Understanding the Role of Mesopores on Porous Fe-N/C Catalysts for Oxygen Reduction Using Electrochemical Impedance Spectroscopy

The Fe-N/C catalysts have been identified as promising alternatives to Pt catalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. The pores in these carbon materials significantly influence the oxygen reduction activity and mechanisms. In this study, electrochemical impedance spectroscopy (EIS) is utilized to study the interfacial structure of porous materials distinguished by pore size. The Fe-N/C and metal-free N/C porous catalysts are synthesized by using an SBA-15 mesoporous silica template, resulting in materials with ordered morphology and a range of pore sizes from micropores to narrow and wide mesopores. Hydrodynamic voltammetry experiments indicate that Fe-N/C catalysts with wide mesopores exhibit better ORR activity and enhanced mass transport compared with metal-free N/C and microporous catalysts. The EIS results show that these catalysts exhibit the lowest overall resistance and the highest adsorption capacitance at the half-wave potential (E1/2), indicating optimal operating conditions for practical applications. The impedance response highly depends on pore size, with the lowest mass transport resistance observed in wide mesoporous materials. The addition of iron precursors creates Fe-Nx sites and produces wide mesopores in the carbon material despite using mesoporous templates. This study underscores the importance of a diverse range of pores in influencing the ORR activity and mechanisms.