Thermodynamic characterization of metal hydrogen systems by assessment of phase diagrams and electrochemical measurements

Abstract

The formation of metal hydrides has been investigated theoretically by the use of ternary thermodynamics and experimentally by applying an electrochemical cell technique. A new theoretical method of describing and investigating metal hydrides has been developed. It involves ternary thermodynamics and interprets the hydrogenation reaction by ternary phase diagrams. This method has the advantage over the conventional “pseudobinary” model in that it allows for disproportionation reactions. Furthermore it describes in a visual way the parameters important for hydrogen storage applications, namely plateau pressure and plateau width. Finally it gives a means of predicting hydrogenation reactions. This method has been applied to the family of Mg-based alloys. A new experimental technique for investigating metal hydrides has also been developed. It involves a nonaqueous low temperature molten salt electrochemical cell. Measurements were done with a saturated solution of NaH in the organometallic salt NaAlEt 4, where the H − ions are the hydrogen transporting species. Several Mg-based alloys were investigated. The thermodynamic calculations are in very good agreement with the experimental results and are in accordance as well with data published in the literature.