excellent properties of their constituent materials, as well as the synergy between them. However, it is challenging to fabricate such structures efficiently. In this study, NiSe nanorods were fabricated using Ni foam as the conductive substrate and reactant via a one-step hydrothermal process, and Ni(OH)₂nanosheets were coated on the surface of the nanorods via one-step electrodeposition. The effect of the structure and morphology on the properties of the material was explored using scanning electron microscopy, x-ray diffraction, and electrochemical technology. The obtained core-shell NiSe/Ni(OH)₂exhibited an areal capacity of 1.89 mAh cm^-2at a current density of 5 mA cm^-2. The assembled NiSe/Ni(OH)₂//AC hybrid supercapacitor exhibited excellent energy and power densities, indicating that NiSe/Ni(OH)₂has great potential for use as a battery-type electrode in energy storage systems.

A robust magnesiothermic reduction combined self-activation strategy towards highly-curved carbon nanosheets for advanced zinc-ion hybrid supercapacitors applications

Aqueous zinc-ion hybrid supercapacitors are considered to be a newly emerging electrochemical energy storage devices. However, the exploration and design of advanced cathode materials remain a huge challenge. Herein, we developed a versatile one-step magnesiothermic reduction and self-activation process for the synthesis of highly-curved carbon nanosheets (HCCNs) with hierarchical pore structures. In this process, low-molecular weight organic potassium salts (e.g. potassium bitartrate, potassium acetate, potassium oxalate, potassium formate, potassium oleate, potassium sorbate), which usually used as the chemical activating reagents, serve as the carbon source whereas the Mg power acts as the reducing agent. The resulting HCCNs possess hierarchical porosity and unique HCCN geometry which can afford abundant active sites for charge accumulation as well as the highly efficient ions diffusion kinetics. Because of the high ratio of surface-controlled capacitive contribution and high ions diffusion coefficient, the optimized sample can exhibit excellent charge storage performance with an impressive reversible capacity (200.2 mAh g^-1), excellent rate capability, and good cyclic stability. The excellent capacitive behaviors combined with the feasible synthetic procedure make the present synthetic protocol a promising choice towards well-designed nanocarbons for electrochemical energy storage applications.

Hollow Biphase Cobalt Nickel Perselenide Spheres Derived from Metal Glycerol Alkoxides for High-Performance Hybrid Supercapacitors

Transition-metal selenides (TMSe) incorporate reversible multielectron Faradaic reactions that can deliver high specific capacitance. Unfortunately, they usually exhibit actual capacitance lower than their theoretical value and suffer from sluggish kinetics, which do not satisfy the demands of hybrid supercapacitors (HSCs), due to poor electron-transmission capability and inferior ion-transport rate. Herein, a kind of hollow biphase and bimetal cobalt nickel perselenide composed of metastable marcasite-type CoSe₂ (m-CoSe₂) and stable pyrite-type NiCoSe₄ (p-NiCoSe₄) is synthesized with metal glycerol alkoxide as precursors by regulating the Ni/Co ratios. This unique hollow biphase structure and bimetallic synergistic effect serves to boost electron-transmission capability and accelerate the ion/electron transfer rate, delivering an excellent specific capacitance of 1008 F g^-1 at 0.5 A g^-1 and a high discharge rate capability of 859 F g^-1 at 20 A g^-1. The capacitance remains around 80% of the initial capacitance after 5000 cycles. Consequently, a HSC based on the cobalt nickel perselenide cathode and a hierarchical porous carbon anode reveals a maximum energy density of 34.8 W h kg^-1 and a maximum power density of 7272 W kg^-1. This polymorphic bimetallic phase engineering provides an advanced and effective guidance for TMSe with high electrochemical properties.

