Fe₅C₂ nanoparticles as low-cost HER electrocatalyst: the importance of Co substitution

Constructing and understanding the doping effect of secondary metal in transition metal carbide (TMC) catalysts is pivotal for the design of low-cost hydrogen evolution reaction (HER) electrocatalysts. In this work, we developed a wet-chemistry strategy for synthesizing Co-modified Fe₅C₂ nanoparticles ((Fe_1-xCo_x)₅C₂ NPs) as highly active HER electrocatalysts in basic solution. The structure of (Fe_1-xCo_x)₅C₂ NPs was characterized by X-ray diffraction (XRD), extended X-ray absorption fine structure spectra (EXAFS) and scanning/transmission electron microscopy (S/TEM), indicating that the isomorphous substitution of cobalt in the lattice of Fe₅C₂. (Fe_0.75Co_0.25)₅C₂ exhibited the best HER activity (174 mV for j = -10 mA/cm²). Computational calculation results indicate that Co provides the most active site for HER. X-ray adsorption spectra (XAS) studies further suggested that the electron transfer in Fe-C bonds are enhanced by the substitution of Co, which modulates the hydrogen adsorption on the adjacent electronic-enriched carbon, and therefore promotes HER activity. Our results affirm the design of low-cost bimetallic TMCs based HER catalysts.