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Facile Synthesis N-Doped FeC@CNT/porous Carbon Hybrid for an Advanced Oxygen Reduction and Water Oxidation Electrocatalyst Pingping Zhao, Wei Xu, Xing Hua, Wei Luo, Shengli Chen, and Gongzhen Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03070 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 11, 2016

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The Journal of Physical Chemistry

Facile Synthesis N-doped Fe3C@CNT/Porous Carbon Hybrid for an Advanced Oxygen Reduction and Water Oxidation Electrocatalyst Pingping Zhao, † Wei Xu, † Xing Hua, † Wei Luo,* ,† ,‡,§ Shengli Chen,* ,† and Gongzhen Cheng†

AUTHOR ADDRESS †

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R.

China. Tel.: +86 2768752366. E-mail address: [email protected]; [email protected]. ‡

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai

University, Tianjin 300071, P. R. China. §

Suzhou Institute of Wuhan University, Suzhou, Jiangsu, 215123, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT: A facile one-step synthesis the hybrid of hierarchical nitrogen-doped porous carbon and Fe3C nanoparticles encapsulated nitrogen-doped carbon nanotube through simply annealing the mixture of FeCl3, o-Phthalic anhydride and melamine at 800 oC in Ar was proposed. Both the specific ratio of these precursors and the selected annealing temperature are key factors for the formation of the unique hybrid structure, while any subtle modulation will result in different morphologies. Thanks to the good conductivity and hierarchical porous diversity, Fe-N-C material obtained at 800 oC exhibit a half-wave potential of 0.880 V, ca. 50 mV more positive than Pt/C for oxygen reduction reaction (ORR) and an overpotential of 0.41 V, ca. 36 mV lower than IrO2 black at the current of 10 mA cm-2 for oxygen evolution reaction (OER).

INTRODUCTION The development of clean and sustainable energy deliveries to substrate the traditional carboncontaining resources was stimulated by a series of modern problem, like environmental contamination as well as the natural resources exhaustion in the near future.1,2 Oxygen reduction reaction (ORR) and oxygen oxidation reaction (OER) are essential for the renewable energy devices, such as fuel cell, rechargeable zinc-air batteries and water splitting.3-5 However, due to the sluggish kinetics, and complicated reaction mechanism, to date, precious metallic catalysts (e.g. Pt-, Ir- or Ru- based catalysts) are still the state-of-the-art for these two reactions.6-8 Thus, the searching for non-precious metal-based catalysts with high catalytic activity toward ORR and OER is highly desirable.9-12


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Over the past years, the Fe−N−C composites, while Fe and/or its carbides, oxides and nitrides nanoparticles encapsulated in various nitrogen-doped carbon materials, have been widely developed as promising candidates.13-15 These materials also exhibit almost no cross effect toward CO, MeOH or other small organic molecules during the catalytic process. It is suggested that the electronic and chemical interactions between Fe-based species and surrounded nitrogen doped carbon are the key factors in boosting their electro-catalytic activity and stability.16 Very recently, a number of Fe species encapsulated in N-doped carbon nanotube (CNT), in a formation of necklace, pod-like or bamboo-like architectures, have been reported with enhanced electrochemical performances.17,18 However, their ORR and OER performances still can hardly match those of the state-of-the-art noble metals, probably due to their low conductivity and porosity. It has been reported that the porous diversity of special hierarchical hybrids could result in a much faster mass transfer and more reactive sites exposure during the catalytic process, and thus boosting their electro-catalytic performances.19,20 Generally, these porous carbons are tend to aggregate during the electrochemical catalytic process, probably due to the strong π-π interaction, which significantly limits their conductivity and further application. Thus, searching for an efficient way to fabricate M-N-C species with both desirable porosity and conductivity through a facile one-step method is highly desired but still of great challenge.21-24 Herein, in this work, we designed a facile one-pot strategy to prepare a hybrid of Fe3C nanoparticles encapsulated N-CNT and hierarchical N-doped porous carbon, by simply pyrolying the mixtures of FeCl3, o-Phthalic anhydride, melamine. By taking the advantage of the hierarchical porosity, electric conductivity, and the high density of the Fe-N active sites, the resulted Fe-N-C hybrids exhibit outstanding ORR and OER performance.


