Hydrogen　storage　has　long　been　a　priority　on　the　renewable　energy　research　agenda.　Due　to　its　high　volumetric　and　gravimetric　hydrogen　density,　MgH2　is　a　desirable　candidate　for　solid-state　hydrogen　storage.　However,　its　practical　use　is　constrained　by　high　thermal　stability　and　sluggish　kinetics.　Here,　PdNi　bilayer　metallenes　are　reported　as　catalysts　for　hydrogen　storage　of　bulk-MgH2　near　ambient　temperature.　Unprecedented　422　K　beginning　dehydrogenation　temperature　and　up　to　6.36　wt.%　reliable　hydrogen　storage　capacity　are　achieved.　Fast　hydrogen　desorption　is　also　provided　by　the　system　(5.49　wt.%　in　1　h,　523　K).　The　in　situ　generated　PdNi　alloy　clusters　with　suitable　d-band　centers　are　identified　as　the　main　active　sites　during　the　de/re-hydrogenation　process　by　aberration-corrected　transmission　electron　microscopy　and　theoretical　simulations,　while　other　active　species　including　Pd/Ni　pure　phase　clusters　and　Pd/Ni　single　atoms　obtained　via　metallene　ball　milling,　also　enhance　the　reaction.　These　findings　present　fundamental　insights　into　active　species　identification　and　rational　design　of　highly　efficient　hydrogen　storage　materials.