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Carbon fiber, with its strong mechanical properties and electrical conductivity, is ideal as a fiber electrode in wearable or structural energy storage devices. However, its energy storage capacity is limited, and coatings like transition metal oxides (TMOs) enhance its electrochemical performance. Metal–organic frameworks (MOFs) are commonly used to grow TMOs on carbon fibers, increasing the surface area for better energy storage. Despite this, TMOs have limited electrical conductivity, so ion exchange is often used to dope them with additional cations, improving both conductivity and energy storage capacity. This study compares different ion-exchange cations in ZIF-L-derived TMO coatings on carbon fiber. Testing both supercapacitor and Li-ion battery applications, Ni-doped samples showed superior results, attributed to their higher exchange ratio with cobalt. As a supercapacitor electrode, the Ni-doped material achieved 13.3 F/g at 50 mA/g—66% higher than undoped samples. For Li-ion battery anodes, it reached a specific capacity of 410.5 mAh/g at 25 mA/g, outperforming undoped samples by 21.4%. [ABSTRACT FROM AUTHOR]

Needle-like CoO nanowires have been successfully synthesized by a facile hydrothermal process on an electrospun carbon nanofibers substrate. The as-prepared sample mesoporous CoO nanowires aligned vertically on the surface of carbon nanofibers and cross-linked with each other, producing loosely porous nanostructures. These hybrid composite electrodes exhibit a high specific capacitance of 1068.3 F g−1 at a scan rate of 5 mV s−1 and a good rate capability of 613.7 F g−1 at a scan rate of 60 mV s−1 in a three-electrode cell. The CoO NWs@CNF//CNT@CNF asymmetric device exhibits remarkable cycling stability and delivers a capacitance of 79.3 F/g with a capacitance retention of 92.1 % after 10,000 cycles. The asymmetric device delivers a high energy density of 37 Wh kg−1 with a power density of 0.8 kW kg−1 and a high power density of 16 kW kg−1 with an energy density of 23 Wh kg−1. This study demonstrated a promising strategy to enhance the electrochemical performance of flexible supercapacitors. [ABSTRACT FROM AUTHOR]

In this paper, we prepared a new type of iron and nitrogen co-doped porous carbon material (WSC-Fe/N) using a carbonization–activation process with wheat straw as a precursor and FeCl3 and NH4Cl as co-doping agents and analyzed the electrochemical properties of the resulting electrode material. Through precise control of the doping elements and carbonization temperature (900 °C), the resulting WSC-Fe/N-900 material exhibits abundant micropores, uniform mesopores, a significant specific surface area (2576.6 m2 g−1), an optimal level of iron doping (1.7 wt.%), and excellent graphitization. These characteristics were confirmed through X-ray diffraction and Raman spectroscopy. Additionally, the WSC-Fe/N-900 electrode demonstrated a specific capacitance of 400.5 F g−1 at a current density of 0.5 A g−1, maintaining a high capacitance of 308 F g−1 even at 10 A g−1. The solid-state symmetric supercapacitor in an aqueous electrolyte achieved an energy density of 9.2 Wh kg−1 at a power density of 250 W kg−1 and maintained an energy density of 6.5 Wh kg−1 at a power density of 5000 W kg−1, demonstrating remarkable synergistic energy–power output characteristics. In terms of structural properties, the porous characteristics of WSC-Fe/N-900 not only enhance the specific surface area of the electrode but also improve the diffusion capability of electrolyte ions within the electrode, thereby enhancing capacitance performance. The reliability of the electrode material demonstrated good performance in long-term cycling tests, maintaining a capacitance retention rate of 93% after 10,000 charge–discharge cycles, indicating excellent electrochemical stability. Furthermore, over time, the aging effect of the WSC-Fe/N-900 electrode material is minimal, maintaining high electrochemical performance even after prolonged use, suggesting that this material is suitable for long-term energy storage applications. This study introduces a novel strategy for producing porous carbon materials for supercapacitors, advancing the development of economically efficient and environmentally friendly energy storage solutions. [ABSTRACT FROM AUTHOR]

This study presents a novel approach to the development of high-performance supercapacitors through 3D printing technology. We synthesized a composite material consisting of silver-doped reduced graphene oxide (rGO) and dodecylbenzenesulfonic acid (DBSA)-doped polyaniline (PANI), which was further blended with polylactic acid (PLA) for additive manufacturing. The composite was extruded into filaments and printed into circular disc electrodes using fused deposition modeling (FDM). These electrodes were assembled into symmetric supercapacitor devices with a solid-state electrolyte. Electrochemical characterization, including cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests, demonstrated considerable mass-specific capacitance values of 136.2 F/g and 133 F/g at 20 mV/s and 1 A/g, respectively. The devices showed excellent stability, retaining 91% of their initial capacitance after 5000 cycles. The incorporation of silver nanoparticles enhanced the conductivity of rGO, while PANI-DBSA improved electrochemical stability and performance. This study highlights the potential of combining advanced materials with 3D printing to optimize energy storage devices, offering a significant advancement over traditional manufacturing methods. [ABSTRACT FROM AUTHOR]

Supercapacitors have gained increased attention in recent years due to their significant role in energy storage devices; their impact largely depends on the electrode material. The diversity of energy storage mechanisms means that various electrode materials can provide unique benefits for specific applications, highlighting the growing trend towards nanocomposite electrodes. Typically, these nanocomposite electrodes combine pseudocapacitive materials with carbon-based materials to form heterogeneous structural composites, often requiring complex multi-step preparation processes. This study introduces a straightforward approach to fabricate a non-carbon-based Mo@MoO2 nanosheet composite electrode using a one-step thermal evaporating vapor deposition (TEVD) method. This novel electrode features Mo at the core and MoO2 as the shell and demonstrates exceptional electrochemical performance. Specifically, at a current density of 1 A g−1, it achieves a storage capacity of 205.1 F g−1, maintaining virtually unchanged capacity after 10,000 charge–discharge cycles at 2 A g−1. The outstanding long-cycle stability is ascribed to the vertical two-dimensional geometry, the superior conductivity, and pseudocapacitance of the Mo@MoO2 core-shell nanosheets. These attributes significantly improve the electrode's charge storage capacity, charge transfer speed, and structural integrity during the cycling process. The development of the one-step grown Mo@MoO2 nanosheets offers a promising way for the advancement of high-performance, non-carbon-based supercapacitor nanocomposite electrodes. [ABSTRACT FROM AUTHOR]

Redox-active porous organic polymers (POPs) demonstrate significant potential in supercapacitors. However, their intrinsic low electrical conductivity and stacking tendencies often lead to low utilization rates of redox-active sites within their structural units. Herein, polyimide POPs (donated as PMTA) are synthesized in situ on multi-walled carbon nanotubes (MWCNTs) from tetramino-benzoquinone (TABQ) and 1,4,5,8-naphthalene tetracarboxylic dianhydride (PMDA) monomers. The strong π–π stacking interactions drive the PMTA POPs and the MWCNTs together to form a PMTA/MWCNT composite. With the assistance of MWCNTs, the stacking issue and low conductivity of PMTA POPs are well addressed, leading to the obvious activation and enhanced utilization of the redox-active groups in the PMTA POPs. PMTA/MWCNT then achieves a high capacitance of 375.2 F g−1 at 1 A g−1 as compared to the pristine PMTA POPs (5.7 F g−1) and excellent cycling stability of 89.7% after 8000 cycles at 5 A g−1. Cyclic voltammetry (CV) and in situ Fourier-Transform Infrared (FT-IR) results reveal that the electrode reactions involve the reversible structural evolution of carbonyl groups, which are activated to provide rich pseudocapacitance. Asymmetric supercapacitors (ASCs) assembled with PMTA/MWCNTs and activated carbon (AC) offer a high energy density of 15.4 Wh kg−1 at 980.4 W kg−1 and maintain a capacitance retention of 125% after 10,000 cycles at 5 A g−1, indicating their good potential for practical applications. [ABSTRACT FROM AUTHOR]

