Highly Porous Reduced Graphene Oxide-Coated Carbonized Cotton Fibers as Supercapacitor Electrodes

Redox-Active Phenanthrenequinone Molecules and Nitrogen-Doped Reduced Graphene Oxide as Active Material Composites for Supercapacitor Applications

Carbon-based supercapacitor electrodes are generally restricted in energy density, as they rely exclusively on electric double-layer capacitance (EDLC). The introduction of redox-active organic molecules to obtain pseudocapacitance is a promising route to develop electrode materials with improved energy densities. In this work, we develop a porous nitrogen-doped reduced graphene oxide and 9,10-phenanthrenequinone composite (N-HtrGO/PQ) via a facile one-step physical adsorption method. The electrochemical evaluation of N-HtrGO/PQ using cyclic voltammetry showed a high capacitance of 605 F g-1 in 1 M H2SO4 when the composite consisted of 30% 9,10-phenanthrenequinone and 70% N-HtrGO. The measured capacitance significantly exceeded pure N-HtrGO without the addition of redox-active molecules (257 F g-1). In addition to promising capacitance, the N-HtrGO/30PQ composite showed a capacitance retention of 94.9% following 20,000 charge/discharge cycles. Based on Fourier transform infrared spectroscopy, we postulate that the strong π-π interaction between PQ molecules and the N-HtrGO substrate enhances the specific capacitance of the composite by shortening pathways for electron transfer while improving structural stability.

Tin Oxide/Vertically Aligned Graphene Hybrid Electrodes Prepared by Sonication-Assisted Sequential Chemical Bath Deposition for High-Performance Supercapacitors

Hybrid electrodes comprising metal oxides and vertically aligned graphene (VAG) are promising for high-performance supercapacitor applications because they enhance the synergistic effect owing to the large contact area between the two constituent materials. However, it is difficult to form metal oxides (MOs) up to the inner surface of a VAG electrode with a narrow inlet using conventional synthesis methods. Herein, we report a facile approach to fabricate SnO₂ nanoparticle-decorated VAG electrodes (SnO₂@VAG) with excellent areal capacitance and cyclic stability using sonication-assisted sequential chemical bath deposition (S-SCBD). The sonication treatment during the MO decoration process induced a cavitation effect at the narrow inlet of the VAG electrode, allowing the precursor solution to reach the inside of the VAG surface. Furthermore, the sonication treatment promoted MO nucleation on the entire VAG surface. Thus, the SnO₂ nanoparticles uniformly covered the entire electrode surface after the S-SCBD process. SnO₂@VAG exhibited an outstanding areal capacitance (4.40 F cm^-2) up to 58% higher than that of VAG electrodes. The symmetric supercapacitor with SnO₂@VAG electrodes showed an excellent areal capacitance (2.13 F cm^-2) and a cyclic stability of 90% after 2000 cycles. These results suggest a new avenue for sonication-assisted fabrication of hybrid electrodes in the field of energy storage.

Co3ZnC@NC Material Derived from ZIF-8 for Lithium-Ion Capacitors

Metal-organic framework (MOF)-derived carbon materials were widely reported as the anodes of lithium-ion capacitors (LICs). However, tunning the structure and electrochemical performance of the MOF-derived carbon materials is still challenging. Herein, metal carbide materials of Co3ZnC@NC-8:2 were obtained by the pyrolysis of the MOF materials of Co0.2Zn0.8ZIF-8 (Zn/Co ratio of 8:2). A half-cell assembled with the Co3ZnC@NC-8:2 electrode exhibits a discharge capacity of the electrode material of 598 mAh g-1 at a current density of 0.1 A g-1. After 100 cycles, the retention rate of discharge specific capacity is about 90%. The high performance of Co3ZnC@NC-8:2 is ascribed to its high crystalline degree and well-defined structure, which facilitates the intercalation/deintercalation of lithium ions and buffers the volume change during the charge/discharge process. The high capacitance contribution ratio calculated by cyclic voltammetry (CV) curves at different scanning rates indicates the pseudocapacitance storage mechanism. LICs constructed from the Co3ZnC@NC-8:2 material have a rectangular CV curve, while the charge-discharge curve has a symmetrical triangular shape. This study indicates that MOF-derived carbon is one of the promising materials for high-performance LICs.

