Highly Uniform Anodically Deposited Film of MnO2 Nanoflakes on Carbon Fibers for Flexible and Wearable Fiber-Shaped Supercapacitors

A Review of Yarn-Based One-Dimensional Supercapacitors

Energy storage in a one-dimensional format is increasingly vital for the functionality of wearable technologies and is garnering attention from various sectors, such as smart apparel, the Internet of Things, e-vehicles, and robotics. Yarn-based supercapacitors are a particularly compelling solution for wearable energy reserves owing to their high power densities and adaptability to the human form. Furthermore, these supercapacitors can be seamlessly integrated into textile fabrics for practical utility across various types of clothing. The present review highlights the most recent innovations and research directions related to yarn-based supercapacitors. Initially, we explore different types of electrodes and active materials, ranging from carbon-based nanomaterials to metal oxides and conductive polymers, that are being used to optimize electrochemical capacitance. Subsequently, we survey different methodologies for loading these active materials onto yarn electrodes and summarize innovations in stretchable yarn designs, such as coiling and buckling. Finally, we outline a few pressing research challenges and future research directions in this field.

Electrospray Deposition of PEDOT:PSS on Carbon Yarn Electrodes for Solid-State Flexible Supercapacitors

The increasing demand for flexible electronic devices has risen due to the high interest in electronic textiles (e-textiles). Consequently, the urge to power e-textiles has sparked enormous interest in flexible energy storage devices. One-dimensional (1D) configuration supercapacitors are the most promising technology for textile applications, but often their production involves complex synthesis techniques and expensive materials. This work unveils the use of the novel electrospray deposition (ESD) technique for the deposition of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). This deposition methodology on conductive carbon yarns creates flexible electrodes with a high surface area. The deposition conditions of PEDOT:PSS were optimized, and their influence on the electrochemical performance of a 1D symmetric supercapacitor with a cellulose-based gel as an electrolyte and a separator was evaluated. The tests herein reported show that these capacitors exhibited a high specific capacitance of 72 mF g^-1, an excellent cyclability of more than 85% capacitance retention after 1500 cycles, and an outstanding capability of bending.

Large Area Millisecond Preparation of High-Quality, Few-Layer Graphene Films on Arbitrary Substrates via Xenon Flash Lamp Photothermal Pyrolysis and Their Application for High-Performance Micro-supercapacitors

We report a method for fast, efficient, and scalable preparation of high-quality, large area, few-layer graphene films on arbitrary substrates via high-intensity pulsed xenon flash lamp photothermal pyrolysis of thin precursor films at ambient conditions in millisecond time frames. The precursors comprised poly(2,2-bis(3,4-dihydro-3-phenyl-1,3-benzoxazine)), and cyclized polyacrylonitrile and possess significant absorption cross section within the bandwidth of the emission spectrum of a xenon flash lamp. By localizing light absorption to the precursor films, the process enabled the preparation of few-layer graphene films on any substrate, including thermally sensitive substrates without the need for any catalytic substrate as in chemical vapor deposition-based approaches or conductive electrodes as in electrochemical method-based approaches. The extent of conversion of the precursor films to graphene was strongly dependent on pulse energy and the local temperature achieved due to photothermal effect, which were controlled via pulse power modulation; it also depended on structural properties of the precursor and to a lesser extent on the substrate. The cPAN showed a higher efficiency for conversion to graphene, as confirmed by Raman spectra (I_D/I_G ∼ 0.3), and sheet resistance of 0.1 Ω cm. To demonstrate the utility of the process, graphene film electrodes prepared photothermally on carbon fiber current collector were used for the fabrication of micro-supercapacitors with a very high areal supercapacitance of 3.5 mF/cm². Subsequent deposition of manganese oxide onto the fabricated electrodes significantly increased the energy storage capability of the supercapacitor, yielding a device with exceptionally high capacitance of 80 F/g at 1 mA current, good rate capability, and long cycle life.