How to address the destruction of the porous structure caused by elemental doping in biochar derived from biomass is still challenging. In this work, the in-situ nitrogen-doped porous carbons (ABPCs) were synthesized for supercapacitor electrode applications through pre-carbonization and activation processes using nitrogen-rich pigskin and broccoli. Detailed characterization of ABPCs revealed that the best simple ABPC-4 exhibited a super high specific surface area (3030.2–3147.0 m2 g−1) and plentiful nitrogen (1.35–2.38 wt%) and oxygen content (10.08–15.35 wt%), which provided more active sites and improved the conductivity and electrochemical activity of the material. Remarkably, ABPC-4 showed an outstanding specific capacitance of 473.03 F g−1 at 1 A g−1. After 10,000 cycles, its capacitance retention decreased by only 4.92% at a current density of 10 A g−1 in 6 M KOH. The assembled symmetric supercapacitor ABPC-4//ABPC-4 achieved a power density of 161.85 W kg−1 at the maximum energy density of 17.51 Wh kg−1 and maintained an energy density of 6.71 Wh kg−1 when the power density increased to 3221.13 W kg−1. This study provides a mixed doping approach to achieve multi-element doping, offering a promising way to apply supercapacitors using mixed biomass. [ABSTRACT FROM AUTHOR]

Poly(3,4-ethylenedioxythiophene) (PEDOT) and PEDOT-functionalized carbon nanoparticles (f-CNPs) were synthesized by in situ chemical oxidative polymerization and pyrolysis methods. f-CNP-PEDOT nanocomposites were prepared by varying the concentration of PEDOT from 1 to 20% by weight (i.e., 1, 2.5, 5, 10, and 20 wt%). Several characterization techniques, such as field-emission scanning electron microscopy (FESEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray diffraction (XRD), N2 Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analyses, as well as cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS), were applied to investigate the morphology, the crystalline structure, the N2 adsorption/desorption capability, as well as the electrochemical properties of these new synthesized nanocomposite materials. FESEM analysis showed that these nanocomposites have defined porous structures, and BET surface area analysis showed that the standalone f-CNP exhibited the largest surface area of 801.6 m2/g, whereas the f-CNP-PEDOT with 20 wt% exhibited the smallest surface area of 116 m2/g. The BJH method showed that the nanocomposites were predominantly mesoporous. CV, GCD, and EIS measurements showed that f-CNP functionalized with 5 wt% PEDOT had a higher capacitive performance compared to the individual f-CNPs and PEDOT constituents, exhibiting an extraordinary specific capacitance of 258.7 F/g, at a current density of 0.25 A/g, due to the combined advantage of enhanced electrochemical activity induced by PEDOT doping, and highly developed porosity of f-CNPs. Symmetric aqueous supercapacitor devices were fabricated using the optimized f-CNP-PEDOT doped with 5 wt% of PEDOT as active material, exhibiting a high capacitance of 96.7 F/g at 1.4 V, holding practically their full charge, after 10,000 charge–discharge cycles at 2 A/g, thus providing the highest electrical electrodes performance. Hereafter, this work paves the way for the potential use of f-CNP-PEDOT nanocomposites in the development of high-energy-density supercapacitors. [ABSTRACT FROM AUTHOR]

With the swift advancement of wearable electronics and artificial intelligence, the integration of electronic devices with the human body has advanced significantly, leading to enhanced real-time health monitoring and remote disease diagnosis. Despite progress in developing stretchable materials with skin-like mechanical properties, there remains a need for materials that also exhibit high optical transparency. Supercapacitors, as promising energy storage devices, offer advantages such as portability, long cycle life, and rapid charge/discharge rates, but achieving high capacity, stretchability, and transparency simultaneously remains challenging. This study combines the stretchable, transparent polymer PEDOT:PSS with MnO2 nanoparticles to develop high-performance, stretchable, and transparent supercapacitors. PEDOT:PSS films were deposited on a PDMS substrate using a spin-coating method, followed by electrochemical deposition of MnO2 nanoparticles. This method ensured that the nanosized MnO2 particles were uniformly distributed, maintaining the transparency and stretchability of PEDOT:PSS. The resulting PEDOT:PSS/MnO2 nanoparticle electrodes were gathered into a symmetric device using a LiCl/PVA gel electrolyte, achieving an areal capacitance of 1.14 mF cm−2 at 71.2% transparency and maintaining 89.92% capacitance after 5000 cycles of 20% strain. This work presents a scalable and economical technique to manufacturing supercapacitors that combine high capacity, transparency, and mechanical stretchability, suggesting potential applications in wearable electronics. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS2), a typical layered material, has important applications in various fields, such as optoelectronics, catalysis, electronic devices, sensors, and supercapacitors. Extensive research has been carried out on few-layered MoS2 in the field of electrochemistry due to its large specific surface area, abundant active sites and short electron transport path. However, the preparation of few-layered MoS2 is a significant challenge. This work presents a simple one-pot hydrothermal method for synthesizing few-layered MoS2. Furthermore, it investigates the exfoliation effect of different amounts of sodium borohydride (NaBH4) as a stripping agent on the layer number of MoS2. Na+ ions, as alkali metal ions, can intercalate between layers to achieve the purpose of exfoliating MoS2. Additionally, NaBH4 exhibits reducibility, which can effectively promote the formation of the metallic phase of MoS2. Few-layered MoS2, as an electrode for supercapacitor, possesses a wide potential window of 0.9 V, and a high specific capacitance of 150 F g−1 at 1 A g−1. This work provides a facile method to prepare few-layered two-dimensional materials for high electrochemical performance. [ABSTRACT FROM AUTHOR]

Ti3C2Tx MXene, as a common two-dimensional material, has a wide range of applications in electrochemical energy storage. However, the surface forces of few-layer or monolayer Ti3C2Tx MXene lead to easy agglomeration, which hinders the demonstration of its performance due to the characteristics of layered materials. Herein, we report a facile method for preparing monolayer Ti3C2Tx MXene on nickel foam to achieve a self-supporting structure for supercapacitor electrodes under high electrostatic fields. Moreover, the specific capacitance varies with the deposition of different-concentration monolayer Ti3C2Tx MXene on nickel foam. As a result, Ti3C2Tx/NF has a high specific capacitance of 319 mF cm−2 at 2 mA cm−2 and an excellent long-term cycling stability of 94.4% after 7000 cycles. It was observed that the areal specific capacitance increases, whereas the mass specific capacitance decreases with the increasing loading mass. Attributable to the effect of the high electrostatic field, the self-supporting structure of the Ti3C2Tx/NF becomes denser as the concentration of the monolayer Ti3C2Tx MXene ink increases, ultimately affecting its electrochemical performance. This work provides a simple way to overcome the agglomeration problem of few-layer or monolayer MXene, then form a self-supporting electrode exhibiting excellent electrochemical performance. [ABSTRACT FROM AUTHOR]

In order to prepare biomass-derived carbon materials with high specific capacitance at a low activation temperature (≤700 °C), nanoporous carbon materials were prepared from zanthoxylum bungeanum peels and seeds via the pyrolysis and KOH-activation processes. The results show that the optimal activation temperatures are 700 °C and 600 °C for peels and seeds. Benefiting from the hierarchical pore structure (micropores, mesopores, and macropores), the abundant heteroatoms (N, S, and O) containing functional groups, and plentiful electrochemical active sites, the PAC-700 and SAC-600 derive the large capacities of ~211.0 and ~219.7 F g−1 at 1.0 A g−1 in 6 M KOH within the three-electrode configuration. Furthermore, the symmetrical supercapacitors display a high energy density of 22.9 and 22.4 Wh kg−1 at 7500 W kg−1 assembled with PAC-700 and SAC-600, along with exceptional capacitance retention of 99.1% and 93.4% over 10,000 cycles at 1.0 A g−1. More significantly, the contribution here will stimulate the extensive development of low-temperature activation processes and nanoporous carbon materials for electrochemical energy storage and beyond. [ABSTRACT FROM AUTHOR]

