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Nitrogen-containing mesoporous carbon cathode for lithium-oxygen batteries: The influence of Nitrogen on oxygen reduction reaction
Accepted Manuscript Title: Nitrogen-containing mesoporous carbon cathode for lithium-oxygen batteries: The influence of Nitrogen on oxygen reduction reaction Author: Hongjiao Nie Yining Zhang Wei Zhou Jing Li Baoshan Wu Tao Liu Huamin Zhang PII: DOI: Reference:
S0013-4686(14)02159-8 http://dx.doi.org/doi:10.1016/j.electacta.2014.10.138 EA 23654
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
3-9-2014 27-10-2014 27-10-2014
Please cite this article as: Hongjiao Nie, Yining Zhang, Wei Zhou, Jing Li, Baoshan Wu, Tao Liu, Huamin Zhang, Nitrogen-containing mesoporous carbon cathode for lithium-oxygen batteries: The influence of Nitrogen on oxygen reduction reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.10.138 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen-containing mesoporous carbon cathode for lithium-oxygen batteries: the influence of Nitrogen on oxygen reduction reaction Hongjiao Nie a,b,Yining Zhang a, Wei Zhoua,b, Jing Lia,b, Baoshan Wua,Tao Liua*and Huamin Zhanga * a
Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
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University of Chinese Academy of Sciences, Beijing 100039, China
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* Corresponding author. Tel.: +86 411 84379072; fax: +86 411 84665057.
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Zhongshan Road 457, Dalian 116023, China
** Corresponding author. Tel.: +86 411 84379535; fax: +86 411 84665057. E-mail address:[email protected] (H.
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Zhang), [email protected] (T. Liu).
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Graphical abstract
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The direct effect of nitrogen content and various nitrogen species on oxygen reduction reaction (ORR)
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activities in nonaqueous Li-O2 batteries are systematically investigated. Mesoporous carbon (MC) with
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various nitrogen species is prepared through heat treatment of N-containing precursor under different
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temperature. The effect of the heat treatment temperature on the performance of carbon materials in Li-O2
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battery is investigated. The bonding state of nitrogen atoms is found to have a significant effect on the ORR
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activity. The ORR activity in Li-O2 battery is proved to be dependent on the quaternary N content while the
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total N content in the carbon material does not play a crucial role in the ORR process.
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Highlights
The role of various N in ORR for Li-O2 battery was investigated.
The total N content does not play an important role in the ORR process.
The ORR activity in Li-O2 battery is dependent on the quaternary N content.
Abstract
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The direct effect of nitrogen content and various nitrogen species on oxygen reduction reaction (ORR) activities in nonaqueous lithium-oxygen (Li-O2) batteries are systematically investigated. Mesoporous carbon (MC) with various nitrogen species is prepared through heat treatment of Ncontaining precursor under different temperature. The effect of the heat treatment temperature on the performance of carbon materials in Li-O2 battery is investigated. The bonding state of nitrogen atoms is found to have a significant effect on the ORR activity. The ORR activity in Li-O2 battery is proved to be dependent on the quaternary N content while the total N content in the carbon material does not play a crucial role in the ORR process.
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Keywords:Lithium-oxygen battery, carbon cathode, nitrogen doping, oxygen reduction reaction activity, quaternary nitrogen
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1. Introduction Li-O2 batteries have been attracting more and more interests due to their extremely high energy density[1]. It has the potential to provide a capacity 5–10 times higher than the state-of-art Li-ion cells[2-4]. However, there are still many challenges, including low capacity, poor cycle stability and low energy efficiency, to be overcome before the practical application of Li-O2 batteries[5]. A typical non-aqueous Li-O2 battery is composed of a Li metal anode, a non-aqueous Li+ conducting electrolyte and a porous air electrode (cathode). During discharge, Li+ combines with reduced oxygen to form insoluble lithium oxide, which would precipitate on the surface or within the pores of the cathode[6]. Once the porosity of the cathode is totally chocked, the discharge process terminates[7]. Therefore, the cathode plays a key role on the cell performance. Currently, cathodes in most Li-O2 batteries are porous carbon materials with large surface area and pore volume. So far, various carbon materials, such as mesoporous carbon foam, carbon aerogel, carbon nanotubes and graphene, have been studied as cathode materials in Li-O2 batteries[8]. In these studies, the performances of carbon cathodes vary according to their microstructures. For high specific capacity, both large pore volume and optimized pore size distribution (PSD) are essential, because it could accommodate more discharge products[9-11]. On the other hand, it cannot be ignored that the carbon cathodes performance also greatly depends on their surface composition. Functionalization of carbon-based materials, such as heteroatom doping, can modify their surface, interfacial, and electronic properties, thereby further improving their performance in Li-O2 batteries. Among possible chemical choices for carbon modification, nitrogen doping has long been a natural and widely studied option. Because this modification allows for the beneficial properties of the carbon to be utilized while finely tuning the final electrical, morphological, and chemical properties of the functionalized carbon network[12]. So
