Carbon Quantum Dot-Induced MnO2 Nanowire Formation and Construction of a Binder-Free Flexible Membrane with Excellent Superhydrophilicity and Enhanced Supercapacitor Performance

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

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

Dual Applications of Cobalt-Oxide-Grafted Carbon Quantum Dot Nanocomposite for Two Electrode Asymmetric Supercapacitors and Photocatalytic Behavior

Carbon materials, such as graphene, carbon nanotubes, and quantum-dot-doped metal oxides, are highly attractive for energy storage and environmental applications. This is due to their large surface area and efficient optical and electrochemical activity. In this particular study, a composite material of cobalt oxide and carbon quantum dots (Co3O4-CQD) was prepared using cobalt nitrate and ascorbic acid (carbon source) through a simple one-pot hydrothermal method. The properties of the composite material, including the functional groups, composition, surface area, and surface morphology, were evaluated by using various methods such as ultraviolet, Fourier transform infrared, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, scanning electron microscopy, and transmission electron microscopy analysis. The electrochemical performance of the Co3O4-CQD composite has been studied using a three-electrode system. The results show that at 1 A g-1, the composite delivers a higher capacitance of 1209 F g-1. The asymmetric supercapacitor (Co3O4-CQD//AC) provided 13.88 W h kg-1 energy and 684.65 W kg-1 power density with a 96% capacitance retention. The Co3O4-CQD composite also demonstrated excellent photocatalytic activity (90% in 60 min) for the degradation of methylene blue dye under UV irradiation, which is higher than that of pristine Co3O4 and CQD. This demonstrates that the Co3O4-CQD composite is a promising material for commercial energy storage and environmental applications.

Electric field-assisted laser ablation fabrication and assembly of zinc oxide/carbon nanocomposites into hierarchical structures for supercapacitor electrodes

One of the major challenges in the field of electrochemical energy storage device performance improvement is the development of suitable synthetic materials for electrodes that can provide high power and high energy density features combined with their long-term stability. Here, we have developed a novel two-step approach based on DC glow discharge plasma pre-treatment of a carbon cloth substrate followed by electric field-assisted laser ablation for the synthesis of ZnO/C nanocomposites in a liquid and their simultaneous assembly into hierarchically organized nanostructures onto the pre-processed carbon cloth to produce a supercapacitor electrode. To form such nanostructures, a processed carbon cloth was included in the electrical circuit as a cathode during laser ablation of zinc in water, while a zinc target served as an anode. A series of studies have been performed to explore the structure, morphology, composition and electrochemical characteristics of the synthesized ZnO/C nanocomposites. Application of the external field provided additional possibilities for tuning the particle morphology. The parameters of the obtained nanostructures were shown to depend on the direction of the applied electric field and liquid composition. SEM studies revealed a nanoflower-like morphology of the prepared nanomaterial having potential in supercapacitor applications due to a large surface area. The ZnO/C nanoflowers, deposited onto a carbon cloth substrate, were tested for energy storage by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) analysis. The results showed a pseudocapacitor behavior with a maximum specific capacitance of about 3045 F g-1 (at a scan rate of 1 mV s-1). These results demonstrate a promising storage efficiency of the synthesized ZnO/C nanocomposite as a material for supercapacitors.

Preparation of MnO2-Carbon Materials and Their Applications in Photocatalytic Water Treatment

Water pollution is one of the most important problems in the field of environmental protection in the whole world, and organic pollution is a critical one for wastewater pollution problems. How to solve the problem effectively has triggered a common concern in the area of environmental protection nowadays. Around this problem, scientists have carried out a lot of research; due to the advantages of high efficiency, a lack of secondary pollution, and low cost, photocatalytic technology has attracted more and more attention. In the past, MnO₂ was seldom used in the field of water pollution treatment due to its easy agglomeration and low catalytic activity at low temperatures. With the development of carbon materials, it was found that the composite of carbon materials and MnO₂ could overcome the above defects, and the composite had good photocatalytic performance, and the research on the photocatalytic performance of MnO₂-carbon materials has gradually become a research hotspot in recent years. This review covers recent progress on MnO₂-carbon materials for photocatalytic water treatment. We focus on the preparation methods of MnO₂ and different kinds of carbon material composites and the application of composite materials in the removal of phenolic compounds, antibiotics, organic dyes, and heavy metal ions in water. Finally, we present our perspective on the challenges and future research directions of MnO₂-carbon materials in the field of environmental applications.

