In situ formed ultrafine metallic Ni from nickel (II) acetylacetonate precursor to realize an exceptional hydrogen storage performance of MgH2Ni-EG nanocomposite

•The ultrafine metallic ni (∼5 nm) was in situ formed in the MgH2Ni-EG nanocomposite by ball milling.•The MgH2Ni-EG nanocomposite can still release extraordinary H2 of 7.03 wt.% within 8.5 min after 10 cycles.•The cycle stability of the MgH2Ni-EG nanocomposite is also excellent.•The evolution process of Ni(acac)2 has been systematically studied.