Yongfu Tang

High-performance asymmetric supercapacitors based on multilayer MnO2 graphene oxide nanoflakes and hierarchical porous carbon with enhanced cycling stability

In this work, MnO(2)/GO (graphene oxide) composites with novel multilayer nanoflake structure, and a carbon material derived from Artemia cyst shell with genetic 3D hierarchical porous structure (HPC), are prepared. An asymmetric supercapacitor has been fabricated using MnO(2)/GO as positive electrode and HPC as negative electrode material. Because of their unique structures, both MnO(2)/GO composites and HPC exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high voltage range of 0-2 V in aqueous electrolyte, which exhibits maximum energy density of 46.7 Wh kg(-1) at a power density of 100 W kg(-1) and remains 18.9 Wh kg(-1) at 2000 W kg(-1). Additionally, such device also shows superior long cycle life along with ∼100% capacitance retention after 1000 cycles and ∼93% after 4000 cycles.

Synthesis of Capsule-like Porous Hollow Nanonickel Cobalt Sulfides via Cation Exchange Based on the Kirkendall Effect for High-Performance Supercapacitors.

To construct a suitable three-dimensional structure for ionic transport on the surface of the active materials for a supercapacitor, porous hollow nickel cobalt sulfides are successfully synthesized via a facile and efficient cation-exchange reaction in a hydrothermal process involving the Kirkendall effect with γ-MnS nanorods as a sacrificial template. The formation mechanism of the hollow nickel cobalt sulfides is carefully illustrated via the tuning reaction time and reaction temperature during the cation-exchange process. Due to the ingenious porous hollow structure that offers a high surface area for electrochemical reaction and suitable paths for ionic transport, porous hollow nickel cobalt sulfide electrodes exhibit high electrochemical performance. The Ni(1.77)Co(1.23)S4 electrode delivers a high specific capacity of 224.5 mAh g(-1) at a current density of 0.25 A g(-1) and a high capacity retention of 87.0% at 10 A g(-1). An all-solid-state asymmetric supercapacitor, assembled with a Ni(1.77)Co(1.23)S4 electrode as the positive electrode and a homemade activated carbon electrode as the negative electrode (denoted as NCS//HMC), exhibits a high energy density of 42.7 Wh kg(-1) at a power density of 190.8 W kg(-1) and even 29.4 Wh kg(-1) at 3.6 kW kg(-1). The fully charged as-prepared asymmetric supercapacitor can light up a light emitting diode (LED) indicator for more than 1 h, indicating promising practical applications of the hollow nickel cobalt sulfides and the NCS//HMC asymmetric supercapacitor.

All-solid-state high performance asymmetric supercapacitors based on novel MnS nanocrystal and activated carbon materials.

All-solid-state high-performance asymmetric supercapacitors (ASCs) are fabricated using γ-MnS as positive electrode and porous eggplant derived activated carbon (EDAC) as negative electrode with saturated potassium hydroxide agar gel as the solid electrolyte. The laminar wurtzite nanostructure of γ-MnS facilitates the insertion of hydroxyl ions into the interlayer space, and the manganese sulfide nanowire offers electronic transportation channels. The size-uniform porous nanostructure of EDAC provides a continuous electron pathway as well as facilitates short ionic transportation pathways. Due to these special nanostructures of both the MnS and the EDAC, they exhibited a specific capacitance of 573.9 and 396 F g−1 at 0.5 A g−1, respectively. The optimized MnS//EDAC asymmetric supercapacitor shows a superior performance with specific capacitance of 110.4 F g−1 and 89.87% capacitance retention after 5000 cycles, a high energy density of 37.6 Wh kg−1 at a power density of 181.2 W kg−1 and remains 24.9 Wh kg−1 even at 5976 W kg−1. Impressively, such two assembled all-solid-state cells in series can light up a red LED indicator for 15 minutes after fully charged. These impressive results make these pollution-free materials promising for practical applications in solid aqueous electrolyte-based ASCs.

