Highly ordered MnO₂ nanopillars for enhanced supercapacitor performance

Advanced three-dimensional hierarchical porousα-MnO2nanowires network toward enhanced supercapacitive performance

Hierarchicalα-MnO2nanowires with oxygen vacancies grown on carbon fiber have been synthesized by a simple hydrothermal method with the assistance of Ti4+ions. Ti4+ions play an important role in controlling the morphology and crystalline structure of MnO2. The morphology and structure of the as-synthesized MnO2could be tuned fromδ-MnO2nanosheets to hierarchicalα-MnO2nanowires with the help of Ti4+ions. Based on its fascinating properties, such as many oxygen vacancies, high specific surface area and the interconnected porous structure, theα-MnO2electrode delivers a high specific capacitance of 472 F g-1at a current density of 1 A g-1and the rate capability of 57.6% (from 1 to 16 A g-1). The assembled symmetric supercapacitor based onα-MnO2electrode exhibits remarkable performance with a high energy density of 44.5 Wh kg-1at a power density of 2.0 kW kg-1and good cyclic stability (92.6% after 10 000 cycles). This work will provide a reference for exploring and designing high-performance MnO2materials.

Intercalation chemistry engineering strategy enabled high mass loading and ultrastable electrodes for High-Performance aqueous electrochemical energy storage devices

Aqueous electrochemical energy storage devices (AEESDs) are considered one of the most promising candidates for large-scale energy storage infrastructure due to their high affordability and safety. Developing electrodes with the merits of high energy density and long lifespan remains a challenging issue toward the practical application of AEESDs. Research attempts at electrode materials, nanostructure configuration, and electronic engineering show the limitations due to the inherent contradictions associated with thicker electrodes and ion-accessible kinetics. Herein, we propose an intercalation chemistry engineering strategy to enhance the electrolyte ion (de)intercalation behaviors during the electrochemical charge-discharge. To validate this strategy, the prototypical model of a high-mass-loading MnO2-based electrode is used with controlled intercalation of Na+ and H2O. Theoretical and experimental results reveal that an optimal content of Na+ and H2O on the MnO2-based electrode exhibits superior electrochemical performance. Typically, the resultant electrode exhibits an impressive areal capacitance of 1551 mF/cm2 with a mass loading of 9.7 mg/cm2 (at 1 mA/cm2). Furthermore, the assembled full-cell with obtained MnO2-based electrode delivers a high energy density of 0.12 mWh/cm2 (at 20.02 mW/cm2) and ultra-high cycling stability with a capacitance retention percentage of 89.63 % (345 mF/cm2) even after 100,000 cycles (tested over 72 days).

Surfactant effect on energy storage performance of hydrothermally synthesized Ni3V2O8

We report a study to improve the ternary oxide Ni3V2O8's electrochemical energy storage capabilities through correct surfactanization during hydrothermal synthesis. In this study, Ni3V2O8nanomaterials were synthesized in three different forms: one with a cationic surfactant (CTAB), one with an anionic surfactant (SLS), and one without any surfactant. FESEM study reveals that all the synthesized Ni3V2O8nanomaterials had a small stone-like morphology. The electrochemical study showed that anionic surfactant-assisted Ni3V2O8(NVSLS) had a maximum of 972 F g-1specific capacitance at 1 A g-1current density, whereas cationic surfactant-assisted Ni3V2O8(NVCTAB) had the lowest specific capacitance of 162 F g-1. The specific capacitance and the capacitance retention of the NVSLS(85% after 4000 cycles) based electrode was much better than that of the NVCTAB(76% after 4000 cycles) based electrode. The improved energy storage properties of the NVSLSelectrode are attributed to its high diffusion coefficient, high surface area, and enriched elemental nickel, as compared to the NVCTABelectrode. All these excellent electrochemical properties of NVSLSelectrode indicates their potential usage in asymmetric supercapacitor application.