Synthesis and Characterization of Supercapacitor Materials from Soy

Renewable resources and their byproducts are becoming of growing interest for alternative energy. Here, we have demonstrated the use of Arkansas’ most important crop, soy, as a carbon precursor for the synthesis of carbonized activated materials for supercapacitor applications. Different soy products (soymeal, defatted soymeal, soy flour and soy protein isolate) were converted into carbonized carbon and co-doped with phosphorus and nitrogen simultaneously, using a facile and time-effective microwave synthesis method. Ammonium polyphosphate was used as a doping agent which also absorbs microwave radiation. The surface morphology of the resulting carbonized materials was characterized in detail using scanning electron microscopy. X-ray photoelectron spectroscopy was also performed, which revealed the presence of a heteroelemental composition, along with different functional groups at the surface of the carbonized materials. Raman spectroscopy results depicted the presence of both a graphitic and defect carbon peak, with defect ratios of over one. The electrochemical performance of the materials was recorded using cyclic voltammetry in various electrolytes including acids, bases and salts. Among all the other materials, soymeal exhibited the highest specific capacitance value of 127 F/g in acidic electrolytes. These economic materials can be further tuned by changing the doping elements and their mole ratios to attain exceptional surface characteristics with improved specific capacitance values, in order to boost the economy of Arkansas, USA.

In Situ Synthesis of CoCeSx Bimetallic Sulfide Nanoparticles on a Bi-Pyrene Terminated Molecular Wire Modified Graphene Surface for Supercapacitors

The excellent electrical conductivity of graphene is due to its highly-conjugated structures. Manipulation of the electronic and mechanical properties of graphene can be achieved by controlling the destruction of its in-sheet conjugation system. Herein, we report the preparation of CoCeS_x -SA@BPMW@RGO through π-π stacking interactions at the molecular level. In this study, sodium alginate was reacted with Co²⁺ and Ce³⁺ , and the composite was loaded onto a graphene surface. The graphene sheets were prepared using a bi-pyrene terminated molecular wire (BPMW) to avoid re-stacking of the grapheme sheets, thereby forming nanoscale spaces between sheets. The angle between the BPMW coplanar pyrene group and the phenyl group was 33.2°, and the graphene layer is supported in an oblique direction. Finally, a three-dimensional porous composite was obtained after annealing and vulcanization. The obtained CoCeS_x -SA@BPMW@RGO exhibited excellent electrical conductivity and remarkable cycle stability. When the current density was 1 A g^-1 , its specific capacitance was as high as 1004 F g^-1 . BPMW modifies graphene through the synergistic effect of π-π stacking interaction and special structure to obtain excellent electrochemical performance. Moreover, a solid-state asymmetric supercapacitor device was fabricated based on the synthesized CoCeS_x -SA@BPMW@RGO hybrid, which exhibited a power density of 979 W kg^-1 at an energy density of 23.96 Wh kg^-1 .

The fabrication of flexible wearable electrodes based on a carbon nanotubes/nickel/nickelous hydroxide ternary composite by facile single-side printing technology

A new material design strategy is developed to prepare high-performance flexible electrochemical electrodes. Carbon nanotubes (CNTs) and nickel/nickelous hydroxide (Ni/Ni(OH)₂) are compounded through a chemical plating method and hydrothermal process. A single-side printing method is used to combine the active material and a flexible cotton substrate. The interfinger microstructure of the textile electrode can greatly facilitate charge/ion transfer at the electrode-electrolyte interface. One side of the fabric, which is untreated, could directly contact with human skin, providing a comfortable and user-friendly surface. With the CNTs/Ni/Ni(OH)₂ ternary composite as a positive electrode and CNTs as a negative electrode, we assembled an in-plane asymmetrical micro-supercapacitor device (SF-NPCs). Thanks to a synergistic effect, SF-NPCs displays a high energy density of 0.29 W h cm^-2 at a power density of 7.2 W cm^-2. The operating window is extended to 1.5 V, and the device displays good potential for applications in the field of smart textiles.

Experimental study on the degradation mechanism of LaCoO3-based symmetric supercapacitors

In this paper, LaCoO₃ powders were prepared by the urea combustion method and used as electrode materials for supercapacitors. The effect of the potential window and the current density on the performance degradation of LaCoO₃ electrodes during the cycling test was analyzed. The degradation mechanism of LaCoO₃-based symmetric supercapacitors was discussed. The results of the cycling test show that: with the increase in potential window and current density, the performance degradation in the cycling test becomes more intense. The results of cyclic voltammetry tests, galvanostatic charge–discharge tests, X-ray photoelectron spectroscopy tests and KOH electrolyte concentration measurements before and after the cycling test show that the degradation of the supercapacitors is mainly caused by the occurrence and accumulation of irreversible redox reactions during the charge and discharge process, which reduces the ratio of Co²⁺/Co³⁺ and the number of oxygen vacancies.

Cycling tests for LaCoO₃-based symmetric supercapacitors were carried out. The change in the valence state of Co in the electrode surface active material, LaCoO₃, before and after the cycling test was investigated by X-ray photoelectron spectroscopy.