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many studies have focused on the study of nitrogen-containing carbon materials for application in LiO2 batteries recently. In these studies, the cell performance is significantly improved due to enhanced oxygen reduction reaction (ORR) activity at the cathode[8, 13-15]. Li et al. investigated the performance of nitrogen-doped graphene nanosheets electrode and found that it not only exhibited higher discharge capacity but also showed higher average discharge plateau than undoped ones at various discharge current densities. At the same time, the onset potentials and the number of electrons transferred during ORR were also improved[8]. The discharge performance of carbon nanotubes and nitrogen-doped carbon nanotubes were also evaluated in Li-O2 battery respectively. With a nitrogen content of 10.2 at. %, the nitrogen-doped carbon nanotubes cathode delivered a specific capacity 1.5 times higher than that of pristine carbon nanotubes cathode[16]. This is also confirmed by Rodrigues et al. who compared the electrochemical performance of carbon materialswith or without nitrogen[17]. Although numerous studies have proved the beneficial effect of nitrogen doping on the Li-O2 battery cathode performance, a systematic study on the direct effect of nitrogen content and various nitrogen species on ORR activities has not been reported. In this paper, mesoporous carbon with different nitrogen content and nitrogen species is used as Li-O2 battery cathode to study the effect of nitrogen content and various nitrogen species on ORR activities. They were all derived from the same starting nitrogen-containing mesoporous carbon (NMC) material. The synthesis of N-MC was carried out by a previous reported hard template method. Through heat treating N-MC at different temperature, the relative amounts of various nitrogen species can be controlled. This approach allowed the direct comparison of the ORR activities of mesoporous carbon cathodes possessing different nitrogen species. The morphologies and microstructure of the prepared carbon powder and carbon cathodes were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) respectively. BrunauerEmmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS) was used to characterize the pore structure and surface nitrogen groups respectively. Finally, the discharge behavior and cyclic voltammetry (CV) performance of N-MC were also evaluated. 2. Experiment 2.1. Preparation of samples Mesoporous carbon with different nitrogen content and nitrogen species was prepared by using a previous reported hard template method[18]. Melamine-formaldehyde (MF) resin and silica sol were employed as carbon precursor and template respectively. Briefly, melamine (3.15 g) was firstly added into the mixture of formaldehyde solution (5 mL, 37 wt %) and deionized water (12.5 mL). Then, the PH was adjusted to 8.5 with Na2CO3 solution. Afterwards, the mixture was heated at 85 ℃ in a water bath with stirring to get a clear solution. Then, silica sol (30 mL, 5 wt %) was mixed with the solution after cooling down to 40 ℃. In order to initiate the condensation reaction, the PH was adjusted to 4.5 by the addition of 2 M HCl. The solution was kept static for 3 h to form a MF resin with silicon dioxide doped. The resin was dried and cured at 180 ℃ for 24 h. To obtain mesoporous carbon with various nitrogen species, the result resin/SiO2 composite was carbonized at 1000, 1200 and 1400 ℃ under Ar atmosphere for 2 h. Then the silica template was removed by 20 % HF for 24 h. They were thoroughly washed in deionized water. Finally, the samples were dried at 80 ℃. In the following discussion, they were denoted as N-MC-x (x=1000, 1200 and 1400), where x stood for the heat treatment temperature.
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2.2. Material characterizations 2.2.1. Transmission electron microscopy (TEM) and Scanning electronic microscopy (SEM) The morphology of carbon material was characterized by transmission electron microscopy (TEM, Tecnai G2 F20) equipped with EDX detector. Scanning electron microscopy (SEM) micrographs were taken on QUANTA 200FEG to get the surface morphology of N-MC-x cathodes. 2.2.2. X-ray diffractionexperiment X-ray diffraction (XRD) analyses of the N-MCx were carried out on a Rigaku Rotalflex (RU200B) diffractometer with a CuKα source (λ=1.54056 Å) and a Ni-filter. The 2θ values of X-ray diffractogramsvaried between 15° and 90° with a scan rate of 7° min-1. 2.2.3. N2 adsorption/desorption experiment N2 adsorption isotherms were measured at 77.3K using an ASAP2010 system. Surface areas and pore volumes were determined using Brunauer-Emmett-Teller (BET) method. The pore size distribution curves were calculated from the desorption branches of nitrogen isotherms using the Barrett- Joyner-Halenda (BJH) model. 2.2.4. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB250 system utilizing Al K-alpha monochromatic (1486.6 eV) with a spot area of 500 μ m. The XPS spectra were peak fit and analyzed using XPeak4.1 (Photoelectron Spectroscopy Lab, Seoul National University). Spectra were calibrated according to the C1s (284.6 eV) peak. 2.3. Preparation of air electrode The N-MC-x cathodes were prepared by casting slurry of the as-prepared N-MC-x with PTFE as a binder (with a weight ratio of 80:20) onto nickel foam (16 mm in diameter) as cathode current collector (both sides).The carbon loading of each electrode was 0.5mg. 2.4.Li-O2 battery construction、 Li-O2 battery was constructed in an argon-filled glove box (H2O＜0.1 ppm, O2＜0.1 ppm). The cell consists of a 0.45 mm thick Li foil (16 mm in diameter) as anode, a carbon electrode as cathode and polypropylene fiber (Novatexx 2471 Freudenberg Filtration Technologies KG) soaked with electrolyte as a separator. The electrolyte employed here is 1.0 M bis(trifluoromethane) sulfonamide lithium (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME, Aladdin). A stainless steel mesh was used as the current collector. All the cell parts were compressed together to ensure good contact and the cell was completely sealed except for the O2 entryway. 2.5. Electrochemical characterizations 2.5.1. Cell discharge performance After exposed to flow pure oxygen for 5 h, the Li-O2 battery was discharged galvanostatically at a rate of 300 mA g-1 on a LAND 2100 system (Wuhan, China) with a cutoff voltage of 2 V. 2.5.2. Cyclic voltammetry (CV) The cyclic voltammetry (CV) tests were conducted in a traditional three-electrode system with glassy carbon (Φ 4 mm) modified with porous carbon film as working electrode (WE), glass filter protected Li foil as counter electrode (CE), and porous ceramic protected Li foil as reference electrode (RE). Porous carbon (1.0 mg) were dissolved in 2-propanol (0.5 mL) and Nafion-Li (5 wt%, 15 μL). After ultrasonication for 30 min, 5 μL of the received carbon ink was drop-casted on the glass carbon (GC) electrode and vacuum-drying at room temperature. 1M LITFSI/TEGDME