Recent Advances and Challenges Toward Application of Fibers and Textiles in Integrated Photovoltaic Energy Storage Devices

Flexible microelectronic devices have seen an increasing trend toward development of miniaturized, portable, and integrated devices as wearable electronics which have the requirement for being light weight, small in dimension, and suppleness. Traditional three-dimensional (3D) and two-dimensional (2D) electronics gadgets fail to effectively comply with these necessities owing to their stiffness and large weights. Investigations have come up with a new family of one-dimensional (1D) flexible and fiber-based electronic devices (FBEDs) comprising power storage, energy-scavenging, implantable sensing, and flexible displays gadgets. However, development and manufacturing are still a challenge owing to their small radius, flexibility, low weight, weave ability and integration in textile electronics. This paper will provide a detailed review on the importance of substrates in electronic devices, intrinsic property requirements, fabrication classification and applications in energy harvesting, energy storage and other flexible electronic devices. Fiber- and textile-based electronic devices for bulk/scalable fabrications, encapsulation, and testing are reviewed and presented future research ideas to enhance the commercialization of these fiber-based electronics devices.

A Review of MnO2 Composites Incorporated with Conductive Materials for Energy Storage

Manganese dioxide (MnO₂ ) has been widely used in the field of energy storage due to its high specific capacitance, low cost, natural abundance, and being environmentally friendly. However, suffering from poor electrical conductivity and high dissolvability, the performance of MnO₂ can no longer meet the needs of rapidly growing technological development, especially for the application as electrode material in metal-ion batteries and supercapacitors. In this review, recent studies on the development of binary or multiple MnO₂ -based composites with conductive components for energy storage are summarized. Firstly, general preparing methods for MnO₂ -based composites are introduced. Subsequently, the binary and multiple MnO₂ -based composites with carbon, conducting polymer, and other conductive materials are discussed respectively. The improvement in their performance is summarized as well. Finally, perspectives on the practical applications of MnO₂ -based composites are presented.

Wide-Spectrum Modulated Electrochromic Smart Windows Based on MnO2/PB Films

Inorganic materials have been extensively studied for visible electrochromism in the past few decades. However, the single inorganic electrochromic (EC) material commonly exhibits a single color change, leading to a narrow spectrum of modulation, which offsets or limits the maximally energy-saving ability. Here, we present a wide-spectrum modulated EC device designed by combining the complementary EC nanocomposite of manganese dioxide (MnO₂) and Prussian blue (PB) for enhanced energy savings. Porous MnO₂ nanostructures serve as host frameworks for the templated growth of PB, resulting in MnO₂/PB nanocomposites. The complementary optical modulation ranges of MnO₂ and PB enable a widen-spectrum modulation across the solar region with the development of the MnO₂/PB nanocomposite. The colored MnO₂/PB device exhibited an optical modulation of 32.1% in the wide solar spectrum range of 320-1100 nm and blocked 72.0% of the solar irradiance. Furthermore, fast switching responses (2.7 s for coloration and 2.1 s for bleaching) and a high coloration efficiency (83.1 cm²·C^-1) of the MnO₂/PB EC device are also achieved. The high EC performance of the MnO₂/PB nanocomposite device provides a new strategy for the design of high-performance energy-saving EC smart windows.

Organic/Inorganic Hybrid Fibers: Controllable Architectures for Electrochemical Energy Applications

Organic/inorganic hybrid fibers (OIHFs) are intriguing materials, possessing an intrinsic high specific surface area and flexibility coupled to unique anisotropic properties, diverse chemical compositions, and controllable hybrid architectures. During the last decade, advanced OIHFs with exceptional properties for electrochemical energy applications, including possessing interconnected networks, abundant active sites, and short ion diffusion length have emerged. Here, a comprehensive overview of the controllable architectures and electrochemical energy applications of OIHFs is presented. After a brief introduction, the controllable construction of OIHFs is described in detail through precise tailoring of the overall, interior, and interface structures. Additionally, several important electrochemical energy applications including rechargeable batteries (lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries), supercapacitors (sandwich-shaped supercapacitors and fiber-shaped supercapacitors), and electrocatalysts (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction) are presented. The current state of the field and challenges are discussed, and a vision of the future directions to exploit OIHFs for electrochemical energy devices is provided.

