The recent trend in research fosters the use of abundant seawater for modifying metal surfaces as electrode materials for energy generation, storage, transport, and water-splitting technologies. Economic and ecofriendly seawater is used as a solvent for modifying the surface of 3D nickel-foam (NiF) to Na2O-NiCl2@NiF as an electrode material in electrochemical supercapacitors and water-splitting electrocatalysis applications. The phase of the as-obtained Na2O-NiCl2 is confirmed from the proposed reaction mechanism, followed various physical measurement tests such as X-ray photoelectron spectroscopy and Fourier transform infrared analysis. The formation of Na2O-NiCl2 is caused by a high operation temperature and pressure of seawater solvent, the presence of lone pair electrons on oxygen, and more reactivity of Na for combining with dissolved oxygen than the lone-pair free Cl (towards Ni). In addition to exceptional HER and OER electrocatalytic activities, i.e., 146.3 mV cm-2 and 217 mV cm-2 at a scan rate of 5 mV s-1 to attain the 10 mA cm-2 current density, the Na2O-NiCl2 has demonstrated moderate energy storage ability with considerable durability, i.e., 2533 F g-1 specific capacitance at 3 A g-1 current density even after 2000 redox cycles. The as-assembled Na2O-NiCl2//Na2O-NiCl2 symmetric electrochemical supercapacitor device has ignited a "CNED" panel consisting of nearly forty LEDs with full brightness, offering applied importance in home appliances. In nutshell, seawater-modified metal surfaces can be used for energy storage and water-splitting applications.