Fe, N codoped porous carbon nanosheets for efficient oxygen reduction reaction in alkaline and acidic media
Graphical abstract
Introduction
The global energy crisis and environmental issues pose major threat to the human community [1], [2], [3], [4], [5]. Developing new energy technologies is the primary path to solve the above noted issues. Fuel cells and metal-air batteries are reliable, efficient and low-polluting new energy technology [6], [7], [8], [9], [10]. The cathode reaction of fuel cells and metal-air batteries is ORR (short for oxygen reduction reaction), and an effective electrocatalyst is critical for the sluggish kinetics of ORR [11], [12], [13], [14], [15]. Carbon supported platinum nanoparticles (Pt/C) is now the commercial electrocatalysts for ORR, whereas the large-scale applications of the fuel cells and metal-air batteries are limited by electrocatalysts, which is attributed to the high cost and poor long term stability of Pt/C. Accordingly, the non-platinum electrocatalysts had become a study focus in recent years, involving heteroatom-doped carbon materials, transition metal compounds, macrocyclic transition metal complexes, etc. [16], [17], [18].
Particularly, heteroatom-doped carbon materials were highly expected by virtue of their low cost, high electrocatalytic activity and stability. Among them, transition metal and nitrogen co-doped carbon materials (M-N/C) were a type of promising non-platinum catalysts with high performance. Especially, in virtue of abundant raw materials and superior catalyst activity for ORR, the Fe/N dual-doped carbon materials have been favored by the researchers. But the role of Fe was not yet very clear. At present, considerable evidences supported that Fe-Nx moieties were the catalytic active sites for ORR. For instance, Kramm et al. [19] demonstrated that Fe–N4 and Fe–N2 moieties were active sites by Mössbauer spectroscopy. Electrocatalysts containing Fe-Nx and Fe−Fe3C@C were prepared by Joo et al. [20]. The study showed that Fe-Nx was the major active site catalyzing the ORR via 4e-process. Besides, heterometalloporphyrinic porous carbons were obtained by heat treating metal-organic frameworks (MOFs) from alternating monomeric iron and cobalt metalloporphyrins, as Bu et al. [21] reported. Because the M-N moieties have been retained during carbonization, the new carbon materials exhibited Pt-like electrocatalytic activity for ORR under alkaline and acidic conditions.
As we know, electrospinning provided a simple and cost-effective approach to fabricate carbon nanomaterial as ORR electrocatalysts [22]. Among these electrocatalysts, metal and nitrogen-doped carbon nanofibers (M-N/CNFs) was most commonly reported and has excellent ORR catalytic activity [23], [24]. In this work, electrospinning was first used to prepare Fe and N doped porous carbon nanosheets (Fe–N/CNs) as ORR electrocatalysts, the process as presented in Fig. 1. Polyacrylonitrile (PAN) nanofibers containing a small number of ferrocenes (Fer-PAN) were produced by electrospinning. Then Fer-PAN were preoxidized at 300 °C in the air, denoted as Fer-PAN-300. During this process, the viscosity and liquidity of PAN rose up with the increment of temperature [25]. 3D network structure of nanofibers began to disappear, and nanosheets were formed and occupied the interspace between nanofibers under the effect of those factors and the sublimation of ferrocene. Meanwhile, a large amount of pores appeared on nanofibers and nanosheets. Fe–N/CNs was finally obtained by carbonized Fer-PAN-300 at 900 °C in N2. Fe–N/CNs had larger surface area than common M-N/CNFs, which was an advantage of the Fe–N/CNs as electrocatalysts [26], [27], [28]. Satisfactorily, Fe–N/CNs took on Pt-like catalytic activity under alkaline and acidic conditions, which was attributed to Fe, N doped and the porous structure of Fe–N/CNs that provided transport channels for reactants and products.
Section snippets
Material
Polyacrylonitrile (PAN, MW = 85,000 g/mol), Ferrocenes, N,N-dimethyl formamide (DMF), Perchloric acid (HClO4), Potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co.,Ltd, China. The above reagents were used without further purification.
Catalyst synthesis
The Fe–N/CNs was synthesized by electrospinning method and subsequent pyrolysis. PAN was used as polymer, carbon and nitrogen sources, and ferrocene (Fe(C5H5)2) was used as iron precursor. To prepare the precursor solution, 1 g PAN was
Characteristics of Fe–N/CNs
The DMF solution containing PAN and ferrocene served as spinning solution in this work. The solvent was volatilized, and PAN containing ferrocene was stretched into nanofibers through high-voltage electric field. Finally, PAN nanofibers mat (Fer-PAN) with 3D network structure was obtained, and the morphology of Fer-PAN was presented in Fig. 2a. PAN nanofibers denoted as Fer-PAN-300 were preoxidized at 300 °C in the muffle furnace. The preoxidation sought to make PAN molecular
Conclusions
In this paper, iron and nitrogen co-doped carbon nanosheets (Fe–N/CNs) was prepared by preoxidation and carbonization of PAN nanofibers containing ferrocene by using the electrospinning technology. Fe–N/CNs had large specific surface area and porous structure. Fe–N/CNs took on prominent catalytic activity for ORR through primarily 4e− pathway in alkaline and acidic media, which was attributed to the formation of Fe3+-N4 moiety and pyridinic-N in framework of graphitic carbon nanosheets.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 51602043), Key Basic Research Program of Hebei Province of China (No. 17964402D).
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