Key Takeaways & Executive Findings
- •• Fe–Mn dual-atom catalysts exhibit superior oxygen reduction reaction (ORR) activity and stability, with high half-wave potentials in both alkaline and acidic conditions. • Synergistic Mn incorporation effectively anchors Fe atoms, mitigates the Fenton reaction, and enhances the durability of ORR catalysts. • Advanced characterization and density-functional theory calculations reveal Mn-induced electronic structure modifications, promoting superior ORR kinetics and active site performance. • (FeMn-DA)-N-C catalysts show remarkable potential for practical fuel cell applications.
Abstract
The ability to unlock the interplay between the activity and stability of oxygen reduction reaction (ORR) represents an important endeavor toward creating robust ORR catalysts for efficient fuel cells. Herein, we report an effective strategy to concurrent enhance the activity and stability of ORR catalysts via constructing atomically dispersed Fe–Mn dual-metal sites on N-doped carbon (denoted (FeMn-DA)–N–C) for both anion-exchange membrane fuel cells (AEMFC) and proton exchange membrane fuel cells (PEMFC). The (FeMn-DA)–N–C catalysts possess ample dual-metal atoms consisting of adjacent Fe-N4 and Mn-N4 sites on the carbon surface, yielded via a facile doping-adsorption-pyrolysis route. The introduction of Mn carries several advantageous attributes: increasing the number of active sites, effectively anchoring Fe due to effective electron transfer to Mn (revealed by X-ray absorption spectroscopy and density-functional theory (DFT), thus preventing the aggregation of Fe), and effectively circumventing the occurrence of Fenton reaction, thus reducing the consumption of Fe. The (FeMn-DA)–N–C catalysts showcase half-wave potentials of 0.92 and 0.82 V in 0.1 M KOH and 0.1 M HClO4, respectively, as well as outstanding stability. As manifested by DFT calculations, the introduction of Mn affects the electronic structure of Fe, down-shifts the d-band Fe active center, accelerates the desorption of OH groups, and creates higher limiting potentials. The AEMFC and PEMFC with (FeMn-DA)–N–C as the cathode catalyst display high power densities of 1060 and 746 mW cm−2, respectively, underscoring their promising potential for practical applications. Our study highlights the robustness of designing Fe-containing dual-atom ORR catalysts to promote both activity and stability for energy conversion and storage materials and devices.
1. Introduction
The past several decades witnessed the advances in platinum group metal-free catalysts as the cathode for hydrogen fuel cells [1–4]. Single-atom catalysts (SACs) have garnered much attention due to their excellent activity, selectivity, and maximum atomic utilization [5–7]. Among them, owing to low cost and outstanding oxygen reduction reaction (ORR) activity, iron–nitrogen co-doped carbon (Fe–N–C) SACs (denoted (Fe-SA)–N–C) with highly dispersed Fe-Nx are regarded as the most promising, low-cost alternative to platinum-based catalysts [8–10]. However, Fe–N–C catalysts are prone to participate in the Fenton reaction, in which dissolved Fe ions combine with H2O2 produced by partial two-electron reaction to produce hydroxyl radicals. As such, it leads to carbon oxidation and demetallization, thereby reducing ORR stability [11–14].
In contrast to SACs, dual-atom catalysts (DACs) carry more complex and flexible synergistic active sites [15–17]. Recent studies revealed that the introduction of additional active sites (e.g., Co, Ni, and Mn) into the Fe-N4 structure could effectively regulate the electronic structure of the latter and optimize the adsorption free energy of intermediates [2, 12, 18–20]. More importantly, the Fenton reaction between Mn ions and H2O2 is difficult to occur, thereby conferring high stability to the catalyst [13, 21–23]. Thus, the introduction of Mn sites into the Fe-N4 sites may inhibit the Fenton reaction and in turn improve the ORR activity.
The key to render the practical application of non-noble metal ORR catalysts for fuel cells lies in improving the effective active sites and accelerating the transfer of oxygen in electrode. Notably, many ORR catalysts composed of M–N–C (M = Fe, Co, Mn, etc.) structures have low performance in real proton exchange membrane fuel cells (PEMFC) and anion-exchange membrane fuel cells (AEMFC), despite their excellent performance on rotating disk electrodes (RDE) [8, 24, 25]. This is because in fuel cell, the active sites of catalysts distributed on the surface layer contact with reactive gases and ionomers, forming a three-phase interface conducive to the reaction.
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Lei Zhang, Yuchen Dong, Lubing Li, Yuchuan Shi, Yan Zhang, Liting Wei, Chung-Li Dong, Zhiqun Lin, Jinzhan Su (2024). Concurrently Boosting Activity and Stability of Oxygen Reduction Reaction Catalysts via Judiciously Crafting Fe–Mn Dual Atoms for Fuel Cells. Nano-Micro Letters. https://doi.org/10.1007/s40820-024-01580-5
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Frequently Asked Questions
What are Fe-Mn dual-atom catalysts and how do they improve ORR performance?
Fe-Mn dual-atom catalysts consist of adjacent Fe-N4 and Mn-N4 sites on N-doped carbon. The Mn incorporation enhances activity by increasing active sites, anchoring Fe to prevent aggregation, and modifying the electronic structure to optimize intermediate adsorption. It also improves stability by mitigating the Fenton reaction.
How does the introduction of Mn affect the Fenton reaction in ORR catalysts?
Mn ions are less prone to participate in the Fenton reaction compared to Fe ions. By incorporating Mn into the Fe-N4 structure, the catalyst reduces the generation of harmful hydroxyl radicals, thereby preventing carbon oxidation and demetallization, which enhances long-term stability.
What are the half-wave potentials achieved by (FeMn-DA)-N-C catalysts?
The (FeMn-DA)-N-C catalysts exhibit half-wave potentials of 0.92 V in 0.1 M KOH and 0.82 V in 0.1 M HClO4, indicating excellent ORR activity in both alkaline and acidic media.
How do (FeMn-DA)-N-C catalysts perform in practical fuel cells?
When used as cathode catalysts, they deliver high power densities of 1060 mW cm−2 in anion-exchange membrane fuel cells (AEMFC) and 746 mW cm−2 in proton exchange membrane fuel cells (PEMFC), demonstrating promising potential for real-world applications.
What is the significance of the doping-adsorption-pyrolysis route in synthesizing these catalysts?
This facile synthesis route enables the formation of atomically dispersed dual-metal sites on N-doped carbon, ensuring high density of active sites and uniform distribution, which is crucial for achieving high ORR performance and stability.
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