Orderly and highly dense polyaniline nanorod arrays fenced on carbon nanofibers for all-solid-state flexible electrochemical energy storage

Carbon quantum dot-anchored polyaniline on electrospun carbon nanofibers as freestanding electrodes for symmetric solid-state supercapacitors

A binder-free and freestanding electrode was designed by uniformly immobilizing carbon quantum dot (CQD)-anchored polyaniline (PANI) heterostructures onto electrospun carbon nanofibers (CNFs) via a facile hierarchical assembly process. The fabricated freestanding CNF/PANI/CQD electrode exhibits a unique three-dimensional (3D) network nanostructure, which accelerates ion migration between the interior and surface of the electrode, thereby enhancing its charging and discharging performance. Moreover, the functional groups on the surface of CQDs could anchor PANI through possible chemical bonding, which not only improves the stability of the PANI/CQD heterojunction but also creates an additional conductive channel for the PANI polymer. As expected, the CQD-anchored electrode exhibits a specific capacitance of 756.5 F g-1 at 0.5 A g-1, which is 1.78 times better than the capacitance of CNFs/PANI and 17.4 times higher than that of CNFs. Benefiting from the excellent electrical conductivity and freestanding structure of the electrode, the assembled symmetric solid-state supercapacitor exhibits an excellent capacitance stability rate of 94.69% following 8000 cycles and remains at 81.25% after 10 000 cycles of operation at 1 A g-1. This remarkable cycling durability may be due to the superior 3D network nanostructure and the stable PANI/CQD heterostructure created through chemical bonding. This work offers new design concepts for fabricating binder-free and freestanding electrodes with unique 3D structures for application in other energy storage devices.

Carbon Nanofibers as Supporting Substrate for Growth of Polyaniline Nanorods on Fe2O3 Nanoneedles toward Electrochemical Energy Storage

Iron-oxide (Fe2O3) nanoneedles were first in situ grown on the surface of carbon nanofibers (CNFs) using hydrothermal and N2 annealing process, and then polyaniline (PANI) was coated on the Fe2O3 nanoneedles to form network-like nanorods through dilute solution polymerization. The PANI/Fe2O3/CNFs binder-free electrode exhibited a high specific capacitance of 603 F/g at 1 A/g with good rate capability. (The capacitance loss was about 48.3% when the current density increased from 1.0 to 5.0 A/g.) It was caused by the fact that the PANI/Fe2O3/CNFs with a well-connected structure could provide a continuous electron transport path and improve the conductivity of the entire electrode. The solid-state hybrid PANI/Fe2O3/CNFs∥PANI/Fe2O3/CNFs symmetric device also achieved a high energy density of 29.85 Wh/kg at a power density of 500 W/kg. This universal compatible synthetic method for the PANI/Fe2O3/CNFs electrode could extend to other supercapacitor electrode systems, making it easy to fabricate various ternary electrodes for supercapacitors.

Dynamically Cross-Linked, Self-Healable, and Stretchable All-Hydrogel Supercapacitor with Extraordinary Energy Density and Real-Time Pressure Sensing

Wearable electronics with flexible, integrated, and self-powered multi-functions are becoming increasingly attractive, but their basic energy storage units are challenged in simultaneously high energy density, self-healing, and real-time sensing capability. To achieve this, a fully flexible and omni-healable all-hydrogel, that is dynamically crosslinked PVA@PANI hydrogel, is rationally designed and constructed via aniline/DMSO-emulsion-templated in situ freezing-polymerization strategy. The PVA@PANI sheet, not only possesses a honeycombed porous conductive mesh configuration with superior flexibility that provides numerous channels for unimpeded ions/electron transport and maximizes the utilization efficiency of pseudocapacitive PANI, but also can conform to complicated body surface, enabling effective detection and discrimination of body movements. As a consequence, the fabricated flexible PVA@PANI sheet electrode demonstrates an unprecedented specific capacitance (936.8 F g-1 ) and the assembled symmetric flexible all-solid-state supercapacitor delivers an extraordinary energy density of 40.98 Wh kg-1 , outperforming the previously highest-reported values of stretchable PVA@PANI hydrogel-based supercapacitors. What is more, such a flexible supercapacitor electrode enables precisely monitoring the full-range human activities in real-time, and fulfilling a quick response and excellent self-recovery. These outstanding flexible sensing and energy storage performances render this emerging PVA@PANI hydrogel highly promising for the next-generation wearable self-powered sensing electronics.

