As-assembled Asymmetric Supercapacitor Research Articles

Green synthesis of MnO2 via plant extracts and its composite with exfoliated graphene for high-performance asymmetric supercapacitors

Pure MnO2 nanostructure oxides were synthesized by a green synthesis method using purified natural extracts of watercress (MW) and spinach (MS) as benign and economical reducing agents for KMnO4. The MnO2/graphene composites (GMW and GMS) were prepared through a physical mixing process, in which MnO2 oxide was uniformly loaded on graphene sheets. The composition, morphology, and microstructure of the as-prepared oxides and composites were characterized using X-ray diffraction (XRD), Thermal gravimetric analysis (TGA), Raman, Scanning electron microscope (SEM), and Transmission electron microscope (TEM). Additionally, the supercapacitive performance was examined by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge/discharge (GCD). The results imply that the composites GMS and GMW exhibit specific capacitances of 334 and 500 F/g at 1 A/g, respectively, and GMW shows a capacitance retention of 71 % after 7000 cycles at 200 mV/s. Herein, the charge storage mechanism in MnO2/graphene composites is discussed in detail. An asymmetric supercapacitor (ASC) was assembled using the GMW composite and graphene as positive and negative electrodes, respectively. The as-assembled ASC device GMW//GE was scanned reversibly at a voltage of 0–1.8 V and showed a specific capacitance of 167 F/g at 0.5 A/g with a rate capability of 77 % (128.3 F/g at 5 A/g). Moreover, the ASC exhibited excellent cycle stability of 94 % after 2000 cycles and an energy density of 18.9 Wh/kg at a power density of 2.25 kW/kg. These encouraging results showed enormous potential in developing supercapacitors with enhanced performance for practical applications.

MOFs-derived niobium oxide nanoparticles/carbon nanofiber hybrid paper as flexible binder-free electrode for solid-state asymmetric supercapacitors

For the first time, a bottom-up approach was adopted to prepare free-standing orthorhombic Nb2O5-anchored carbon nanofibers (Nb2O5 @CNFs) hybrid paper via in-situ solvothermal synthesis of Nb-metal organic framework@CNFs hybrid and subsequent pyrolysis process. The nanocrystalline Nb2O5 decorated on the CNF surfaces improved the charge storage performance through synergistic effect between porous CNFs (EDLC) and Nb2O5 (pseudocapacitance). Moreover, the high surface area and core-shell structure of Nb2O5 @CNF hybrid reduce diffusion limitations and facilitate ionic transport. The Nb2O5 @CNF hybrid paper as a binder-free electrode delivered greater specific capacitance of 633.6 F/g (975.7 F/cm3) in 1 M H2SO4, compared to Li2SO4 electrolyte (524.3 F/g) at 0.5 A/g. However, the Nb2O5 @CNF electrode showed better cycling stability in neutral Li2SO4 electrolyte than in acidic H2SO4 electrolyte. The solid-state asymmetric supercapacitor (ASC) based on Nb2O5 @CNF paper and activated carbon (AC) electrodes delivered higher energy density of 62.1 Wh kg−1 at 292.7 W kg−1 in Li2SO4 electrolyte due to a wider working potential window (0–1.5 V), compared to H2SO4 electrolyte (55 Wh kg−1). The Li2SO4-based ASC cell is also capable of producing an energy density of up to 45.6 Wh kg−1, even at a high-power density of up to 5853.6 W kg−1. The as-assembled ASC demonstrated low self-discharge rate as well as excellent bending stability, indicating its excellent prospects in real-life applications.

NiCoP/MXene nanocomposites via electrostatic self-assembly for high-performance supercapacitor electrodes.