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were used as electrolyte. All the measurements were conducted between 2.0 V to 4.3 V using an AFCBP1 bipotentiostat (Pine Research Instrumentation, USA) in an argon-filled glove box (H2O＜0.1 ppm, O2＜0.1 ppm) at room temperature. 3. Results and discussion 3.1 Structure characterization The detailed structure and the element distribution of the resulting N-MC-x powder are shown in Figure 1a-f. The TEM images show similar morphology of N-MC-x with numerous threedimensionally interconnected pores. These pores aggregate randomly all through the carbon network. The formation of the highly porous structure is mainly caused by the addition of silica template, as previously reported[19]. In addition, the graphitization degrees of N-MC-x do not change much with the heat treatment temperature carried out in this paper, which can be confirmed by the XRD pattern in Figure 2. By employing the nitrogen-containing MF resin as carbon source, N-MC-x is “born” rich in nitrogen. From EDX mapping, it could be obviously seen that the total nitrogen content decreases when the samples are heat treated at higher temperature, similar to results from the literatures[20]. Still, N atoms disperse homogeneously for all of N-MC-x. It is reasonable because N atoms are incorporated through in-situ doping instead of post-treatment such as NH3 calcination and so on. A more detailed analysis on the surface nitrogen compositions of the N-MC-x samples using XPS analysis will be presented later in this paper. SEM test was also used to characterize the morphology of Li-O2battery cathodes fabricated using N-MC-x. Figure 1g-i shows that heat treatment at different temperature do not change the morphology of N-MC-x cathodes significantly. The porous structure is perfectly preserved. Disordered open pores in the range of tens of nanometers spread over the electrode matrix, which would be fully exploited during discharge owing to their applicable size. And the size of these pores could be controlled by proper template selection. 3.2 Porosity analysis The pore structure of N-MC-x was further evaluated by N2 adsorption/desorption method. All of N-MC-1000, N-MC-1200 and N-MC-1400 possess a typical type-IV isotherm with hysteresis loop following the IUPAC classication, indicating that they all have highly mesoporous structure. The PSD curves derived from the desorption branch of the isotherm were also shown in Figure 2b. As the same silica template was used, the mesopore size distributions of the obtained N-MC-x are similar. Their mean mesopore sizes are ca. 23 nm, approximately equal to the average diameter of the silica nanoparticle. The pore parameters are summarized in Table 1. The BET specific surface areas and pore volume decrease from 817 to 566 m2g−1 and 2.24 to 2.02 cm3g−1, respectively, with the change of heat treatment temperature from 1000 to 1400 ℃. However, for the pores larger than 10 nm, the pore volume changes slightly, less than 3 %. So the decrease of specific surface areas and pore volume probably mainly comes from the collapse of micropores smaller than 10 nm. Previous studies by Xia et al. have proved that pores with several tens of nanometers should be the major contributor to the discharge capacity[21]. So in this study, the impact of pore structure of N-MC-x on discharge performance is supposed to be small. It allows us to study the effect of surface N atoms on battery performance independently. 3.3 The analysis of N composition
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To gain more insight into the composition of surface N atoms in N-MC-x, XPS measurements were performed. When improving the heat treatment temperature, the surface N content obviously decreases, which is determined to be 8.62, 4.37 and 1 % for N-MC-1000, N-MC-1200 and N-MC1400 respectively. After deconvolution, the N1s peak can be fitted to four dominant peaks (pyridine N, pyrrolic N, quaternary N and pyridine N-oxide) as shown in Figure 4. Pyridinic N refers to N atoms at the edges of graphene planes, where it is bonded to two carbon atoms, donates one pelectron to the aromatic π system[20, 22]. Pyrrolic N atoms are incorporated into five-membered heterocyclic rings, which are bonded to two carbon atoms and contribute two p-electrons to the π system. Quaternary N atoms are within a graphene plane and bonded to three carbon atoms. Pyridine N-oxide forms after the sample is exposed to air, which is bonded to two carbon atoms and one oxygen atom. The pyridinic and pyrrolic N are always located at the graphitic edge, whereas quaternary N can be both ‘‘edge-N’’ and ‘‘bulk-like-N’’[23]. From quantitative analysis, the binding energy and relative composition ratio of N in N-MC-x were listed in Table 2. For N-MC-1000, only two peaks, pyridinic N and pyrrolic N, appear. Upon further increasing heat treatment temperature, it can be seen that for N-MC-1200 in Figure 4 and Table 2, the intensities of pyridinic N and pyrrolic N peaks become much weaker and their proportion are obviously smaller than those of N-MC-1000, while the intensity of quaternary N peak largely increase. At 1400 ℃, pyridinic N and pyrrolic N peaks are not observed and only quaternary N peak are found. It agrees with reported results regarding favorable formation of pyridine N at higher nitrogen content[24, 25]. 