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MATERIALS AND METHODS Materials FeCl3·6H2O, o-Phthalic anhydride, melamine, isopropanol and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd (China). All the agents were used without further purification. Nafion (5 wt.% in isopropanol) was purchased from Sigma-Aldrich. The reference commercial catalysts 20 % Pt/C and pure IrO2 black were from the Johnson-Matthey (UK) and the Shanxi Kaida Chemical (China), respectively. Synthesis of Fe-base catalysts The preparation of Fe-base catalysts was divided into three part: (1) 0.18g FeCl3·6H2O mixed with 1g melamine and 1.5g o-Phthalic anhydride, then added 30mL ultrapure water to the system. After stirring for 24h at 50 oC, the mixture was dried by rotary evaporation. (2) The precursor mixture was then transferred to a tube furnace, and annealed under argon atmosphere with the temperature programmed at a heating rate of 10 oC/min. In the process of temperature control, the mixture was exposed to a constant temperature at 550 oC for 3h, then heated to the o

desired temperature (700-900

C) for 2h, followed by naturally cooling to the room

temperature.(3) Using 0.5M H2SO4 leached for 48h, the catalysts were washed three times with ultrapure water. Final product is dried after freeze-drying. In the comparison experiments, the method to obtain the samples were similar except that the ratio of the precursor was altered.


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Characterization All the electrochemical experiments were carried out on a CHI760e work station (Shanghai, China). Scanning electronic microcopy (SEM) images were investigated by a Zeiss Sigma electron microscope. Transmission electron microscopy (TEM) images were obtained through Tecnai G20 U-Twin. Powder X-ray diffraction (PXRD) data was detected by a D8-Advance instrument using Cu Kα as the radiation source (Bruker X-ray diffractometer PAN analytical Xray system, German). The electric current and voltage was controlled at 40 mA and 40 KV respectively, and the scanning velocity was kept at 5o min-1. The porosity data was detected by a Quantachrome Instruments QUADRASORB evo automated surface area & pore size analyzer at the temperature of 77K. The elemental information analyzed by XPS was detected by a Kratos XSAM 800 spectrophotometer. Renishaw inVia Plus instrument was used to get the information of Raman spectrum. Electrochemical measurements All the electrochemical measurements were carried out at 30 oC, and the characterization of all the catalysis were performed on the modified surface of the glassy carbon electrode (GCE, φ = 5 mm) as the working electrode, while the reference electrode was Hg/HgO electrode (SEC) immersed in alkaline system. Platinum foil, dimension of which was 1cm*1cm*0.02cm, was used as the counter electrode. Hence, all the potential that calculated by electrochemical workstation and measured against SEC were finally converted to the potential versus the reversible hydrogen electrode (RHE).By using the glassy carbon electrode coated with Pt/C as the working electrode, the potential scale from SEC to RHE was calibrated with the H2 add to the system. Saturated with hydrogen, the


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electrochemical system was applied to the hydrogen electrode reactions (HERs) at 5 mV per second. After measurements, the RHE zero potential was chosen form the potentials at which the current crossed zero. To ensure the catalyst was coated on the glassy carbon electrode uniformly, 5 mg catalyst was perfectly dispersed in 1mL 0.1% Nafion isopropanol solution by ultrasonic vibration. Then, 36 µL catalytic mixture, divided into 6 parts evenly, was placed on the GCE. After dring naturally, the modified GCE was obtained with a loading content of 900 µg cm-1. Pt/C samples was performed in the same way, while the volume plated on GCE was subtracted to 6 µL, and the corresponding loading content was 150 µg cm-1. Before the modification of glassy carbon electrode, all the electrodes was polished by gamma Alumina powders with the average size of 1, 0.5, 0.05µm respectively to remove the impurities on the electrodes, then cleaned with ultrapure water and ethanol to remove the Alumina powders. The solutions of electrochemical system were saturated by highly purified Ar (for CVs), H2 (for HER polarization curves), or O2 (for ORR polarization curves) for about 30 minutes. For RRDE experiments, the constant ring potential of ORR was set at 1.26 V to detect the mediate product of H2O2. In the OER test, the constant ring potential was set to be 1.5 V and 0.6 V to detect the content of mediate product and the oxygen reduction reaction respectively. Before measuring the ORR and OER performances, all the electrodes were electrochemical activated firstly by cycling in the corresponding potential range with a scanning rate of 50 mV/s till relatively reproducible voltammetric responses were obtained.