Bimetallic phosphides have attracted a significant amount of attention in the field of energy storage due to their high specific capacity. However, the cycle stability of supercapacitors has been hampered by the volume expansion and sluggish reaction kinetics of phosphides during charge and discharge. Herein, NiCoP/MXene was successfully prepared by the solvothermal method and subsequent phosphidization. The impacts of the amount of MXene nanosheets on the electrochemical properties of the NiCoP/MXene composite were investigated. The optimized electrode (NCP/MX-20/CC) exhibited a high specific capacity of 848.83 C g-1 at 1 A g-1 and outstanding cyclic stability (86.57% retention after 5000 cycles). The enhanced charge storage performance observed after forming composites with MXene originates from the larger specific surface area, accelerated diffusion process, and enhanced conductivity. These factors lead to an increased number of electrochemically accessible sites and more facile redox kinetics. The NCP/MX-20/CC exhibits battery-type behavior, and surface-controlled processes provide primary support for the charge storage mechanism. The as-assembled asymmetric supercapacitor (NCP/MX-20//activated carbon ASC) delivers an energy density of 49.7 W h kg-1 at 800.1 W kg-1, and it is durable towards repetitive charge-discharge cycles. This work demonstrates that NiCoP/MXene composite materials are potential candidates for use as supercapacitor electrodes.

Azure B microspheres/nitrogen-doped reduced graphene oxide: Non-covalent interactions based crosslinking fabrication for high-performance supercapacitors

Organic molecules with conjugated π-bonds are promising electrode materials for supercapacitors due to their outstanding redox reversibility and structural stability. Yet they are limited for applications in supercapacitors on account of poor conductivity and inferior cyclability. In this work, a series of azure B/nitrogen-doped reduced graphene oxide (AB/N-rGO) composites are prepared via hydrothermal process and following freeze-drying. As expected, non-covalent bonds between N-rGO and AB allow them to link together and form three-dimensional porous structure, exhibiting enhanced specific capacitance and cycle life. The supercapacitive properties of AB/N-rGO composites are optimized by controlling mass ratio of graphene oxide and AB raw materials. The prepared AB/N-rGO-5 electrode exhibits optimal specific capacitance of 371 F g − 1 at 1 A g − 1 and excellent cyclic stability (94% capacitance retention after 10,000 cycles) in 1.0 M H2SO4 electrolyte. Besides, the as-assembled asymmetric supercapacitor constructed with AB/N-rGO-5 cathode and WO3 anode not only displays wide voltage window (1.55 V) and high energy density of 25.0 Wh kg−1 at power density of 387.9 W kg−1, but also exhibits prominent cyclic stability (81% capacity retention after 10,000 cycles). This work offers ideas for applications of more electroactive molecules with excellent capacitive properties in supercapacitors.

Solvent-assisted assembly of reduced graphene oxide/MXene-polypyrrole composite film for flexible supercapacitors

Two-dimensional (2D) materials represented by graphene and MXene have attracted extensive attention in the field of energy storage. However, the automatic stacking and poor stability of 2D materials considerably limit their electrochemical performance. In this article, we apply a design strategy based on combining the ternary components of reduced graphene oxide (rGO), MXene, and polypyrrole (PPy) into one electrode to form a flexible film with a sandwich structure. As a result, the resulting rGO/MXene-PPy composite electrode inherits the characteristics of high conductivity, robust mechanical properties, and pseudocapacitance. In addition to providing capacitive contributions, the PPy serves as a blocker to prevent face-to-face restacking of the 2D nanosheets and also as a coating layer to significantly protect MXene from oxidation. Consequently, the rGO/MXene-PPy electrode exhibits a high specific capacitance of 408.2 F g−1 and a superior rate performance, with 67.3% capacitance retention at an increased current density of 10.0 A g−1. Furthermore, the as-assembled asymmetric supercapacitor possesses a pronounced energy density of 11.3 Wh kg−1 (35.5 Wh L−1) at a power density of 500.0 W kg−1 (1570.0 W L−1) and remarkable cycling stability, with 8.8% capacitance deterioration after 10,000 cycles. This work demonstrates the potential for application of as-prepared rGO/MXene-PPy electrodes in flexible energy storage devices with high volumetric/gravimetric energy and power densities.

