Gaind P. Pandey

Ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate-based gel polymer electrolyte for electrochemical capacitors

A novel ionic liquid gel polymer electrolyte (ILGPE) based on low viscosity ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate (EMImTCB) entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) copolymer has been prepared for its application as an electrolyte in electrochemical double layer supercapacitors. The ILGPE shows excellent thermal stability up to 310 °C, a wider electrochemical window of ∼3.8 V (vs. Ag) and a high ionic conductivity of ∼9 × 10−3 S cm−1 at room temperature. The solid-state electrochemical double layer capacitors (EDLCs) have been fabricated with MWCNT electrodes and ILGPE films. The EDLC cells have also been fabricated with pure ionic liquid EMImTCB for comparison. The performance characteristics have been evaluated by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge studies. The ILGPE based EDLCs show comparable capacitance values (∼34.4 F g−1) and the values of the resistive parameters as compared to the pure ionic liquid based capacitor cells. The solid-state EDLC cell shows a specific energy of ∼3.5 W h kg−1 and a specific power of ∼4.2 kW kg−1 of MWCNT. The performance studies on EDLCs indicate the excellent potential of the ionic liquid EMImTCB based gel polymer films as electrolytes.

Chemical vapor-deposited carbon nanofibers on carbon fabric for supercapacitor electrode applications

Entangled carbon nanofibers (CNFs) were synthesized on a flexible carbon fabric (CF) via water-assisted chemical vapor deposition at 800°C at atmospheric pressure utilizing iron (Fe) nanoparticles as catalysts, ethylene (C2H4) as the precursor gas, and argon (Ar) and hydrogen (H2) as the carrier gases. Scanning electron microscopy, transmission electron microscopy, and electron dispersive spectroscopy were employed to characterize the morphology and structure of the CNFs. It has been found that the catalyst (Fe) thickness affected the morphology of the CNFs on the CF, resulting in different capacitive behaviors of the CNF/CF electrodes. Two different Fe thicknesses (5 and 10 nm) were studied. The capacitance behaviors of the CNF/CF electrodes were evaluated by cyclic voltammetry measurements. The highest specific capacitance, approximately 140 F g−1, has been obtained in the electrode grown with the 5-nm thickness of Fe. Samples with both Fe thicknesses showed good cycling performance over 2,000 cycles.

Mesoporous Hybrids of Reduced Graphene Oxide and Vanadium Pentoxide for Enhanced Performance in Lithium-Ion Batteries and Electrochemical Capacitors

Mesoporous hybrids of V2O5 nanoparticles anchored on reduced graphene oxide (rGO) have been synthesized by slow hydrolysis of vanadium oxytriisopropoxide using a two-step solvothermal method followed by vacuum annealing. The hybrid material possesses a hierarchical structure with 20-30 nm V2O5 nanoparticles uniformly grown on rGO nanosheets, leading to a high surface area with mesoscale porosity. Such hybrid materials present significantly improved electronic conductivity and fast electrolyte ion diffusion, which synergistically enhance the electrical energy storage performance. Symmetrical electrochemical capacitors with two rGO-V2O5 hybrid electrodes show excellent cycling stability, good rate capability, and a high specific capacitance up to ∼466 F g(-1) (regarding the total mass of V2O5) in a neutral aqueous electrolyte (1.0 M Na2SO4). When used as the cathode in lithium-ion batteries, the rGO-V2O5 hybrid demonstrates excellent cycling stability and power capability, able to deliver a specific capacity of 295, 220, and 132 mAh g(-1) (regarding the mass of V2O5) at a rate of C/9, 1C, and 10C, respectively. The value at C/9 rate matches the full theoretical capacity of V2O5 for reversible 2 Li(+) insertion/extraction between 4.0 and 2.0 V (vs Li/Li(+)). It retains ∼83% of the discharge capacity after 150 cycles at 1C rate, with only 0.12% decrease per cycle. The enhanced performance in electrical energy storage reveals the effectiveness of rGO as the structure template and more conductive electron pathway in the hybrid material to overcome the intrinsic limits of single-phase V2O5 materials.

Advanced Physical Chemistry of Carbon Nanotubes

The past decade has seen a surge of exciting research and applications of carbon nanotubes (CNTs) stimulated by deeper understanding of their fundamental properties and increasing production capability. The intrinsic properties of various CNTs were found to strongly depend on their internal microstructures. This review summarizes the fundamental structure-property relations of seamless tube-like single- and multiwalled CNTs and conically stacked carbon nanofibers, as well as the organized architectures of these CNTs (including randomly stacked thin films, parallel aligned thin films, and vertically aligned arrays). It highlights the recent development of CNTs as key components in selected applications, including nanoelectronics, filtration membranes, transparent conductive electrodes, fuel cells, electrical energy storage devices, and solar cells. Particular emphasis is placed on the link between the basic physical chemical properties of CNTs and the organized CNT architectures with their functions and performance in each application.