Facile Synthesis and Electrochemical Studies of Mn2O3/Graphene Composite as an Electrode Material for Supercapacitor Application

A simplified sol-gel method that can be scaled up for large-scale production was adopted for the preparation of manganese oxide nanocrystals. Prepared Mn₂O₃ exhibited micron-sized particles with a nanoporous structure. In the present study, a simple and low-cost strategy has been employed to fabricate nanoporous Mn₂O₃ with an increased surface area for an electrode/electrolyte interface that improved the conduction of Mn₂O₃ material. The crystal phase and morphology of the prepared material was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The prepared electrode materials were deposited on a nickel foam substrate to investigate the electrochemical properties. The galvanostatic charge/discharge (GCD), cyclic voltammetry (CV), and complex impedance studies confirmed excellent specific capacitance and capacitive behavior of the prepared material. The synthesized Mn₂O₃/graphene composites exhibited an excellent specific capacitance of 391 F/g at a scan rate of 5 mV/S. Moreover, a specific capacitance of 369 F/g was recorded at a current density of 0.5 A/g using the galvanostatic charge/discharge test. The high porosity of the materials provided a better electrolyte-electrode interface with a larger specific area, thus suggesting its suitability for energy storage applications.

Low cost, high efficiency flexible supercapacitor electrodes made from areca nut husk nanocellulose and silver nanoparticle embedded polyaniline

Energy storage is a key aspect in the smooth functioning of the numerous gadgets that aid easy maneuvering through modern life. Supercapacitors that store energy faradaically have recently emerged as potential inventions for which mechanical flexibility is an absolute requirement for their future applications. Flexible supercapacitors based on nanocellulose extracted from easily available waste materials via low cost methods have recently garnered great attention. In the present work, we discuss the construction of flexible, binder-free supercapacitive electrodes using nanocellulose extracted from locally available areca nut husks and polyaniline embedded with silver nanoparticles. The prepared electrodes were characterized using SEM, TEM, XRD, FTIR, EDX and electrochemical characterization techniques such as CV, galvanostatic charge-discharge, chronoamperometry and EIS. A specific capacitance of 780 F g-1 was obtained for the silver nanoparticle embedded polyaniline-nanocellulose (Ag-PANI-NC) substrate supported electrodes, which is ∼4.2 times greater than that of bare polyaniline-nanocellulose electrodes. We attributed this enhancement to a lowering of the activation energy barrier of correlated electron hopping among localized defect states in the composite matrix by the Ag nanoparticles. An energy density value of 15.64 W h kg-1 and a power density of 244.8 W kg-1 were obtained for the prepared electrodes. It was observed that the Ag-PANI-NC based electrode can retain ∼98% of its specific capacitance upon recovery from mechanical bending to extreme degrees.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Analyzing extended STIRPAT model of urbanization and CO2 emissions in Asian countries

CO₂ emissions tend to increase more rapidly in underdeveloped economies as compared to developed countries mainly in China, India, and Asia. One of the aspects that accounts for the increasing CO₂ emissions is urbanization (UR) and it is increasing all over the world particularly in Asian and African regions. The present study examines the impact of energy use and UR on carbon emissions over the period 1995 to 2018 while using the extended STIRPAT model for Asian countries. Panel co-integration techniques and Granger causality test are applied on selected variables. FMOLS and DOLS methods are also applied to check for robustness. Findings confirm the presence of long-run co-integration among variables. The outcomes propose that energy consumption and UR have positive impact on CO₂ emissions and output. Outcomes also reveal that financial development (FD) has negative effect on emissions of CO₂ but positive effect on economic growth. Results of Granger causality technique indicate that long-run causality association exists among emissions of CO₂, economic growth, and UR. In the short run (SR), bidirectional causal relationship has been found between trade openness and FD.

Atomic Modulation of 3D Conductive Frameworks Boost Performance of MnO2 for Coaxial Fiber-Shaped Supercapacitors

