• A simple one-step high-temperature pyrolysis method fabricates hierarchical NiX@CNT catalysts with tunable non-metallic element doping.
• Twelve catalyst variants were prepared by temperature adjustment, enabling systematic optimization of HER and OER activity.
• Ni(NO3)2@CNT-900 demonstrates superior bifunctional performance with low overpotentials (145 mV for HER, 300 mV for OER at 10 mA/cm²).
• The optimized catalyst exhibits excellent durability in alkaline electrolyte, highlighting its potential for practical water splitting.