Marc A. Anderson

Novel Electrode Materials for Thin‐Film Ultracapacitors: Comparison of Electrochemical Properties of Sol‐Gel‐Derived and Electrodeposited Manganese Dioxide

Thin films of manganese dioxide were formed on nickel foils by electrodeposition and by both dip‐coating and drop‐coating with manganese dioxide suspensions (sols) and their subsequent gelation and calcination. The performance of these films as ultracapacitors was studied by cyclic voltammetry in the range 0.0–0.9 V (SCE) and by chronopotentiometry in unbuffered solution. The cyclic voltammograms of ultrathin, dip‐coated sol‐gel‐derived films indicated better capacitive behavior and gave differential specific capacitance values as high as 698 F/g compared to values half to two‐thirds as great for the electrodeposited films. Multilayer drop‐coated sol‐gel films were prepared to attain film thicknesses comparable to the electrodeposited films, and these were found to provide charge‐storage capacity as high as , more than three times greater than that of the electrodeposited films. All films, except electrodeposited films that were not thermally cured, exhibited good cycling stability, losing not much more than 10% of capacity after 1500 cycles.

Porous Nickel Oxide/Nickel Films for Electrochemical Capacitors

Nano-sized NiO/Ni composite films have been found to perform as superior electrodes in electrochemical capacitor applications. These films can provide a specific capacitance of 50 to 64 F/g. The specific energy and specific power of these films were 25 to 40 kJ/kg and 4 to 17 kW/kg, respectively. These specific quantities are dependent on the microstructure of the films. Superior performance can be obtained from samples having rough surfaces and consisting of larger secondary particles (ca. 100 to 120 nm in diam).

Titania and alumina ceramic membranes

This paper discusses the physical-chemical properties of γ-Al2O3 and TiO2 ceramic membranes which have been prepared by sol-gel techniques from alkoxide starting materials. Mean pore size and pore size distributions are examined using N2 sorption analysis. Particle size of sols as measured by in situ quasi-elastic light scattering is compared with SEM analysis of sintered membranes. Crack-free unsupported monoliths with thicknesses up to 120 μm have been prepared of γ-Al2O3. Supported TiO2 membranes have been fabricated with thicknesses up to 0.5 μm. The γAl2O3 membranes are obtained from “particulate” sols, while for TiO2 both “particulate” and “polymeric” sols have been used to produce membranes.

Photoelectrocatalytic degradation of formic acid using a porous titanium dioxide thin-film electrode.

The degradation of formic acid (HCOOH) using titanium dioxide (TiO[sub 2]) in photocatalytic and photoelectrocatalytic reactions was investigated in order to determine whether electrical biasing could improve the efficiency of photocatalytic reactions. This study addressed the effects of film thickness, biasing potential, presence of oxygen, and added inorganic electrolytes on the photocatalytic degradation of HCOOH. The results of these experiments showed that the degradation of HCOOH in this system was due only to the photocatalytic as opposed to homogeneous photolysis reactions. Degradation efficiency of the photocatalytic reaction was roughly proportional to the TiO[sub 2] film thickness. In the photoelectrocatalytic reaction, positive potentials (vs saturated calomel electrode, SCE) improved the degradation efficiency and +0.0 V (vs SCE) was enough to obtain a maximum efficiency. The supply of oxygen was essential in the photocatalytic reaction, while the photoelectrocatalytic reaction was not significantly affected by the removal of oxygen. The presence of inorganic electrolytes lowered the efficiency of the photocatalytic degradation of HCOOH. However, the efficiency of photoelectrocatalytic degradation was not affected by inorganic electrolytes. Overall, when used with the bias, the system showed efficient degradation over a wide range of conditions. 21 refs., 9 figs.

Three-dimensional high-density hierarchical nanowire architecture for high-performance photoelectrochemical electrodes.

Three-dimensional (3D) nanowire (NW) networks are promising for designing high-performance photoelectrochemical (PEC) electrodes owing to their long optical path for efficient light absorption, high-quality one-dimensional conducting channels for rapid electron-hole separation and charge transportation, as well as high surface areas for fast interfacial charge transfer and electrochemical reactions. By growing titanium dioxide (TiO(2)) nanorods (NRs) uniformly on dense Si NW array backbones, we demonstrated a novel three-dimensional high-density heterogeneous NW architecture that could enhance photoelectrochemical efficiency. A 3D NW architecture consisting of 20 μm long wet-etched Si NWs and dense TiO(2) NRs yielded a photoelectrochemical efficiency of 2.1%, which is three times higher than that of TiO(2) film-Si NWs having a core-shell structure. This result suggests that the 3D NW architecture is superior to straight NW arrays for PEC electrode design. The efficiency could be further improved by optimizing the number of overcoating cycles and the length/density of NW backbones. By implementing these 3D NW networks into electrode design, one may be able to advantageously impact PEC and photovoltaic device performance.

