Volumetric Capacitance Research Articles

What about electrochemical behaviors for aurivillius-phase bismuth tungstate? Capacitive or pseudocapacitive.

Researchers mainly explore the mechanism of pseudocapacitance through studying electrode materials with Faraday pseudocapacitive behavior. Here, we found that Bi2WO6, a typical Aurivillius phase material with pseudo-perovskite structure, showed nearly ideal pseudocapacitive behavior. The cyclic voltammetry curve is approximately rectangular in shape, with no redox peaks, which is similar to that of carbon materials. And the shape of the galvanostatic charge-discharge curve is close to an isosceles triangle. In addition, the kinetic analysis demonstrated that the electrochemical process of the A-Bi2WO6 electrode is dominated by surface processes, not diffusion. The A-Bi2WO6 electrode material presents a great volumetric specific capacitance of 466.5 F cm-3 at 0.5 A g-1. These electrochemical properties confirm that the Bi2WO6 material can serve as an ideal support material to explore pseudocapacitive energy storage. This work also provides guidance for the development of new pseudocapacitive materials.

Chalcogen (S, Se, and Te) decorated few-layered Ti3C2Tx MXene hybrids: modulation of properties through covalent bonding.

2D carbides and nitrides of transition metals (MXenes) have shown great promise in a variety of energy storage and energy conversion applications. The extraordinary properties of MXenes are because of their excellent conductivity, large carrier concentration, vast specific surface area, superior hydrophilicity, high volumetric capacitance, and rich surface chemistry. However, it is still desired to synthesize MXenes with specific functional groups that deliver the required characteristics. This is due to the fact that a considerable amount of metal atoms is exposed on the surface of MXenes during their synthesis through an etching procedure; hence, other anions and cations are uncontrollably implanted on their surfaces. Because of this situation, the first invented Ti3C2Tx MXene suffers from low photoresponsivity and detectivity, large overpotential, and small sensitivity in photoelectrochemical (PEC) photodetectors, hydrogen evolution reaction (HER), and sensing applications. Therefore, surface modification of the MXene structure is required to develop the device's performance. On the other hand, there is still a lack of understanding of the MXene mechanism in such cutting-edge applications. Thus, the manipulations of MXenes are highly dependent on understanding the device mechanism, suitable modification elements, and modification methods. This study for the first time reveals the conjugation effect of pre-selected S, Se, and Te chalcogen elements on a few-layered Ti3C2Tx MXene to synthesize new composites for PEC photodetector, HER, and vapor sensor applications. Also, the mechanism of the chalcogen decorated few-layered Ti3C2Tx MXene composites for each application is discussed. The selection of a few-layered Ti3C2Tx MXene is due to its fascinating characteristics which make it capable to be considered as an appropriate substrate and incorporating chalcogen atoms. The Te-decorated few-layered Ti3C2Tx MXene composite provides better performances in PEC photodetector and vapor sensing applications. Although the potential value of the Se-decorated few-layered Ti3C2Tx composite is slightly lower than that of the Te-decorated sample in HER application, its overpotential is still greater than that of the Te-decorated sample. The acquired results show that the S-decorated few-layered Ti3C2Tx composite demonstrates the lowest performance in all three examined applications in comparison with the other two samples.

Injectable and tissue-conformable conductive hydrogel for MRI-compatible brain-interfacing electrodes

