Ni-doped versus undoped Mg–MgH2 materials for high temperature heat or hydrogen storage

Abstract

A comparative study of the behavior of various Ni-doped and undoped Mg–MgH2 materials to be utilized for reversible (thermochemical) high temperature heat or hydrogen storage has for the first time been conducted over a broad range of hydrogenation/dehydrogenation (cycling) conditions (see Fig. 5). The storage capacity losses observed in the course of cyclic tests are found to be sensitively dependent to all the details of the applied cycling conditions and can be of temporary (reversible) or persistent (irreversible) nature. Based upon investigations via optical microscopy, the reversible capacity losses appear to be associated with an excessively high formation rate of MgH2-nucleation sites on the surface of Ni-doped Mg particles under intensified cycling conditions; irreversible capacity losses, especially pronounced in the case of Ni-doped materials, are the result of sintering of the material particles in the dehydrogenated (metallic) form upon prolonged cycling at higher temperatures. Ni-doped Mg–MgH2 materials have excellent cyclic stability and high hydrogenation rates even under very mild pressure/temperature cycling conditions (so-called standard cycling conditions or below them [B. Bogdanović, Th. Hartwig, B. Spliethoff, Int. J. Hydrogen Energy 18 (1993) 575; Final Report of Project No. 0328939 C, Federal Ministry for Research and Technology of the F.R.G., Bonn (1992)]) suitable for applications such as solar generation of heat and cold, heat pumps, hydrogen storage, and the like. On the other hand, based on their cyclic stability and sufficient reaction rates under severe reaction conditions, neat Mg powders produced by brushing can be used as cheap materials for the purpose of reversible thermochemical high temperature heat storage in the temperature range of 450–500°C with heat storage capacities amounting to 0.6–0.7 kWh/kg Mg, applicable for solar power generation via Stirling engines or storage of industrial heat in the above temperature ranges.