Mesoscale topology descriptor governing oxygen reduction reaction activity in Fe3C@C catalysts: Role of nanoshell and warped graphene layer multiplicity

• Carbon nanoshells with warped graphene layers (WGLs) were synthesized. • Fe₃C content and N species do not determine activity. • A composite descriptor reflecting mesoscale topology ( f sharp × WGL number) shows strong correlation with ORR activity (R² ≈ 0.94). The catalytic origin of Fe₃C-encapsulated carbon (Fe₃C@C) catalysts remains debated despite their promising oxygen reduction reaction (ORR) performance as non-precious metal alternatives to platinum. Activity has been attributed to Fe₃C cores, interfacial effects, or defect-rich carbon shells; however, no universal descriptor has been established. Here, Fe₃C@C nanoshell catalysts synthesized from glucose–urea–FeCl₃ precursors under controlled carbonization were systematically investigated. Structural analyses using TEM, XRD, Raman spectroscopy, and XPS revealed that the ORR activity could not be explained by the nitrogen content, Fe₃C fraction, or shell thickness. Instead, the activity is governed by the mesoscale carbon topology, particularly by curved, disordered graphitic domains-warped graphene layers (WGLs) decorating the nanoshell surfaces. Quantitative descriptors—the nanoshell development parameter ( f sharp ) from XRD and the WGL number from TEM—were strongly correlated with ORR activity, while their combined form (fsharp × WGL number) provided the most reliable prediction (R² ≈ 0.94). These findings indicate that Fe-based catalytic behavior can be rationalized without explicit reference to Fe sites, emphasizing mesoscale topology as the key factor governing activity. This study establishes a curvature-driven carbon architecture as a universal design principle for Pt-free carbon catalysts.