In this contribution, we report a facile preparation method of nickel cobalt oxide–reduced graphite oxide (NiCo2O4–rGO) composite material. A fast Faradic process has been realized by sodium dodecyl sulfate (SDS)-induced ultrasmall NiCo2O4 nanocrystals on rGO. As a result, this composite material gives a high specific capacitance of 1222 F g−1 at 0.5 A g−1 and 768 F g−1 at 40 A g−1, showing an outstanding rate capability. An asymmetric supercapacitor device with high energy and power densities has been successfully assembled based on NiCo2O4–rGO composite material and activated carbon. The optimized device shows a high energy density of 23.32 Wh kg−1 at a power density of 324.9 W kg−1. In addition, this asymmetric device shows good stability towards multistage current charge–discharge cycles.

Dodecyl Sulfate Induced Fast Faradic Process in Nickel Cobalt Oxide/Reduced Graphite Oxide Composite Material and Its Application for Asymmetric Supercapacitor Device

Carbon-containing NiCo2S4 hollow-nanoflake structures were fabricated by a one-step solvothermal method using CS2 as a single source for sulfidation and carbonization. The reaction mechanism for the hollow structure with carbon residues was explored based on the formation of a bis(dithiocarbamate)–metal complex and the Kirkendall effect during solvothermal synthesis. The NiCo2S4 nanoflake electrode exhibited a high specific capacitance of 1722 F g−1 (specific capacity 688.8 C g−1) at a current density of 1 A g−1 and an excellent cycling stability (capacity retention of 98.8% after 10 000 cycles). The as-fabricated asymmetric supercapacitor based on NiCo2S4 nanoflakes and activated carbon electrodes revealed a high energy density of 38.3 W h kg−1 and a high power density of 8.0 kW kg−1 with a capacitance retention of 91.5% and a coulombic efficiency of 95.6% after 5000 cycles, highlighting its great potential for practical supercapacitor applications.

One-step synthesis of hollow C-NiCo2S4 nanostructures for high-performance supercapacitor electrodes.

Supercapacitors or ultracapacitors are considered as one of the potential candidates in the domain of energy storage devices for the forthcoming generations. These devices have earned their significance in numerous applications, viz., to power hybrid electric/electric vehicles and other power and electronic systems which require electrical energy for their operation. Supercapacitors are the most versatile devices which are most widely used for delivery of electrical energy in short time and in arenas which demand long shelf life. Therefore, the development of supercapacitors has huge market requirements, and long-term progress is needed for their successful advancement and commercialization. Meanwhile, supercapacitors are also facing challenges such as technical problems, establishing electrical parameter models, consistency testing, and establishing industrial standards. In this paper, the above challenges and the future development opportunities of supercapacitors are introduced in detail. This perspective will provide corresponding guidance and new directions for the development of supercapacitors.

Challenges and opportunities for supercapacitors

With the ever increasing demand for clean, sustainable energy, electrochemical supercapacitors with the advantages of high power density, high efficiency and long life expectancy have become one of the major devices for energy storage and power supply, and have found wide application in hybrid power sources, backup power sources, starting power for fuel cells and burst-power generation in electronic devices. Metal phosphates with the advantages of abundance, environmental friendliness and low cost are emerging as a novel class of promising electrode materials for supercapacitors. This review summarizes recent progress with respect to cobalt and nickel phosphate based micro/nanomaterials applied as supercapacitors, covering ammonium/bimetallic phosphates, cobalt phosphates, nickel phosphates and cobalt/nickel phosphates. Much progress has been made using metal phosphate materials as supercapacitors; however, there is still much room for further improvement.

Metal (M = Co, Ni) phosphate based materials for high-performance supercapacitors

Beyond conventional hybrid ion capacitors (HICs) based on porous carbon capacitive materials, the introduction of faradaic pseudocapacitance in HICs is an effective method to enhance energy density. Herein, we prepare methylene blue functionalized graphene (MB-X) composites taking advantage of electrostatic and extensive π-conjugated interaction. The test results confirm that the composites have high rate capability and excellent cycle stability on account of the synergistic effect between graphene and MB molecules. In order to improve the safety of HICs, an aqueous electrolyte is a promising candidate as long as the cell voltage can be efficiently increased. Therefore, we skillfully employ aqueous solutions with different pH values as asymmetric electrolytes to assemble HICs, where polyimide acts as the battery material and MB-X serves as the capacitive material. Thanks to the asymmetric electrolytes, the constructed aqueous HICs can operate at a cell voltage of 1.9 V, much larger than the decomposition voltage of water. Moreover, the flexible structure, and reversible and fast redox reactions of both the anode and cathode make the aqueous HICs display a maximum energy density of 48.6 W h kg−1 and a maximum power density of 19 kW kg−1, as well as a long-cycle lifetime with a decay of 0.00172% per cycle.

Metal-free energy storage systems: combining batteries with capacitors based on a methylene blue functionalized graphene cathode

Interface-rich core-shell ammonium nickel cobalt phosphate for high-performance aqueous hybrid energy storage device without a depressed power density

A multihierarchical structure with (NH4)(Ni, Co)PO4·067H2O microplatelets and (Ni, Co)3(PO4)2·8H2O ultrathin nanopieces anchored on reduced graphene oxide (NCNP/RGO) is synthesized via a mild hydrothermal approach This unique interface-rich structure is suitable for a high power energy storage device by providing efficient pathways for both electronic conduction and ionic transportation, which are effective ways to improve the electrochemical performance Specifically, a specific capacity of 993 F g–1 is obtained in the three-electrode measurement, with ultrahigh capacity retention of 812% (807 F g–1) from 05 to 32 A g–1 The hybrid device constructed with the as-prepared NCNP/RGO as anode and a hierarchical porous carbon (HPC) as cathode offers a very superior energy density of 421 Wh kg–1 at a power density of 73 W kg–1, which remains 32 Wh kg–1 at 14 kW kg–1 Meanwhile, the as-prepared hybrid capacitor exhibits a remarkable cycling stability (965% capacitance retention after 10 000 cycles) The c

Multihierarchical Structure of Hybridized Phosphates Anchored on Reduced Graphene Oxide for High Power Hybrid Energy Storage Devices

An ultrathin MnO2 sheet assembled three-dimensional flower microsphere grown on nitrogen-doped graphene is synthesized through a hydrothermal treatment method. When tested as an anode in a lithium-ion battery, the obtained material exhibits a high discharge capacity of 993 mAh g–1 in the second cycle at 0.1 A g–1, which goes up to 2243 mAh g–1 gradually after 135 charge/discharge cycles. This phenomenon turns out to be related to the deep coupling between nitrogen-doped graphene and MnO2 caused by morphology evolution of the composite upon cycling. Also, kinetics analysis reveals the elevated capacitive contribution after cyclic reaction, indicating the ever enhanced phase interface charge storage mechanism associated with the morphology evolution. Otherwise, the first-principles calculations also indicate the electronic structure of MnO2 can be efficiently modulated by coupling with a conductive graphene substrate, through a covalent C–Mn or N–Mn bond; thus, the deep coupling between nitrogen-doped graph...

Miao Wang

Papers

Interface-rich core-shell ammonium nickel cobalt phosphate for high-performance aqueous hybrid energy storage device without a depressed power density

Multihierarchical Structure of Hybridized Phosphates Anchored on Reduced Graphene Oxide for High Power Hybrid Energy Storage Devices

Largely Increased Lithium Storage Ability of Mangnese Oxide through a Continuous Electronic Structure Modulation and Elevated Capacitive Contribution