Portugaliae
Electrochimica Acta

Nickel secondary systems have high energy density, power density, rugged physical structure and good low temperature performance. Nickel secondary systems are mainly used in electric vehicles as well as in military and commercial aircraft. Electrochemical impregnation of active material onto porous nickel foam electrode containing different additives has been studied to give best electrochemical reversibility. The development of additives for nickel electrode is mainly aimed at increasing the discharge capacity and minimizing self-discharge. Porous nickel electrodes have been evaluated using cyclic voltammetry, potentiodynamic polarization and AC impedance measurements. All the addition agents favour the reversibility of the charge-storage reaction of nickel electrode. Addition of copper and thiourea increases the capacity values.

The redox reaction of V(V) in ammonia buffer solution of pH = 8.60 was studied by means of square-wave (SWV) and cyclic (CV) voltammetry. The redox reaction studied exhibits properties of a surface redox process in which both reactant and product of the redox reaction are immobilized to the electrode surface. A mathematical model for the electrode mechanism proposed was developed under conditions of square-wave voltammetry. In agreement with the theoretical findings, the phenomena of "split SW peaks" and "quasi-reversible maximum" are demonstrated experimentally. These unique features of the SWV response of a surface redox reaction are utilized for characterization of the redox process of V(V) in ammonia buffer. The following values of the kinetic parameters of the investigated redox reaction were estimated: standard rate constant Ks = 120 ± 10 s-1 and electron transfer coefficient = 0.4 ± 0.05. The formal potential of the redox couple V(V)/V(IV) in ammonia buffer with pH = 8.6 is E° = -0.56 V vs. Ag/AgCl (sat. KC1).

Electrochemical noise measurements and impedance analyses were performed on A516-70 carbon steel in chromate/chloride solutions. The noise data were used to calculate the noise resistance in the time domain, and the spectral noise resistance and spectral noise impedance in the frequency domain. It is shown that the spectral noise impedance is equivalent to the modules of electrode impedance over the experimental frequency range. This relationship is independent of the corrosion types (passivity, general corrosion and metastable pitting). The noise resistance coincides with the charge-transfer resistance and the spectral noise resistance only under certain conditions, such as passive state or general corrosion, where the noise resistance provides an indication of the corrosion resistance of the material under study. During pitting corrosion, the noise resistance is much lower than the charge-transfer resistance and only qualitatively, rather than quantitatively, tracks the change of corrosion resistance. The spectral noise resistance is consistent with the charge-transfer resistance and can be used to indicate the corrosivity of carbon steel under studied conditions.

A Nafion-coated wall-jet mercury film electrode was tested to determine cadmium and lead at trace levels in flowing systems by square wave anodic stripping voltammetry. Optimization of the experimental conditions and square wave parameters is discussed. For a 60 s deposition time, the detection limits restricted by the amount of cadmium and lead in the blank solution were lxlO"9 M and lxlO"10 M respectively. In comparison with the conventional mercury film electrode, the resistance to Triton X-100 interference was found to be superior with the Nafion film coating. The modified electrode was applied to trace metal detenninations in real environmental samples.

Sodium benzoate is a salt widely used in industry as preserver. Its activity coefficients in aqueous solutions can be estimated by Pitzer theory, once the necessary parameters are known. Electromotive force (emf) measurements on galvanic cells without liquid junction have been used for the determination of mean activity coefficients of sodium chloride in the ternary system sodium chloride + sodium benzoate (NaB) + water. Pitzer theory has been applied to this system and those data enabled the evaluation of the specific parameters for sodium benzoate at 298.15 K by linear regression. The values obtained for beta(NaB)(0) and beta(NaB)(1) parameters, respectively 0.177 kg mol-1 and 0.34 kg mol-1 are close to those published already for other carboxylic acid salts.