Due to advancements in technology, the energy demand is becoming more intense with time. The rapid fossil fuels consumption and environmental concerns triggered intensive research for alternative renewable energy resources, including sunlight and wind. Yet, due to their time‐dependent operations, significant electric energy storage systems are required to store substantial energy. In this regard, electrochemical energy storage devices, like batteries and supercapacitors (SCs), have recently attracted much research attention. Recent developments in SCs demonstrated that hybrid SCs (HSCs), which combine the excellent properties of batteries and SCs, increase the specific energy, specific power, specific capacitance, and life span. Carbonaceous and redox‐active materials have been explored as efficient electrode materials for applications in HSCs, ultimately enhancing their electrochemical performances. The HSCs performance significantly depends on the porosity, specific surface area, and conductivity of the electrode materials. This review article gives an overview of recent advances in developing HSCs with high specific power, specific energy, and long cyclic‐life. The fabrication of various HSCs materials using carbonaceous and redox‐active nanoarchitectures and their characterization are explored in‐depth, including electrode development, basic principles, and device engineering. A proper investigation has been conducted regarding state‐of‐the‐art materials as HSC electrodes. This review focuses on the most up‐to‐date, cutting‐edge, electrode materials for HSCs and their performance. The possibilities for novel electrode materials and their impact on the HSCs performance for future energy storage are also discussed.

Recent Progress in Carbonaceous and Redox‐Active Nanoarchitectures for Hybrid Supercapacitors: Performance Evaluation, Challenges, and Future Prospects

In order to aid the research and development of Al2O3-based materials, a comprehensive study of the structural properties of CaO–Al2O3–SiO2 systems was carried out via Raman and MAS-NMR spectroscopies. The spectral results showed that Raman scattering coefficients of Q2(Al), Q3(Al), Q4(Al), and [AlO6] units calibrated by 27Al MAS-NMR were 1, 0.554, 0.374, and 1.468, respectively. Using these Raman scattering coefficients to quantitatively analyze the aluminum structures, the results showed that CaO could promote the transformation from six-coordinated aluminum to four- and five-coordinated state with simultaneous depolymerization of four-coordinated aluminum. And CaO could exert a stronger depolymerization on [AlO4]-tetrahedrons than [SiO4]-tetrahedrons. Moreover, non-bridging oxygen number per coordinated atom (NBO/T) corresponding to transition of aluminum speciation were acquired. The values of NBO/T of the transformations from coexistences of [AlO6] and [AlO4] to coexistences of [AlO6], [AlO5], and [AlO4] to coexistences of [AlO5] and [AlO4] were 0.94 and 1.38, respectively.

Quantitative insight into aluminum structures in CaO–Al2O3–SiO2 system via Raman and 27Al MAS-NMR spectroscopies

Performance and transition mechanism from acidity to basicity of amphoteric oxides (Al2O3 and B2O3) in SiO2–CaO–Al2O3–B2O3 system: A molecular dynamics study

Al2O3 and B2O3 possess properties of amphoteric oxide, so Al2O3 and B2O3 have many different influences on the structure and properties of silicate melts, while its influence mechanism in atomic scale has not yet been fully understood. Molecular dynamics (MD) simulation was conducted to gain a clear understanding about the influences of Al2O3 and B2O3 on the structure and properties of SiO2-CaO-Al2O3 (SiO2-CaO-B2O3) silicate melts at 1773 K. The results show that when the Al2O3 content is less than 25 wt.%, the Al2O3 in the system is almost acidic in the studied system, and the bridging oxygen content and polymerization degree of the system increase with the increase of Al2O3 content. When the content of Al2O3 exceeds 25 wt.%, part of Al2O3 in the system becomes basic, which makes the bridging oxygen content of the system decrease and the growth rate of polymerization degree decrease. The effect of B2O3 on the network structure of SiO2-CaO-B2O3 (SCB) system is similar to that of Al2O3 in SiO2-CaO-Al2O3 (SCA) system, but the inflection point from acid to basic is lower (21 wt.%) and the basicity is stronger. Moreover, through two suitable viscosity calculation models, combined with experimental data, it shows that the viscosity of SCA system increases with the increase of Al2O3 content, while that of SCB system is just the opposite.

Effects of amphoteric oxide (Al2O3 and B2O3) on the structure and properties of SiO2-CaO melts by molecular dynamics simulation

Supercapacitor (SC) is generally regarded as a promising electrochemical device in the field of energy storage. Electrode materials, as one of the components of SCs, play an important role in the electrochemical performance of energy storage devices. Thus, it is essential to look for or synthesize new electrode materials. Metal-organic frameworks (MOFs) material, as a crucial classification of new materials in metal-organic materials (MOM) has been widely studied in SCs due to their multifunctional diversities, inherent porosity, large specific surface area, tunable pore size distribution, and simple synthesis method. In this review, we present recent research advances in developing various support materials (e.g., carbon nanotube, carbon nanofiber, and metal supports) to enhance the electrochemical performance of SCs. Specially, we provide a detailed overview of the latest research advances in MOF-derived materials and composites in SCs. At last, current developments are summarized and critical challenges for driving these intriguing developments toward practical applications are discussed together with promising solutions. 

Recent advances in metal-organic framework-based electrode materials for supercapacitors: A review

Supercapacitors have gained e wide attention because of high power density, fast charging and discharging, as well as good cycle performance. Recently, expanded graphite (EG) has been widely investigated as an effective electrode material for supercapacitors owing to its excellent physical, chemical, electrical, and mechanical properties. Based on charge storage mechanism, supercapacitors have been divided into symmetric, asymmetric, and hybrid supercapacitors. Here, we review the study progress of EG-based materials to be electrode materials. Furthermore, we discuss the application prospects and challenges of EG-based materials in supercapacitors.

