Effect of alloying elements (Al, Co, Fe, Ni) on the solubility of hydrogen in vanadium: A thermodynamic calculation

Abstract

CALPHAD type thermodynamic assessments for the V–M–H (M = Al, Co, Fe, Ni) hydrogen membrane systems have been carried out on the basis of a newly assessed Co–H binary description, a partly modified Fe–H description and existing thermodynamic descriptions for the other M–H (V–H, Al–H, Ni–H) and V–M (V–Al, V–Co, V–Fe, V–Ni) binary systems. A special attention was paid to estimate the thermodynamic descriptions for the non-existing bcc Al–H, Co–H and Ni–H alloys. Thermodynamic parameters for those binary alloys were estimated by using a theoretical approach (atomistic computation) and fitting limited amount of experimental data for the hydrogen solubility in V-rich bcc ternary alloys. The proposed thermodynamic descriptions predict phase equilibria, especially the effect of alloying elements on the hydrogen solubility in the V-rich bcc alloys, in good agreement with available experimental data. The present thermodynamic descriptions can be easily extended to higher order alloy systems and can provide useful information for alloy design of metallic hydrogen membranes with well-balanced hydrogen permeability and mechanical properties.

Introduction

Demand for hydrogen, a typical alternative energy resource, has grown continuously in recent years [1], [2], [3], [4], [5]. With the application of hydrogen-based energy, the development of the hydrogen-separation technology has become increasingly important [6], [7]. Palladium and its alloys are the most commonly used metallic materials for hydrogen-separation membrane because of its high hydrogen permeability [8], [9]. From an economic perspective, Pd-based membranes are prohibitively expensive to use on large-scale industrial processes. Recently, vanadium has been noticed as a replacement for palladium because of its low cost and high hydrogen permeable characteristic. The intrinsic hydrogen permeability (defined as a product of solubility and diffusivity) of vanadium is much higher than that of palladium–silver alloys which have been commercially used for hydrogen-separation membranes [10]. However, high hydrogen solubility of vanadium causes a hydrogen embrittlement problem that reportedly comes from the formation of hydrides or change of lattice parameter, etc., especially at low temperatures [11], [12]. Therefore, reducing the hydrogen solubility for a given thermodynamic condition could be thought as a way to maintain mechanical properties of membranes.

Alloying can be an effective way to control the hydrogen solubility without changing other thermodynamic conditions: the hydrogen pressure or temperature. Many researchers have carried out experimental studies to investigate the alloying effect on the hydrogen solubility in vanadium alloys such as V–Al [7], V–Co [13], [14], V–Fe [13], [14], V–Ni [10], [15], etc. Although many experiments at various thermodynamic conditions have been done, the information is still limited and not enough to be used for a design of membrane alloys. Meanwhile, the shortage of experimental information can be supplemented by using a thermodynamic calculation technique known as the CALPHAD method [16], [17].

The CALPHAD method [16], [17] is a semi-empirical critical assessment method where thermodynamic properties (Gibbs free energy) of individual phases are modeled and the model parameters are optimized by fitting all available experimental (or theoretical) thermodynamic information. By this, existing experimental thermodynamic property data can be effectively evaluated and a self-consistent thermodynamic description of the relevant system can be obtained. Once the thermodynamic description for a certain system is obtained, various thermodynamic properties can be calculated under arbitrary thermodynamic conditions. For example, Huang et al. [18] carried out a thermodynamic assessment for the Cu–Pd–H system. The calculated phase equilibria using the assessed thermodynamic parameters were in a good agreement with corresponding experimental data. More importantly, the assessed thermodynamic description allowed a calculation of hydrogen solubility under an arbitrary thermodynamic condition where experimental information was not available, and thus provided a strong tool for alloy design of membranes with controlled solubility or permeability.

It would be helpful for a design of vanadium-based alloys with an improved balance between hydrogen permeability and mechanical properties, if such thermodynamic descriptions are available also for vanadium–metal–hydrogen (V–M–H) systems. Indeed such a thermodynamic assessment has been done for the V–H binary system by Ukita et al. [19]. However, thermodynamic descriptions for V–M–H ternary systems, which are essential for the investigation of the effect of alloying elements on the solubility of hydrogen in vanadium, are not available. Therefore, the purpose of the present study is to provide critically assessed thermodynamic descriptions for ternary systems, V–M–H (M = Al, Co, Fe, Ni). Thermodynamic model parameters are determined by an atomistic computation and by fitting available experimental information on the hydrogen solubility in V–M (M = Al, Co, Fe, Ni) binary alloys. The quality of thermodynamic description is demonstrated by comparing thermodynamic calculations with relevant experimental information. The applicability of the present thermodynamic approach to alloy design of vanadium-based membrane alloys will also be discussed.