Sponge-like 3D flower-like core-shell heterostructure CuCo2O4@CuCo2S4 as advanced electrodes for high-performance supercapacitor

The design of nanocomposite electrode materials with reasonable core-shell heterostructures is vital to upgrade the energy density and structural stability of asymmetric supercapacitors. Herein, a unique sponge-like 3D flower-like core-shell CuCo2O4@CuCo2S4 (CCO@CCS-4) heterostructure is engineered and synthesized on a carbon cloth substrate by hydrothermal assisted electrodeposition method, which reveals that the externally interconnected CCS nanosheets as shell is uniformly anchored on the vertically aligned CCO nanoneedles, not only spatially separating the redox reaction but also improving the stability of the composite. Benefiting from its exclusive structure and the heterogeneity synergies of two components, the as-obtained CCO@CCS-4 electrode material exhibits specific capacity (1459.15 C g−1, 2.91 C cm−2), together with ascendant cycle stability of only 6.77% loss after 10,000 cycles. Moreover, the as-assembled asymmetric supercapacitor device, composed of CCO@CCS-4 as the cathode and activated carbon as the anode, provides considerable energy density (92.06 Wh kg−1 at 799.99 W kg−1). These properties indicate that there is a great potential in the application of high-performance supercapacitors.

NiCo-MOF directed NiCoP and coconut shell derived porous carbon as high-performance supercapacitor electrodes

Bimetallic mickel/cobalt metal-organic frameworks(MOFs) have showed important applications as supercapacitor electrode materials due to their rich redox active sites, regular pore structure and huge specific surface area. But their specific energy and capacitance are limited. In this paper, a MOFs phosphorization strategy was proposed for preparing high-performance NiCo bimetallic phosphide nanosheets (NiCoP) by simple OH− ion etching and phosphorization. The formed nanosheet structure possesses a huge specific surface area, which is beneficial for the Faraday reaction. The as-prepared Ni1Co1-10P nanosheets present significant synergy among transition metal ions, which contributes to enhanced electrochemical performance. Because of structural and composition advantages, the optimized Ni1Co1-10P nanosheets show a superior specific capacitance (1188.4 F g−1 at 1 A g−1) than corresponding hydroxide (Ni1Co1-OH) and Ni1Co1-MOF precursors. The as-assembled asymmetric supercapacitor (ASC) using Ni1Co1-10P nanomaterial as the positive electrode and coconut shell derived porous carbon (EC) with huge surface area and electric double layer capacitor (EDLC) property as negative electrode delivers a high specific capacitance of 135.5 F g−1 at 1 A g−1 and a maximum energy density of 55.01 Wh kg−1 at 849.94 W kg−1 as well as excellent capacitance retention (86.12 % after 10,000 cycles). Therefore, the prepared Ni1Co1-10P electrode materials have a broad application in high-performance supercapacitors.

Double ion-buffering reservoirs of advanced NiCo//G-PANI asymmetric supercapacitors with high performance

Asymmetric supercapacitors (ASCs) have been designed due to their lower cost effectiveness, better safety, long cycle life, and high energy/power density. Herein, the ASC of double “ion-buffering reservoirs” NiCo2O4-CNT-c-PEGm//rGO-PANI (NiCo//G-PANI) with high capacity and ultralong lifetime were fabricated and evaluated. The three-dimensional hierarchical flower-like NiCo2O4-CNT-c-PEGm microspheres were prepared via solvothermal method. In these composites, the incorporated CNTs were modified with methoxypolyethylene glycol (mPEG) polymer by Cu(I)-catalyzed click chemistry. The NiCo2O4-CNT-c-PEGm composites with typical honeycomb-like “ion-buffering reservoir” structures exhibited the specific capacity of 191.3 mAh g−1 at 0.5 A g−1. Moreover, the rGO-PANI composites with hierarchical interconnected porous network structures as “ion-buffering reservoir” were prepared via in situ polymerization and chemical reduction. Further, the rGO-PANI electrode materials could achieve the specific capacity of 81.5 mAh g−1 at 1 A g−1. Owing to such excellent attributes, the as-assembled ASC NiCo//G-PANI exhibit the specific capacity of 84.7 mAh g−1 at 1 A g−1 and high energy density of 67.7 Wh Kg−1 at a power density of 800 W Kg−1 for the potential applications in electric energy storage technologies. As a result, this study promotes an innovative strategy to synthesize the metal oxides modified with carbon materials as positive electrode materials to assemble the new type of ASCs.