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Expanded Graphite-Based Materials for Supercapacitors: A Review

Amphoteric oxide Al2O3 is generally considered to be acidic or alkaline in different environment. The effects of Al2O3 on the structure and dynamic property of CaO–SiO2–Al2O3 melt were investigated by MD simulation in this work. The results showed that as the content of Al2O3 increased, the contents of the tri-coordinate O and high-coordinate Al increased to balancing charge. When the basicity reaches 1, the viscosity of the melt increases with the addition of Al2O3 and reaches a maximum at 26 mol% of Al2O3. The higher concentration of Al2O3 leads to the relative deficiency of Ca atoms and the increase of tri-coordinate O and five-coordinate Al content, instability of the structure, and increase in DAl and DO. Al2O3 is considered to be acidic and forms network structures in the environment with sufficient alkaline cation. In contrast, Al2O3 becomes alkaline with insufficient basic cations environment, thus providing Oxygen atoms and balancing charge.

Effects of the amphoteric behavior of Al2O3 on the structure and properties of CaO–SiO2–Al2O3 melts by molecular dynamics

In steel continuous casting with submerged entry nozzles at a domestic steel plant, nozzle clogging was largely alleviated and defect rates of hot-rolled and cold-rolled sheets relating to slag entrainment were reduced for a nozzle with an approximately oval exit relative to a nozzle with a rectangular exit. Numerical and physical models were built to investigate the above phenomena. A turbulent model was applied in large eddy simulation to obtain the evolution of turbulent features within the nozzle and mold, while particle image velocimetry was adopted to monitor fluid flow beneath the slag–steel interface. Main results were that the flow speed of the main stream from the approximately oval exit was higher than that from the rectangular exit, while the zone of a velocity magnitude below 0.2 m/s within the approximately oval exit was smaller than that within the rectangular exit, indicating that inclusions tended to reside for longer in the latter. At the same casting speed, that slag entrainment occurred more frequently for the nozzle with a rectangular exit was found to have no direct relationship with the velocity magnitude and vorticity at 1/4 mold width on the top surface. Meanwhile, slag entrainment always occurred at the moment that vorticity changed most rapidly. The current study thus provided new insight into the structural parameter optimization of the submerged entry nozzle.

Effect of Exit Shape of Submerged Entry Nozzle on Flow Field and Slag Entrainment in Continuous Casting Mold

The four-electrode method was employed to examine the electrical conductivity and viscosity of CaO–Al2O3-based mold slags designed especially for continuous casting of high-Al steels. The effect of equal mass substitutions of SiO2 by Al2O3 and CaO by BaO on the electrical conductivity and viscosity of the mold slags was investigated in combination with the microstructure analysis using Raman spectroscopy. The equal mass substitutions of SiO2 by Al2O3 and CaO by BaO in the molten slags caused the electrical conductivity to decrease and the viscosity to increase, which was attributed to the increased polymerization degree of the melts and the depletion of the cations. The polymerization degree of the melts increased with Al2O3 replacing SiO2 because of the generation of Al-related networks by the charge compensation effect of Al3+. The equal mass substitution of CaO by BaO decreased the number of available network modifiers and accelerated the structural complexity of the [SiO4] tetrahedra. Meanwhile, Ba2+ would preferentially promote the [AlO4] tetrahedra configuration because it has a stronger ability of charge compensation than Ca2+. So the polymerization degree of slag melts increased with BaO replacing CaO. Nonlinear equations between the electrical conductivity and the polymerization degree of the molten slags were obtained to accurately predict the electrical conductivity of the melts at different temperatures in the current slag compositions range.

Electrical Conductivity, Viscosity and Structure of CaO-Al 2 O 3 -Based Mold Slags for Continuous Casting of High-Al Steels

The fast hydration rate of aluminate makes the slurry of CaO–Al2O3-based mold flux coagulates fast, which hinders the production and large-scale application in steel plants. This study selected four dispersants and investigated their mechanism and influence on the dispersibility of CaO–Al2O3-based mold powder slurry. Main results showed that the stability of CaO–Al2O3-based mold powder slurry was poorer than CaO–SiO2-based mold powder slurry. The addition of non-ionic dispersant, OP-10 or glycerol, could make the viscosity of the CaO–Al2O3-based slurry become larger, and the macromolecule dispersant, Poly Vinyl Pyrrolidone, resulted in unstable variation of the viscosity of the mold powder slurry. The anionic dispersant, tannic acid, performed well, as it was able to release negative charges covering the powder particles, and the electrostatic repulsion made the aggregation among particles difficult to occur. The 5 wt% addition of tannic acid was found to effectively improve the dispersion of the CaO–Al2O3-based slurry.

Weijie Pan

Papers

Expanded Graphite-Based Materials for Supercapacitors: A Review

Effects of the amphoteric behavior of Al2O3 on the structure and properties of CaO–SiO2–Al2O3 melts by molecular dynamics

Effect of Exit Shape of Submerged Entry Nozzle on Flow Field and Slag Entrainment in Continuous Casting Mold

Electrical Conductivity, Viscosity and Structure of CaO-Al 2 O 3 -Based Mold Slags for Continuous Casting of High-Al Steels

Effect of Dispersant on the Dispersibility of CaO–Al2O3-Based Mold Powder Slurry