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

Abstract 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@MN x (M = Co, Fe, Ni) encapsulated in N‐doped carbon layers supported with N‐doped graphene sheets (denoted as M@MN x @NC‐NG) are synthesized by facile hydrothermal reaction and nitridation process. During the nitriding process, metal ions in M(tzbc) 2 (H 2 O) 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 MN x shells; the tzbc ligands are in‐situ carbonized to form outermost N‐doped carbon (NC) layers that connect with inner MN x via M–N–C motifs inherited from the complex precursors and inhibit the transition of MN x from amorphous to crystalline phase. The Co@CoN x @NC‐NG catalyst exhibits excellent HER activity with small overpotentials of 45 and 64 mV at a cathode current density of 10 mA·cm −2 and low Tafel slopes of 40 and 85 mV·dec −1 in 0.5 mol·L −1 H 2 SO 4 and 1.0 mol·L −1 KOH electrolytes, respectively. The Co@CoN x @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@CoN x that redistributes electrons around the heterointerfaces, facilitating the conversion process of H + /H 2 O to hydrogen and thereby promoting HER kinetics. The outermost NC layers serve as the armor of Co@CoN x , and graphene nanosheets act as carriers of egg‐like Co@CoN x @NC and conduction paths for electron shuttles, ensuring stable and continuous electrocatalytic hydrogen production.