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

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 foun d to be sensitively dependent to all the details of the applied cycling con ditions and can be of temporary (reversible) or persistent (irreversible) n ature. Based upon investigations via optical microscopy, the reversible cap acity losses appear to be associated with an excessively high formation rat e of MgH2-nucleation sites on the surface of Ni-doped Mg particles under in tensified cycling conditions; irreversible capacity losses, especially pron ounced in the case of Ni-doped materials, are the result of sintering of th e material particles in the dehydrogenated (metallic) form upon prolonged c ycling at higher temperatures. Ni-doped Mg-MgH2 materials have excellent cyclic stability and high hydroge nation rates even under very mild pressure/temperature cycling conditions ( so-called standard cycling conditions or below them [B. Bogdanovie, Th. Har twig; 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 o f heat and cold, heat pumps, hydrogen storage, and the Like. On the other h and, based on their cyclic stability and sufficient reaction rates under se vere reaction conditions, neat Mg powders produced by brushing can be used as cheap materials for the purpose of reversible thermochemical high temper ature heat storage in the temperature range of 450-500 degrees C with heat storage capacities amounting to 0.6-0.7 kWh/kg Mg, applicable for solar pow er generation via Stirling engines or storage of industrial heat in the abo ve temperature ranges. (C) 1999 Elsevier Science S.A. All rights reserved.