Rational design of high-surface-area carbon nanotube/microporous carbon core-shell nanocomposites for supercapacitor electrodes

Recent Development on Transition Metal Oxides-Based Core-Shell Structures for Boosted Energy Density Supercapacitors

In recent years, nanomaterials exploration and synthesis have played a crucial role in advancing energy storage research, particularly in supercapacitor development. Researchers have diversified materials, including metal oxides, chalcogenides, and composites, as well as carbon materials, to enhance energy and power density. Balancing energy density with electrochemical stability remains challenging, driving intensified efforts in advancing electrode materials. This review focuses on recent progress in designing and synthesizing core-shell materials tailored for supercapacitors. The core-shell architecture offers advantages such as increased surface area, redox active sites, electrical conductivity, ion diffusion kinetics, specific capacitance, and cyclability. The review explores the impact of core and shell materials, specifically transition metal oxides (TMOs), on supercapacitor electrochemical behavior. Metal oxide choices, such as cobalt oxide as a preferred core and manganese oxide as a shell, are discussed. The review also highlights characterization techniques for assessing structural, morphological, and electrochemical properties of core-shell materials. Overall, it provides a comprehensive overview of ongoing TMOs-based core-shell material research for supercapacitors, showcasing their potential to enhance energy storage for applications ranging from gadgets to electric vehicles. The review outlines existing challenges and future opportunities in evolving TMOs-based core-shell materials for supercapacitor advancements, holding promise for high-efficiency energy storage devices.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Low-cost supercapacitor based on multi-walled carbon nanotubes and activated carbon derived from Moringa Oleifera fruit shells

An electric double-layer capacitor (EDLC) was fabricated using multi-walled carbon nanotubes (MWCNT) and activated carbon (AC) derived from Moringa Oleifera fruit shells as electrode material. The carbonization temperature and the weight ratio of the fruit shells to the activating agent were varied to determine the best condition in the fabrication of the electrodes. Activation of the carbonized fruit shells by ZnCl2 resulted in the formation of pores as verified by the scanning electron micrographs. Energy dispersive X-ray analyses show that the washing of the carbonized sample resulted in the removal of zinc and chlorine residues. The supercapacitor electrodes were fabricated by adding polyvinylidene fluoride and N-methylpyrrolidone to the MWCNT-AC mixture to form a slurry and was cast onto a nickel foam. The capacitance of the fabricated electrodes was determined using a potentiostat. The activated carbon with a carbonization temperature of 800 °C and a 1:2 weight ratio between the fruit shells and ZnCl2 was observed to have the highest capacitance of 130 F g-1 and was duplicated to fabricate the supercapacitor electrodes. A glass microfiber filter was soaked in 3 M KOH and placed in between the two electrodes. The specific capacitance of the EDLC was found to be 122 F g-1 at a current density of 0.5 A g-1, average energy density of 17 W h kg-1, average power density of 1.5 kW kg-1 and an equivalent series resistance of 1.6 Ω. After 100 scans with a scan rate of 0.1 V s-1, the percent decrease in capacitance was calculated to be 2.65 % of its original capacitance.

Core-shell nanomaterials: Applications in energy storage and conversion

Materials with core-shell structures have attracted increasing attention in recent years due to their unique properties and wide applications in energy storage and conversion systems. Through reasonable adjustments of their shells and cores, various types of core-shell structured materials can be fabricated with favorable properties that play significant roles in energy storage and conversion processes. The core-shell material can provide an effective solution to the current energy crisis. Various synthetic strategies used to fabricate core-shell materials, including the atomic layer deposition, chemical vapor deposition and solvothermal method, are briefly mentioned here. A state-of-the -art review of their applications in energy storage and conversion is summarized. The involved energy storage includes supercapacitors, li-ions batteries and hydrogen storage, and the corresponding energy conversion technologies contain quantum dot solar cells, dye-sensitized solar cells, silicon/organic solar cells and fuel cells. In addition, the correlation between the core-shell structures and their performance in energy storage and conversion is introduced, and this finding can provide guidance in designing original core-shell structures with advanced properties.

Biowaste-Derived Hierarchical Porous Carbon Nanosheets for Ultrahigh Power Density Supercapacitors

Low-cost activated carbons with high capacitive properties remain desirable for supercapacitor applications. Herein, a three-dimensional scaffolding framework of porous carbon nanosheets (PCNSs) has been produced from a typical biowaste, namely, ground cherry calyces, the specific composition and natural structures of which have contributed to the PCNSs having a very large specific surface area of 1612 m²  g^-1 , a hierarchical pore size distribution, a turbostratic carbon structure with a high degree graphitization, and about 10 % oxygen and nitrogen heteroatoms. A high specific capacitance of 350 F g^-1 at 0.1 A g^-1 has been achieved in a two-electrode system with 6 m KOH; this value is among the highest specific capacitance of biomass-derived carbon materials. More inspiringly, a high energy density of 22.8 Wh kg^-1 at a power density of 198.8 W kg^-1 can be obtained with 1 m aqueous solution of Li₂ SO₄ , and an ultrahigh energy density of 81.4 Wh kg^-1 at a power density of 446.3 W kg^-1 is realized with 1-ethyl-3-methylimidazolium tetrafluoroborate electrolyte.