Hierarchical NiCo-LDH core/shell homostructural electrodes with MOF-derived shell for electrochemical energy storage

Constructing hierarchical structure is an effective strategy to boost the electrochemical performance of layered double hydroxide (LDH) materials, but the rational design of such delicate architectures is still challenging. Herein, a unique hierarchical core/shell homostructure with NiCo-LDH nanorods (NCNRs) as core and NiCo-LDH nanosheets (NCNSs) as shell is constructed via in-situ ZIF shell growth and subsequent ion exchange-coprecipitation process. Such novel hierarchical structure provides a large accessible surface area and more exposed electrochemical active sites. The in-situ growth and conversion process contribute to the formation of robust adhesion between the core and the shell, which could facilitate the effective charge and ion diffusion, as well as improve the mechanical stability. Benefiting from the unique structure, the NCNRs@NCNSs electrode exhibits a high capacitance of 2640.2 F g−1, along with the good rate performance and cyclic stability. Furthermore, the as-assembled asymmetric supercapacitor of NCNRs@NCNSs//AC device displays a high energy density of 22.81 Wh kg−1 at the power density of 374.95 W kg−1. This work demonstrates a new strategy for designing hierarchical LDH with core/shell structure as electrode materials for superior electrochemical energy storage.

Asymmetric supercapacitors based on porous MnMoS4 nanosheets-anchored carbon nanofiber and N, S-doped carbon nanofiber electrodes

Electrode materials with high electrochemical activity and a favorable morphology are highly desired for improving the energy density of supercapacitors. A ternary metal sulfide with higher electrochemical activity and capacity than mono-metal sulfides offers immense promise as an energy storage material. In the present investigation, an advanced flexible hybrid electrode material composed of porous manganese molybdenum sulfide (MnMoS4) nanosheets supported on flexible carbon nanofiber (CNF) mat has been prepared via three sequential steps: (i) electrospinning, (ii) stabilization/carbonization, and (iii) hydrothermal reaction. Advantages from the rich electrochemical redox properties of MnMoS4 and the 3D interconnected network architecture of porous CNF mat, a large specific capacitance of 2187.5 F/g (at 1 A/g), and a good capacity retention ability (>87%) were achieved for the MnMoS4 @CNF hybrid electrode. In addition, asymmetric supercapacitor (ASC) devices were assembled by utilizing two different binder-free electrodes, i.e., MnMoS4 @CNF mat as the positive electrode and N, S doped CNF mat as the negative electrode, and evaluated their capacitive performances in two different electrolytes, i.e., KOH and Na2SO4. As-assembled ASC with 1 M Na2SO4 electrolyte delivered a high energy density of 72.5 Wh kg−1 and a power density of 2.7 kW kg−1 together with a capacity retention of 93.5% after 5000 cycles. The overall outcome of this investigation indicates that the binder-free nanostructured MnMoS4 @CNF hybrid mat has great potential for the development of next-generation supercapacitor devices.

Flexible nickel disulfide nanoparticles-anchored carbon nanofiber hybrid mat as a flexible binder-free cathode for solid-state asymmetric supercapacitors

