Free-standing Supercapacitor Electrode Research Articles

A rigid free-standing supercapacitor electrode material made from fat coal-based activated carbon foam

A free-standing electrode material is a competitive candidate for supercapacitor due to its excellent capacity and volumetric energy density. Currently, most of the studies are focused on the preparations and performances of flexible free-standing electrode materials, whereas few on the development of rigid free-standing electrode materials. Herein, a vitrinite concentrate of fat coal is used as a raw material and carbon black as an addition agent to prepare a carbon foam by way of high-pressure pyrolysis. The prepared carbon foam is activated by KOH solution to further achieve an activated carbon foam by which we have fabricated a rigid free-standing electrode material. The assembled symmetric device with the rigid free-standing electrode material demonstrates an extremely high areal specific capacitance of 2304.41 mF cm−2 at 4 mA cm−2, an outstanding cycling stability of 100 % after 40 000 cycles, and an excellent volume energy density of 10.23 mWh cm−3 at 31.78 mW cm−3. These fascinating electrochemical performances of the electrode material are attributed to unique 3D interconnected porous structure of the activated carbon foam. The micron-sized 3D interconnected pores facilitate the transport of electrolyte ions within the electrodes. Meanwhile, micropores on the 3D interconnected pore wall provide abundant active sites for electrolyte ions.

Construction of novel coaxial electrospun polyetherimide@polyaniline core-shell fibrous membranes as free-standing flexible electrodes for supercapacitors

Designing porous and flexible electrodes with excellent electrochemical properties is crucial to fabricate high-performance flexible supercapacitors. Conductive polymer-based electrospun fibers are expected to be an ideal material for constructing flexible electrodes due to the enormous specific surface area, unique pore structure and inherent flexibility. However, the exploration of such electrospun fibers with facile preparation methods and high areal capacitance remains challenging. Herein, a novel polyetherimide@polyaniline core-shell fibrous membranes (PEI@PANI CFMs) is fabricated via simply coaxial electrospinning and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) secondary doping. With the aid of polyetherimide, highly concentrated emeraldine base polyaniline (EB-PANI) can be perfectly wrapped on the surface of fibers without any spinning assistants, ensuring the well-developed pore structures, superior hydrophilicity, good electrical conductivity and outstanding mechanical flexibility. Benefiting from the synergistic effect of structure and constituents, such flexible PEI@PANI CFMs as electrode materials can provide more accessible electrochemical active sites and decrease the ion diffusion resistance. Therefore, the PEI@PANI CFMs present outstanding areal capacitance of 1159.9 mF cm−2 and remarkable energy density of 46.91 μW h cm−2 in the electrodes and symmetric supercapacitors, demonstrating great potential for applications in flexible energy storage field. This fabrication strategy provides a simple and practicable way to manufacture polyaniline-based fibrous membranes for high-performance flexible electrode materials.

3D hierarchical Ti3C2Tx@NiCo2S4–RGO heterostructure hydrogels as free-standing electrodes for high-performance supercapacitors

A high-energy supercapacitor device based on 3D hierarchical free-standing Ti3C2Tx@NiCo2S4–RGO heterostructure hydrogel electrodes is assembled.

Hydrothermal growth of FeMoO4 nanosheets on electrospun carbon nanofibers as freestanding supercapacitor electrodes

In this study, an electrospinning method was used to produce highly conductive freestanding carbon nanofibers (CNFs). The freestanding CNFs are compatible with hierarchical growth techniques, such as hydrothermal processes, for the nanostructure engineering of supercapacitor electrodes with enhanced electrochemically active sites. Therefore, the flower-like FeMoO4@CNF nanosheets were investigated to improve the electrochemical performance using aqueous and neutral electrolytes (Na2SO4 and K2SO4). The optimized FeMoO4@CNF electrode exhibits areal capacitances of 252 and 220 mF·cm−2 at a high current density of 2.5 mA·cm−2 with Na2SO4 and K2SO4 electrolytes, respectively. The wide potential window (1.6 V) of the symmetric supercapacitor delivered maximum energy densities of 22.4 and 19.6 μWh·cm−2 at a power density of 2 mW·cm−2 for Na2SO4 and K2SO4 electrolytes, respectively.