3.4 The effects of N species on discharge and CV performance Although there are many reports that the doping of N can effectively improve the ORR catalytic activity of cathodesin Li-O2 battery, the identity and role of the electrocatalytically active center are still controversial as its contribution to catalytic activity is not well defined. To investigate the electrocatalytic properties of N-MC-x cathodesin Li-O2 batteries, the discharge and CV curvesare acquired. Figure 5a shows that as raising the heat treatment temperature from 1000 to 1200 ℃, the discharge capacity has a 68 % increase, from 6250 to 10488 mAh g-1. When the heat treatment temperature further improves to 1400 ℃, the discharge capacity starts to decrease (8536 mAh g-1). At the same time, the voltage of discharge flat holds the same trend. It is identified to be 2.61, 2.69 and 2.64 V for N-MC-1000, N-MC-1200 and N-MC-1400 respectively. Both the highest discharge capacity and the highest voltage plateau are observed for Li-O2 battery with N-MC-1200 cathode. Figure 5b displays the CV curves for the reduction of O2-saturated electrolyte at a scan rate of 10 mV s-1. The reduction peak current (ip) for N-MC-1000, N-MC-1200 and N-MC-1400 firstly increase and then decrease, which is in accordance with the change of discharge capacity in Figure 5a. Both the cell discharge behavior and CV curves demonstrate that N-MC-1200 has the best performance of ORR among the carbon materials investigated, while N-MC-1000 has the poor performance. Generally, it has been believed that the pore structure and N-containing functional groups in the carbon materials play key roles for the different activity. As mentioned above, N-MC-x shows no obvious difference in the structure and morphology, as well as in the mesoporosity and pore size distribution (Fig. 1, Fig.2 and Tab.1). Therefore, the difference of ORR activity for the samples in this paper is speculated to be caused by N content and N species proportion. Based on the analysis of XPS and electrochemical performance (Fig. 4, Tab. 2 and Fig. 5a, b), no obvious
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dependence of the ORR activity on the total content of N is observed. But this investigation showed that quaternary N atoms seem to be the most important species for the ORR due to the matching relationship between activity and quaternary N contents. Previous studies have reported the ORR catalytic activity of quaternary N both experimentally and theoretically[26-28]. As the first step of ORR, the adsorption of O2 is important[29]. It has been shown that the O2 molecule is preferentially adsorbed associatively at C sites on graphene-like zigzag edges if a quaternary-N is located nearby[26, 30]. This is probably because carbon at the zig-zag edge has sharp density of states near the Fermi level called “the edge states”, which is crucially important for O2 adsorption [31, 32]. In addition, the relative electronegativity of quaternary N atoms reduces the electron density on the adjacent C nuclei, which helps electrons transfer from the adjacent C to N atoms, and N backdonates electrons to adjacent C pz orbitals. The donation and backdonation processes facilitate O2 dissociation on the adjacent C atoms[33]. This is in accordance with our experiment results that the samples containing more quaternary N show higher ORR activities. Thus, effective doping with quaternary N should be a practical guideline for the synthesis of active carbon cathode materials in Li-O2 battery. 4. Conclusion N-containing carbon is a promising cathode material in Li-O2 battery, but a deeper understanding of the relationship between the concentration and type of N species and ORR activity is needed. In this study, N-doped mesoporous carbon (N-MC) was prepared through hard template method by employing N-containing polymer as carbon source. Heat treatment of N-MC precursor under different temperature gives products with various nitrogen species. Most importantly, the influence of material microstructure on electrochemical performance could be excluded because the morphology, crystalline and pore structure of all samples remained nearly unchanged. The total N content in the carbon material does not play an important role in the ORR processin Li-O2 battery and the ORR activity was proved to be dependent on the quaternary N content. References [1] K.M. Abraham, Z. Jiang, J. Electrochem. Soc., 143 (1996) 1. [2] P.G. Bruce, Solid State Ionics, 179 (2008) 752. [3] T. Ogasawara, A. Débart, M. Holzapfel, P. Nova´k, P.G. Bruce, J. Am. Chem. Soc., 128 (2006) 1390. [4] A. Débart, A.J. Paterson, J. Bao, P.G. Bruce, Angew. Chem. Int. Ed., 47 (2008) 4521. [5] Y.-C. Lu, D.G. Kwabi, K.P. Yao, J.R. Harding, J. Zhou, L. Zuin, Y. Shao-Horn, Energy Environ. Sci., 4 (2011) 2999. [6] S.S. Zhang, D. Foster, J. Read, J. Power Sources, 195 (2010) 1235. [7] J. Read, J. Electrochem. Soc., 153 (2006) A96. [8] Y. Li, J. Wang, X. Li, D. Geng, M.N. Banis, R. Li, X. Sun, Electrochem. Commun., 18 (2012) 12. [9] M. Mirzaeian, P.J. Hall, Electrochim. Acta, 54 (2009) 7444. [10] M. Mirzaeian, P.J. Hall, J.Power Sources, 195 (2010) 6817. [11] J. Xiao, D. Wang, W. Xu, D. Wang, R.E. Williford, J. Liu, J.-G. Zhang, J. Electrochem. Soc., 157 (2010) A487. [12] K.N. Wood, R. O'Hayre, S. Pylypenko, Energy Environ. Sci., 7 (2014) 1212.
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[13] P. Kichambare, S. Rodrigues, J. Kumar, ACS Appl. Mater. Interfaces, 4 (2012) 49. [14] W. Xia, J. Masa, M. Bron, W. Schuhmann, M. Muhler, Electrochem. Commun., 13 (2011) 593. [15] E. Yoo, J. Nakamura, H. Zhou, Energy Environ. Sci., 5 (2012) 6928. [16] Y. Li, J. Wang, X. Li, J. Liu, D. Geng, J. Yang, R. Li, X. Sun, Electrochem. Commun., 13 (2011) 668. [17] P. Kichambare, J. Kumar, S. Rodrigues, B. Kumar, J. Power Sources, 196 (2011) 3310. [18] H. Nie, H. Zhang, Y. Zhang, T. Liu, J. Li, Q. Lai, Nanoscale, 5 (2013) 8484. [19] W. Li, D. Chen, Z. Li, Y. Shi, Y. Wan, G. Wang, Z. Jiang, D. Zhao, Carbon, 45 (2007) 1757. [20] L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin and R. S. Ruoff, Energy Environ. Sci., 5 (2012) 7936. [21] X.-h. Yang, P. He, Y.-y. Xia, Electrochem. Commun., 11 (2009) 1127. [22] K.A. Kurak, A.B. Anderson, J. Phys. Chem. C, 113 (2009) 6730. [23] E.J. Biddinger, U.S. Ozkan, J. Phys.Chem. C, 114 (2010) 15306. [24] M. Dos Santos, F. Alvarez, Phys. Rev. B, 58 (1998) 13918. [25] M. Terrones, H. Terrones, N. Grobert, W. Hsu, Y. Zhu, J. Hare, H. Kroto, D. Walton, P. KohlerRedlich, M. Rühle, Appl. Phys. Lett., 75 (1999) 3932. [26] H. Niwa, K. Horiba, Y. Harada, M. Oshima, T. Ikeda, K. Terakura, J.-i. Ozaki, S. Miyata, J. Power Sources, 187 (2009) 93. [27] G. Liu, X. Li, P. Ganesan, B.N. Popov, Appl. Catal., B: Environ., 93 (2009) 156. [28] H. Niwa, M. Kobayashi, K. Horiba, Y. Harada, M. Oshima, K. Terakura, T. Ikeda, Y. Koshigoe, J.-i. Ozaki, S. Miyata, S. Ueda, Y. Yamashita, H. Yoshikawa, K. Kobayashi, J. Power Sources, 196 (2011) 1006. [29] G. Liu, X. Li, P. Ganesan, B.N. Popov, Electrochim. Acta, 55 (2010) 2853. [30] T. Ikeda, M. Boero, S.-F. Huang, K. Terakura, M. Oshima, J.-i. Ozaki, J. Phys. Chem. C, 112 (2008) 14706. [31] M. Fujita, K. Wakabayashi, K. Nakada, K. Kusakabe, J. Phys. Soc. Jpn., 65 (1996) 1920. [32] D.E. Ramaker, M. Teliska, Y. Zhang, A.Yu. Stakheev, D.C. Koningsberger, Phys. Chem. Chem. Phys., 5 (2003) 4492. [33] D. Deng, X. Pan, L. Yu, Y. Cui, Y. Jiang, J. Qi, W.-X. Li, Q. Fu, X. Ma, Q. Xue, Chem. Mater., 23 (2011) 1188.
