O₂-Accessible Fe-N₄ Active Site Density Boosts Efficient Oxygen Reduction to Fuel-Cell Level

Area of Science:

• Materials Science
• Catalysis
• Nanotechnology

Background:

• Intrinsic catalytic activity is often limited by mass transport in practical applications.
• Hierarchical nanostructures are crucial for overcoming diffusion limitations and enhancing catalyst efficacy.
• Atomically dispersed Fe-NC catalysts are promising for oxygen reduction reactions (ORR).

Purpose of the Study:

• To investigate the impact of nanostructure design on oxygen accessibility and ORR performance.
• To develop a pH-dependent strategy for creating hierarchical Fe-NC structures (solid, yolk-shell, hollow).
• To compare the mass transport properties and catalytic activity of different Fe-NC architectures.

Main Methods:

• Synthesis of Fe-NC catalysts with varying nanostructures (solid, yolk-shell, hollow) via pH-dependent manipulation.
• Characterization of Fe-N4 site density and porosity.
• Electrochemical evaluation of ORR activity, including half-wave potential and diffusion-limited current density (j_d).
• Testing in a fuel cell setup to assess power density.

Main Results:

• Yolk-shell Fe-NC (y-Fe/NC) structures exhibited optimized oxygen-accessible active site density (ASD) due to enhanced porosity and connectivity.
• The y-Fe/NC structure demonstrated a higher j_d than theoretical values, attributed to local recirculation effects and increased O2-accessible ASD.
• y-Fe/NC achieved a half-wave potential of 0.82 V and a j_d of 7.66 mA cm⁻², outperforming other structures and state-of-the-art catalysts.
• The optimized y-Fe/NC catalyst maintained high performance in a fuel cell, delivering a power density of 1.03 W cm⁻².

Conclusions:

• Rationally designed hierarchical nanostructures, specifically yolk-shell architectures, are essential for maximizing catalyst performance by ensuring sufficient oxygen accessibility.
• The study highlights the critical role of nanostructure engineering in overcoming mass transport limitations for efficient oxygen reduction reactions.
• Optimized Fe-NC catalysts with hierarchical structures show great potential for applications in fuel cells and other energy conversion technologies.