Self-standing Na3MnTi(PO4)3/carbon nanofiber (CNF) electrodes are successfully synthesized by electrospinning. A pre-synthesized Na3MnTi(PO4)3 is dispersed in a polymeric solution, and the electrospun product is heat-treated at 750 °C in nitrogen flow to obtain active material/CNF electrodes. The active material loading is 10 wt%. SEM, TEM, and EDS analyses demonstrate that the Na3MnTi(PO4)3 particles are homogeneously spread into and within CNFs. The loaded Na3MnTi(PO4)3 displays the NASICON structure; compared to the pre-synthesized material, the higher sintering temperature (750 °C) used to obtain conductive CNFs leads to cell shrinkage along the a axis. The electrochemical performances are appealing compared to a tape-casted electrode appositely prepared. The self-standing electrode displays an initial discharge capacity of 124.38 mAh/g at 0.05C, completely recovered after cycling at an increasing C-rate and a coulombic efficiency ≥98%. The capacity value at 20C is 77.60 mAh/g, and the self-standing electrode exhibits good cycling performance and a capacity retention of 59.6% after 1000 cycles at 1C. Specific capacities of 33.6, 22.6, and 17.3 mAh/g are obtained by further cycling at 5C, 10C, and 20C, and the initial capacity is completely recovered after 1350 cycles. The promising capacity values and cycling performance are due to the easy electrolyte diffusion and contact with the active material, offered by the porous nature of non-woven nanofibers. [ABSTRACT FROM AUTHOR]

Biowaste conversion into activated carbon is a sustainable and inexpensive approach that relieves the pressure on its disposal. Here, we prepared micro-mesoporous activated carbons (ACs) from cucumber peels through carbonization at 600 °C followed by thermal activation at different temperatures. The ACs were tested as supercapacitors for the first time. The carbon activated at 800 °C (ACP-800) showed a high specific capacitance value of 300 F/g at a scan rate of 5 mV/s in the cyclic voltammetry and 331 F/g at the current density of 0.1 A/g in the galvanostatic charge–discharge analysis. At the current density of 1 A/g, the specific discharge capacitance was 286 F/g and retained 100% capacity after 2000 cycles. Their properties were analyzed by scanning electron microscopy, energy-dispersive X-ray analysis, porosity, thermal analysis, and Fourier-transform infrared spectroscopy. The specific surface area of this sample was calculated to be 2333 m2 g−1 using the Brunauer–Emmett–Teller method. The excellent performance of ACP-800 is mainly attributed to its hierarchical porosity, as the mesopores provide connectivity between the micropores and improve the capacitive performance. These electrochemical properties enable this carbon material prepared from cucumber peels to be a potential source for supercapacitor materials. [ABSTRACT FROM AUTHOR]

Zinc-ion hybrid capacitors (ZICs) can achieve high energy and power density, ultralong cycle life, and a wide operating voltage window, and they are widely used in wearable devices, portable electronics devices, and other energy storage fields. The design of advanced ZICs with high specific capacity and energy density remains a challenge. In this work, a novel kind of V, N dual-doped Ti3C2 film with a three-dimensional (3D) porous structure (3D V-, N-Ti3C2) based on Zn-ion pre-intercalation can be fabricated via a simple synthetic process. The stable 3D structure and heteroatom doping provide abundant ion transport channels and numerous surface active sites. The prepared 3D V-, N-Ti3C2 film can deliver unexpectedly high specific capacitance of 855 F g−1 (309 mAh g−1) and demonstrates 95.26% capacitance retention after 5000 charge/discharge cycles. In addition, the energy storage mechanism of 3D V-, N-Ti3C2 electrodes is the chemical adsorption of H+/Zn2+, which is confirmed by ex situ XRD and ex situ XPS. ZIC full cells with a competitive energy density (103 Wh kg−1) consist of a 3D V-, N-Ti3C2 cathode and a zinc foil anode. The impressive results provide a feasible strategy for developing high-performance MXene-based energy storage devices in various energy-related fields. [ABSTRACT FROM AUTHOR]

Developing novel supercapacitor electrodes with high energy density and good cycle stability has aroused great interest. Herein, the vertically aligned CoNiO2/Co3O4 nanosheet arrays anchored on boron doped diamond (BDD) films are designed and fabricated by a simple one-step electrodeposition method. The CoNiO2/Co3O4/BDD electrode possesses a large specific capacitance (214 mF cm−2) and a long-term capacitance retention (85.9% after 10,000 cycles), which is attributed to the unique two-dimensional nanosheet architecture, high conductivity of CoNiO2/Co3O4 and the wide potential window of diamond. Nanosheet materials with an ultrathin thickness can decrease the diffusion length of ions, increase the contact area with electrolyte, as well as improve active material utilization, which leads to an enhanced electrochemical performance. Additionally, CoNiO2/Co3O4/BDD is fabricated as the positive electrode with activated carbon as the negative electrode, this assembled asymmetric supercapacitor exhibits an energy density of 7.5 W h kg−1 at a power density of 330.5 W kg−1 and capacity retention rate of 97.4% after 10,000 cycles in 6 M KOH. This work would provide insights into the design of advanced electrode materials for high-performance supercapacitors. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) are hybrid materials that are being explored as active electrode materials in energy storage devices, such as rechargeable batteries and supercapacitors (SCs), due to their high surface area, controllable chemical composition, and periodic ordering. However, the facile and controlled synthesis of a pure MOF phase without impurities or without going through a complicated purification process (that also reduces the yield) are challenges that must be resolved for their potential industrial applications. Moreover, various oxide formations of the Ni during Ni-MOF synthesis also represent an issue that affects the purity and performance. To resolve these issues, we report the controlled synthesis of nickel-based metal–organic frameworks (NiMOFs) by optimizing different growth parameters during hydrothermal synthesis and by utilizing nickel chloride as metal salt and H2bdt as the organic ligand, in a ratio of 1:1 at 150 °C. Furthermore, the synthesis was optimized by introducing a magnetic stirring stage, and the reaction temperature varied across 100, 150, and 200 °C to achieve the optimized growth of the NiMOFs crystal. The rarely used H2bdt ligand for Ni-MOF synthesis and the introduction of the ultrasonication stage before putting it in the furnace led to the formation of a pure phase without impurities and oxide formation. The synthesized materials were further characterized by powder X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), and UV–vis spectroscopy. The SEM images exhibited the formation of nano NiMOFs having a rectangular prism shape. The average size was 126.25 nm, 176.0 nm, and 268.4 nm for the samples (1:1)s synthesized at 100 °C, 150 °C, and 200 °C, respectively. The electrochemical performances were examined in a three-electrode configuration, in a wide potential window from −0.4 V to 0.55 V, and an electrolyte concentration of 2M KOH was maintained for each measurement. The charge–discharge galvanostatic measurement results in specific capacitances of 606.62 F/g, 307.33 F/g, and 287.42 F/g at a current density of 1 A/g for the synthesized materials at 100 °C, 150 °C, and 200 °C, respectively. [ABSTRACT FROM AUTHOR]

Lithium–sulfur batteries offer the potential for significantly higher energy density and cost-effectiveness. However, their progress has been hindered by challenges such as the "shuttle effect" caused by lithium polysulfides and the volume expansion of sulfur during the lithiation process. These limitations have impeded the widespread adoption of lithium–sulfur batteries in various applications. It is urgent to explore the high-performance sulfur host to improve the electrochemical performance of the sulfur electrode. Herein, bimetallic NiFe hydroxide (NiFe-LDH)-modified carbon nanotubes (CNTs) are prepared as the sulfur host materials (NiFe-CNT@S) for loading of sulfur. On the one hand, the crosslinked CNTs can increase the electron conductivity of the sulfur host as well as disperse NiFe-LDHs nanosheets. On the other hand, NiFe-LDHs command the capability of strongly adsorbing lithium polysulfides and also accelerate their conversion, which effectively suppresses the shuttle effect problem in lithium polysulfides. Hence, the electrochemical properties of NiFe-CNT@S exhibit significant enhancements when compared with those of the sulfur-supported pure NiFe-LDHs (NiFe-LDH@S). The initial capacity of NiFe-CNT@S is reported to be 1010 mAh g−1. This value represents the maximum amount of charge that the material can store per gram when it is first synthesized or used in a battery. After undergoing 500 cycles at a rate of 2 C (1 C = 1675 mA g−1), the NiFe-CNT@S composite demonstrates a sustained capacity of 876 mAh g−1. Capacity retention is a measure of how well a battery or electrode material can maintain its capacity over repeated charge–discharge cycles, and a higher retention percentage indicates better durability and stability of the material. [ABSTRACT FROM AUTHOR]

