The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Hybrid supercapacitors with their improved performance in energy density without altering their power density have been in trend since recent years. The hybrid supercapacitor delivers higher specific capacitance in comparison to the existing electric double layer capacitor (EDLC) and pseudocapacitors. Generally, the asymmetric behavior of hybrid supercapacitors which is the combination of EDLC and pseudocapacitor acts as an enhancer in its respective capacitance values. This asymmetric approach marks a new beginning towards the much-needed pollution free, long lasting and proficient energy-storing performance. Corresponding to their utilization in hybrid electric vehicles and similar sort of power necessity based devices; the research in developing new advanced storage devices finds an enormous and vast future ahead. The most significant factor for the energy efficient applications demands a considerably higher ratio of surface to the volume by incorporation of new materials. This review article gives an overview of recent advances in the development of hybrid supercapacitors, storage mechanism, criteria of formation, components, different electrode and electrolyte materials, electrochemical profile assessment, design fabrication and their applications.

A review on recent advances in hybrid supercapacitors: Design, fabrication and applications

MnO2-based materials have been intensively investigated for use in pseudocapacitors due to their high theoretical specific capacitance, good chemical and thermal stability, natural abundance, environmental benignity and low cost. In this review, several main factors that affect the electrochemical properties of MnO2-based electrodes are presented. Various strategic design and synthetic methods of MnO2-based electrode materials for enhanced electrochemical performance are highlighted and summarized. Finally, the challenges and future directions toward the development of MnO2-based nanostructured electrode materials for high performance supercapacitors (SCs) are discussed.

MnO2-based nanostructures for high-performance supercapacitors

Spinel ferrite magnetic adsorbents: Alternative future materials for water purification?

Metal–organic frameworks (MOFs) have obtained increasing attention as a kind of novel electrode material for energy storage devices. Yet low capacity in most MOFs largely thwarts their application. In this study, an effective strategy was developed to improve the conductivity of MOFs by partially substituting Ni2+ in the Ni-MOF with Co2+ or Zn2+. The mixed-metal organic frameworks (M-MOFs) showed excellent electrochemical performance, which is attributed not only to the favorable paths for charge transport due to the presence of free pores, but also to the raised electrochemical double-layer capacitance (EDLC) at the enlarged specific surface area of the material. Meanwhile, the cycling stability of the assembled hybrid supercapacitors (M-MOFs//CNTs–COOH) is enhanced due to the alleviation of phase transformation during electrochemical cycling tests. More interestingly, the Co/Ni-MOF//CNTs–COOH also exhibited an excellent energy density (49.5 W h kg−1) and power density (1450 W kg−1) simultaneously. These values demonstrated the better performance of all the MOF materials in supercapacitors at present. In addition to broadening the application of MOFs, our study may open a new avenue for bridging the performance gap between batteries and supercapacitors.

Mixed-metallic MOF based electrode materials for high performance hybrid supercapacitors

ZnCo2O4 nanowire cluster arrays (NWCAs) were directly grown on Ni foam via a facile hydrothermal method. The resulting products were analyzed by using X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The ZnCo2O4 NWCAs on Ni foam were directly used as integrated electrodes for supercapacitors and exhibited a high specific capacitance of 4.05 F cm−2 at 20 mA cm−2 (1620 F g−1 at 8 A g−1) in 3 M KOH aqueous solution, and an excellent cycling ability at various current densities up to 100 mA cm−2 (40 A g−1); 90% of the initial capacitance remained after 6000 cycles. Moreover, the asymmetric supercapacitor had a high energy density of 41.00 W h kg−1 at a power density of 384 W kg−1 and 16.63 W h kg−1 at a high power density of 2561 W kg−1. Such excellent rate properties and superior cycling life suggest that ZnCo2O4 NWCAs can not only be applied in high energy density fields, but also used in high power density applications.

Facile synthesis of ZnCo2O4 nanowire cluster arrays on Ni foam for high-performance asymmetric supercapacitors

A novel hybrid structure of core/shell NiMoO4@MnO2 was directly synthesized on carbon cloth by a facile two-step hydrothermal route for supercapacitors. The smart combination of NiMoO4 and MnO2 shows a synergistic effect for capacitors with greatly enhanced performance. The NiMoO4@MnO2 electrode yields high-capacitance performance with a high areal capacitance of 3.90 F cm−2 at a charge and discharge current density of 8 mA cm−2 and 3.22 F cm−2 at 24 mA cm−2 with a desirable cycling ability (90.5% of the initial specific capacitance remains after 4000 cycles). Such core/shell hybrid nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and excellent long-term cycling stability, which could be promising pseudocapacitive electrode materials for high-performance supercapacitors.

NiMoO4 nanowire @ MnO2 nanoflake core/shell hybrid structure aligned on carbon cloth for high-performance supercapacitors

Hierarchical CoFe2O4/MnO2/C nanotubes were prepared by electrospinning and a subsequent hydrothermal method. Their electrochemical properties were investigated for use as anode materials for lithium-ion batteries. The CoFe2O4/MnO2/C composite displayed a stable cycling performance with a reversible capacity of 713.6 mA h g−1 after 250 discharge–charge cycles at a current density of 100 mA g−1, much higher than that of CoFe2O4/C nanofibers, which displayed a capacity of 200 mA h g−1 after 90 cycles. The improved electrochemical performance could be ascribed to the stable hierarchical structure of the nanocomposite, the existence of carbon, the high active surface area, and the high theoretical capacity of MnO2. Given their enhanced electrochemical performance, the CoFe2O4/MnO2/C heterostructure composite can be considered as a promising anode for lithium-ion batteries.

Weiji Ren

Papers

Facile synthesis of ZnCo2O4 nanowire cluster arrays on Ni foam for high-performance asymmetric supercapacitors

NiMoO4 nanowire @ MnO2 nanoflake core/shell hybrid structure aligned on carbon cloth for high-performance supercapacitors

Enhanced electrochemical performance of hierarchical CoFe2O4/MnO2/C nanotubes as anode materials for lithium-ion batteries

Green and rapid synthesis of 3D Fe2(MoO4)3 by microwave irradiation to detect H2S gas