Self-discharge of supercapacitors based on carbon nanotubes with different diameters

A Ce3+/4+ Redox Couple-Promoted Aqueous Zinc-Ion Hybrid Capacitor: Design Strategies and Mechanistic Insights

Aqueous zinc-ion hybrid capacitors (ZIHCs) are emerging as promising energy storage devices owing to several desirable attributes including good safety, high power density, and good stability. However, the limited energy density mainly caused by the low specific capacity of porous carbon cathodes hinders the practical application of ZIHCs. A Ce3+/4+ redox couple-promoted aqueous ZIHC (Ce-ZIHC) is designed with the addition of Ce3+/4+ electrolyte additives. The Ce3+/4+ redox couple is shown to markedly increase the specific capacity of the porous carbon cathode energy and enhance the stability of Zn2+ stripping/plating at the Zn metal anode. Notably, the as-constructed Ce-ZIHC performs more than twice the energy density of the ZIHC with the commercial activated carbon cathode. Furthermore, the Ce-ZIHC shows a low self-discharge rate and can work stably for more than 60 000 cycles at 5.0 A g-1. This work highlights the great potential of Ce3+/4+ redox couple in improving the overall performance of ZIHCs toward practical application.

In situ synthesis of bimetallic chalcogenides with highly conductive carbon nanotubes for efficient symmetric hybrid supercapacitors

Achieving high energy density and long cycle stability in energy storage devices necessitates excellent electrochemical performance, which often relies on the innovative structural design of the materials under investigation. Therefore, hybrid supercapacitors are crucial in the realm of energy storage devices. The elevated energy and power densities, combined with various energy storage mechanisms, significantly improve electrochemical performance. Here, we developed a highly efficient electrode material, carbon nanotube-metal chalcogenides (CNT-CuNiSe2), using a simple one-pot reflux method (in situ). The enhanced energy storage performance was achieved by synergising CuNiSe2 with the pi-cloud of CNTs, resulting in enhanced specific capacitance retention over prolonged cycling stability. The hybrid supercapacitor electrode was formed by combining conducting carbon cloth (CC) with CNT-CuNiSe2 as a hybrid material, referred to as the CC/CNT-CuNiSe2 material. The fabricated hybrid electrode materials demonstrated excellent potential for energy storage. CC/CNT-CuNiSe2 exhibited excellent energy storage capabilities, achieving a specific capacitance of 957.06 F g-1 at 1 A g-1. Hybrid supercapacitors with high energy and power density were developed using conducting carbon cloth and CNT-CuNiSe2, designated as CC/CNT-CuNiSe2//CC/CNT-CuNiSe2. The hybrid capacitor device demonstrated a capacitance of 265.586 F g-1, along with an energy density of 82.99 W h kg-1 at a power density of 1511.35 W kg-1. When charged and discharged at 4 A g-1, the hybrid capacitor device displayed an impressive capacitance retention of 101.3% over 6000 continuous cycles.

Electrochemical Performance of Corn Waste Derived Carbon Electrodes Based on the Intrinsic Biomass Properties

The exploration of cost-effective and sustainable biomass-derived carbon materials as electrodes for energy conversion and storage has gained extensive attention in recent research studies. However, the selection of the biomass and the electrochemical performance regulation of the derived biochar, as well as their interrelationship still remain challenging for practical application. Herein, corn wastes with high carbon content (>40%), corn cob and corn silk, were selected as precursors for the preparation of high value-added and high yield carbon materials via a modified synthetic process. Uniquely, this work put emphasis on the theoretical and experimental investigations of how the biomass properties influence the composition and nanostructure regulation, the electrolyte ion adsorption free energy, and the electrical conductivity of the derived carbon materials as well as their electrochemical performance optimization. Owing to the favorable specific surface area, the hierarchical porous structure, and the diverse elemental distribution, corn cob and corn silk derived carbon materials (CBC and SBC) present great potential as promising electrodes for alkaline aqueous zinc batteries and supercapacitors. The assembled CBC//Zn and SBC//Zn zinc batteries deliver high energy densities of 63.0 Wh kg^-1 and 39.1 Wh kg^-1 at a power density of 575 W kg^-1, with excellent cycling performance of 91.1% and 84.3% capacitance retention after 10,000 cycles. As for the assembled symmetric supercapacitors, high energy densities of 14.9 Wh kg^-1 and 13.6 Wh kg^-1, and superior long-term cycling stability of 99.3% and 96.6% capacitance retention after 20,000 cycles could be achieved. This study highlights the advantages of utilizing corn cob and corn silk as carbon sources on the designed synthesis of carbon electrodes, and presents a meaningful perspective in the investigation of biomass-derived carbon materials and their potential applications in rechargeable devices.