Sodium-ion batteries (SIBs) as a replaceable energy storage technology have attracted extensive attention in recent years. The design and preparation of advanced anode materials with high capacity and excellent cycling performance for SIBs still face enormous challenges. Herein, a solution method is developed for in situ synthesis of anti-aggregation tellurium nanorods/reduced graphene oxide (Te NR/rGO) composite. The material working as the sodium-ion battery (SIB) anode achieves a high reversible capacity of 338 mAh g−1 at 5 A g−1 and exhibits up to 93.4% capacity retention after 500 cycles. This work demonstrates an effective preparation method of nano-Te-based composites for SIBs. [ABSTRACT FROM AUTHOR]

The development of efficient energy storage systems is critical in the transition towards sustainable energy solutions. In this context, the present work investigates the viability of using orange juice, as a promising and sustainable precursor, for the synthesis of activated carbon electrodes for supercapacitor technologies. Through the carbonization-activation process and controlling the preparation parameters (KOH ratio and activation time), we have tailored the specific surface area (SSA) and pore size distribution (PSD) of the resulting carbon materials—crucial parameters that support supercapacitive performance. Several spectroscopic, morphological, and electrochemical techniques are used to characterize the obtained carbon materials. In particular, our optimization efforts revealed that a 5:1 KOH ratio with an activation time up to 120 min produced the highest SSA of about 2203 m2/g. Employing these optimal conditions, we fabricated symmetric coin cell supercapacitors using Na2SO4 as the electrolyte, which exhibited interesting specific capacitance (~56 F/g). Durability testing over 5000 cycles sustained the durability of the as-made activated carbon electrodes, suggesting an excellent retention of specific capacitance. This study not only advances the field of energy storage by introducing a renewable material for electrode fabrication but also contributes to the broader goal of waste reduction through the repurposing of food byproducts. [ABSTRACT FROM AUTHOR]

An effective approach for the large-scale fabrication of conducting polyaniline (PANI) using in situ anodic electrochemical polymerization on nickel foam which had been coated in aryl diazonium salt (ADS)-modified graphene (ADS-G). In the present work, ADS-G was used as a high surface-area support material for the electrochemical polymerization of PANI. The electrochemical performances of the ADS-G/PANI composites exhibited better suitability as supercapacitor electrode materials than those of the PANI. The ADS-G/PANI composites achieved a specific capacitance of 528 F g−1, which was higher than that of PANI (266 F g−1) due to excellent electrode–electrolyte interaction and the synergistic effect of electrical conductivity between ADS-G and PANI in the composites. These findings suggest that the ADS-G/PANI composites are a suitable composite for potential supercapacitor applications. [ABSTRACT FROM AUTHOR]

Transparent ZnMn2O4 thin films on indium tin oxide (ITO) were prepared through spray pyrolysis and implemented as electrodes in symmetric supercapacitors (SSCs). A specific capacitance value of 752 F g−1 at 0.5 A g−1 and a 70% retention over 3000 galvanostatic charge–discharge (GCD) cycles were reached with a 1.0 M Na2SO4 electrolyte in a three-electrode electrochemical cell. Analysis of the cycled electrodes with 1.0 M Na2SO4 revealed a local loss of electrode material; this loss increases when electrodes are used in SCCs. To avoid this drawback, solid polyvinylpyrrolidone-LiClO4 (PVP-LiClO4) and quasi-solid polyvinylpyrrolidone-ionic liquid (PVP-ionic liquid) electrolytes were tested in SSCs as substitutes for aqueous Na2SO4. An improvement in capacitance retention without a loss of electrode material was observed for the PVP-ionic liquid and PVP-LiClO4 electrolytes. With these non-aqueous electrolytes, the tetragonal structure of the ZnMn2O4 spinel was maintained throughout the cyclic voltammetry (CV) cycles, although changes occurred in the stoichiometry from ZnMn2O4 to Mn-rich Zn1−xMn3−xO4. In the case of the electrolyte 1.0 M Na2SO4, the loss of Zn2+ led to the formation of MnO2 via Zn1-xM3-xO4. The location of the three SCCs in the Ragone plot shows supercapacitor behavior. The electrochemical results prove that the pseudocapacitance is the major contributor to the electrode capacitance, and the SCCs can therefore be considered as pseudocapacitors. [ABSTRACT FROM AUTHOR]

A supercapacitor is an energy storage device that has the advantage of rapidly storing and releasing energy compared to traditional batteries. One powerful method for creating a wide range of materials is combustion synthesis, which relies on self-sustained chemical reactions. Specifically, solution combustion synthesis involves mixing reagents at the molecular level in an aqueous solution. This method allows for the fabrication of various nanostructured materials, such as binary and complex oxides, sulfides, and carbon-based nanocomposites, which are commonly used for creating electrodes in supercapacitors. The solution combustion synthesis offers flexibility in tuning the properties of the materials by adjusting the composition of the reactive solution, the type of fuel, and the combustion conditions. The process takes advantage of high temperatures, short processing times, and significant gas release to produce well crystalline nanostructured materials with a large specific surface area. This specific surface area is essential for enhancing the performance of electrodes in supercapacitors. Our review focuses on recent publications in this field, specifically examining the relationship between the microstructure of materials and their electrochemical properties. We discuss the findings and suggest potential improvements in the properties and stability of the fabricated composites based on the results. [ABSTRACT FROM AUTHOR]

The development of coal-based activated carbon for supercapacitors provides a robust and effective approach toward the clean and efficient use of coal, and it also offers high-quality and low-cost raw materials for energy storage devices. However, the one-step activation method for preparing coal-based activated carbon has problems, such as difficulty in introducing surface-functional groups and high KOH dosage. In our work, activated carbon was prepared through an effective strategy of oxidation and KOH activation with a low KOH content by employing coal-based carbon dots as raw material. The influence of temperature during the KOH activation of carbon dots on a specific surface area, pore structure, and various quantities and types of surface-functional groups, as well as on the electrochemical performance of supercapacitors, was systematically studied. The as-prepared sample, with the alkali–carbon ratio of 0.75, processes a large specific surface area (1207 m2 g−1) and abundant surface-functional groups, which may provide enormous active sites and high wettability, thus bringing in high specific capacitance and boosted electrochemical performances. The oxygen and nitrogen content of the activated carbon decreases while the carbon content increases, and the activation temperature also increases. The as-prepared activated carbon reaches the highest specific capacitance of 202.2 F g−1 in a 6 M KOH electrolyte at a current density of 10 A g−1. This study provides new insight into the design of high-performance activated carbon and new avenues for the application of coal-based carbon dots. [ABSTRACT FROM AUTHOR]

References 1 Dai M.Z., Zhao D.P., Wu X. Research progress on transition metal oxide based electrode materials for asymmetric hybrid capacitors. 10.1021/acsaem.0c01055 3 Wang X.W., Sun Y.C., Zhang W.C., Wu X. Flexible CuCo2O4@Ni-Co-S hybrids as electrode materials for high-performance energy storage devices. 2023; 34: 107593 4 Sun Y.C., Wang X.W., Wu X. High-performance flexible hybrid capacitors by regulating NiCoMoS@Mo0.75-LDH electrode structure. Besides the above supercapacitor reports, we collected some work about metal ion batteries, such as Li-ion, Zn ion, and sodium batteries. [Extracted from the article]

