Supercapacitors are increasingly used for energy conversion and storage systems in sustainable nanotechnologies. Graphite is a conventional electrode utilized in Li-ion-based batteries, yet its specific capacitance of 372 mA h g−1 is not adequate for supercapacitor applications. Interest in supercapacitors is due to their high-energy capacity, storage for a shorter period and longer lifetime. This review compares the following materials used to fabricate supercapacitors: spinel ferrites, e.g., MFe2O4, MMoO4 and MCo2O4 where M denotes a transition metal ion; perovskite oxides; transition metals sulfides; carbon materials; and conducting polymers. The application window of perovskite can be controlled by cations in sublattice sites. Cations increase the specific capacitance because cations possess large orbital valence electrons which grow the oxygen vacancies. Electrodes made of transition metal sulfides, e.g., ZnCo2S4, display a high specific capacitance of 1269 F g−1, which is four times higher than those of transition metals oxides, e.g., Zn–Co ferrite, of 296 F g−1. This is explained by the low charge-transfer resistance and the high ion diffusion rate of transition metals sulfides. Composites made of magnetic oxides or transition metal sulfides with conducting polymers or carbon materials have the highest capacitance activity and cyclic stability. This is attributed to oxygen and sulfur active sites which foster electrolyte penetration during cycling, and, in turn, create new active sites.

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Advanced materials and technologies for supercapacitors used in energy conversion and storage: a review

Porous N-doped carbon nanofibers assembled with nickel ferrite nanoparticles as efficient chemical anchors and polysulfide conversion catalyst for lithium-sulfur batteries.

Supercapacitors are widely accepted as one of the energy storage devices in the realm of sustainable and renewable energy storage. Supercapacitors have emerged as a good alternative to traditional capacitors and fuel cells due to their higher energy density and power density compared to batteries and fuel cells. However, supercapacitors have some drawbacks such as low energy density and poor cycle life compared to batteries. To overcome these issues, researchers are paying much attention to the fabrication of metal oxide nanostructures and their modification by different approaches such as doping, introducing oxygen vacancies, and hybridization with nanomaterials of carbon allotropes for enhanced electrochemical properties. In this review article, we have presented the above-mentioned topics with the aid of recently reported works. Moreover, we have provided theoretical insights from density functional theory for the electrochemical behavior of the electrode materials from the published works. This review concisely presents the advancement in the supercapacitor energy storage field and the different approaches involved in the fabrication of supercapacitor electrode materials, which will be very handy to the researchers working in the field of energy storage. Further, the challenges and future perspectives of this exciting research field are discussed in detail.

Recent advances in engineered metal oxide nanostructures for supercapacitor applications: experimental and theoretical aspects

By using a facile hydrothermal method, we synthesized Ni1−xMnxFe2O4 nanoparticles as supercapacitor electrode materials and studied how the incremental substitution of Ni with Mn would affect their structural, electronic, and electrochemical properties. X-ray diffractometry confirmed the single-phase spinel structure of the nanoparticles. Raman spectroscopy showed the conversion of the inverse structure of NiFe2O4 to the almost normal structure of MnFe2O4. Field-emission scanning electron microscopy showed the spherical shape of the obtained nanoparticles with a size in the range of 20–30 nm. Optical bandgaps were found to decrease as the content of Mn increased. Electrochemical characterizations of the samples indicated the excellent performance and the desirable cycling stability of the prepared nanoparticles for supercapacitors. In particular, the specific capacitance of the prepared Ni1−xMnxFe2O4 nanoparticles was found to increase as the content of Mn increased, reaching the highest specific capacitance of 1,221 F/g for MnFe2O4 nanoparticles at the current density of 0.5 A/g with the corresponding power density of 473.96 W/kg and the energy density of 88.16 Wh/kg. We also demonstrated the real-world application of the prepared MnFe2O4 nanoparticles. We performed also a DFT study to verify the changes in the geometrical and electronic properties that could affect the electrochemical performance.