MoO3 thin layers on NiCo2S4 substrate for efficient electrochemical charge storage

Ternary oxides/sulfides have long been investigated as promising electrode materials for charge storage applications. However, it is important to rationally design nanostructured hybrid composites for superior charge storage performance as electrodes in devices. In this work, MoO₃@NiCo₂S₄ hybrid composites materials are synthesized by the hydrothermal method followed by annealing at different temperatures. The charge storage properties of these materials are tested by cyclic voltammetry, galvanostatic charge-discharge curves and electrochemical impedance spectroscopy. It is found that the structure of the hybrid composite material not only assists electron and charge transportation but also precisely control the volume expansion during redox reactions, contributing to superior electrochemical behavior. Among all the electrodes, the electrode fabricated with MoO₃@NiCo₂S₄ composite material annealed at 400 °C (MoO₃@NiCo₂S₄-400) is the best for charge storage applications. At 400 °C, MoO₃ spreads as a thin layer of surface polymeric molybdates on NiCo₂S₄ as seen in the XRD pattern. Significantly, it delivers the highest capacitance of 1622 F g^-1 at 1 A g^-1 in 2 M aqueous KOH electrolyte compared to other hybrid composite electrodes, NiCo₂S₄ (962 F g^-1), MoO₃@NiCo₂S₄-500 (1412 F g^-1) and MoO₃@NiCo₂S₄-600 (970 F g^-1), under the same measurement conditions. Furthermore, the MoO₃@NiCo₂S₄-400 hybrid electrode shows better cyclic stability with 93% capacitance retention after 3000 charge-discharge cycles at 8 A g^-1. The synergistic effect of two components and annealing temperature plays important role in enhancing the charge storage performance. This work shows the importance of the synthesis temperature on the functional character of ternary sulfide/oxide composite materials for charge storage applications.

Flexible supercapacitor electrodes using metal-organic frameworks

Advancements in the field of flexible and wearable devices require flexible energy storage devices to cater their power demands. Metal-ion batteries (such as lithium-ion batteries, sodium-ion batteries, etc.) and electrochemical capacitors (also called supercapacitors or ultracapacitors) have achieved great interest in the recent past due to their superior energy storage characteristics like high power density and long cycle life. A major bottleneck of using metal-ion batteries in wearable devices is their lack of flexibility. Low power density, toxicity and flammability due to organic electrolytes inhibit them from safe on-body device applications. On the other hand, supercapacitors can be made with aqueous electrolytes, making them a safer alternative for wearable applications. Metal-organic frameworks (MOFs) are novel candidates as electrode materials due to their salient features such as large surface area, three-dimensional porous architecture, permeability to foreign entities, structural tailorability, etc. Though pristine MOFs suffer from poor intrinsic conductivity, this can be rectified by preparing composites with other electronically conducting materials. MOF-based electrodes are highly promising for flexible and wearable supercapacitors since they exhibit good energy and power densities. This review focuses on the new developments in the field of MOF-based composite electrodes for developing flexible supercapacitors.

High-performance asymmetric supercapacitor based hierarchical NiCo2O4@ carbon nanofibers//Activated multichannel carbon nanofibers

Synthesis of rational nanostructure design of hybrid materials including uniformly growing, stable and highly porous structures have received a great deal of attention for many energy storage applications. In this study, the positive electrode of the uniform distribution of NiCo2O4 nanorods anchored on carbon nanofibers has been successfully prepared by in-situ growth under the hydrothermal process. Whereas, the activated multichannel carbon nanofibers (AMCNFs) have been fabricated via electrospinning followed by alkaline activation as the negative electrode. The crystal phase, morphological structure for the proposed electrode materials were characterized by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Moreover, the electrochemical behaviors were investigated using cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements. Compared to the neat CNFs and the pristine NiCo2O4, the NiCo2O4@CNFs hybrid electrodes showed better electrochemical performance and achieved a high specific capacitance up to 649 F g-1 at a current density of 3 A g-1. The optimized NiCo2O4@CNFs//AMCNFs asymmetric device achieved a high energy density of 38.5 Wh kg-1 with a power density of 1.6 kW kg-1 and possessed excellent recyclability with 93.1% capacitance retention over 6000 charging/discharging cycles. Overall, the proposed study introduces a facile strategy for the robust design of hybrid structured as effective nanomaterials based electrode for high-performance electrochemical supercapacitors.