Effective Infiltration of Gel Polymer Electrolyte into Silicon-Coated Vertically Aligned Carbon Nanofibers as Anodes for Solid-State Lithium-Ion Batteries

This study demonstrates the full infiltration of gel polymer electrolyte into silicon-coated vertically aligned carbon nanofibers (Si-VACNFs), a high-capacity 3D nanostructured anode, and the electrochemical characterization of its properties as an effective electrolyte/separator for future all-solid-state lithium-ion batteries. Two fabrication methods have been employed to form a stable interface between the gel polymer electrolyte and the Si-VACNF anode. In the first method, the drop-casted gel polymer electrolyte is able to fully infiltrate into the open space between the vertically aligned core-shell nanofibers and encapsulate/stabilize each individual nanofiber in the polymer matrix. The 3D nanostructured Si-VACNF anode shows a very high capacity of 3450 mAh g(-1) at C/10.5 (or 0.36 A g(-1)) rate and 1732 mAh g(-1) at 1C (or 3.8 A g(-1)) rate. In the second method, a preformed gel electrolyte film is sandwiched between an Si-VACNF electrode and a Li foil to form a half-cell. Most of the vertical core-shell nanofibers of the Si-VACNF anode are able to penetrate into the gel polymer film while retaining their structural integrity. The slightly lower capacity of 2800 mAh g(-1) at C/11 rate and ∼1070 mAh g(-1) at C/1.5 (or 2.6 A g(-1)) rate have been obtained, with almost no capacity fade for up to 100 cycles. Electrochemical impedance spectroscopy does not show noticeable changes after 110 cycles, further revealing the stable interface between the gel polymer electrolyte and the Si-VACNFs anode. These results show that the infiltrated flexible gel polymer electrolyte can effectively accommodate the stress/strain of the Si shell due to the large volume expansion/contraction during the charge-discharge processes, which is particularly useful for developing future flexible solid-state lithium-ion batteries incorporating Si-anodes.

Effects of the catalyst and substrate thickness on the carbon nanotubes/nanofibers as supercapacitor electrodes

The different growth conditions of carbon nanotubes (CNTs)/carbon nanofibers (CNFs) which lead to different characteristics when used as supercapacitor electrodes are reported. A layer of SiO2 was coated onto the Si substrate and then a layer of Ti was evaporated as a current collector. CNTs/CNFs were synthesized on the Ti surface via a water-assisted chemical vapor deposition method at 800 °C and at atmospheric pressure utilizing iron (Fe) nanoparticles as catalysts, ethylene (C2H4) as the precursor gas and argon (Ar) and hydrogen (H2) as the carrier gases. The effects of different thicknesses of the catalyst (5 and 10 nm) and Ti substrate layer (10, 30 and 150 nm) on the specific capacitance of the CNFs were studied and the capacitance of the CNTs/CNFs-based device was dependent on CNT/CNF morphology of the CNFs that varied for different combinations of the catalyst and Ti layer thicknesses. The characterization of CNTs/CNFs was carried out using scanning electron microscopy, electron dispersive spectroscopy, transmission electron microscopy and electron diffraction. The specific capacitance was measured using cyclic voltammetry via a three-electrode system. The highest specific capacitance (60 F g−1) was obtained in the sample grown with 5 nm of Fe catalyst onto 10 nm of Ti substrate.

High-rate lithium-ion battery anodes based on silicon-coated vertically aligned carbon nanofibers

A multiscale hierarchical lithium-ion battery (LIB) anode composed of Si shells coaxially coated on vertically aligned carbon nanofibers has been explored. A high Li storage capacity of ~3,000-3,500 mAh (gSi)-1 and > 99% Coulombic efficiency have been obtained. Remarkable stability over 500 charge-discharge cycles have been demonstrated. Particularly, this electrode present a high-rate capability that the capacity remains within ~7% as the C-rate was increased from ~C/10 to ~8C. Electron microscopy, Raman spectroscopy and electrochemical impedance spectroscopy revealed that the electrode structure remains stable during long cycling. This high-rate property is likely associated with the unique nanocolumnar microstructure of Si in the shell. It reveals an exciting potential to develop high-performance LIBs.

Self-Organization of Ions at the Interface between Graphene and Ionic Liquid DEME-TFSI

Electrochemical effects manifest as nonlinear responses to an applied electric field in electrochemical devices, and are linked intimately to the molecular orientation of ions in the electric double layer (EDL). Herein, we probe the origin of the electrochemical effect using a double-gate graphene field effect transistor (GFET) of ionic liquid N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) top-gate, paired with a ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) back-gate of compatible gating efficiency. The orientation of the interfacial molecular ions can be extracted by measuring the GFET Dirac point shift, and their dynamic response to ultraviolet-visible light and a gate electric field was quantified. We have observed that the strong electrochemical effect is due to the TFSI anions self-organizing on a treated GFET surface. Moreover, a reversible order-disorder transition of TFSI anions self-organized on the GFET surface can be triggered by illuminating the interface with ultraviolet-visible light, revealing that it is a useful method to control the surface ion configuration and the overall performance of the device.