Realizing an Asymmetric Supercapacitor Employing Carbon Nanotubes Anchored to Mn3O4 Cathode and Fe3O4 Anode

A metal-organic framework-derived α-MnS/MWCNT composite as a promising pseudocapacitive material for a flexible quasi-solid-state asymmetric supercapacitor device

The low conductivity of traditionally used pseudocapacitive materials like transition metal oxides has forced researchers to look for alternative materials. Transition metal sulfides are being investigated as viable alternative materials and have shown promising results. In this work, an α-MnS/MWCNT composite is selected as the active material for supercapacitor application. α-MnS has better conductivity than many transition metal oxides but has an extremely low specific surface area (10.5 m2 g-1), which reduces its specific capacitance. Metal-organic framework (MOF)-derived materials are known to possess higher specific surface area and favorable pore size distribution. Herein, α-MnS/MWCNT composites are synthesized via two routes: the conventional solvothermal technique and the MOF route, and their performance is compared. It is proved that the α-MnS/MWCNT composite synthesized through the MOF route shows a favorable porous structure and better performance than the composite synthesized through the conventional route. It shows a specific surface area of 47.6 m2 g-1 and a specific capacitance of 546.3 F g-1 at 1 A g-1 with a mass loading of 1.5 mg cm-2 in 3 M KOH under a 3-electrode configuration. A flexible quasi-solid-state asymmetric supercapacitor device is fabricated with MOF-derived α-MnS/MWCNT as the positive electrode material, and the device achieved a potential window of 1.4 V, a specific capacitance of 82.5 F g-1 at 1 A g-1 and a capacitance retention of 90.1% after 5000 cycles at 10 A g-1. The results clearly indicate that transition metal sulfides like MOF-derived α-MnS can be a viable alternative to traditional materials like transition metal oxides. The assembled device has the potential to power flexible, wearable electronics.

Review on recent advancements in the role of electrolytes and electrode materials on supercapacitor performances

Supercapacitors currently hold a prominent position in energy storage systems due to their exceptionally high power density, although they fall behind batteries and fuel cells in terms of energy density. This paper examines contemporary approaches aimed at enhancing the energy density of supercapacitors by adopting hybrid configurations, alongside considerations of their power density, rate capability, and cycle stability. Given that electrodes play a pivotal role in supercapacitor cells, this review focuses on the design of hybrid electrode structures with elevated specific capacitance, shedding light on the underlying mechanisms. Factors such as available surface area, porosity, and conductivity of the constituent materials significantly influence electrode performance, prompting the adoption of strategies such as nanostructuring. Additionally, the paper delves into the impact of novel bio-based hybrid electrolytes, drawing upon literature data to outline the fabrication of various hybrid electrode materials incorporating conducting polymers like polyaniline and polypyrrole, as well as metal oxides, carbon compounds, and hybrid electrolytes such as ionic liquids, gel polymers, aqueous, and solid polymer electrolytes. The discussion explores the contributions of different components and methodologies to overall capacitance, with a primary emphasis on the mechanisms of energy storage through non-faradic electrical double-layer capacitance and faradaic pseudo-capacitance. Furthermore, the paper addresses the electrochemical performance of hybrid components, examining their concentrations and functioning via diverse charge storage techniques.

A novel catalyst derived from Co-ZIFs to grow N-doped carbon nanotubes for all-solid-state supercapacitors with high performance

Carbon nanotubes (CNTs) have been widely used as electrode materials for electrochemical energy storage devices (e.g., supercapacitors) due to their excellent chemical and physical properties. However, conventional approaches (e.g., electron-beam vapor deposition and atomic layer deposition) to fabricate catalysts for the growth of CNTs are complex and demand high energy consumption. Herein, we report a facile method to synthesize catalysts derived from cobalt-containing zeolitic imidazolate frameworks (Co-ZIFs), which is exploited to in situ construct the three-dimensional (3D) CNT hybrid materials for all-solid-state supercapacitors. In brief, Co-ZIFs with a controllable structure is first grown on the inner porous surface of carbon foams pyrolyzed from commercial melamine foams, followed by thermal annealing and chemical vapor deposition to grow CNTs, achieving 3D free-standing CNT-based hybrids. The well-distributed Co-ZIFs in the carbon foam enable the grown CNTs with uniform structures and morphologies. Using the fabricated CNT-based hybrid as electrodes, the assembled all-solid-state supercapacitors show a high specific capacitance of 19.4 mF cm-2 at a current density of 0.5 mA cm-2, which could be further optimized to as high as 871.8 mF cm-2 by incorporating the pseudocapacitive material of manganese dioxide in CNT-based hybrids. This study provides a facile solution approach to fabricate the catalyst for the growth of a CNT inner porous substrate; the resultant 3D free-standing hybrids could be used as efficient electrodes for high-performance energy storage devices beyond supercapacitors.

