Key Takeaways & Executive Findings
- •• • FeN5@N-C hollow microplates achieve a half-wave potential of 0.93 V vs. RHE, surpassing Pt/C (typically 0.85–0.90 V) and enabling a 225.3 mW cm−2 peak power density in Zn-air batteries, directly addressing the performance bottleneck of non-precious ORR catalysts. • • The axial nitrogen coordination (Fe-N5) induces additional 3d-2p orbital hybridization that weakens OH* binding compared to planar Fe-N4, as confirmed by theoretical calculations, providing a mechanistic basis for enhanced intrinsic activity. • • The polydopamine-assisted hollowing strategy is versatile, successfully encapsulating Ni, Co, Mn, and Cu single atoms into N-doped carbon hollow microplates, demonstrating a platform for synthesizing diverse single-atom catalysts with tailored coordination environments. • • Zn-air batteries using FeN5@N-C air-cathode exhibit stable cyclability up to 400 h, indicating excellent durability under operational conditions, a critical requirement for practical energy storage applications.
Abstract
Metal single-atoms with optimized coordination structure on highly accessible substrate can maximize the metal utilization efficiency along with enhancing catalytic activities. Herein, axial nitrogen-coordinated Fe-N5 sites on N-doped carbon (denoted as FeN5@N-C) hollow microplates are fabricated via a unique Fe3+-chelated polydopamine assisted hollowing strategy using ZIF-L microplates as multifunctional templates. Due to the powerful chelating and adhesive ability of polydopamine, this hollow-carbon strategy can be extended to fabricate single-atom Fe-N-C hollow structures with different shapes and encapsulate other transition-metal single atoms (Ni, Co, Mn, and Cu) into the N-doped carbon hollow microplates. The FeN5@N-C hollow microplates exhibit outstanding oxygen reduction reaction (ORR) capability with an impressive half-wave potential of 0.93 V vs. reversible hydrogen electrode and high stability, which can serve as air-cathode catalysts for high-performance Zn-air batteries with high peak power density of 225.3 mW cm−2 and stable cyclability of up to 400 h. Comprehensive analysis and theoretical calculations elucidate that axial nitrogen coordination in Fe-N5 catalytic sites, unlike the planar Fe-N4 configuration, can compete well with the bonding of OH* through additional 3d-2p orbital hybridization, thereby giving moderate bonding strength to enhance the ORR activity.
1. Introduction
Single-atom catalysts (SACs) have emerged as a frontier in electrocatalysis, offering maximum atom utilization and tunable activity. However, conventional Fe-N-C SACs, typically derived from solid ZIF-8 precursors, suffer from limited accessibility of metal sites deeply embedded in carbon matrices, leading to underutilization and suboptimal ORR performance. The challenge is to design catalysts with highly exposed active sites and optimized coordination environments to balance intermediate adsorption and desorption.
This work introduces a Fe3+-chelated polydopamine-assisted hollowing strategy using ZIF-L microplates as templates, yielding Fe-N5 sites on N-doped carbon hollow microplates. The axial nitrogen coordination in Fe-N5, unlike planar Fe-N4, modulates the electronic structure via additional 3d-2p orbital hybridization, weakening OH* binding and enhancing ORR kinetics. The resulting catalyst exhibits a half-wave potential of 0.93 V and excellent stability in Zn-air batteries, offering a scalable route to high-performance non-precious metal catalysts.
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Fei-Xiang Ma, Jianghua Wu, Xiongyi Liang, Guobin Zhang, Zheng-Qi Liu, Hong-Shuang Fan, Jian Lu, Cheng-Yan Xu, Xiao Cheng Zeng, Yang Yang Li (2026). Axial orbital hybridization enables single-atom Fe-N-C hollow microplates for efficient oxygen reduction. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3682-6
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Frequently Asked Questions
What is the specific role of axial nitrogen coordination in enhancing ORR activity compared to planar Fe-N4?
Axial nitrogen coordination in Fe-N5 introduces an additional 3d-2p orbital hybridization that modulates the d-band center, weakening the binding strength of OH* intermediates. This moderate bonding strength facilitates the desorption of OH*, a rate-limiting step in ORR, thereby enhancing the overall activity. Theoretical calculations confirm that Fe-N5 sites have a lower energy barrier for OH* reduction compared to Fe-N4.
How does the hollow microplate structure improve the accessibility of active sites and mass transport?
The hollow structure provides a high surface area and exposes the Fe-N5 sites on both the inner and outer surfaces, increasing the number of accessible active sites. Additionally, the hollow architecture shortens diffusion pathways for reactants and products, improving mass transport and reducing concentration overpotentials, which is critical for high-current-density operation.
What is the stability of FeN5@N-C under prolonged operation in Zn-air batteries?
Zn-air batteries assembled with FeN5@N-C as the air-cathode exhibit stable cyclability for up to 400 hours without significant voltage decay, indicating excellent durability. This stability is attributed to the strong covalent Fe-N bonds and the robust carbon hollow structure that resists degradation under repeated charge-discharge cycles.
Can the synthesis strategy be scaled up for industrial production?
The synthesis uses ZIF-L microplates and polydopamine, both of which are amenable to scalable production. The Fe3+-chelated polydopamine coating process is solution-based and can be performed in batch reactors. The versatility to encapsulate various transition metals (Ni, Co, Mn, Cu) further enhances its industrial relevance for producing a family of single-atom catalysts.
How does the performance of FeN5@N-C compare to commercial Pt/C in terms of half-wave potential and power density?
FeN5@N-C exhibits a half-wave potential of 0.93 V vs. RHE, which is 30-50 mV higher than typical Pt/C (0.85-0.90 V) in alkaline media. In Zn-air battery tests, it achieves a peak power density of 225.3 mW cm−2, outperforming Pt/C-based cathodes under similar conditions, demonstrating its potential as a cost-effective alternative.
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