Uniform Co9S8 nanosheets on carbon nanotube arrays grown on Ni mesh as free-standing electrodes for asymmetric supercapacitors

A free-standing 3D core/shell composite material CNTs@Co9S8 on nickel mesh has been prepared by a simple two-step method. Carbon nanotubes (CNTs) grown vertically on the surface of nickel mesh by chemical vapor deposition (CVD) method act as conductive skeleton. Then Co9S8 nanosheets uniformly aligned on the surface of the CNTs by solvothermal method are interconnected to form a honeycomb structure. Owing to the close contact of the CNTs@Co9S8 hybrid with the nickel mesh and the hierarchical 3D porous structure, the prepared Ni@CNTs@Co9S8 electrodes exhibit low resistance and excellent charge storage capability with a specific capacitance of 1305.4 F g−1 at 1 A g−1. The composite delivers an excellent rate capability with 84.3 % capacitance retention at 10 A g−1. An asymmetric supercapacitor composed of Ni@CNTs@Co9S8 and nitrogen-doped carbon nanotubes (NCNTs) shows an energy density of 23.8 Wh kg−1 and a high power density of 8007.4 W kg−1. The specific capacitance of the ASC device remained 87.31 % after 10,000 charge-discharge cycles and two tandem ASCs can power two red LED diodes for at least 7 min, suggesting the potential application of the obtained free-standing electrode for supercapacitors.

Hierarchical porous 3D Ni3N-CoN/NC heterojunction nanosheets with nitrogen vacancies for high-performance flexible supercapacitor

In the realm of everyday existence, the allure of flexible portable electronic devices has captured widespread attention. Consequently, the imperative to conceive and fabricate flexible energy storage/conversion systems assumes paramount significance. Transition metal nitrides (TMNs), distinguished by their extraordinary electrochemical properties, emerge as compelling candidates for electrode materials in high-performance energy storage devices. This study undertakes the synthesis of three-dimensional Ni3N-CoN/NC heterojunction nanosheets via a meticulous two-step in situ growth process involving metal organic frameworks (MOFs) on carbon cloth (CC), succeeded by annealing in an ammonia-rich environment. The ensuing nitridation engenders copious ion pathways, engrossing a considerable specific surface area of 233.2 m2 g−1 and endowing nitrogen vacancies in abundance. The augmented conductivity of the nitrogen-vacancy-rich heterojunction is corroborated through density functional theory calculations. Employed as a flexible freestanding supercapacitor electrode, the Ni3N-CoN/NC/CC configuration evinces a marked enhancement in electrochemical performance attributable to its superior intrinsic conductivity and heightened active sites. Notably, the synthesized Ni3N-CoN/NC/CC flexible electrode showcases a remarkable specific capacitance of 468.3 mA h g−1 at a current density of 3 A g−1. Moreover, the integration of the Ni3N-CoN/NC/CC cathode with an activated carbon (AC) anode in a flexible asymmetric supercapacitor (ASC) yields an impressive energy density of 0.2144 mWh cm−2 and a maximum power density of 80 mW cm−2, exhibiting exceptional cycling stability of 92.3% even after 15,000 cycles. The exceptional performance exhibited by the synthesized freestanding Ni3N-CoN/NC/CC composite underscores its tremendous potential in the development of flexible energy storage devices.

Experimental and theoretical exploration of nickel ion-implanted metal-organic frameworks (ZIF-67) as free-standing electrodes for hybrid supercapacitors

Ion beam irradiation is a non-destructive method to tune the composition and morphology of nanoarchitectures. It is a promising tool owing to the creation of crystal defects such as dislocations and vacancies in addition to ion implantation. Due to the advantages of structural versatility, tunability of chemical composition and high theoretical capacity, Metal organic frameworks are considered as a potential class of electrode materials for supercapacitors. We have in situ grown ZIF-67, without binder on carbon cloth and modified the interface by implanting with Ni ions. Benefiting from the Ni+ ion implanted ZIF-67 (CC/Ni@ZIF67-4) as a binder-free positive electrode delivered an areal capacitance of 1010 mF cm−2 at 1 mA cm−2 with profound cyclic retention of 93.5%. The hybrid supercapacitor assembled with CC/Ni@ZIF-67-4//CC/O,N,S@AC composition delivered a maximum voltage window of 1.5 V, exhibited a specific capacitance of 68 F g−1 at a current density of 2 Ag-1 energy density, an energy density of 21.26 Wh kg−1 and power density of 13,500 W kg−1 as well as a remarkable electrochemical stability up to 10,000 (96.4% retention at 10 Ag-1). The density functional theory calculations show a change in the Fermi level after the inclusion of Ni+ ions supporting the experimental data.