The development of flexible and stretchable materials has led to advances in implantable bio-integrated electronic devices that can sense physiological signals or deliver electrical stimulation to various organs in the human body. Such devices are particularly useful for neural interfacing systems that monitor neurodegenerative diseases such as Parkinson’s disease or epilepsy in real time. However, coupling current brain-interfacing devices with magnetic resonance imaging (MRI) remains a practical challenge due to resonance frequency variations from inorganic metal-based devices. Thus, organic conductive materials, such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), have recently been considered as promising candidates. Nonetheless, their conformability on curvilinear tissues remains questionable. In this study, we developed an injectable conductive hydrogel (ICH) composed of tyramine-conjugated hyaluronic acid (HATYR) and PEDOT:PSS for MRI-compatible brain-interfacing electrodes. Our ICH produced low impedance around 5 kΩ even under 10 Hz, demonstrating high confidence volumetric capacitance. Due to HATYR’s biocompatibility, histological and cytotoxicity assays showed almost no inflammation and toxicity, respectively; in addition, ICH was able to degrade into 40% of its original volume within four weeks in vivo. An electrocorticogram (ECoG) array was also patternable by syringe injections of ICH on a stretchable and flexible elastomeric substrate layer that conformed to curvy brain tissues and successfully recorded ECoG signals under light stimulation. Furthermore, MRI imaging of implanted devices did not show any artifacts, indicating the potential of the MRI-compatible hydrogel electrodes for advanced ECoG arrays. This study provides a promising solution for MRI-compatible neural electrodes, enabling the advancement of chronic neural interfacing systems for monitoring neurodegenerative diseases.

Super-tough and self-healable all-cellulose-based electrolyte for fast degradable quasi-solid-state supercapacitor

Recyclable and degradable supercapacitors have promising applications for a sustainable energy storage industry. Herein, we prepare a dual-physical crosslinking (DP) carboxymethyl cellulose (CMC) hydrogel with high-toughness, healability, and electric conductivity by integrating abundant ions into the matrix. The prepared hydrogel displays a maximum compressive fracture stress of 4.42 MPa, fast healing in five seconds, and full degradation within eight days. Moreover, the fabricated supercapacitor shows high specific capacitance (309 F g−1) and volumetric capacitance (2.60 F cm−3). The supercapacitor achieves a healing efficiency of 93.9 % after five cuttings, and exhibits a cycling stability of 84.6 % capacitance retention after 1000 cycles. These merits ensure that the all-cellulose-based supercapacitor can operate in case of sudden collision and deformation, which contribute to reducing the environmental hazards from supercapacitor's preparation to its abandonment.

Aramid Nanofibers/Reduced Graphene Oxide Composite Electrodes with High Mechanical Properties

In this work, aramid nanofibers (ANFs)/reduced graphene oxide (ANFs/RGO) film electrodes were prepared by vacuum-assisted filtration, followed by hydroiodic acid reduction. Compared with thermal reduced ANFs/RGO, these as-prepared film electrodes exhibit a combination of mechanical and electrochemical properties with a tensile strength of 184.5 MPa and a volumetric specific capacitance of 134.4 F/cm3 at a current density of 0.125 mA/cm2, respectively. In addition, the film electrodes also show a superior cycle life with 94.6% capacitance retention after 5000 cycles. This kind of free-standing film electrode may have huge potential for flexible energy-storage devices.

Nanocellulose-based electrodes and separator toward sustainable and flexible all-solid-state supercapacitor

Nanocellulose, as the most abundant natural nanomaterial with sustainability, biodegradability, and excellent mechanical properties, has been widely applied in modern electronic systems, particularly, in the flexible electrochemical energy storage devices. Herein, a reduced graphene oxide (RGO)/cellulose nanocrystal/cellulose nanofiber (RCC) composite membrane was prepared by using a one-pot method. Compared to the pure RGO membranes, the RCC composite membranes exhibited better mechanical properties and hydrophilicity. Furthermore, due to the synergistic effect of nanocellulose and RGO sheets, the RCC composite membrane exhibited a specific capacitance as high as 171.3 F·cm−3. Consequently, a nanocellulose-based symmetric flexible all-solid-state supercapacitor (FASC) was constructed, in which two RCC composite membranes served as electrodes and a porous cellulose nanofiber membrane acted as separator. This fabricated FASC demonstrated a high volumetric specific capacitance of 164.3 F·cm−3 and a satisfactory energy density of 3.7 mW·h·cm−3, which exceeded that of many other FASCs ever reported. This work may open a new avenue in design of next-generation nanocellulose based, sustainable and flexible energy storage device.

