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.