Resistivity changes during hydrogenation of Pd80Ag20 alloy in non-equilibrium circumstances (original) (raw)

Effects of cathodic charging on hydrogen permeation in a Pd 80Rh 20 alloy

Journal of Alloys and Compounds, 2004

Samples of Pd 80 Rh 20 alloy were submitted to electrochemical hydrogen permeation tests at 40 • C, using a 0.1N NaOH electrolyte, to study hydrogen permeation in a Pd 80 Rh 20 alloy for different cathodic hydrogen generating currents varying from 0.1 to 20 mA. It was found that the apparent hydrogen diffusivity and the hydrogen flux increased with increasing cathodic charging. For high levels of applied current, equal to or above 5 mA the hydrogen permeation curves present a double sigmoidal shape which corresponds to hydride formation during the test. The hydrides were characterized by X-ray diffraction after exposure of the sample to hydrogen. Phase separation was also observed in the alloy, having been provoked, for high applied currents, by the formation of a PdRhH hydride and a Rh-rich phase.

Hydrogen Absorption in Pd–Ag Systems: A TPD and Electrical Resistivity Study

Materials

Hydrogen retention in Pd–Ag (silver 21 wt. %) thin foil has been tested by means of temperature-programmed desorption (TPD) in the temperature range 25–200 °C and compared to the resistivity measurements for the purpose of explaining the characteristic S-shaped resistivity curve and its minimum observed in the same temperature range. The TPD results indicated that the highest uptake of hydrogen was between 65 °C and 105 °C, with a maximum at ~85 °C. Furthermore, in all examined cases, the hydrogen desorption peak was between 140 °C and 180 °C. The resistivity measurements in argon, hydrogen, and vacuum allowed us to examine the influence of hydrogen on the resistivity of a Pd–Ag alloy. The results showed evidence of two kinds of hydrides: (1) a weak absorption at low temperature (T < 70 °C) with the hydrogen present mainly in tetrahedral sites, and (2) a strong absorption up to 150 °C with the hydrogen present mainly in octahedral sites. The behaviour of the electrical resistivit...

Peculiarities of the electrolytic hydrogenation of Pd as revealed by resistivity measurements

Journal of Alloys and Compounds, 2005

In this work, the electrochemical hydrogen doping and withdrawal process of Pd metal was examined with respect to the applicability of Faraday's law. It was found that after a suitable electrolytic pre-treatment, the normalized resistivity (R/R 0 ) versus hydrogen concentration (c F = H/M) curves where c F was determined from Faraday's law by assuming that the 100% current efficiency is independent of the value of the applied charging current density in a wide range. In the ␣-Pd(H) phase (up to c(␣ max ) = 0.015H/M), a good quantitative agreement was found with previous resistivity studies by both electrolytic and gas-phase charging. However, for c > c(␣ max ) our R/R 0 versus c F data did not agree with the R/R 0 versus c H curve from previously reported electrochemical charging experiments of Pd metal where the doped amount of hydrogen, c H , was determined by direct methods. The two sets of resistivity data can be brought into good agreement by assuming that the current efficiency for the absorption process is η a = 1.55. Similarly, for the electrolytic desorption process, an agreement with the previously reported R/R 0 versus c H curve could be achieved by taking η d = 1.9. These current efficiency values higher than unity would imply that each hydrogen absorbed/desorbed by electrolytic charge/discharge processes is accompanied by the absorption/desorption of some excess H atoms by processes not requiring electric charge transfer (i.e., only neutral H atoms are involved). At present, we do not have a physical/chemical picture about the nature of such processes.

Pd–Ag hydrogen content and electrical resistivity: Temperature and pressure effect

International Journal of Hydrogen Energy, 2012

A PdeAg (silver 21 wt.%) thin sheet has been tested in order to measure its electrical resistivity by means of electrochemical impedance spectroscopy under different hydrogenation conditions in the temperature range 25e350 C. The metal sheet has been assembled with the electrical contacts in a gas tight module where pure hydrogen has been fed at a pressure of 100 and 200 kPa. The electrical resistivity vs. both the temperature and hydrogen pressure presents a characteristic S-shape curve with a minimum and a maximum of the resistivity. This behaviour permitted to recognize three stages: (1) introduction of H into PdeAg lattice up to a well-defined H/M ratio increases the resistivity; (2) further uptake of hydrogen with a decrease of resistivity until a higher H/M ratio; (3) above this higher H/M ratio the resistivity increases sharply. The behaviour of the electrical resistivity is discussed in details by considering the hydrogen uploading into the metal lattice, its effect on the conduction electrons and the scattering of the hydrogen atoms into the metal lattice.

