Je Moon Yun

Vertically stacked bilayer CuCo2O4/MnCo2O4 heterostructures on functionalized graphite paper for high-performance electrochemical capacitors

Cobaltite systems with spinel structures are promising cathode materials for next-generation high-performance electrochemical capacitors because of their high electrochemical stability. However, increasing the mass loading of active materials without sacrificing the geometry of the nanostructures remains a challenge. In this study, we propose vertically stacked bilayer spinel heterostructures constructed from hierarchical CuCo2O4/MnCo2O4 on graphite paper as highly capable supercapacitor electrodes. A two-step hydrothermal method with post annealing treatment is used in the preparation of the heterostructures. The CuCo2O4/MnCo2O4 electrode delivers a remarkable specific capacitance of 1434 F g−1 at 0.5 A g−1, considerable high-rate capability (810 F g−1 at 15 A g−1), and an excellent cycling stability, maintaining 81.4% at 10 A g−1 after 5000 cycles. An electrochemical capacitor device operating at 1.6 V is also constructed using CuCo2O4/MnCo2O4 and graphene as positive and negative electrodes, respectively. The device shows a high energy density of 42.1 W h kg−1 at a power density of 400 W kg−1, as well as good cycling stability (88.4% retention after 10 000 cycles). The concept of stacking heteronanostructures can potentially enrich the electrochemical performance of metal oxides for next-generation electrochemical capacitors.

Facile synthesis of manganese carbonate quantum dots/Ni(HCO3)2–MnCO3 composites as advanced cathode materials for high energy density asymmetric supercapacitors

We have developed a high performance supercapacitor cathode electrode composed of well dispersed MnCO3 quantum dots (QDs, ∼1.2 nm) decorated on nickel hydrogen carbonate–manganese carbonate (Ni(HCO3)2–MnCO3) hedgehog-like shell@needle (MnCO3 QDs/NiH–Mn–CO3) composites directly grown onto a 3D macro-porous nickel foam as a binder-free supercapacitor electrode by a facile and scalable hydrothermal method. The MnCO3 QDs/NiH–Mn–CO3 composite electrode exhibited a remarkable maximum specific capacitance of 2641.3 F g−1 at 3 A g−1 and 1493.3 F g−1 at 15 A g−1. Moreover, the asymmetric supercapacitor with MnCO3 QDs/NiH–Mn–CO3 composites as the positive electrode and graphene as the negative electrode showed an energy density of 58.1 W h kg−1 at a power density of 900 W kg−1 as well as excellent cycling stability with 91.3% retention after 10 000 cycles, which exceeded the energy densities of most previously reported nickel or manganese oxide/hydroxide-based asymmetric supercapacitors. The ultrahigh capacitive performance is attributed to the presence of the high surface area core–shell nanostructure, the well dispersed MnCO3 quantum dots, and the high conductivity of MnCO3 quantum dots as well as the synergetic effect between multiple transition metal ions. The superior supercapacitive performance of the MnCO3 QDs/NiH–Mn–CO3 composites makes them promising cathode materials for high energy density asymmetric supercapacitors.

High volumetric energy density annealed-MXene-nickel oxide/MXene asymmetric supercapacitor

A Ti3C2Tx MXene electrode decorated with NiO nanosheets was synthesized by a facile and cost-effective hydrothermal method. The NiO nanosheets were grown and immobilized on the carbon-supported TiO2 layer which was derived from Ti3C2Tx-MXene during a thermal annealing process. An electrode based on the NiO-grown derived-TiO2/C-Ti3C2Tx-MXene nanocomposite (Ni-dMXNC) exhibited a remarkable maximum specific capacity of 92.0 mA h cm−3 at 1 A g−1 and 53.9 mA h cm−3 at 10 A g−1. Furthermore, an asymmetric supercapacitor (ASC) device composed of Ni-dMXNC as the positive electrode and Ti3C2Tx MXene as the negative electrode was demonstrated to be better with a high energy density of 1.04 × 10−2 W h cm−3 at a power density of 0.22 W cm−3, and cycling stability with 72.1% retention after 5000 cycles, compared to ASCs using previously reported Ti3C2Tx MXene materials. The enhanced capacitive performance is attributed to the newly formed high-surface-area multilayers of the Ni-dMXNC architecture, the active surface of NiO layer, and a favourable synergetic behaviour of the Ti3C2Tx MXene negative electrode.

3D yolk–shell NiGa2S4 microspheres confined with nanosheets for high performance supercapacitors

Recent advances in the development of two-dimensional transition-metal chalcogenides (2D TMCs) have opened up new avenues for supercapacitor applications. However, they still suffer from limited specific capacitance and poor rate capability due to their poor interfacial properties and simple geometry. Here, we propose a facile strategy for the synthesis of yolk–shell NiGa2S4 microspheres comprising crumpled nanosheets supported on nickel foam. The robust structure not only highly facilitates the electron and charge transportation but also efficiently alleviates the volume expansion during redox reactions, contributing to excellent electrochemical behaviors in terms of specific capacitance and rate capability. Significantly, an asymmetric supercapacitor based on the prepared NiGa2S4 as the positive electrode and N,S-codoped graphene/Fe2O3 (N,S-G/Fe2O3) as the negative electrode delivers a high energy density of 43.6 W h kg−1 at a power density of 961 W kg−1 and retains an energy density of 22.2 W h kg−1 even at 15974 W kg−1. These impressive results may provide a new perspective to develop high energy and power density storage systems for practical applications.

