Jim P. Zheng

Hydrous Ruthenium Oxide as an Electrode Material for Electrochemical Capacitors

The hydrous ruthenium oxide has been formed by a sol-gel process. The precursor was obtained by mixing aqueous solutions of RuCl{sub 3}{center_dot}xH{sub 2}O and alkalis. The hydrous ruthenium oxide powder was obtained by annealing the precursor at low temperatures. The crystalline structure and the electrochemical properties of the powder have been studied as a function of the annealing temperature. At lower annealing temperatures the powder is in an amorphous phase with a high specific capacitance. Specific capacitance as high as 720 F/g was measured for the powder formed at 150 C. when the annealing temperature exceeded 175 C, the crystalline phase was formed, and the specific capacitance dropped rapidly. The surface area of the powder and the resistivity of the pellet made from these powders have also been studied. The specific surface area and the resistivity decreased as the annealing temperature increased. A capacitor was made with electrodes comprised of hydrous ruthenium oxide and H{sub 2}SO{sub 4} electrolyte. The energy density of 96 J/g (or 26.7 Wh/kg), based on electrode material only, was measured for the cell using hydrous ruthenium oxide electrodes. It was also found that hydrous ruthenium oxide is stable in H{sub 2}SO{sub 4} electrolyte.

A New Charge Storage Mechanism for Electrochemical Capacitors

The hydrous form of ruthenium oxide (RuO[sub 2][center dot]xH[sub 2]O) has been demonstrated to be an excellent electrode material for electrochemical capacitors. This material, as prepared by a sol-gel process at low temperatures, is amorphous and electrically conductive. The specific capacitance is over 720 F/g. This value is at least two times higher than the highest value ever reported for such materials. The charge storage mechanism is believed to involve bulk electrochemical protonation of the oxide. This discovery opens a new avenue of research in the field of high energy density electrochemical capacitors.

High energy and high power density electrochemical capacitors

Abstract High energy density electrochemical capacitors were built with a newly discovered electrode material (amorphous RuO 2 · x H 2 O). Energy densities as high as 96 J/g (26 Wh/kg) were obtained based on the RuO 2 · x H 2 O electrode material alone. However, the power density of the capacitor is low. By mixing RuO 2 · x H 2 O powders with about 20% weight of carbon black, power densities greater than 10 kW/kg could be achieved at a delivered energy density of about 72 J/g (20 Wh/kg). Capacitance as a function of cycle life was studied for up to 60 000 cycles. The temperature dependence of capacitance and resistance of the capacitor are also reported in this paper.

Lithium-Air Batteries Using SWNT/CNF Buckypapers as Air Electrodes

Li-air cells based on Li foil as an anode electrode, freestanding carbon nanotube/nanofiber mixed buckypaper as an air (cathode) electrode, and organic electrolyte were assembled. The air electrode was made with single-wall carbon nanotube (SWNT) and carbon nanofiber (CNF) without any binder. The discharge capacity was strongly dependent on both the discharge current density and the thickness of the air electrode. A discharge capacity as high as 2500 mAh/g was obtained for an air electrode at a thickness of 20 μm with a discharge current density of 0.1 mA/cm 2 ; however, it was reduced to 400 mAh/g when the thickness of the air electrode was increased to 220 μm. For a 66 μm thick air electrode, the discharge capacity decreased from 1600 to 340 mAh/g when the discharge current density increased from 0.1 to 0.5 mA/cm 2 . The scanning electron microscope images on surfaces of the air electrode from a fully discharged cell showed that the voids at the air side were almost fully filled by the solid deposition; however, the voids at the membrane side were still wide open.

