Abstract: A novel material Mg-4%Ni-1%NiO for hydrogen storage applications was fabricated by mechanical milling at cryogenic temperature(cryomilling).The effects of ball milling time on the structure morphology and the hydrogen storage performance of the newly developed materials were investigated at cryogenic temperature.The phase structure and surface morphology were analyzed by X-ray diffraction(XRD) and scanning electron microscopy(SEM).The hydrogenation and dehydrogenation characteristics were studied by a pressure-composition-temperature(P-C-T) apparatus.It is shown that the phase structure of the materials remains almost unchanged even after 2,4,and 7h of ball milling,respectively and only a small amount of Mg₂Ni forms.However,a sharp depreciation in the average particle size of the alloy was observed with the ball milling time prolonging.Furthermore,there are Ni and NiO particles in the Mg matrix.In company with the changes above,the activation performance and absorption performance of the materials are gradually improved.The materials ball-milled for 7h can reach the maximal desorption rate after one activation,the onset desorption temperature is 60℃,the compounds exhibit a hydrogen storage capacity of 6.4%(mass fraction) at 200℃ under the hydrogen pressure of 4.0 MPa,and they can absorb 80% of their full hydrogen capacity in 60s and 90% in 10 min.As the ball milling time is prolonged to 4h,the performance of the materials is stable,the onset desorption temperature is 310℃ under 0.1 MPa,and the materials are able to desorb about 80% of their full hydrogen capacity in 500s at 350℃ under the hydrogen pressure of 0.1 MPa.