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Hierarchical Porous Carbon Doped with Iron-NitrogenSulfur for Efficient Oxygen Reduction Reaction Issa Kone, Ao Xie, Yang Tang, Yu Chen, Jia Liu, Yongmei Chen, Yanzhi Sun, Xiao Jin Yang, and Pingyu Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

Hierarchical Porous Carbon Doped with IronNitrogen-Sulfur for Efficient Oxygen Reduction Reaction Issa Kone, † ,& Ao Xie, ‡,& Yang Tang,*,‡ Yu Chen,† Jia Liu,‡ Yongmei Chen,‡ Yanzhi Sun,† Xiaojin Yang,‡ Pingyu Wan*, † †

National Fundamental Research Laboratory of New Hazardous Chemicals Assessment &

Accident Analysis, Beijing University of Chemical Technology, 100029 Beijing, PR China ‡

Institute of Applied Electrochemistry, Beijing University of Chemical Technology, 100029

Beijing, PR China &

These authors contributed equally to this work and should be considered co-first authors

KEYWORDS: hierarchical porous carbon, graphitic nitrogen, active sites, oxygen reduction reaction, in-situ generated template.

ABSTRACT: Hierarchical porous Fe/N/S-doped carbon with dominance of graphitic nitrogen (FeNS/HPC) has been successfully synthesized by a facile dual-template method. FeNS/HPC shows not only macro-pores resulting from the dissolution of SiO2 template, but also abundant meso-pores were obtained after removing of the in-situ generated Fe2O3 nanoparticles on the

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ultra-thin (~4 nm) carbon shell of macro-pores. Moreover, micro-pores are produced during the thermal pyrolysis of carbon precursors. With respect to electrochemical performance towards oxygen reduction reaction (ORR), FeNS/HPC not only exceeds other prepared porous carbon materials completely, but also shows higher onset potential (0.97 vs. 0.93 V), half-wave potentials (0.87 vs. 0.83 V) and diffusion current density (5.5 vs. 5.3 mA cm-2) than Pt/C. Furthermore, the FeNS/HPC also exhibits outstanding stability and methanol tolerance, making it a competent candidate for ORR. The following aspects contribute to its excellent ORR performance. (1) High content of graphitic-N (5.1 %) and co-doping of pyridinic-N species, thiophene-S, FeNx and graphitic carbon encapsulated iron nanoparticles, providing the highly active sites. (2) The hierarchical porous mesh structures with micro-, meso- and macro-porosity, accelerating the mass-transfer and facilitating the full utilization of active sites. (3) The high specific surface area (1148 m2 g-1) composite with graphitic carbon, assuring large interface and rapid electron conduction for ORR.

1. Introduction Oxygen reduction reaction (ORR) plays a crucial role in advanced energy conversion and storage technologies, such as metal-air batteries and fuel cells 1-3. However, the performance of these techniques is limited by the sluggishness of ORR kinetics 4. Platinum (Pt) and Pt-based materials are regarded to be the most active electrocatalysts for ORR, but suffer from the disadvantages of scarcity, prohibitive cost and poor stability.

5,6

Nowadays, noticeable progress has been made in developing low cost and high effective non-

precious metal electrocatalyst, especially the Fe, Co and heteroatoms (N, S, P, B or F)-doped carbon.7-13 In the past five years, most investigated catalysts for ORR were related to nitrogen-doped carbon. 10, 14-17 Nitrogen, with an atomic size similarity, has a higher electronegativity (3.04) than that of carbon (2.55).

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Thus, doping nitrogen atoms can change the electronic structures while minimizing the lattice mismatch of carbon materials, thus influencing the ORR activity with low destruction of the carbon lattice. It is reported


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that nitrogen in the form of graphitic and pyridinic types induces the electropositivity of adjacent carbon and facilitates the reduction process of oxygen consequently.

18, 19

Specifically, the pyridinic-N is mainly

responsible for exposing planar edges or defect sites and improving the onset potential.

