Ni3S2 nanorods and three-dimensional reduced graphene oxide electrodes-based high-performance all-solid-state flexible asymmetric supercapacitors

A flexible asymmetric supercapacitor (ASC) with high electrochemical performance was constructed using reduced graphene oxide (rGO)-wrapped redox-active metal oxide-based negative and positive electrodes. Thin layered rGO functionality on the positive and the negative electrode surfaces has promoted the feasible surface-active sites and enhances the electrochemical response with a wide operating voltage window. Herein we report the controlled growth of rGO-wrapped tubular FeMoO4 nanofibers (NFs) via electrospinning followed by surface functionalization as a negative electrode. The tubular structure offers the ultrathin-layer decoration of rGO inside and outside of the tubular walls with uniform wrapping. The rGO-wrapped tubular FeMoO4 NF electrode exhibited a high specific capacitance of 135.2 F g−1 in Na2SO4 neutral electrolyte with an excellent rate capability and cycling stability (96.45% in 5000 cycles) at high current density. Meanwhile, the hydrothermally synthesized binder-free rGO/MnO2 nanorods on carbon cloth (rGO-MnO2@CC) were selected as cathode materials due to their high capacitance and high conductivity. Moreover, the ASC device was fabricated using rGO-wrapped FeMoO4 on carbon cloth (rGO-FeMoO4@CC) as the negative electrode and rGO-MnO2@CC as the positive electrode (rGO-FeMoO4@CC/rGO-MnO2@CC). The rationally designed ASC device delivered an excellent energy density of 38.8 W h kg−1 with a wide operating voltage window of 0.0–1.8 V. The hybrid ASC showed excellent cycling stability of 93.37% capacitance retention for 5000 cycles. Thus, the developed rGO-wrapped FeMoO4 nanotubes and MnO2 nanorods are promising hybrid electrode materials for the development of wide-potential ASCs with high energy and power density.

Partially nitrogenized mesoporous Co3O4 nanoflakes as a binder-free positive electrode for high-performance flexible solid-state asymmetric supercapacitors

Hierarchical FeCo2O4@NiCo layered double hydroxide core/shell nanowires for high performance flexible all-solid-state asymmetric supercapacitors

Decoration of carbon nanofibers with NiCo2S4 nanoparticles for flexible asymmetric supercapacitors

Porous NiCo2O4 nanowires supported on carbon cloth for flexible asymmetric supercapacitor with high energy density

Flexible energy storage devices are in great demand since the advent of flexible electronics. Until now, flexible supercapacitors based on graphene analogues usually have had low operating potential windows. To this end, two dissimilar electrode materials with complementary potential ranges are employed to obtain an optimum cell voltage of 1.8 V. A low-temperature organic sol-gel method is used to prepare two different types of functionalized reduced graphene oxide aerogels (rGOA) where Ag nanorod functionalized rGOA acts as a negative electrode while polyaniline nanotube functionalized rGOA acts as a positive electrode. Both materials comprehensively exploit their unique properties to produce a device that has high energy and power densities. An assembled all-solid-state asymmetric supercapacitor gives a high energy density of 52.85 W h kg-1 and power density of 31.5 kW kg-1 with excellent cycling and temperature stability. The device also performs extraordinarily well under different bending conditions, suggesting its potential to meet the requirements for flexible electronics.

