Zn3[Fe(CN)6]2 derived Fe/Fe5C2@N-doped carbon as a highly effective oxygen reduction reaction catalyst for zinc-air battery

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Highlights

  • A zinciferous Prussian blue analogue Zn3[Fe(CN)6]2 was prepared and used as the precursor to synthesize a highly active ORR catalyst Fe/Fe5C2@N-C.

  • The resultant Fe/Fe5C2@N-C catalyst displayed a high ORR activity with a half-wave potential (E1/2) of 0.85 V (vs. RHE) in alkaline media, more active than commercial Pt/C.

  • The Fe/Fe5C2@N-C catalyst was used as a air-cathode material of a zinc-air battery battery, exhibiting a high powder density and large capacity.

  • The synergism of the rich iron species and the N-doped carbon layer endowed the catalyst with high activity and superior durability.

Abstract

We fabricated a highly effective oxygen reduction reaction (ORR) catalyst consisting of Fe/Fe5C2 species wrapped in N-doped carbon (Fe/Fe5C2@N-C), derived from annealing decomposition of a zinciferous Prussian blue analogue Zn3[Fe(CN)6]2, which can be prepared through a facile precipitation reaction. The rationally designed carbon layer-encapsulated structure with iron/iron carbide encased inside the N-doped graphitic carbon shells is favorable for ORR, wherein the iron species not only facilitate the graphitization process but also improve the ORR activity together with outside N-doped graphic carbon. The resultant catalyst demonstrates a positive half-wave potential (E1/2) of 0.85 V (vs. RHE) and a nearly four-electron pathway in 0.1 M KOH solution, comparable to that of the commercial Pt/C. With superior stability to that of Pt/C, Fe/Fe5C2@N-C suffers almost no performance attenuation after 5000 potential cycles. The zinc-air battery using Fe/Fe5C2@N-C catalyst exhibits high power density and capacity, holding great promise for the practical application of metal-air batteries.

Introduction

Large numbers of researches have been carried out to develop alternative energy storage and conversion devices such as fuel cells and metal-air batteries. For the mass production of these promising devices, highly active cathode catalyst to enhance the efficiency of oxygen reduction reaction (ORR) is distinctly important because the sluggish kinetics at the cathode is the major technical challenge. Pt-based materials have been still by far the most efficient electrocatalyst while the scarcity, high cost and poor stability greatly limit their widespread applications. Recently intensive research efforts in reducing or replacing Pt-based catalysts have led to the development of new ORR electrocatalysts, including Pt-based alloys [[1], [2], [3]], metal-free carbon materials [4,5], conducting polymers [6], and transition metal/heteroatom doped carbon composites [7]. Among them, transition metal (e.g., nickel, cobalt, iron) and nitrogen doped carbon (Me-N-C) has attracted tremendous research interest as a promising electrocatalyst toward ORR [[8], [9], [10], [11]]. However, in terms of the activity and durability, significant gaps between the Me-N-C and precious metal catalysts remain to be eliminated for viable application. The active sites of Me-N-C catalysts are generally speculated to be associated with N coordinated metal structures (M-Nx) with average coordination number x from 2 to 4 through X-ray absorption or Mössbauer spectroscopy techniques [12,13]. A key method for the Me-N-C catalysts to enhance the electrocatalytic activity is to offer the sufficient exposure of the active Me-Nx sites in carbon composite materials by accurate control of the catalyst composition and structure. Despite tremendous efforts, the relatively slow kinetics of the ORR continues to be a bottleneck because of the complicated multielectron transfer process. Furthermore, some catalysts always suffer from structural degradation or catalytic centers poisoning during electrochemical processes, thus resulting in poor durability. For the Me-N-C catalysts, the metal sites are easy to fall off from the carbon matrix [14]. It is beneficial to design a highly active and durable catalyst with rich metal sites and rational core-shell structure, in which the metal sites are encased by the thin graphitic layer, not only protecting the metal sites from aggregation and dissolution, but also coupling metal sites and N-doped carbon acting as the active sites. Many researches have reported that carbon encapsuled metal or metal carbide is in favor of electrocatalysis [[15], [16], [17], [18]], due to the synergistic effect between metal/metal carbide and protective nitrogen-doped graphitic layers, thus facilitating interfacial charge transfer and improving proton reduction ability [19,20].

Recently, metal-organic frameworks (MOFs) are attracting lots of interests in catalytic fields due to the unique features such as large specific surface area, controllable pore texture and tuneable composition. For example, Zhang et.al synthesized novel Co@N-C bifunctional catalysts derived from a pair of enantiotopic chiral 3D MOFs for highly efficient zinc-air battery and water splitting [17]. Prussian blue analogue, as a cyano ligand-bridged MOF, has been widely used as the precursor material in the fields of water splitting and energy storage [[21], [22], [23]]. Here we developed a facile strategy to synthesize highly active Fe/Fe5C2@N-C ORR electrocatalyst from a Prussian blue analogue Zn3[Fe(CN)6]2 and applied it to the zinc-air battery. The iron species facilitate the graphitization process and improve the ORR activity together with outside N-doped graphic carbon. In addition, the volatile zinc help to produce a porous structure and improve the dispersity of iron species during the pyrolysis process. As a result, Fe/Fe5C2@N-C-1000 (annealed in 1000 °C) was tested to have the highest ORR activity, comparable to that of Pt/C.

Section snippets

Catalysts synthesis

The Prussian blue analogue Zn3[Fe(CN)6]2 was synthesized with a precipitation method. First, 6 mmol ZnCl2 (0.818 g) was dissolved in 50 mL deioinized water. Then 4 mmol K3[Fe(CN)6] (1.317 g) together with 0.6 g PVP (Polyvinyl Pyrrolidone, K30) was dissolved in 50 mL deioinized water. The above two solutions were mixed together with 30 min magnetic stirring and kept in the dark for 6 h. The resultant brown precipitates were collected by centrifugation, consecutively washed with distilled water

Results and discussion

The fabrication of Fe/Fe5C2@N-C is on the base of wet precipitation method and subsequent heat-treatment as illustrated in Fig. 1. The Zn3[Fe(CN)6]2 precipitate comes into being after the mixture of aqueous ZnCl2 and K3[Fe(CN)6], forming spherical particles at the assistance of PVP. Zn3[Fe(CN)6]2 is constructed by the cross-linking of zinc and the nitrogen atom within the hexacyanoferrate [Fe(CN)6]3+ group, thus forming a 3D continuous framework with abundant Fe, N, C and Zn. Fig. S1 shows the

Conclusions

In summary, Fe/Fe2C5 wrapped in N-doped carbon (Fe/Fe5C2@N-C) was fabricated by annealing decomposition of a zinciferous Prussian blue analogue Zn3[Fe(CN)6]2. The obtained Fe/Fe5C2@N-C-1000 catalyst exhibits an excellent ORR activity and a superior stability, especially in alkaline electrolyte, which are due to the rich iron/iron carbide particles and protective N-doped graphitic carbon shells. In addition, served as an air cathode material for zinc-air battery, Fe/Fe5C2@N-C-1000 demonstrates a

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (11575084 and 51602153), the Natural Science Foundation of Jiangsu Province (BK20160795), Nanjing University of Aeronautics and Astronautics PhD Short-term Visiting Scholar Project, Funding of Jiangsu Innovation Program for Graduate Education (KYCX17_0250), Funding for Outstanding Doctoral Dissertation in NUAA (BCXJ17-09) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher

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