Surface　segregation　constitutes　an　efficient　approach　to　enhance　the　alkaline　hydrogen　evolution　reaction　(HER)　activity　of　bimetallic　PtxNiy　nanoalloys.　Herein,　a　new　strategy　is　proposed　by　utilizing　the　small　gas　molecule　of　H-2　as　the　structure　directing　agent　(SDA)　to　in　situ　induce　Pt　surface　segregations　over　a　series　of　PtNi5-n　samples　with　extremely　low　Pt　doping　(Pt/Ni　=　0.2).　Impressively,　the　sample　of　PtNi5-0.3　synthesized　under　0.3　MPa　H-2　delivers　an　extremely　low　overpotential　of　26.8　mV　(-10　mA　cm(-2))　and　Tafel　slope　of　19.2　mV　dec(-1),　which　is　superior　to　most　of　the　previously　reported　PtxNiy　electrocatalysts.　This　is　substantially　related　to　the　strong　H-2　in　situ　inducing　effect　to　generate　Pt-rich@Ni-rich　core-shell　nanostructure　of　PtNi5-0.3　with　an　ultrahigh　Pt　surface　content　of　46%.　The　specific　mechanistic　effects　of　H-2　during　the　PtNi5-n　synthesis　process　are　well　illustrated　based　on　the　combined　experimental　and　theoretical　studies.　The　density　functional　theory　mechanism　simulations　further　unravel　that　the　evolved　active　site　of　PtNi5-n　can　efficiently　reduce　the　reaction　Gibbs　free　energies;　especially　for　the　scenario　of　PtNi5-0.3,　the　downward-shifted　d　band　center　of　the　Pt　active　site　significantly　reduces　the　Pt-H　bond　strength,　eventually　resulting　in　the　lowest　absolute　value　of　Delta　G(H).