2D quasi-ordered nitrogen-enriched porous carbon nanohybrids for high energy density supercapacitors

Sandwich construction of chitosan/reduced graphene oxide composite as additive-free electrode material for high-performance supercapacitors

The sandwich construction of chitosan (CS)/reduced graphene oxide (rGO) composite was synthesized through microwave-assisted hydrothermal method without further carbonization or activation process (CRG). CS homogeneous attached between the rGO slice sheet and improve the dispersion of CRG effectively, which can increase its specific surface area with hierarchical porous structure. Dehydration condensation occurred between CS and rGO, forming NHCO groups that can promote the wettability and conductivity of the composites. CRG exhibited improved degree of order and reduced graphitization defect, N-5 and OI groups were the dominant nitrogen and oxygen-containing groups. When used as additive-free electrode, CRG exhibited a high specific capacitance of 274 F g^-1 at the current density of 0.5 A g^-1 with good rate performance in a three-electrode system using 1 M H₂SO₄ electrolyte. Solid-state supercapacitor device was assembled with CRG electrode and lignin hydrogel electrolytes, high gravimetric energy densities of 8.4 Wh kg^-1 at the power density of 50 W kg^-1 was achieved.

N/O co-doped porous interconnected carbon nanosheets from the co-hydrothermal treatment of soybean stalk and nickel nitrate for high-performance supercapacitors

Porous interconnected carbon nanosheets (PICNs) with high electrochemical performance were prepared by doping urea and a co-hydrothermal precursor derived from soybean stalk (SS) and nickel nitrate. The specific surface area and average pore diameter of the as-synthesized PICNs are 2226.29 m2 g-1 and 1.89 nm, and their N and O contents are 5.08% and 9.4%, respectively, which is beneficial for increasing pseudocapacitance. Furthermore, the doping of the metal Ni increases the graphitization degree of the PICNs and promotes the conversion of pyridine-N to graphitized-N. Therefore, the PICNs possess a high specific capacitance of 407 F g-1 at a current density of 0.5 A g-1, a high capacitance retention of 78.62% even at 20 A g-1, and an outstanding cycling stability (over 93% retention rate after 10,000 charge/discharge cycles). Moreover, an energy density of 36.11 W h kg-1 is achieved at a power density of 517.8 W kg-1 during a two-electrode system test, and a retention rate of 87.5% is obtained after 10,000 cycles. This co-hydrothermal treatment as well as nitrogen-doping approach for preparing porous interconnected carbon from SS not only represents an alternative strategy for carbon-based supercapacitor materials but also provides a new option for the utilization of waste SS.

Integrating ultrathin and modified NiCoAl-layered double-hydroxide nanosheets with N-doped reduced graphene oxide for high-performance all-solid-state supercapacitors

As one class of important electroactive materials, layered double-hydroxide (LDH) nanostructures show great promise for application in the fields of electrocatalysis, secondary batteries, and supercapacitors. Nonetheless, the synthesis of ultrathin multi-metallic-based LDH nanosheets or related nanohybrids (NHs) remains a challenge, and their supercapacitive performances need to be further improved for achieving both high energy and high power densities. Herein, ultrathin and modified NiCoAl-LDH (m-LDH) nanosheets and N-doped reduced graphene oxide (NRG) NHs were synthesized by an alkaline etching of pre-synthesized ultrathin NiCoAl-LDH nanosheets, followed by electrostatic assembly with NRG. The alkaline etching could efficiently modulate the chemical states of the active Ni/Co elements and create more oxygen vacancies in the m-LDH nanosheets. After integrating m-LDH with NRG, the strong interaction or efficient electronic coupling of those two constituents further mediated the surface electronic structure of the m-LDH nanosheets, improving the interfacial charge transport and offering more available electrochemical active sites for surface faradaic reactions. Thus, the obtained m-LDH/NRG NHs manifested greatly enhanced specific capacitance (1877.0 F g^-1 at 1 A g^-1), which was superior to that of pure m-LDH and most other reported electrode materials. Moreover, using such NHs as the positive electrode and activated carbon as the negative electrode, a fabricated asymmetric all-solid-state supercapacitor device delivered a high energy density of 19.9 W h kg^-1 at a power density of 319.8 W kg^-1 together with good cycling stability (76.5% capacitance retention after over 5000 cycles). Remarkably, even at a power density up to 1637.5 W kg^-1, it could still retain an energy density of 13.1 W h kg^-1, superior to recently reported asymmetric supercapacitors devices based on Ni, Co, and other transition metal compounds.

Design and fabrication of polypyrrole/expanded graphite 3D interlayer nanohybrids towards high capacitive performance

Polypyrrole/expanded graphite (PPy/EG) nanohybrids, with a hierarchical structure of a three dimensional EG framework with a thick PPy coating layer, have been synthesized via a vacuum-assisted intercalation in situ oxidation polymerization method. In the synthesis, pyrrole monomers were intercalated into the irregular pores of EG with the assistance of a vacuum pump. Subsequently, the intercalated pyrrole monomers assembled on both sides of the EG nanosheets and formed PPy by an in situ polymerization method. As electrode materials, the typical PPy/EG10 sample with an EG content of 10% had a high specific capacitance of 454.3 F g⁻¹ and 442.7 F g⁻¹ (1.0 A g⁻¹), and specific capacitance retention rate of 75.9% and 73.3% (15.0 A g⁻¹) in 1 M H₂SO₄ and 1 M KCl electrolytes, respectively. The two-electrode symmetric supercapacitor showed a high energy density of 47.5 W h kg⁻¹ at a power density of 1 kW kg⁻¹, and could retain superb stability after 2000 cycles. The unique self-supporting structure feature and homogeneous PPy nanosphere coating combined the contributions of electrochemical double layer capacitance and pseudo-capacitance, which made the nanohybrids an excellent electrode material for high performance energy storage devices.