Study of the hydrogen absorption in Pd in alkaline solution

Electrochimica Acta, 2008

Hydrogen adsorption/absorption in palladium thin deposits on gold electrode, in 0.1 M NaOH solution, was studied. The contributions of adsorption and absorption to the total charge of hydrogen are separated from the total isotherms at different deposit thicknesses. The adsorbed hydrogen charge increases to a plateau of ∼73.5 C cm −2 , which corresponds to the surface coverage ratio by adsorbed hydrogen of 0.36. The absorbed hydrogen charge agreed with that obtained from the permeation experiments at 50 m Pd foil, at potentials between +100 and +300 mV vs. RHE. EIS was carried out at thin Pd deposits. The kinetics of hydrogen sorption is slower in alkaline solutions than in acids and the isotherms are shifted towards more negative potentials. address: a.lasia@usherbrooke.ca (A. Lasia).

A mechanistic analysis of hydrogen entry into metals during cathodic hydrogen charging

Scripta Metallurgica, 1988

Entry of hydrogen into metals is of serious concern to metallurgists and engineers, since it severely degrades the mechanical properties of metals (1). These problems arise in the cathodic protection of metals, power plants, and in environments where H2S is present as in petroleum refining (2) where the hydrogen evolution reaction (h.e.r.) and hydrogen permeation reaction take place in the corroding or cathodically polarized metal. By performing hydrogen charging experiments of thin samples using the Devanathan-Stachursld cell, the permeation characteristics have been extensively studied (3,4). The present paper seeks to analyze the h.e.r, mechanism and to predict the relationship between the permeation flux and the charging and evolution (recombination) fluxes. A thorough development of the model and actual computations of rate constants and hydrogen coverages will appear elsewhere (5).

The mathematical model of the stripping voltammetry hydrogen evolution/dissolution process on Pd layer

Electrochimica Acta, 2010

The advanced two-plate mathematical model of electrochemical hydrogen evolution/dissolution process has been presented and discussed. The model, with Langmuir adsorption equation, has been experimentally verified by the use of the glassy carbon/Pd layer electrode system at different scan rates. The two cathodic-anodic stages of hydrogen evolution/dissolution process in 0.1 M and 0.001 M HCl solutions have been interpreted and discussed. The thickness of the layer and the way of deposition were also investigated. The fundamental kinetic problem of a change of electrode properties during electrode process as an effect of the elementary hydrogen presence in the solid electrode is presented and interpreted. The isopotential point phenomenon, an electrochemical analog of isosbestic point in absorption spectroscopy, was unexpectedly discovered as experimental effect of hydrogen adsorption and ␣ variability.

HYDROGEN DIFFUSION IN PALLADIUM BY GALVANOSTATIC CHARGING

The diffusion of hydrogen in x-palladium at ambient temperature has been studied by a galvanostatic technique. Single transient and multiple transient hydrogen charging experiments were conducted on specimens over a range of charging currents and specimen thicknesses. The mathematical description of multiple transient charging is developed for the galvanostatic technique from the general solution for diffusion.

ChemInform Abstract: Electrochemical Study of Hydrogen Absorption in Polycrystalline Palladium

ChemInform, 1991

The hydrogen reactions on polycrystalline Pd in 0.1 M NaOH at 25 o C have been studied by using transients at constant potential, and by impedance spectroscopy and X-ray diffraction techniques. At potentials, E,, more positive than the reversible potential, E,, for the Hz evolution reaction, the current-time response and the impedance data indicate H atom diffusion into the bulk Pd. The X-ray diffraction pattern of electrodes cathodized during 20 min at these potentials are similar to those obtained for Pd. At E, < E,, the current transients exhibit a current maximum which increases as Es moves in the negative direction. The Nyquist plot for the rising part of the transients indicates the H atom diffusion into the bulk metal and Hz evolution on the Pd surface at high frequencies. The X-ray diffraction pattern of the electrodes cathodized at Es < E, shows the presence of the pPdH phase and Pd. The experimental results indicate that different reactions take place simultaneously in this potential range: (i) Hz evolution, (ii) H diffusion into the bulk Pd, (iii) nucleation and diffusion-controlled growth of the pPdH phase. Taking into account the contribution of these reactions, a model, which is able to reproduce the experimental current transients, is presented.