Creation of nanopores and nitrogen doping in the surface layers of reduced graphene oxide electrode via ions implantation resulting in enhanced electrochemical performance for supercapacitor

Nitrogen doping is an effective way to enhance the electrochemical performance of carbon materials. In this study, the surface layers of graphene oxide (GO) film was firstly implanted by nitrogen ions, and then annealed at nitrogen atmosphere to obtain the nitrogen-doped reduced GO (NrGO) electrode for supercapacitor. Uniformly distributed nanopores are created in the surface layers of NrGO electrode, and also as high as 9.9 % nitrogen content is doped into its surface layers. The synergistic effects of these surface modifications significantly improve its electrochemical performance. The specific capacitance is increased from 148 F g−1 (original reduced GO (rGO)) to 270 F g−1 (NrGO), tested at current density of 0.5 A g−1 in symmetrical cells with 6 M KOH electrolyte. The volumetric capacitance of NrGO reaches 416 F cm−3, and its volumetric energy density is 14.3 Wh L−1 with power density of 191.7 W L−1. This study provides an exploration via nitrogen ions implantation to fabricate graphene-based electrodes with high electrochemical performance.

Effects of Side-Chain Length and Functionality on Polar Poly(dioxythiophene)s for Saline-Based Organic Electrochemical Transistors.

Understanding the impact of side chains on the aqueous redox properties of conjugated polymers is crucial to unlocking their potential in bioelectrochemical devices, such as organic electrochemical transistors (OECTs). Here, we report a series of polar propylenedioxythiophene-based copolymers functionalized with glyme side chains of varying lengths as well as an analogue with short hydroxyl side chains. We show that long polar side chains are not required for achieving high volumetric capacitance (C*), as short hydroxy substituents can afford facile doping and high C* in saline-based electrolytes. Furthermore, we demonstrate that varying the length of the polar glyme chains leads to subtle changes in material properties. Increasing the length of glyme side chain is generally associated with an enhancement in OECT performance, doping kinetics, and stability, with the polymer bearing the longest side chains exhibiting the highest performance ([μC*]OECT = 200 ± 8 F cm-1 V-1 s-1). The origin of this performance enhancement is investigated in different device configurations using in situ techniques (e.g., time-resolved spectroelectrochemistry and chronoamperometry). These studies suggest that the performance improvement is not due to significant changes in C* but rather due to variations in the inferred mobility. Through a thorough comparison of two different architectures, we demonstrate that device geometry can obfuscate the benchmarking of OECT active channel materials, likely due to contact resistance effects. By complementing all electrochemical and spectroscopic experiments with in situ measurements performed within a planar OECT device configuration, this work seeks to unambiguously assign material design principles to fine-tune the properties of poly(dioxythiophene)s relevant for application in OECTs.

Cyclic synthesis of lignin anthraquinone electrolytes for aqueous redox flow batteries

Lignin, which is rich in phenolic hydroxyl/methoxy groups as redox active groups, is a potential electrolyte material for aqueous redox flow batteries (ARFBs). This work demonstrated to the synthesis of lignin-derived electrolytes via cyclization with 1,4-dihydroxyanthraquinone (1,4-DHAQ), in the absence of hazardous or noble metal catalysts in mild conditions (0 °C, 1 atm). The structure of lignin anthraquinone derivatives (LAQDs) cyclized in basis alkaline solution was experimentally determined. An exhaustive comparative study was conducted with respect to the electrochemical properties, charging-discharging tests and cycling performances. The initially volumetric capacitance, the capacity retention rate and coulombic efficiency of two LAQDs were determined to be 148.0 mAh.L−1, 89.3 % and 99.0 % for coniferaldehyde-anthraquinone derivative [LAQD(G)], and 132.1 mAh.L−1, 81.2 % and 99.0 % for sinusaldehyde-anthraquinone derivative [LAQD(S)], respectively. The theoretical value calculated by DFT is consistent with the actual value. Such LAQDs can be used as organic electrolyte materials, which can overcome poor chemical stability of anthraquinone, while improving the electrochemical activity of lignin-based electrolyte materials. This technology provides a pathway to prepare organic electrolyte for the development of environment friendly and better energy storage performance electrolytes for ARFBs.