With the rapid development of nanotechnology, the study of nanocomposites as electrode materials has significantly enhanced the scope of research towards energy storage applications. Exploring electrode materials with superior electrochemical properties is still a challenge for high-performance supercapacitors. In the present research article, we prepared a novel nanocomposite of tungsten trioxide nanoparticles grown over supported graphene oxide sheets and embedded with a poly(3,4-ethylenedioxythiophene) matrix to maximize its electrical double layer capacitance. The extensive characterization shows that the poly(3,4-ethylenedioxythiophene) matrix was homogeneously dispersed throughout the surface of the tungsten trioxide–graphene oxide. The poly(3,4-ethylenedioxythiophene)@tungsten trioxide–graphene oxide exhibits a higher specific capacitance of 478.3 F·g−1 at 10 mV·s−1 as compared to tungsten trioxide–graphene oxide (345.3 F·g−1). The retention capacity of 92.1% up to 5000 cycles at 0.1 A·g−1 shows that this ternary nanocomposite electrode also exhibits good cycling stability. The poly(3,4-ethylenedioxythiophene)@tungsten trioxide–graphene oxide energy density and power densities are observed to be 54.2 Wh·kg−1 and 971 W·kg−1. The poly(3,4-ethylenedioxythiophene)@tungsten trioxide–graphene oxide has been shown to be a superior anode material in supercapacitors because of the synergistic interaction of the poly(3,4-ethylenedioxythiophene) matrix and the tungsten trioxide–graphene oxide surface. These advantages reveal that the poly(3,4-ethylenedioxythiophene)@tungsten trioxide–graphene oxide electrode can be a promising electroactive material for supercapacitor applications. [ABSTRACT FROM AUTHOR]

Printed energy storage components attracted attention for being incorporated into bendable electronics. In this research, a homogeneous and stable ink based on vanadium dioxide (VO2) is hydrothermally synthesized with a non-toxic solvent. The structural and morphological properties of the synthesized material are determined to be well-crystalline monoclinic-phase nanoparticles. The charge storage mechanisms and evaluations are specified for VO2 electrodes, gold (Au) electrodes, and VO2/Au electrodes using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The VO2 electrode shows an electrical double layer and a redox reaction in the positive and negative voltage ranges with a slightly higher areal capacitance of 9 mF cm−2. The VO2/Au electrode exhibits an areal capacitance of 16 mF cm−2, which is double that of the VO2 electrode. Due to the excellent electrical conductivity of gold, the areal capacitance 18 mF cm−2 of the Au electrode is the highest among them. Based on that, Au positive electrodes and VO2 negative electrodes are used to build an asymmetric supercapacitor. The device delivers an areal energy density of 0.45 μWh cm−2 at an areal power density of 70 μW cm−2 at 1.4 V in the aqueous electrolyte of potassium hydroxide. We provide a promising electrode candidate for cost-effective, lightweight, environmentally friendly printed supercapacitors. [ABSTRACT FROM AUTHOR]

The NiMn2O4/graphene oxide (GO) nanocomposite material was in situ grown on the surface of a nickel foam 3D skeleton by combining the solvent method with the microwave-assisted hydrothermal method and annealing; then, its performance was investigated as a superior supercapacitor electrode material. When nickel foam was soaked in GO aqueous or treated in nickel ion and manganese ion solution by the microwave-assisted hydrothermal method and annealing, gauze GO film or flower-spherical NiMn2O4 was formed on the nickel foam surface. If the two processes were combined in a different order, the final products on the nickel surface had a remarkably different morphology and phase structure. When GO film was first formed, the final products on the nickel surface were the composite of NiO and Mn3O4, while NiMn2O4/GO nanocomposite material can be obtained if NiMn2O4 was first formed (immersed in 2.5 mg/L GO solution). In a 6M KOH solution, the specific capacitance of the latter reached 700 F/g at 1 A/g which was superior to that of the former (only 35 F/g). However, the latter's specific capacitance was still inferior to that of in-situ grown NiMn2O4 on nickel foam (802 F/g). Though the gauze-formed GO film, almost covering the preformed flower-spherical NiMn2O4, can also contribute a certain specific capacitance, it also restricted the electrolyte diffusion and contact with NiMn2O4, accounting for the performance decrease of the NiMn2O4/GO nanocomposite. A convenient method was raised to fabricate the nanocomposite of carbon and double metal oxides. [ABSTRACT FROM AUTHOR]

Bimetallic selenide compounds show great potential as supercapacitor electrode materials in energy storage and conversion applications. In this work, a coral-like MnCo selenide was grown on nickel foam using a facile electrodeposition method to prepare binder-free supercapacitor electrodes. The heating temperature was varied to tune the morphology and crystal phase of these electrodes. Excellent electrochemical performance was achieved due to the unique coral-like, dendritic- dispersed structure and a bimetallic synergistic effect, including high specific capacitance (509 C g−1 at 1 A g−1) and outstanding cycling stability (94.3% capacity retention after 5000 cycles). Furthermore, an asymmetric supercapacitor assembled with MnCo selenide as the anode and active carbon as the cathode achieved a high specific energy of 46.2 Wh kg−1 at 800 W kg−1. The work demonstrates that the prepared coral-like MnCo selenide is a highly promising energy storage material. [ABSTRACT FROM AUTHOR]

The emergence of the Internet of things stimulates the pursuit of flexible and miniaturized supercapacitors. As an advanced technology, screen printing displays vigor and tremendous potential in fabricating supercapacitors, but the adoption of high-performance ink is a great challenge. Here, hierarchical V3O7 with rodlike texture was prepared via a facile template–solvothermal route; and the morphology, component, and valence bond information are characterized meticulously. Then, the screen-printed inks composed of V3O7, acetylene black, and PVDF are formulated, and the rheological behaviors are studied detailedly. Benefitting from the orderly aligned ink, the optimal screen-printed electrode can exhibit an excellent specific capacitance of 274.5 F/g at 0.3 A/g and capacitance retention of 81.9% after 5000 cycles. In addition, a flexible V3O7 symmetrical supercapacitor (SSC) is screen-printed and assembled on the Ag current collector, exhibiting a decent areal specific capacitance of 322.5 mF/cm2 at 0.5 mA/cm2, outstanding cycling stability of 90.8% even after 5000 cycles, satisfactory maximum energy density of 129.45 μWh/cm2 at a power density of 0.42 mW/cm2, and remarkable flexibility and durability. Furthermore, a single SSC enables the showing of an actual voltage of 1.70 V after charging, and no obvious self-discharge phenomenon is found, revealing the great applied value in supply power. Therefore, this work provides a facile and low-cost reference of screen-printed ink for large-scale fabrication of flexible supercapacitors. [ABSTRACT FROM AUTHOR]

The intersection between the field of hybrid materials and that of electrochemistry is a quickly expanding area. Hybrid combinations usually consist of two constituents, but new routes toward more complex and versatile electroactive hybrid designs are quickly emerging. The objective of the present work is to explore novel triple hybrid material integrating polyoxometalates (POMs), silver nanoparticles (Ag0 NPs), and activated carbon (AC) and to demonstrate its use as a hybrid electrode in a symmetric supercapacitor. The tri-component nanohybrid (AC/POM-Ag0 NPs) was fabricated through the combination of AC with pre-synthesized ∼27 nm POM-protected Ag0 NPs (POM-Ag0 NPs). The POM-Ag0 NPs were prepared using a green electrochemical method and characterized via UV-vis and IR spectroscopy, electron microscopy, dynamic light scattering (DLS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Afterward, the AC/POM-Ag0 NPs ternary nanocomposite material was constructed and characterized. The electrochemical behavior of AC/POM-Ag0 NPs' modified electrodes reveal that the nanomaterial is electroactive and exhibits a moderately higher specific capacitance (81 F/g after 20 cycles) than bare AC electrodes (75 F/g) in a symmetrical supercapacitor configuration in the voltage range 0 to 0.75 V and 20 mV/s, demonstrating the potential use of this type of tri-component nanohybrid for electrochemical applications. [ABSTRACT FROM AUTHOR]