MnO2-based materials for supercapacitor electrodes: challenges, strategies and prospects

Manganese dioxide (MnO₂) has always been the ideal electrode material for supercapacitors due to its non-toxic nature and high theoretical capacity (1370 F g^-1). Over the past few years, significant progress has been made in the development of high performance MnO₂-based electrode materials. This review summarizes recent research progress in experimental, simulation and theoretical studies for the modification of MnO₂-based electrode materials from different perspectives of morphology engineering, defect engineering and heterojunction engineering. Several main approaches to achieve enhanced electrochemical performance are summarized, respectively increasing the effective active site, intrinsic conductivity and structural stability. On this basis, the future problems and research directions of electrode materials are further envisaged, which provide theoretical guidance for the adequate design and synthesis of MnO₂-based electrode materials for use in supercapacitors.

The Consequences of Water Interactions with Nitrogen-Containing Carbonaceous Quantum Dots-The Mechanistic Studies

Despite the importance of quantum dots in a wide range of biological, chemical, and physical processes, the structure of the molecular layers surrounding their surface in solution remains unknown. Thus, knowledge about the interaction mechanism of Nitrogen enriched Carbonaceous Quantum Dots' (N-CQDs) surface with water-their natural environment-is highly desirable. A diffusive and Stern layer over the N-CQDs, characterized in situ, reveals the presence of anionic water clusters [OH(H2O)n]-. Their existence explains new observations: (i) the unexpectedly low adsorption enthalpy (ΔHads) in a pressure range below 0.1 p/ps, and ΔHads being as high as 190 kJ/mol at 0.11 p/ps; (ii) the presence of a "conductive window" isolating nature-at p/ps below 0.45-connected to the formation of smaller clusters and increasing conductivity above 0.45 p/ps, (iii) Stern layer stability; and (iv) superhydrophilic properties of the tested material. These observables are the consequences of H2O dissociative adsorption on N-containing basic centers. The additional direct application of surfaces formed by N-CQDs spraying is the possibility of creating antistatic, antifogging, bio-friendly coatings.

Modification strategies of membranes with enhanced Anti-biofouling properties for wastewater Treatment: A review

This review addresses composite membranes used for wastewater treatment, focusing heavily on the anti-biofouling properties of such membranes. Biofouling caused by the development of a thick biofilm on the membrane surface is a major issue that reduces water permeance and reduces its lifetime. Biofilm formation and adhesion are mitigated by modifying membranes with two-dimensional or zero-dimensional carbon-based nanomaterials or their modified substituents. In particular, nanomaterials based on graphene, including graphene oxide and carbon quantum dots, are mainly used as nanofillers in the membrane. Functionalization of the nanofillers with various organic ligands or compositing the nanofiller with other materials, such as silver nanoparticles, enhances the bactericidal ability of composite membranes. Moreover, such membrane modifications reduce biofilm adhesion while increasing water permeance and salt/dye rejection. This review discusses the recent literature on developing graphene oxide-based and carbon quantum dot-based composite membranes for biofouling-resistant wastewater treatment.

Electromagnetic Interference Shielding Effectiveness of Room Temperature Fabricated Manganese Dioxide/Carbon Dots Nanocomposites

The present work is focused on the fabrication of manganese dioxide/carbon dots (MnO₂/CDs) nanocomposites at room temperature in situ co-participation method in an aqueous medium and characterized. Our study showed that the concentration of CDs controls the morphology of MnO₂/CDs nanocomposite and also acted as a reducing agent to convert potassium permanganate (KMnO₄) to MnO₂. Subsequently, nanoflowers, quasi-spherical particles, broken, and interconnected chain type of morphology was observed by adding dispersion of 0.5, 1.0, 1.5, and 2.0 ml CDs in acetone to 1 mmol KMnO₄ aqueous solution in the corresponding MnO₂/CDs-0.5, MnO₂/CDs-1.0, MnO₂/CDs-1.5, and MnO₂/CDs-2.0 composites, respectively. A plausible mechanism on the transformation of morphology of MnO₂/CDs with CDs concentration is also provided. Further, the present work also focused for the first time on the application in the electromagnetic interference (EMI) shielding of MnO₂/CD nanocomposites due to the high dielectric and conductivity. Interestingly, MnO₂/CDs-2.0 (nanochains) exhibited the highest total EMI shielding efficiency (SE_T) of ~39.4 dB following reflection as dominant shielding mechanism due to the high aspect ratio, highest conductivity, high dielectric loss, and impendence mismatch.