CoCx‑induced interfacial octahedral Co2+ sites of NiCo-LDH electrode with improved faradic reactivity toward high-performance supercapacitor

Battery-like electrode materials are characterized by large theoretical capacitance but suffer from poor surface reactivity and insufficient electroactive sites thus limiting their practical charge storage capacity. To overcome this challenge, an effective strategy for vacancy modulation on battery-like electrode materials is necessary. Herein, we report for the first time an elaborately designed three-dimensional (3D) hierarchical heterostructure consisting of CoCx@NiCo-LDH on conductive nickel foam as a freestanding supercapacitor electrode. Benefiting from the weakening of the coordination of CoO bonds, the CoCx structure induces in-situ reconstruction of the NiCo-LDH lattice, resulting in the formation of abundant oxygen vacancies (interfacial octahedral Co2+ sites) that lower the OH− adsorption energy as determined by the density functional theory (DFT) calculation. The resulting CoCx@NiCo-LDH/NF electrode exhibits an ultrahigh rate capability (2330 mF cm−2 at 0.3 mA cm−2, with capacitance retention of 51.5 % at 30 mA cm−2) and remarkable cycling performance (capacitance retention of 81.6 % after 10,000 cycles). Additionally, the assembled asymmetric devices deliver an extremely high energy density of 246 μWh cm−2 at the power density of 798 μW cm−2, with 87.8 % capacitance retention after 10,000 cycles at 8 mA cm−2. Overall, this study presents a simple yet effective strategy to construct high-performance battery-like electrodes for potential applications in energy storage, transportation, and communication.

A morphology control engineered strategy of Ti3C2Tx/sulfated cellulose nanofibril composite film towards high-performance flexible supercapacitor electrode

2D Ti3C2Tx MXene is an ideal material for fabricating supercapacitor electrodes due to its excellent physical-chemical properties. However, the inherent self-stacking, narrow interlayer spacing, and low general mechanical strength limit its application in flexible supercapacitors. Herein, facile structural engineering strategies by drying (vacuum drying, freeze drying, and spin drying) were proposed to fabricate 3D high-performance Ti3C2Tx/sulfated cellulose nanofibril (SCNF) self-supporting film supercapacitor electrodes. Compared with other composite films, the freeze-dried Ti3C2Tx/SCNF composite film exhibited a looser interlayer structure with more space which was conducive to charge storage and ion transport in the electrolyte. Therefore, the freeze-dried Ti3C2Tx/SCNF composite film exhibited a higher specific capacitance (220 F/g) compared to the vacuum-dried Ti3C2Tx/SCNF composite film (191 F/g) and the spin-dried Ti3C2Tx/SCNF composite film (211 F/g). After 5000 cycles, the capacitance retention rate of the freeze-dried Ti3C2Tx/SCNF film electrode was close to 100 %, showing excellent cycle performance. Meanwhile, the tensile strength of freeze-dried Ti3C2Tx/SCNF composite film (13.7 MPa) was much greater than that of the pure film (7.4 MPa). This work demonstrated a facile strategy for control of Ti3C2Tx/SCNF composite film interlayer structure by drying for fabricating well-designed structured flexible and free-standing supercapacitor electrodes.

High-performance Zn-ion hybrid supercapacitors enabled by rationally designed hierarchical SiC@C quasi-aligned nanoarrays

In the present work, we report the exploration of rationally designed electrodes based on quasi-aligned nanoarrays composited by the cores of SiC nanowires and shells of sulfur-doped porous carbons (SiC@PC-S), which are directly employed as the free-standing electrodes for Zn-ion hybrid supercapacitors. They behave an ultrahigh areal capacitance of 38.24 mF/cm2 at a scan rate of 10 mV/s, with an exceptional rate capability (~20.67 mF/cm2 even at 1000 mV/s). The as-constructed hybrid supercapacitors deliver a remarkable energy of 10.6 μWh/cm2, which is the highest one among SiC-based supercapacitors ever reported. Moreover, such hybrid devices exhibit a high power density of 7.2 mW/cm2 and a robust cycle stability with ~97 % capacitance retention after 5000 cycles at 2 mA/cm2, representing their promising practical applications.