Recent Advances in Rechargeable Metal-CO2 Batteries with Nonaqueous Electrolytes

This review article discusses the recent advances in rechargeable metal-CO2 batteries (MCBs), which include the Li, Na, K, Mg, and Al-based rechargeable CO2 batteries, mainly with nonaqueous electrolytes. MCBs capture CO2 during discharge by the CO2 reduction reaction and release it during charging by the CO2 evolution reaction. MCBs are recognized as one of the most sophisticated artificial modes for CO2 fixation by electrical energy generation. However, extensive research and substantial developments are required before MCBs appear as reliable, sustainable, and safe energy storage systems. The rechargeable MCBs suffer from the hindrances like huge charging-discharging overpotential and poor cyclability due to the incomplete decomposition and piling of the insulating and chemically stable compounds, mainly carbonates. Efficient cathode catalysts and a suitable architectural design of the cathode catalysts are essential to address this issue. Besides, electrolytes also play a vital role in safety, ionic transportation, stable solid-electrolyte interphase formation, gas dissolution, leakage, corrosion, operational voltage window, etc. The highly electrochemically active metals like Li, Na, and K anodes severely suffer from parasitic reactions and dendrite formation. Recent research works on the aforementioned secondary MCBs have been categorically reviewed here, portraying the latest findings on the key aspects governing secondary MCB performances.

Microwave-Assisted Rational Designed CNT-Mn3 O4 /CoWO4 Hybrid Nanocomposites for High Performance Battery-Supercapacitor Hybrid Device

Extensive research interest in hybrid battery-supercapacitor (BSH) devices have led to the development of cathode materials with excellent comprehensive electrochemical properties. In this work, carbon nanotube (CNT)-Mn3 O4 /CoWO4 triple-segment hybrid electrode is synthesized by using a two-step microwave-assisted hydrothermal route. Systematic physical characterization revealed that, with the assistance of microwave, granular Mn3 O4 and spheroid-like CoWO4 with preferred orientation, and oxygen vacancies are stacked or arranged on CNTs skeletons to construct a rational designed hybrid nanocomposite with abundant heterointerfaces and interfacial chemical bonds. Electrochemical evaluations show that the synergistic cooperation in CNT-Mn3 O4 /CoWO4 resulted in an ultra-high specific capacity (1907.5 C g-1 /529.8 mA h g-1 at 1 A g-1 ), a wide operating voltage window (1.15 V), the satisfactory rate capability (capacity maintained at 1016.5 C g-1 /282.3 mA h g-1 at 15 A g-1 ), and excellent cycling stability (117.2% initial capacity retention after 13000 cycles at 15 A g-1 ). In addition, the assembled CNT-Mn3 O4 /CoWO4 //N doped porous carbon (NC) BSH device delivered a stable working voltage of 2.05 V and superior energy density of 67.5 Wh kg-1 at power density of 1025 W kg-1 , as well as excellent stability (92.2% capacity retained at 5 A g-1 for 12600 cycles). This work provides a new and feasible tactic to develop high-performance transition metal oxide-based cathodes for advanced BSH devices.

Spontaneous intra-electron transfer within rGO@Fe2O3-MnO catalyst promotes long-term NOx reduction at ambient conditions

Iron (Fe)-based catalysts are widely used for taming nitrogen oxides (NO_x) containing flue gas, but the regeneration and long-term reusability remains a concern. The reusability can be acquired by external additives, and resultantly can not only increase the cost but can also add to process complexity as well as secondary pollutants. Herein, a self-sustainable material is designed to regenerate the catalyst for long-term reusability without adding to process complexity. The catalyst is based on reduced graphene-oxide impregnated by Fe₂O₃-MnO (rGO@Fe₂O₃-MnO; G-F-M) for spontaneous intra electron (e^-)-transfer from Mn to Fe. The developed catalyst; G-M-F exhibited 93.7% NO_x reduction, which suggests its high catalytic activity. The morphological and structure characterizations confirmed the Fe/Mn loading, contributing to e^--transfer between Mn and Fe due to its conductivity. The synthesized G-F-M showed higher NO_x reduction about 2.5 folds, than rGO@Fe₂O₃ (G-FeO) and rGO@MnO_x (G-MnO_x). The performance of G-M-F without and with an electrochemical system was also compared, and the difference was only 5%, which is an evidence of the spontaneous e^- transfer between the Mn and Fe-NO_x complex. The designed catalyst can be used for a long time without external assistance, and its efficiency was not affected significantly (<3.7%) in the presence of high oxygen contents (8%). The as-prepared G-M-F catalyst has great potential for executing a dual role NOx removal and self-regeneration of catalyst (SRC), promoting a sustainable remediation approach for large-scale applications.

