Comprehensive Insight into the Mechanism, Material Selection and Performance Evaluation of Supercapatteries.

Electrochemical energy storage devices (EESs) play a crucial role for the construction of sustainable energy storage system from the point of generation to the end user due to the intermittent nature of renewable sources. Additionally, to meet the demand for next-generation electronic applications, optimizing the energy and power densities of EESs with long cycle life is the crucial factor. Great efforts have been devoted towards the search for new materials, to augment the overall performance of the EESs. Although there are a lot of ongoing researches in this field, the performance does not meet up to the level of commercialization. A further understanding of the charge storage mechanism and development of new electrode materials are highly required. The present review explains the overview of recent progress in supercapattery devices with reference to their various aspects. The different charge storage mechanisms and the multiple factors involved in the performance of the supercapattery are described in detail. Moreover, recent advancements in this supercapattery research and its electrochemical performances are reviewed. Finally, the challenges and possible future developments in this field are summarized.

Freeze-dried MoS2 sponge electrodes for enhanced electrochemical energy storage.

In the present study, we have synthesized high surface area MoS2 sponge electrodes via a facile hydrothermal method followed by a freeze drying process. The performance of the MoS2 based symmetric capacitor showed a high specific capacitance value of around 128 F g-1 at a scan rate of 2 mV s-1, and also a single electrode showed a specific capacitance of 510 F g-1, which is a remarkable value to be reported for a MoS2 based material in a symmetric device configuration. Also, a high energy density of around 6.15 Wh kg-1 and a good cyclic stability over 4000 cycles are obtained for the symmetrical cell.

Amorphous MoSx thin-film-coated carbon fiber paper as a 3D electrode for long cycle life symmetric supercapacitors.

Amorphous MoSx thin-film-coated carbon fiber paper as a binder-free 3D electrode was synthesized by a facile hydrothermal method. The maximum specific capacitance of a single electrode was 83.9 mF cm(-2), while it was 41.9 mF cm(-2) for the symmetric device. Up to 600% capacitance retention was observed for 4750 cycles.

Molybdenum diselenide/reduced graphene oxide based hybrid nanosheets for supercapacitor applications.

In the present study, molybdenum diselenide/reduced graphene oxide (MoSe2/rGO) nanosheets were synthesized via a facile hydrothermal process and the electrochemical performance of the nanosheets was evaluated for supercapacitor applications. The MoSe2 nanosheets were uniformly distributed on the surface of the rGO matrix. The MoSe2/rGO nanosheet electrode exhibited an enhanced specific capacitance (211 F g(-1)) with excellent cycling stability, compared with pristine MoSe2. The enhanced electrochemical performance of the MoSe2/rGO nanosheet electrode is mainly attributed to the improved electron and ion transfer mechanism involving the synergistic effects of pseudocapacitance (from the MoSe2 nanosheets) and the electric double layer charge (EDLC, from the rGO nanosheets) storage behavior. These results demonstrate that the enhanced electrochemical performance of MoSe2/rGO nanosheets could be obtained via a facile and scalable approach.

Few-layered MoSe2 nanosheets as an advanced electrode material for supercapacitors.

We report the synthesis of few-layered MoSe2 nanosheets using a facile hydrothermal method and their electrochemical charge storage behavior. A systematic study of the structure and morphology of the as-synthesized MoSe2 nanosheets was performed. The downward peak shift in the Raman spectrum and the high-resolution transmission electron microscopy images confirmed the formation of few-layered nanosheets. The electrochemical energy-storage behavior of MoSe2 nanosheets was also investigated for supercapacitor applications in a symmetric cell configuration. The MoSe2 nanosheet electrode exhibited a maximum specific capacitance of 198.9 F g(-1) and the symmetric device showed 49.7 F g(-1) at a scan rate of 2 mV s(-1). A capacitance retention of approximately 75% was observed even after 10 000 cycles at a high charge-discharge current density of 5 A g(-1). The two-dimensional MoSe2 nanosheets exhibited a high specific capacitance and good cyclic stability, which makes it a promising electrode material for supercapacitor applications.

Importance of hydrophilic pretreatment in the hydrothermal growth of amorphous molybdenum sulfide for hydrogen evolution catalysis.

Amorphous molybdenum sulfide (MoSx) has been identified as an excellent catalyst for the hydrogen evolution reaction (HER). It is still a challenge to prepare amorphous MoSx as a more active and stable catalyst for the HER. Here the amorphous MoSx catalysts are prepared on carbon fiber paper (CFP) substrates at 200 °C by a simple hydrothermal method using molybdic acid and thioacetamide. Because the CFP is intrinsically hydrophobic due to its graphene-like carbon structure, two kinds of hydrophilic pretreatment methods [plasma pretreatment (PP) and electrochemical pretreatment (EP)] are investigated to convert the hydrophobic surface of the CFP to be hydrophilic prior to the hydrothermal growth of MoSx. In the HER catalysis, the MoSx catalysts grown on the pretreated CFPs reach a cathodic current density of 10 mA/cm(2) at a much lower overpotential of 231 mV on the MoSx/EP-CFP and 205 mV on the MoSx/PP-CFP, compared to a high overpotential of 290 mV on the MoSx of the nonpretreated CFP. Turnover frequency per site is also significantly improved when the MoSx are grown on the pretreated CFPs. However, the Tafel slopes of all amorphous MoSx catalysts are in the range of 46-50 mV/dec, suggesting the Volmer-Heyrovsky mechanism as a major pathway for the HER. In addition, regardless of the presence or absence of the pretreatment, the hydrothermally grown MoSx catalyst on CFP exhibits such excellent stability that the degradation of the cathodic current density is negligible after 1000 cycles in a stability test, possibly due to the relatively high growth temperature.