ZnO is a potential candidate for providing an economic and environmentally friendly substitute for energy storage materials. Therefore, in this work, Fe-doped ZnO nanostructures prepared using the microwave irradiation procedure were investigated for structural, morphological, magnetic, electronic structural, specific surface area and electrochemical properties to be used as electrodes for supercapacitors. The X-ray diffraction, high-resolution transmission electron microscopy images, and selective-area electron diffraction pattern indicated that the nanocrystalline structures of Fe-doped ZnO were found to possess a hexagonal wurtzite structure. The effect of Fe doping in the ZnO matrix was observed on the lattice parameters, which were found to increase with the dopant concentration. Rods and a nanosheet-like morphology were observed via FESEM images. The ferromagnetic nature of samples is associated with the presence of bound magnetic polarons. The enhancement of saturation magnetization was observed due to Fe doping up to 3% in correspondence with the increase in the number of bound magnetic polarons with an Fe content of up to 3%. This behavior is observed as a result of the change in the oxidation state from +2 to +3, which was a consequence of Fe doping ranging from 3% to 5%. The electrode performance of Fe-doped ZnO nanostructures was studied using electrochemical measurements. The cyclic voltammetry (CV) results inferred that the specific capacitance increased with Fe doping and displayed a high specific capacitance of 286 F·g−1 at 10 mV/s for 3% Fe-doped ZnO nanostructures and decreased beyond that. Furthermore, the stability of the Zn0.97Fe0.03O electrode, which was examined by performing 2000 cycles, showed excellent cyclic stability (85.0% of value retained up to 2000 cycles) with the highest specific capacitance of 276.4 F·g−1, signifying its appropriateness as an electrode for energy storage applications. [ABSTRACT FROM AUTHOR]

Chitosan, a biomass raw material, was utilized as a carbon skeleton source and served as a nitrogen (N) atom dopant in this study. By co-doping phosphorus (P) atoms from H3PO4 and nitrogen (N) atoms with a carbon (C) skeleton and hybridizing them with Mn3O4 on a carbon fiber cloth (CC), an Mn3O4@NPC/CC electrode was fabricated, which exhibited an excellent capacitive performance. The N, P-codoped carbon polycrystalline material was hybridized with Mn3O4 during the chitosan carbonization process. This carbon polycrystalline structure exhibited an enhanced conductivity and increased mesopore content, thereby optimizing the micropore/mesopore ratio in the electrode material. This optimization contributed to the improved storage, transmission, and diffusion of electrolyte ions within the Mn3O4@NPC electrode. The electrochemical behavior was evaluated via cyclic voltammetry and galvanostatic charge–discharge tests using a 1 M Na2SO4 electrolyte. The capacitance significantly increased to 256.8 F g−1 at 1 A g−1, and the capacitance retention rate reached 97.3% after 5000 charge/discharge cycles, owing to the higher concentration of the P-dopant in the Mn3O4@NPC/CC electrode. These findings highlight the tremendous potential of flexible supercapacitor electrodes in various applications. [ABSTRACT FROM AUTHOR]

The composites of NiCo2O4 with unique structures were substantially investigated as promising electrodes. In this study, the unique structured nanosheets anchored on nickel foam (Ni foam) were prepared under the hydrothermal technique of NiCo2O4 and subsequent preparation of Co(OH)2. The Co(OH)2@NiCo2O4 nanosheet composite has demonstrated higher specific capacitances owing to its excellent specific surface region, enhanced rate properties, and outstanding electrical conductivities. Moreover, the electrochemical properties were analyzed in a three-electrode configuration to study the sample material. The as-designed Co(OH)2@NiCo2O4 nanosheet achieves higher specific capacitances of 1308 F·g−1 at 0.5 A·g−1 and notable long cycles with 92.83% capacity retention over 6000 cycles. The Co(OH)2@NiCo2O4 nanosheet electrode exhibits a long life span and high capacitances compared with the NiCo2O4 and Co(OH)2 electrodes, respectively. These outstanding electrochemical properties are mainly because of their porous construction and the synergistic effects between NiCo2O4 and Co(OH)2. Such unique Co(OH)2@NiCo2O4 nanosheets not only display promising applications in renewable storage but also reiterate to scientists of the unlimited potential of high-performance materials. [ABSTRACT FROM AUTHOR]

In recent years, supercapacitors have been widely used in the fields of energy, transportation, and industry. Among them, electrical double-layer capacitors (EDLCs) have attracted attention because of their dramatically high power density. With the rapid development of computational methods, theoretical studies on the physical and chemical properties of electrode materials have provided important support for the preparation of EDLCs with higher performance. Besides the widely studied double-layer capacitance (CD), quantum capacitance (CQ), which has long been ignored, is another important factor to improve the total capacitance (CT) of an electrode. In this paper, we survey the recent theoretical progress on the CQ of two-dimensional (2D) electrode materials in EDLCs and classify the electrode materials mainly into graphene-like 2D main group elements and compounds, transition metal carbides/nitrides (MXenes), and transition metal dichalcogenides (TMDs). In addition, we summarize the influence of different modification routes (including doping, metal-adsorption, vacancy, and surface functionalization) on the CQ characteristics in the voltage range of ±0.6 V. Finally, we discuss the current difficulties in the theoretical study of supercapacitor electrode materials and provide our outlook on the future development of EDLCs in the field of energy storage. [ABSTRACT FROM AUTHOR]

Biochar derived from waste biomass has proven to be an encouraging novel electrode material in supercapacitors. In this work, luffa sponge-derived activated carbon with a special structure is produced through carbonization and KOH activation. The reduced graphene oxide (rGO) and manganese dioxide (MnO2) are in-situ synthesized on luffa-activated carbon (LAC) to improve the supercapacitive behavior. The structure and morphology of LAC, LAC-rGO and LAC-rGO-MnO2 are characterized by the employment of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), BET analysis, Raman spectroscopy and scanning electron microscopy (SEM). The electrochemical performance of electrodes is performed in two and three-electrode systems. In the asymmetrical two-electrode system, the LAC-rGO-MnO2//Co3O4-rGO device shows high specific capacitance (SC), high-rate capability and excellent cycle reversibly in a wide potential window of 0–1.8 V. The maximum specific capacitance (SC) of the asymmetric device is 586 F g−1 at a scan rate of 2 mV s−1. More importantly, the LAC-rGO-MnO2//Co3O4-rGO device exhibits a specific energy of 31.4 W h kg−1 at a specific power of 400 W kg−1. Overall, the synergistic effect between the ternary structures of microporous LAC, rGO sheets and MnO2 nanoparticles leads to the introduction of high-performance hierarchical supercapacitor electrodes. [ABSTRACT FROM AUTHOR]

To meet the growing demand for efficient and sustainable power sources, it is crucial to develop high-performance energy storage systems. Additionally, they should be cost-effective and able to operate without any detrimental environmental side effects. In this study, rice husk-activated carbon (RHAC), which is known for its abundance, low cost, and excellent electrochemical performance, was combined with MnFe2O4 nanostructures to improve the overall capacitance of asymmetric supercapacitors (ASCs) and their energy density. A series of activation and carbonization steps are involved in the fabrication process for RHAC from rice husk. Furthermore, the BET surface area for RHAC was determined to be 980 m2 g−1 and superior porosities (average pore diameter of 7.2 nm) provide abundant active sites for charge storage. Additionally, MnFe2O4 nanostructures were effective pseudocapacitive electrode materials due to their combined Faradic and non-Faradic capacitances. In order to assess the electrochemical performance of ASCs extensively, several characterization techniques were employed, including galvanostatic charge –discharge, cyclic voltammetry, and electrochemical impedance spectroscopy. Comparatively, the ASC demonstrated a maximum specific capacitance of ~420 F/g at a current density of 0.5 A/g. The as-fabricated ASC possesses remarkable electrochemical characteristics, including high specific capacitance, superior rate capability, and long-term cycle stability. The developed asymmetric configuration retained 98% of its capacitance even after 12,000 cycles performed at a current density of 6A/g, demonstrating its stability and reliability for supercapacitors. The present study demonstrates the potential of synergistic combinations of RHAC and MnFe2O4 nanostructures in improving supercapacitor performance, as well as providing a sustainable method of using agricultural waste for energy storage. [ABSTRACT FROM AUTHOR]