Improving the Supercapacitor Performance by Dispersing SiO2 Microspheres in Electrodes

This paper describes a simple, reproducible, and scalable procedure for the preparation of a SiO₂-containing supercapacitor with high cycle stability. A carbon mesoporous material (CMM) with a high specific surface area, CMK-3, was adopted as an electric double-layer capacitor (EDLC) active material for the preparation of electrodes for the supercapacitor. The optimized SiO₂ content decreased as the microsphere diameter decreased, and the optimal specific capacitance was obtained with 6 wt % SiO₂ microspheres (100 nm size). The capacitance improved from 133 to 298 F/g. The corresponding capacitance retention rate after 1000 cycles increased from 68.04 to 91.53%. In addition, the energy density increased from 21.05 to 26.25 Wh/kg with a current density of 1 A/g. Finally, similar results based on active carbon, CeO₂/CMK-3, and graphene/CNT/MnO_v composite electrodes demonstrated that the proposed method exhibits wide compatibility with diverse electrode materials.

Supercapatteries as High-Performance Electrochemical Energy Storage Devices

Abstract

The development of novel electrochemical energy storage (EES) technologies to enhance the performance of EES devices in terms of energy capacity, power capability and cycling life is urgently needed. To address this need, supercapatteries are being developed as innovative hybrid EES devices that can combine the merits of rechargeable batteries with the merits of supercapacitors into one device. Based on these developments, this review will present various aspects of supercapatteries ranging from charge storage mechanisms to material selection including electrode and electrolyte materials. In addition, strategies to pair different types of electrode materials will be discussed and proposed, including the bipolar stacking of multiple supercapattery cells internally connected in series to enhance the energy density of stacks by reducing the number of bipolar plates. Furthermore, challenges for this stack design will also be discussed together with recent progress on bipolar plates.

Graphic Abstract

Supercapattery is an innovated hybrid electrochemical energy storage (EES) device that combines the merit of rechargeable battery and supercapacitor characteristics into one device. This article reviews supercapatteries from the charge storage mechanisms to the selection of materials including the materials of electrodes and electrolytes. Strategies for pairing different kinds of electrode materials and device engineering are discussed.

Electronegativity principles in metal oxides based supercapacitors

To meet growing demands for energy consumptions in modern society, it is necessary to develop different energy sources. Renewable energy such as wind and solar sources are intermittent, therefore, energy storage devices become more and more important to store energy for use when no wind or no light. Supercapacitors play a key role in energy storage, mainly due to their high power density and long cycling life. However, supercapacitors are facing the obstacle of low energy density, one of the most intensive approaches is to rationally design new electrode materials. In this review, we focus on metal oxides-based materials and present an electronegativity criterion for the design and appropriate selection of new electrode chemical compositions. Metal elements with proper electronegativity scale have the potential to transfer electron for energy storage. Suitable positive and negative electrodes matching can enhance many properties of supercapacitors, which may overcome many related obstacles. Furthermore, electronegativity scale may also help people to find novel metal oxides based supercapacitors.

Nitridation Temperature Effect on Carbon Vanadium Oxynitrides for a Symmetric Supercapacitor

