Electrochemical hydrogen storage properties of Mg100−xNix produced by hydriding combustion synthesis and mechanical milling

Abstract

In this work, Mg-based hydrogen storage composites with an initial 100-x: x (x=25, 32.3, 50, 66.7) of Mg:Ni molar ratio were prepared by HCS+MM and their phase compositions and electrochemical performances were investigated in detail. The results show that the composites with desirable constituents can be achieved by adjusting the molar ratio of the starting materials in the HCS process. Particularly, the HCS product of Mg67.7Ni32.3 consists of the principal phase Mg2NiH4 and minor phase Mg2NiH0.3. The dominate phase varies from Mg2NiH0.3 and MgH2 for the Mg enriched sample (x<32.3) to MgNi2 and Ni for the Ni enriched sample (x>32.3). The MM modification not only brings about grain refinement of the alloys, but also leads to phase transformation of part Mg2NiH4 to Mg2NiH0.3 in the Mg67.7Ni32.3 sample. Electrochemical tests indicate that each sample can reach its maximum discharge capacity at the first cycle. Mg67.7Ni32.3 displays the highest discharge capacity as well as a superior electrochemical kinetics owing to its excellent H atom diffusion ability and lower charge-transfer resistance. The Mg67.7Ni32.3 provides the most optimized Mg/Ni atomic ratio considering the comprehensive electrochemical properties of all samples.