Covalently Assembled Black Phosphorus/Conductive C3N4 Hybrid Material for Flexible Supercapacitors Exhibiting a Superlong 30,000 Cycle Durability

Black phosphorus (BP) has been demonstrated as a promising electrode material for supercapacitors. Currently, the main limitation of its practical application is the low electrical conductivity and poor structure stability. Hence, BP-based supercapacitors usually severely suffer from low capacitance and poor cycling stability. Herein, a chemically bridged BP/conductive g-C3N4 (BP/c-C3N4) hybrid is developed via a facile ball-milling method. Covalent P-C bonds are generated through the ball-milling process, effectively preventing the structural distortion of BP induced by ion transport and diffusion. In addition, the overall electrical conductivity is significantly enhanced owing to the sufficient coupling between BP and highly conductive c-C3N4. Moreover, the imbalanced charge distribution around the C atom can induce the generation of a local electric field, facilitating the charge transfer behavior of the electrode material. As a result, the BP/c-C3N4-20:1 flexible supercapacitor (FSC) exhibits an outstanding volumetric capacitance of 42.1 F/cm3 at 0.005 V/s, a high energy density of 5.85 mW h/cm3, and a maximum power density of 15.4 W/cm3. More importantly, the device delivers excellent cycling stability with no capacitive loss after 30,000 cycles.

Origin and Regulation of Self‐Discharge in MXene Supercapacitors

AbstractMXene‐based supercapacitors are promising electrochemical energy‐storage devices due to their ultrahigh volumetric capacitance, high‐power characteristics, and excellent cyclability. However, they suffer from severe self‐discharging behavior while the underlying self‐discharging mechanism is still unclear. Here, the self‐discharge behavior of MXene‐based supercapacitors from surface electronic structure of MXenes is disclosed, and a novel method to mitigate it is proposed. A superficial engineering strategy based on bio‐thermal treatment is developed to effectively tailor surface electronic structure of Ti3C2Tx MXenes by eliminating hydroxyl terminations. With the evolution of surface electronic structure, as revealed by Kelvin probe force microscope and synchrotron radiation X‐ray absorption fine structure analysis, MXene‐based supercapacitors with common aqueous electrolytes show >20% decline in self‐discharge rate. This decline mechanism originates from the increased work function that induces higher zero‐charge potential after the removal of hydroxyl groups in MXenes. Meanwhile, the strengthened surface dipole leads to higher surface free energy between MXene and electrolytes. These two positive effects endow MXenes with weaker self‐discharge kinetics. Specifically, the activation‐controlled self‐discharge process is greatly suppressed. Illuminating the relevance between electronic structure and self‐discharge accompanying superficial engineering suppression strategy can guide to development of high‐performance energy storage devices.

A continuum theory of organic mixed ionic-electronic conductors of phase separation

Organic mixed ionic-electronic conductors (OMIECs) are the core functioning component in the emerging flexible, bio-, and optoelectronics owning to their unique capability of mixed conduction. Of all types, two-phase OMIECs exhibit exceptional performance due to their high stretchability and balanced ionic-electronic conduction. However, the electron-conducting phase may segregate from the ion-conducting phase in a two-phase OMIEC, changing the conducting path and eventually leading to degraded performance and dysfunction of the devices. In this work, we formulate a continuum theory following the thermodynamics framework of a two-phase OMIEC undergoing phase separation. The free energy consists of contributions from the deformation of the polymer chains, the mixing of the polymer with salts and solvents, the electrostatic field, and the two-phase interfaces. The equilibrium conditions and kinetics equations are derived with the constraint of mass conservation, thermodynamics laws, and electrostatics. We implement the theory into a finite element model and study the mechanics and electrochemistry of the OMIEC channel in an electrolyte-gated organic electrochemical transistor (OECT) device. The computational model captures the concurrent transport of charge carriers, mechanical swelling, and phase separation in the OMIEC and replicates the transfer curves of an OECT which agree well with the experiments. More specifically, we reveal the origin of the volumetric capacitance as the accumulation of charge carriers at the two-phase interfaces. We examine the parametric space to elucidate experimental observations such as molecular size-dependent conductivity and substrate-dependent phase separation. The swelling behavior and the transfer curves of OECTs under stretched, free, and constrained states are compared, demonstrating the effects of deformation on the phase dynamics and the electron-conducting behavior. We show that, for volumetric swelling and the electrochemical transfer curves, the effect of stress-transport coupling dominates while the effect of the Maxwell stress is negligible. This work provides a theoretical basis for the mechanics and electrochemistry of two-phase OMIECs.