Layer-by-Layer Heterostructure of MnO2@Reduced Graphene Oxide Composites as High-Performance Electrodes for Supercapacitors

In this paper, δ-MnO₂ with layered structure was prepared by a facile liquid phase method, and exfoliated MnO₂ nanosheet (e-MnO₂) was obtained by ultrasonic exfoliation, whose surface was negatively charged. Then, positive charges were grafted on the surface of MnO₂ nanosheets with a polycation electrolyte of polydiallyl dimethylammonium chloride (PDDA) in different concentrations. A series of e-MnO₂@reduced graphene oxide (rGO) composites were obtained by electrostatic self-assembly combined with hydrothermal chemical reduction. When PDDA was adjusted to 0.75 g/L, the thickness of e-MnO₂ was ~1.2 nm, and the nanosheets were uniformly adsorbed on the surface of graphene, which shows layer-by-layer morphology with a specific surface area of ~154 m²/g. On account of the unique heterostructure, the composite exhibits good electrochemical performance as supercapacitor electrodes. The specific capacitance of e-MnO₂-0.75@rGO can reach 456 F/g at a current density of 1 A/g in KOH electrolyte, which still remains 201 F/g at 10 A/g. In addition, the capacitance retention is 98.7% after 10000 charge-discharge cycles at 20 A/g. Furthermore, an asymmetric supercapacitor (ASC) device of e-MnO₂-0.75@rGO//graphene hydrogel (GH) was assembled, of which the specific capacitance achieves 94 F/g (1 A/g) and the cycle stability is excellent, with a retention rate of 99.3% over 10000 cycles (20 A/g).

One-Step Hydrothermal Synthesis of Nitrogen-Doped Reduced Graphene Oxide/Hausmannite Manganese Oxide for Symmetric and Asymmetric Pseudocapacitors

In this paper, the pseudocapacitive performance of nitrogen-doped and undoped reduced graphene oxide/tetragonal hausmannite nanohybrids (N-rGO/Mn₃O₄ and rGO/Mn₃O₄) synthesized using a one-pot hydrothermal method is reported. The nanohybrid electrode materials displayed exceptional electrochemical performance relative to their respective individual precursors (i.e., reduced graphene oxide (rGO), nitrogen-doped reduced graphene oxide (N-rGO), and tetragonal hausmannite (Mn₃O₄)) for symmetric pseudocapacitors. Among the two nanohybrids, N-rGO/Mn₃O₄ displayed greater performance with a high specific capacitance of 345 F g^-1 at a current density of 0.1 A g^-1, excellent specific energy of 12.0 Wh kg^-1 (0.1 A g^-1), and a high power density of 22.5 kW kg^-1 (10.0 A g^-1), while rGO/Mn₃O₄ demonstrated a high specific capacitance of 264 F g^-1 (0.1 A g^-1) with specific energy and power densities of 9.2 Wh kg^-1 (0.1 A g^-1) and 23.6 kW kg^-1 (10.0 A g^-1), respectively. Furthermore, the N-rGO/Mn₃O₄ nanohybrid exhibited an impressive pseudocapacitive performance when fabricated in an asymmetric configuration, having a stable potential window of 2.0 V in 1.0 M Na₂SO₄ electrolyte. The nanohybrid showed excellent specific energy and power densities of 34.6 Wh kg^-1 (0.1 A g^-1) and 14.01 kW kg^-1 (10.0 A g^-1), respectively. These promising results provide a good substance for developing novel carbon-based metal oxide electrode materials in pseudocapacitor applications.

In situ synthesis of nanostructured Fe3O4@TiO2 composite grown on activated carbon cloth as a binder-free electrode for high performance supercapacitors

Transition metal oxide (TMO) nanomaterials with regular morphology have received widening research attention as electrode materials due to their improved electrochemical characteristics. In this study we present the successful fabrication of an Fe₃O₄/TiO₂ nanocomposite grown on a carbon cloth (Fe₃O₄/TiO₂@C) used as a high-efficiency electrochemical supercapacitor electrode. Flexible electrodes are directly used for asymmetric supercapacitors without any binder. The increased specific surface area of the TiO₂ nanorod arrays provides sufficient adsorption sites for Fe₃O₄ nanoparticles. An asymmetric supercapacitor composed of Fe₃O₄/TiO₂@C is tested in 1 M Na₂SO₃ electrolyte, and the synergistic effects of fast reversible Faraday reaction on the Fe₃O₄/TiO₂ surface and the highly conductive network formed by TiO₂@C help the electrode to achieve a high areal capacitance of 304.1 mF cm⁻² at a current density of 1 mA cm⁻² and excellent cycling stability with 90.7% capacitance retention at 5 mA cm⁻² after 10 000 cycles. As a result, novel synthesis of a binder-free Fe₃O₄/TiO₂@C electrode provides a feasible approach for developing competitive candidates in supercapacitor applications.