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RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of the preparation of FeNC-800.

Figure 1 (a) TEM, (b) HAADF-STEM, (c-e) element mapping and (f-h) HRTEM images and (i) SAED pattern of the FeNC-800 hybrid.


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The facial synthesis strategy of N-doped Fe3C@CNT/porous carbon hybrid was illustrated in Scheme 1 and described in detail in the Experimental section. After removement of the unstable iron species by sulfate acid, the resulted FeNC-800 catalysts were obtained (FeNC-X means the samples obtained at a target temperature of X oC). The morphology of the as-synthesized FeNC800 was characterized by transmission electron microscopy (TEM) and scanning electron microscope (SEM) images. Complexes of N-doped porous carbon nanosheet crossed by a large amount of N-doped carbon nanotube were observed. At the same time, a deep structure of CNT in a length of ca. 10 µm, along with a diameter of ca. 20-40 nm was observed (Figure 1 and Figure S1), which is in the typical range of mesoporous materials. Fe3C nanoparticles were encapsulated at the tip or inside of CNT. The high-angle annular dark-field scanning TEM (HAADF-STEM) image along with energy-filtered TEM (EFTEM) images exhibited a pattern of uniform distribution of C and N in both carbon nanosheet and CNT, while Fe species are located in the CNT only, indicating that Fe3C nanoparticles were encapsulated in N-CNT. Highresolution TEM (HRTEM) images showed a lattice distance of 0.21 nm, corresponding to (211) plane of Fe3C,25 as shown in Figure 1g. Moreover, Fe3C nanoparticle was wrapped by highly graphitized carbon shell with a lattice distance of 0.34 nm, agree well with the (002) plane of graphic sheet.26 Based on the large amount of previous report including the DFT calculation and experimental measurement, it is suggested that the active sites toward electro-catalytic performance may locate at these highly graphic sheets. Fe3C influenced the electronic structure of the graphic sheets surround and further improve the activities of ORR and OER (vide infra). The corresponding selected area electron diffraction (SAED) pattern of Fe3C demonstrated a single crystal structure (Figure 1i).27 In the investigation of the morphological influence of temperature, elevating or decreasing the target temperature would lead to the absence of CNT, as


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shown in Figure S2a-b. At the same time, a mixed metallic phase corresponded to Fe3O4 and Fe3C was determined in FeNC-700 (Figure S2c),28 and the metallic species in FeNC-900 can be assigned to be Fe3C (Figure S2d). It is thus believed that the calcined temperatures are essential for the synthesis of the unique structure of Fe3C@CNT/porous carbon hybrid. Based on the kinetics study on CNT formation reported by several groups, it is no doubt that Fe-based species could catalyze such a 1D nanostructure formation. The graphene nanoshells with Fe species encapsulated were first formed, and then assembled to form a 1D nanostructure under the assistant of some soft template or the surface functional groups. 29-31


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Figure 2 The characterization of FeNC-800 hybrid. (a) XRD, (b) TGA, (c-d) XPS of Fe 2p and N 1s, (e) Raman spectrum (f) N2 adsorption-desorption isotherms, (inset) the pore size distribution by DFT method.