A high mass loading flexible electrode with a sheet-like Mn3O4/NiMoO4@NiCo LDH on a carbon cloth for supercapacitors

Mass loading is an important parameter to evaluate the application potential of active materials in high-capacity supercapacitors. Synthesizing active materials with high mass loading is a promising strategy to improve high performance energy storage devices. Preparing electrode materials with a porous structure is of significance to overcome the disadvantages brought by high mass loading. In this work, a Mn3O4/NiMoO4@NiCo layered double hydroxide (MO/NMO/NiCo LDH) positive electrode is fabricated on a carbon cloth with a high mass loading of 20.4 mg cm-2. The MO/NMO/NiCo LDH presents as a special three-dimensional porous nanostructure and exhibits a high specific capacitance of 815 F g-1 at 1 A g-1. Impressively, the flexible supercapacitor based on the MO/NMO/NiCo LDH positive electrode and an AC negative electrode delivers a maximum energy density of 22.5 W h kg-1 and a power density of 8730 W kg-1. It also retains 60.84% of the original specific capacitance after bending to 180° 600 times. Moreover, it exhibits 76.92% capacitance retention after 15 000 charge/discharge cycles. These results make MO/NMO/NiCo LDH one of the most attractive candidates of positive electrode materials for high-performance flexible supercapacitors.

In Situ Growth of MnO2 Nanosheets on a Graphite Flake as an Effective Binder-Free Electrode for High-Performance Supercapacitors

In this work, manganese dioxide (MnO2) nanosheets in situ loaded on a high-purity graphite flake (GF) were prepared by one-step hydrothermal deposition. It was found that the specific capacitance value of a single MnO2/GF electrode was 882 F/g at a current density of 1.0 A/g in a KOH electrolyte, and the specific capacitance retention of the MnO2/GF electrode can reach about 90.1% after 5000 charge-discharge cycles at a current density of 10 A/g. Furthermore, a MnO2/GF∥MnO2/GF symmetric supercapacitor device was fabricated with two pieces of MnO2/GF electrodes and ordinary filter paper with a 1 M KOH/PVA gel electrolyte as a separator. The single symmetric device displayed a high energy density of 64.2 Wh/kg at a power density of 400 W/kg within an applied voltage of 1.6 V, and this value was superior to those of previously reported MnO2-based systems. A tandem device consisting of a five-series tandem device (the applied voltage of a single device was 0.7 V) and a three-series tandem device (the applied voltage of a single device was 1.6 V) was prepared to drive a red light-emitting diode (LED). These findings open up application prospects for MnO2-based composite electrode materials for high-performance supercapacitors.

Designing flexible, smart and self-sustainable supercapacitors for portable/wearable electronics: from conductive polymers

The rapid development of portable/wearable electronics proposes new demands for energy storage devices, which are flexibility, smart functions and long-time outdoor operation. Supercapacitors (SCs) show great potential in portable/wearable applications, and the recently developed flexible, smart and self-sustainable supercapacitors greatly meet the above demands. In these supercapacitors, conductive polymers (CPs) are widely applied due to their high flexibility, conductivity, pseudo-capacitance, smart characteristics and moderate preparation conditions. Herein, we'd like to introduce the CP-based flexible, smart and self-sustainable supercapacitors for portable/wearable electronics. This review first summarizes the flexible SCs based on CPs and their composites with carbon materials and metal compounds. The smart supercapacitors, i.e., electrochromic, electrochemical actuated, stretchable, self-healing and stimuli-sensitive ones, are then presented. The self-sustainable SCs which integrate SC units with energy-harvesting units in one compact configuration are also introduced. The last section highlights some current challenges and future perspectives of this thriving field.

