High‐Density Strain‐Free Janus Heterojunctions Enable Enhanced Electron Transport Kinetics in CeNbO ₄ /NaNbO ₃ /C Nanocomposite for Superior Overall Water Splitting

Abstract Although sodium niobate (NaNbO 3 ) demonstrates exceptional chemical stability and highly tunable electronic properties for electrochemical overall water splitting, it is fundamentally constrained by intrinsically sluggish charge carrier mobility. Hence, a high‐density strain‐free Janus heterojunction is proposed, which strategically incorporated CeNbO 4 into NaNbO 3 system through a molten salt method to establish higher‐order 4 f‐ 4 d‐ 2 p orbital hybridization. Significant electron accumulation at high‐density strain‐free Janus heterojunctions, driven by charge redistribution from niobium/cerium/sodium to oxygen, activates niobium sites and facilitates efficient electronic transfer between CeNbO 4 and NaNbO 3 . Additionally, strain‐free behavior at the interface enhances stability by eliminating lattice strain and preventing distortion. Accordingly, CeNbO 4 /NaNbO 3 /C demonstrates noteworthy overpotential for hydrogen evolution reaction (70.0 mV at 10 mA cm −2 ) and oxygen evolution reaction (281.4 mV at 10 mA cm −2 ) in alkaline electrolyte. Additionally, the CeNbO 4 /NaNbO 3 /C‐based electrolyze acquires favorable overall water splitting activity (1.65 V) at 10 mA cm −2 , while maintaining impeccable durability for 168 h. Overall, the above consequences demonstrate that high‐density strain‐free Janus heterojunctions provide a novel pathway for developing superior electrocatalysts for next‐generation green energy conversion systems.