An Effective Electrodeposition Mode for Porous MnO₂/Ni Foam Composite for Asymmetric Supercapacitors

Hierarchical Activated Carbon-MnO2 Composite for Wide Potential Window Asymmetric Supercapacitor Devices in Organic Electrolyte

The consumption of electrical energy grows alongside the development of global industry. Generating energy storage has become the primary focus of current research, examining supercapacitors with high power density. The primary raw material used in supercapacitor electrodes is activated carbon (AC). To improve the performance of activated carbon, we used manganese dioxide (MnO₂), which has a theoretical capacitance of up to 1370 Fg^-1. The composite-based activated carbon with a different mass of 0-20% MnO₂ was successfully introduced as the positive electrode. The asymmetric cell supercapacitors based on activated carbon as the anode delivered an excellent gravimetric capacitance, energy density, and power density of 84.28 Fg^-1, 14.88 Wh.kg^-1, and 96.68 W.kg^-1, respectively, at 1 M Et₄NBF₄, maintaining 88.88% after 1000 test cycles.

NiCoSe4nanoparticles derived from nickel-cobalt Prussian blue analogues on N-doped reduced graphene oxide for high-performance asymmetric supercapacitors

Synthesis of NiHCCo precursors via simple co-precipitation and nickel-cobalt tetraselenide composites grown on nitrogen-doped reduced graphene oxide (NiCoSe₄/N-rGO) were fabricated using solvothermal method. The introduction of N-rGO used as a template effectively prevented agglomeration of NiCoSe₄nanoparticles and provided more active sites, which greatly increased the electrochemical and electrical conductivity for NiCoSe₄/N-rGO. NiCoSe₄/N-rGO-20 presents a remarkably elevated specific capacity of 120 mA h g^-1under current density of 1 A g^-1. NiCoSe₄/N-rGO-20 demonstrates an excellent cycle life and achieves a remarkable 83% retention rate over 3000 cycles with 10 A g^-1. NiCoSe₄/N-rGO-20//N-rGO asymmetric supercapacitor was constructed based on the NiCoSe₄/N-rGO-20 as an anode, N-rGO as cathode by using 2 mol l^-1KOH as an electrolyte. NiCoSe₄/N-rGO-20//N-rGO ASC demonstrates an ultra-big energy density of 14 Wh kg^-1and good circulation stability in the power density of 902 W kg^-1. It is doubled in comparison to the NiCoSe₄/N-rGO-20//rGO asymmetric supercapacitor (7 Wh kg^-1). The NiCoSe₄/N-rGO-20//N-rGO ASC capacity retention is still up to 93% over 5000 cycles (5 A g^-1). The results reveal that this device would be a prospective cathode material of supercapacitors in actual applications.

Stereolithography-Derived Three-Dimensional Pyrolytic Carbon/Mn3O4 Nanostructures for Free-Standing Hybrid Supercapacitor Electrodes

The development of permeable three-dimensional (3D) macroporous carbon architectures loaded with active pseudocapacitive nanomaterials offers hybrid supercapacitor (SC) materials with higher energy density, shortened diffusion length for ions, and higher charge-discharge rate capability and thereby is highly relevant for electrical energy storage (EES). Herein, structurally complex and tailorable 3D pyrolytic carbon/Mn₃O₄ hybrid SC electrode materials are synthesized through the self-assembly of MnO₂ nanoflakes and nanoflowers onto the surface of stereolithography 3D-printed architectures via a facile wet chemical deposition route, followed by a single thermal treatment. Thermal annealing of the MnO₂ nanostructures concurrent with carbonization of the polymer precursor leads to the formation of a 3D hybrid SC electrode material with unique structural integrity and uniformity. The microstructural and chemical characterization of the hybrid electrode reveals the predominant formation of crystalline hausmannite-Mn₃O₄ after the pyrolysis/annealing process, which is a favorable pseudocapacitive material for EES. With the combination of the 3D free-standing carbon architecture and self-assembled binder-free Mn₃O₄ nanostructures, electrochemical capacitive charge storage with very good rate capability, gravimetric and areal capacitances (186 F g^-1 and 968 mF cm^-2, respectively), and a long lifespan (>92% after 5000 cycles) is demonstrated. It is worth noting that the gravimetric capacitance value is obtained by considering the full mass of the electrode including the carbon current collector. When only the mass of the pseudocapacitive nanomaterial is considered, a capacitance value of 457 F g^-1 is achieved, which is comparable to state-of-the-art Mn₃O₄-based SC electrode materials.