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Figure captions Fig. 1. Structure characterization, a. TEM images of N-MC-1000, b. TEM images of N-MC-1200, c. TEM images of N-MC-1400, d. EDX mapping of C and N in N-MC-1000, e. EDX mapping of C and N in N-MC-1200, f. EDX mapping of C and N in N-MC-1400, g. SEM images of N-MC-1000 cathode, h. SEM images of N-MC-1200 cathode, i. SEM images of N-MC-1400 cathode. Fig.2. XRD pattern of N-MC-1000, N-MC-1200 andN-MC-1400. Fig. 3. Pore structure of samples treated at different temperature, a. N2 adsorption/desorption isotherms, b. PSD curves. Fig. 4. XPS analysis of N1s signals, a. N-MC-1000, b. N-MC-1200, c. N-MC-1400. Fig. 5. Electrochemical performance of N-MC-x, a. the first cycle discharge curves at 300 mA g-1 in 1.2 atm O2, b. the first cycle CV curves for the reduction of O2-saturated 1.0 M LiTFSI/TEGDME on GC electrode at sweep rate 10 mV s-1 .
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Table 1 Porosity parameters of N-MC-x.
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Table 2 Detail breakdown of N1s signal showing the peak position and relative composition of different surface nitrogen groups.
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S0013-4686(14)02159-8 http://dx.doi.org/doi:10.1016/j.electacta.2014.10.138 EA 23654
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
3-9-2014 27-10-2014 27-10-2014
Please cite this article as: Hongjiao Nie, Yining Zhang, Wei Zhou, Jing Li, Baoshan Wu, Tao Liu, Huamin Zhang, Nitrogen-containing mesoporous carbon cathode for lithium-oxygen batteries: The influence of Nitrogen on oxygen reduction reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.10.138 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen-containing mesoporous carbon cathode for lithium-oxygen batteries: the influence of Nitrogen on oxygen reduction reaction Hongjiao Nie a,b,Yining Zhang a, Wei Zhoua,b, Jing Lia,b, Baoshan Wua,Tao Liua*and Huamin Zhanga * a
Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
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University of Chinese Academy of Sciences, Beijing 100039, China
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* Corresponding author. Tel.: +86 411 84379072; fax: +86 411 84665057.
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Zhongshan Road 457, Dalian 116023, China
** Corresponding author. Tel.: +86 411 84379535; fax: +86 411 84665057. E-mail address:[email protected] (H.
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Zhang), [email protected] (T. Liu).
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Graphical abstract
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The direct effect of nitrogen content and various nitrogen species on oxygen reduction reaction (ORR)
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activities in nonaqueous Li-O2 batteries are systematically investigated. Mesoporous carbon (MC) with
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various nitrogen species is prepared through heat treatment of N-containing precursor under different
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temperature. The effect of the heat treatment temperature on the performance of carbon materials in Li-O2
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battery is investigated. The bonding state of nitrogen atoms is found to have a significant effect on the ORR
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activity. The ORR activity in Li-O2 battery is proved to be dependent on the quaternary N content while the
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total N content in the carbon material does not play a crucial role in the ORR process.
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Highlights
The role of various N in ORR for Li-O2 battery was investigated.
The total N content does not play an important role in the ORR process.
The ORR activity in Li-O2 battery is dependent on the quaternary N content.
Abstract
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The direct effect of nitrogen content and various nitrogen species on oxygen reduction reaction (ORR) activities in nonaqueous lithium-oxygen (Li-O2) batteries are systematically investigated. Mesoporous carbon (MC) with various nitrogen species is prepared through heat treatment of Ncontaining precursor under different temperature. The effect of the heat treatment temperature on the performance of carbon materials in Li-O2 battery is investigated. The bonding state of nitrogen atoms is found to have a significant effect on the ORR activity. The ORR activity in Li-O2 battery is proved to be dependent on the quaternary N content while the total N content in the carbon material does not play a crucial role in the ORR process.
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Keywords:Lithium-oxygen battery, carbon cathode, nitrogen doping, oxygen reduction reaction activity, quaternary nitrogen
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1. Introduction Li-O2 batteries have been attracting more and more interests due to their extremely high energy density[1]. It has the potential to provide a capacity 5–10 times higher than the state-of-art Li-ion cells[2-4]. However, there are still many challenges, including low capacity, poor cycle stability and low energy efficiency, to be overcome before the practical application of Li-O2 batteries[5]. A typical non-aqueous Li-O2 battery is composed of a Li metal anode, a non-aqueous Li+ conducting electrolyte and a porous air electrode (cathode). During discharge, Li+ combines with reduced oxygen to form insoluble lithium oxide, which would precipitate on the surface or within the pores of the cathode[6]. Once the porosity of the cathode is totally chocked, the discharge process terminates[7]. Therefore, the cathode plays a key role on the cell performance. Currently, cathodes in most Li-O2 batteries are porous carbon materials with large surface area and pore volume. So far, various carbon materials, such as mesoporous carbon foam, carbon aerogel, carbon nanotubes and graphene, have been studied as cathode materials in Li-O2 batteries[8]. In these studies, the performances of carbon cathodes vary according to their microstructures. For high specific capacity, both large pore volume and optimized pore size distribution (PSD) are essential, because it could accommodate more discharge products[9-11]. On the other hand, it cannot be ignored that the carbon cathodes performance also greatly depends on their surface composition. Functionalization of carbon-based materials, such as heteroatom doping, can modify their surface, interfacial, and electronic properties, thereby further improving their performance in Li-O2 batteries. Among possible chemical choices for carbon modification, nitrogen doping has long been a natural and widely studied option. Because this modification allows for the beneficial properties of the carbon to be utilized while finely tuning the final electrical, morphological, and chemical properties of the functionalized carbon network[12]. So
