Electrospinning Equipment for Research| Active-Sites-ntegrated Hierarchical Porous Nanofbers for lmproved Oxygen Reduction in Fuel Cells

Professor Xiang Zhonghua's Team from Beijing University of Chemical Technology: Hierarchical Porous Nanofibers Integrated with Active Sites for Efficient Oxygen Reduction in Fuel Cells

With the rapid development of the hydrogen energy industry, the oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells (PEMFCs) heavily relies on precious metal platinum catalysts, resulting in high costs. Although Fe-N-C non-precious metal catalysts exhibit excellent activity in rotating disk electrode (RDE) tests, their practical performance in membrane electrode assemblies (MEAs) is limited by mass transfer resistance caused by thick electrode layers and structural defects dominated by micropores, making it difficult to meet the stringent gas-liquid-solid three-phase interface requirements of fuel cells.

Recently, Professor Xiang Zhonghua's team from Beijing University of Chemical Technology published a research paper titled "Active-Sites-Integrated Hierarchical Porous Nanofibers for Improved Oxygen Reduction in Fuel Cells" in Small. The team successfully prepared a self-supporting hierarchical porous carbon nanofiber membrane catalyst through a synergistic strategy combining electrospinning and ammonia etching. This material integrates atomically dispersed sites with iron atomic clusters and constructs interconnected meso/macroporous networks, significantly improving fuel cell mass transfer efficiency. The MEA based on this catalyst achieved a maximum power density of 0.79 W cm-2, representing a 43% improvement over traditional ZIF-based catalysts, opening new pathways for high-performance non-precious metal catalyst design.

Fig. 1: Fabrication and morphology of FeSA/AC-N-PCNFs self-supporting electrocatalyst

The ZIF-loaded nanofiber precursor (with rough surfaces distinct from smooth pure polymer fibers) was prepared by electrospinning (containing Fe/Zn-ZIFs, PVP, PAN). After NH₃-assisted pyrolysis activation, the FeSA/AC-N-PCNFs catalyst was obtained. NH₃ preferentially etched disordered carbon components, significantly increasing surface porosity and specific surface area (BET: 570 m² g⁻¹ vs. 381 m² g⁻¹ for FeSA-N-PCNFs). More importantly, its pore distribution exhibited a unique hierarchical structure dominated by macropores and mesopores (Fig. 1g, 1h), contrasting sharply with traditional ZIF-only FeSA-N-C catalysts (mainly producing micropores). This optimized hierarchical porous structure originated from nanofiber 3D network engineering, effectively promoting pore connectivity. This meso/macroporous-dominated structure significantly reduced mass transfer resistance within the catalyst layer, laying the foundation for subsequent MEA performance improvement.

Elemental mapping confirmed uniform distribution of Fe, C, N, O. 

Fig. 2: Active site analysis of FeSA/AC-N-PCNFs self-supporting membrane

As shown in Fig. 2a, HAADF-STEM observed coexistence of Fe atomic clusters (ACs) and FeN₄ sites on the FeSA/AC-N-PCNFs carbon support. XPS analysis revealed that compared to FeSA-N-PCNFs, FeSA/AC-N-PCNFs catalyst had higher graphitic nitrogen content (38.62% vs 17.25%) and lower pyridinic nitrogen content (26.18% vs 63.88%), contrary to conventional understanding (NH₃ etching increases defects). Research showed NH₃ preferentially etched unstable defective carbon associated with pyridinic nitrogen and formed additional graphitic nitrogen through carbon volatilization redeposition, thereby improving carbon matrix graphitization (confirmed by reduced Raman ID/IG ratio). This graphitization degree first increased then decreased (with NH₃ time variation), enhancing carbon corrosion resistance and significantly improving catalyst stability.

Fig. 3: Three-electrode electrochemical performance analysis of FeSA/AC-N-PCNFs self-supporting membrane

RDE tests compared three Fe-N-C catalysts' ORR performance in acidic media. FeSA/AC-N-PCNFs exhibited optimal activity, with half-wave potential (0.80 V vs. RHE) approaching Pt/C (0.82 V). This was attributed to: 1) NH₃ etching increased porosity and specific surface area, improving accessible active site density; 2) Synergy between Fe atomic clusters and FeN₄ sites promoted charge transfer; 3) High graphitization degree (lowest Tafel slope) improved electron conduction. RRDE tests confirmed highest selectivity (H₂O₂ yield <2%, electron transfer number ≈4) and largest electrochemical active surface area (427.6 m² g⁻¹). Fe clusters significantly improved FeN₄ sites' intrinsic activity (60% TOF increase) and mass activity. More importantly, high graphitization effectively suppressed carbon corrosion, with only 18 mV E₁/₂ decay after 10,000 CV cycles, far exceeding FeSA-N-PCNFs and traditional FeSA-N-C catalysts in durability, outstanding among acidic ORR catalysts.

Fig. 4: Theoretical analysis of ORR active sites in FeSA/AC-N-PCNFs catalyst

DFT calculations revealed synergistic effects between Fe atomic clusters (Fe₄-N₆) and single-atom Fe-N₄ sites. A Fe-N₄/Fe₄-N₆ coexistence model was constructed (Fig. 4a). Gibbs free energy calculations showed that compared to single Fe-N₄ sites (overpotential 0.67 eV), Fe-N₄/Fe₄-N₆ (0.63 eV) and Fe-N₄-OH/Fe₄-N₆ (0.63 eV) significantly reduced ORR rate-determining step (OH* → H₂O) overpotential (Fig. 4b). Fe₄-N₆ clusters promoted charge transfer (charge density difference analysis, Fig. 4c) and optimized O* adsorption energy (especially for Fe-N₄-OH/Fe₄-N₆). Density of states (DOS) analysis (Fig. 4d) showed enhanced bonding between Fe 3d and N 2p orbitals in coexistence systems, with significantly downshifted d-band centers (Fe-N₄: -1.54 eV; Fe-N₄/Fe₄-N₆: -2.17 eV; Fe-N₄-OH/Fe₄-N₆: -2.40 eV). According to d-band center theory, d-band centers farther from Fermi level weakened intermediate adsorption strength, fundamentally reducing ORR overpotential.

Fig. 5: Fuel cell performance and porous structure simulation analysis of FeSA/AC-N-PCNFs

FeSA/AC-N-PCNFs (labeled as FeSA/AC-N-CNFs in original text) was assembled into MEAs as cathode catalyst for practical performance evaluation. Under H₂-air conditions, it exhibited excellent performance: open-circuit voltage reached 0.95 V, peak power density up to 0.79 W cm⁻², significantly better than traditional FeSA-N-C catalyst (0.87 V, 0.37 W cm⁻²). This performance improvement mainly originated from unique hierarchical fiber network structure: synergistic effects between fiber macropores and micron-sized macropores between fiber networks greatly optimized oxygen diffusion/mass transfer and improved active site utilization (EIS analysis used equivalent circuit model fitting). Additionally, during long-term stability testing at 0.2 A cm⁻² current density, FeSA/AC-N-PCNFs maintained 83.3% initial activity after 117 hours, demonstrating good stability in practical fuel cell applications.

This work achieved synergistic optimization of active sites and mass transfer channels through electrospinning technology, providing a new electrode design paradigm for energy devices like fuel cells and metal-air batteries. Lightweight flexible nanofiber membranes can be directly integrated into MEAs, promoting cost reduction in new energy vehicles and distributed power generation fields.

Paper link: https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202504253