Development of high efficient and stable water oxidation catalysts is essential for the realization of in-dustrial water-splitting systems.Herein,a novel approach involving an in-situ transformation of Co(OH)2 nanosheets into NH4CoPO4·H2O nanoplates on Co foil is reported.As a 3D self-supported oxygen rev-olution reaction(OER)electrocatalyst,the as-prepared NH4CoPO4·H2O/Co exhibits remarkable catalytic activity and exceptional stability.Specifically,it can deliver a current density of 10 mA cm-2 at a quite low overpotential of 254 mV with a small Tafel slope of 64.4 mV dec-1 in alkaline electrolyte.Through experimental study and theoretical analysis,the excellent OER performance can be attributed to enriched exposed active sites,favorable electron/proton transfer and mass transport,and its unique asymmetric local atomic and electronic structure.Thus,this present research not only provides a practicable in-situ transformation strategy to design 3D self-supported electrocatalysts,but also enlightens a new way of developing transition-metal phosphates for efficient and stable water oxidation at atomic level.