The gas-phase photocatalytic mineralization of benzene on porous titania-based catalysts

Photocatalytic degradation of benzene in oxygen-containing gaseous feed streams was investigated using titania and platinized (0.1 wt.-%) titania photocatalysts. The titania catalyst was synthesized using sol-gel techniques. Results of this study indicate that, when using this particular photocatalyst, benzene was oxidized to carbon dioxide and water without forming any detectable organic reaction products in the reactor effluent, although only some of the benzene reacted. Both the overall conversion of benzene and its mineralization were improved by platinizing the titania. When the titania catalyst was platinized, both photocatalytic and thermocatalytic reactions were promoted. Rates of photocatalytic reactions were significantly enhanced at reaction temperatures between 70 and 90°C, while at temperatures above 90°C the rates of thermocatalytic oxidation reactions were noticeably increased. It proved possible to obtain the continuous and essentially complete mineralization of benzene by using the platinized titania catalyst and optimizing such parameters as the reaction temperature, space time, and the concentrations of oxygen and water vapor in the feed stream.

Dynamic phenomena during the photocatalytic oxidation of ethanol and acetone over nanocrystalline TiO2: simultaneous FTIR analysis of gas and surface species

Photocatalytic oxidation of acetone and ethanol over nanocrystalline TiO2 powder was studied under batch conditions using an UV-illuminated DRIFTS chamber as a photoreactor. In this way, we could study the evolution of the reaction by examining changes in the species at the surface of the photocatalyst under UV irradiation. In addition, we were able to simultaneously analyze the gas-phase composition of this reaction by means of a multiple reflection FTIR gas cell. Results obtained indicate that ethanol adsorbs on the TiO2 surface either molecularly or in the form of ethoxide complexes. Under UV irradiation, these species are progressively removed, and acetate and formate complexes slowly accumulate on the TiO2 surface. Acetaldehyde builds up in the gas phase during the photocatalytic oxidation of ethanol, although this molecule is scarcely found on the TiO2 surface. The spectroscopic data provided are consistent with the existence of two parallel reaction pathways for the photocatalytic oxidation of ethanol: one yielding acetaldehyde vapor and the other leading to CO2 through adsorbed ethoxide species. On the other hand, acetone is adsorbed exclusively in a molecular form on TiO2. Its photocatalytic oxidation yields acetate and formate complexes, along with adsorbed acetaldehyde and formic acid. These adsorbed molecules can act as intermediates species in the photooxidation of acetone. In addition, participation of specific hydroxyls groups on the TiO2 surface during this photocatalytic reaction can also be observed using the DRIFTS system.

Novel electrode materials for electrochemical capacitors: Part II. Material characterization of sol-gel-derived and electrodeposited manganese dioxide thin films

Material characterization of sol-gel-derived and electrodeposited MnO 2 thin films showed that their microstructures are highly porous in nature. While sol-gel-derived films are nanoparticulate, electrodeposited films showed macropores of random and irregular platelike structures, comprising much denser surface layers and highly porous underlying layers. On the basis of calculated and theoretical density values of 1 and 4.99 g/cm 3 , respectively, the porosity of sol-gel-derived MnO 2 films was determined to be as high as 80%, which is substantially higher than electrodeposited films at 67%. Apart from their higher specific capacitance, sol-gel-derived MnO2 films appeared to exhibit higher cycling stability and reversibility than electrodeposited MnO 2 films. In the case of sol-gel films, thinner films appeared to exhibit higher cycling stability than thicker films. There was less alteration in surface morphology and microstructure, and the rate of loss in charge-storage capacity upon voltammetric cycling was not as significant for sol-gel MnO2 thin films.

A Kinetic study of the photocatalytic degradation of 3-Chlorosalicylic acid over TiO2 membranes supported on glass

Abstract The photocatalytic degradation of 3-chlorosalicylic acid to HCl and CO 2 on glass-supported TiO 2 has been investigated in a photoreactor designed to permit both continuous throughput of gaseous and liquid feed streams and frequent exchange of the catalyst. The data for the photodegradation reaction can best be correlated in terms of the rate expression r = kK 1 [ O 2 ]K 2 [ S ] {(1 + K 1 [ O 2 ])(1 + K 2 [ S ])} where [O 2 ] and [S] represent the concentrations of oxygen and 3-chlorosalicylic acid, respectively.

Synthesis of porosity controlled ceramic membranes

Porosity control in ceramic membranes has been achieved by controlling particle packing densities in sol-gel processing. TiO 2 xerogels with two mean pore radii of 0.7 and 1.7 nm and a porosity varying from 30% to 52% have been obtained. ZrO 2 xerogels with a mean pore radius of 0.7 nm and a porosity varying from 7% to 34% have also been prepared. The principle of controlling porosity is to make spongy aggregates and to control the degree of aggregation. Experiments have been conducted to show that spongy aggregates can be produced by gradually removing protons from the strongly charged particles. Viscosity techniques have been used to measure the relative volume fraction of the dispersed phase which, in turn, provides information on aggregate structures. Two aggregation models have been proposed to explain different structural aggregates formed through thermal destabilization in the highly charged system and through charge neutralization by gradually removing charge from the particles in the system.