MOF-derived carbon (MDC) and metal oxide (MDMO) are superior materials for supercapacitor electrodes due to their high specific capacitances, which can be attributed to their high porosity, specific surface area (SSA), and pore volume. To improve the electrochemical performance, the environmentally friendly and industrially producible MIL-100 (Fe) was prepared using three different Fe sources through hydrothermal synthesis. MDC-A with micro- and mesopores and MDC-B with micropores were synthesized through carbonization and an HCl washing process, and MDMO (α-Fe2O3) was obtained by a simple sintering in air. The electrochemical properties in a three-electrode system using a 6 M KOH electrolyte were investigated. These novel MDC and MDMO were applied to an asymmetric supercapacitor (ASC) system to overcome the disadvantages of traditional supercapacitors, enhancing energy density, power density, and cyclic performance. High SSA materials (MDC-A nitrate and MDMO iron) were selected for negative and positive electrode material to fabricate ASC with KOH/PVP gel electrolyte. As-fabricated ASC resulted in high specific capacitance 127.4 Fg−1 at 0.1 Ag−1 and 48.0 Fg−1 at 3 Ag−1, respectively, and delivered superior energy density (25.5 Wh/kg) at a power density 60 W/kg. The charging/discharging cycling test was also conducted, indicating 90.1% stability after 5000 cycles. These results indicate that ASC with MDC and MDMO derived from MIL-100 (Fe) has promising potential in high-performance energy storage devices. [ABSTRACT FROM AUTHOR]

This investigation is motivated by interest in nanostructured FeOOH anodes for aqueous asymmetric supercapacitors operating in Na2SO4 electrolyte. The research goal is the fabrication of anodes with high active mass loading of 40 mg cm−2, high capacitance and low resistance. The influence of high-energy ball milling (HEBM), capping agents and alkalizer on the nanostructure and capacitive properties is investigated. HEBM promotes the crystallization of FeOOH, which results in capacitance reduction. Capping agents from the catechol family, such as tetrahydroxy-1,4-benzoquinone (THB) and gallocyanine (GC), facilitate the fabrication of FeOOH nanoparticles, eliminate the formation of micron size particles and allow the fabrication of anodes with enhanced capacitance. The analysis of testing results provided insight into the influence of the chemical structure of the capping agents on nanoparticle synthesis and dispersion. The feasibility of a conceptually new strategy for the synthesis of FeOOH nanoparticles is demonstrated, which is based on the use of polyethylenimine as an organic alkalizer-dispersant. The capacitances of materials prepared using different nanotechnology strategies are compared. The highest capacitance of 6.54 F cm−2 is obtained using GC as a capping agent. The obtained electrodes are promising for applications as anodes for asymmetric supercapacitors. [ABSTRACT FROM AUTHOR]

We present a facile low-cost method to produce nitrogen-doped holey graphene (N-HGE) and its application to supercapacitors. A composite of N-HGE and activated carbon (AC) was used as the electrode active material in organic-electrolyte supercapacitors, and the performances were evaluated. Melamine was mixed into graphite oxide (GO) as the N source, and an ultra-rapid heating method was used to create numerous holes during the reduction process of GO. X-ray photoelectron spectra confirmed the successful doping with 2.9–4.5 at.% of nitrogen on all samples. Scanning electron micrographs and Raman spectra revealed that a higher heating rate resulted in more holes and defects on the reduced graphene sheets. An extra annealing step at 1000 °C for 1 h was carried out to further eliminate residual oxygen functional groups, which are undesirable in the organic electrolyte system. Compared to the low-heating-rate counterpart (N-GE-15), N-HGE boosted the specific capacity of the supercapacitor by 42 and 22% at current densities of 0.5 and 20 A/g, respectively. The effects of annealing time (0.5, 1, and 2 h) at 1000 °C were also studied. Longer annealing time resulted in higher capacitance values at all current densities due to the minimized oxygen content. Volumetric specific capacitances of 49 and 24 F/cm3 were achieved at current densities of 0.5 and 20 A/g, respectively. For the high-power-density operation at 31,000 W/kg (or 10,000 W/L), an energy density as high as 11 Wh/kg (or 3.5 Wh/L) was achieved. The results indicated that N-HGE not only improved the conductivity of the composite supercapacitors but also accelerated ion transport by way of shortened diffusion paths through the numerous holes all over the graphene sheets. [ABSTRACT FROM AUTHOR]

A metal–organic framework (MOF) is a highly porous material with abundant redox capacitive sites for intercalation/de-intercalation of charges and, hence, is considered promising for electrode materials in supercapacitors. In addition, dopants can introduce defects and alter the electronic structure of the MOF, which can affect its surface reactivity and electrochemical properties. Herein, we report a copper-doped iron-based MOF (Cu@Fe-MOF/NF) thin film obtained via a simple drop-cast route on a 3D-nickel foam (NF) substrate for the supercapacitor application. The as-deposited Cu@Fe-MOF/NF electrodes exhibit a unique micro-sized bipyramidal structure composited with nanoparticles, revealing a high specific capacitance of 420.54 F g−1 at 3 A g−1 which is twice compared to the nano-cuboidal Fe-MOF/NF (210 F g−1). Furthermore, the asymmetric solid-state (ASSSC) supercapacitor device, derived from the assembly of Cu@Fe-MOF/NFǁrGO/NF electrodes, demonstrates superior performance in terms of energy density (44.20 Wh.kg−1) and electrochemical charge–discharge cycling durability with 88% capacitance retention after 5000 cycles. This work, thus, demonstrates a high potentiality of the Cu@Fe-MOF/NF film electrodes in electrochemical energy-storing devices. [ABSTRACT FROM AUTHOR]

New NiSn(OH)6 hexahydroxide nanoparticles were synthesised through a co-precipitation method using various concentrations of Ni2+ and Sn4+ ions (e.g., 1:0, 0:1, 1:2, 1:1, and 2:1; namely, N, S, NS-3, NS-2, and NS-1) with an ammonia solution. The perovskite NiSn(OH)6 was confirmed from powder X-ray diffraction and molecule interactions due to different binding environments of Ni, Sn, O, and water molecules observed from an FT-IR analysis. An electronic transition was detected from tin (Sn 3d) and nickel (Ni 2p) to oxygen (O 2p) from UV-Vis/IR spectroscopy. Photo luminescence spectroscopy (PL) identified that the emission observed at 400–800 nm in the visible region was caused by oxygen vacancies due to various oxidation states of Ni and Sn metals. A spherical nanoparticle morphology was observed from FE-SEM; this was due to the combination of Ni2+ and Sn4+ increasing the size and porosity of the nanoparticle. The elemental (Ni and Sn) distribution and binding energy of the nanoparticle were confirmed by EDAX and XPS analyses. Among the prepared various nanoparticles, NS-2 showed a maximum specific capacitance of 607 Fg−1 at 1 Ag−1 and 56% capacitance retention (338 Fg−1 and 5 Ag−1), even when increasing the current density five times, and excellent cycle stability due to combining Ni2+ with Sn4+, which improved the ionic and electrical conductivity. EIS provided evidence for NS-2's low charge transfer resistance compared with other prepared samples. Moreover, the NS-2//AC (activated carbon) asymmetric supercapacitor exhibited the highest energy density and high-power density along with excellent cycle stability, making it the ideal material for real-time applications. [ABSTRACT FROM AUTHOR]

Two-dimensional nanomaterials hold great promise as electrode materials for the construction of excellent electrochemical energy storage and transformation apparatuses. In the study, metallic layered cobalt sulfide was, firstly, applied to the area of energy storage as a supercapacitor electrode. By a facile and scalable method for cathodic electrochemical exfoliation, metallic layered cobalt sulfide bulk can be exfoliated into high-quality and few-layered nanosheets with size distributions in the micrometer scale range and thickness in the order of several nanometers. With a two-dimensional thin sheet structure of metallic cobalt sulfide nanosheets, not only was a larger active surface area created, but also, the insertion/extraction of ions in the procedure of charge and discharge were enhanced. The exfoliated cobalt sulfide was applied as a supercapacitor electrode with obvious improvement compared with the original sample, and the specific capacitance increased from 307 F∙g−1 to 450 F∙g−1 at the current density of 1 A∙g−1. The capacitance retention rate of exfoliated cobalt sulfide enlarged to 84.7% from the original 81.9% of unexfoliated samples while the current density multiplied by 5 times. Moreover, a button-type asymmetric supercapacitor assembled using exfoliated cobalt sulfide as the positive electrode exhibits a maximum specific energy of 9.4 Wh∙kg−1 at the specific power of 1520 W∙kg−1. [ABSTRACT FROM AUTHOR]