Nickel disulfide nanoparticles (NiS2NPs)-anchored carbon nanofibers (NiS2NPs@CNF) hybrid mats were fabricated via the sequential process of stabilization and carbonization of electrospun polyacrylonitrile-based fibers followed by hydrothermal growth of NiS2NPs on the porous surface of CNFs. The vertical growth of NiS2NPs on entire surfaces of porous CNFs appeared in the SEM images of hybrid mat. The hierarchical NiS2NPs@CNF core–shell hybrid nanofibers with 3D interconnected network architecture can endow continuous channels for easy and rapid ionic diffusion to access the electroactive NiS2NPs. The conductive and interconnected CNF core could facilitate electron transfer to the NiS2 shell. Moreover, the porous CNF as a buffering matrix can resist volumetric deformation during the long-term charge–discharge process. The NiS2NPs@CNF electrode can yield high specific capacitance (916.3 F g−1 at 0.5 A g−1) and reveal excellent cycling performances. The solid-state asymmetric supercapacitor (ASC) was fabricated with NiS2NPs@CNF mat as a binder-free positive electrode and activated carbon cloth as a negative electrode. As-assembled ASC not only produce high specific capacitance (364.8 F g−1 at 0.5 A g−1) but also exhibit excellent cycling stability (∼92.8% after 5000 cycles). The ASC delivered a remarkably high energy density of 129.7 Wh kg−1 at a power density of 610 W kg−1. These encouraging results could make this NiS2NPs@CNF hybrid mat a good choice of cathode material for the fabrication of flexible solid-state ASC for various flexible/wearable electronics.

Metal-organic framework derived hollow rod-like NiCoMn ternary metal sulfide for high-performance asymmetric supercapacitors

In this work, metal–organic frameworks (MOFs) derived hollow rod-like NiCoMn ternary metal sulfides nanosheets are successfully synthesized by the etching/ion-exchange reaction and additional sulfurization. The formed hollow structure exhibits a unique hierarchical architecture constructed by NiCoMn-S nanosheets and internal porous channels, which is beneficial for the penetration of electrolyte and the electron/ion transportation. The as-prepared NiCoMn-S nanosheets present significant synergy among transition metal ions, which contributes to enhanced electrochemical performances. Owing to structural and compositional advantages, the optimized NiCoMn-S nanosheets show a superior specific capacitance (2098.2 F g−1 at 1 A g−1) than corresponding hydroxide precursor (NiCoMn-OH) and other controlled sulfides with different ratio of Ni/Co/Mn. The as-assembled asymmetric supercapacitor (ASC) using NiCoMn-S nanomaterials as positive electrode and activated carbon (AC) as negative electrode delivers a high specific capacitance of 124.5 F g−1 at 1 A g−1 and a maximum energy density of 50.0 Wh kg−1 at 850.0 W kg−1 as well as excellent capacitance retention (73.6% after 6000 cycles). Therefore, the prepared NiCoMn-S electrode materials have a broad application prospect in high-performance supercapacitors.

Cabbage-Like NiCo2S4/MoS2 Assemblies as Electrode Materials for Asymmetric Hybrid Capacitors

In this work, cabbage-like NiCo2S4/MoS2 assemblies are prepared via a simple hydrothermal process. The as-prepared product as electrode material possesses a specific capacitance of 612 C g−1 at 1 A g−1 and 94% capacitance retention after 10000 cycles at 10 A g−1. Furthermore, the as-assembled asymmetric supercapacitor exhibits an energy density of 73.3 Wh kg−1 at a power density of 453.4 W kg−1. It demonstrates that the cabbage-like structures present promising applications in future energy storage devices.

Branched nanosheets-interlaced structure of Co2+/Co3+-doped Ni(OH)2 originating from Ni3(NO3)2(OH)4 template with significantly boosted electrochemical performance

The cycling stability of electrode materials is a key for Faraday supercapacitors (FSs), especially the structure-unstable Ni(OH)2. There exist various strategies for stabilizing the structure of Ni(OH)2 such as cobalt ions doping and mechanical support from substrate. However, these strategies have their own disadvantages. On the other hand, the combination of two or more strategies has the serious issues in materials synthesis. In this work, a branched nanosheets-interlaced structure of Co2+/Co3+-doped Ni(OH)2 is built by a method of precursor template, which effectively applies two strategies including ions doping and self-supportive structure. Due to the synergistic contribution of these two strategies, the cycling stability of as-prepared material is significantly enhanced. This material also presents excellent high-rate capacity, and the as-assembled asymmetric supercapacitors (ASCs) show both high energy and power densities. These results prove that the effective combination of ions doping and self-supportive structure strategies is a promising route for the preparation of Ni(OH)2-based electrode materials with high performance.

