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Quantitative evaluation of hydrogen absorption by detecting non-absorbed hydrogen in electrochemical hydrogen permeation test
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Hydrogen evolution reaction, HER, is the simplest electrocatalytic reaction. With development of renewable energy sources electrolytic production of hydrogen becomes an alternative way of hydrogen production for internal combustion engines and fuel cells. HER
proceeds through the Volmer, Heyrovsky and Tafel reaction mechanism. The formal kinetics of HER is described briefly in many books but detailed description is only found in research papers. In this review detailed kinetic description of HER is presented for stationary
measurements, electrochemical impedance spectroscopy, open circuit potential decay and current step techniques. It is illustrated by examples and limitations of the determination of kinetic parameters are discussed. Finally, experimental conditions necessary for obtaining good experimental data are presented.
The Devanathan-Stachurski-cell is the most commonly used electrochemical technique for the investigation of the hydrogen trapping properties of a material. In this set-up, a flux of hydrogen atoms diffuses through a metal membrane under study. The atoms are subjected to the heterogeneities naturally present in the lattice of the metal, acting as traps. This matter influences the diffusion of hydrogen, which is strongly microstructure related. In this paper, we apply a numerical model to different iron-alloys to determine the hydrogen trapping parameters characterizing the materials. We highlight the complexity of the hydrogen trapping mechanism which needs to be further investigated.
The behavior of hydrogen in metals has attracted considerable interest in various areas, particularly materials degradation and energy conversion. The adverse effect of hydrogen on the mechanical properties of steels and other alloys has long been known and has prompted numerous studies of hydrogen in metals and alloys aimed at understanding the processes leading to hydrogen embrittlement.1–3 The use of hydrogen as a reactant in batteries and fuel cells has also promoted research on the storage of hydrogen either in the hydride or molecular form.4–7 In both the embrittlement and energy areas, the demand for improved performance of materials has created an emphasis on acquiring a knowledge of the absorption and transport characteristics of hydrogen in metals.
The mobility of dissolved hydrogen in an iron lattice having a population of extraordinary, or trapping, sites for hydrogen is analyzed under the assumption of local equilibrium between the mobile and the trapped populations. It is shown that at low coverage of the trapping sites the usual solutions of the diffusion equations can be used to analyze the experimental results and that the effective diffusivity is a function of trap density and of the magnitude of the trap depth. In the domain of coverage in which the activity of trapped hydrogen is not linear in the fractional occupation of the trap population, the concept of phenomenological diffusivity becomes non-operational and the diffusion equation must be solved with terms for sources and sinks, as done by McNabb and Foster.
A sensitive electrochemical technique, which permits the recording of the instantaneous rate of permeation of electrolytic hydrogen through palladium, is described. Results were obtained under conditions required by theory for the diffusion of hydrogen with the use of electronic potentiostats. Analysis of the results shows the validity of the equations previously deduced for the diffusion of hydrogen. No anomalies in the diffusion have been found under these conditions. Thus the diffusion constant is found to be independent of thickness in the range 0.0035 to 0.054 cm. The permeation has been found to be inversely proportional to thickness as required by theory. The diffusion constant for a hydrogen poor alpha-palladium has been found to be 1.30 ± 0.20 x 10-7 cm^2 s-1 at room temperature. Reasons for permeation anomalies reported in the literature for diffusion of hydrogen from the gas phase are discussed. Attention is drawn to errors in the classical time lag determination which unless corrected, can give rise to spurious thickness and temperature dependence of the diffusion constant.
Analysis was made of the influence of reversible and irreversible traps on the diffusion of hydrogen in a system of low hydrogen concentration and of low coverage of trap sites. It is shown that the steady state hydrogen concentration in a slab of thickness a in permeation test is influenced by the trap strength of irreversible traps, k, and deviates from linearity, being given by co sinh(√kxa) sinh√k The maximum attainable permeation flux is shown to be given by ( Dc0 a). (√ k sinh √ K), being smaller than Dc0 a. The influence of reversible and irreversible traps on the apparent diffusion constant, Da, of hydrogen as measured by the 'time lag' method is multiplicative: Da = D (1 + Nk P)f(k), where f(k) = 3 k( √k tanh√k-1); parameters appearing above are defined in the text.
Recent studies of the characteristics and mechanism of hydrogen related failure in steels are overviewed. Based on an analysis of the states of hydrogen in steels, the role of hydrogen in reducing ductile crack growth resistance is attributed to the increased creation of vacancies on straining. Cases showing the involvement of strain induced vacancies in susceptibility to fracture are presented. The function of hydrogen is ascribed to an increase in the density of vacancies and their agglomeration, rather than hydrogen itself, through interactions between vacancies and hydrogen. The newly proposed mechanism of hydrogen related failure is supported by a recent finding of amorphisation associated with crack growth.
The dependence of the exchange current for the electrolytic evolution of hydrogen on metals (i0,H) on the work function () is analyzed on the basic of a new list offor polycrystalline surfaces. It is shown that log i0,H is linearly related toirrespective of the detailed nature of the mechanism involved in the rate determining step (rds). i0,H on sp metals depends on the sign of the surface charge. Results confirm previous suggestions that the main difference in double layer structure between sp and transition metals arises as a result of hindered rotation of water molecules on the latter. If the strength of the M−H bond is taken into consideration, then metals divide into two groups: (a) sp metals, with slow discharge at usual overvoltages, and probably slow hydrogen removal close to equilibrium, and (b) transition metals, with slow hydrogen removal as the rate determining step. Mn is anomalous and cannot be assigned to either class in correlations. Dependence of M−H bond strength onis also shown and discussed.
A new analysis of the diffusion of hydrogen in iron and ferritic steels
- McNabb