Deepak P. Dubal

Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor

Different nanostructures (Ns), such as nanobelts, nanobricks and nanosheets, of polypyrrole (PPy) were successfully fabricated on stainless steel substrates by simply varying the scan rate of deposition in the potentiodynamic mode. These PPy Ns were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and surface area measurement. The XRD analysis showed the formation of amorphous PPy thin films, and the FTIR studies confirmed characteristic chemical bonding in the PPy materials. SEM images depicted that a high scan rate of deposition can form multilayer nanosheets with high porosity leading to a system with excellent processability. The PPy nanosheets possess a higher Brunauer-Emmett-Teller (BET) surface area of 37.1 m 2 g -1 than PPy nanobelts and nanobricks. The supercapacitive performances of different PPy Ns were evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge techniques in 0.5 M H 2SO 4. A maximum specific capacitance of 586 F g -1 was obtained for multilayer nanosheets at a scan rate of 2 mV s -1. In addition, impedance measurements of the different Ns of PPy electrodes were performed suggesting that the PPy electrodes with multilayer nanosheets are promising materials for the next generation high performance electrochemical supercapacitors.

Temperature influence on morphological progress of Ni(OH)2 thin films and its subsequent effect on electrochemical supercapacitive properties

The temperature dependent morphological evolution and its effect on the electrochemical supercapacitive properties of Ni(OH)2 thin films have been systematically investigated. A temperature dependent growth mechanism model is proposed for the changes in microstructure. Different nanostructures of Ni(OH)2 thin films such as nanoplates, stacked nanoplates, nanobelts and nanoribbons have been fabricated by varying the deposition temperature. An X-ray diffraction study discloses the orientations of different nanostructures and the formation of nanocrystalline β-Ni(OH)2. Further, these Ni(OH)2 nanostructures demonstrate excellent surface properties like uniform surface morphology, good surface area, pore volume and uniform pore size distribution. The electrochemical supercapacitive properties of Ni(OH)2 nanostructures have been investigated by cyclic voltammetry, charge–discharge and electrochemical impedance spectroscopy techniques. The electrochemical studies of the Ni(OH)2 samples show an obvious influence of surface properties on the pseudocapacitance. The maximum specific capacitance of 357 F g−1 was evaluated for nanoplates at a scan rate of 5 mV s−1. Furthermore, all these Ni(OH)2 samples show good long-term cycling performances in KOH electrolyte. The Ragone plots ascertain good power and energy densities of all Ni(OH)2 nanostructured samples. Subsequently, electrochemical impedance measurements for the different nanostructures of Ni(OH)2 electrodes are assessed indicating that the Ni(OH)2 nanoplates structured electrodes are suitable for good capacity electrochemical supercapacitors.

Architectured morphologies of chemically prepared NiO/MWCNTs nanohybrid thin films for high performance supercapacitors.

The preparation of nanostructured metal oxide decorated on multiwalled carbon nanotubes (MWCNTs) nanohybrid films through simple, scalable, additive-free, binderless, and cost-effective route has fascinated significant attention not only in fundamental research areas but also its commercial applications, in order to reduce the growing environmental pollution and the cost of electrode fabrication. Here, we report the fabrication of highly flexible electrode with NiO/MWCNTs nanohybrid thin films directly on stainless steel substrate using successive ionic layer adsorption and reaction (SILAR) method. The impact of ratio of adsorption and reaction cycles on structural, surface areas and electrochemical properties of NiO/MWCNTs nanohybrids was investigated. X-ray diffraction measurements confirm the hybridization and face centered cubic (FCC) crystal structure of NiO in NiO/MWCNTs nanohybrids. In addition, these nanohybrids exhibit excellent surface properties such as uniform surface morphology, good surface area, pore volume, and uniform pore size distribution. The electrochemical tests demonstrate the highest specific capacitance of 1727 F g(-1) at 5 mA cm(-2) of current density with 91% capacitance retention after 2000 cycles. In addition, the Ragone plot confirms the better power and energy densities for all NiO/MWCNTs nanohybrids. The attractive electrochemical capacitive activity revealed by NiO/MWCNTs nanohybrid electrode proposes that it is an auspicious respondent for future energy storage application.