Electrodeposited Polyaniline Nanofibers and MoO3 Nanobelts for High-Performance Asymmetric Supercapacitor with Redox Active Electrolyte.

Transition molybdenum oxides (MoO3) and conductive polymer (polyaniline, PANI) nanomaterials were fabricated and asymmetric supercapacitor (ASC) was assembled with MoO3 nanobelts as negative electrode and PANI nanofibers as a positive electrode. Branched PANI nanofibers with a diameter of 100 nm were electrodeposited on Ti mesh substrate and MoO3 nanobelts with width of 30–700 nm were obtained by the hydrothermal reaction method in an autoclave. Redox active electrolyte containing 0.1 M Fe2+/3+ redox couple was adopted in order to enhance the electrochemical performance of the electrode nano-materials. As a result, the PANI electrode shows a great capacitance of 3330 F g−1 at 1 A g−1 in 0.1 M Fe2+/3+/0.5 M H2SO4 electrolyte. The as-assembled ASC achieved a great energy density of 54 Wh kg−1 at power density of 900 W kg−1. In addition, it displayed significant cycle stability and its capacitance even increased to 109% of the original value after 1000 charge–discharge cycles. The superior performance of the capacitors indicates their promising application as energy storage devices.

Core–shell crystalline ZIF-67@amorphous ZIF for high-performance supercapacitors

Zeolitic imidazolate frameworks (ZIFs)-based core–shell structured nanomaterials were synthesized. As supercapacitor electrode materials, they exhibit excellent performance due to the advanced intrinsic properties of ZIFs, such as high porosity, good stability and large surface area. The materials were characterized by SEM, TEM, XRD, FT-IR, XPS and BET. When synthesized core–shell structured nanomaterial was tested in a 3-electrode system, it exhibited a high specific capacitance of 1176.8 F g−1 at a current density of 1 A g−1 and a good cycling performance with 98.3% initial capacity retention after 1000 continuous charge–discharge cycles. An asymmetric supercapacitor (ASC) device was assembled by employing ZIF-67@amorphous ZIF and activated carbon as positive and negative electrode materials, respectively. The as-assembled ASC devices exhibited excellent stability that retained almost 100% of the initial capacity after 2000 cycles. To demonstrate the practical application of the ASC, two simple fully charged ASCs can light up the LED for over 2 min, which proved that the core–shell structured ZIF-67@amorphous ZIF material has a suitable potential for supercapacitor applications.

Three dimensional flower-like CuO/Co3O4/r-GO heterostructure for high-performance asymmetric supercapacitors

The supercapacitors have been widely used in the field of electronics appliances and automobiles, but its utility is limited by low energy density and poor specific capacitance. Energy density and specific capacitance of the supercapacitor is invariably determined by the electrode materials. Herein, a novel 3D flower-like CuO/Co3O4/r-GO heterostructure was synthesized by the hydrothermal method and used as an anode material for high-performance supercapacitors in an alkaline electrolyte solution. The as-prepared 3D flower-like CuO/Co3O4/r-GO heterostructure with high surface area provides vast interfacial contact area between electrode and electrolyte solution which offers a fast surface reaction kinetics and improved conductivity, thereby the remarkable electrochemical performance is achieved. The prepared CuO/Co3O4/r-GO heterostructure material shows an ultra-high specific capacitance of 1458 Fg-1 at a current density of 0.5 Ag-1, exceptional rate capability, and stable long- term cycling performance (97% retention after 10000 successive charge-discharge cycles at 5 Ag-1). In as-assembled asymmetric supercapacitor, CuO/Co3O4/r-GO heterostructure is an anode material and r-GO is a cathode material. The assembled device shows a high specific capacitance of 198 Fg-1 at 2 Ag-1, an ultra-high power density of 10510.30 W kg−1, and a high energy density of 34.20 Wh kg−1. Meanwhile, the ASC device exhibits excellent rate capability (97%) after 10000 successive charge-discharge cycles at 5 Ag-1 in the PVA-KOH gel electrolyte.