Highly stable polyaniline array@ partially reduced graphene oxide hybrid fiber for high-performance flexible supercapacitors

The development of flexible supercapacitors (SCs) with high energy density, excellent deformation and cycling stability has great meaning for flexible electronic. Herein, polyaniline array@carbon nanotubes/partially reduced graphene oxide hybrid fiber (PANI@CNT/PRGO) was rationally designed and fabricated. The PANI array were vertically and uniformly grown on CNT/PRGO fiber by controlling the nucleation sites on the fiber. This unique structure enabled the ion to rapid diffusion and the electron to fast transport and sufficient utilization of active sites. The strong interfacial interaction between PANI and CNT/PRGO fiber can reduce the risk of PANI shedding from the fiber during mechanical deformation and long-term cycling. As a consequence, a fiber-shaped SC fabricated by PANI@CNT/PRGO fiber can deliver high volumetric capacitance of 50.2 F cm−3 at a current density of 60 mA cm−3 with 62.5% capacitance retention at 2000 mA cm−3. More encouragingly, the device not only possesses outstanding flexibility and deformation stability (Retention 96.6% after 2000 bending cycles) but also owns prominent long-term stability (Retention 95.8% after 20,000 cycles). Furthermore, the device can yield an outstanding energy density up to 6.97 mWh cm−3. This findings may carve out a new path for regulating the nucleation sites to control the distribution of nanomaterials on graphene fiber, thus obtain hybrid fiber electrodes with outstanding electrochemical performances.

Synergistic effect of two-dimensional additives on carbon nanotube film electrodes towards high-performance all-solid-state flexible supercapacitors

Flexible supercapacitors (SCs) have attracted growing interest as the power source for portable and wearable electronics. In the present work, we report the CNTs (Carbon Nanotubes) buckypaper integrated with electroactive 2D materials including graphene, graphene oxide (GO), and transition metal sulphides (TMDs), especially tin sulphide (SnS2). The flexible and free-standing, interface-enhanced CNT paper having homogeneously dispersed 2D materials was prepared using a traditional scalable casting method. The hybridized CNT films were thoroughly characterized to elucidate their structural and surface properties. Finally, such hybrid CNT films were used as free-standing electrodes without any binder for the fabrication of flexible-solid-state supercapacitor (FSSSs) devices. In the FSSS, the electrochemical interactions between hybrid CNT paper electrodes and a polymer gel electrolyte were studied. In the symmetric configuration, the CNT-SnS2 electrodes reach the highest areal and volumetric capacitance of 533 mF/cm2 and 63 mF/cm3, respectively, almost four times that of pristine CNT electrodes, whereas CNT-GO-based electrodes display high-rate capability. The CNT-SnS2-based symmetric FSSS devices exhibit an extended voltage window of 1.5 V with a high capacitance of 133 mF/cm2 and show high cyclic stability for 5000 cycles under 180 0 bendings.