Enhanced electromagnetic wave absorption of Fe3O4@MnO2@Ni-Co/C composites derived from Prussian blue analogues

Herein, Fe₃O₄@MnO₂@Ni-Co/C composites derived from PBAs were successfully fabricated. Firstly, Ni-Co Prussian blue analogues (Ni-Co PBAs) were used as precursors to derive a carbon layer on their surface by annealing treatment and subsequently translated into MnO₂@Ni-Co/C nanocubes after hydrothermal reactions. Fe₃O₄@MnO₂@Ni-Co/C composites were finally obtained after depositing Fe₃O₄ nanoparticles through the annealing process. Their electromagnetic wave (EMW) absorption performance apparently enhanced, thanks to the excellent impedance matching and strong attenuation derived from the synergy between the dielectric loss and the magnetic loss. In particular, the minimum reflection loss (RL_min) of Fe₃O₄@MnO₂@Ni-Co/C reached -41.2 dB with a thickness of 4.0 mm and the effective absorption bandwidth (EAB) reached 7.1 GHz with a thickness of 2.0 mm. Therefore, the results could be significant for synthesizing EMW absorbers with excellent performance, a broad bandwidth, strong absorption, thin thickness and light weight.

Organic Supercapacitors as the Next Generation Energy Storage Device: Emergence, Opportunity, and Challenges

Harnessing new materials for developing high-energy storage devices set off research in the field of organic supercapacitors. Various attractive properties like high energy density, lower device weight, excellent cycling stability, and impressive pseudocapacitive nature make organic supercapacitors suitable candidates for high-end storage device applications. This review highlights the overall progress and future of organic supercapacitors. Sustainable energy production and storage depend on low cost, large supercapacitor packs with high energy density. Organic supercapacitors with high pseudocapacitance, lightweight form factor, and higher device potential are alternatives to other energy storage devices. There are many recent ongoing research works that focus on organic electrolytes along with the material aspect of organic supercapacitors. This review summarizes the current research status and the chemistry behind the storage mechanism in organic supercapacitors to overcome the challenges and achieve superior performance for future opportunities.

Polypyrrole-coated copper@graphene core-shell nanoparticles for supercapacitor application

The performance of supercapacitors strongly depends on the electrochemical characterizations of electrode materials. Herein, a composite material consisted of polypyrrole (PPy) and multilayer graphene-wrapped copper nanoparticles (PPy/MLG-Cu NPs) is fabricated on a flexible carbon cloth (CC) substrate via two-step synthesis process for supercapacitor application. Where, MLG-Cu NPs are prepared on CC by one-step chemical vapor deposition synthesis approach; thereafter, the PPy is further deposited on the MLG-Cu NPs/CC via electropolymerization. The related material characterizations of PPy/MLG-Cu NPs are well investigated by scanning electron microscopic, high resolution transmission electron microscopy, Raman spectrometer and x-ray photoelectron spectroscopy; the electrochemical behaviors of the pertinent electrodes are studied by cyclic voltammogram, galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements. The flexible electrode with PPy/MLG-Cu NPs composites exhibits the best specific capacitance of 845.38 F g^-1at 1 A g^-1, which is much higher than those of electrodes with PPy (214.30 F g^-1), MLG-Cu NPs (6.34 F g^-1), multilayer graphene hollow balls (MLGHBs; 52.72 F g^-1), and PPy/MLGHBs (237.84 F g^-1). Finally, a supercapacitor system consisted of four PPy/MLG-Cu NPs/CC electrodes can efficiently power various light-emitting diodes (i.e. red, yellow, green and blue lighs), demonstrating the practical application of PPy/MLG-Cu NPs/CC electrode.

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.

Multimetallic transition metal phosphide nanostructures for supercapacitors and electrochemical water splitting

Transition metal phosphides (TMPs) have recently emerged as an important class of functional materials and been demonstrated to be outstanding supercapacitor electrode materials and catalysts for electrochemical water splitting. While extensive investigations have been devoted to monometallic TMPs, multimetallic TMPs have lately proved to show enhanced electrochemical performance compared to their monometallic counterparts, thanks to the synergistic effect between different transition metal species. This topical review summarizes recent advance in the synthesis of new multimetallic TMP nanostructures, with particular focus on their applications in supercapacitors and electrochemical water splitting. Both experimental reports and theoretical understanding of the synergy between transition metal species are comprehensively reviewed, and perspectives of future research on TMP-based materials for these specific applications are outlined.