Low-cost flexible supercapacitors with high-energy density based on nanostructured MnO2 and Fe2O3 thin films directly fabricated onto stainless steel.

The facile and economical electrochemical and successive ionic layer adsorption and reaction (SILAR) methods have been employed in order to prepare manganese oxide (MnO2) and iron oxide (Fe2O3) thin films, respectively with the fine optimized nanostructures on highly flexible stainless steel sheet. The symmetric and asymmetric flexible-solid-state supercapacitors (FSS-SCs) of nanostructured (nanosheets for MnO2 and nanoparticles for Fe2O3) electrodes with Na2SO4/Carboxymethyl cellulose (CMC) gel as a separator and electrolyte were assembled. MnO2 as positive and negative electrodes were used to fabricate symmetric SC, while the asymmetric SC was assembled by employing MnO2 as positive and Fe2O3 as negative electrode. Furthermore, the electrochemical features of symmetric and asymmetric SCs are systematically investigated. The results verify that the fabricated symmetric and asymmetric FSS-SCs present excellent reversibility (within the voltage window of 0–1 V and 0–2 V, respectively) and good cycling stability (83 and 91%, respectively for 3000 of CV cycles). Additionally, the asymmetric SC shows maximum specific capacitance of 92 Fg−1, about 2-fold of higher energy density (41.8 Wh kg−1) than symmetric SC and excellent mechanical flexibility. Furthermore, the “real-life” demonstration of fabricated SCs to the panel of SUK confirms that asymmetric SC has 2-fold higher energy density compare to symmetric SC.

3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li- doped ionic liquid

Building of hierarchical core-shell hetero-structures is currently the subject of intensive research in the electrochemical field owing to its potential for making improved electrodes for high-performance micro-supercapacitors. Here we report a novel architecture design of hierarchical MnO2@silicon nanowires (MnO2@SiNWs) hetero-structures directly supported onto silicon wafer coupled with Li-ion doped 1-Methyl-1-propylpyrrolidinium bis(trifluromethylsulfonyl)imide (PMPyrrBTA) ionic liquids as electrolyte for micro-supercapacitors. A unique 3D mesoporous MnO2@SiNWs in Li-ion doped IL electrolyte can be cycled reversibly across a voltage of 2.2 V and exhibits a high areal capacitance of 13 mFcm−2. The high conductivity of the SiNWs arrays combined with the large surface area of ultrathin MnO2 nanoflakes are responsible for the remarkable performance of these MnO2@SiNWs hetero-structures which exhibit high energy density and excellent cycling stability. This combination of hybrid electrode and hybrid electrolyte opens up a novel avenue to design electrode materials for high-performance micro-supercapacitors.

Hierarchical 3D-flower-like CuO nanostructure on copper foil for supercapacitors

We report a trouble-free chemical synthesis of copper oxide (CuO) nanoflowers on flexible copper foil (Cu) and their use as electrodes for supercapacitors. Various characterization techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and Brunauer–Emmett–Teller (BET) analysis, have been used to characterize CuO nanostructure. Supercapacitive properties show that CuO electrodes exhibit a high specific capacitance of about 498 F g−1 at 5 mV s−1, with a high energy density of 26 W h kg−1 in KOH electrolyte. Moreover, impedance analysis showed lower ESR value, high power performance, and an excellent rate as well as frequency response for the CuO electrodes. The excellent electrochemical properties of the CuO electrodes indicate that they have many potential applications in high-performance supercapacitors.

Conversion of Chemically Prepared Interlocked Cubelike Mn3O4 to Birnessite MnO2 Using Electrochemical Cycling

Interlocked cubelike Mn3 O4 thin films have been prepared by a simple and low temperature chemical bath deposition method. These interlocked cubelike Mn3 O4 thin films are further converted into nanoflakes of birnessite MnO2 using voltammetric cycling in aqueous Na2 SO4 electrolyte. The process is dynamic potential activated, which causes the formation of sheet-shaped nanoflakes. The films are characterized by X-ray diffraction, field-emission-scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform IR spectrum, and wettability test. Impedance spectroscopy studies revealed that charge-transfer resistance of the birnessite MnO 2 structure has a lower value than that of the Mn3 O 4 structure. The effect of different numbers of potential cycles on structure, surface morphology, valence states, and contact angles has been investigated. During the cycling process, the supercapacitance of manganese oxide increased by more than 10 times. The maximum supercapacitance achieved at 5 mV s-1 is 223 F g-1. The effect of scan rate on the specific capacitance of birnessite MnO2 electrode has been studied.

