Electrochemical hydrogen storage properties of Mg_100−xNi_x produced by hydriding combustion synthesis and mechanical milling

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 Mg_67.7Ni_32.3 consists of the principal phase Mg₂NiH₄ and minor phase Mg₂NiH_0.3. The dominate phase varies from Mg₂NiH_0.3 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 Mg₂NiH_0.3 in the Mg_67.7Ni_32.3 sample. Electrochemical tests indicate that each sample can reach its maximum discharge capacity at the first cycle. Mg_67.7Ni_32.3 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 Mg_67.7Ni_32.3 provides the most optimized Mg/Ni atomic ratio considering the comprehensive electrochemical properties of all samples.