Polypyrrole/expanded graphite nanohybrids with a hierarchical structure were synthesized as electrode materials, and showed outstanding energy storage performance.

High-performance asymmetric supercapacitor based on hierarchical nanocomposites of polyaniline nanoarrays on graphene oxide and its derived N-doped carbon nanoarrays grown on graphene sheets

Activated carbon (AC), as a material for asymmetric supercapacitor (ASC), is the most widely used as negative electrode. However, AC has some electrode kinetic problems which are corresponded to inner-pore ion transport that restrict the maximum specific energy and power that can be attained in an energy storage system. Therefore, it is an important topic for researchers to extend the carbonaceous material with qualified structure for negative electrode supercapacitor. In this work, novel promoted ASC have been fabricated using nanoarrays of polyaniline grown on graphene oxide sheets (PANI-GO) as positive electrode and also, carbonized nitrogen-doped carbon nanoarrays grown on the surface of graphene (CPANI-G) as negative electrode. The porous structure of the as-synthesized CPANI-G can enlarge the specific surface area and progress ion transport into the interior of the electrode materials. From the other point of view, nitrogen doping can impressively improve the wettability of the carbon surface in the electrolyte and upgrade the specific capacitance by a pseudocapacitive effect. Because of the high specific capacitance and distinguished rate performance of PANI-GO and CPANI-G and moreover, the synergistic effects of the two electrodes with the optimum potential window, the ASC display excellent electrochemical performances. In comparison with the symmetric cell based on PANI-GO (40 Wh kg^-1), the fabricated PANI-GO//CPANI-G ASC exhibits a remarkably enhanced maximum energy density of 52 Wh kg^-1. Furthermore, ASC electrode exhibits excellent cycling durability, with 90.3% specific capacitance preserving even after 5000 cycles. These admirable results show great possibilities in developing energy storage devices with high energy and power densities for practical applications.

High-performance solid state supercapacitors assembling graphene interconnected networks in porous silicon electrode by electrochemical methods using 2,6-dihydroxynaphthalen

The challenge for conformal modification of the ultra-high internal surface of nanoporous silicon was tackled by electrochemical polymerisation of 2,6-dihydroxynaphthalene using cyclic voltammetry or potentiometry and, notably, after the thermal treatment (800 °C, N₂, 4 h) an assembly of interconnected networks of graphene strongly adhering to nanoporous silicon matrix resulted. Herein we demonstrate the achievement of an easy scalable technology for solid state supercapacitors on silicon, with excellent electrochemical properties. Accordingly, our symmetric supercapacitors (SSC) showed remarkable performance characteristics, comparable to many of the best high-power and/or high-energy carbon-based supercapacitors, their figures of merit matching under battery-like supercapacitor behaviour. Furthermore, the devices displayed high specific capacity values along with enhanced capacity retention even at ultra-high rates for voltage sweep, 5 V/s, or discharge current density, 100 A/g, respectively. The cycling stability tests performed at relatively high discharge current density of 10 A/g indicated good capacity retention, with a superior performance demonstrated for the electrodes obtained under cyclic voltammetry approach, which may be ascribed on the one hand to a better coverage of the porous silicon substrate and, on the other hand, to an improved resilience of the hybrid electrode to pore clogging.

Performance of Partially Exfoliated Nitrogen-Doped Carbon Nanotubes Wrapped with Hierarchical Porous Carbon in Electrolytes

The preparation of highly conductive, high-surface-area, heteroatom-doped, porous carbon nanocomposite materials with enhanced electrochemical performance for sustainable energy-storage technologies, such as supercapacitors, is challenging. Herein, a route for the large-scale synthesis of nitrogen-doped porous carbon wrapped partially exfoliated carbon nanotubes (N-PPECNTs) with an interconnected hierarchical porous structure, as an advanced electrode material that can realize several potential applications for energy storage, is presented. Polypyrrole conductive polymer acts as both nitrogen and carbon sources that contribute to the pseudocapacitance. Partially exfoliated carbon nanotubes (PECNTs) provide a high specific surface area for ion and charge transportation and act as a conductive matrix. The derived porous N-PPECNT displays a nitrogen content of 6.95 at %, with a specific surface area of 2050 m²  g^-1 , and pore volume of 1.13 cm³  g^-1 . N-PPECNTs, as an electrode material for supercapacitors, exhibit an excellent specific capacitance of 781 F g^-1 at 2 A g^-1 , with a high cycling stability of 95.3 % over 10 000 cycles. Furthermore, the symmetric supercapacitor exhibits remarkable energy densities as high as 172.8, 62.7, and 53.55 Wh kg^-1 in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]), organic, and aqueous electrolytes, respectively. Also, biocompatible hydrogel and polymer gel electrolyte based, stable, flexible supercapacitors with excellent electrochemical performance could be demonstrated.