Hydrogen Solubility in Metal Membranes: Critical Review and Re-Elaboration of Literature Data

This study undertakes a thorough examination of hydrogen solubility within various metal-alloy membranes, including those based on palladium (Pd), vanadium (V), niobium (Nb), tantalum (Ta), amorphous alloys and liquid gallium (Ga). The analysis aims to outline the strengths and weaknesses of each material in terms of solubility and permeability performance. The investigation began by acknowledging the dual definitions of solubility found in literature: the "<i>secant method</i>", which calculates solubility based on the hydrogen pressure corresponding to a specific sorbed hydrogen loading, and the "<i>tangent method</i>", which evaluates solubility as the derivative (differential solubility) of the sorption isotherm at various square root values of hydrogen partial pressure. These distinct methodologies yield notably different outcomes. Subsequently, a compilation of experimental data for each membrane type is gathered, and these data are re-analysed to assess both solubility definitions. This enabled a clearer comparison and a deeper analysis of membrane behaviour across different conditions of temperature, pressure, and composition in terms of hydrogen solubility in the metal matrix. The re-evaluation presented in this study serves to identify the most suitable membranes for hydrogen separation or storage, as well as to pinpoint the threshold of embrittlement resulting from hydrogen accumulation within the metal lattice. Lastly, recent research has indicated that particularly promising membranes are those fashioned as "sandwich" structures using liquid gallium. These membranes demonstrate resistance to embrittlement while exhibiting superior performance characteristics.