FeNC-800 hybrid was further characterized by Powder X-ray diffraction (PXRD), Thermogravimetric analysis (TGA), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) as well as N2 adsorption-desorption analysis. Figure 2a showed the XRD pattern, a remarkable peak at 26.2o corresponded to (002) plane of graphite was observed, indicating the highly graphitization. Besides, a series peaks observed in the range of 40-50o were attributed to Fe3C.29 According to TGA results (Figure 2b), the content of Fe in FeNC-800 can be determined with a value of 9.41%. XPS spectroscopy was used to further investigate the surface information. From the high-resolution spectra, the binding energy of N 1s could be divided into five types: graphic N (400.7 eV, 30.7%), pyridinic N (398.2 eV, 29.72%), oxidized N (404.6 eV, 11.8%), quaternary N (401.8 eV, 11.6%) and FeNx (399.4 eV, 13.7%).13 Among all the types of N 1s, pyridinic N and FeNx was considered to be the major contributor toward the catalytic process during electrochemical performance.32,33 In Fe 2p regions, Fe (II) and Fe (III) type can be distinguished easily (Figure 2d). The binding energy at 712.9 eV should be an indicator of Fe coordinated to N.34 The XPS spectroscopies of FeNC-700 and FeNC-900 were also studied for comparison, as shown in Figure S3 and Figure S4, the FeNC-900 has the lowest nitrogen content, which resulted in the lowest catalytic activity (vide infra). Raman spectroscopy can be used to measure the disorder and graphitization through the ratio of D band (1360 cm−1) and G band (1590 cm−1).35 Among the FeNC-X samples, FeNC-800 showed the highest graphitization with an ID/IG value of 0.83, which may be caused by the large amount of CNT (Figure 2e and S5).29


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The higher graphitization is considered to be an evaluating indicator of good electrical conductivity, which is a major factor that causes the high catalytic activity in the electro-catalytic test (vide infra). Also, we tested the electrochemical impedance spectroscopy (EIS) of the obtained FeNC-X materials in the ORR region at 0.96 V to measure their charge-transfer resistances. As shown in Figure S6, the Nyquist plots indicated that a highest electrical conductivity was determined for FeNC-800 among all the FeNC-X samples, agreement well with its high graphitization determined by Raman spectroscopy.36-38 The porous property of FeNC800 was tested by N2 adsorption-desorption method. A typical type-IV isotherm was observed, indicating a mesoporous structure.20 From the DFT pore size distribution (inset), a wide range spanned from micropores to mesopores were observed, demonstrated a hierachical architechture, which is necessary to enhance the mass transfer efficiently during the catalytic process toward ORR and OER. As for comparisons, the FeNC-700 and FeNC-900 samples were also characterized by N2 adsorption-desorption, shown in Figure S7 and Table S1. As a result, FeNC800 exhibited the largest average pore size.


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Figure 3 (a) Steady-state current-potential responses on disk (lower pannel) and ring (upper pannel) electrodes, (b) H2O2 yield and electron transfer number, (c) Tafel plots and (d) Time evolution of currents on electrodes for the ORR obtained with the FeNC-800 and Pt/C in an O2saturated 0.1 M KOH solution with a rotating rate of 1600 rpm and a constant potential of 1.26 V (vs. RHE).

Oxygen reduction reaction (ORR) is highly essential in fuel cell to achieve the transformation of chemical energy into electrochemical energy. Thus, a series relative test of FeNC-800 was carried out to meet the actual application. Figure 3 showed the ORR activities of FeNC-800, commercial Pt/C was investigated at the same time as a comparison. An onset potential ca. 1.1 V along with a half-wave potential of 0.88 V (ca. 50 mV higher than Pt/C) can be achieved. At the same time, a higher diffusion limiting current of FeNC-800 was detected, indicating a desirable mass transfer process occurred. From the current recorded by ring electrode, no oxidation peak