Microstructures and electrochemical characterization of graphene oxide/carboxymethylated cellulose nanofibril-derived hybrid carbon aerogels for freestanding supercapacitor electrodes

We herein report the morphological, thermal, electrical, and electrochemical properties of porous hybrid carbon aerogels prepared from carboxymethylated cellulose nanofibrils (c-CNF) and graphene oxide (GO, 1–10 wt %) via a facile process of freeze-drying, thermal stabilization, and carbonization at different temperatures of 800–1000 °C. During the carbonization, GO and c-CNF in the freeze-dried aerogel transformed into reduced GO (rGO) and porous carbon gel (CA), respectively. Morphological studies revealed that the porous hybrid carbon aerogels (rGO/CAs) had interconnected pores. The maximal electrical conductivity was achieved for the rGO/CA fabricated at the highest carbonization temperature of 1000 °C. With an increase in the amount of rGO, the electrical conductivity of the rGO/CAs increased to 0.91 S cm−1. For a three-electrode system in a PVA/H3PO4 gel electrolyte, the rGO/CA with 5 wt % rGO loading exhibited a high specific capacitance of 96.5 F g−1 at 0.5 A g−1. Also, the rGO/CA with 5 wt % rGO exhibited a rate capability of 10.3 % at 2 A g−1 and an energy density of 2.14 Wh kg−1 at a power density of 49.9 W kg−1. These results reveal that GO/c-CNF-derived hybrid carbon aerogels can be utilized as a potential material for the free-standing electrode of supercapacitors.

Hierarchical N-Ti3C2TX@NiCo2S4 core-shell nanosheets assembled into 3D porous hydrogel as free-standing electrodes for high-performance supercapacitors

Designing new electrode structures with excellent electrochemical properties and finding facile synthesis methods are challenges for energy storage devices. Herein, we constructed a heterogeneous N-doped 2D structure of Ti3C2TX with tubular NiCo2S4 and formed a 3D hydrogel structure using graphene oxide as a binder, which could prevent the natural packing of Ti3C2TX and increase the specific surface area. This electrode combines the metallic conductivity of Ti3C2TX with the pseudocapacitance property of NiCo2S4 to achieve exceptional electrochemical performance. As a result, the electrode achieves an ultra-high specific capacitance of 517.11 F g−1 at 1 A g−1 and 92.6 % capacitance retention after 10,000 cycles. Moreover, a high energy density of 27.78 Wh kg−1 is obtained at a power density of 596 W kg−1. This new N-Ti3C2TX@NiCo2S4/RGO electrode sheet provides more active sites with higher charge transfer rates and improved charge transfer kinetics, boosting the level of material processing of supercapacitors.

Growing hierarchical CoNi-based phosphates sandwiched Ni metal on carbon cloth for advanced supercapacitors

The rational design of electrode materials plays an important role in the development of supercapacitors. Although CoNi-based phosphate as active material for supercapacitor shows great potential including high theoretical capacitance, low cost, and environmental friendliness, the further investigation of CoNi-based phosphate with structural design to achieve high performance is of great urgent. In this work, hierarchical CoNi-based phosphates sandwiched Ni are directly grown on carbon cloth (CC@CoNi-PO/Ni/CoNi-PO) as freestanding electrode for supercapacitors. The introduction of Ni into hierarchical CoNi-based phosphates with abundant redox active sites can improve the integral electrochemical performance. In three-electrode system, the CC@CoNi-PO/Ni/CoNi-PO sustains a cycling stability of 94.7% after 1000 cycle (20 mV s−1), which is better than those of CC@CoNi-PO (85.9%) and CC@CoNi-PO/CoNi-PO (91.8 %). Matched with activated carbon as negative electrode, the hybrid supercapacitor delivers considerable stability of 88.3% capacitance retention after 2000 cycles at 100 mA cm−2.

Construction of NiCo2O4 nanoflake arrays on cellulose-derived carbon nanofibers as a freestanding electrode for high-performance supercapacitors

Construction of NiCo2O4 nanoflake arrays on cellulose-derived carbon nanofibers as a freestanding electrode for high-performance supercapacitors

A high-performance and flexible electrode film based on bacterial cellulose/polypyrrole/nitrogen-doped graphene for supercapacitors

With the development and popularity of portable electronic devices, there is an urgent need for flexible energy storage devices suitable for mass production. We report freestanding paper electrodes for supercapacitors fabricated via a simple but efficient two-step method. Nitrogen-doped graphene (N-rGO) was first prepared via a hydrothermal method. This not only obtained nitrogen atom-doped nanoparticles but also formed reduced graphene oxide. Pyrrole (Py) was then deposited on the bacterial cellulose (BC) fibers as a polypyrrole (PPy) pseudo-capacitance conductive layer by in situ polymerization and filtered with nitrogen-doped graphene to prepare a self-standing flexible paper electrode with a controllable thickness. The synthesized BC/PPy/N15-rGO paper electrode has a remarkable mass specific capacitance of 441.9 F g−1, a long cycle life (96 % retention after 3000 cycles), and excellent rate performance. The BC/PPy/N15-rGO-based symmetric supercapacitor shows a high volumetric specific capacitance of 244 F cm−3 and a max energy density of 67.9 mWh cm−3 with a power density of 1.48 W cm−3, suggesting that they will be promising materials for flexible supercapacitors.