Morphology engineering of biomass-derived porous carbon from 3D to 2D towards boosting capacitive charge storage capability

Carbon morphology significantly affects the capacitive performance of porous carbons. Biomass-derived porous carbons are usually restricted by inferior capacitive performance owing to their inherently three-dimensional (3D) blocked morphologies. Fabricating two-dimensional (2D) sheet-like morphology is expected to expose more inner space for better electrochemical performance, however, it needs to overcome the self-aggregation of biomass. The comprehensive understanding of how 2D morphology boosts capacitive performance remains challenging. Herein, we provide a morphology-regulating strategy to prepare 2D and 3D porous carbons and investigate the morphology effect on charge storage capability via both experimental data and theoretical simulations. 2D carbon exhibits better capacitance than 3D carbon in both electric double-layer capacitors (254 versus 211F g^-1) and zinc-ion hybrid supercapacitors (320 versus 232F g^-1), because the 2D carbon morphology not only improves the pore accessibility for higher double-layer capacitance, but also facilitates the exposure of active functional groups for more pseudocapacitance. Moreover, 2D morphology shortens pore length, leading to better anti-self-discharge capability. This study is beneficial to understanding the relationship between carbon morphology and capacitive performance and provides a facile strategy to upgrade biomass-derived porous carbons via morphology engineering.

Phase-Transitional Ionogel-Based Supercapacitors for a Selective Operation

As the demand for energy storage devices increases, the importance of electrolytes for supercapacitors (SCs) is further emphasized. However, since ions in electrolytes are always in an active state, it is difficult to store energy for a long time due to ion diffusion. Here, we have synthesized a phase-transitional ionogel and fabricated an SC based on the ionogel. The 1-ethyl-3-methylimidazolium nitrate ([EMIM]+[NO3]-) ionogel changes its phase from crystal to amorphous when the temperature was elevated above its phase transition temperature (∼44 °C). When the temperature is elevated from 25 to 45 °C, the resistivity of the gel is decreased from 2318.4 kΩ·cm to 43.2 Ω·cm. At the same time, the capacitance is boosted from 0.02 to 37.35 F g-1, and this change was repeatable. Furthermore, the SC exhibits an energy density of 7.77 Wh kg-1 with a power density of 4000 W kg-1 at 45 °C and shows a stable capacitance retention of 87.5% after 3000 cycles of test. The phase transition can switch the SCs from "operating mode" to "storage mode" when the temperature drops. A degree of self-discharge is greatly suppressed in the storage mode, storing 89.51% of charges after 24 h in self-discharge tests.

Scalable CNTs/NiCoSe2 Hybrid Films for Flexible All-Solid-State Asymmetric Supercapacitors

The rapidly developing wearable flexible electronics makes the development of high-performance flexible energy storage devices, such as all-solid-state supercapacitors (SCs), particularly important. Herein, we report the fabrication of CNTs/NiCoSe₂ hybrid films on carbon cloth (CC) through a facile co-electrodeposition method based on flexible electrodes for all-solid-state SCs. The NiCoSe₂ sheets grown on CNTs uniformly with a diameter of 50-100 nm act as the active materials. The CNTs in the hybrid films act as the scaffold to offer more deposition sites for NiCoSe₂ and provide a conductive network to facilitate the transfer of electrons. Moreover, the one-step electrodeposition process avoids the usage of any organic binders. Benefiting from the high intrinsic reactivity and unique 3D architecture, the obtained CNTs/NiCoSe₂ electrode delivers high specific capacity (218.1 mA h g^-1) and satisfactory durability (over 5000 cycles). Remarkably, the CNTs/NiCoSe₂//AC flexible all-solid-state (FASS) ASC provides remarkable energy density (112.2 W h kg^-1) within 0-1.7 V and maintains 98.1% of its initial capacity after 10,000 cycles. In addition, this flexible ASC device could be fabricated at a large scale (5 × 6 cm²), and the LED arrays (>3.7 V) can be easily lighted up by three ASCs in series, showing its potential practical application.

Towards High-Energy and Anti-Self-Discharge Zn-Ion Hybrid Supercapacitors with New Understanding of the Electrochemistry

Aqueous Zn-ion hybrid supercapacitors (ZHSs) are increasingly being studied as a novel electrochemical energy storage system with prominent electrochemical performance, high safety and low cost. Herein, high-energy and anti-self-discharge ZHSs are realized based on the fibrous carbon cathodes with hierarchically porous surface and O/N heteroatom functional groups. Hierarchically porous surface of the fabricated free-standing fibrous carbon cathodes not only provides abundant active sites for divalent ion storage, but also optimizes ion transport kinetics. Consequently, the cathodes show a high gravimetric capacity of 156 mAh g^-1, superior rate capability (79 mAh g^-1 with a very short charge/discharge time of 14 s) and exceptional cycling stability. Meanwhile, hierarchical pore structure and suitable surface functional groups of the cathodes endow ZHSs with a high energy density of 127 Wh kg^-1, a high power density of 15.3 kW kg^-1 and good anti-self-discharge performance. Mechanism investigation reveals that ZHS electrochemistry involves cation adsorption/desorption and Zn₄SO₄(OH)₆·5H₂O formation/dissolution at low voltage and anion adsorption/desorption at high voltage on carbon cathodes. The roles of these reactions in energy storage of ZHSs are elucidated. This work not only paves a way for high-performance cathode materials of ZHSs, but also provides a deeper understanding of ZHS electrochemistry.