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appeared for FeNC-800 until near 0.5 V (vs. RHE). However, besides the main peak at 0.5 V, an obvious oxidation peak started at ca. 0.75 V (vs. RHE) was detected by Pt/C, indicating a much weaker ability to stable the intermediate. Thus, a much more desirable catalytic process was suggested by FeNC-800. The H2O2 yield of FeNC-800 was lower than 3% in the range of 0.20.9V and the corresponding electron transfer number was calculated to be 3.99 (Figure 3b). Tafel slope of FeNC-800 was determined to be 85 mV/dec, which is close to Pt/C (78 mV/dec), indicating a similar kinetics happened (Figure 3c). In the time evolution of currents (i-t) test, FeNC-800 remained 95% current after 5000 s, much higher than that of Pt/C (74%) (Figure 3d). After 3000 cycles, half-wave potential of only 25 mV decreased much smaller than that of Pt/C (90 mV decreased), as shown in Figure S8. Moreover, in Direct Methanol Fuel Cell (DMFC), it is inevitable that methanol molecules diffused toward the cathode, thus the tolerance toward MeOH was tested, shown in Figure S9. FeNC-800 exhibited no crossover effect in MeOHcontained solution, however an oxidation peak emerged for Pt/C, highly indicating the selection of FeNC-800. To the best of our knowledge, the ORR activity of the as-synthesized FeNC-800 in this work is among the highest values ever reported for non-precious metal based catalysts (Table S2). All the data above revealed an ideal catalytic process toward ORR occurred of FeNC-800, which can be a suitable candidate to substrate commercial Pt/C.


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Figure 4 (a) Steady-state polarization curves and (b) RRDE measurement recorded at the platinum ring potential maintained at 0.6 V and 1.5 V (vs. RHE), (c) Tafel plots and (d) Chronoamperometric (constant potential) measurement at the constant current 10 mA cm-2 for the OER obtained with FeNC-800 and IrO2 in an Ar-saturated 0.1 M KOH solution with a rotating rate of 1600 rpm.

Oxygen evolution reaction (OER), occurred as half-reaction on anode in water splitting for producing hydrogen. Low overpotential during hydrogen evolution has been widely achieved through a catalytic process by non-precious metal. However, due to its sluggish kinetics, large overpotential and the large reaction barrier during OER, are still the key issue for their actual application in water splitting. Figure 4 showed the catalytic activity of FeNC-800 hybrid toward OER, while commercial IrO2 black was also studied as comparison. An overpotential of 0.41 V was achieved at the current density of 10 mA cm-2 (40 mV lower than IrO2 black). The current on the ring electrode at 0.6 and 1.5 V (vs. RHE) was measured, as shown in Figure 4b. In the range


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tested, a negligible current was observed when the ring electrode was set to be 1.5 V, indicating the negligible intermediate production during water oxidation. Thus, a 4-electron route happened during O2 evolution, which is considered to be desirable from a scope of energy conservation. On the other hand, an obvious oxygen reduction raction peak starting at around 1.35 V was observed when the constant potential of platinum ring electrode was set to be 0.6 V, agree well with the the onset potential detected by disk electrode. Tafel slope of FeNC-800 was calculated to be 137 mV/dec, close to that of IrO2 black (117 mV/dec), indicating a desirable kinetics occured (Figure 4c). Also, the stability was investigated through Chronoamperometric (constant potential) measurement at the constant current 10 mA cm-2, slightly overpotential increased after 5000 s, as shown in Figure 4d. The OER performance of the FeNC-800 hybrid also surpassed most of the recently reported values for non-precious metal catalysts (Table S3).

Figure 5 Steady-state polarization curves of (a) FeNC-X and (b) samples obtained with different precursors at the same target temperature of 800 oC: a FeNC-800, b with adding 50% o-Phthalic anhydride, c with reducing 50% o-Phthalic anhydride, d without FeCl3, e without melamine, f without o-Phthalic anhydride.