Transition metal oxide (TMO) nanomaterials with regular morphology have received widening research attention as electrode materials due to their improved electrochemical characteristics.

Laser Synthesis of MOF-Derived Ni@Carbon for High-Performance Pseudocapacitors

Although nanosizing of multiphase pseudocapacitive nanomaterials could dramatically improve their electrochemical properties, a proper way to simultaneously control both the size and the phase of the pseudocapacitive materials is still elusive. Herein, we employed a commercial CO₂ laser engraver to do the transformation of a metal-organic framework (MOF-74(Ni)) into size-controlled Ni nanoparticles (4-12 nm) in porous carbon. The produced Ni@carbon hybrid showed the best specific capacitance of 925 F/g with excellent cycling stability when the particle size is 5.5 nm. We found that the highly redox-active α-Ni(OH)₂ is more predominantly formed than the less redox-active β-Ni(OH)₂ as the particle size becomes smaller. Our results substantiate that various MOFs could be created into high-performance pseudocapacitive materials with the controlled size and phase. It is believed that the laser-based synthesis could also serve as a powerful tool for the discovery of new MOF-derived materials in the field of energy storage and catalysis.

Electrode Materials for Supercapacitors: A Review of Recent Advances

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

Boron-Induced Nitrogen Fixation in 3D Carbon Materials for Supercapacitors

Nitrogen-rich carbon materials attract great attention because of their admirable performance in energy storage and electrocatalysis. However, their conductivity and nitrogen content are somehow contradictory because good conductivity requires high-temperature heat treatment, which decomposes most of the nitrogen species. Herein, we propose a facile method to solve this problem by introducing boron (B) to fix the nitrogen in a three-dimensional (3D) carbon material even at 1000 °C. Besides, this N-rich carbon material has a high content of pyrrolic nitrogen due to the selective stabilization of B, which is favorable in electrochemical reactions. Density functional theory (DFT) investigation demonstrates that B reduces the energy level of neighboring N species (especially pyrrolic nitrogen) in the graphene layer, making it difficult to escape. Thus, this carbon material simultaneously, achieves high conductivity (30 S cm^-1) and nitrogen content (7.80 atom %), thus showing an outstanding capacitance of 412 F g^-1 and excellent rate capability.

Enhanced capacitive properties of all-metal-oxide-nanoparticle-based asymmetric supercapacitors

The major problem of transition metal oxide (TMO)-based supercapacitors is their low specific energy (E_sp) due to the poor electrical conductivity of the TMO electrodes and narrow operating voltage window. To solve these limitations simultaneously, we propose asymmetric supercapacitors (ASCs) consisting of two composite TMO electrodes working in different potential ranges. Titanium dioxide (TiO₂) nanoparticle (NP)-incorporated iron oxide (Fe₂O₃) and manganese oxide (MnO₂) NPs were used as electrode materials covering the negative and positive potential window, respectively. The specific capacitance (C_sp) of this asymmetric TiO₂–Fe₂O₃‖TiO₂–MnO₂ supercapacitor is comparable to that of the symmetric TiO₂–MnO₂‖TiO₂–MnO₂ supercapacitor. However, the ASC can operate over a doubly extended voltage range, which resulted in a significant enhancement in the specific energy of the device. The E_sp value of the ASC at a specific power of 1000 W kg⁻¹ is 48.6 W h kg⁻¹, which is 34.1 and 8.1 times, respectively, larger than that of the two symmetric devices.

This study reports on an asymmetric supercapacitor consisting of two composite transition metal oxide (TiO₂–Fe₂O₃ and TiO₂–MnO₂) nanoparticles-based electrodes working in different potential ranges.

Asymmetric supercapacitors with high energy densities

The low energy densities of supercapacitors (SCs) are generally limited by the used anodes. To develop SCs with high energy densities, Fe3+ modified V2O5@GQDs (m-V2O5@GQDs) and ZIF-67-derived nanoporous carbon loaded with Mn3O4 (C/N-Mn3O4) were synthesized. After their detailed characterization using electron microscopy, X-ray methods and electrochemical techniques, they were further utilized as the anode and the cathode, respectively, to construct asymmetric supercapacitors (ASCs). The as-synthesized m-V2O5@GQDs improve the poor conductivity of V2O5, contributing greatly to a specific capacitance of 761 F g-1 at a current density of 2 A g-1. With application of a cell voltage of 2 V, an energy density of up to 99.4 W h kg-1 is achieved at a power density of 1000 W kg-1. Such ASCs also exhibit outstanding cycling performance (95% of initial capacitance even after 10 000 charging/discharging cycles). This study thus provides a new way to design and construct ASCs with high energy densities.