A review of functionalized carbon nanotubes and graphene for heavy metal adsorption from water: Preparation, application, and mechanism

Carbon-based nanomaterials, especially carbon nanotubes and graphene, have drawn wide attention in recent years as novel materials for environmental applications. Notably, the functionalized derivatives of carbon nanotubes and graphene with high surface area and adsorption sites are proposed to remove heavy metals via adsorption, addressing the pressing pollution of heavy metal. This critical revies assesses the recent development of various functionalized carbon nanotubes and graphene that are used to remove heavy metals from contaminated water, including the preparation and characterization methods of functionalized carbon nanotubes and graphene, their applications for heavy metal adsorption, effects of water chemistry on the adsorption capacity, and decontamination mechanism. Future research directions have also been proposed with the goal of further improving their adsorption performance, the feasibility of industrial applications, and better simulating adsorption mechanisms.

Construction of Hierarchically One-Dimensional Core-Shell CNT@Microporous Carbon by Covalent Bond-Induced Surface-Confined Cross-Linking for High-Performance Supercapacitor

A covalent bond-induced surface-confined cross-linking is reported to construct one-dimensional coaxial CNT@microporous carbon composite (CNT@micro-C). Octaphenyl polyhedral oligomeric silsesquioxane (Ph-POSS) composed of eight phenyls and a -Si₈O₁₂ cage was selected as precursor for microporous carbon. The layer-by-layer cross-linking of phenyl anchored Ph-POSS on the surface of CNT; after carbonization and etching of -Si₈O₁₂ cages, CNT@micro-C including CNT core and microporous carbon shell was harvested. The thickness of microporous carbon shell can be well tailored from 6.0 to 20.0 nm, and the surface area of CNT@micro-C can reach 1306 m² g^-1. CNT@micro-C combines the structural advantages of CNT and microporous carbon, presenting large surface area, high electrical conductivity, fast ion transfer speed, and short ion transfer distance. When used as electrode material, CNT@micro-C reveals superior supercapacitive performance; for example, its capacitance can reach 243 F g^-1 at 0.5 A g^-1 and slightly decreases to 209 F g^-1 at 10 A g^-1, indicating a capacitance retention of 86%. Even at a very high scan rate of 50 A g^-1, a high capacitance of 177 F g^-1 is retained, giving a capacitance retention of 73%.

Nitrogen-Doped Mesoporous Carbons for Supercapacitor Electrodes with High Specific Volumetric Capacitance

To pursue the miniaturization of supercapacitors in practical use, it is critical to construct an efficient but limited porosity of a nanocarbon-based electrode for simultaneously obtaining a high utilization of energy storage places and high coating density. However, current studies dominantly focus on the enhancement of specific mass capacitance (C_m) by increasing the pore volume and surface area, leading to a low coating density and, thereby, resulting in a low specific volumetric capacitance (C_V). We report herein the fabrication of a nitrogen-doped mesoporous carbon (NNCM), whose tunable pore volume coupled with the fixed mesopore size offers us the possibility to control the coating density, thus optimizing the C_V and C_m for different application purposes. As a result, NNCM with the highest pore volume and surface area of 2.11 cm³ g^-1 and 663 m² g^-1 demonstrates the highest C_m (190 F g^-1) but lowest C_V (124 F cm^-3) because the overhigh porosity reduces the coating density greatly. NNCM with moderate pore volume and surface area of 1.22 cm³ g^-1 and 489 m² g^-1 shows the highest C_V of 200 F cm^-3, although it presents a low C_m of 147 F g^-1. These results may raise concerns about constructing a suitable porosity to realize a target-oriented use, particularly those targeting miniaturized devices.

The influence of surface area, porous structure, and surface state on the supercapacitor performance of titanium oxynitride: implications for a nanostructuring strategy

Underlying factors governing the capacitance and stability of titanium oxynitride are revealed.