Polypyrrole-coated Fe2O3 nanotubes constructed from nanoneedles as high-performance anodes for aqueous asymmetric supercapacitors.

Asymmetric supercapacitors (ASCs) show promising potential for electrochemical energy storage applications. However, the energy density of ASCs is limited by the poor electrochemical performance of anodes. To achieve high-performance ASCs, herein, Fe2O3 nanotubes constructed from Fe2O3 nanoneedles were fabricated by employing MnO2 nanotubes as a self-sacrificing template, and then a layer of polypyrrole (PPy) was coated through an in situ chemical oxidative polymerization method to enhance their performance. The electrochemical tests indicate that the resultant PPy-coated Fe2O3 nanotubes (Fe2O3@PPy) exhibit a high areal capacitance of 530 mF cm-2 at 1 mA cm-2 and good cycling stability, which are superior to those of the Fe2O3 nanotubes. The superior performance of the Fe2O3@PPy nanotubes can be attributed to the synergistic effect between the PPy shell and Fe2O3 core, in which the conducting PPy shell not only works as a superhighway for charge transport, but also stabilizes the Fe2O3 nanotubes during charge-discharge processes. When the Fe2O3@PPy nanotubes were assembled with MnO2 nanotubes, the as-assembled ASCs possess a high cell voltage of 2.0 V and deliver a high energy density of up to 51.2 Wh kg-1 at a power density of 285.4 W kg-1 with an excellent cycling stability (83.5% capacitance retention over 5000 cycles).

Nickel hexacyanoferrate on graphene sheets for high-performance asymmetric supercapacitors in neutral aqueous electrolyte

Nickel hexacyanoferrate (NiHCF) with rigid open framework is promising for energy storage devices, yet still suffer from inferior conductivity and low energy density. Herein, NiHCF supported on diallyldimethylammonium chloride (PDDA) modified graphene (PRGO-NiHCF) was successfully synthesized via a facile co-precipitation method. The size of NiHCF nanoparticals decreased from 50 nm to 20 nm with the introduction of PDDA-induced reduced graphene oxide. The unique structure can not only reversibly accommodate hydrated alkali cations with 3D open-framework, but also facilitate the electrons transportation through the graphene basal plane. Consequently, the as-obtained PRGO-NiHCF exhibits higher specific capacitances of 294 F g−1 at 2 mV s−1 compared to pure NiHCF (176 F g−1). Moreover, under the combination of PRGO-NiHCF positive electrode and the interconnected sandwiched graphene carbon (SGC) negative electrode, the as-assembled asymmetric supercapacitor delivers a wide operation potential of 1.8 V and a high energy density of 25.4 Wh kg−1.

Constructing hierarchical lanthanum-doped nickel hydroxidenitrate nanoflowers with dilated lattice spacing for improving asymmetric supercapacitors

The novel lanthanum-doped nickel hydroxidenitrate (LaNiHN) nanoflowers are prepared by a facile one-step solvothermal growth method. The doping of La ions into the lattice of the NiHN nanoflowers can effectively increase the electrical conductivity, lattice volume and c-axis spacing of NiHN nanosheets and lead to rapid transfer of electrolyte ions and electrons between the nanosheet channels, resulting in a high specific capacity of 154 mAh g−1 (at 1 A g−1) and high rate performance for supercapacitor positive electrode. Moreover, the Fe2O3 supported on graphene nanosheets (Fe2O3/GNS) with high specific capacity and excellent stability is selected as negative electrode to match the LaNiHN nanoflowers positive electrode for assembly of a novel LaNiHN//Fe2O3/GNS asymmetric supercapacitor (ASC). By virtue of their excellent electrochemical properties and complementary working voltage, the as-assembled ASC possesses a wide operating voltage of 1.65 V, high energy density of 51 Wh kg−1 at a power density of 825 W kg−1, as well as the acceptable cycling stability of 87% capacity retention after 5000 cycles.