Chu-Nan Cao

On electrochemical techniques for interface inhibitor research

Abstract The electrochemical behavior related to different modes of inhibition effect is discussed for interface inhibitors. The action coefficients of an inhibitor on the anodic and cathodic reactions (fa and fc, respectively) of a corrosion process are defined. An equation is deduced which denotes that if the inhibition effect is caused by geometrically blocking the surface of metal electrode by adsorbed inhibitive species, the corrosion potential in the inhibitor-containing solution will be nearly equal to that in the solution without the inhibitor. In this case the inhibition efficiency (η) equals the coverage (Θ) of the adsorbed species and can be estimated by polarization resistance (Rp) measurements. In addition, the plot of η vs the “relative coverage” (μ) estimated from interfacial capacitance measurements will be a straight line through the original point of the coordinate. If the inhibition is not due to the geometric blocking effect, which can be judged by the noticeable shift of corrosion potential as the inhibitor is added into the solution, then η cannot be estimated by Rp and does not equal Θ. Instead, transfer resistance (Rt) can always be employed to estimate inhibition efficiency whatever the mode of the inhibition effect. It is argued that theoretically no Tafelian straight lines can be measured in solutions with interface inhibitors unless Θ is independent of the electrode potential E or fa and fc of the inhibitor change linearly with E. Equations of faradaic admittance at the corrosion potential in inhibitor-containing solution for different modes of inhibition effect are deduced. In addition to the estimation of Rt, Rp and interfacial capacitance, information on the adsorption of the inhibitive species can also be gathered by EIS measurement. In the case of the geometric blocking effect, the EIS display in the inhibitor-containing solution will be similar to that in the black solution when η is low, while a single capacitive loop will display when η is high. If the inhibition is not due to the geometric blocking effect, two time constants will be involved in the EIS display measured in the inhibitive-containing solution no matter what the displays of EIS in the blank solution are, and in this case the effect of E on Θ can be predicted based on the EIS and polarization curve measurements.

Synthesis of foam-like freestanding Co3O4 nanosheets with enhanced electrochemical activities

We report a facile and simple strategy to synthesize freestanding Co(3)O(4) nanosheets on conductive substrates. The as-prepared product shows two-dimensional hexagons in morphology with mesoporous inside architecture.

Studies of impedance models and water transport behaviors of polypropylene coated metals in NaCl solution

Using electrochemical impedance spectroscopy (EIS) technique, the water uptake process and the evolution of impedance models of polypropylene coated mild steels and LY12 Al alloys immersed in 3.5% (weight fraction) NaCl solution were studied. The equivalent electrical circuit (EEC) fitting results suggested that there was one single semi-circle in the impedance spectra of immersed samples in the initial period of immersion, which was corresponded to one time constant. Along with the immersion time, the number of time constant in EEC for metal/coating electrode system increased. The diffusion process towards solution of corrosion products forming on the metal/coating interface might be restrained due to the influence of coating barrier, resulting in the initiation of diffusion component in EEC. This diffusion did not act as a Warburg behavior, leading to a deviation of dispersive number, n, from 0.5. The studies revealed that the water transport in polypropylene clear varnish followed the Fickian diffusion during the initial time of exposure to the solution. After a certain time of immersion, the water uptake reached a saturation value. According to the variation of coating capacitance with time, the water diffusion coefficient was deduced. The experimental results indicated that the water diffusion coefficients in polypropylene coatings were independent of substrate materials used and coating thickness. Relative dielectric constant of dry coatings (ed) and water volume fraction of the coatings at saturation (φ∞) were also calculated. The similar results were found in time dependence of coating capacitance calculated from the impedance imaginary component at a fixed high frequency and that obtained from the fitting result by EEC, which indicated that the coating capacitance from the impedance imaginary component was reliable to evaluate barrier performance of coatings against water.

Effects of metallic cobalt addition on the performance of pasted nickel electrodes

Abstract A study is made on the performance of Ni-foam pasted nickel electrodes prepared from co-precipitated Ni 1− x Zn x (OH) 2 with and without cobalt powder. The addition of cobalt provides higher performance in terms of both discharge capacity and cycle life. Cobalt increases remarkably the active-material utilization through enhancement of the discharge depth due to the formation of conducting CoOOH during the first activation charging. The superior capacity stability during cycling is attributed to the synergistic action of zinc and cobalt, especially to the co-precipitation of zinc hydroxide which is considered to inhibit the formation of γ-NiOOH.

Study of the performance of secondary alkaline pasted zinc electrodes

Calcium zincate was prepared by a chemical coprecipitation method and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical performance of pasted zinc electrodes with bismuth and calcium additives was investigated by the charge–discharge method. The addition of metallic bismuth powder improves the discharge performance of zinc electrodes due to the formation of an electronic conduction matrix. The calcium-containing zinc electrodes showed higher discharge capacity, less shape change and longer cycle lifetime. Moreover, zinc electrodes using calcium zincate as active material show better electrochemical performance than those with the physical mixture of zinc oxide and calcium hydroxide.