In this work, porous carbon-vanadium oxynitride (C-V₂NO) nanostructures were obtained at different nitridation temperature of 700, 800 and 900 °C using a thermal decomposition process. The X-ray diffraction (XRD) pattern of all the nanomaterials showed a C-V₂NO single-phase cubic structure. The C-V₂NO obtained at 700 °C had a low surface area (91.6 m² g⁻¹), a moderate degree of graphitization, and a broader pore size distribution. The C-V₂NO obtained at 800 °C displayed an interconnected network with higher surface area (121.6 m² g⁻¹) and a narrower pore size distribution. In contrast, at 900 °C, the C-V₂NO displayed a disintegrated network and a decrease in the surface area (113 m² g⁻¹). All the synthesized C-V₂NO yielded mesoporous oxynitride nanostructures which were evaluated in three-electrode configuration using 6 M KOH aqueous electrolyte as a function of temperature. The C-V₂NO@800 °C electrode gave the highest electrochemical performance as compared to its counterparts due to its superior properties. These results indicate that the nitridation temperature not only influences the morphology, structure and surface area of the C-V₂NO but also their electrochemical performance. Additionally, a symmetric device fabricated from the C-V₂NO@800 °C displayed specific energy and power of 38 W h kg⁻¹ and 764 W kg⁻¹, respectively, at 1 A g⁻¹ in a wide operating voltage of 1.8 V. In terms of stability, it achieved 84.7% as capacity retention up to 10,000 cycles which was confirmed through the floating/aging measurement for up to 100 h at 10 A g⁻¹. This symmetric capacitor is promising for practical applications due to the rapid and easy preparation of the carbon-vanadium oxynitride materials.

Electrospun poly(acrylonitrile-co-itaconic acid) as a porous carbon precursor for high performance supercapacitor: study of the porosity induced by in situ porogen activity of itaconic acid

An acrylonitrile based copolymer, poly(acrylonitrile-co-itaconic acid), P(AN-co-IA) was synthesized with different amounts of itaconic acid (IA) to study in situ porogen activity of IA to produce porous carbon nanofibers (CNFs) without any subsequent physical or chemical activation. The concept developed here avoids unnecessary and complex extra activation steps when fabricating CNFs which ultimately lead to lower char yields and uncontrollable pore sizes. The ability of COOH in P(AN-co-IA) to act as an in situ porogen by releasing CO₂ during carbonization was verified by simultaneous thermogravimetric analysis-mass spectrometry compared to polyacrylonitrile (PAN). The specific surface area of PAN CNFs (27 m² g^-1) dramatically increases to 1427 m² g^-1 upon addition of ∼8 wt% IA without any ex situ activation. Furthermore, we confirmed that the porosity could be tuned by changing the IA content. The best electrochemical performance was obtained from the copolymer containing ∼8 wt% of IA, which gives a maximum specific capacitance of ∼93 F g^-1 at a scan rate of 10 mV s^-1 and energy density of ∼46 Wh kg^-1 at 1 A g^-1 without any subsequent physical or chemical activation.

Enhanced capacitive properties of all-metal-oxide-nanoparticle-based asymmetric supercapacitors

The major problem of transition metal oxide (TMO)-based supercapacitors is their low specific energy (E_sp) due to the poor electrical conductivity of the TMO electrodes and narrow operating voltage window. To solve these limitations simultaneously, we propose asymmetric supercapacitors (ASCs) consisting of two composite TMO electrodes working in different potential ranges. Titanium dioxide (TiO₂) nanoparticle (NP)-incorporated iron oxide (Fe₂O₃) and manganese oxide (MnO₂) NPs were used as electrode materials covering the negative and positive potential window, respectively. The specific capacitance (C_sp) of this asymmetric TiO₂–Fe₂O₃‖TiO₂–MnO₂ supercapacitor is comparable to that of the symmetric TiO₂–MnO₂‖TiO₂–MnO₂ supercapacitor. However, the ASC can operate over a doubly extended voltage range, which resulted in a significant enhancement in the specific energy of the device. The E_sp value of the ASC at a specific power of 1000 W kg⁻¹ is 48.6 W h kg⁻¹, which is 34.1 and 8.1 times, respectively, larger than that of the two symmetric devices.

This study reports on an asymmetric supercapacitor consisting of two composite transition metal oxide (TiO₂–Fe₂O₃ and TiO₂–MnO₂) nanoparticles-based electrodes working in different potential ranges.

Lithium ferrite (α-LiFe5O8) nanorod based battery-type asymmetric supercapacitor with NiO nanoflakes as the counter electrode

The fabricated battery-type NiO//α-LiFe₅O₈ cell could deliver a specific energy of 30 W h Kg⁻¹ at a specific power of 621 W kg⁻¹ with 90.5% capacity retention at the end of 5000 GCD cycles.