In Situ Binder-Free and Hydrothermal Growth of Nanostructured NiCo2S4/Ni Electrodes for Solid-State Hybrid Supercapacitors

Herein, we report a comparison of the electrochemical performance of two kinds of NiCo2S4-based electrodes for solid-state hybrid supercapacitors (HSCs). For the binder-free electrode, NiCo2S4 was grown on Ni foam by the chemical bath deposition (CBD) method. For the binder-using electrode, NiCo2S4 powder was synthesized by the hydrothermal method. FESEM images depicted the hierarchical nanostructure of NiCo2S4 synthesized by the hydrothermal method and uniform distribution of nanostructured NiCo2S4 grown on Ni foam by the CBD method. Half-cell studies of both NiCo2S4 electrodes showed them exhibiting battery-type charge storage behavior. To assemble HSCs, NiCo2S4 and activated carbon were used as a positive and negative electrode, respectively. Electrochemical studies of the HSCs showed that the accessible potential window was wide, up to 2.6 V, through cyclic voltammetry (CV) analysis. Chronopotentiometry (CP) studies revealed that the energy and power densities of binder-using HSC were 51.24 Wh/kg and 13 kW/kg at 1 Ag−1, respectively, which were relatively higher than those of the binder-free HSC. The binder-free HSC showed 52% cyclic stability, relatively higher than that of the binder-using HSC. Both HSCs, with unique benefits and burdens on energy storage performance, are discussed in this work.

Investigation on the Mass Distribution and Chemical Compositions of Various Ionic Liquids-Extracted Coal Fragments and Their Effects on the Electrochemical Performance of Coal-Derived Carbon Nanofibers (CCNFs)

Coal-derived carbon nanofibers (CCNFs) have been recently found to be a promising and low-cost electrode material for high-performance supercapacitors. However, the knowledge gap still exists between holistic understanding of coal precursors derived from different solvents and resulting CCNFs’ properties, prohibiting further optimization of their electrochemical performance. In this paper, assisted by laser desorption/ionization (LDI) and gas chromatography–mass spectrometry (GC–MS) technologies, a systematic study was performed to holistically characterize mass distribution and chemical composition of coal precursors derived from various ionic liquids (ILs) as extractants. Sequentially, X-ray photoelectron spectroscopy (XPS) revealed that the differences in chemical properties of various coal products significantly affected the surface oxygen concentrations and certain species distributions on the CCNFs, which, in turn, determined the electrochemical performances of CCNFs as electrode materials. We report that the CCNF that was produced by an oxygen-rich coal fragment from C₆mimCl ionic liquid extraction showed the highest concentrations of quinone and ester groups on the surface. Consequentially, C₆mimCl-CCNF achieved the highest specific capacitance and lowest ion diffusion resistance. Finally, a symmetric carbon/carbon supercapacitor fabricated with such CCNF as electrode delivered an energy density of 21.1 Wh/kg at the power density of 0.6 kW/kg, which is comparable to commercial active carbon supercapacitors.