1. Introduction Carbon cloth (CC), which is composed of numerous uniform carbon fibers, has attracted significant attention in recent years as an electrode for use in flexible supercapacitors, owing to the low cost of this material, along with its unique 3D structure, high surface area, chemical stability, electrical conductivity and flexibility. However, because the active part of CC consists solely of an exfoliated carbon shell, as-purchased CC exhibits lower gravimetric capacitance (1~2 F g-1) than other carbonaceous materials, such as graphene and carbon nanotubes (100~200 F g-1). Thus, CC is effective when used as a scaffold to support highly capacitive materials. In the present study, commercial CC pieces were first treated using facile wet processes and the resulting material served as the substrate for the direct growth of a birnessite-type MnO2 film.1 During this process, the mass loading of the MnO2 was optimized in terms of the areal capacitance (capacitance normalized by the unit area of the electrode) and the electrochemical utilization. An asymmetric supercapacitor was subsequently built by assembling the optimized MnO2/CC together with activated carbon (AC)-coated CC as the positive and negative electrodes, respectively. We report the fabrication of a MnO2-modified CC electrode with optimized loading and the construction of a device with high performance and good stability. 2. Experimental MnO2 was deposited electrochemically in 2 mM MnSO4 aqueous solution containing 50 mM KCl, in which the treated and untreated CC pieces were used as the working electrodes. The CC piece with the optimized mass loading of MnO2 was used as the positive electrode together with an activated carbon-coated CC (AC/CC) as the negative electrode to build a two-electrode asymmetric cell. First, this cell was assembled in an open beaker, such that the potentials of both electrodes could be monitored with respect to a Ag/AgCl reference electrode while the voltage between the two electrodes was controlled within the range of 0 to 2.0 V. After a stable response was observed, the equilibrated electrodes were detached and re-assembled in an acrylic holder with a cellulose filter paper (pre-soaked in 0.5 M Na2SO4 electrolyte) sandwiched between the MnO2- and AC-modified CC pieces to construct a solid-state supercapacitor (The cell volume, 0.07 cm3). 3. Results and discussion A simple wet process was developed to enhance the EDLC performance of commercial CC, including oxidation followed by reduction with NaBH4. This treatment was found to enhance the areal capacitance from 4 to 78 mF cm-2. The mass loading of birnessite-type MnO2 film could be varied by controlling the delivered charge density during electrodeposition. Based on the magnitude of areal capacitance (C A) and the electrochemical utilization, the optimal MnO2 loading appeared to be approximately 4 mg cm-2. The MnO2-modified CC positive electrode with the optimized mass loading was combined with an AC-modified CC negative electrode to build an asymmetric supercapacitor operating at a cell voltage of 2.0 V, in which the potential windows of 0.73 and 1.27 V were assigned to the MnO2 and AC electrodes, respectively. The volumetric capacitance (C vol) values of each experimental cell were collected as a function of scan rate. The C vol at 2 mV s-1 increased in the order symmetric AC/CC < asymmetric MnO2/CC//AC/CC < symmetric MnO2/CC, which can largely be attributed to the capacitance of the single electrode. With increasing scan rate, the symmetric MnO2/CC cell exhibited a significantly decreased C vol due to slow kinetics. In contrast, the asymmetric cell showed a gentle decrease, equivalent to better rate-capability, indicating a contribution from the EDLC due to the AC negative electrode. The volumetric capacitance of the MnO2//AC asymmetric supercapacitor at scan rates above 20 mV s-1 was much higher than those of the MnO2 and AC symmetric supercapacitors. The asymmetric MnO2/CC//AC/CC cell exhibited high volumetric energy density (0.978 mW cm-3) and power density (0.158 W cm-3) with excellent cycling stability (94.4% after 10,000 cycles). This exceptional performance per unit volume can be attributed to the following reasons: an increase in the mass loading of MnO2 resulting from the pre-treatment of the commercial CC,an improvement in the rate capability, due to the specific structure of the electrodeposited MnO2 (birnessite) and the optimization of the loading mass based on considering the electrochemical utilization of the MnO2 on the CC, andavoidance of side reactions at the positive and negative electrodes in the asymmetric cell. Reference 1) M. Nakayama et al, “A Direct Electrochemical Route to Construct a Polymer/Manganese Oxide Layered Structure”, Inorg. Chem. , 43, 8215-8217 (2004).

In recent years, a flexible supercapacitor with high storage properties, it is no longer concerned because of its low voltage window. Here, we design and synthesize CoFe 2 O 4 nanoparticles grow on Carbon Fiber Cloth (CFC) via a mild hydrothermal and spay the ultra‐conductive polymer Poly (3,4‐ ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) on the surface of CoFe 2 O 4 nanolayer. The as‐fabricated PEDOT:PSS /Fe 2 O 3 ‐CNTs /CFC and PEDOT:PSS/CoFe 2 O 4 /CFC electrodes as the negative and positive pseudocapacitor electrode show an excellent capacitance of 426 and 472.5 F g −1 at 1 A g −1 , respectively. An aqueous flexible asymmetric supercapacitor (AFAS) at 2.0 V and achieve well specific capacitance of 181.3 F g −1 at a current density of 1 A g −1 and the maximum energy density is 25.17 Wh kg −1 at power density of 620.7 W kg −1 . To the aqueous flexible asymmetric supercapacitor, both positive and negative electrodes are important for the electrochemical performance. The power storage capacity per unit area of two electrodes should be similar, because the area of negative and positive electrode of flexible asymmetric supercapacitor must be similarity. The advantages of AFAS, according to its low‐cost, high capacitances and simple fabrication process, show that AFAS can be one of the excellent candidate flexible asymmetric supercapacitor for next‐generation high‐performance supercapacitors.