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The outermost

graphitic-N can inclusively facilitates O2 reduction through the 4 e- pathway rather than the 2 e- by supplying the most favorable energetic and electron-transfer conditions,

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thus improving the onset potential and

diffusion limiting current. With respect to sulfur, although the electronegativity of S (χ = 2.58) is almost equal to that of C (χ = 2.55), it is found that the sulfur species (e.g. thiophene-S) at the zigzag and armchair edges of the carbon framework can induce “electron spin” redistribution, contributing to the enhancement of ORR activity. 22 These meaningful investigations indicate that the species of doped heteroatoms are vital toward the performance of electrocatalyst. Transition metals, especially the Fe and Co have been reported to enable the introduction of N and S into carbon materials and the formation of metal-nitrogen or metal/metal-carbides encapsulated by graphitic carbon shells

23-28

by using proper precursors and optimal conditions. During the high temperature pyrolysis process,

organic precursors decomposed and the Fe precursor was reduced to high active Fe nanoparticles, then the carbon atoms diffused inside/outside the lattice of the Fe Nps, generating Fe3C or graphitic carbon shell/nanotubes 29,30. In the case of nitrogen-doped carbon, nitrogen species especially in the form of pyridinicN and pyrrolic-N have the opportunity to combine with Fe atoms, contributing to the formation of well-known FeNx species. Fe3C and FeNx, have been proved to be highly active sites toward ORR.

31,32

Therefore,

transition-metals are also employed in some N-doped carbon materials as direct or indirect catalyst sites. 7, 17, 25, 26, 33-37

In this work, the contribution of transition metals (Fe) to the pore structure, specific surface areas and

formation of active sites was fully studied. The excellent electrocatalysts for ORR should also possess high surface area, optimal pore structure and good electron conductivity. 38 The former facilitates the loading of enormous active sites, and the latter two provide path for mass and electron transfer, respectively. Except for the graphene and nanotubes, most of high surface area porous carbon materials show the typical pore size of only several nanometers, even below 2 nm. 39-41

It must be mentioned that the active sites on the wall of micro-pore deep inside the carbon materials are


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difficult to interact with diffused oxygen and ions, making the ‘active sites’ become ‘inactive sites’ during the practical ORR process. 42, 43 To solve this problem, hierarchically porous carbon materials with well distributed micro-, meso- and macro-pores have been caught much attention.

44-47, 32

Macro and large meso-pore serve as

reservoirs and transport paths for oxygen and electrolyte, while those enormous micro-pores and small mesopores adjacent to macro-pore and big meso-pore serve as the main active sites due to the convenient transport and large surface area.

48-50

In order to obtain nitrogen doping hierarchically porous carbon, complicated

procedure (two steps of high temperature graphitization), several kinds of template, or toxic and dangerous reagents (NH3, HF) are often involved. Therefore, designing and building hierarchical porous carbon with high surface area, high conductivity and efficient loading of active sites by a cost effective and efficient way is a meaningful and interesting challenge. In this study, hierarchical porous carbon was synthesized by a facile method in which glucose and thiourea were pyrolyzed in the presence of SiO2 particles (~100 nm) and iron nitrate, followed by alkaline leaching of SiO2 and acidic leaching of the in-situ generated Fe2O3 to construct macro-pore and meso-pore structure. The prepared N/S/Fe-CHP catalyst also shows abundant micro-pores from the thermal pyrolysis of organic precursors, in which thiourea acts as the sole precursor of N and S, while glucose acts as cheap and green precursor of carbon. The graphitization of carbon matrix and the interaction among N, S and C are enhanced by Fe species during the high temperature pyrolysis, which favors the formation of active species, such as high percentages of graphitic- & pyridinic-N (91 %), thiophene-S, FeNx and graphitic carbon encapsulated Fe Nps. With the synergistically contributions of above aspects, the hierarchically porous FeNS/HPC materials undoubtedly show extremely high activity, long-term durability and excellent methanol tolerance for electrochemical reduction of O2 through 4e- path. This work provides a promising non precious metal oxygen reduction catalyst for fuel cells and metal air batteries.


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2. Experimental procedures 2.1 Chemical and reagents Glucose (C6H12O6), sodium hydroxide (NaOH), hydrochloric acid (HCl, 37%) and iron nitrate (Fe(NO3)3⋅9H2O) were purchased from Beijing Chemical Works. Thiourea (CS(NH2)2) and urea (CO(NH2)2) were purchased from Tianjin Fuchen Chemical Reagents Factory. Sulfur and silica oxide were purchased from Alfa Aesar. All the chemicals are analytical grade as received. Commercial Pt/C (20 wt %) was purchased from Johnson Matthey Company. Nafion solution (5 wt %) was purchased from DuPont, Inc.

