This study presents a three-dimensional (3D) core-shell metal-nitride catalyst, NiMoN@NiFeN, for efficient alkaline seawater electrolysis. The catalyst consists of NiFeN nanoparticles uniformly decorated on NiMoN nanorods supported on Ni foam, which serves as an active and durable oxygen evolution reaction (OER) catalyst. Combined with an efficient hydrogen evolution reaction (HER) catalyst of NiMoN nanorods, the catalyst achieves current densities of 500 and 1000 mA cm⁻² at record low voltages of 1.608 and 1.709 V, respectively, for overall alkaline seawater splitting at 60°C. The catalyst demonstrates excellent stability and durability, with minimal degradation over 48 hours of operation. The 3D core-shell nanostructure provides a large surface area with numerous active sites, efficient charge transfer, and rapid gaseous product release, contributing to the enhanced OER performance. The catalyst also exhibits strong corrosion resistance to chloride anions in seawater. The study highlights the potential of non-noble metal-nitride based electrocatalysts for large-scale hydrogen production from seawater. The electrolyzer can be driven by an AA battery or a commercial thermoelectric (TE) module, demonstrating great potential and flexibility in utilizing a broad range of power sources. The results show that the NiMoN@NiFeN catalyst is highly efficient for alkaline seawater splitting, with a current density of 100 and 500 mA cm⁻² at overpotentials of 307 and 369 mV, respectively, in natural seawater. The catalyst also exhibits excellent HER activity, with a Tafel slope of 45.6 mV dec⁻¹. The study demonstrates the potential of non-noble metal-nitride based electrocatalysts for efficient and sustainable seawater electrolysis.This study presents a three-dimensional (3D) core-shell metal-nitride catalyst, NiMoN@NiFeN, for efficient alkaline seawater electrolysis. The catalyst consists of NiFeN nanoparticles uniformly decorated on NiMoN nanorods supported on Ni foam, which serves as an active and durable oxygen evolution reaction (OER) catalyst. Combined with an efficient hydrogen evolution reaction (HER) catalyst of NiMoN nanorods, the catalyst achieves current densities of 500 and 1000 mA cm⁻² at record low voltages of 1.608 and 1.709 V, respectively, for overall alkaline seawater splitting at 60°C. The catalyst demonstrates excellent stability and durability, with minimal degradation over 48 hours of operation. The 3D core-shell nanostructure provides a large surface area with numerous active sites, efficient charge transfer, and rapid gaseous product release, contributing to the enhanced OER performance. The catalyst also exhibits strong corrosion resistance to chloride anions in seawater. The study highlights the potential of non-noble metal-nitride based electrocatalysts for large-scale hydrogen production from seawater. The electrolyzer can be driven by an AA battery or a commercial thermoelectric (TE) module, demonstrating great potential and flexibility in utilizing a broad range of power sources. The results show that the NiMoN@NiFeN catalyst is highly efficient for alkaline seawater splitting, with a current density of 100 and 500 mA cm⁻² at overpotentials of 307 and 369 mV, respectively, in natural seawater. The catalyst also exhibits excellent HER activity, with a Tafel slope of 45.6 mV dec⁻¹. The study demonstrates the potential of non-noble metal-nitride based electrocatalysts for efficient and sustainable seawater electrolysis.