Monolayer Nickel Cobalt Hydroxyl Carbonate for High Performance All-Solid-State Asymmetric Supercapacitors

The emergence of atomically thick nanolayer materials, which feature a short ion diffusion channel and provide more exposed atoms in the electrochemical reactions, offers a promising occasion to optimize the performance of supercapacitors on the atomic level. In this work, a novel monolayer Ni-Co hydroxyl carbonate with an average thickness of 1.07 nm is synthesized via an ordinary one-pot hydrothermal route for the first time. This unique monolayer structure can efficiently rise up the exposed electroactive sites and facilitate the surface dependent electrochemical reaction processes, and thus results in outstanding specific capacitance of 2266 F g(-1). Based on this material, an all-solid-state asymmetric supercapacitor is developed adopting alkaline PVA (poly(vinyl alcohol)) gel (PVA/KOH) as electrolyte, which performs remarkable cycling stability (no capacitance fade after 19 000 cycles) together with promising energy density of 50 Wh kg(-1) (202 μWh cm(-2)) and high power density of 8.69 kW kg(-1) (35.1 mW cm(-2)). This as-assembled all-solid-state asymmetric supercapacitor (AASC) holds great potential in the field of portable energy storage devices.

Synthesis of graphene oxide anchored porous manganese sulfide nanocrystals via the nanoscale Kirkendall effect for supercapacitors

Graphene oxide (GO) anchored porous manganese sulfide nanocrystals (MnS/GO-NH3) were obtained via a facile hydrothermal method based on the Kirkendall effect. The honeycomb-like manganese sulfide nanocrystals (40–80 nm) and the three-dimensional sandwich structure endow the MnS/GO-NH3 with high supercapacitive performance when it was used as a supercapacitor material. The MnS/GO-NH3 electrode exhibits high specific capacitance (390.8 F g−1 at 0.25 A g−1), high rate capacity (78.7% retention at 10 A g−1) and stable cycle life (81.0% retention after 2000 cycles), which are superior to those of GO anchored MnS floccules (MnS/GO) and manganese hydroxide (Mn(OH)2/GO). As a novel material for supercapacitors, the charge–discharge mechanism of the MnS/GO-NH3 composite is proposed via detailed investigation. Asymmetric supercapacitors, assembled with MnS/GO-NH3 as the positive material and activated carbon as the negative electrode, reveal a high specific capacitance (73.63 F g−1), a high energy density of 14.9 W h kg−1 at 66.5 W kg−1 and even 12.8 W h kg−1 at a high power density of 4683.5 W kg−1.

Hybridized Phosphate with Ultrathin Nanoslices and Single Crystal Microplatelets for High Performance Supercapacitors

A novel hybridized phosphate is developed through a mild hydrothermal method to construct high performance asymmetric supercapacitor. Single layered (Ni,Co)3(PO4)2·8H2O nanoslices (∼1 nm) and single crystal (NH4)(Ni,Co)PO4·0.67H2O microplatelets are obtained through a template sacrificial method and dissolution recrystallization approach respectively in one step. This unique hybridized structure delivers a maximum specific capacitance of 1128 F g−1 at current density of 0.5 A g−1. The asymmetric supercapacitor (ASC) based on the hybrid exhibits a high energy density of 35.3 Wh kg−1 at low power density, and still holds 30.9 Wh kg−1 at 4400 W kg−1. Significantly, the ASC manifests very high cycling stability with 95.6% capacitance retention after 5000 cycles. Such excellent electrochemical performance could be attributed to the synergistic effect of the surface redox reaction from the ultrathin nanoslices and ion intercalation from the single crystal bulk structure. This material represents a novel kind of electrode material for the potential application in supercapacitors.

Molybdenum Carbide-Derived Chlorine-Doped Ordered Mesoporous Carbon with Few-Layered Graphene Walls for Energy Storage Applications

In this work, we propose a one-step process to realize the in situ evolution of molybdenum carbide (Mo2C) nanoflakes into ordered mesoporous carbon with few-layered graphene walls (OMG) by chloridization and self-organization, and simultaneously the Cl-doping of OMG (OMG-Cl) by modulating chloridization and annealing processes is fulfilled. Benefiting from the improvement of electroconductivity induced by Cl-doping, together with large specific surface area (1882 cm2 g-1) and homogeneous pore structures, as anode of lithium ion batteries, OMG-Cl shows remarkable charge capacity of 1305 mA h g-1 at current rate of 50 mA g-1 and fast charge-discharge rate within dozens of seconds (a charge time of 46 s), as well as retains a charge capacity of 733 mA h g-1 at a current rate of 0.5 mA g-1 after 100 cycles. Furthermore, as a promising electrode material for supercapacitors, OMG-Cl holds the specific capacitances of 250 F g-1 in 1 M H2SO4 solution and 220 F g-1 at a current density of 0.5 A g-1 in 6 M KOH solution, which are ∼40% and 20% higher than those of undoped OMG electrode, respectively. The high capacitive performance of OMG-Cl material can be due to the additional fast Faradaic reactions induced from Cl-doping species.