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Incremental substitution of Ni with Mn in NiFe 2 O 4 to largely enhance its supercapacitance properties

Deciphering of the charge storage mechanism of conventional supercapacitors (SCs) can be a humongous stride towards developing high energy density and prolonged cyclability based SCs which can cease the energy crisis to a greater extent. Though ex-situ characterization techniques have helped to puzzle out the charge storage mechanism of the SCs, voluminous untrodden grey areas with unknown ensembles are still out there which cannot be left behind. Lately, in situ analytical characterization tools such as in situ X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), in situ X-ray photoelectron spectroscopy (XPS), in situ Raman, in situ infrared/Fourier-transform infrared spectroscopy (IR/FTIR), in situ nuclear magnetic resonance (NMR), in situ atomic force microscopy (AFM), in situ scanning electron microscopy (SEM), in situ tunnelling electron microscopy (TEM), in situ electrochemical quartz crystal microbalance (EQCM) techniques etc. have exclusively come into picture to throw some light on the charge storage mechanism of SCs over the past decade or so. This review solely emphases on the insights of the charge storage mechanism interpreted from in situ characterization techniques along with the theoretical investigation validations. Various charge storage parameters obtained from electronic structure simulations like quantum capacitance, voltage induced by electrolyte ions, diffusion energy barrier of electrolyte ions etc. have been particularized with pertinent examples. Both the amalgamation of in situ techniques as well as theoretical simulations can efficiently elucidate the ion dynamics and charge transfer of the SC electrode systems with a whole new perspective. However, the comprehensive classification of SCs based on mechanism, choice of electrodes and device configuration, enlightenment of charge storage mechanism by in situ/operando techniques along with theoretical explorations can be retrieved in this audit.

Understanding the charge storage mechanism of supercapacitors: in situ/operando spectroscopic approaches and theoretical investigations

Manganese ferrite (MnFe2O4) nanoparticles were synthesized via a hydrothermal method and combined with exfoliated MoS2 nanosheets, and the nanocomposite was studied as a supercapacitor. X-ray diffractometry and Raman spectroscopy confirmed the crystalline structures and structural characteristics of the nanocomposite. Transmission electron microscopy images showed the uniform size distribution of MnFe2O4 nanoparticles (~ 13 nm) on few-layer MoS2 nanosheets. UV-visible absorption photospectrometry indicated a decrease in the bandgap of MnFe2O4 by MoS2, resulting in a higher conductivity that is suitable for capacitance. Electrochemical tests showed that the incorporation of MoS2 nanosheets largely increased the specific capacitance of MnFe2O4 from 600 to 2093 F/g (with the corresponding energy density and power density of 46.51 Wh/kg and 213.64 W/kg, respectively) at 1 A/g, and led to better charge-discharge cycling stability. We also demonstrated a real-world application of the MnFe2O4/MoS2 nanocomposite in a two-cell asymmetric supercapacitor setup. A density functional theory study was also performed on the MnFe2O4/MoS2 interface to analyze how a MoS2 monolayer can enhance the electronic properties of MnFe2O4 towards a higher specific capacitance.

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Highly improved supercapacitance properties of MnFe2O4 nanoparticles by MoS2 nanosheets.

Effect of Co2+ content on supercapacitance properties of hydrothermally synthesized Ni1-xCoxFe2O4 nanoparticles

Graphene quantum dots (GQDs) with their very large specific surface area and numerous edges can offer promising novel features as metal-free and inexpensive electrodes of supercapacitors. Here, we investigated the effect of boron and nitrogen doping in GQDs on their electrochemical energy storage properties. The GQDs as well as B-GQDs and N-GQDs were synthesized by a facile hydrothermal method and characterized by X-ray diffractometry, UV-Vis absorption spectrophotometry, transmission electron microscopy, and energy-dispersive X-ray mapping techniques. Electrochemical properties of the nanoparticles were investigated in a three-electrode setup using cyclic voltammetry and galvanostatic charge/discharge techniques in KOH electrolyte. The N-GQDs electrode showed the highest specific capacitance of 283 F/g at 1 A/g as compared to the GQDs and B-GQDs electrodes. The N-GQDs electrode exhibited the specific energy and specific power of 6.2 Wh/Kg and 108 W/Kg, respectively. The N-GQDs electrode showed also better cycling stability (80% capacity retention) than the GQDs and B-GQDs electrodes.

Samira Sharifi

Papers

Incremental substitution of Ni with Mn in NiFe 2 O 4 to largely enhance its supercapacitance properties

Highly improved supercapacitance properties of MnFe2O4 nanoparticles by MoS2 nanosheets.

Effect of Co2+ content on supercapacitance properties of hydrothermally synthesized Ni1-xCoxFe2O4 nanoparticles

Boron- and nitrogen-doped graphene quantum dots with enhanced supercapacitance

Effect of substituting the ionic radius in the structural and optoelectrical properties of spin-coated thin film synthesized by solvothermal method