Coaxial fiber-shaped supercapacitors are a promising class of energy storage devices requiring high performance for flexible and miniature electronic devices. Yet, they are still struggling from inferior energy density, which comes from the limited choices in materials and structure used. Here, Zn-doped CuO nanowires were designed as 3D framework for aligned distributing high mass loading of MnO₂ nanosheets. Zn could be introduced into the CuO crystal lattice to tune the covalency character and thus improve charge transport. The Zn-CuO@MnO₂ as positive electrode obtained superior performance without sacrificing its areal and gravimetric capacitances with the increasing of mass loading of MnO₂ due to 3D Zn-CuO framework enabling efficient electron transport. A novel category of free-standing asymmetric coaxial fiber-shaped supercapacitor based on Zn_0.11CuO@MnO₂ core electrode possesses superior specific capacitance and enhanced cell potential window. This asymmetric coaxial structure provides superior performance including higher capacity and better stability under deformation because of sufficient contact between the electrodes and electrolyte. Based on these advantages, the as-prepared asymmetric coaxial fiber-shaped supercapacitor exhibits a high specific capacitance of 296.6 mF cm^-2 and energy density of 133.47 μWh cm^-2. In addition, its capacitance retention reaches 76.57% after bending 10,000 times, which demonstrates as-prepared device's excellent flexibility and long-term cycling stability.

Hierarchical CoO@Ni(OH)2 core-shell heterostructure arrays for advanced asymmetric supercapacitors

Constructing multicomponent electrode materials with a rational structure is an effective route to develop high-performance supercapacitors. We herein report a novel nickel-foam-supported hierarchical CoO@Ni(OH)₂ nanowire-nanosheet core-shell heterostructure array synthesized by a facile hydrothermal-electrodeposition strategy. The core CoO nanowire arrays with good electrical conductivity and shell Ni(OH)₂ nanosheets with thickness of ∼ 2 nm synergistically contributes to increased active sites, fast mass transfer, and improved structural stability. Consequently, the optimal CoO@Ni(OH)₂-400 s architecture delivers a high specific capacitance of 1418.2 F g^-1 at 1 A g^-1 and 93.7% retention after 5000 cycles. Furthermore, the CoO@Ni(OH)₂//activated carbon asymmetric supercapacitor could achieve an outstanding energy density of up to 92.47 W h kg^-1 at 800 W kg^-1. This simple but effective strategy provides insight into the development of core-shell hierarchical architectures for constructing high-performance supercapacitors.

Binder Free and Flexible Asymmetric Supercapacitor Exploiting Mn3O4 and MoS2 Nanoflakes on Carbon Fibers

Emerging technologies, such as portable electronics, have had a huge impact on societal norms, such as access to real time information. To perform these tasks, portable electronic devices need more and more accessories for the processing and dispensation of the data, resulting in higher demand for energy and power. To overcome this problem, a low cost high-performing flexible fiber shaped asymmetric supercapacitor was fabricated, exploiting 3D-spinel manganese oxide Mn₃O₄ as cathode and 2D molybdenum disulfide MoS₂ as anode. These asymmetric supercapacitors with stretched operating voltage window of 1.8 V exhibit high specific capacitance and energy density, good rate capability and cyclic stability after 3000 cycles, with a capacitance retention of more than 80%. This device has also shown an excellent bending stability at different bending conditions.

A Review on Nano-/Microstructured Materials Constructed by Electrochemical Technologies for Supercapacitors

The article reviews the recent progress of electrochemical techniques on synthesizing nano-/microstructures as supercapacitor electrodes. With a history of more than a century, electrochemical techniques have evolved from metal plating since their inception to versatile synthesis tools for electrochemically active materials of diverse morphologies, compositions, and functions. The review begins with tutorials on the operating mechanisms of five commonly used electrochemical techniques, including cyclic voltammetry, potentiostatic deposition, galvanostatic deposition, pulse deposition, and electrophoretic deposition, followed by thorough surveys of the nano-/microstructured materials synthesized electrochemically. Specifically, representative synthesis mechanisms and the state-of-the-art electrochemical performances of exfoliated graphene, conducting polymers, metal oxides, metal sulfides, and their composites are surveyed. The article concludes with summaries of the unique merits, potential challenges, and associated opportunities of electrochemical synthesis techniques for electrode materials in supercapacitors.

Metal Oxide and Hydroxide-Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need-Tailored Devices

Energy storage devices that efficiently use energy, in particular renewable energy, are being actively pursued. Aqueous redox supercapacitors, which operate in high ionic conductivity and environmentally friendly aqueous electrolytes, storing and releasing high amounts of charge with rapid response rate and long cycling life, are emerging as a solution for energy storage applications. At the core of these devices, electrode materials and their assembling into rational configurations are the main factors governing the charge storage properties of supercapacitors. Redox-active metal compounds, particularly oxides and hydroxides that store charge via reversible valence change redox reactions with electrolyte ions, are prospective candidates to optimize the electrochemical performance of supercapacitors. To address this target, collaborative investigations, addressing different streams, from fundamental charge storage mechanisms and electrode materials engineering to need-tailored device assemblies, are the key. Over the last few years, significant achievements in metal oxide and hydroxide-based aqueous supercapacitors have been reported. This work discusses the most recent achievements and trends in this field and brings into the spotlight the authors' viewpoints.