Synthesis of peanut-like hierarchical manganese carbonate microcrystals via magnetically driven self-assembly for high performance asymmetric supercapacitors

To construct a suitable structure for both electronic conduction and ionic transport towards supercapacitors, peanut-like hierarchical manganese carbonate (MnCO3) microcrystals assembled with floss-like nanowires are synthesized via a hydrothermal process and primarily used as an active material for supercapacitors. The formation mechanism is illustrated by means of a dissolution–recrystallization process and magnetically driven self-assembly. The electrode with peanut-like hierarchical MnCO3 microcrystals exhibits a high specific capacitance of 293.7 F g−1 and a superior cycle stability of 71.5% retention after 6000 cycles, which are higher than those of the reported Mn-based active materials in alkaline electrolytes. The asymmetric supercapacitor, assembled with the peanut-like MnCO3 electrode as the positive electrode and a home-made porous carbon electrode as the negative electrode, exhibits an energy density of 14.7 W h kg−1 at a power density of 90.2 W kg−1 and an energy density of up to 11.0 W h kg−1 at 3.3 kW kg−1. An as-assembled all-solid-state supercapacitor series can light up a LED indicator for 10 min, indicating a promising practical application of peanut-like MnCO3 microcrystals.

A highly ordered multi-layered hydrogenated TiO2-II phase nanowire array negative electrode for 2.4 V aqueous asymmetric supercapacitors with high energy density and long cycle life

To increase the active material loading coupled with the improvement of electrical conductivity and ionic transport in the TiO2 based negative electrode for 2.4 V aqueous asymmetric supercapacitors (ASCs), a highly ordered binder-free multi-layered hydrogenated TiO2-II phase nanowire array (ML-HTO) electrode is synthesized via a multi-step method. Due to the highly ordered scaffold-like multi-layered structure, a hydrogenated TiO2-II phase, small TiO2 grains (4.4 nm), high oxygen vacancy (Ti3+) concentration and functionalized hydrophilic surfaces, endowing the electrode with high electrical conductivity (low band gap of 2.39 eV), more positive flat-band potential (−0.35 V) and a high Na+ diffusion coefficient (3.08 × 10−8 cm2 s−1), the as-prepared ML-HTO electrode with high active material loading (3.0 mg cm−2) exhibits a high areal specific capacitance (710.7 mF cm−2), which is much higher than that of a pristine TiO2 nanowire array (TO) electrode (28.3 mF cm−2) and the single layered hydrogenated TiO2 nanowire array (HTO) electrode (258.7 mF cm−2). Consequently, a 2.4 V aqueous ASC was firstly assembled with the ML-HTO electrode as the negative electrode, presenting a high energy density of 90.3 W h kg−1 at 349.0 W kg−1, as well as a high volumetric energy density of 9.6 mW h cm−3 at 61.0 mW cm−3, which is much higher than most of previously reported aqueous ASCs. The capacitance retention of the as-prepared MnO2//ML-HTO ASC is 65.7% after 10 000 cycles, demonstrating its long cycle life.

Comparative study on three commercial carbons for supercapacitor applications

Three commercial carbon materials for supercapacitor were investigated by physicochemical characterization, electrochemical measurements and surface treatment to explore the effects of specific surface area, electrolyte and surface functional groups on the specific capacitance, charge storage mode and high rate performance of carbon materials. Results indicate that the specific surface area of carbon material plays dominate role in the specific capacitance. The electrolytes have remarkable effects on the specific capacitance and high rate performance. Investigation of HNO3 treated Vulcan XC-72 carbon material reveals that the treatment can increase the specific surface area and surface functional groups, which observably improve the specific capacitance of the XC-72 carbon material. The surface functional groups contribute to the pseudo-capacitance of the carbon material.