Crystalline-amorphous M@MNx (M = Co, Fe, Ni) encapsulated in nitrogen-doped carbon for enhanced efficient and durable hydrogen evolution reaction

Developing earth-abundant electrocatalysts for hydrogen evolution reaction (HER) is important for the sustainable energy economy. Herein, efficient and stable heterocatalysts consisting of crystalline-amorphous M@MNx (M = Co, Fe, Ni) encapsulated in N-doped carbon layers supported with N-doped graphene sheets (denoted as M@MNx@NC-NG) are synthesized by facile hydrothermal reaction and nitridation process. During the nitriding process, metal ions in M(tzbc)(2)(H2O)(4) (tzbc = 4-(1H-1,2,4-triazol-1-yl) benzoic acid) complexes are reduced to crystalline M cores, accompanied by the formation of amorphous MNx shells; the tzbc ligands are in-situ carbonized to form outermost N-doped carbon (NC) layers that connect with inner MNx via M-N-C motifs inherited from the complex precursors and inhibit the transition of MNx from amorphous to crystalline phase. The Co@CoNx@NC-NG catalyst exhibits excellent HER activity with small overpotentials of 45 and 64 mV at a cathode current density of 10 mA<middle dot>cm(-2) and low Tafel slopes of 40 and 85 mV<middle dot>dec(-1) in 0.5 mol<middle dot>L-1 H2SO4 and 1.0 mol<middle dot>L-1 KOH electrolytes, respectively. The Co@CoNx@NC-NG retains 97% of the initial overpotential after 100,000 s in both acidic and alkaline media. Such outstanding HER performance originates from the crystalline-amorphous Co@CoNx that redistributes electrons around the heterointerfaces, facilitating the conversion process of H+/H2O to hydrogen and thereby promoting HER kinetics. The outermost NC layers serve as the armor of Co@CoNx, and graphene nanosheets act as carriers of egg-like Co@CoNx@NC and conduction paths for electron shuttles, ensuring stable and continuous electrocatalytic hydrogen production.