For the first time, WO2.72 nanowires were in-situ grown on carbon cloth by a simple solvothermal reaction. The nanowire WO2.72/carbon cloth (NW WO2.72/CC) electrode showed good electrochemical performance with specific capacitance (Cs) reaching up to 398 F g−1 at a current density of 2 A g−1. The capacitance of 240 F g−1 was retained at a high current density of 16 A g−1. To further evaluate the energy storage performance, flexible asymmetric supercapacitors (FASCs) were fabricated using the activated carbon/carbon cloth (AC/CC) as negative electrode and NW WO2.72/CC as positive electrode, respectively. The FASCs delivered a high energy density of 28 Wh kg−1 at a power density of 745 W kg−1 and 13 Wh kg−1 even at a high power density of 22.5 kW kg−1. More impressively, 81% of the specific capacitance of the FASCs was retained after 10,000 cycles, indicating excellent cycle stability. This work indicates the NW WO2.72/CC holds a great potential for application in energy storage devices.

Supercapacitors have attracted enormous attention for energy storage in both academic and industrial sectors in the past years. In this study, all‐solid‐state flexible asymmetric supercapacitors (ASCs) without any binder, incorporated with the hydrophilic carbon cloth (HCC) with MnO2 nanocomposite (HCC@MnO2) as the positive electrode, the HCC with polypyrrole (PPy) (HCC@PPy) as the negative electrode, and polyvinyl alcohol (PVA)–LiCl gel as both gel electrolyte and separator, are reported. The HCC@MnO2 and HCC@PPy electrodes are prepared by direct deposition of either MnO2 nanoparticles or PPy nanofilms on the HCC through a simple, facile, and controllable electrochemical deposition method, respectively. The HCC@MnO2 and HCC@PPy electrodes provide rich contact area for gel electrolyte, facilitating the rapid delivery of electrolyte ions, and also minimize the resistance of ASCs. As a result, all‐solid‐state flexible binder‐free HCC@MnO2//HCC@PPy ASCs exhibit a large operating voltage of 1.8 V, high energy density of 28.2 Wh kg−1 at the power density of 420.5 W kg−1, and excellent cycling stability (91.2% capacitance retention after 5000 cycles). The present study provides a facile, scalable, and efficient approach to fabricate all‐solid‐state ASCs with high electrochemical storage performance for flexible electronics.

Composite of CoS1.97 nanoparticles decorated CuS hollow cubes with rGO as thin film electrode for high-performance all solid flexible supercapacitors

