Fe, N, S-codoped carbon frameworks derived from nanocrystal superlattices towards enhanced oxygen reduction activity
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Recently, iron, nitrogen and sulfur codoped carbon-based materials have gained increasing attention for their synergistic effect towards superior electrocatalytic oxygen reduction performance. To gain insight into the contributions of the heteroatoms, we developed a facile and reproducible method for constructing Fe, N, S-codoped carbon frameworks derived from self-assembled Fe3O4 nanocrystal superlattices. The material constructed by the suggested method exhibited excellent ORR activity with more positive half-wave potential (∼ 0.869 V, vs RHE), higher diffusion-limiting current density (∼ 5.88 mA/cm2) and smaller Tafel slope (45 mV/dec) compared with Fe, N-codoped carbon frameworks and Pt/C. Notably, Fe3O4 nanocrystals served as both the building blocks for constructing carbon frameworks and the source of Fe residues leaving in the frameworks at the same time. By artificially tailoring the doping type and level as well as the homogeneousness of heteroatoms, the results discussed herein prove the importance of each kind of heteroatom in boosting ORR activity.
KeywordsSelf-assembly Nanocrystals Heteroatom doping Single atom Oxygen reduction reaction
To mitigate environmental problems, more and more efforts have been devoted into searching desired green energy, among which fuel cells and metal-air batteries show tremendous potential [1, 2]. However, the energy efficiency is greatly hindered by the cathodic oxygen reduction reaction (ORR) due to the intrinsic sluggish kinetics . By far, Pt/C and Pt-based noble metal materials are still commonly used as the best commercial ORR catalyst . The drawbacks of Pt-based catalysts, such as the scarcity, high cost, poor durability, and low methanol crossover tolerance, have greatly motivated the research on metal-free and nonprecious-metal-based ORR catalysts in the last few years [5, 6].
Novel carbon-based materials with high surface area, structural stability, as well as morphological diversity have been studied extensively in the field of electrochemical research [7, 8, 9]. In particular, the introduction of transition metal atoms (e.g., Fe, Co and Ni) and nonmetal heteroatoms (e.g., N, S, P and B) has been proved to be efficacious in endowing these carbon-based materials with superior ORR catalytic activity, which is attributed to the change in charge and spin densities and increasing defects of the carbon matrix [10, 11, 12]. In another word, the electrocatalytic activity for ORR can be artificially tailored by rational design via screening the type of heteroatoms, the doping level, and other relevant factors.
Recently, iron, nitrogen and sulfur codoped carbon-based materials have gained increasing attention for its synergistic effect. For example, sulfur-doped Fe/N/C nanosheets , porous Fe–N–S/C catalyst  and Fe1–xS/Fe3O4/N, S-doped porous carbon  have been designed and studied. The unique nature of S enables the modification of the electronic structure of iron and nitrogen codoped carbon materials, which leads to the boost of the ORR reactivity [16, 17, 18]. However, miscellaneous Fe species (e.g., Fe-based sulfides, carbides, and oxides) exist in most of these catalysts complicating the process of verifying heteroatom-induced performance differences. Thus, to gain insight into the roles of heteroatoms, it is of great significance to place more emphasis on designing ORR catalysts with homogeneously dispersed doping atoms.
In this work, we developed a facile and reproducible method for constructing Fe, N, S-codoped carbon frameworks (denoted as Fe–N–S/CFs) via acid etching of Fe3O4 superlattices derived from self-assembled nanocrystals and subsequent heteroatom-doping with thiourea. Notably, Fe atoms still remained well dispersed rather than agglomeration after doping treatment at the temperature of 900 °C. Compared with Fe, N-codoped carbon frameworks (denoted as Fe–N/CFs) prepared by the same method, Fe–N–S/CFs possessed more competitiveness towards enhanced oxygen reduction reactivity due to the unique structure. When evaluated as electrocatalyst on glassy carbon electrode in alkaline conditions, Fe–N–S/CFs exhibited excellent ORR activity in terms of onset potential, half-wave potential and long-term durability, which outperformed Fe–N/CFs and even commercial Pt/C catalyst (20 wt%). RRDE measurements and Tafel analysis also manifest the facile ORR kinetics of Fe–N–S/CFs. Further results prove the effectiveness of tailoring electrocatalytic activity by controlling the doping level of heteroatoms.