Facile Activation of Commercial Carbon Felt as a Low-Cost Free-Standing Electrode for Flexible Supercapacitors

High cost, low capacitance, and complicated synthesis process are still the key limitations for carbon-negative materials to meet their industrial production and application in high-energy-density asymmetric supercapacitors (ASCs). In this work, we demonstrate the facile preparation of ultrahigh-surface-area free-standing carbon material from low-cost industrial carbon felt (CF) and its application for flexible supercapacitor electrode with outstanding performance. Through a simple freeze-drying-assisted activation method, the as-prepared activated CF (ACF) was endowed with satisfactory flexibility, ultrahigh specific surface area of 2109 m² g^-1, good electric conductivity (311 S m^-1), and excellent wettability to aqueous electrolyte. Owing to these merits, the ACF expressed an ultrahigh areal capacitance of 1441 mF cm^-2, a high specific capacitance ( C_s) of 280 F g^-1 based on the mass of the whole electrode, and an impressive cycling stability (87% retention after 5000 cycles). When applied as a flexible freestanding electrode for MnO₂//ACF ASCs, the ACF-based device provided satisfactory areal energy densities of 0.283 and 0.104 mWh cm^-2 in aqueous and quasi-solid electrolytes, respectively. The values outperform many previously reported carbon-based electrochemical devices. The low cost of raw material and the facile fabrication process, together with the high electrochemical performance, make our ACF electrode highly applicable for the mass production of flexible energy-storage devices.

Nitrogen and Oxygen Co-doped Graphitized Carbon Fibers with Sodiophilic-Rich Sites Guide Uniform Sodium Nucleation for Ultrahigh-Capacity Sodium-Metal Anodes

Sodium (Na) metal is an ideal anode for high-energy Na batteries due to the low cost and natural abundance of Na metal. Nevertheless, issues regarding dendritic and mossy Na metal deposits have prevented their practical application. Herein, nitrogen and oxygen co-doped graphitized carbon fibers (DGCF) have been developed as the Na plating hosts to direct Na metal homogeneous nucleation and suppress the growth of Na dendrites. We show experimental results as well as first-principles calculations demonstrating that the uniformly doped nitrogen and oxygen function as sodiophilic sites that direct the sodium-metal nucleation to a smooth dendrite-free anode. The resultant DGCF-Na anode can be cycled stably at 1 mA cm^-2 for a high areal capacity of 12.7 mA h cm^-2 with an average Coulombic efficiency of 99.8%, and a Na|Na symmetrical cell can be cycled with long-term durability for more than 1200 h at 2 mA cm^-2. When coupled with P2-Na_2/3Ni_1/3Mn_1/3Ti_1/3O₂ and Na₃V₂(PO₄)₃ cathodes, the DGCF-Na composite demonstrates good feasibility in full cells.

Patterning Islandlike MnO2 Arrays by Breath-Figure Templates for Flexible Transparent Supercapacitors

Although plenty of active materials could be used as supercapacitor electrodes, only limited ones have been engineered to construct transparent supercapacitors. Specially, it is a great challenge to make opaque metal oxides, which often own high energy density, into transparent films. Here, we demonstrate a novel approach to fabricate transparent MnO₂ films for flexible transparent supercapacitors. By utilizing breath-figure polymer films with ordered pores as template, arrays of MnO₂ islands were electrochemically deposited, with high light transmission. The thickness and interspace distance of MnO₂ island arrays could be adjusted by tuning deposition time so that the capacitance and transparency of the electrodes are changed accordingly. Such island array structure can effectively eliminate the internal stress existing in the composite film to avoid cracks during bending operation. The assembled transparent supercapacitor shows a transmittance of 44% at 550 nm and can yield a high capacitance of 4.73 mF/cm² at a current density of 50 μA/cm², demonstrating high flexibility and stability.