AbstractElectrocatalytic synergy is a functional yet underrated concept in electrocatalysis. Often, it materializes as intermetallic interaction between different metals. We demonstrate interphasic synergy in monometallic structures is as much effective. An interphasic synergy between Ni(OH)₂ and Ni-N/Ni-C phases is reported for alkaline hydrogen evolution reaction that lowers the energy barriers for hydrogen adsorption-desorption and facilitates that of hydroxyl intermediates. This makes ready-to-serve Ni active sites and allocates a large amount of Ni d-states at Fermi level to promote charge redistribution from Ni(OH)₂ to Ni-N/Ni-C and the co-adsorption of H_ads and OH_ads intermediates on Ni-N/Ni-C moieties. As a result, a Ni(OH)₂@Ni-N/Ni-C hetero-hierarchical nanostructure is developed, lowering the overpotentials to deliver −10 and −100 mA cm⁻² in alkaline media by 102 and 113 mV, respectively, compared to monophasic Ni(OH)₂ catalyst. This study unveils the interphasic synergy as an effective strategy to design monometallic electrocatalysts for water splitting and other energy applications.