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many studies have focused on the study of nitrogen-containing carbon materials for application in LiO2 batteries recently. In these studies, the cell performance is significantly improved due to enhanced oxygen reduction reaction (ORR) activity at the cathode[8, 13-15]. Li et al. investigated the performance of nitrogen-doped graphene nanosheets electrode and found that it not only exhibited higher discharge capacity but also showed higher average discharge plateau than undoped ones at various discharge current densities. At the same time, the onset potentials and the number of electrons transferred during ORR were also improved[8]. The discharge performance of carbon nanotubes and nitrogen-doped carbon nanotubes were also evaluated in Li-O2 battery respectively. With a nitrogen content of 10.2 at. %, the nitrogen-doped carbon nanotubes cathode delivered a specific capacity 1.5 times higher than that of pristine carbon nanotubes cathode[16]. This is also confirmed by Rodrigues et al. who compared the electrochemical performance of carbon materialswith or without nitrogen[17]. Although numerous studies have proved the beneficial effect of nitrogen doping on the Li-O2 battery cathode performance, a systematic study on the direct effect of nitrogen content and various nitrogen species on ORR activities has not been reported. In this paper, mesoporous carbon with different nitrogen content and nitrogen species is used as Li-O2 battery cathode to study the effect of nitrogen content and various nitrogen species on ORR activities. They were all derived from the same starting nitrogen-containing mesoporous carbon (NMC) material. The synthesis of N-MC was carried out by a previous reported hard template method. Through heat treating N-MC at different temperature, the relative amounts of various nitrogen species can be controlled. This approach allowed the direct comparison of the ORR activities of mesoporous carbon cathodes possessing different nitrogen species. The morphologies and microstructure of the prepared carbon powder and carbon cathodes were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) respectively. BrunauerEmmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS) was used to characterize the pore structure and surface nitrogen groups respectively. Finally, the discharge behavior and cyclic voltammetry (CV) performance of N-MC were also evaluated. 2. Experiment 2.1. Preparation of samples Mesoporous carbon with different nitrogen content and nitrogen species was prepared by using a previous reported hard template method[18]. Melamine-formaldehyde (MF) resin and silica sol were employed as carbon precursor and template respectively. Briefly, melamine (3.15 g) was firstly added into the mixture of formaldehyde solution (5 mL, 37 wt %) and deionized water (12.5 mL). Then, the PH was adjusted to 8.5 with Na2CO3 solution. Afterwards, the mixture was heated at 85 ℃ in a water bath with stirring to get a clear solution. Then, silica sol (30 mL, 5 wt %) was mixed with the solution after cooling down to 40 ℃. In order to initiate the condensation reaction, the PH was adjusted to 4.5 by the addition of 2 M HCl. The solution was kept static for 3 h to form a MF resin with silicon dioxide doped. The resin was dried and cured at 180 ℃ for 24 h. To obtain mesoporous carbon with various nitrogen species, the result resin/SiO2 composite was carbonized at 1000, 1200 and 1400 ℃ under Ar atmosphere for 2 h. Then the silica template was removed by 20 % HF for 24 h. They were thoroughly washed in deionized water. Finally, the samples were dried at 80 ℃. In the following discussion, they were denoted as N-MC-x (x=1000, 1200 and 1400), where x stood for the heat treatment temperature.
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2.2. Material characterizations 2.2.1. Transmission electron microscopy (TEM) and Scanning electronic microscopy (SEM) The morphology of carbon material was characterized by transmission electron microscopy (TEM, Tecnai G2 F20) equipped with EDX detector. Scanning electron microscopy (SEM) micrographs were taken on QUANTA 200FEG to get the surface morphology of N-MC-x cathodes. 2.2.2. X-ray diffractionexperiment X-ray diffraction (XRD) analyses of the N-MCx were carried out on a Rigaku Rotalflex (RU200B) diffractometer with a CuKα source (λ=1.54056 Å) and a Ni-filter. The 2θ values of X-ray diffractogramsvaried between 15° and 90° with a scan rate of 7° min-1. 2.2.3. N2 adsorption/desorption experiment N2 adsorption isotherms were measured at 77.3K using an ASAP2010 system. Surface areas and pore volumes were determined using Brunauer-Emmett-Teller (BET) method. The pore size distribution curves were calculated from the desorption branches of nitrogen isotherms using the Barrett- Joyner-Halenda (BJH) model. 2.2.4. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB250 system utilizing Al K-alpha monochromatic (1486.6 eV) with a spot area of 500 μ m. The XPS spectra were peak fit and analyzed using XPeak4.1 (Photoelectron Spectroscopy Lab, Seoul National University). Spectra were calibrated according to the C1s (284.6 eV) peak. 2.3. Preparation of air electrode The N-MC-x cathodes were prepared by casting slurry of the as-prepared N-MC-x with PTFE as a binder (with a weight ratio of 80:20) onto nickel foam (16 mm in diameter) as cathode current collector (both sides).The carbon loading of each electrode was 0.5mg. 2.4.Li-O2 battery construction、 Li-O2 battery was constructed in an argon-filled glove box (H2O＜0.1 ppm, O2＜0.1 ppm). The cell consists of a 0.45 mm thick Li foil (16 mm in diameter) as anode, a carbon electrode as cathode and polypropylene fiber (Novatexx 2471 Freudenberg Filtration Technologies KG) soaked with electrolyte as a separator. The electrolyte employed here is 1.0 M bis(trifluoromethane) sulfonamide lithium (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME, Aladdin). A stainless steel mesh was used as the current collector. All the cell parts were compressed together to ensure good contact and the cell was completely sealed except for the O2 entryway. 2.5. Electrochemical characterizations 2.5.1. Cell discharge performance After exposed to flow pure oxygen for 5 h, the Li-O2 battery was discharged galvanostatically at a rate of 300 mA g-1 on a LAND 2100 system (Wuhan, China) with a cutoff voltage of 2 V. 2.5.2. Cyclic voltammetry (CV) The cyclic voltammetry (CV) tests were conducted in a traditional three-electrode system with glassy carbon (Φ 4 mm) modified with porous carbon film as working electrode (WE), glass filter protected Li foil as counter electrode (CE), and porous ceramic protected Li foil as reference electrode (RE). Porous carbon (1.0 mg) were dissolved in 2-propanol (0.5 mL) and Nafion-Li (5 wt%, 15 μL). After ultrasonication for 30 min, 5 μL of the received carbon ink was drop-casted on the glass carbon (GC) electrode and vacuum-drying at room temperature. 1M LITFSI/TEGDME