Hierarchically nanostructured Zn0.76C0.24S@Co(OH)2 for high-performance hybrid supercapacitor

It is a great challenge to achieve both high specific capacity and high energy density of supercapacitors by designing and constructing hybrid electrode materials through a simple but effective process. In this paper, we proposed a hierarchically nanostructured hybrid material combining Zn_0.76Co_0.24S (ZCS) nanoparticles and Co(OH)₂ (CH) nanosheets using a two-step hydrothermal synthesis strategy. Synergistic effects between ZCS nanoparticles and CH nanosheets result in efficient ion transports during the charge-discharge process, thus achieving a good electrochemical performance of the supercapacitor. The synthesized ZCS@CH hybrid exhibits a high specific capacity of 1152.0 C g^-1 at a current density of 0.5 A g^-1 in 2 M KOH electrolyte. Its capacity retention rate is maintained at ∼ 70.0% when the current density is changed from 1 A g^-1 to 10 A g^-1. A hybrid supercapacitor (HSC) assembled from ZCS@CH as the cathode and active carbon (AC) as the anode displays a capacitance of 155.7 F g^-1 at 0.5 A g^-1, with a remarkable cycling stability of 91.3% after 12,000cycles. Meanwhile, this HSC shows a high energy density of 62.5 Wh kg^-1 at a power density of 425.0 W kg^-1, proving that the developed ZCS@CH is a promising electrode material for energy storage applications.

Exploring flowery MnO2/Ag nanocomposite as an efficient solar-light-driven photocatalyst

An efficient approach was developed to boost the solar light driven photocatalytic efficacy of pristine flowery MnO2 NCs through the immobilization of Ag NPs, which in turn produces MnO2/Ag NCs.

Role of Oxygen Vacancy Ordering and Channel Formation in Tuning Intercalation Pseudocapacitance in Mo Single-Ion-Implanted CeO2-x Nanoflakes

Metal oxide pseudocapacitors are limited by low electrical and ionic conductivities. The present work integrates defect engineering and architectural design to exhibit, for the first time, intercalation pseudocapacitance in CeO_2-x. An engineered chronoamperometric electrochemical deposition is used to synthesize 2D CeO_2-x nanoflakes as thin as ∼12 nm. Through simultaneous regulation of intrinsic and extrinsic defect concentrations, charge transfer and charge-discharge kinetics with redox and intercalation capacitances together are optimized, where reduction increases the gravimetric capacitance by 77% to 583 F g^-1, exceeding the theoretical capacitance (562 F g^-1). Mo ion implantation and reduction processes increase the specific capacitance by 133%, while the capacitance retention increases from 89 to 95%. The role of ion-implanted Mo⁶⁺ is critical through its interstitial solid solubility, which is not to alter the energy band diagram but to facilitate the generation of electrons and to establish the midgap states for color centers, which facilitate electron transfer across the band gap, thus enhancing n-type semiconductivity. Critically, density functional theory simulations reveal, for the first time, that the reduction causes the formation of ordered oxygen vacancies that provide an atomic channel for ion intercalation. These channels enable intercalation pseudocapacitance but also increase electrical and ionic conductivities. In addition, the associated increased active site density enhances the redox such that the 10% of the Ce³⁺ available for redox (surface only) increases to 35% by oxygen vacancy channels. These findings are critical for any oxide system used for energy storage systems, as they offer both architectural design and structural engineering of materials to maximize the capacitance performance by achieving accumulative surface redox and intercalation-based redox reactions during the charge/discharge process.

Fabrication of hierarchical MoO3@NixCo2x(OH)6x core-shell arrays on carbon cloth as enhanced-performance electrodes for asymmetric supercapacitors

Advanced integrated electrode materials with designedcore-shell nanostructureplay a crucial role for the application in alternative energy storage system. Herein, hierarchical MoO₃@Ni_xCo_2x(OH)_6x core-shell arrays were equably grown on face of carbon cloth after a series of hydrothermal growth and electrochemical deposition processes. This core-shell arrays structure can not only provide large electroactive surface areas and high speed ion transport paths, but also keep the material structure stable during the process of redox reactions. Thus MoO₃@Ni_xCo_2x(OH)_6x displays excellent electrochemical performance (4.7 F cm^-2 at 10 mA cm^-2). Moreover, the asymmetric supercapacitor is assembled with MoO₃@Ni_xCo_2x(OH)_6x and carbon nanotubes, which delivers a maximal energy density of 0.50 mWh cm^-2 at 4.25 mW cm^-2, high specific capacitance and superior cycling stability (94.5% capacitance retention after 5000 cycles). We believe that MoO₃@Ni_xCo_2x(OH)_6x arrays could be a great prospective candidate energy storage materials.

