The structure and electrochemical hydrogen storage properties of the ballmilled Pr5Mg41 alloy with Ni powders in the range of 100, 150, 200, and 250 wt % were investigated. It was found that with the increase of Ni content, the discharge capacity first increases and then decreases. The ballmilled Pr5Mg41+200 wt % Ni composite has the highest initial discharge capacity (1016.2 mAh/g). Electrochemical impedance spectroscopy and cyclic voltammetry measurements show that the initial increase of the capacity is due to the reduced hydrogen diffusion resistance with the introduction of Ni. However, further increase of the Ni content leads to the reduced discharge capacity due to the excessively higher charge transfer resistance. Because of the lower Mg content in Pr5Mg41 alloy, the ballmilled Pr5Mg41+200 wt % Ni composite has lower maximum discharge capacity and a better cycling retention rate as compared to the previously reported results of the ballmilled PrMg12+200 wt % Ni composite. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2994178] All rights reserved.