Molecular Strain Accelerates Electron Transfer for Enhanced Oxygen Reduction.

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Iron-nitrogen-carbon (Fe-N-C) materials are promising alternatives to platinum catalysts for the oxygen reduction reaction (ORR).
• Sluggish ORR kinetics in Fe-N-C materials lead to high overpotentials, limiting their efficiency in energy conversion devices.

Purpose of the Study:

• To investigate the effect of local molecular strain on the ORR performance of Fe-N-C catalysts.
• To elucidate the mechanism by which strain influences ORR kinetics using iron phthalocyanine (FePc) as a model system.

Main Methods:

• Density functional theory (DFT) calculations to predict the ORR mechanism and energy barriers.
• Experimental synthesis and electrochemical characterization of strained FePc catalysts on single-walled carbon nanotubes.
• Integration of the optimized catalyst into a zinc-air battery for performance evaluation.

Main Results:

• DFT calculations revealed that molecular strain accelerates the reductive desorption of *OH by decreasing the energy barrier by approximately 60 meV.
• Experimentally, strained FePc achieved a half-wave potential (E1/2) of 0.952 V and a Tafel slope of 35.7 mV dec⁻¹, competitive with state-of-the-art Fe-N-C catalysts.
• A 70 mV shift in E1/2 and distinct Tafel slopes were observed for flat versus curved FePc configurations, aligning with theoretical predictions.

Conclusions:

• Molecular strain is an effective strategy for enhancing the ORR activity of Fe-N-C materials.
• The findings provide a pathway for designing high-performance catalysts for renewable energy applications by controlling catalyst structure.
• The strained FePc catalyst demonstrated excellent performance in a zinc-air battery, achieving a maximum power density of 350.6 mW cm⁻².