Multishelled NiO Hollow Microspheres for High-performance Supercapacitors with Ultrahigh Energy Density and Robust Cycle Life

Multishelled NiO hollow microspheres for high-performance supercapacitors have been prepared and the formation mechanism has been investigated. By using resin microspheres to absorb Ni²⁺ and subsequent proper calcinations, the shell numbers, shell spacing and exterior shell structure were facilely controlled via varying synthetic parameters. Particularly, the exterior shell structure that accurately associated with the ion transfer is finely controlled by forming a single shell or closed exterior double-shells. Among multishelled NiO hollow microspheres, the triple-shelled NiO with an outer single-shelled microspheres show a remarkable capacity of 1280 F g⁻¹ at 1 A g⁻¹, and still keep a high value of 704 F g⁻¹ even at 20 A g⁻¹. The outstanding performances are attributed to its fast ion/electron transfer, high specific surface area and large shell space. The specific capacitance gradually increases to 108% of its initial value after 2500 cycles, demonstrating its high stability. Importantly, the 3S-NiO-HMS//RGO@Fe₃O₄ asymmetric supercapacitor shows an ultrahigh energy density of 51.0 Wh kg⁻¹ at a power density of 800 W kg⁻¹, and 78.8% capacitance retention after 10,000 cycles. Furthermore, multishelled NiO can be transferred into multishelled Ni microspheres with high-efficient H₂ generation rate of 598.5 mL H₂ min⁻¹ g⁻¹_Ni for catalytic hydrolysis of NH₃BH₃ (AB).

Layered NiO/reduced graphene oxide composites by heterogeneous assembly with enhanced performance as high-performance asymmetric supercapacitor cathode

Layered NiO/RGO composites by heterogeneous assembly show high energy density as an asymmetric supercapacitor.

Fabrication and electrochemical performance of novel hollow microporous carbon nanospheres

A new class of hollow microporous carbon nanospheres with good electrochemical performances was fabricated through a facile hypercrosslinking strategy.

Crumpled Nitrogen-Doped Graphene for Supercapacitors with High Gravimetric and Volumetric Performances

Graphene is considered a promising electrochemical capacitors electrode material due to its high surface area and high electrical conductivity. However, restacking interactions between graphene nanosheets significantly decrease the ion-accessible surface area and impede electronic and ionic transfer. This would, in turn, severely hinder the realization of high energy density. Herein, we report a strategy for preparation of few-layer graphene material with abundant crumples and high-level nitrogen doping. The two-dimensional graphene nanosheets (CNG) feature high ion-available surface area, excellent electronic and ion transfer properties, and high packing density, permitting the CNG electrode to exhibit excellent electrochemical performance. In ionic liquid electrolyte, the CNG electrode exhibits gravimetric and volumetric capacitances of 128 F g(-1) and 98 F cm(-3), respectively, achieving gravimetric and volumetric energy densities of 56 Wh kg(-1) and 43 Wh L(-1). The preparation strategy described here provides a new approach for developing a graphene-based supercapacitor with high gravimetric and volumetric energy densities.

Nitrogen-Doped Carbon Nanocoil Array Integrated on Carbon Nanofiber Paper for Supercapacitor Electrodes

Integrating a nanostructured carbon array on a conductive substrate remains a challenging task that presently relies primarily on high-vacuum deposition technology. To overcome the problems associated with current vacuum techniques, we demonstrate the formation of an N-doped carbon array by pyrolysis of a polymer array that was electrochemically grown on carbon fiber paper. The resulting carbon array was investigated for use as a supercapacitor electrode. In-depth surface characterization results revealed that the microtextural properties, surface functionalities, and degree of nitrogen incorporated into the N-doped carbon array can be delicately controlled by manipulating carbonization temperatures. Furthermore, electrochemical measurements showed that subtle changes in these physical properties resulted in significant changes in the capacitive behavior of the N-doped carbon array. Pore structures and nitrogen/oxygen functional groups, which are favorable for charge storage, were formed at low carbonization temperatures. This result showed the importance of having a comprehensive understanding of how the surface characteristics of carbon affect its capacitive performance. When utilized as a substrate in a pseudocapacitive electrode material, the N-doped carbon array maximizes capacitive performance by simultaneously achieving high gravimetric and areal capacitances due to its large surface area and high electrical conductivity.

Nanostructured Electrode Materials for Electrochemical Capacitor Applications

The advent of novel organic and inorganic nanomaterials in recent years, particularly nanostructured carbons, conducting polymers, and metal oxides, has enabled the fabrication of various energy devices with enhanced performance. In this paper, we review in detail different nanomaterials used in the fabrication of electrochemical capacitor electrodes and also give a brief overview of electric double-layer capacitors, pseudocapacitors, and hybrid capacitors. From a materials point of view, the latest trends in electrochemical capacitor research are also discussed through extensive analysis of the literature and by highlighting notable research examples (published mostly since 2013). Finally, a perspective on next-generation capacitor technology is also given, including the challenges that lie ahead.