Recent Developments of Transition Metal Compounds-Carbon Hybrid Electrodes for High Energy/Power Supercapacitors

Due to their rapid power delivery, fast charging, and long cycle life, supercapacitors have become an important energy storage technology recently. However, to meet the continuously increasing demands in the fields of portable electronics, transportation, and future robotic technologies, supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged. Transition metal compounds (TMCs) possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors. However, the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process, which greatly impede their large-scale applications. Most recently, the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges. Herein, we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies, including conductive carbon skeleton, interface engineering, and electronic structure. Furthermore, the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.

Surface chemical functionality of carbon dots: influence on the structure and energy storage performance of the layered double hydroxide

As a kind of zero-dimensional material, carbon dots (CDs) have become a kind of promising novel material due to their incomparable unique physical and chemical properties. Despite the optical properties of CDs being widely studied, their surface chemical functions are rarely reported. Here we propose an interesting insight into the important role of surface chemical properties of CDs in adjusting the structure of the layered double hydroxide (LDH) and its energy storage performance. It was demonstrated that CDs with positive charge (p-CDs) not only reduce the size of the flower-like LDH through affecting the growth of LDH sheets, but also act as a structure stabilizer. After calcination, the layered double oxide (LDO) maintained the morphology of the LDH and prevented the stacking of layers. And the superiority of the composite in lithium-ion batteries (LIBs) was demonstrated. When used as an anode of LIBs, composites possess outstanding specific capacity, cycle stability and rate performance. It presents the discharge capacity of 1182 mA h g-1 and capacity retention of 94% at the current density of 100 mA g-1 after 100 cycles. Our work demonstrates the important chemical functions of CDs and expands their future applications.

Carbon-Based Quantum Dots for Supercapacitors: Recent Advances and Future Challenges

Carbon-based Quantum dots (C-QDs) are carbon-based materials that experience the quantum confinement effect, which results in superior optoelectronic properties. In recent years, C-QDs have attracted attention significantly and have shown great application potential as a high-performance supercapacitor device. C-QDs (either as a bare electrode or composite) give a new way to boost supercapacitor performances in higher specific capacitance, high energy density, and good durability. This review comprehensively summarizes the up-to-date progress in C-QD applications either in a bare condition or as a composite with other materials for supercapacitors. The current state of the three distinct C-QD families used for supercapacitors including carbon quantum dots, carbon dots, and graphene quantum dots is highlighted. Two main properties of C-QDs (structural and electrical properties) are presented and analyzed, with a focus on the contribution to supercapacitor performances. Finally, we discuss and outline the remaining major challenges and future perspectives for this growing field with the hope of stimulating further research progress.

A reasonable design of polypyrrole nanotubes interconnected Ni–Co layered double hydroxide-based composites via ZIF templates for high performance supercapacitors

Hybrids PPy/Ni–Co LDH are used for supercapacitors and show high specific capacitances via synergistic effects between Ni–Co LDH and PPy.

Hydrangea-like α-Ni1/3Co2/3(OH)2 Reinforced by Ethyl Carbamate "Rivet" for All-Solid-State Supercapacitors with Outstanding Comprehensive Performance

Improving the self-conductivity and structural stability of electrode materials is a key strategy to improve the energy density, rate performance, and cycle life of supercapacitors. Controlled intercalation of ethyl carbamate (CH₃CH₂OCONH₂) as the rivet between Ni-Co hydroxide layers can be used to obtain sufficient ion transport channels and robust structural stability of hydrangea-like α-Ni_1/3Co_2/3(OH)₂ (NC). Combining the improved electronic conductivity offered by the coexistence of Ni²⁺ and Co²⁺ optimizing itself electronic conductivity and the addition of carbon nanotubes (CNTs) as the electron transport bridge between the active material and the current collector and the large specific surface area (296 m² g^-1) reducing the concentration polarization, the capacitance retention ratio of NC-CNT from 0.2 to 20 A g^-1 is up to 93.4% and its specific capacitance is as high as 1228.7 F g^-1 at 20 A g^-1. The large total hole volume (0.40 cm³ g^-1) and wide crystal plane spacing (0.71 nm) provide an adequate space to withstand structure deformation during charge/discharge processes and enhance the structural stability of the NC material. The capacitance fading ratio of NC-CNT is only 4.5% at 10 A g^-1 for 10 000 cycles. The aqueous supercapacitor (NC-CNT//AC) and all-solid-state supercapacitor (PVA-NC-CNT//PVA-AC) exhibit high energy density (35.2 W h kg^-1 at 100.0 W kg^-1 and 35.4 W h kg^-1 at 100.7 W kg^-1), ultrahigh rate performance (the specific capacitances at 20 A g^-1 are 92.8 and 87.2% compared to that at 0.5 A g^-1), and long cycling life span (the specific capacitances after 100 000 cycles at 10 A g^-1 are 91.5 and 90.8% compared with that of their initial specific capacitances), respectively. Therefore, hydrangea-like NC could be a promising material for advanced next-generation supercapacitors.

