Surface carbon layer controllable Ni₃Fe particles confined in hierarchical N-doped carbon framework boosting oxygen evolution reaction

Developing high-efficiency and low-cost catalysts towards oxygen evolution reaction (OER) is extremely important for overall water splitting and rechargeable metal−air batteries. Herein we propose a promising organometallic coordination polymer (OCP) induced strategy to construct hierarchical N-doped carbon framework with NiFe nanoparticles encapsulated inside (N_xFe@N–C) as a highly active and stable OER catalyst. The synthesis of OCP precursor depends on the unique molecular structure of iminodiacetonitrile (IDAN), which can coordinate with metal ions to form Ni₂Fe(CN)₆ with prussian blue analogs (PBA) structure. Unlike previous PBA-induced methods, the thickness of the carbon layer covering the surface of the metal core can be well controlled during the pyrolysis through adjusting the amount of IDAN, which builds a wonderful bridge for investigating the relationship between carbon layer thickness and catalytic performance. Both the experimental characterizations and theoretical studies validate that a suitable carbon layers thickness leads to optimal OER activity and stability. By optimizing the structure and composition, the optimized Ni₃Fe@N–C with hierarchical framework exhibits the low overpotentials (260 ​mV at 10 ​mA ​cm⁻²; 320 ​mV at 50 ​mA ​cm⁻²), improved kinetics (79 ​mV dec⁻¹), and robust long-term stability, which exceeds those of benchmark RuO₂.