Fe Loading Triggers High-Spin State: Expediting H₂O/O₂ Swap in Fuel Cell Cathodes

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Fe-N-C catalysts exhibit sluggish oxygen reduction reaction (ORR) kinetics due to strong binding with reaction intermediates.
• Modulating the spin state of active Fe centers is a key strategy to overcome kinetic limitations in ORR.
• Understanding the electronic structure and spin state is crucial for designing efficient Fe-N-C catalysts.

Purpose of the Study:

• To investigate the role of high-spin (HS) Fe2+ active centers in enhancing ORR kinetics.
• To demonstrate the transition from low-spin (LS) Fe3+ to HS Fe2+ via controlled Fe site density.
• To correlate electronic structure modifications with improved catalytic activity and performance.

Main Methods:

• Synthesis of Fe-N-C catalysts with controlled Fe site density.
• Synchrotron-based Fe Kβ X-ray emission spectroscopy (XES) and L-edge soft X-ray absorption spectroscopy (sXAS) for electronic structure analysis.
• Single-cell testing to evaluate electrochemical performance, including power density and current density at a specific potential.

Main Results:

• Successfully achieved a transition from LS Fe3+ to HS Fe2+ (t2g4eg2) by increasing Fe site density, facilitated by the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism.
• Directly observed the redistribution of electrons in t2g and eg orbitals using XES and sXAS, confirming the emergence of unpaired electrons in the dz2 orbital.
• Demonstrated that HS Fe2+ sites promote antibonding orbital occupancy, leading to reduced activation energy and higher turnover frequencies compared to LS states.
• Achieved a peak power density of 1.31 W cm-2 and a current density of 65.1 mA cm-2 at 0.9 V (iR-free) in single-cell tests.

Conclusions:

• High-spin Fe2+ active centers significantly reduce the ORR energy barrier through synergistic regulation of H2O-O2 adsorption.
• The observed electronic structure changes directly correlate with enhanced catalytic activity and improved electrochemical performance.
• The developed Fe-N-C catalyst surpasses the DOE 2025 target, showcasing its potential for advanced energy applications.