Electrochemical energy storage devices are one of the main protagonists in the ongoing technological advances in the energy field, whereby the development of efficient, sustainable, and durable storage systems aroused a great interest in the scientific community. Batteries, electrical double layer capacitors (EDLC), and pseudocapacitors are characterized in depth in the literature as the most powerful energy storage devices for practical applications. Pseudocapacitors bridge the gap between batteries and EDLCs, thus supplying both high energy and power densities, and transition metal oxide (TMO)-based nanostructures are used for their realization. Among them, WO3 nanostructures inspired the scientific community, thanks to WO3's excellent electrochemical stability, low cost, and abundance in nature. This review analyzes the morphological and electrochemical properties of WO3 nanostructures and their most used synthesis techniques. Moreover, a brief description of the electrochemical characterization methods of electrodes for energy storage, such as Cyclic Voltammetry (CV), Galvanostatic Charge–Discharge (GCD), and Electrochemical Impedance Spectroscopy (EIS) are reported, to better understand the recent advances in WO3-based nanostructures, such as pore WO3 nanostructures, WO3/carbon nanocomposites, and metal-doped WO3 nanostructure-based electrodes for pseudocapacitor applications. This analysis is reported in terms of specific capacitance calculated as a function of current density and scan rate. Then we move to the recent progress made for the design and fabrication of WO3-based symmetric and asymmetric supercapacitors (SSCs and ASCs), thus studying a comparative Ragone plot of the state-of-the-art research. [ABSTRACT FROM AUTHOR]

Supercapacitors are candidates with the greatest potential for use in sustainable energy resources. Extensive research is being carried out to improve the performances of state-of-art supercapacitors to meet our increased energy demands because of huge technological innovations in various fields. The development of high-performing materials for supercapacitor components such as electrodes, electrolytes, current collectors, and separators is inevitable. To boost research in materials design and production toward supercapacitors, the up-to-date collection of recent advancements is necessary for the benefit of active researchers. This review summarizes the most recent developments of water-in-salt (WIS) and deep eutectic solvents (DES), which are considered significant electrolyte systems to advance the energy density of supercapacitors, with a focus on two-dimensional layered nanomaterials. It provides a comprehensive survey of 2D materials (graphene, MXenes, and transition-metal oxides/dichalcogenides/sulfides) employed in supercapacitors using WIS/DES electrolytes. The synthesis and characterization of various 2D materials along with their electrochemical performances in WIS and DES electrolyte systems are described. In addition, the challenges and opportunities for the next-generation supercapacitor devices are summarily discussed. [ABSTRACT FROM AUTHOR]

Hydrotalcite, first found in natural ores, has important applications in supercapacitors. NiCoAl-LDH, as a hydrotalcite-like compound with good crystallinity, is commonly synthesized by a hydrothermal method. Al 3 + plays an important role in the crystallization of hydrotalcite and can provide stable trivalent cations, which is conducive to the formation of hydrotalcite. However, aluminum and its hydroxides are unstable in a strong alkaline electrolyte; therefore, a secondary alkali treatment is proposed in this work to produce cation vacancies. The hydrophilicity of the NiCoAl-OH surface with cation vacancy has been greatly improved, which is conducive to the wetting and infiltration of electrolyte in water-based supercapacitors. At the same time, cation vacancies generate a large number of defects as active sites for energy storage. As a result, the specific capacity of the NiCoAl-OH electrode after 10,000 cycles can be maintained at 94.1%, which is much better than the NiCoAl-LDH material of 74%. [ABSTRACT FROM AUTHOR]

Designing and preparing dual-functional Dawson-type polyoxometalate-based metal–organic framework (POMOF) energy storage materials is challenging. Here, the Dawson-type POMOF nanomaterial with the molecular formula CoK4[P2W18O62]@Co3(btc)2 (abbreviated as {P2W18}@Co-BTC, H3btc = 1,3,5-benzylcarboxylic acid) was prepared using a solid-phase grinding method. XRD, SEM, TEM et al. analyses prove that this nanomaterial has a core–shell structure of Co-BTC wrapping around the {P2W18}. In the three-electrode system, it was found that {P2W18}@Co-BTC has the best supercapacitance performance, with a specific capacitance of 490.7 F g−1 (1 A g−1) and good stability, compared to nanomaterials synthesized with different feedstock ratios and two precursors. In the symmetrical double-electrode system, both the power density (800.00 W kg−1) and the energy density (11.36 Wh kg−1) are greater. In addition, as the electrode material for the H2O2 sensor, {P2W18}@Co-BTC also exhibits a better H2O2-sensing performance, such as a wide linear range (1.9 μM–1.67 mM), low detection limit (0.633 μM), high selectivity, stability (92.4%) and high recovery for the detection of H2O2 in human serum samples. This study provides a new strategy for the development of Dawson-type POMOF nanomaterial compounds. [ABSTRACT FROM AUTHOR]

The primary purpose of this study was to produce an ore-based high-capacity supercapacitor electrode. For this, chalcopyrite ore was first leached with nitric acid, and then metal oxide synthesis was carried out immediately on nickel foam using a hydrothermal technique from the solution. Cauliflower-patterned CuFe2O4 with a wall thickness of about 23 nm was synthesized on the Ni foam surface, characterized by XRD, FTIR, XPS, SEM, and TEM investigations. The produced electrode also displayed a feature of a battery-like charge storage mechanism with a specific capacity of 525 mF cm−2 at 2 mA cm−2 current density, energy of 8.9 mWh cm−2, and a power density of 233 mW cm−2. Additionally, even after 1350 cycles, this electrode still performed at 109% of its original capacity. The performance of this finding is 255% higher than that of the CuFe2O4 in our earlier investigation; despite being pure, it performs far better than some of its equivalents in the literature. Obtaining such performance from an electrode made from ore indicates that the use of ore has a lot of potential for supercapacitor production and property improvement. [ABSTRACT FROM AUTHOR]

Supercapacitors have become a popular form of energy-storage device in the current energy and environmental landscape, and their performance is heavily reliant on the electrode materials used. Carbon-based electrodes are highly desirable due to their low cost and their abundance in various forms, as well as their ability to easily alter conductivity and surface area. Many studies have been conducted to enhance the performance of carbon-based supercapacitors by utilizing various carbon compounds, including pure carbon nanotubes and multistage carbon nanostructures as electrodes. These studies have examined the characteristics and potential applications of numerous pure carbon nanostructures and scrutinized the use of a wide variety of carbon nanomaterials, such as AC, CNTs, GR, CNCs, and others, to improve capacitance. Ultimately, this study provides a roadmap for producing high-quality supercapacitors using carbon-based electrodes. [ABSTRACT FROM AUTHOR]

As a typical binary transition metal oxide, ZnFe2O4 has attracted considerable attention for supercapacitor electrodes due to its high theoretical specific capacitance. However, the reported synthesis processes of ZnFe2O4 are complicated and ZnFe2O4 nanoparticles are easily agglomerated, leading to poor cycle life and unfavorable capacity. Herein, a facile microwave hydrothermal process was used to prepare ZnFe2O4/reduced graphene oxide (rGO) nanocomposites in this work. The influence of rGO content on the morphology, structure, and electrochemical performance of ZnFe2O4/rGO nanocomposites was systematically investigated. Due to the uniform distribution of ZnFe2O4 nanoparticles on the rGO surface and the high specific surface area and rich pore structures, the as-prepared ZnFe2O4/rGO electrode with 44.3 wt.% rGO content exhibits a high specific capacitance of 628 F g−1 and long cycle life of 89% retention over 2500 cycles at 1 A g−1. This work provides a new process for synthesizing binary transition metal oxide and developing a new strategy for realizing high-performance composites for supercapacitor electrodes. [ABSTRACT FROM AUTHOR]