Carbon coating on metal oxide materials for electrochemical energy storage

During the past decades, nano-structured metal oxide electrode materials have received growing attention due to their low development cost and high theoretical specific capacity, accordingly, quite a lot of metal oxide electrode materials are being used in electrochemical energy storage devices. However, the further development was limited by the relatively low electrical conductivity and the volume expansion during electrochemical reactions. Thus, many approaches have been proposed to obtain high-efficiency metal oxide electrode materials, such as designing nanomaterials with ideal morphology and high specific surface area, optimizing with carbon-based materials (such as graphene and glucose) to prepare nanocomposites, combining with conductive substrates to enhance the conductivity of electrodes, etc. Owning to the advantages of low cost and high chemical stability of carbon materials, core-shell structure formed by carbon-coated metal oxides is considered to be a promising solution to solve these problems. Therefore, this review mainly focuses on recent research advances in the field of carbon-coated metal oxides for energy storage, summarizing the advantages and disadvantages of common metal oxides and different types of carbon sources, and proposing methods to optimize the material properties in terms of structure and morphology, carbon layer thickness, coating method, specific surface area and pore size distribution, as well as improving electrical conductivity. In addition, the double or multi-layer coating strategy is also a reflection of the continuous development of carbon coating method. Hopefully, this rereview may provide a new direction for the renewal and development of future energy storage electrode materials.

Electrodeposition of the MnO2 on the Ag/Au Core-Shell Nanowire and Its Application to the Flexible Supercapacitor

Supercapacitors have received considerable attention as energy storage devices owing to their high power density, fast charge/discharge rate, and long cyclic life. Especially with an increasing demand for flexible and wearable devices, research on flexible supercapacitors has surged in recent years. The silver nanowire (Ag NW) network has been used as a flexible electrode owing to its excellent mechanical and electrical properties; however, its use as an electrode for flexible supercapacitors has been limited due to insufficient electrochemical stability. In this study, we proposed a method to resolve this issue. We employed a solution process that enabled the coating of the surface of Ag NW by a thin Au shell of ≈ 5 nm thickness, which significantly improved the electrochemical stability of the Ag NW network electrodes. Furthermore, we confirmed for the first time that MnO₂, which is one of the most widely used capacitive materials, can be directly electroplated on the AACS NW network electrode. Finally, we fabricated a high-performance and flexible solid-state supercapacitor using the suggested Ag/Au/MnO₂ core–shell NW network electrodes.

Materials Combining Asymmetric Pore Structures with Well-Defined Mesoporosity for Energy Storage and Conversion

Porous materials design often faces a trade-off between the requirements of high internal surface area and high reagent flux. Inorganic materials with asymmetric/hierarchical pore structures or well-defined mesopores have been tested to overcome this trade-off, but success has remained limited when the strategies are employed individually. Here, the attributes of both strategies are combined and a scalable path to porous titanium nitride (TiN) and carbon membranes that are conducting (TiN, carbon) or superconducting (TiN) is demonstrated. These materials exhibit a combination of asymmetric, hierarchical pore structures and well-defined mesoporosity throughout the material. Fast transport through such TiN materials as an electrochemical double-layer capacitor provides a substantial improvement in capacity retention at high scan rates, resulting in state-of-the-art power density (28.2 kW kg^-1) at competitive energy density (7.3 W-h kg^-1). In the case of carbon membranes, a record-setting power density (287.9 kW kg^-1) at 14.5 W-h kg^-1 is reported. Results suggest distinct advantages of such pore architectures for energy storage and conversion applications and provide an advanced avenue for addressing the trade-off between high-surface-area and high-flux requirements.

