Overall seawater electrolysis is an important direction for the development of hydrogen energy conversion. The key issues include how to achieve high selectivity, activity, and stability in seawater electrolysis reactions. In this report, the heterostructures of graphdiyne-RhO _x -graphdiyne (GDY/RhO _x /GDY) were constructed by in situ-controlled growth of GDY on RhO _x nanocrystals. A double layer interface of sp -hybridized carbon-oxide-Rhodium ( sp -C∼O-Rh) was formed in this system. The microstructures at the interface are composed of active sites of sp -C∼O-Rh. The obvious electron-withdrawing surface enhances the catalytic activity with orders of magnitude, while the GDY outer of the metal oxides guarantees the stability. The electron-donating and withdrawing sp -C∼O-Rh structures enhance the catalytic activity, achieving high-performance overall seawater electrolysis with very small cell voltages of 1.42 and 1.52 V at large current densities of 10 and 500 mA cm ⁻² at room temperatures and ambient pressures, respectively. The compositional and structural superiority of the GDY-derived sp -C-metal-oxide active center offers great opportunities to engineer tunable redox properties and catalytic performance for seawater electrolysis and beyond. This is a typical successful example of the rational design of catalytic systems.