Due to the global warming and energy depletion issues, developing sustainable and renewable energy resources has become a critical concern among researchers. The constantly growing demand for energy has urged researchers to develop highly improved energy storage devices. In relation to relevant energy storage systems, supercapacitor technology has drawn burgeoning interest in recent years owing to its environmentally safe and cost effective advantages. Especially, its high power density (>10 kW/kg), fast charge/discharge characteristics, and excellent cycle stability are highly beneficial in storing renewable energy. Currently, the rapid development of portable electronic devices and the expanding renewable energy systems have paved the way for energy storage systems to play an important role in human society. Many significant breakthroughs for the next-generation supercapacitors have been achieved in terms of material synthesis, device innovations, and multifunctional device designs. Therefore, this paper summarizes the latest progress on new materials and novel device assemblies. First, to fully assess the effect of device assembly on supercapacitor performance, five types of supercapacitor structures and their assembly principles are discussed in detail, including the three-electrode (also known as semi-battery) device, two-electrode supercapacitor, flexible solid-state supercapacitor, fiber supercapacitor, and planar (micro-) supercapacitor. Among them, the three-electrode system and the two-electrode device are still the most widely used types at present. The three-electrode cell is suitable for characterizing electrode materials or investigating the electrochemical storage processes. The structure of a two-electrode system is closer to an actual supercapacitor. Other specially designed cells, which are fabricated with the aim of meeting the requirements of flexible and lightweight energy sources, have also been proposed in recent years. The results indicate that the structural innovative design could provide a fascinating way to enhance the energy density of devices while also holding huge potential to enhance the compatibility between supercapacitor component and various portable or wearable electronic devices. A careful interpretation and rigorous scrutiny of the electrochemical characteristics of every supercapacitor is also conducted in this work. Moreover, the design principles for enhancing the supercapacitor performance are highlighted through a comprehensive analysis of the literature. The main challenges in the structural innovations for enhancing the electrochemical performance are analyzed. The solutions to overcome these challenges are proposed. Second, supercapacitors still suffer from a lower energy density compared with Li-ion batteries. Among various efforts to build high-performance supercapacitors in recent years, major improvements have been made in electrode materials with rational designs. This review article also examines the latest methodologies and performance evaluation metrics for several emerging electrode materials in terms of their improved electrochemical properties, including carbon materials, binary transition metal oxides (NiCo2O4, Ni3V2O8, and Co3V2O8, among others), transition metal chalcogenides/selenide/phosphide positive electrodes, and VN, Fe2O3 negative electrode materials. This paper also highlights the electrode material design principle, which is the fundamental understanding of the relationships between structural design, structural properties and components of electrode materials and their electrochemical performances. Moreover, we also summarized the latest contributions and progress in multifunctional supercapacitors, which include the transparent flexible supercapacitor, self-healable supercapacitor, piezoelectric supercapacitors, self-charging supercapacitors, and so on. Several methods to realize the abilities of transparent, folded, wearable, self-healable, and even self-chargeable supercapacitors with almost no performance degradation are discussed. This paper also analyzes the compatibility of a multifunctional supercapacitor with industrial manufacturing, and offers a paradigm for developing portable and wearable energy storage devices and systems. Furthermore, the operating principles, system design/engineering, and the rational optimization of the multifunctional supercapacitors are also analyzed in this review. Finally, the major challenges faced by next-generation supercapacitors, along with the future research prospects, are discussed at the end of the paper.

In this article, we report a novel electrode of NiCo2O4 nanowire arrays (NWAs) on carbon textiles with a polypyrrole (PPy) nanosphere shell layer to enhance the pseudocapacitive performance. The merits of highly conductive PPy and short ion transport channels in ordered NiCo2O4 mesoporous nanowire arrays together with the synergistic effect between NiCo2O4 and PPy result in a high specific capacitance of 2244 F g(-1), excellent rate capability, and cycling stability in NiCo2O4/PPy electrode. Moreover, a lightweight and flexible asymmetric supercapacitor (ASC) device is successfully assembled using the hybrid NiCo2O4@PPy NWAs and activated carbon (AC) as electrodes, achieving high energy density (58.8 W h kg(-1) at 365 W kg(-1)), outstanding power density (10.2 kW kg(-1) at 28.4 W h kg(-1)) and excellent cycling stability (∼89.2% retention after 5000 cycles), as well as high flexibility. The three-dimensional coaxial architecture design opens up new opportunities to fabricate a high-performance flexible supercapacitor for future portable and wearable electronic devices.

Metal organic frameworks (MOFs) with binder-free electrodes have shown promise for portable electrochemical energy storage applications. However, their low specific capacitance and challenges associated with the attachment of active materials to the substrate constrain their practical utility. In this research, we prepared a CoNi0.5-MOF/CC electrode by in situ growth of CoNi0.5-MOF on an H2O2-pretreated carbon cloth (CC) without using any binder. It exhibits a higher specific capacitance of 1337.5 F g-1 than that of CoNi0.5-MOF (∼578 F g-1) at a current density of 1 A g-1 and an excellent rate ability of 88% specific capacitance retention at a current density of 10 A g-1 after 6000 cycles. The as-assembled flexible asymmetric solid-state supercapacitor based on the CoNi0.5-MOF/CC positive electrode and a nitrogen-doped graphene (N-Gr) negative electrode exhibits an energy density of 61.46 W h kg-1 at a power density of 1244.56 W kg-1 and holds a stable capacitance of ∼125 F g-1 at 1 A g-1 when the flexible supercapacitor is bent, showing great potential for flexible electronics application. The H2O2 is indicated to play an important role, enhancing the adhesion of CoNi0.5-MOF on CC and reducing its charge transfer resistance by functionalizing the carbon fiber during the pretreatment of the CC matrix. The results provide a great way to prepare a flexible asymmetric solid-state supercapacitor with both high power density and high energy density for practical application.

Chapter 13 - Summary and future perspectives