Enhanced Activity of Hierarchical Nanostructural Birnessite-MnO2-Based Materials Deposited onto Nickel Foam for Efficient Supercapacitor Electrodes

Hierarchical porous birnessite-MnO₂-based nanostructure composite materials were prepared on a nickel foam substrate by a successive ionic layer adsorption and reaction method (SILAR). Following composition with reduced graphene oxide (rGO) and multiwall carbon nanotubes (MWCNTs), the as-obtained MnO₂, MnO₂/rGO and MnO₂/rGO-MWCNT materials exhibited pore size distributions of 2-8 nm, 5-15 nm and 2-75 nm, respectively. For the MnO₂/rGO-MWCNT material in particular, the addition of MWCNT and rGO enhanced the superb distribution of micropores, mesopores and macropores and greatly improved the electrochemical performance. The as-obtained MnO₂/rGO-MWCNT/NF electrode showed a specific capacitance that reached as high as 416 F·g^-1 at 1 A·g^-1 in 1 M Na₂SO₄ aqueous electrolyte and also an excellent rate capability and high cycling stability, with a capacitance retention of 85.6% after 10,000 cycles. Electrochemical impedance spectroscopy (EIS) analyses showed a low resistance charge transfer resistance for the as-prepared MnO₂/rGO-MWCNT/NF nanostructures. Therefore, MnO₂/rGO-MWCNT/NF composites were successfully synthesized and displayed enhanced electrochemical performance as potential electrode materials for supercapacitors.

Biomass-Derived Porous Carbons Derived from Soybean Residues for High Performance Solid State Supercapacitors

A series of heteroatom-containing porous carbons with high surface area and hierarchical porosity were successfully prepared by hydrothermal, chemical activation, and carbonization processes from soybean residues. The initial concentration of soybean residues has a significant impact on the textural and surface functional properties of the obtained biomass-derived porous carbons (BDPCs). SRAC5 sample with a BET surface area of 1945 m2 g-1 and a wide micro/mesopore size distribution, nitrogen content of 3.8 at %, and oxygen content of 15.8 at % presents the best electrochemical performance, reaching 489 F g-1 at 1 A g-1 in 6 M LiNO3 aqueous solution. A solid-state symmetric supercapacitor (SSC) device delivers a specific capacitance of 123 F g-1 at 1 A g-1 and a high energy density of 68.2 Wh kg-1 at a power density of 1 kW kg-1 with a wide voltage window of 2.0 V and maintains good cycling stability of 89.9% capacitance retention at 2A g-1 (over 5000 cycles). The outstanding electrochemical performances are ascribed to the synergistic effects of the high specific surface area, appropriate pore distribution, favorable heteroatom functional groups, and suitable electrolyte, which facilitates electrical double-layer and pseudocapacitive mechanisms for power and energy storage, respectively.

In situ transformation of ZIF-67 into hollow Co2V2O7 nanocages on graphene as a high-performance cathode for aqueous asymmetric supercapacitors

A Co₂V₂O₇/G composite with MOF-derived hollow Co₂V₂O₇ nanocages uniformly distributed on graphene exhibits excellent electrochemical performance for supercapacitors.

Electrochemically grown MnO2 nanowires for supercapacitor and electrocatalysis applications

In this study, MnO₂ nanowires are electrochemically grown over a 3D nickel foam (NF) substrate using cyclicvoltammetry at 27 °C; furthermore, their potential for applications in supercapacitors and oxygen evolution reaction (OER) is highlighted.

Li2TiO3/Ni foam composite as high-performance electrode for energy storage and conversion

Li₂TiO₃/Ni foam composites were prepared by a solid-state reaction process. They crystallized in the monoclinic Li₂TiO₃ structure with C2/c space group. SEM images show that the Li₂TiO₃ particles are monodispersed crystallites of average size 49 nm, infused into porous scaffold Ni foam. As an anode in lithium battery, the composite delivered a discharge capacity of 153 mAh g^-1 in an aqueous electrolyte and retained 95% of its initial capacity after 30 cycles. Moreover, the Li₂TiO₃/Ni foam composite as a negative electrode of pseudo-supercapacitor delivered a specific capacitance of 593 F g^-1 and retained 95% of its initial capacitance after 1000 cycles. The enhanced capacity of Li₂TiO₃/Ni composite is due to porous scaffold Ni foam, which provides high conductivity to the Li₂TiO₃ particles and high effective surface area for redox reactions. The performance of the Li₂TiO₃/Ni foam as an electrode material for both lithium-ion batteries (LIBs) and supercapacitors (SCs) shows that this composite is promising for energy storage devices.

