Enhanced hydrogen sorption capacities and kinetics of Mg 2 Ni alloys by ball-milling with carbon and Pd coating
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A. Nazri R&D, General Motors, Warren, Michigan 48090
J.M. Tarascon Laboratoire de Réactivité et Chimie des Solides, Unite´ Mixte de Recherche (UMR) 6007, Université de Picardie, 33 rue St. Leu, 80039 Amiens, France (Received 5 March 2003; accepted 14 May 2003)
Solid-state hydrogen storage alloys are becoming a practical method to transport and utilize hydrogen as fuel for various technologies. In this paper, the kinetics and capacity of hydrogen desorption from Mg-based alloys have markedly been enhanced by tuning the surface composition of alloy particles. Mg2Ni–Ct,x composites (where t refers to the pregrinding time and x to the Brunauer–Emmet–Teller specific surface area) were prepared by ball-milling the alloy in the presence of preground graphite, and Pd-coated Mg2Ni alloy powders were obtained by controlled chemical deposition of Pd on the alloy surface. We have found that the optimization of the pregrinding step of carbon is a determinant factor in enhancing the hydrogen desorption capacity of the Mg2Ni–10 wt.% C10,320 composites to 2.6 wt.% at 150 °C, the maximum performance so far reported on desorption for Mg-based alloys. Such value can even be raised to 2.8 wt.% by applying Pd deposition on the composite.
With the regaining interest in fuel-cell technologies for transportation, the need to develop a safe hydrogen storage system is crucial. Among many intermetallic alloys exhibiting a reversible hydrogen absorption– desorption, the Mg-based alloys are the most promising due to their high hydrogen storage capacities (up to 7.6 wt.% for pure Mg) and lower cost, as compared with alloys containing rare-earth elements. Unfortunately, the Mg-based alloys have very slow desorption kinetics, particularly below 250 °C,1,2 which limit their practical applications. Improvement in performance of Mg-based alloys has been reported by substitution of Mg or Ni by other elements3,4 and by optimizing the alloy particle size to nanometer scale. For instance, an increase in sorption akinetics at low temperature was achieved by mechanomilling of Mg2Ni alloys,5 increasing the number of grain boundaries and generating a suitable pathway for hydrogen diffusion. However, the performance of these ball-milled alloys remains far from the values required for transportation applications. It is generally known that the permeation of hydrogen through the alloy surface is the rate-controlling step for the sorption process. Slow absorption kinetics a)
e-mail: [email protected] J. Mater. Res., Vol. 18, No. 8, Aug 2003
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can be improved by activating the surface by grinding the Mg–Ni alloy with graphite 6–8 or Pd. 9 In both cases, the mechanically alloyed composites present a faster solid-gas hydrogen absorption as compared to the starting alloys whereas the benefit on desorption is only documented at high temperature (T > 250 °C).10 On the contrary, recent work states that carbon addition induces a negative effect on the electrochemical hydrogenation–dehydrogenation of
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