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were used as electrolyte. All the measurements were conducted between 2.0 V to 4.3 V using an AFCBP1 bipotentiostat (Pine Research Instrumentation, USA) in an argon-filled glove box (H2O＜0.1 ppm, O2＜0.1 ppm) at room temperature. 3. Results and discussion 3.1 Structure characterization The detailed structure and the element distribution of the resulting N-MC-x powder are shown in Figure 1a-f. The TEM images show similar morphology of N-MC-x with numerous threedimensionally interconnected pores. These pores aggregate randomly all through the carbon network. The formation of the highly porous structure is mainly caused by the addition of silica template, as previously reported[19]. In addition, the graphitization degrees of N-MC-x do not change much with the heat treatment temperature carried out in this paper, which can be confirmed by the XRD pattern in Figure 2. By employing the nitrogen-containing MF resin as carbon source, N-MC-x is “born” rich in nitrogen. From EDX mapping, it could be obviously seen that the total nitrogen content decreases when the samples are heat treated at higher temperature, similar to results from the literatures[20]. Still, N atoms disperse homogeneously for all of N-MC-x. It is reasonable because N atoms are incorporated through in-situ doping instead of post-treatment such as NH3 calcination and so on. A more detailed analysis on the surface nitrogen compositions of the N-MC-x samples using XPS analysis will be presented later in this paper. SEM test was also used to characterize the morphology of Li-O2battery cathodes fabricated using N-MC-x. Figure 1g-i shows that heat treatment at different temperature do not change the morphology of N-MC-x cathodes significantly. The porous structure is perfectly preserved. Disordered open pores in the range of tens of nanometers spread over the electrode matrix, which would be fully exploited during discharge owing to their applicable size. And the size of these pores could be controlled by proper template selection. 3.2 Porosity analysis The pore structure of N-MC-x was further evaluated by N2 adsorption/desorption method. All of N-MC-1000, N-MC-1200 and N-MC-1400 possess a typical type-IV isotherm with hysteresis loop following the IUPAC classication, indicating that they all have highly mesoporous structure. The PSD curves derived from the desorption branch of the isotherm were also shown in Figure 2b. As the same silica template was used, the mesopore size distributions of the obtained N-MC-x are similar. Their mean mesopore sizes are ca. 23 nm, approximately equal to the average diameter of the silica nanoparticle. The pore parameters are summarized in Table 1. The BET specific surface areas and pore volume decrease from 817 to 566 m2g−1 and 2.24 to 2.02 cm3g−1, respectively, with the change of heat treatment temperature from 1000 to 1400 ℃. However, for the pores larger than 10 nm, the pore volume changes slightly, less than 3 %. So the decrease of specific surface areas and pore volume probably mainly comes from the collapse of micropores smaller than 10 nm. Previous studies by Xia et al. have proved that pores with several tens of nanometers should be the major contributor to the discharge capacity[21]. So in this study, the impact of pore structure of N-MC-x on discharge performance is supposed to be small. It allows us to study the effect of surface N atoms on battery performance independently. 3.3 The analysis of N composition
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To gain more insight into the composition of surface N atoms in N-MC-x, XPS measurements were performed. When improving the heat treatment temperature, the surface N content obviously decreases, which is determined to be 8.62, 4.37 and 1 % for N-MC-1000, N-MC-1200 and N-MC1400 respectively. After deconvolution, the N1s peak can be fitted to four dominant peaks (pyridine N, pyrrolic N, quaternary N and pyridine N-oxide) as shown in Figure 4. Pyridinic N refers to N atoms at the edges of graphene planes, where it is bonded to two carbon atoms, donates one pelectron to the aromatic π system[20, 22]. Pyrrolic N atoms are incorporated into five-membered heterocyclic rings, which are bonded to two carbon atoms and contribute two p-electrons to the π system. Quaternary N atoms are within a graphene plane and bonded to three carbon atoms. Pyridine N-oxide forms after the sample is exposed to air, which is bonded to two carbon atoms and one oxygen atom. The pyridinic and pyrrolic N are always located at the graphitic edge, whereas quaternary N can be both ‘‘edge-N’’ and ‘‘bulk-like-N’’[23]. From quantitative analysis, the binding energy and relative composition ratio of N in N-MC-x were listed in Table 2. For N-MC-1000, only two peaks, pyridinic N and pyrrolic N, appear. Upon further increasing heat treatment temperature, it can be seen that for N-MC-1200 in Figure 4 and Table 2, the intensities of pyridinic N and pyrrolic N peaks become much weaker and their proportion are obviously smaller than those of N-MC-1000, while the intensity of quaternary N peak largely increase. At 1400 ℃, pyridinic N and pyrrolic N peaks are not observed and only quaternary N peak are found. It agrees with reported results regarding favorable formation of pyridine N at higher nitrogen content[24, 25]. 