Rational design of asymmetric supercapacitors via a hierarchical core–shell nanocomposite cathode and biochar anode

Hierarchical MnO₂ nanosheets attached on hollow NiO microspheres have been designed by a facile hydrothermal process. The core–shell structure is achieved by decorating an MnO₂ nanosheet shell on a hollow NiO sphere core. The highly hollow and porous structure exhibits a high surface area, shortened ion diffusion length, outstanding electrochemical properties (558 F g⁻¹ at a current density of 5 mA cm⁻²), and excellent cycling stability (83% retention after 5000 cycles). To further evaluate the NiO/MnO₂ core–shell composite electrode for real applications, three asymmetric supercapacitors (NiO/MnO₂//pomelo peel (PPC), NiO/MnO₂//buckwheat hull (BHC), and NiO/MnO₂//activated carbon (AC)) are assembled. The results demonstrated that NiO/MnO₂//BHC delivered a substantial energy density (20.37 W h kg⁻¹ at a power density of 133.3 W kg⁻¹) and high cycling stability (88% retention after 5000 cycles) within a broad operating potential window of 1.6 V.

Hierarchical MnO₂ nanosheets standing on hollow NiO microspheres have been designed by a facile hydrothermal process. Furthermore, asymmetric supercapacitors via core/shell NiO/MnO₂ cathode and biochar anode were assembled.

Sandwich-like NiCo layered double hydroxide/reduced graphene oxide nanocomposite cathodes for high energy density asymmetric supercapacitors

Lab-synthesized sandwich-like LDH/rGO composites were assembled into asymmetric supercapacitors exhibiting high energy density and excellent cycling stability.

Dandelion-like nickel/cobalt metal-organic framework based electrode materials for high performance supercapacitors

Metal-organic frameworks (MOFs), serving as a promising electrode material in the supercapacitors, have attracted tremendous interests in recent years. Here, through modifying the molar ratio of the Ni²⁺ and Co²⁺, we have successfully fabricated Ni-MOF and Ni/Co-MOF by a facile hydrothermal method. The Ni/Co-MOF with a dandelion-like hollow structure shows an excellent specific capacitance of 758 F g^-1 at 1 A g^-1 in the three-electrode system. Comparing with Ni-MOF, the obtained Ni/Co-MOF has a better rate capacitance (89% retention at 10 A g^-1) and cycling life (75% retention after 5000 circulations). Besides, the assembled asymmetric supercapacitor based on Ni/Co-MOF and active carbon exhibits a high specific energy density of 20.9 W h kg^-1 at the power density of 800 W kg^-1. All these results demonstrate that the mixed-metal strategy is an effective way to optimize the morphology and improve the electrochemical property of the MOFs.

Three-Dimensional Bi-Continuous Nanoporous Gold/Nickel Foam Supported MnO2 for High Performance Supercapacitors

A three-dimensional bi-continuous nanoporous gold (NPG)/nickel foam is developed though the electrodeposition of a gold–tin alloy on Ni foam and subsequent chemical dealloying of tin. The newly-designed 3D metal structure is used to anchor MnO₂ nanosheets for high-performance supercapacitors. The formed ternary composite electrodes exhibit significantly-enhanced capacitance performance, rate capability, and excellent cycling stability. A specific capacitance of 442 Fg⁻¹ is achieved at a scan rate of 5 mV s⁻¹ and a relatively high mass loading of 865 μg cm⁻². After 2500 cycles, only a 1% decay is found at a scan rate of 50 mV s⁻¹. A high power density of 3513 W kg⁻¹ and an energy density of 25.73 Wh kg⁻¹ are realized for potential energy storage devices. The results demonstrate that the NPG/nickel foam hybrid structure significantly improves the dispersibility of MnO₂ and makes it promising for practical energy storage applications.