2.2. Catalyst preparation Glucose, thiourea, iron nitrate and silica (d~100 nm) with the weight ratio of 1:4:0.3:4 in solid state were mixed together and grounded into solid powder. The powder was heated up to 900 °C and kept for 2h in a quartz tube furnace under an argon atmosphere (80 mL mn-1) at heating rate of 5 °C min-1. In the heating process, the presence of –OH bound on the porous silica surface favored the polycondensation with the heteroatoms (N, S, O) functional groups of glucose and thiourea. 51, 52 Fe species are easy to interact with the heteroatoms of organic precursors during high temperature treatment, and further promote the graphitization and facilitate the anchoring of heteroatoms in carbon materials. After the decomposition and carbonization, the surface of SiO2 was covered by the carbonized products with the insertion of Fe/Fe2O3 species. The obtained FeNSC@SiO2 composite was leached by 2 M NaOH for 12h to obtain the FeNS/PC, while FeNS/HPC was obtained by the following acid washing (6 M HCl) of FeNS/PC for 6h to remove inorganic Fe species. The FeNS/HPC prepared at different temperature was denoted as FeNS/HPC–x00, where x00 represents the temperature (x00=700, 800, 900 or 1000). Without special marks, the pyrolysis temperature is 900 °C for all catalysts in this work. The synthesis approaches for FeN/HPC and FeS/HPC were the same as that for FeNS/HPC, except for replacement of thiourea by urea and replacement of thiourea by sulfur powders, respectively. The synthesis approaches for NS/HPC were the same as that for FeNS/HPC, except for without using iron nitrate.


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2.3. Physical characterization The surface area and meso/macro-pore size distributions of the as-prepared materials were determined by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. Nitrogen adsorption-desorption isotherm tests were performed on a gas adsorption analyzer (Tristar II 3020, Micromeritics, USA). The morphology of the as-prepared materials was studied using scanning transmission electron microscopy (SEM, Zeiss-SPURA 55) with energy dispersive spectrometer (EDS) Mapping, transmission electron microscopy (TEM, Hitachi-800) and high-resolution TEM (HRTEM, JEOL JEM-2100). X-ray diffraction (XRD) patterns were recorded by using an X-ray diffractometer (Bruker D8 Advance). Surface chemical compositions were investigated by using X-ray photoelectron spectroscopy (XPS, Thermo Electron ESCALAB 250). Thermogravimetric analysis was carried by a TGA instrument (Netzsch, STA449F3) with feeding synthetic air (20% O2, 80% N2, 75 mL min-1) at a heating rate of 10 oC min-1 from 25°C to 950°C.

2.4. Electrochemical measurements The ORR activities of the as-prepared catalysts were tested by cyclic voltammetry (CV, 50 mV s−1), linear sweep voltammetry (LSV, 5 mV s−1) and dual electrode voltammetry (DECV, 5 mV s−1) on an electrochemical work station (PINE, USA) with a typical three-electrode system in Ar or O2-saturated 0.1 M KOH electrolyte. Rotating ring disk electrode (RRDE, PINE) with catalyst modification was used as working electrode. 5 mg of the prepared sample, 600 µL of DMF and 400 µL of 0.5 w % Nafion solution were mixed together and ultra-sonicated for 30 min. The obtained homogeneous catalyst ink with volume of 10 µL was dripped onto the surface of glassy carbon disk (0.247 cm2) of RRDE and then dried to obtain a typical nonprecious catalyst loading of 0.2 mg cm-2. Commercial Pt/C, with a typical loading of approximately 0.1 mg cm-2 (Pt: 20 µg cm-2), was used for comparison with precious metal catalyst. A platinum wire and saturated calomel electrode (SCE) was used as counter and reference electrodes, respectively. All potentials (vs. SCE) in this work were calibrated to the RHE (E (RHE) = E (SCE) + 0.998 V). The ORR LSV curves for all catalyst were recorded by subtracting the current in Ar-saturated electrolyte from the current in O2-saturated


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electrolyte. All LSV curves were recorded after 85% iR compensation. The details for calculation of the number of electrons transferred (n) of ORR are described in supporting information.

