Marcella Cappadonia

Conductance of Nafion 117 membranes as a function of temperature and water content

Abstract The conductance of Nafion membranes was investigated by means of impedance spectroscopy as a function of temperature and of sample treatment. In addition to other treatments, the hot-pressing of Nafion membranes was also considered, because of its relevance for making membrane-electrode assemblies (MEA) for proton exchange membrane fuel cells (PEMFC). An Arrhenius-type analysis of the conductance shows two regimes, with a change in activation energy observed at transition temperatures between 225 and 260 K which depends on the water content.

Proton conduction of Nafion® 117 membrane between 140 K and room temperature

Abstract Nafion® 117 membranes were investigated by measuring the impedance response of the Au/membrane/Au cell between 140 K and room temperature in the frequency range 10−2 to 106 Hz. The conductance and its activation energy were obtained for membranes aged either in air or in water after different pretreatments. Two different water environments can be identified in the membrane, and a relationship between the water content and the conductance has been observed.

Quasielastic neutron scattering study of proton diffusion in SrCe0.95Yb0.05H0.02O2.985

Abstract By means of quasielastic neutron scattering (QENS) we have studied proton diffusion in SrCe 0.95 Yb 0.05 H 0.02 O 2.985 on microscopic scales in space and time. The diffusion process consists of a sequence of free diffusion and trapping/escape events. The resulting diffusion coefficient agrees with the diffusion coefficient determined by impedance spectroscopy measurements which were performed on the same sample batch.

THE IMPEDANCE BEHAVIOR OF THE CELL AU/HCLO4.5.5 H2O/AU IN THE TEMPERATURE RANGE 4.2-300 K

Abstract The impedance of the cell Au/HClO4-5.5 H2O/Au was investigated in the frequency range 1 to 105 Hz between 4.2 and 300 K. The analysis of the data enables an evaluation of important electrolyte properties such as conductivity and dielectric constant in a wide range of temperatures, predominantly in the solid state of the electrolyte HClO4-5.5 H2O (Tf = 228 K). The double layer capacity of the gold electrodes was also determined; it shows a qualitatively similar result compared with previous measurements. In the solid state, the ionic conductivity exhibits two distinct activation energies of 0.37 and 0.54 eV corresponding to the two phases present in HClO4-5.5 H2O above and below 170 K. Below 120 K the activation energy becomes very small and tends to zero around 80 K indicating possible tunneling processes in the rigid H2O structure. At about the same temperature the dielectric constant reaches its low temperature limit with a value ϵ∞ ≈ 11 which is considerably higher than the value of pure ice of ϵ∞ ≈ 3.

Proton conductive perovskite solid solutions with enhanced mechanical stability

Abstract Perovskite solid solutions with formula SrYb0.05(Ce1 − xZrx)0.95O3 − α (x = 0, 0.25, 0.5, 0.75 and 1) were prepared. Their electrical behaviour was investigated using impedance spectroscopy between 200 and 1000 °C in different atmospheres, with and without hydrogen. Conductivities and their activation energies were obtained. The perovskites showed a high mechanical stability and a considerable high proton conductivity. Yb doped SrCe0.75Zr0.25O3 − α seems to be a suitable electrolyte for solid oxide fuel cells.

Oxygen evolution at nickel anodes in concentrated alkaline solution

Abstract Oxide coated nickel electrodes were investigated in concentrated alkaline solutions. Their electrochemical behaviour was studied in the potential regime between the double layer capacity region and the oxygen evolution, at 20, 50 and 80°C. Cyclic voltammograms and quasi-steady-state current-potential curves change remarkably with temperature, correspondingly, the impedance of the system changes strongly with potential, but also with temperature. The experimental results do not support the Krasilshchikov mechanism suggested by many authors.

Electrical and electrochemical processes at low temperatures

The effect of isotopic exchange (D vs H) on the bulk electric conductivity and the static dielectric constant of acid polyhydrates, having a clathrate structure, were determined between 10 and 228 K (melting point). Protonated and deuterated acids show virtually identical Arrhenius plots of the conductivity above 120 K. Below that temperature they differ, the deuterated compound exhibits a smaller conductivity and a smaller dielectric constant as compared with the protonated form. The hydrogen and the deuterium evolution reactions were investigated at the interface Ag-solid 5.5 hydrated perchloric acid between 140 and 228 K. The gas evolution was found to depend strongly on temperature and isotope (D vs H). At the solid-solid interface the hydrogen evolution occurs faster than the deuterium evolution at the same overpotential. A temperature dependence of the charge transfer coefficient is observed with nα decreasing with decreasing temperature.

Proton transport in solid electrolytes with clathrate structure

Abstract The bulk electric conductivity and the static dielectric constant of acid polyhydrates, having a clathrate structure, were determined between 4 and 300 K. The effect of isotopic exchange (D versus H) was investigated in the same temperature range. Protonated and deuterated acids show virtually identical Arrhenius plots of the conductivity between room temperature and 125 K, below that temperature they differ. The deuterated compound exhibits a smaller conductivity and a smaller dielectric constant as compared with the protonated one.

Study of the electrical state of the phase boundary between polyelectrolyte gel and electrolyte solution by measurements of membrane potential and volta potential

The concentration dependence of the membrane potential of a film of a polyelectrolyte gel (Nafion) separating two aqueous KCl solutions is measured. Thus, the range of electrolyte concentrations is determined in which the membrane potential is given by the difference of the two equilibrium potential drops (the “Donnan potentials”) across the phase boundaries between the Nafion film and the electrolyte solutions. The results are compared with the concentration dependence of the Volta potential difference between electrodes coated with the same Nafion matrix and a reference metal (Kelvins vibrating capacitor plate). For these experiments the coated electrodes are withdrawn under potential control from the electrolytes. It is demonstrated that both experimental approaches yield similar information on the equilibrium Donnan potential across the membrane/electrolyte interface, as long as the electrolyte concentration remains below the fixed-charge concentration. At higher electrolyte concentrations, at which the measured membrane potential includes a diffusion potential developing within the membrane phase, the corresponding Volta potential measurements show a measurable change in the surface dipole potential of the withdrawn membrane surface.

In-situ X-ray surface diffraction of copper underpotential deposition on Au(100)

Abstract In order to investigate the copper underpotential deposition (upd) on gold single crystals. X-ray surface diffraction (XRSD) measurements were performed under in-situ conditions. After formation of a complete Cu upd layer, Cu was found to adsorb in a commensurate p (1 × 1) structure in fourfold hollow sites with a vertical distance of 1.4 A from the Au(100) surface. The coverage obtained by XRSD was (65 ± 8)%. The structure of the Cu upd layer was studied as a function of applied potential and of the Cu-ion concentration in solution. The Cu-ion concentration did not affect the structure of the Cu upd layer. As a function of the applied potential, it was possible to investigate the formation of the Cu upd layer by XRSD. From a comparison between the XRSD results and the cyclic voltammetry, the formation of an ionic adlayer is inferred for the Cu upd on Au(100) in perchloric acid.