Development of hybrid materials based on sponge supported reduced graphene oxide and transition metal hydroxides for hybrid energy storage devices

Earnest efforts have been taken to design hybrid energy storage devices using hybrid electrodes based on capacitive (rGO) and pseudocapacitive (Ni(OH)2 and Co(OH)2) materials deposited on the skeleton of 3D macroporous (indicate sponge material) sponge support. Conducting framework was formed by coating rGO on macroporous sponge on which subsequent deposition of Ni(OH)2 and Co(OH)2 was carried out. The synergetic combination of rGO and Ni(OH)2 or Co(OH)2) provides dual charge-storing mechanisms whereas 3D framework of sponge allows excellent accessibility of electrolyte to hybrid electrodes. Moreover, to further increase the energy density, hybrid devices have been fabricated with SP@rGO@Ni or SP@rGO@Co and SP@rGO as positive and negative electrodes, respectively. These hybrid devices operate with extended operating voltage windows and achieve remarkable electrochemical supercapacitive properties which make them truly promising energy storage devices for commercial production.

A high voltage solid state symmetric supercapacitor based on graphene–polyoxometalate hybrid electrodes with a hydroquinone doped hybrid gel-electrolyte

In pursuit of high capacitance and high energy density storage devices, hybrid materials have quickly garnered well-deserved attention based on their power to merge complementary components and properties. Here, we report the fabrication of all-solid state symmetric supercapacitors (ASSSC) based on a double hybrid approach combining a hybrid electrode (reduced graphene oxide-phoshomolybdate, rGO-PMo12) and a hybrid electrolyte (hydroquinone doped gel-electrolyte). To begin with, a high-performance hybrid electrode based on H3PMo12O40 nanodots anchored onto rGO was prepared (rGO-PMo12). Later, an all-solid state symmetric cell based on these rGO-PMo12 electrodes, and making use of a polymer gel-electrolyte was assembled. This symmetric cell showed a significant improvement in cell performance. Indeed, it allowed for an extended potential window by 0.3 V that led to an energy density of 1.07 mW h cm-3. Finally, we combined these hybrid electrodes with a hybrid electrolyte incorporating an electroactive species. This is the first proof-of-design where a redox-active solid-state gel-electrolyte is applied to rGO-PMo12 hybrid supercapacitors to accomplish a significant enhancement in the capacitance. Strikingly, a further excellent increase in the device performance (energy density of 1.7 mW h cm-3) was realized with the hybrid electrode-hybrid electrolyte combination cell as compared to that of the conventional electrolyte cell. Thus, this unique symmetric device outclasses the high-voltage asymmetric counterparts under the same power and represents a noteworthy advance towards high energy density supercapacitors.

Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors

This investigation demonstrates the surfactant assisted fabrication of nanosheet clusters of caddice clew, yarn ball and cabbage slash-like microstructures of copper oxide (CuO) in thin film form directly grown onto a stainless steel substrate using a binder free hydrothermal approach. The impact of organic surfactants such as Triton X-100 (TRX) and polyvinyl alcohol (PVA) on the structural, morphological, surface area and electrochemical properties of CuO is investigated. The X-ray diffraction study reveals the structure-directing ability of the organic surfactants and confirms the nanocrystalline nature of the CuO thin films. Additionally, these CuO microstructures show excellent surface properties like uniform surface morphology, good surface area and a uniform pore size distribution. The electrochemical tests manifest a high specific capacitance of 535 F g−1 at a scan rate of 5 mV s−1 with 90% capacitive retention after 1000 cycles and low dissolution and charge transfer resistance of the yarn ball-like structured CuO thin film. This approach renders a plain picture of the process–structure–property relationship in thin film synthesis and provides significant schemes to boost the performance of supercapacitor electrodes.