One-step hydrothermal synthesis of graphene decorated V2O5 nanobelts for enhanced electrochemical energy storage.

Graphene-decorated V2O5 nanobelts (GVNBs) were synthesized via a low-temperature hydrothermal method in a single step. V2O5 nanobelts (VNBs) were formed in the presence of graphene oxide, a mild oxidant, which also enhanced the conductivity of GVNBs. From the electron energy loss spectroscopy analysis, the reduced graphene oxide (rGO) are inserted into the layered crystal structure of V2O5 nanobelts, which further confirmed the enhanced conductivity of the nanobelts. The electrochemical energy-storage capacity of GVNBs was investigated for supercapacitor applications. The specific capacitance of GVNBs was evaluated using cyclic voltammetry (CV) and charge/discharge (CD) studies. The GVNBs having V2O5-rich composite, namely, V3G1 (VO/GO = 3:1), showed superior specific capacitance in comparison to the other composites (V1G1 and V1G3) and the pure materials. Moreover, the V3G1 composite showed excellent cyclic stability and the capacitance retention of about 82% was observed even after 5000 cycles.

Graphene oxide assisted spontaneous growth of V2O5 nanowires at room temperature.

Graphene-decorated single crystalline V2O5 nanowires (G-VONs) have been synthesized by mixing graphene oxide (GO) and V2O5 suspensions at room temperature. In this process, V2O5 nanowires (VONs) are formed spontaneously from commercial V2O5 particles with the aid of GO. The as-formed one dimensional G-VONs were characterized by using a X-ray diffractometer, a X-ray photoelectron spectrometer, a scanning electron microscope, and a transmission electron microscope. GO plays a vital role in the VON formation with the simultaneous reduction of GO. A single G-VON showed superior electrical conductivity compared with that of the pure VONs obtained from the sol-gel method. This could be ascribed to the insertion of rGO sheets into the V2O5 layered structure, which was further confirmed by electron energy loss spectroscopy.

Metal substrate based electrodes for flexible dye-sensitized solar cells: fabrication methods, progress and challenges.

A step towards commercialization of dye-sensitized solar cells (DSSCs) requires more attention to engineering aspects, such as flexibility, the roll to roll fabrication process, the use of cost effective materials, etc. In this aspect, advantages of flexible DSSCs attracted many researchers to contemplate the transparent conducting oxide coated flexible plastic substrates and the thin metallic foils. In this feature article, the pros and cons of these two kinds of substrates are compared. The flexible dye-sensitized solar cells fabricated using metal substrates are briefly discussed. The working electrodes of DSSCs fabricated on various metal substrates, their fabrication methods, the effect of high temperature calcination and drawbacks of back illumination are reviewed in detail. A few reports on the flexible metal substrate based counter electrodes that could be combined with the plastic substrate based working electrodes are also covered at the end.

Efficiency enhancement of dye-sensitized solar cells by the addition of an oxidizing agent to the TiO(2) paste.

The addition of various amounts of a strong oxidizing agent (3,5-dinitrosalicyclic acid, DNSA) to TiO2 paste enhances the solar-to-electrical-energy conversion efficiency of the corresponding dye-sensitized solar cells (DSSCs). Maximum performance was obtained from a device that was fabricated by using a TiO2 paste with 2 wt % DNSA, which showed a short-circuit current density of 17.88 mA cm(-2) , an open-circuit voltage of 0.78 V, and an overall conversion efficiency of 9.62 %, which was an improvement in comparison to reference cells without DNSA. This improvement was rationalized in terms of the amount of residual carbon (formed due to the oxidation of binders) remaining on the TiO2 surface. Addition of a larger amount of oxidizing agent led to a smaller amount of residual carbon on the TiO2 surface. This smaller amount of residual carbon enhanced the adsorption of a larger number of dye molecules on the TiO2 surface. The addition of an oxidizing agent facilitated the removal of more residual organic species during the high-temperature calcination process while causing no change in the surface morphology and microstructure of the TiO2 film.

Improvement of dye-sensitized solar cells toward the broader light harvesting of the solar spectrum.

Dye-sensitized solar cells (DSSCs) have been extensively evolved for the past two decades in order to improve their cell performance. From the commercialization point of view, the overall solar to electrical energy conversion efficiency should compete with other solar cells. But, due to structural restrictions of DSSC using the liquid electrolyte and a space requirement between two electrodes, the direct tandem construction of DSSCs by stacking of repeating units is highly limited. In this feature article, important research trials to overcome these barriers and a recent research trend to improve the light harvesting strategies mainly panchromatic engineering, various tandem approaches such as parallel tandem, series tandem, p-n tandem etc., have been briefly reviewed.