2.4. Synthesis of Ni2P/NC

First, 1 mmol NiCl₂·6H₂O and 1.5 mmol NaH₂PO₂ were dissolved in 25 mL deionized water. Then NC in different amounts (20 mg, 40 mg, 60 mg, 80 mg) were added, and the mixture was stirred for 4 h. Then the mixture was put in an ultrasonic bath for 60 min and freeze-dried for 18 h. The mixture was annealed in Ar atmosphere and kept at 300 °C for 30 min with a 5 °C/min heating rate. Finally, the powder was washed with deionized water and ethanol several times, and dried under 120 °C for 6 h. We define the names of materials according to the amount of NC added, which are Ni₂P/NC-20, Ni₂P/NC-40, Ni₂P/NC-60 and Ni₂P/NC-80, respectively. For comparison, Ni₂P/C-60 and Ni₂P/NG-60 catalysts were prepared by using triphenylmethane and NG [35] as carbon sources in the same way.