Ultra-long cycle life and binder-free manganese-cobalt oxide supercapacitor electrodes through photonic nanostructuring

We report a novel photonic processing technique as a next-generation cost-effective technique to instantaneously synthesize nanostructured manganese-cobalt mixed oxide reduced graphitic oxide (Mn-Co-rGO) for supercapacitor electrodes in energy storage applications. The active material was prepared directly on highly conductive Pt-Si substrate, eliminating the need for a binder. Surface morphological analysis showed that the as-prepared electrodes have a highly porous and resilient nanostructure that facilitates the ion/electron movement during faradaic redox reactions and buffers the volume changes during charge-discharge processes, leading to the improved structural integrity of the electrode. The presence of distinct redox peaks, due to faradaic redox reactions, at all scan rates in the cyclic voltammetry (CV) curves and non-linear nature of the charge-discharge curves suggest the pseudocapacitive charge storage mechanism of the electrode. The electrochemical stability and the life cycle were examined by carrying out galvanostatic charge-discharge (GCD) measurements at 0.40 mA cm^-2 constant areal current density for 80 000 cycles, and the electrode showed 95% specific capacitance retention, exhibiting excellent electrochemical stability and an ultra-long cycle life. Such remarkable electrochemical performance could be attributed to the enhanced conductivity of the electrode, the synergistic effect of metal ions with rGO, and the highly porous morphology, which provides large specific surface area for electrode/electrolyte interaction and facilitates the ion transfer.

Birnessite based nanostructures for supercapacitors: challenges, strategies and prospects

In the past few years, intensive attention has been focused on birnessite based electrodes for supercapacitors. Much progress has been achieved in developing birnessite based nanostructures with high electrochemical performance. However, challenges still remain in taking full advantage of birnessite and building smart structures to overcome the gap between the obtained capacitance and its theoretical capacitance. In this review, the basic information on birnessite and its preparation strategies are summarized and the current challenges are put forward. Finally, some new strategies for preparing high electrochemical performance birnessite based nanostructures are highlighted.

Hollow nanostructures of metal oxides as emerging electrode materials for high performance supercapacitors

Comparative study of TMO based hollow and solid nanostructures for supercapacitor applications.

Electrochemically grown MnO2 nanowires for supercapacitor and electrocatalysis applications

In this study, MnO₂ nanowires are electrochemically grown over a 3D nickel foam (NF) substrate using cyclicvoltammetry at 27 °C; furthermore, their potential for applications in supercapacitors and oxygen evolution reaction (OER) is highlighted.

The effects of Ni ions' charge disproportionation on the high electrochemical performance of Ni1−xCoxO nanoparticles

The outstanding electrochemical properties of Ni_1−xCo_xO electrode materials can be attributed to the Ni ion charge disproportionation, which is caused by Co atom doping.

Intelligent electrochromic-supercapacitor based on effective energy level matching poly(indole-6-carboxylicacid)/WO3 nanocomposites

A high-quality electrochromic-supercapacitor based on poly(indole-6-carboxylicacid)/WO₃ nanocomposites can intelligently monitor the energy storage state by changing the color of the device.

Enhanced adsorption of Cd(II) using a composite of poly(acrylamide-co-sodium acrylate) incorporated LDH@MoS24

The Mg/Al layered double hydroxide (Mg/Al-LDH) intercalated with the [Formula: see text] (MoS₄-LDH) impregnated into poly(acrylamide-co-sodium acrylate) (PP) was synthesized as layered double hydroxides-PP (LDHS-PP), whose characterization, adsorption properties and mechanisms were investigated. The maximum adsorption capacity (q_m) for Cd(II) was ∼2789.58 mg/g by 1% LDHS-PP, while it was ∼1893.09 mg/g by PP, which indicated that the MoS₄-LDH greatly improved the Cd(II) uptake for PP in aqueous solution. In strongly acidic conditions (∼pH 3.0), there was still a good removal efficiency of about 45.65% by the 1% LDHS-PP, while that of PP was almost zero. At pH 5.0 the removal efficiency increased to 85.17% by the 1% LDHS-PP. The sorption kinetics for the 1% LDHS-PP was described well by a pseudo-second-order kinetic model. X-ray photoelectron spectrometry (XPS) and elemental distribution maps further confirmed the presence of MoS₄-LDH in the PP and most of the Cd(II) chemisorption based on the Cd-S bonding. Due to its high removal efficiency and acid resistance, LDHS-PP is a promising in-situ fixation agent for the remediation of agricultural soil polluted with Cd(II) at low pH.

