Mariana Ciureanu

Direct Electron Transfer for Hemoglobin in Surfactant Films Cast on Carbon Electrodes

Direct electron transfer is reported for hemoglobin in liquid-crystal films of didodecyldimethylammonium bromide (DDAB) cast on glassy-carbon electrodes. Cyclic and square-wave voltammograms were obtained for two types of electrodes with DDAB films in the presence and in the absence of oxygen. At pH 7.1, in the absence of oxygen, a redox couple with a large cathodic and a small anodic peak was obtained, while for pH 5.5, in the presence of oxygen, only the cathodic wave could be obtained up to scan rates of 1 V/s. Based on the voltammogram pattern and the scan rate dependence of the peak potential and current, the mechanism of the electroreduction process was discussed in terms of the equilibrium between the T (tense) and R (relaxed) conformations. Two different reaction pathways were found, depending on the pH value and oxygen concentration. In neutral solutions and in the absence of oxygen, the electroreduction is followed by a slow release of the water molecule bound in the sixth coordination site of the heme, which determines the appearance of the reduction in a potential range characteristic for the R state. In acidic media (pH 5.5) and in the presence of oxygen, the electrode process involves successive dissociation of the water molecule, electroreduction, and oxygenation of deoxyhemoglobin; in this case, the cathodic peak potential is shifted to positive values, as expected for an increased contribution of the T conformation.

Composite hydride electrode materials

Abstract A new class of electrode materials — composite hydride materials — is proposed for anodes in hydride-based rechargeable batteries. The composites were synthesized by mechanical mixing of two components: a major component having good hydrogen storage properties and a minor component used as surface activator. The major component was selected among nonconventional or conventional hydride electrode materials; alloys and intermetallic compounds of the AB 2 or AB 5 type. The minor component was amorphous Mg x Ni 1− x ( x =0.4, 0.5) or a 1:1 mixture of Mg x Ni 1− x and LmM 5 (where Lm is a La-rich mischmetal and M 5 is Ni 3.6 Co 0.8 Mn 0.3 Al 0.3 ). The role of the minor component was to improve the kinetics of electrochemical charging and to eliminate the initial activation of the major component. Experimental results have shown that the electrodes manufactured from composite materials have the following advantages: (1) elimination of the initial activation; (2) a significant improvement of electrode kinetics, resulting in better rate capabilities and higher charging efficiency; (3) a drop of the charging overpotential, resulting in better energy efficiency of the battery; (4) better cycling behavior; (5) in most cases, a considerable increase of the discharge capacity with respect to that of the major component.

Electrochemical Adsorption-Desorption of Hydrogen on Amorphous Ni[sub 40]Nb[sub 60] in Alkaline Media

New interest has arisen in the last decade in the formation and properties of metal hydrides, particularly with respect to their applications as rechargeable hydride anodes in high energy density batteries. The electrochemical adsorption-desorption of hydrogen on amorphous Ni[sub 40]Nb[sub 60] is strongly alkaline media was investigated by cyclic voltammetry and chronopotentiometry. After cathodic polarization, the voltammogram in the [minus]1.0 to [minus]0.6 V range (vs Hg/HgO) show a new cathodic wave (C) and two anodic waves (A[prime] and B[prime]), due, respectively, to adsorption and desorption of H atoms at the surface of the alloy. The dependence of these waves on the sweep range and sweep potential is presented and discussed. The overpotential dependence on time at constant anodic current presents two plateaus, corresponding to the discharge of the two types of species on the surface of the alloy.

A discussion on the equilibrium potential and exchange current of hydride electrodes

Abstract A theoretical treatment is presented to account for the dependence of the equilibrium potential and exchange current on the bulk hydrogen occupancy in hydride electrodes. The model is based upon considering the equilibrium between adsorbed and absorbed hydrogen at the interface. Several characteristic parameters can be obtained for the hydride electrode from the fit of the experimental data ( ϵ e and I 0 ) to the theoretical concentration dependence: the maximum value of the storable hydrogen concentration, the solubility of H in the bulk at 1 atm, the equilibrium potential at X = 0.5, the symmetry coefficient for the electrochemical reaction. Also, it is possible to estimate the constant for the transfer through the interface, a parameter on which there are virtually very little information. The model was found to fit very well the experimental data obtained on crystalline TiMn 1.5 . A discussion of the effect of lateral HH interactions on the ϵ e and I 0 values was made.