Phase‐Controlled Synthesis of Nickel‐Iron Nitride Nanocrystals Armored with Amorphous N‐Doped Carbon Nanoparticles Nanocubes for Enhanced Overall Water Splitting

Abstract Transition metal nitrides (TMNs) nanostructures possess distinctive electronic, optical, and catalytic properties, showing great promise to apply in clean energy, optoelectronics, and catalysis fields. Nonetheless, phase‐regulation of NiFe‐bimetallic nitrides nanocrystals or nanohybrid architectures confronts challenges and their electrocatalytic overall water splitting (OWS) performances are underexplored. Herein, novel pure‐phase Ni _{2+ x} Fe _{2− x} N nanocrystals armored with amorphous N‐doped carbon (NC) nanoparticles nanocubes (NPNCs) are obtained by controllable nitridation of NiFe‐Prussian‐blue analogues derived oxides/NC NPNCs under Ar/NH ₃ atmosphere. Such Ni _{2+ x} Fe _{2− x} N/NC NPNCs possess mesoporous structures and show enhanced electrocatalytic activity in 1 m KOH electrolyte with the overpotential of 101 and 270 mV to attain 10 and 50 mA cm ^–2 current toward hydrogen and oxygen evolution reactions, outperforming their counterparts (mixed‐phase NiFe ₂ O ₄ /Ni ₃ FeN/NC and NiFe oxides/NC NPNCs). Remarkably, utilizing them as bifunctional catalysts, the assembled Ni _{2+ x} Fe _{2− x} N/NC||Ni _{2+ x} Fe _{2− x} N/NC electrolyzer only needs 1.51 V cell voltage for driving OWS to approach 10 mA cm ^–2 water‐splitting current, exceeding their counterparts and the‐state‐of‐art reported bifunctional catalysts‐based devices, and Pt/C||IrO ₂ couples. Additionally, the Ni _{2+ x} Fe _{2− x} N/NC||Ni _{2+ x} Fe _{2− x} N/NC manifests excellent durability for OWS. The findings presented here may spur the development of advanced TMNs nanostructures by combining phase, structure engineering, and hybridization strategies and stimulate their applications toward OWS or other clean energy fields.