Papers by Author: S. Mentus

Abstract: Magnesium based composites MgH2 + X (X=Ti, Co) were synthesized by ball milling in an argon atmosphere using stainless steel vial and balls. The crystallographic behavior of the resulting powders was examined by XRD. Thermal stability and hydrogen desorption properties were investigated by thermal analysis methods. In order to obtain a deeper insight into bonding mechanisms of the transition metal in MgH2 relaxed structure, ab initio electronic structure calculation of MgH2 + X (X=Ti, Co) was performed using Full Potential Linearized Augmented Plane Wave method, implemented in WIEN2K code. DOS analysis, confirmed by DTA measurements, resulted in the conclusion that, in the composite, in comparison to MgH2, the bonding Mg-H was weakened, on account of the shortening of interatomic distances hydrogentransition metal.

Abstract: Olivine-type lithium iron phosphate (LiFePO4) powders were synthesized applying three different methods: solid state reaction at high temperature, ultrasonic spray pyrolysis, and sonochemical treatment. The samples were characterized by X-ray powder diffraction (XRPD). Particle morphologies of the obtained powders were determined by scanning electron microscopy (SEM). It was found that structural and microstructural parameters of this material were strongly dependent on the synthesis conditions. We present here the results obtained upon optimization of each procedure for designing this cathode material.

Abstract: Potentiodynamic polarization of a mechanically polished titanium electrode in a diluted solution of Rhodium(III) chloride in 0.1 M perchloric acid was performed, resulting in simultaneous formation of both Rh and TiO2 films. The morphology of obtained Rh/TiO2 composite film followed the morphology of titanium support, as evidenced by SEM technique. This composite surface was examined by cyclic voltammetry in both acidic and alkaline solutions, in the potential region of both hydrogen and oxygen underpotential deposition. The charge related to hydrogen underpotential deposition corresponded to a surface roughness of 43. As a consequence of high surface roughness, the diffusion current of oxygen reduction in an oxygen saturated 0.1 M NaOH solution, measured by voltammetry on rotating disc electrode, was found to be comparable to the current of hydrogen underpotential deposition.

Abstract: The mixtures NiO-WO3 were synthesized by the combustion of gels obtained by drying common solutions of nickel nitrate, polytungstic acid and citric acid. The X-ray diffractograms of oxide mixtures confirmed mutual interaction of oxides during synthesis leading to a new phase, NiWO4. The reduction of oxide mixtures in hydrogen atmosphere was investigated thermogravimetrically. The temperature of reduction of the oxide mixture lies between the temperatures of reduction of pure oxides and monotonously increases with the increase in WO3 mole fraction; however, the reduction itself is a multi-step process, preferably in the composition region rich in NiO. The X-ray diffractometry of metallic residues evidenced Ni-W alloys and tungsten excess to be the reduction products.

Abstract: The exchange current density and the peak current density of the hydrogen electrode reaction on LmNi3.55Co0.75Mn0.4Al0.3 alloy electrode in an aqueous 1M KOH solution, at low hydrogen contents (<0.1 H/M) and elevated temperatures (42-60°C), were studied using potentiostatic polarization techniques. Both measured quantities increased linearly with the increase in hydrogen content in the alloy. On the basis of the difference between the activation energy of the charge transfer and the hydrogen diffusion, it was concluded that the charge transfer reaction represented the rate-determining step of the hydrogen electrode reaction under applied experimental conditions. The decrease of each of these two activation energies with increasing hydrogen content was evidenced, which indicated the existence of other processes occurring in the electrode bulk accompanying the hydrogen diffusion.