Activation-induced layered structure in NiCoAl by atomic modulation for energy storage application

The surface activation of alloys favors their electrochemical interactions, ion diffusivity, and the rapid kinetics of ions and electrons, leading to the formation of self-supported layered double hydroxides (LDHs) in them. However, the formation of LDHs at different depths in the alloy upon activation, their electronic/atomic structures, and their electrochemical charge storage mechanism, have not been thoroughly explored. Herein, Ni ion-substituted CoAl alloys are prepared by arc melting and activated by KOH electrolyte, which is responsible for the modulation of the atomic configuration as confirmed by XRD. Raman depth mapping demonstrates how the LDHs vary with depth upon activation and that the octahedral and tetrahedral symmetry sites of CoO and Co3O4 are responsible for the formation of the layered structures of CoOOH and Co(OH)2, respectively. The activated Ni10Co85Al5 has a superior volumetric capacitance of 4.15 F/cm3 at 0.5 mA/g, which is 38.6 times that of an unactivated one, and excellent cyclic stability up to 5000 cycles, and a voltage of 0.54 V generated from a fabricated supercapacitor cell. X-ray Absorption Spectroscopy (XAS) analysis indicates greater charge transfer by Co than by Ni and the modulation of the local atomic structures facilitates electrochemical charge storage in Ni10Co85Al5. This work presents an easy route for the development of advanced LDHs, and the mechanism of electrochemical charge storage in them.

Nanostructured zig-zag γ-Mo2N thin films produced by glancing angle deposition for flexible symmetrical solid-state supercapacitors

The performance of electrochemical capacitors strongly depends on their accessible surface area, chemical stability, and electrical conductivity. Simple columnar and zig-zag γ-Mo2N thin-film electrodes were prepared by magnetron sputtering. The latter is obtained by glancing angle deposition (GLAD), which is known for open porous structures due to the ballistic shadowing effect. As intended, the zig-zag structured γ-Mo2N electrode presents an outstanding area capacitance of 248 mF/cm2 at a scan speed of 50 mV/s, which is 4 times higher than that of the simple columnar one. Both of them exhibit excellent cycling stability of 95 % over 20,000 cycles (at 200 mV/s). The symmetrical solid-state supercapacitor prepared with the zig-zag structured γ-Mo2N thin film delivers an excellent power density of 107.1 W/cm3 at 33.8 mWh/cm3, and its volumetric capacitance is ∼ 3.5 times higher than the simple columnar structured γ-Mo2N device. Bending tests of such solid-state γ-Mo2N supercapacitors proved their mechanical flexibility to a bending angel of even 107°. Based on these studies we can conclude that the highly porous zig-zag structured γ-Mo2N-based electrodes prepared by GLAD combine outstanding electrochemical energy storage capabilities with excellent mechanical flexibility.

Morphology-controlled synthesis of metal-organic frameworks derived hollow nanostructures for supercapacitor application

The design and construction of nanostructured electrode materials are regarded as a key factor for the supercapacitor. In this paper, we demonstrated a self-templated method which hollow structure with different morphologies derived from ZIF-67 (Zeolitic imidazolate frameworks-67) as electrode materials can be synthesized. By the assistance of surfactant and proper sulfurization, the hollow cobalt sulfides with dodecahedron, cube, and plate-like morphologies can be obtained. The optimized hollow cobalt sulfide dodecahedron electrode displayed a high specific capacitance of 684.08 F g−1 at a scan rate of 5 mV s−1, while the hollow cobalt sulfide nanoplate, in contrast, showed a highest capacitive retention. Also, they were further employed as the positive electrode in asymmetric supercapacitors. The assembled hollow cobalt sulfide dodecahedron//activated carbon asymmetric supercapacitor device exhibited a maximum operating potential window of 1.5 V, high volumetric capacitance of 3511 mF cm−3, the maximum energy density of 1.17 Wh cm−3 and the maximum power density of 0.697 W cm−3. This part of the result revealed the relationship of different morphologies-hollow metal−organic framework structures with the electrochemical performance in supercapacitor application.