2 Results and discussion
2.1 Fabrication procedure
2.2 Electrocatalytic performance
The successful fabrication of different heteroatom doping electrocatalysts with unique structure, including highly ordered mesoporosity, atomically dispersed Fe dopants and well-retained carbon frameworks, allows a detailed study on the role of heteroatoms and their intrinsic ORR activities.
To gain further insight into the catalytic process, LSV measurements at different electrode rotation speed were recorded. As shown in Fig. 4d, the diffusion-limiting current density of Fe–N–S/CFs increased with the speed increasing, which is attributable to the improved oxygen diffusion behavior . Both of the heteroatom-doped carbon frameworks show good linearity in Koutecky–Levich (K–L) plots (Fig. 4e), suggesting the first-order oxygen reduction kinetics . Based on the K–L equation, electron transfer number (n) of Fe–N–S/CFs is calculated to be 4.0, close to the theoretical value of ideal catalyst following 4e reduction pathway and higher than the value of Fe–N/CFs (calculated to be 3.8). Rotating ring-disk electrodes (RRDE) measurement was adopted to further quantify the materials’ ORR efficiency. Remarkably, although Fe–N–S/CFs exhibited larger reaction current on the disk, the peroxide yield is still over 1% less than sulfur-free Fe–N/CFs at 0.3 V vs. RHE (Fig. 4f). It clearly indicates that a higher proportion of oxygen is directly reduced into OH− without intermediate peroxides on Fe–N–S/CFs, which corresponds with values of n calculated by RRDE results (Fig. 4f). The above comparison of the ORR performance can strongly prove the introduction of S dopants synergistic with Fe–N–C is an ideal method to enhance oxygen reduction reactivity.
Apart from the high activity and efficiency for ORR, Fe–N–S/CFs also show excellent long-term stability as indicated by chronoamperometric measurements. As shown in Additional file 1: Figure S3d, Fe–N–S/CFs can maintain a high current retention of 98% after 48,000 s of continuous operation (0.7 V vs. RHE) in O2-saturated 0.1 M KOH, higher than that of Fe–N/CFs (93%) and Pt/C (88%) tested under the same conditions. The ordered structure retained in Fe–N–S/CFs after durability test showing the stability of carbon frameworks, as confirmed by TEM image (Additional file 1: Figure S4).
The doping level of heteroatoms is another factor affecting electrocatalytic performance. Thus, the samples prepared with different mass ratios of carbon frameworks and doping precursors were assessed by RDE measurements (Additional file 1: Figure S6) and K–L analysis. The N and S contents were measured to be 2.96, 3.77, 4.18 and 4.96 wt% and 1.05, 1.65, 2.97 and 4.76 wt%, respectively, when the precursor-to-carbon framework mass ratio is 1:0.2, 1:1, 1:10 and 1:20 (Additional file 1: Table S3). The increasing doping level led to an obvious ORR performance enhancement with the mass ratio ranging from 1:0.2 to 1:10 at the rotating speed of 1600 rpm (Fig. 5c). When the ratio further increases to 1:20, LSV results show almost the same half-wave potential with Fe–N–S/CFs doping with 1:10 ratio. Based on the K–L equation, the values of n are calculated to be 3.4, 3.6, 4.0 and 3.9, respectively with the ratio increasing (Fig. 5d). The enhancement of oxygen reduction reactivity can be ascribed to the increasing content of N and S as evidenced by EDS results. When the amount of thiourea is over ten times higher than carbon frameworks, the increment of heteroatoms would be less efficacious in further lifting oxygen reduction performance.
In summary, we have demonstrated a facile and reproducible strategy to fabricate Fe, N, S-codoped carbon frameworks derived from self-assembled Fe3O4 nanocrystal superlattices with superior ORR performance, which outperformed Fe–N/CFs and even commercial Pt/C catalyst. Considering the homogeneous dispersion of heteroatoms, facile tunability of doping type and level as well as detailed study on structure and electrocatalytic reactivity, the results discussed herein provide an important perspective to understand the role of each kind of heteroatom in boosting ORR activity. With further screening relevant factors, the study witnessed a good opportunity to figure out the intrinsic mechanism which is of significance to rationally design a desirable catalyst in the future.