Electrodeposited Porous Mn1.5Co1.5O₄/Ni Composite Electrodes for High-Voltage Asymmetric Supercapacitors

Mesoporous Mn_1.5Co_1.5O₄ (MCO) spinel films were prepared directly on a conductive nickel (Ni) foam substrate via electrodeposition and an annealing treatment as supercapacitor electrodes. The electrodeposition time markedly influenced the surface morphological, textural, and supercapacitive properties of MCO/Ni electrodes. The (MCO/Ni)-15 min electrode (electrodeposition time: 15 min) exhibited the highest capacitance among three electrodes (electrodeposition times of 7.5, 15, and 30 min, respectively). Further, an asymmetric supercapacitor that utilizes (MCO/Ni)-15 min as a positive electrode, a plasma-treated activated carbon (PAC)/Ni electrode as a negative electrode, and carboxymethyl cellulose-lithium nitrate (LiNO₃) gel electrolyte (denoted as (PAC/Ni)//(MCO/Ni)-15 min) was fabricated. In a stable operation window of 2.0 V, the device exhibited an energy density of 27.6 Wh·kg⁻¹ and a power density of 1.01 kW·kg⁻¹ at 1 A·g⁻¹. After 5000 cycles, the specific energy density retention and power density retention were 96% and 92%, respectively, demonstrating exceptional cycling stability. The good supercapacitive performance and excellent stability of the (PAC/Ni)//(MCO/Ni)-15 min device can be ascribed to the hierarchical structure and high surface area of the (MCO/Ni)-15 min electrode, which facilitate lithium ion intercalation and deintercalation at the electrode/electrolyte interface and mitigate volume change during long-term charge/discharge cycling.

Anion exchange strategy to synthesis of porous NiS hexagonal nanoplates for supercapacitors

A facile anion exchange strategy was applied to the synthesis of porous NiS hexagonal nanoplates (NiS HNPs) as an electrode material for supercapacitors. It was found that Na₂S concentration is a key factor to achieve porous NiS hexagonal nanoplates with well-defined architecture. Porous NiS hexagonal nanoplates exhibited a specific capacitance of 1897 F g^-1 at a current density of 1 A g^-1. NiS HNPs//activated carbon (AC) asymmetric supercapacitor (ASC) shows a long cycle lifespan (about 100% capacity retention after 4000 cycles at a current density of 3 A g^-1) with a maximum energy density of 11.6 Wh kg^-1 at a large loading mass of about 30 mg. Impressively, two NiS HNPs//AC ASCs in series could light up a red LED for about 30 min. The remarkable electrochemical performance of NiS HNPs is ascribed to their unique hierarchical porous architectures. The anion exchange method is a facile and versatile strategy for the synthesis of metal sulfides with high performance for energy storage.

New Supercapacitors Based on the Synergetic Redox Effect between Electrode and Electrolyte

Redox electrolytes can provide significant enhancement of capacitance for supercapacitors. However, more important promotion comes from the synergetic effect and matching between the electrode and electrolyte. Herein, we report a novel electrochemical system consisted of a polyanilline/carbon nanotube composite redox electrode and a hydroquinone (HQ) redox electrolyte, which exhibits a specific capacitance of 7926 F/g in a three-electrode system when the concentration of HQ in H₂SO₄ aqueous electrolyte is 2 mol/L, and the maximum energy density of 114 Wh/kg in two-electrode symmetric configuration. Moreover, the specific capacitance retention of 96% after 1000 galvanostatic charge/discharge cycles proves an excellent cyclic stability. These ultrahigh performances of the supercapacitor are attributed to the synergistic effect both in redox polyanilline-based electrolyte and the redox hydroquinone electrode.