Theoretical Energy Density of Li–Air Batteries

A model for predication of the gravimetric and volumetric energy densities of Li-air batteries using aqueous electrolytes is developed. The theoretical gravimetric/volumetric capacities and energy densities are calculated based on the minimum weight of the electrolyte and volume of air electrode needed for completion of the electrochemical reaction with Li metal as an anode electrode. It was determined that both theoretical gravimetric/volumetric capacities and energy densities are dependent on the porosity of the air electrode. For instance, at a porosity of 70%, the maximum theoretical cell capacities are 435 mAh/g and 509 mAh/cm 3 in basic electrolyte, and 378 mAh/g and 452 mAh/cm 3 in acidic electrolyte. The maximum theoretical cell energy densities are 1300 Wh/kg and 1520 Wh/L in basic electrolyte, and 1400 Wh/kg and 1680 Wh/L in acidic electrolyte. The significant deduction of cell capacity from specific capacity of Li metal is due to the bulky weight requirement from the electrolyte and air electrode materials. In contrast, the Li-air battery using a nonaqueous electrolyte does not consume electrolyte during the discharge process and has high cell energy density. For Li-air batteries using both aqueous and nonaqueous electrolytes, the weight increases by 8-13% and the volume decreases by 8-20% after the cell is fully discharged.

Highly sensitive photodetector using porous silicon

A highly sensitive photodetector was made with a metal‐porous silicon junction. The spectral response was measured for the wavelength range from 400 nm to 1.075 μm. It was demonstrated that close to unity quantum efficiency could be obtained in the wavelength range of 630–900 nm without any antireflective coating. The detector response time was about 2 ns with a 9 V reverse bias. The possible mechanisms are discussed.

Generation of high‐energy atomic beams in laser‐superconducting target interactions

High‐energy atomic beams with Mach numbers as high as 5 were observed in excimer laser‐superconducting target interactions. The velocity distributions of the Y, Ba, Cu, and O atoms and ions could be described very well by a supersonic expansion‐type mechanism similar to a molecule beam. The physics of the atomic beam formation process is discussed.

The Limitations of Energy Density of Battery/Double-Layer Capacitor Asymmetric Cells

The formula describing the energy density of asymmetric cells, which consists of a battery-type electrode (such as lithium intercalated compound) and an electrochemical capacitor-type electrode (such as activated carbon), was derived. From the formula, the optimal mass (or volume) ratio of battery electrode to capacitor electrodes and electrolyte can be obtained for achieving the maximum theoretical gravimetric (or volumetric) energy density. The voltage swing of the cell during charge and discharge cycles was also described. Relationships between the energy density, ion concentration of the electrolyte, specific capacity of battery electrode, specific capacitance of capacitor electrode, and maximum operational voltage were also given. Three specific asymmetric systems, including carbon/LiPF 6 ethylene carbonate:dimethyl carbonate (EC:DMC)/Li x Ti 5 O 12 , carhon/LiPF 6 EC:DMC/WO 2 , and Ni(OH) 2 /KOH H 2 O/carbon were evaluated for their maximum theoretical energy density and swing voltage. It was found that for asymmetric cells using nonaqueous electrolyte, the maximum energy density (about 30 Wh/kg) was limited mainly by the electrolyte due to the low ion concentration; however, for asymmetric cells using aqueous electrolytes, the maximum energy density (about 40 Wh/kg) was limited mainly by the capacitor electrode. The maximum operational voltage always plays an important role in the maximum energy density.

Low resistivity indium tin oxide films by pulsed laser deposition

Indium tin oxide films were grown by pulsed laser deposition on glass substrates. The electrical and optical properties of these films were studied. At optimized oxygen pressures, films with resistivity values of 1.4×10−4 and 5.6×10−4 Ω cm were deposited at substrate temperatures of 310 and 20 °C, respectively. Films with a thickness of 180 nm had a transmission of nearly 100% for the wavelength range of 600–800 nm.

Role of the oxygen atomic beam in low-temperature growth of superconducting films by laser deposition

An oxygen jet placed near the target during plasma‐assisted laser deposition produces a strong atomic oxygen beam with kinetic energies of 5.6 eV, simultaneous with the laser‐induced atomic beams of Ba, Cu, and Y from the target. All atomic beams can be well characterized by a supersonic expansion mechanism. The behavior of the velocity distributions was studied as a function of the distance from the target and laser energy fluence. A target‐substrate separation of 7 cm was found to be optimum in terms of producing the best as‐deposited films. At that distance, the velocity distributions of all atomic beams become nearly the same.