Titanium dioxide as photocatalyst on porous nickel : Adsorption and the photocatalytic degradation of sulfosalicylic acid

The commonly used photocatalyst, TiO2 (anatase), has been immobilized on porous nickel using 3 wt.% polyvinyl alcohol (PVA) as the binder. The results show that sulfosalicylic acid (SSal) can be degraded on the developed catalytic system. The adsorption characteristics on TiO2-Ni system have been investigated. The observance of photocalytic degradation of SSal under pH values and initial concentrations can be explained by the adsorption behavior of SSal. The parameters of the Langmuir-Hinshelwood expression have been determined by different experimental ways and the results are satisfactory.

Studies on the electrochemical characteristics of K2Sr(FeO4)2 electrode

K2Sr(FeO4)(2) was prepared from K2FeO4 and characterized by SEM. XRD, IR and AAS. The measured solubility of K2Sr(FeO4)(2) is lower than that of K2FeO4 either in various concentrations of aqueous potassium hydroxide at room temperature (25degreesC) or in saturated KOH solution at different temperatures. Discharge performance of K2Sr(FeO4)(2) electrode under different conditions was studied by the constant current discharge method. The discharge efficiency of K2Sr(FeO4)(2) electrode is higher than that of K2FeO4 electrode at the same discharge condition. The electrochemical characteristics of K2Sr(FeO4)(2) electrode were also investigated by means of electrochemical impedance spectroscopy (EIS)

Effects of high-energy ball milling (HEBM) on the structure and electrochemical performance of nickel hydroxide

A method of high-energy ball milling (HEBM) was first used to modify the structure and electrochemical performance of nickel hydroxide for the active material of Ni/MH battery. The experimental results showed that HEBM is an effective method to improve the electrochemical performance of beta-Ni(OH)(2) such as specific discharge capacity, discharge potential and cycle performance. But the performance of alpha-Ni(OH)(2) was greatly reduced by HEBM processing. The results of electrochemical impedance spectroscopy, powder X-ray diffraction and particle size distribution indicated that the improvement of the performance of beta-Ni(OH)(2) through HEBM was attributed to the reduction of the charge-transfer resistance (R-t), the diffusion impedance (Z(w)), which was resulted from the decrease of the crystallite and particle size and the increase of interlayer spacing. However, for alpha-Ni(OH)(2) samples, those structure changes after HEBM resulted in the increase of the R-1 and Z(w) and the deterioration of its electrochemical performance

Effects of coprecipitated zinc on the structure and electrochemical performance of Ni/Al-layered double hydroxide

The effects of zinc on the structure and electrochemical performance of Ni/Al-layered double hydroxides (LDHs) prepared by a coprecipitation method were studied. The experimental results showed that the addition of zinc in Ni/Al-LDHs improved the electrochemical performance of the samples such as discharge potential, cycle stability and reversibility. The maximum specific discharge capacity of 425 mAh g(-1) of nickel hydroxide was obtained for the Ni/Al-LDHs without zinc. The maximum discharge mediate potential of 413 mV and the lowest capacity deterioration rate of < 5% after 398 charge-discharge cycles at 1C rate were gained for the 6.4 wt% Zn-containing sample. The results of electrochemical impedance spectroscopy and powder X-ray diffraction measurements indicated that the advantageous performance of Ni/Al-LDHs with high zinc content resulted from the decrease of electron transfer resistance (R-t) and the number of cells per crystallite in the direction of c-axis and slow dissolution of zinc in alkaline solution

Microstructure, electrochemical surface and electrocatalytic properties of IrO2+Ta2O5 oxide electrodes

X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to characterize physical structure of IrO2+Ta2O5 films over the whole composition range by thermodecomposition of chloride solutions heated at 450°C. Solid solubilization between Ta component and IrO2 rutile in the mixed films was measured, and three typical surface morphologies of the oxide coatings were observed. The surface electrochemical properties of Ti/IrO2-Ta2O5 electrodes were studied by cyclic voltammetry at varying potential scan rate, and a ‘double-layer’ electrochemical structure containing the ‘inner’ and ‘outer’ layers has been distinguished. The voltammetric charge appears to decline with the decrease of grain size of oxide coatings as a result of the effect of surface tension. However, the coatings of 70% IrO2+30% Ta2O5 with the finest grains still exhibit the highest apparent activity for oxygen evolution evaluated by the anodic current at a constant potential. This result is interpreted by the measurements of open-circuit potential (Eoc) and double-layer capacitance (Cdl) using electrochemical impedance spectroscopy (EIS). Thereby, the reliability of voltammetric charge obtained in ‘double-layer’ potential region in determining the real electrocatalytic activity for O2 evolution has been discussed.