Designing Ultrasmall Carbon Nanospheres with Tailored Sizes and Textural Properties for High-Rate High-Energy Supercapacitors.

The present work demonstrates the efficient design of ultrasmall porous carbon nanospheres with tailored sizes (5-40 nm in diameter) and optimized intrasphere textural properties for high-rate high-energy supercapacitor application. The carbon nanospheres are synthesized via a miniemulsion polymerization technique followed by KOH activation. It is shown that dual-step activation renders enlarged intrasphere micropores/mesopores, facilitating enhanced ion transports. Meanwhile, a decrease in nanosphere size from 40 to 5 nm significantly improves the rate performance, demonstrating the pronounced size effects due to enhanced intrasphere ion transport. The optimum dual-step-activated carbon nanosphere sample with an average sphere size of 5 nm, ACNS5-2, shows the high specific capacitances along with outstanding high-rate capabilities in both aqueous (272 F g-1 at 0.5 A g-1 and 81.6% of retention at 200 A g-1) and EMIMBF4 (223 F g-1 at 0.5 A g-1 and 67.2% of retention at 100 A g-1) electrolytes in symmetrical two-electrode cells. In EMIMBF4, ACNS5-2 displays a high energy density of 48 Wh kg-1 at a high power density of 14 kW kg-1, suggesting excellent energy storage efficiency. Moreover, the performance of ACNS5-2 competes well with or is superior to some best-performing porous carbon-based materials reported in the literature for supercapacitor applications even at lowered temperatures (at -20 °C: 150 F g-1 at 0.5 A g-1 with a capacitance retention of 64% at 10 A g-1) and high mass loading (8 mg cm-2: 205 F g-1 at 0.5 A g-1 with a capacitance retention of 64.5% at 20 A g-1). Our results, combined with structure-performance relationships, offer valuable guidelines for the rational design of carbon nanomaterials of optimum supercapacitive performances.

Macromolecular Polyethynylbenzonitrile Precursor-Based Porous Covalent Triazine Frameworks for Superior High-Rate High-Energy Supercapacitors.

Porous covalent triazine framework (CTF)-based carbon materials have gained increasing attention in energy-storage applications because of their tunable structure, high chemical stability, and rich heteroatom contents. However, CTFs have thus far been exclusively synthesized from small-molecular precursors and generally show unsatisfactory supercapacitive performance. We report herein the construction of a novel range of CTFs of significantly improved supercapacitive performance from polyethynylbenzonitrile (PEBN) as a unique macromolecular precursor for the first time by ionothermal synthesis. CTF-800 synthesized at the optimized condition (800 °C; ZnCl2/PEBN mass ratio of 3:1) shows a nanosheet-like morphology with a high yield (∼90%), high nitrogen content (>5.8%), high specific surface area (1954 m2 g-1), and optimized micropore to meso/macropore surface area ratio (42:58). As the electrode material for supercapacitor application, CTF-800 exhibits a high specific capacitance of 628 F g-1 at 0.5 A g-1, high-rate performance (71% of capacitance retention at 50 A g-1), and excellent cyclic stability (96% of capacitance retention over 20 000 cycles) in a three-electrode system with aqueous 1 M H2SO4 electrolyte. Symmetric supercapacitor devices have been further fabricated with CTF-800 in aqueous 1 M H2SO4, [EMIM][BF4], and LiPF6 electrolytes separately. The device with the aqueous electrolyte shows the highest capacitance of 448 F g-1 (at 0.5 A g-1) and a high energy density of 15.5 W h kg-1. The devices with [EMIM][BF4] and LiPF6 electrolytes exhibit exceptional energy densities of 70 and 78 W h kg-1, respectively, and retain energy densities of 41 and 45 W h kg-1, respectively, even at the high power density of 15 000 W kg-1, confirming their high-rate high-energy performance. Meanwhile, the device with [EMIM][BF4] electrolyte has also been demonstrated to operate well at various temperatures ranging from -20 to 60 °C with remarkable energy-storage performance.

Utilizing Waste Thermocol Sheets and Rusted Iron Wires to Fabricate Carbon-Fe3 O4 Nanocomposite-Based Supercapacitors: Turning Wastes into Value-Added Materials.

The synthesis of porous activated carbon (specific surface area=1883 m2 g-1 ), Fe3 O4 nanoparticles, and carbon-Fe3 O4 (C-Fe3 O4 ) nanocomposites from local waste thermocol sheets and rusted iron wires is demonstrated herein. The resulting carbon, Fe3 O4 nanoparticles, and C-Fe3 O4 composites are used as electrode materials for supercapacitor applications. In particular, C-Fe3 O4 composite electrodes exhibit a high specific capacitance of 1375 F g-1 at 1 A g-1 and longer cyclic stability with 98 % capacitance retention over 10 000 cycles. Subsequently, an asymmetric supercapacitor, namely, C-Fe3 O4 ∥Ni(OH)2 /carbon nanotube device, exhibits a high energy density of 91.1 Wh kg-1 and a remarkable cyclic stability, with 98 % capacitance retention over 10 000 cycles. Thus, this work has important implications not only for the fabrication of low-cost electrodes for high-performance supercapacitors, but also for the recycling of waste thermocol sheets and rusted iron wires for value-added reuse.

Anchoring Ultrafine ZnFe2O4/C Nanoparticles on 3D ZnFe2O4 Nanoflakes for Boosting Cycle Stability and Energy Density of Flexible Asymmetric Supercapacitor.

Heterostructure-based metal oxide thin films are recognized as the leading material for new generation, high-performance, stable, and flexible supercapacitors. However, morphologies, like nanoflakes, nanotubes, nanorods, and so forth, have been found to suffer from issues related to poor cycle stability and energy density. Thus, to circumvent these problems, herein, we have developed a low-cost, high surface area, and environmentally benign self-assembled ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructure electrode via anchoring ZnFe2O4 and carbon nanoparticles using an in situ biomediated green rotational chemical bath deposition approach for the first time. The synthesized ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructure thin films demonstrate an excellent specific capacitance of 1884 F g-1 at a current density of 5 mA cm-2. Additionally, all solid-state flexible asymmetric supercapacitor devices were designed on the basis of ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructures as the negative electrode and reduced graphene oxide and energy density of 81 Wh kg-1 at a power density of 3.9 kW kg-1. Similarly, the asymmetric device exhibits ultralong cycle stability of 35 000 cycles by losing only 2% capacitance. The excellent performance of the device is attributed to the self-assembled organization of the heterostructures. Moreover, the in situ biomediated green strategy is also applicable for the synthesis of other metal oxide and carbon-based heterostructure electrodes.

Contact angle measurements: a preliminary diagnostic tool for evaluating the performance of ZnFe2O4 nano-flake based supercapacitors.

Contact angle measurements (surface wettability) of the electrolytes (1 M KOH, NaOH and LiOH) and their combination (1 M 1 : 1 v/v LiOH + KOH, NaOH + KOH and LiOH + NaOH) in contact with ZnFe2O4 nano-flake based electrodes is used as an empirical diagnostic tool to pre-evaluate the performance of a supercapacitor prior to actual fabrication of the device.