Recent Advances in Layer-by-Layer Assembled Conducting Polymer Based Composites for Supercapacitors

Development of well-designed electrodes is the key to achieve high performance supercapacitors. Therefore, as one of the effective methods, a layer-by-layer (LBL) approach is often fruitfully employed for the fabrication of electrode material. Benefiting from a tunable parameter of the LBL approach, this approach has paved a way to design a highly ordered nanostructured electrode material with excellent performance. Conducting polymers (CPs) are the frontrunners in supercapacitors and notably, the LBL assembly of CPs is attracting extensive attention. Therefore, this critical review covers a comprehensive discussion on the research progress of CP-based composites with special importance on the LBL approach predominately for supercapacitors. Following a brief discussion on supercapacitors and CPs, the most up-to-date techniques used in LBL are highlighted.

Interwoven Carbon Nanotube Wires for High-Performing, Mechanically Robust, Washable, and Wearable Supercapacitors

An energy storage system with large storage capacity, rapid power release, and simultaneous tolerance to harsh mechanical stresses is a major bottleneck for realizing self-sustaining, wearable electronics. Addressing this, we demonstrate carbon nanotube wire (CNT-wire) interwoven solid-state supercapacitive energy storage devices (sewcaps) exhibiting superior storage capacity (30 Wh/kg, compared to electrochemical capacitors at ∼10 Wh/kg) and 14-fold higher power density (3511 W/kg) compared to Li-ion batteries (∼250 W/kg). While the high specific surface area and electrical conductivity of CNT-wires and high ionic conductivity of the electrolyte enable high energy density, the device design enables the combination of planar and radial diffusive pathways for ultralow interface resistance (∼0.2 mΩ/sewcap) and rapid charging-discharging ability (τ = 1.16 ms). Thus, this versatile approach of interweaving to form functional devices provides tunable power delivery across six orders of magnitude (2 μW to 2 W) through reconfiguration of the interweaving pattern and density. Importantly, such textile-integrated sewcaps exhibit unaltered performance (>95% retention across 4000 charge-discharge cycles) under extreme mechanical punishments such as repeated laundering, flexing (∼68°), rolling (360°), and crushing (∼21.8 kPa), implying direct interfacing with wearable platforms.

Silver-Quantum-Dot-Modified MoO3 and MnO2 Paper-Like Freestanding Films for Flexible Solid-State Asymmetric Supercapacitors

Free-standing paper-like thin-film electrodes have great potential to boost next-generation power sources with highly flexible, ultrathin, and lightweight requirements. In this work, silver-quantum-dot- (2-5 nm) modified transition metal oxide (including MoO₃ and MnO₂ ) paper-like electrodes are developed for energy storage applications. Benefitting from the ohmic contact at the interfaces between silver quantum dots and MoO₃ nanobelts (or MnO₂ nanowires) and the binder-free nature and 0D/1D/2D nanostructured 3D network of the fabricated electrodes, substantial improvements on the electrical conductivity, efficient ionic diffusion, and areal capacitances of the hybrid nanostructure electrodes are observed. With this proposed strategy, the constructed asymmetric supercapacitors, with Ag quantum dots/MoO₃ "paper" as anode, Ag quantum dots/MnO₂ "paper" as cathode, and neutral Na₂ SO₄ /polyvinyl alcohol hydrogel as electrolyte, exhibit significantly enhanced energy and power densities in comparison with those of the supercapacitors without modification of Ag quantum dots on electrodes; present excellent cycling stability at different current densities and good flexibility under various bending states; offer possibilities as high-performance power sources with low cost, high safety, and environmental friendly properties.

