Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes.

Charge storage in electrochemical double-layer capacitors (EDLCs) is via the adsorption of electrolyte counterions in their positive and negative electrodes under an applied potential. This study investigates the EDLC-type charge storage in carbon nanotubes (CNT) electrodes in aqueous acidic (NaHSO4), basic (NaOH), and neutral (Na2SO4) electrolytes of similar cations but different anions as well as similar anions but different cations (Na2SO4 and Li2SO4) in a two-electrode Swagelok-type cell configuration. The physicochemical properties of ions, such as mobility/diffusion and solvation, are correlated with the charge storage parameters. The neutral electrolytes offer superior charge storage over the acidic and basic counterparts. Among the studied ions, SO4 2- and Li+ showed the most significant capacitance owing to their larger solvated ion size. The charge stored by the anions and cations follows the order SO4 2- > HSO4 - > OH- and Li+ > Na+, respectively. Consequently, the CNT//Li2SO4//CNT cell displayed outstanding charge storage indicators (operating voltage ∼0-2 V, specific capacitance ∼122 F·g-1, specific energy ∼67 W h·kg-1, and specific power ∼541 W·kg-1 at 0.5 A·g-1) than the other cells, which could light a red light-emitting diode (2.1 V) for several minutes. Besides, the CNT//Li2SO4//CNT device showed exceptional rate performance with a capacitance retention of ∼95% at various current densities (0.5-2.5 A·g-1) after 6500 cycles. The insights from this work could be used to design safer electrochemical capacitors of high energy density and power density.

Graphene Oxide as a Dielectric and Charge Trap Element in Pentacene-Based Organic Thin-Film Transistors for Nonvolatile Memory.

In this report, the dielectric nature of graphene oxide (GO) was exploited for the successful implementation of low-power pentacene thin-film transistors suitable for nonvolatile memory applications. Two different types of devices were fabricated on indium tin oxide-coated glass substrates with two different metals, viz., gold and aluminum, as the source and drain contacts. The performance of the devices was analyzed from their field-effect characteristics. Both the devices showed dominant p-type charge transport behavior. The breakdown electric field was determined to be 1.02 × 108 V/m. The current transport mechanism was explained from the output characteristics using the Fowler-Nordheim tunneling theory. Capacitance-voltage (C-V) measurements have been employed to determine the value of the oxide capacitance and to examine the memory effect. The hysteresis behavior observed from the C-V characteristics show the suitability of the device for memory applications with a low operating voltage of 3 V. The charge trapping behavior of GO was explained by the energy band diagram. Frequency-dependent C-V measurements in the range 100 kHz to 1 MHz were also performed to account for the memory window obtained in the devices. The charge retention and endurance characteristics were evaluated under a constant voltage stress to check the reliability of device operation.