Electrospun porous carbon nanofibers derived from bio-based phenolic resins as free-standing electrodes for high-performance supercapacitors

Electrospun porous carbon nanofibers derived from bio-based phenolic resins as free-standing electrodes for high-performance supercapacitors

Rational construction of hierarchical nanocomposites by growing dense polyaniline nanoarrays on carbon black-functionalized carbon nanofiber backbone for freestanding supercapacitor electrodes

Designing freestanding electrodes for supercapacitors has attracted tremendous attention since avoiding the use of inactive additives is cost-effective and process-efficient for the industrialization and commercialization of supercapacitors. To accommodate the vigorous electrochemical process and facilitate charge transfer, robust and conductive backbones are required to fabricate freestanding electrodes. Herein, a carbon black (CB)-functionalized electrospun carbon nanofiber (CNF) backbone is fabricated for growing redox-active dense polyaniline (PANI) nanoarrays to construct hierarchical carbon black/carbon nanofiber/PANI nanocomposites (CCNFP) as freestanding supercapacitor electrodes. The CB particles act as nanofillers to functionalize the CNF backbone, not only providing additional conductive networks and pores inside each nanofiber but also increasing the surface roughness to facilitate PANI deposition; whereas PANI nanoarrays offer pseudocapacitive properties and improve the wettability of the electrodes. The synthesized CCNF7P24 electrode exhibits a high specific capacitance of 1062.7 F g−1 and excellent retention of 72.2 % at current densities of 1 and 20 A g−1, respectively. A symmetric supercapacitor constructed from two freestanding CCNF7P24 electrodes delivered a maximum energy density of 54.3 Wh kg−1, benefiting from the excellent electrochemical performance of the electrodes. This work provides a reliable approach to fabricating nanofiber-based hierarchical composites for freestanding electrodes.

A facile “thick to thin” strategy for integrating high volumetric energy density and excellent flexibility into MXene/wood free-standing electrode for supercapacitors

Wood-based thick electrodes (>400 μm) always exhibit low volumetric loading of active material and poor flexibility. Herein, a “thick to thin” strategy including densification and low-temperature heating treatment is innovatively proposed and then applied in a MXene (Ti3C2Tx)/wood thick electrode (500 μm) to simultaneously realize large volumetric loading and excellent flexibility. It is found that the densification can significantly increase the volumetric mass loading of Ti3C2Tx by 10 times owing to the great reduction of the electrode thickness (50 μm), and endow the electrode with excellent flexibility by changing porous microstructure to compact microstructure. Furthermore, the low-temperature heating treatment greatly improves the electrochemical performances of the electrode by generating meso-/macropores for ion transport and exposing more active sites to the electrolyte. After conducting our proposed strategy, the optimized electrode exhibits 9 times higher volumetric capacitance (reaching 187F cm−3) than the control electrode, and still has a high rate capability of 77 % when the current density increases by 100 folds. Additionally, the optimized free-standing electrode is used to fabricate a quasi-solid-state supercapacitor which demonstrates stable capacitive behaviors during bending and a large volumetric energy density of 3 mWh cm−3 at 30.6 mW cm−3.

Ti3C2Tx MXene-embedded MnO2-based hydrophilic electrospun carbon nanofibers as a freestanding electrode for supercapacitors.

Herein, MnO2 nanoflowers are electrodeposited on a self-supported and electroconductive electrode in which 2D Ti3C2Tx nanosheets are encased in carbon nanofibers (MnO2@Ti3C2Tx/CNFs). This improves the conductivity and hydrophilicity of the MnO2 composite electrode. The asymmetric supercapacitor shows a high energy density of 46.4 W h kg-1 and a power density of 4 kW kg-1.

Recent advances in wood-based electrode materials for supercapacitors

The hierarchical structures of wood at different levels from top down, and their functions and applications in freestanding supercapacitor electrodes with multiple dimensions.