Scalable, large-area synthesis of heteroatom-doped few-layer graphene-like microporous carbon nanosheets from biomass for high-capacitance supercapacitors

Ginger-derived biomass ultrathin graphene-like carbon nanosheets for supercapacitor applications

Metal-Organic Framework Template-Directed Fabrication of Well-Aligned Pentagon-like Hollow Transition-Metal Sulfides as the Anode and Cathode for High-Performance Asymmetric Supercapacitors

Given the exceptional specific surface area, geometry, and periodic porosity, transition-metal sulfides derived from crystalline metal-organic frameworks have spurred great interest in energy storage systems. Herein, employing a different sulfurization process, well-aligned NiCo₂S₄ and CoS₂ nanoarrays with a hollow/porous configuration derived from pentagon-like ZIF-67 are successfully designed and constructed on Ni foam. The hollow/porous structure grown on a conductive matrix can significantly improve electroactive sites, shorten charge/ion diffusion length, and enhance mass/electron transfer. Consequently, the obtained NiCo₂S₄ possesses an excellent specific capacitance of 939 C/g, a fast charge/discharge rate, and a favorable life span. An advanced asymmetrical supercapacitor is fabricated by engaging NiCo₂S₄ and CoS₂ as cathode and anode materials, respectively, with a well-separated potential window. The obtained device delivers an exceptional energy density of 55.8 W h/kg at 695.2 W/kg, which is highly considerable to the recent transition metal sulfide-based devices. This facile tactic could be employed to construct other electrode materials with superior electrochemical properties.

Facile synthesis of Al-doped NiO nanosheet arrays for high-performance supercapacitors

Electrode material design is the key to the development of asymmetric supercapacitors with high electrochemical performances and stability. In this work, Al-doped NiO nanosheet arrays were synthesized using a facile hydrothermal method followed by a calcination process, and the synthesized arrays exhibited a superior pseudocapacitive performance, including a favourable specific capacitance of 2253 ± 105 F g-1 at a current density of 1 A g-1, larger than that of an undoped NiO electrode (1538 ± 80 F g-1). More importantly, the arrays showed a high-rate capability (75% capacitance retention at 20 A g-1) and a high cycling stability (approx. 99% maintained after 5000 cycles). The above efficient capacitive performance benefits from the large electrochemically active area and enhanced conductivity of the arrays. Furthermore, an assembled asymmetric supercapacitor based on the Al-doped NiO arrays and N-doped multiwalled carbon nanotube ones delivered a high specific capacitance of 192 ± 23 F g-1 at 0.4 A g-1 with a high-energy density of 215 ± 15 Wh kg-1 and power density of 21.6 kW kg-1. Additionally, the asymmetric device exhibited a durable cyclic stability (approx. 100% retention after 5000 cycles). This work with the proposed doping method will be beneficial to the construction of high-performance supercapacitor systems.

Manganese-Oxide-Based Electrode Materials for Energy Storage Applications: How Close Are We to the Theoretical Capacitance?

Of the transition metals, Mn has the greatest number of different oxides, most of which have a special tunnel structure that enables bulk redox reactions. The high theoretical capacitance and capacity results from a greater number of accessible oxidation states than other transition metals, wide potential window, and the high natural abundance make MnO_x species promising electrode materials for energy storage applications. Although MnO_x electrode materials have been intensely studied over the past decade, their electrochemical performance is still insufficient for practical applications. Currently, there is a trade-off between specific capacitance and loading mass. MnO_x species have intrinsically poor electrical conductivity, and current structural designs are not sophisticated enough to accommodate enough redox-active sites. Recent studies have certainly made progress in increasing capacitance through making use of electrically conductive components and controlling the morphology of the MnO_x species to expose more surface area. To increase the capacitance of MnO_x electrodes to the largest extent without limiting loading mass, further structural design at the nanoscale and manipulation of the electrically conductive component are required. An ideal nanostructure is proposed to guide future research toward closing the gap between achieved and theoretical capacitance, without limiting the loading mass.

Fiber-Type Solar Cells, Nanogenerators, Batteries, and Supercapacitors for Wearable Applications

Wearable electronic devices represent a paradigm change in consumer electronics, on-body sensing, artificial skins, and wearable communication and entertainment. Because all these electronic devices require energy to operate, wearable energy systems are an integral part of wearable devices. Essentially, the electrodes and other components present in these energy devices should be mechanically strong, flexible, lightweight, and comfortable to the user. Presented here is a critical review of those materials and devices developed for energy conversion and storage applications with an objective to be used in wearable devices. The focus is mainly on the advances made in the field of solar cells, triboelectric generators, Li-ion batteries, and supercapacitors for wearable device development. As these devices need to be attached/integrated with the fabric, the discussion is limited to devices made in the form of ribbons, filaments, and fibers. Some of the important challenges and future directions to be pursued are also highlighted.