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Abstract:The most critical disadvantages of the Zn-air flow battery system are corrosion of the zinc, which appears as a high self-discharge current density and a short cycle life due to the non-uniform, dendritic, zinc electrodeposition that can lead to internal short-circuit. In our efforts to find a dendrite-free Zn electrodeposition which can be utilized in the Zn-air flow battery, the surface morphology of the electrolytic Zn deposits on a polished polymer carbon composite anode in alkaline, additive-free solutions was studied. Experiments were carried out with 0.1 M, 0.2 M and 0.5 M zincate concentrations in 8 M KOH. The effects of different working conditions such as: elevated temperatures, different current densities and different flow velocities, on current efficiency and dendrite formation were investigated. Specially designed test flow-cell with a central transparent window was employed. The highest Coulombic efficiencies of 80%-93% were found for 0.5 M ZnO in 8 M KOH, at increased temperatures (50-70 °C), current densities of up to 100 mA·cm -2 and linear electrolyte flow velocities higher than 6.7 cm·s -1 .

Low loading PtAu nanoparticles supported on high area carbon were synthesized by water-in-oil microemulsion method and examined for formic acid and methanol oxidation.Prepared catalyst powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). These techniques revealed that the catalyst contains rather agglomerated quasi-spherical particles, ~4 nm diameter, composed of a solid solution of Pt and Au with only ~4 at% of Au. In spite of such low Au content, both onset and peak potentials for CO oxidation are shifted some 150 mV to more positive values in comparison to Pt synthesized in the same manner due to stronger binding of CO as a result of notable electronic effect. It is important that this small quantity of Au also significantly influences oxidation of formic acid promoting direct path and suppressing indirect path in formic acid oxidation in a degree as expected by a much larger quantity of Au. Such improvement could be due exclusively by ensemble effect of high number of small Pt domains which formation could be possible only by very fine dispersion of such low Au quantity. High number of small Pt domains is corroborated by lower activity for methanol oxidation in comparison to Pt catalyst synthesized by the same procedure. These results emphasize the importance of the Au dispersion on the surface of Pt over its quantity in PtAu catalyst with regards to both, the ensemble and the electronic effects.

Highlights-A complementary electrochemical -microscopic study addresses the shape evolution of Pt nanoparticles synthesised by water-in-oil microemulsion method with addition of HCl and supported on Vulcan XC-72R -W/O microemulsion method was altered with the addition of catalyst support in the system and change in the catalyst cleaning steps, which enabled the application of synthesized catalysts in their as prepared state, without the electrochemical treatment. -XRD and TEM results revealed that synthesized particles are smaller in size (4-8 nm) compared to some previously reported -Concave cubic particles show improved catalytic properties and the contribution of their preferentially oriented {100} facets is electrochemically detectable.

Glassy metallic Hf thin films were obtained using electron beam deposition at room temperature due to the low energy received by Hf atoms during the film formation process. The amorphous nature of the Hf films suggested by XRD was confirmed by low temperature electrical conductivity measurements where a negative temperature coefficient of resistivity was identified. Anodic oxidations using a scanning droplet cell microscopy were performed on a typical crystalline (hexagonal) Hf film obtained by sputtering and on the glassy Hf. A decrease of the oxide formation factor by 30% (from 2.4 to 1.7 nm V −1 ) was evidenced on the amorphous Hf. Electrochemical impedance spectroscopy on both samples revealed almost identical capacitances while the electrical permittivities were found to differ by 40%. The dielectric constant of the HfO 2 decreased from 33.5 on the crystalline parent metal sample to 19.8 on the amorphous one. The unexpected 10% deviation was attributed to a smoother defect-free oxide/metal interface in the case of Hafnium dioxide has a high dielectric constant, a relatively large bandgap, larger heat of formation as compared to SiO 2 , good chemical and thermal stability on Si and large barrier heights at interfaces with Si. Its modern applications are diverse, most recent ones focused on electronic devices. The chosen methods for obtaining hafnia thin films are also varied, most common one being reactive sputtering.1 Doping of sputtered hafnia with elements such as Ce has led to an increase in the dielectric constant and a decrease of the leakage currents, while Si doping revealed intrinsic ferroelectricity due to a non-centrosymmetric orthorhombic phase with direct applications in ferroelectric memories.2,3 Cathodoluminescence due to oxygen vacancies was also recently found in ion beam sputtered HfO 2 and first steps toward fabrications of hafnia micro-light emitting devices were already taken. 4,5 The resistance switching behavior due to formation and rupture of conductive paths within HfO 2 deposited by atomic layer deposition was exploited for random access memory fabrication. 6 Since these are only a few examples of most recent applications of HfO 2 , still the predominant effort is put into using it as high-k material for gate oxides for replacing SiO 2 gates in MOS-FET structures.7 For a typical operation voltage of 1-1.5 V, the leakage current through HfO 2 dielectric films was found to be several orders of magnitude lower than that of SiO 2 for the same equivalent oxide thickness of 0.9-2 nm.8-10 However, clearly a weak point in using thermally grown or vapour phase deposited HfO 2 as gate dielectrics arises from its poor interfacing with semiconductors which has led to a fervent investigation of hafnium-based complex oxides in recent years. 7Nowadays it is common knowledge that the anodic oxides on valve metals (e.g. Al, Hf, Ta, Nb etc.) are some of the most compact oxides available due to their growth mechanism obeying a high field regime.11 However, their use for electronic devices is weakly ex...

Pt, PtSn and PtSnO2 catalysts supported on high surface area carbon synthesized by microwave assisted polyol procedure were tested for methanol oxidation. Based on TGA, EDX and XRD analysis, PtSn/C is composed of Pt and Pt3Sn phase while the rest of Sn is present in a form of very small tin oxide particles. This paper focuses on structure-activity relationships for CO tolerance and methanol oxidation reactions after addition of Sn to Pt catalysts. Alloying of Sn with Pt improves the rate of CO oxidation despite the fact that the pure Sn does not react with CO and therefore activity for methanol oxidation increases ~ 2 times in comparison to Pt/C catalyst. PtSn/C catalyst shows small advantage in comparison with PtSnO2/C catalyst due to the alloyed Sn and its electronic effect. Long term stability tests also confirmed that PtSn/C catalyst is somewhat better in comparison to PtSnO2/C.

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