CoNi2S4 nanosheets on nitrogen-doped carbon foam as binder-free and flexible electrodes for high-performance asymmetric supercapacitors

Investigation on pore structure regulation of activated carbon derived from sargassum and its application in supercapacitor

In order to realize the effective regulation of the pore structure of activated carbon and optimize its pore structure properties as electrode material, the effects of activation temperature, activation time and impregnation ratio on the specific surface area, total pore volume and average pore diameter of activated carbon prepared by sargassum are studied by orthogonal experiment. In addition, the electrochemical properties of sargassum-based activated carbon (SAC) and the relationship between the gravimetric capacitance and specific surface area of SAC are also studied. The SACs prepared under all conditions have high specific surface area (≥ 2227 m2 g-1) and developed pore structure, in which the pore diameter of micropores mainly concentrated in 0.4 ~ 0.8 nm, the pore diameter of mesopores mainly concentrated in 3 ~ 4 nm, and the number of micropores is far more than that of mesopores. In the activation process, the impregnation ratio has the greatest effect on the specific surface area of SAC, the activation temperature and impregnation ratio have significant effect on the total pore volume of SAC, and the regulation of the average pore diameter of SAC is mainly realized by adjusting the activation temperature. The SACs exhibit typical electric double layer capacitance performances on supercapacitors, delivering superior gravimetric capacitance of 237.3 F g-1 in 6 mol L-1 KOH electrolyte system at current density of 0.5 A g-1 and excellent cycling stability of capacitance retention of 92% after 10,000 cycles. A good linear relationship between gravimetric capacitance and specific surface area of SAC is observed.

Carbon nanotubes interpenetrating MOFs-derived Co-Ni-S composite spheres with interconnected architecture for high performance hybrid supercapacitor

Recently, carbon nanotubes (CNT)-based interconnected architectures exhibit promising prospects in supercapacitors due to their flexibility and high electrical conductivity. Herein, a three-dimensional (3D) interconnected network structure combined with conductive carbon nanotubes interpenetrating MOFs-derived Co-Ni-S composite spheres (Co-Ni-S/CNTs) was synthesized. Such 3D interconnected architecture significantly leads to a favorable electronic structure, fast charge-transfer capacity, and more pseudocapacitive. The Co-Ni-S/CNTs-based hybrid electrode exhibits an extraordinary specific capacitance of 540.6C g^-1 at 1 A g^-1 and competitive rate performance (capacity retention rate of 69.9% when the current density increases to 10 times). Subsequently, a hybrid supercapacitor is assembled using Co-Ni-S/CNTs as the positive electrode and commercial activated carbon as negative electrode. The device delivers a high energy density of 63.5 W h kg^-1 at 800 W kg^-1 and keeps 83.0% initial capacitance retention after 10,000 cycles. The encouraging performances demonstrate the significant contribution of the 3D interconnected architecture for the future energy storage.

MOF-derived ZnCo2O4@NiCo2S4@PPy core-shell nanosheets on Ni foam for high-performance supercapacitors

The ZnCo₂O₄@NiCo₂S₄@PPy core-shell nanosheets material is prepared by directly growing leaf-like ZnCo₂O₄ nanosheets derived from the metal-organic framework (MOF) on Ni foam (NiF) via chemical bath deposition and annealing methods and then combining with NiCo₂S₄ and PPy via electrodeposition methods. The special core-shell structure formed by MOF-derived ZnCo₂O₄, NiCo₂S₄ and PPy creates a bi-interface, which could significantly promote the contact between electrode and electrolyte, provide more active sites and accelerate electron/ion transfer. And the combination of these three materials also produces a strong synergistic effect, which could further improve the capacitive performance of the electrode. Therefore, the ZnCo₂O₄@NiCo₂S₄@PPy/NiF electrode exhibits the maximum areal capacitance (3.75 F cm^-2) and specific capacitance (2507.0 F g^-1) at 1 mA cm^-2 and 0.5 A g^-1, respectively. Moreover, its capacitance retention rate is still 83.2% after 5000 cycles. In addition, a coin-type hybrid supercapacitor is assembled and displays a high energy density of 44.15 Wh kg^-1 and good cycling performance.

Controlling and optimizing the morphology and microstructure of 3D interconnected activated carbons for high performance supercapacitors

For an active electrode material, the morphology, microstructure and the effective specific surface area derived from them, have a dominant effect for the high performance supercapacitors. In this study, 3D interconnected activated carbons with controlled and optimized morphologies and porous structures were prepared from accessible carbon source and graphene oxide by a hydrothermal carbonization and following an activation method. Through optimizing the ratios of the precursors and reaction conditions, an electrode material with excellent specific surface area of 2318 m² g^-1, meso-/macro-pore ratio of 63.2% (meso-/macro-pore volume reached to 0.83 cm³ g^-1), as well as an outstanding electrical conductivity of 46.6 S m^-1, was obtained. The materials exhibit superior double-layer capacitive performances on a symmetric supercapacitor, delivering superior specific capacitance of 157 F g^-1 in organic electrolyte system at current density of 0.5 A g^-1, excellent energy density of 37.6 W h kg^-1 with a power density of 7.1 kW kg^-1 and good cycling stability of capacitance retention of 94% over 7000 cycles. These results offer a practical method to prepare the desired carbon electrode materials with controlled morphology and structure for high efficiency electrochemical energy storage devices.

Hierarchical Co(OH)2@NiMoS4 nanocomposite on carbon cloth as electrode for high-performance asymmetric supercapacitors

Hierarchical Co(OH)₂@NiMoS₄ nanocomposites were successfully prepared on a carbon cloth by using a simple two-step hydrothermal method coupled with a room-temperature vulcanization method. The resulting nanocomposites were composed of large-scale uniform Co(OH)₂ nanowires fully covered with ultrathin vertical NiMoS₄ nanoflakes. Because of the synergetic effect between Co(OH)₂ and NiMoS₄, the nanocomposites exhibited good electrochemical performance as a supercapacitor electrode. In particular, a specific capacity of 2229 F g⁻¹ was achieved at a current density of 1 A g⁻¹. In addition, an asymmetrical supercapacitor fabricated using activated carbon as the negative electrode and the as-synthesised nanocomposite as the positive electrode exhibited a maximum energy density of 59.5 W h kg⁻¹ at a power density of 1 kW h kg⁻¹ and excellent cycling stability (100% capacitance retention after 5000 cycles). These results indicate that the hierarchical Co(OH)₂@NiMoS₄ nanocomposite has great potential for practical application in high-performance energy storage devices.

A hierarchical Co(OH)₂@NiMoS₄ nanocomposite was prepared on the surface of carbon cloth, which exhibited good electrochemical performance as a supercapacitor electrode.

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.