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Eftekhari A.,National Institute of Arts and science | Jafarkhani P.,National University of Singapore
Journal of Physical Chemistry C | Year: 2013

While almost all attempts for bulk synthesis of graphene result in agglomeration of flat graphene layers, a simple solvothermal route is reported for the preparation of a graphene powder consisting of very flexible sheets. Curvature of these graphene sheets avoids vital agglomeration, which normally blocks the accessible surface of graphene sheets. The thickness of the graphene sample varies from one to a few layers, but the spaces among the graphene layers are sufficiently large enough to provide internal accessibility. Thus, this graphene sample shows the general (bulk) properties of graphene single sheets. For instance, it has a high specific surface area of ca. 1168 m2 g-1 and starts to burn at 350 C because of a large interlayer spacing of graphene sheets (i.e., 5.1 Å). It should be considered that this specious interlayer is mostly due to the graphene curvature rather than to filling with functional groups (as in the case of graphite oxide). Owing to the high accessibility and active sites over a large surface area, this graphene sample shows a superior electrochemical behavior for hydrogen storage. © 2013 American Chemical Society. Source


Eftekhari A.,National Institute of Arts and science | Jafarkhani P.,National University of Singapore
Journal of Electroanalytical Chemistry | Year: 2014

Galvanodynamic synthesis is introduced as a new flexible electrochemical method for the preparation of electroactive materials, as utilized for electropolymerization of aniline in the present seminal work. It is indeed a missing method in the four possible electrochemical methods namely potentiodynamic, potentiostatic, galvanostatic, and finally galvanodynamic. This explicit comparative study shows the flexibility of this method in controlling the physical and electrochemical properties of the polyaniline film. Electrochemical polymerization was performed by scanning the current in a given range. Different approaches for this purpose were also introduced as the synthesis can be performed under different conditions through linear scanning, cyclic scanning, and repetitive cyclic scanning. Effects of various controllable parameters, such as scan rate, the current range, and the scan direction were also examined. In comparison with other electrochemical methods, galvanodynamic synthesis has higher flexibility to alter controllable parameters in favor of a specific application. For instance, synthesis of polyaniline to display characteristic redox peaks suitable for battery performance or strong capacitive behavior ideal for supercapacitors. © 2014 Elsevier B.V. All rights reserved. Source


Eftekhari A.,National Institute of Arts and science | Molaei F.,National Institute of Arts and science
Journal of Power Sources | Year: 2015

In electrochemical synthesis of manganese oxide, current density has a substantial influence on redox behavior, but less effect on the film morphology. Here we report that a small amount (even not detectable by electron microscopy) of carbon nanotubes can significantly affect the film growth pathway. Although, the galvanostatic syntheses of manganese oxide were similar in the absence and presence of carbon nanotubes, the morphological structures are totally different. This difference is also valid for electrochemical behavior in favor of the formation of a single redox couple. The influence of carbon nanotubes on potentiodynamic electrodeposition of manganese oxide was also similar, leading to the appearance of a strong redox couple without any noticeable capacitive behavior. It should be emphasized that this influence is only valid for the experimental condition under consideration (i.e., low current densities), and electrodeposition at higher current densities in the presence of carbon nanotubes may strengthen the capacitive behavior in favor of supercapacitors (as discussed in the second part). The interesting point is that this tiny additive can predominantly control the electrochemical properties of the system under consideration: supercapacitor or battery. © 2013 Elsevier B.V. All rights reserved. Source


Eftekhari A.,National Institute of Arts and science | Molaei F.,National Institute of Arts and science
Journal of Power Sources | Year: 2015

A practical approach for controlling the morphology and electrochemical properties of electroactive materials is proposed. In this study, manganese oxide films were galvanostatically deposited in the presence of a small amount of carbon nanotube (CNT). The resulting film cannot be considered as a CNT-based nanocomposite, as no CNT is detected by electron microscopy. However, the manganese oxide electrodeposited delivers an excellent pseudo-capacitive behavior to be used as a superior supercapacitor. The samples prepared by applying a current density of 3.0 mA cm-2 showed a specific capacitance of 280 F g-1. As it seems that the capacitance of this electrode is related to the chemisorption of the alkali cation, an extremely high specific capacitance of 434 F g-1 was achieved in a saturated medium of Li electrolyte. This high specific capacitance can be attributed to a bulk process. The presence of carbon nanotubes results in the formation of nanostructured films which provide a better accessibility for capacitive behavior. Although the exact mechanism for this phenomenon is still vague, the presence of carbon nanotubes (probably as a solid charge carrier) close to the electrode surface is apparently responsible for a different pathway for the electrodeposition process. © 2013 Elsevier B.V. All rights reserved. Source

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