Oleic acid (OA, 90%),1-octadecene (ODE, 90%) sodium oleate (CP), thiourea (99%) and Nafion (5 wt%, containing 15 ~ 20 wt% water) were purchased from Aldrich. Iron(III) chloride hexahydrate (FeCl3·6H2O, 99.0%) was purchased from J&K Chemical Co., Ltd. Anhydrous ethanol, isopropanol, ammonium chloride (NH4Cl), sodium sulfocyanate (NaSCN) and hexane were obtained from Sinopharm Chemical Reagent Co., Ltd (China). All chemicals were used as received without further purification.
4.2 Synthesis of Fe3O4 nanocrystals
Monodispersed Fe3O4 nanocrystals were synthesized according to a literature method . Firstly, iron oleate was obtained by the reaction between FeCl3·6H2O and sodium oleate. In a typical synthesis for 14 nm Fe3O4 nanocrystals, 18 g of iron oleate and 4.3 g of OA were dissolved in 120 g of ODE in a three-neck flask. The mixture was degassed under vacuum at 120 °C for 0.5 h, heated up to 320 °C under N2 atmosphere and kept at this temperature for 1 h. The as-synthesized Fe3O4 nanocrystals were precipitated from the reaction solution by addition of isopropanol and ethanol. After centrifugation. The precipitated Fe3O4 nanocrystals were re-dispersed in hexane with a suitable concentration.
4.3 Fabrication of heteroatom-doped carbon frameworks
Fe3O4 superlattices were obtained by the evaporation of the solution containing Fe3O4 nanocrystals under room condition via drying-induced self-assembly. After following heat treatment at 500 °C in Ar atmosphere for 2 h, repeated acid treatment with HCl was adopted to remove Fe3O4 nanocrystals. To realize N and S doping, the as-obtained carbon frameworks and thiourea with a mass ratio of 1:10 were mixed and heated up to 900 °C in Ar atmosphere and kept for 1 h. Fe–N/CFs were fabricated in the same way using NH4Cl as doping precursor. Doping level was adjusted by changing the mass ratio from 1:0.2 to 1:20.
Scanning electron microscopy (SEM) was taken on a Zeiss Ultra 55 microscope operated at 5 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning TEM (STEM), energy-dispersive X-ray spectroscopy (EDS) and elemental mapping were conducted by a FEI Tecnai G2 F20 S-TWIN microscope operated at 200 kV. Cs-corrected HAADF-STEM measurement was carried out on a Titan G2 60–300 microscope operated at 300 kV. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin Elmer PHI-5000C ESCA system. Small-angle X-ray scattering (SAXS) was conducted on a Nanostar U small angle X-ray scattering system using Cu Ka radiation (40 kV, 35 mA). Raman spectra were recorded at room temperature with an XploRA Raman system.
4.5 Electrochemical tests
All of the catalysts inks were prepared by homogeneously mixing 1 mg of catalyst, 0.25 mg of Carbon ECP, 6 μL of Nafion, and 250 μL of anhydrous ethanol. A certain volume of the ink was dropped on glassy carbon electrode and dried at room temperature. Electrochemical measurements were carried out on CHI 760E electrochemical station in 0.1 M KOH electrolyte. A carbon rod and a saturated calomel electrode (SCE) electrode were selected to be the counter electrode and reference electrode. Oxygen or nitrogen flow was used for certain measurements. Cyclic voltammetry (CV) experiments were recorded at a sweep rate of 50 mV/s, and the Linear sweep voltammetry (LSV) tests were measured with a scan rate of 20 mV/s under various rotation rates.
The durability of the catalysts was tested in the O2-saturated 0.1 M KOH electrolyte at room temperature by applying chronoamperometric measurements at 0.7 V vs. RHE for 48000 s.
JZ and BW designed and wrote the manuscript. BZ, YY and WH helped in performing the experiment. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Funding for this work has been received from NSFC (21872038, 21733003), MOST (2017YFA0207303), and Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (17JC1400100).
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