Mechanism of anodic oxidation of tungsten in neutral sulphate-fluoride solutions

Anodic Oxidation of Tungsten under Illumination-Multi-Method Characterization and Modeling at the Molecular Level

Tungsten oxide has received considerable attention as photo-anode in photo-assisted water splitting due to its considerable advantages such as significant light absorption in the visible region, good catalytic properties, and stability in acidic and oxidative conditions. The present paper is a first step in a detailed study of the mechanism of porous WO3 growth via anodic oxidation. In-situ electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS) during oxidation of W illuminated with UV and visible light are employed to study the ionic and electronic processes in slightly acidic sulfate-fluoride electrolytes and a range of potentials 4-10 V. The respective responses are discussed in terms of the influence of fluoride addition on ionic and electronic process rates. A kinetic model is proposed and parameterized via regression of experimental data to the EIS and IMPS transfer functions.

Spatio-temporal Characterization Analysis and Water Quality Assessment of the South-to-North Water Diversion Project of China

In this article, a data matrix of 20 indicators (6960 observations) was obtained from 29 water quality monitoring stations of the Middle Route (MR) of the South-to-North Water Diversion Project of China (SNWDPC). Multivariate statistical techniques including analysis of variance (ANOVA), correlation analysis (CA), and principal component analysis (PCA) were applied to understand and identify the interrelationships between the different indicators and the most contributive sources of anthropogenic and natural impacts on water quality. The water quality index (WQI) was used to assess the classification and variation of water quality. The distributions of the indicators revealed that six heavy-metal indicators including arsenic (As), mercury (Hg), cadmium (Cd), chromium (Cr), selenium (Se), and lead (Pb) were within the Class I standard, while the As, Pb, and Cd displayed spatial variation. Moreover, some physicochemical indicators such as dissolved oxygen, 5-day biochemical oxygen demand (as BOD₅), and total phosphorus (TP) had spatio-temporal variability. The correlation analysis result demonstrated that As, Hg, Cd, Cr, Se, Pb, copper (Cu), and zinc (Zn) had high correlation coefficients. The PCA result extracted three principal components (PC) accounting for 82.67% of the total variance, while the first PC was indicative of the mixed sources of anthropogenic and natural contributions, the second and the third PCs were mainly controlled by human activities and natural sources, respectively. The calculation results of the WQI showed an excellent water quality of the MR of the SNWDPC where the values of the stations ranged from 10.49 to 17.93, while Hg was the key indicator to determine the WQI > 20 of six stations, which indicated that the Hg can be the main potential threat to water quality and human health in this project. The result suggests that special attention should be paid to the monitoring of Hg, and the investigation and supervision within the areas of high-density human activities in this project should be taken to control the impacts of urban and industrial production and risk sources on water quality.

Anion-incorporation model proposed for interpreting the interfacial physical origin of the faradaic pseudocapacitance observed on anodized valve metals--with anodized titanium in fluoride-containing perchloric acid as an example

With anodized titanium in fluoride-containing perchloric acid solution as an example in the present Article, the physical origin of the faradaic pseudocapacitance C(0) frequently observed in electrochemical impedance spectroscopy (EIS) spectra at low frequencies was interpreted for the first time by the incorporation/insertion of some electrolyte anions (i.e., F(-)-ions in this work) into the oxide film under the electric field during film growth. It was shown that the emergence of the faradaic pseudocapacitive behavior not only indicates a significant incorporation of anions into the oxide films but also reflects the arrival of them at the metal/oxide interface. The anion incorporation/transfer process was depicted with a modified point-defect model and simulated by a blocking, restricted finite-length diffusion impedance model (which has been widely used for studying the ion-insertion electrodes). The fast inward migration of the incorporated electrolyte species, along with the accumulation of them at the metal/film interface (due to the blocking effect of this interface), is responsible for the low-frequency capacitance behavior revealed by a vertical capacitive line in EIS spectra at low frequencies for anodically growing oxide films on valve metals. Many related published results for anodized valve metals (such as Ti, Bi, Ta, Al, etc.) are in good accordance with the model predictions. This work also suggests that the electrochemical method for the preparation of F(-)-doped TiO(2) thin film materials should be an easy, low-cost, and even more effective one to be used.