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The enhanced activity toward electrochemical performance may originated from a balance of compositions, conductivity as well as hierarchical porosity. During the synthesis, the desired amount of precursors and selected annealing temperature were considered to be the necessary factors for the formation of the Fe-N-C hybrid. Any subtle modulation will lead to the difference of metallic crystal structure, morphology and further influence their ORR and OER activities. The corresponding steady-state current-potential curves of Fe-N-C materials obtained at different temperatures in the range of 0-1.85 V (vs. RHE) were shown in Figure 5a. A slightly lower activities were detected by FeNC-700 and FeNC-900 samples both in ORR or OER performance. In the Fe-based catalyst, Fe3O4 was considered to be an inferior choice toward the electrochemical catalytic reactions due to its spinel structure and poor conductivity.39 In the cases of altering the amount of o-Phthalic anhydride, it will lead to a hybrid of almost pure CNT or the absence of most CNT (Figure S10). The corresponding performance in Figure 5b indicated that a proper ratio of porous carbon and CNT was essential for the enhancement of electrochemical activity. Furthermore, the absence one of melamine, o-Phthalic anhydride, or FeCl3 will lead to irregularly nanoparticles, short CNT or N-doped carbon, respectively, as shown in Figure S11a-c. From their XRD patterns in Figure S10d-e, in the case of no melamine, Fe3C was formed, while in the absence of o-Phthalic anhydride, Fe/Fe3C was observed. The corresponding catalytic performance of these materials toward ORR and OER were shown in Figure 5b, which are all inferior to those of FeNC-800. The excellent activity of FeNC-800 toward ORR and OER may be due to its fantastic structure with a unique hybrid composition, excellent conductivity as well as a hierarchical porosity. However, it is still not clear whether the active centers were located at N, C or Fe-N during oxygen electrocatalytic performance in such system. Very recently, DFT calculations and


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experimental data indicated that pyridine N is determined to be the reason for the enhanced the ORR activities in N/C materials.32,33 N may change the electrostructure of the surrounded carbons and thus further enhance the ORR activities.40 Fe-Nx with a planar structure was also demonstrated to be the active sites for ORR through EXAFS spectrum and Mossbauer spectroscopy.16,41,42 Nakanishi suggested that the ORR active sites of the M/C/N catalyst are a combination of N/C and transition metal.22 Moreover, N/C material itself can serve as active catalysts for the OER.43-44 Nakanishi’s group suggested no obvious enhancement was attributed by metal-based species during OER catalysis.22 Schuhmann proposed that nitrogen functionalized carbon groups, N-metal moieties, and the spinels of the respective metals are responsible for the remarkable enhancement of OER.45

CONCLUSION In summary, a hybrid of nitrogen-doped carbon and Fe3C nanoparticles encapusated N-CNT was synthesized through a facile one-step strategy, by simply annealing the mixture of melamine, o-Phthalic anhydride, and FeCl3. Thanks to the uniformly distribution of Fe3C nanoparticles, good conductivity, hierarchical porous diversity as well as sufficient active site exposure derived from the synergistic effect between CNT and hierarchical porous carbon, the as-synthesized FeNC-800 hybrid exhibited an outstanding performance toward ORR and OER in alkaline media, which surpassed most of the non-precious metal catalysts ever reported. Besides, this simple in situ strategy to construct hybrid of metal nanoparticles encapsulated N-doped carbon nanotube and hierarchical porous carbon with both desirable porosity and conductivity


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may open up a promising avenue for the development of multifunctional non-precious-metal electrocatalysts for various applications.

Supporting Information Available The Supporting information is free of charge on the ACS Publications website. Characterizations of the catalysts obtained. The performance toward the ORR and OER catalysis as well as the electrochemical impedance spectroscopy (EIS) data. Reports on the non-precious metal-based catalysts.

AUTHOR INFORMATION Corresponding Author * W. Luo. Email: [email protected]; S. L. Chen. Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology of China under the National Basic Research Program (Grant nos. 2012CB215500 and 2012CB932800), National Natural Science Foundation of China (Grant nos. 21201134 and 21571145), the Natural Science Foundation of Jiangsu Province (BK20130370), the Creative Research Groups of Hubei


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Province (2014CFA007) and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.

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