Feeling the power: robust supercapacitors from nanostructured conductive polymers fostered with Mn2+ and carbon dots

Polyaniline (PANI) is considered one of the most preferred electrically conductive polymers (CPs), which is widely studied as an electrode material in designing next-generation energy storage devices due to their chemical stability, fast redox reactions between the polymer and the electrolytes, high electrical conductivity, excellent electrochemical performance, and low cost. However, the inferior stability of PANI limits its application. In this work, the benefit of carbon dots (CDots) as light-weight and spherical carbon-based electrodes and fillers that allow the maintenance of the nanostructure of PANI while easing the ionic transport was studied together with the effect of manganese(ii) (Mn²⁺) doping on the overall capacitive properties of PANI. The integration of N-doped spherical, nanosized carbon dots (N-CDots) in the copolymerization of nanostructured PANI in the presence of varying concentrations of Mn²⁺ as a dopant synergistically improved the overall conductivity and specific surface area of the PANI-based electrode and showed surface double layer ion exchange. Pseudocapacitance mechanisms were observed when the dopant concentration was kept at a molar percentage of Mn²⁺ to aniline of 1, which displayed exceptionally high specific capacitances of up to 595 F g^-1. The asymmetric supercapacitor devices made with N-CDot and nanostructured hybrid electrodes could reveal the great potential in the development of cheap yet efficient battery-sized supercapacitor devices. In addition to extensive electrochemical performance, advanced EPR spectroscopy revealed detailed information regarding the defect structures of electrode materials in terms of understanding the conduction behavior of defect centers.

Iron oxide-based nanomaterials for supercapacitors

As highly efficient and clean electrochemical energy storage devices, supercapacitors (SCs) have drawn widespread attention as promising alternatives to batteries in recent years. Among various electrode materials, iron oxide materials have been widely studied as negative SC electrode materials due to their broad working window in negative potential, ideal theoretical specific capacitance, good redox activity, abundant availability, and eco-friendliness. However, iron oxides still suffer from the problems of low stability and poor conductivity. In this review, recent progress in iron oxide-based nanomaterials, including Fe₂O₃, Fe₃O₄, Fe_xO_y, and FeOOH, as electrode materials of SCs, is discussed. The nanostructure design and various synergistic effects of nanocomposites for improving the electrochemical performance of iron oxides are emphasized. Research on iron oxide-based symmetric/asymmetric SCs is also discussed. Future outlooks regarding iron oxides for SCs are likewise proposed.

Sandwich-like NiCo layered double hydroxide/reduced graphene oxide nanocomposite cathodes for high energy density asymmetric supercapacitors

Lab-synthesized sandwich-like LDH/rGO composites were assembled into asymmetric supercapacitors exhibiting high energy density and excellent cycling stability.

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

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

A Novel Radiation Method for Preparing MnO₂/BC Monolith Hybrids with Outstanding Supercapacitance Performance

A novel facile process for fabrication of amorphous MnO₂/bamboo charcoal monolith hybrids (MnO₂/BC) for potential supercapacitor applications using γ-irradiation methods is described. The structural, morphological and electrochemical properties of the MnO₂/BC hybrids have been investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. The combination of BC (electrical double layer charge) and MnO₂ (pseudocapacitance) created a complementary effect, which enhanced the specific capacitance and good cyclic stability of the MnO₂/BC hybrid electrodes. The MnO₂/BC hybrids showed a higher specific capacitance (449 F g⁻¹ at the constant current density of 0.5 A g⁻¹ over the potential range from –0.2 V to 0.8 V), compared with BC (101 F g⁻¹) in 1 M of Na₂SO₄ aqueous electrolyte. Furthermore, the MnO₂/BC hybrid electrodes showed superior cycling stability with 78% capacitance retention, even after 10,000 cycles. The experimental results demonstrated that the high performance of MnO₂/BC hybrids could be a potential electrode material for supercapacitors.