3. Results and discussion 3.1. TEM/XRD results of structural characterization The morphology and microstructure of FeNS/PC and FeNS/HPC were firstly investigated by TEM and HRTEM. As shown in Figure 1a, FeNS/PC shows porous structure with large numbers of nanoparticles distributed uniformly on the carbon matrix. As amplified in Figure S1, most of the nanoparticles are smaller than 20 nm, in which a larger number are smaller than 10 nm. Figure 1b and the inset show the HRTEM images of nanoparticles and their lattice fringe, respectively. The lattice distance of 0.25 nm is corresponding to (311) plane of Fe2O3. As shown in XRD patterns of FeNS/PC (Figure 2a), several peaks at 30.2, 35.7, 54.0, 57.6 and 61.1o are well associated to the γ-Fe2O3 (JCPDS, No.39-1346). 53 The high intensities for low angles (below 15o) indicate the presence of micro-pore inside amorphous carbon atoms while the peak at 25o indicates existence of typical graphitic carbon in FeNS/PC. As shown in Figure 1c and 1d, most of these Fe2O3 nanoparticles disappear after acid leaching. As verified by TG results in Figure S2, the residual weight of FeNS/HPC (＜5%) is much lower that of FeNS/PC (35%). There are no diffraction peaks of Fe2O3 in the XRD patterns of FeNS/HPC. The removal of most Fe species after acid leaching in the samples of FeNS/HPC prepared at 700, 800, 900 and 1000 °C are all verified by XRD investigation (see Figure S3a and S3b). Figure 1c and 1d show the macro-pore structure of FeNS/HPC at low and high magnification, respectively. The diameter of some macro-pore in the prepared FeNS/HPC is different with that of SiO2 template (Figure S4). It is inferred that some of carbon shells shrink or collapse after the NaOH washing and acid leaching process. The dark curves in Fig 1d represent the cross profiles of the carbon shell surrounding the pore space. The carbon shell is very thin with the thickness around 4 nm and the statistics of thickness is shown in the inset of Figure 1d. As a result, carbon shells are almost transparent in X-Y direction and some are cracked.


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Figure 1. TEM images of (a) FeNS/PC and (c) FeNS/HPC; HRTEM images of (b) FeNS/PC and (d), (e), (f) and (g) FeNS/HPC.


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The most important finding in Figure 1c and 1d is that numerous meso-pores with the size of several nanometers to ten nanometers appear on the carbon matrix, indicating that the acid leaching of Fe2O3 nanoparticles leads to the formation of abundant meso-pores. Figure 1e further exhibits the meso-pore clearly at high magnification. The carbon surrounding the pores shows the lattice of graphitic carbon layer with coexistence of amorphous carbon due to the competition between the Fe-promoted graphitization and the heteroatoms-induced disordering during the high temperature pyrolysis. On one hand, Fe can improve the graphitization of carbon materials during high temperature pyrolysis, which is verified by the growth of graphited carbon near the Fe nanoparticles. 30 On the other hand, the mismatch of lattice and electron structure between carbon atoms and doped heteroatoms (nitrogen, sulfur, phosphorous or fluorine) can break the integrity of carbon structure. As a result, the doping of heteroatoms not only introduces active sites, but also leads to a certain extent of disordering. Both the above Fe-promoted graphitization and heteroatoms-induced disordering take place during high temperature pyrolysis. As shown in Figure S5, Raman spectra reflect the coexistence of graphitic and disordered carbon in FeNS/HPC after pyrolysis. It is found that a few nanoparticles were still present in FeNS/HPC after strong acid leaching. As shown in Figures 1f and Figure 1g, the residual nanoparticles have a diameter of 6-10 nm and are surrounded by several layers (2~5 layers) of graphitic carbon. The crystalline lattice distances of the encapsulated nanoparticles and the carbon layer are 0.21 nm and 0.34 nm, respectively, which are associated to the (111) plane of Fe (JCPDS, No.65-4150) and (002) plane of graphitic carbon. It is inferred that the surrounding graphitic carbon protects the encapsulated Fe metallic nanoparticles from dissolution in acid and oxidation in air.

3.2. BET measurement of surface area and pore structure Nitrogen isothermal adsorption-desorption measurements were carried out to further investigate the pore structure and BET specific surface area of FeNS/HPC and FeNS/PC. As shown in Figure 2b, both catalysts have type II isotherms and H4 loop according to the IUPAC classification. The sharp increase in N2 adsorption at low pressure (p/p0 < 0.01) indicates the existence of abundant micro-pores while the sharp increase at high pressure (p/p0 ＞ 0.9) verifies the existence of larger quantity of macro-pores. As compared with FeNS/PC, FeNS/HPC shows not only higher N2 adsorption, but also an extend loop at medium pressure (0.2

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