Abstract
In this paper, an overview on thermodynamic aspects related to hydrogen-metal systems in non-ideal conditions is provided, aiming at systematically merging and analyzing information achieved from several different studies present in the open literature. In particular, the relationships among inner morphology, dissolved hydrogen and internal stresses are discussed in detail, putting in evidence the conformation complexity and the various types of dislocations induced by the presence of H-atoms in the lattice. Specifically, it is highlighted that the octahedral sites are preferentially occupied in the FCC metals (such as palladium), whereas tetrahedral sites are more energetically favored in BCC-structured ones (such as vanadium). These characteristics are shown to lead to a different macroscopic behavior of the two classes of metals, especially in terms of solubility and mechanical failure due to the consequent induced stresses. Furthermore, starting from the expression of the chemical potential generally presented in the literature, a new convenient expression of the activity of the H-atoms dissolved into the metal lattice as a function of the H-concentration is achieved. Such an activity expression is then used in the dissolution equilibrium relationship, which is shown to be the overall result of two different phenomena: (i) dissociative adsorption of molecular hydrogen onto the surface; and (ii) atomic hydrogen dissolution from the surface to the metal bulk. In this way, the obtained expression for equilibrium allows a method to calculate the equilibrium composition in non-ideal conditions (high pressure), which are of interest for real industrial applications.
Highlights
Separation processes based on metal membranes with high hydrogen permeance and selectivity have been identified as a promising technology for efficiency improvement and cost reduction of hydrogen production
During the past two decades, the research on potential field of the (Pd)-alloy membranes has led to a technology maturity that appears ready for up-scaling for applications at operating temperatures less than about 550 ◦ C
The present paper aims at providing a critical review of thermodynamics related to metal-H systems, giving general guidelines to improve the performances of metal membranes
Summary
Separation processes based on metal membranes with high hydrogen permeance and selectivity have been identified as a promising technology for efficiency improvement and cost reduction of hydrogen production. Based on the behavior of hydrogen dissolution into the metal lattice, metals can be classified into materials in which solubility is favored by temperature and materials showing the opposite trend The former group, including palladium and vanadium, absorbs hydrogen by means of an exothermic process generating a metal-hydride phase, whereas the latter (e.g., Pt, Fe, Cu, Ni, Ag) absorbs hydrogen endothermically forming a covalent hydride phase [3]. In the fabrication of membranes and hydrogen-storage material, it is important to limit the hydrogen solubility to avoid embrittlement and mechanical failure For this purpose, the main metal is alloyed with others such as Cr, Co and Ni [4]. Current research on metal membranes has been looking for the best combination of materials and manufacturing approaches Such a kind of research risks to turn into a trial-and-error activity if it is not driven by the knowledge of the phenomena involved in the hydrogen-metal system. The present paper aims at providing a critical review of thermodynamics related to metal-H systems, giving general guidelines to improve the performances of metal membranes
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