Axial Phosphate Coordination Driven Spin State Change in FeN₄ Site for Stable Oxygen Reduction

Area of Science:

• Electrocatalysis
• Materials Science
• Nanotechnology

Background:

• Understanding structure-performance relationships in single-atom catalysts (SACs) under operating conditions is crucial but challenging.
• Fe single-atom sites on cellulose-derived carbon are promising electrocatalysts.

Purpose of the Study:

• To investigate the effect of axial phosphate groups on the electronic structure and catalytic performance of Fe-based SACs.
• To enhance the activity, selectivity, and stability of SACs for the oxygen reduction reaction (ORR).

Main Methods:

• Introduction of axial phosphate groups (PO4) into the Fe-SAC microenvironment (P-FeN4/CC).
• Modulation of the Fe center's crystal field (D4h to C4v) and spin state.
• Electrocatalytic testing for ORR, including half-wave potential and current density measurements.
• Long-term stability tests and application in zinc-air batteries (ZABs).
• Theoretical calculations and in situ spectroscopy for mechanistic insights.

Main Results:

• The P-FeN4/CC catalyst exhibited a half-wave potential of 0.97 V and an ultra-high kinetic current density of 179.2 mA cm-2 at 0.85 V.
• The catalyst maintained over 90.5% of its current after 136 hours, outperforming commercial Pt/C-20%.
• Liquid ZABs demonstrated a peak power density of 280.1 mW cm-2 and 11,130 cycles, while flexible ZABs achieved 81 mW cm-2 and stable operation for over 160 hours.

Conclusions:

• Axial phosphate groups dynamically modulate the Fe center's electronic structure and spin state, significantly enhancing ORR performance.
• The ligand-induced spin tunability and magnetic field control offer a new strategy for designing high-performance SACs.
• The P-FeN4/CC catalyst shows great potential for practical applications in energy storage devices like ZABs.