A review on the origin of nanofibers/nanorods structures and applications

In this review work, we highlight the origin of morphological structures such as nanofibers/nanorods in case of various materials in nano as well as bulk form. In addition, a discussion on different cations of different ionic radii and other intrinsic factors is provided. The materials (ceramic titanates, ferrites, hexaferrites, oxides, organic/inorganic composites, etc.,) exhibiting the nanofibers/nanorods like morphological structures are tabulated. Furthermore, the significance of nanofibers/nanorods obtained from distinct materials is elucidated in multiple scientific and technological fields. At the end, the device applications of these morphological species are also described in the current technology. The nucleation and growth mechanism of α-MnO₂ nanorods using natural extracts from Malus domestica and Vitis vinifera [3].

High-power-energy proton supercapacitor based on interface-adapted durable polyaniline and hexagonal tungsten oxide

Supercapacitors are high power energy storage devices, however, their application are remain limited by the low energy density. Developing high capacity electrode materials and constructing devices with high operating voltage are effective ways to solve this problem. Herein, performance of polyaniline (PANI) electrode materials is dramatically enhanced by engineering robust PANI/carbon interfaces, through assembling PANI nanorod array on rose petals derived carbon network (RPDCN). The structure of the PANI is optimized by adjusting the concentration of the aniline precursor. The unique structure enables the prepared PANI/RPDCN composite show a high capacitance of 636 F g^-1 at 0.5 A g^-1, based on the total weight of PANI and RPDCN substrate. The robust interface effectively prolonged the composite electrode stably cycled for over 2000 cycles at 2 A g^-1 with a capacity retention of 89%. When coupled with a hexagonal tungsten oxide (h-WO₃) anode, a high-power asymmetric proton supercapacitor with high energy densities (29.0 Wh kg^-1/0.61 kW kg^-1 and 21.4 Wh kg^-1/19.51 kW kg^-1) was assembled. This work provides an effective and eco-friendly route toward superior PANI electrodes and proposes a promising high-power energy storage system using proton as working ion.

In Situ Growth of Oriented Polyaniline Nanorod Arrays on the Graphite Flake for High-Performance Supercapacitors

Polyaniline with oriented nanorod arrays could provide high surface area and relaxed nanostructure to optimize ion diffusion paths, thus enhancing the performance of the device. In this paper, we designed an all-solid symmetrical supercapacitor with good performance based on polyaniline nanorod arrays in situ-grown on a graphite flake free-standing substrate. The specific capacitance, cycle stability, and energy density of the prepared supercapacitor device were 135 F/g, 75.4% retention after 1500 cycles, and the energy density is 18.75 W h/kg at a power density of 500 W/kg. The good performance of the supercapacitor device was obviously related to the oriented nanorod arrays of polyaniline/graphite flakes. In order to find the application of the prepared supercapacitor device, the tandem device consisting of three single supercapacitor devices connected in series had been used to drive small electronic equipment. The red light-emitting diode and chronograph could be easily driven by the 3-series supercapacitor devices. These results indicated that the prepared supercapacitor device based on the polyaniline/graphite flake electrode had potential applications in energy storage devices.

Preparation of Y-Doped La2Ti2O7 Flexible Self-Supporting Films and Their Application in High-Performance Flexible All-Solid-State Supercapacitor Devices

Flexible all-solid-state supercapacitors have drawn more attention owing to the rapid growth of wearable electronic equipments. Herein, we have succeeded in synthesizing a series of Y-doped lanthanum titanate flexible self-supporting films (LSF-x, 0.1 ≤ x ≤ 0.5) and investigating the change of microstructures, morphological characteristics, and lattice structures of these films affected by different Y-doping contents. To further determine the optimum Y-doping content, we have explored the electrochemical properties of working electrodes prepared by LSF-x (0.1 ≤ x ≤ 0.5) samples as the main active material. As the LSF-0.2 electrode has the best areal capacitance of 1.3 F·cm-2 at 2 mA·cm-2, we use the LSF-0.2 electrodes and PVA-Na2SO4 gel to fabricate a flexible all-solid-state supercapacitor device. This device has a high areal capacitance of 255.9 mF·cm-2 at a current density of 2 mA·cm-2 with a high cell voltage of 2.1 V, while the corresponding energy density is 156.8 μWh·cm-2 with a power density of 2.1 mW·cm-2. Moreover, it also shows a long cycling life and outstanding flexibility. Therefore, the LSF-0.2 sample can be used as an excellent energy-storage material for a wearable electronic device.