Spin-regulated Fe-N-C catalyst enabled by adjusting coordination nitrogen species for robust oxygen reduction

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Iron-nitrogen-carbon (Fe-N-C) catalysts are promising alternatives to platinum for the oxygen reduction reaction (ORR).
• Conventional Fe-N₄ active sites often exhibit low-spin configurations, leading to strong oxygen intermediate adsorption and limiting ORR efficiency.
• Optimizing the electronic structure of active sites is crucial for enhancing catalyst performance.

Purpose of the Study:

• To investigate the influence of adjacent graphitic nitrogen (N_GC) on the spin state of Fe-N₄ sites in Fe-N-C catalysts.
• To develop a cost-effective method for preparing Fe-N-C catalysts with modulated spin states for improved ORR activity.
• To evaluate the performance of these optimized catalysts in electrochemical applications, including Zn-air batteries.

Main Methods:

• Theoretical calculations to elucidate the effect of N_GC on Fe spin state and oxygen intermediate binding.
• Synthesis of Fe-N-C catalysts using electrospinning and controlled thermal annealing with inexpensive precursors.
• Electrochemical characterization of ORR activity and durability using rotating disk electrode (RDE) voltammetry.
• Performance evaluation in Zn-air batteries (ZABs) and quasi-solid ZABs.
• In-situ characterization techniques to confirm spin-state-promoted performance enhancement.

Main Results:

• Adjacent N_GC effectively regulates the spin state of Fe sites, facilitating electron transfer and optimizing oxygen intermediate desorption.
• The synthesized Fe-N-C catalyst exhibits superior ORR activity compared to benchmark Pt/C.
• The catalyst demonstrates excellent durability with minimal voltage decay over 10,000 cycles.
• Optimized catalysts achieve a peak power density of 225 mW cm^-2 in Zn-air batteries.
• Quasi-solid ZABs with the catalyst show robust performance under bending conditions, powering electronic devices.

Conclusions:

• Modulating the Fe spin state via adjacent N_GC is an effective strategy to enhance ORR performance in Fe-N-C catalysts.
• The developed Fe-N-C catalysts offer a cost-effective and high-performance alternative for ORR and energy storage applications.
• The findings provide a pathway for designing advanced electrocatalysts for wearable electronics and other energy devices.