Nitrogen-enriched activated carbons via dual N-doping processes: Electrode material for high gravimetric- and volumetric-performance supercapacitor

N/O co-doped activated carbons are prepared via a hydrothermal synthesis (HTS) using glucose and egg solution (egg white + egg yolk) as precursors, followed by an ammonia activation. Controlling the egg amount for the HTS processing and the ammonia concentration for activation, we tailor the N and O contents in the ACs and achieve a N amount of 6.44 at.%. The comprehensive electrochemical performances of the symmetrical supercapacitors (SCs) prepared with the N/O co-doped ACs are investigated with 1 M H2SO4 as electrolyte and show positive dependence on the N amount. The SCs with the ACs of the highest N amount possess a gravimetric capacitance of 364 F/g and a volumetric capacitance of 276 F/cm3 as well as good rate performances of 72 % in the capacitance retention at 10 A/g and remarkable cycling stability of ~95 % after 10,000 GCD cycles at 5 A/g. The positive effects on the intrinsic electrochemical properties of the ACs are further confirmed by an optimized specific capacitance per surface area of 0.28 F/m2 and a highest nominal diffusion coefficient of 0.146 × 10−6 cm2/s which are achieved by the ACs with highest amount of N contents.

Structural transformation of porous and disordered carbon during ball-milling

Mechanical ball-milling is a commonly used method to shattering crystals or introducing defects in carbon materials. But herein, we observe that the highly porous and disordered carbon lattice of activated carbon is converted to graphitic structure during ball-milling, along with the introduction of oxygen in the bulk phase. The well-developed pores are gradually eliminated, leading to obviously decreased surface area from 1364 to 156 m2 g-1 and improved compaction density from 0.573 to 1.423 g cm-3. Due to the continuous particle cleavage, re-assembly, and cold-welding, the as-formed carbon blocks show 2 times larger in size and 4 times higher bulk oxygen content. This simultaneous crystallinity enhancement and oxygen functionalization is barely observed in other methods. The mechanism is that highly reactive carbon radicals generated by mechanical cleavage of carbon skeleton enable atom rearrangement to form large graphitic domains or being terminated by oxygenated groups. Interestingly, the capacitive energy storage performance of the ball-milled activated carbon is changed from electric-double-layer to pseudo-capacitance in aqueous electrolyte, showing significantly improved volumetric capacitances and high-rate performance. Our work provides a theoretical guidance for mechanochemistry of porous and disordered carbon materials.

Wet oxidative functionalization of carbon nanotube cloth to boost its performance as a flexible supercapacitor electrode

Electrochemical recharging behavior of a binder-free flexible carbon nanotube cloth (CNTC) material in respect to aqueous supercapacitor applications is reported. To provide high enough pseudocapacitance, the surface of CNTC was exposed to wet oxidative functionalization by a KMnO4 treatment in an acidic medium. We report the effect of a KMnO4 to CNT molar ratio on capacitance, electrical resistivity and specific surface area of the CNTC material. The change in the composition of oxygen-containing interfacial functional groups attached to the carbon surface is also investigated by X-ray photoelectron spectroscopy and temperature-programmed desorption methods. For the most oxidized CNTC sample the capacitance of ca. 71 F g−1 (29 µF cm−2BET) in 0.5 M H2SO4 (slightly decreasing with a scan rate up to 100 mV s−1) with the retention of ca. 99% during 30,000 recharging cycles as well as specific resistivity of ca. 31 µΩ m are achieved. Finally, two flexible symmetric supercapacitor prototypes based on a 38% H2SO4 solution and operating at 1.3 or 1.5 V were assembled from the freestanding and graphite foil supported functionalized CNTC electrodes, respectively. For the former device, the volumetric capacitance of ca. 22 F cm−3, power density of ca. 10 kW l−1 and energy density of ca. 3 Wh l−1 (per total volume of both electrodes) are demonstrated. The issues of the long-term stability and self-discharge behavior of assembled prototypes are also considered. Reported results allow us to consider the functionalized CNTC material as a promising electrode material for flexible aqueous supercapacitors.