Enhanced electrochemical activity of perforated graphene in nickel-oxide-based supercapacitors and fabrication of potential asymmetric supercapacitors

We synthesize a hierarchically porous electrode material with a high specific capacity composed of nickel oxide (NiO) nanosheets and perforated graphene (PG) sheets grown on a three-dimensional (3D) macroporous nickel foam via a facile hydrothermal method. Employing perforated graphene instead of non-perforated graphene greatly improves the electrochemical performance of the composite by increasing the specific surface area of the NiO/PG composite owing to the small perforations in the graphene and by improving the mechanical strength and electrical conductivity of NiO due to the graphene covering layer. The PG also provides (a) the accessibility and diffusion of ions onto NiO and PG surfaces through the perforations; (b) the electrolyte cages in the spaces below the perforations to allow ion-reversible adsorption to the inner surface of the graphene; (c) the inevitable edge defects around the holes causing reactivity with ions. After assembling an asymmetric supercapacitor coin cell composed of NiO/PG as the positive electrode, a separator, PG as the negative electrode, and a 1 M KOH electrolyte, the coin cell exhibits a high energy density of 57.8 W h kg−1 at a power density of 1030.9 W kg−1 and excellent cycling stability (82.1%) after 10 000 cycles.

Polycrystalline and Mesoporous 3-D Bi2O3 Nanostructured Negatrodes for High-Energy and Power-Asymmetric Supercapacitors: Superfast Room-Temperature Direct Wet Chemical Growth

Superfast (≤10 min) room-temperature (300 K) chemical synthesis of three-dimensional (3-D) polycrystalline and mesoporous bismuth(III) oxide (Bi2O3) nanostructured negatrode (as an abbreviation of negative electrode) materials, viz., coconut shell, marigold, honey nest cross section and rose with different surface areas, charge transfer resistances, and electrochemical performances essential for energy storage, harvesting, and even catalysis devices, are directly grown onto Ni foam without and with poly(ethylene glycol), ethylene glycol, and ammonium fluoride surfactants, respectively. Smaller diffusion lengths, caused by the involvement of irregular crevices, allow electrolyte ions to infiltrate deeply, increasing the utility of inner active sites for the following electrochemical performance. A marigold 3-D Bi2O3 electrode of 58 m2·g-1 surface area has demonstrated a specific capacitance of 447 F·g-1 at 2 A·g-1 and chemical stability of 85% even after 5000 redox cycles at 10 A·g-1 in a 6 M KOH electrolyte solution, which were higher than those of other morphology negatrode materials. An asymmetric supercapacitor (AS) device assembled with marigold Bi2O3 negatrode and manganese(II) carbonate quantum dots/nickel hydrogen-manganese(II)-carbonate (MnCO3QDs/NiH-Mn-CO3) positrode corroborates as high as 51 Wh·kg-1 energy at 1500 W·kg-1 power and nearly 81% cycling stability even after 5000 cycles. The obtained results were comparable or superior to the values reported previously for other Bi2O3 morphologies. This AS assembly glowed a red-light-emitting diode for 20 min, demonstrating the scientific and industrial credentials of the developed superfast Bi2O3 nanostructured negatrodes in assembling various energy storage devices.

Highly porous nitrogen-doped carbon for superior electric double-layer capacitors

In recent years, the research of doping heteroatoms in a carbon framework for supercapacitive electrodes has drawn tremendous attention due to the highly active electrochemical performance characteristics of the resulting materials. Here, we present a method to synthesize highly porous nitrogen-doped carbon nanomaterials derived from a polyvinylpyrrolidone (PVP) material via a relatively low-temperature simultaneous activation/calcination process. PVP fine powder was mixed with sodium hydroxide as a carbon-activation agent and calcined at a relatively low temperature of 600 °C for one hour under a nitrogen atmosphere. By this process, we obtained a highly porous nitrogen-doped carbon material, possessing a specific surface area of 2400 m2 g−1, which was formed with an amorphous and graphitic structure incorporating ultrathin large sheets. The resultant material displays an excellent specific capacitance (478 F g−1 at 1 A g−1) and a high retention rate of 99.6% after 10 000 cycles at 10 A g−1. The symmetric supercapacitor exhibits high energy densities of 14.2 W h kg−1 and 5.7 W h kg−1 at power densities of 720 W kg−1 and 6035 W kg−1, respectively.

Silver particle-loaded nickel oxide nanosheet arrays on nickel foam as advanced binder-free electrodes for aqueous asymmetric supercapacitors

Conductive metal loading is an efficient approach for enhancing the electric conductivity of redox-active transition-metal oxide electrodes. In this study, Ag particle-loaded NiO/nickel foam (Ag-NiO/NF) composites were fabricated using a green chemistry method. The anchored Ag particles standing on the surface of NiO nanosheet arrays helped improve the electrical conductivity and were thus beneficial for supercapacitors (SCs). The as-developed Ag-NiO/NF electrode delivered a specific capacitance of 1254.9 F g−1 at the current density of 1 A g−1, which is higher than that of NiO/NF electrode (946.3 F g−1). Using such Ag-NiO/NF as a positive electrode, we further fabricated aqueous asymmetric SC devices with reduced graphene oxide as a negative electrode. Owing to the efficient electron transport and short ion diffusion paths in the designed electrode, the devices exhibited a high specific capacitance of 97.9 F g at 1 A g−1 with a high energy density of 26.7 W h kg−1 at a power density of 1017.1 W kg−1 and great cyclic stability.