3.4 The effects of N species on discharge and CV performance Although there are many reports that the doping of N can effectively improve the ORR catalytic activity of cathodesin Li-O2 battery, the identity and role of the electrocatalytically active center are still controversial as its contribution to catalytic activity is not well defined. To investigate the electrocatalytic properties of N-MC-x cathodesin Li-O2 batteries, the discharge and CV curvesare acquired. Figure 5a shows that as raising the heat treatment temperature from 1000 to 1200 ℃, the discharge capacity has a 68 % increase, from 6250 to 10488 mAh g-1. When the heat treatment temperature further improves to 1400 ℃, the discharge capacity starts to decrease (8536 mAh g-1). At the same time, the voltage of discharge flat holds the same trend. It is identified to be 2.61, 2.69 and 2.64 V for N-MC-1000, N-MC-1200 and N-MC-1400 respectively. Both the highest discharge capacity and the highest voltage plateau are observed for Li-O2 battery with N-MC-1200 cathode. Figure 5b displays the CV curves for the reduction of O2-saturated electrolyte at a scan rate of 10 mV s-1. The reduction peak current (ip) for N-MC-1000, N-MC-1200 and N-MC-1400 firstly increase and then decrease, which is in accordance with the change of discharge capacity in Figure 5a. Both the cell discharge behavior and CV curves demonstrate that N-MC-1200 has the best performance of ORR among the carbon materials investigated, while N-MC-1000 has the poor performance. Generally, it has been believed that the pore structure and N-containing functional groups in the carbon materials play key roles for the different activity. As mentioned above, N-MC-x shows no obvious difference in the structure and morphology, as well as in the mesoporosity and pore size distribution (Fig. 1, Fig.2 and Tab.1). Therefore, the difference of ORR activity for the samples in this paper is speculated to be caused by N content and N species proportion. Based on the analysis of XPS and electrochemical performance (Fig. 4, Tab. 2 and Fig. 5a, b), no obvious
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dependence of the ORR activity on the total content of N is observed. But this investigation showed that quaternary N atoms seem to be the most important species for the ORR due to the matching relationship between activity and quaternary N contents. Previous studies have reported the ORR catalytic activity of quaternary N both experimentally and theoretically[26-28]. As the first step of ORR, the adsorption of O2 is important[29]. It has been shown that the O2 molecule is preferentially adsorbed associatively at C sites on graphene-like zigzag edges if a quaternary-N is located nearby[26, 30]. This is probably because carbon at the zig-zag edge has sharp density of states near the Fermi level called “the edge states”, which is crucially important for O2 adsorption [31, 32]. In addition, the relative electronegativity of quaternary N atoms reduces the electron density on the adjacent C nuclei, which helps electrons transfer from the adjacent C to N atoms, and N backdonates electrons to adjacent C pz orbitals. The donation and backdonation processes facilitate O2 dissociation on the adjacent C atoms[33]. This is in accordance with our experiment results that the samples containing more quaternary N show higher ORR activities. Thus, effective doping with quaternary N should be a practical guideline for the synthesis of active carbon cathode materials in Li-O2 battery. 4. Conclusion N-containing carbon is a promising cathode material in Li-O2 battery, but a deeper understanding of the relationship between the concentration and type of N species and ORR activity is needed. In this study, N-doped mesoporous carbon (N-MC) was prepared through hard template method by employing N-containing polymer as carbon source. Heat treatment of N-MC precursor under different temperature gives products with various nitrogen species. Most importantly, the influence of material microstructure on electrochemical performance could be excluded because the morphology, crystalline and pore structure of all samples remained nearly unchanged. The total N content in the carbon material does not play an important role in the ORR processin Li-O2 battery and the ORR activity was proved to be dependent on the quaternary N content. References [1] K.M. Abraham, Z. Jiang, J. Electrochem. Soc., 143 (1996) 1. [2] P.G. Bruce, Solid State Ionics, 179 (2008) 752. [3] T. Ogasawara, A. Débart, M. Holzapfel, P. Nova´k, P.G. Bruce, J. Am. Chem. Soc., 128 (2006) 1390. [4] A. Débart, A.J. Paterson, J. Bao, P.G. Bruce, Angew. Chem. Int. Ed., 47 (2008) 4521. [5] Y.-C. Lu, D.G. Kwabi, K.P. Yao, J.R. Harding, J. Zhou, L. Zuin, Y. Shao-Horn, Energy Environ. Sci., 4 (2011) 2999. [6] S.S. Zhang, D. Foster, J. Read, J. Power Sources, 195 (2010) 1235. [7] J. Read, J. Electrochem. Soc., 153 (2006) A96. [8] Y. Li, J. Wang, X. Li, D. Geng, M.N. Banis, R. Li, X. Sun, Electrochem. Commun., 18 (2012) 12. [9] M. Mirzaeian, P.J. Hall, Electrochim. Acta, 54 (2009) 7444. [10] M. Mirzaeian, P.J. Hall, J.Power Sources, 195 (2010) 6817. [11] J. Xiao, D. Wang, W. Xu, D. Wang, R.E. Williford, J. Liu, J.-G. Zhang, J. Electrochem. Soc., 157 (2010) A487. [12] K.N. Wood, R. O'Hayre, S. Pylypenko, Energy Environ. Sci., 7 (2014) 1212.
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[13] P. Kichambare, S. Rodrigues, J. Kumar, ACS Appl. Mater. Interfaces, 4 (2012) 49. [14] W. Xia, J. Masa, M. Bron, W. Schuhmann, M. Muhler, Electrochem. Commun., 13 (2011) 593. [15] E. Yoo, J. Nakamura, H. Zhou, Energy Environ. Sci., 5 (2012) 6928. [16] Y. Li, J. Wang, X. Li, J. Liu, D. Geng, J. Yang, R. Li, X. Sun, Electrochem. Commun., 13 (2011) 668. [17] P. Kichambare, J. Kumar, S. Rodrigues, B. Kumar, J. Power Sources, 196 (2011) 3310. [18] H. Nie, H. Zhang, Y. Zhang, T. Liu, J. Li, Q. Lai, Nanoscale, 5 (2013) 8484. [19] W. Li, D. Chen, Z. Li, Y. Shi, Y. Wan, G. Wang, Z. Jiang, D. Zhao, Carbon, 45 (2007) 1757. [20] L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin and R. S. Ruoff, Energy Environ. Sci., 5 (2012) 7936. [21] X.-h. Yang, P. He, Y.-y. Xia, Electrochem. Commun., 11 (2009) 1127. [22] K.A. Kurak, A.B. Anderson, J. Phys. Chem. C, 113 (2009) 6730. [23] E.J. Biddinger, U.S. Ozkan, J. Phys.Chem. C, 114 (2010) 15306. [24] M. Dos Santos, F. Alvarez, Phys. Rev. B, 58 (1998) 13918. [25] M. Terrones, H. Terrones, N. Grobert, W. Hsu, Y. Zhu, J. Hare, H. Kroto, D. Walton, P. KohlerRedlich, M. Rühle, Appl. Phys. Lett., 75 (1999) 3932. [26] H. Niwa, K. Horiba, Y. Harada, M. Oshima, T. Ikeda, K. Terakura, J.-i. Ozaki, S. Miyata, J. Power Sources, 187 (2009) 93. [27] G. Liu, X. Li, P. Ganesan, B.N. Popov, Appl. Catal., B: Environ., 93 (2009) 156. [28] H. Niwa, M. Kobayashi, K. Horiba, Y. Harada, M. Oshima, K. Terakura, T. Ikeda, Y. Koshigoe, J.-i. Ozaki, S. Miyata, S. Ueda, Y. Yamashita, H. Yoshikawa, K. Kobayashi, J. Power Sources, 196 (2011) 1006. [29] G. Liu, X. Li, P. Ganesan, B.N. Popov, Electrochim. Acta, 55 (2010) 2853. [30] T. Ikeda, M. Boero, S.-F. Huang, K. Terakura, M. Oshima, J.-i. Ozaki, J. Phys. Chem. C, 112 (2008) 14706. [31] M. Fujita, K. Wakabayashi, K. Nakada, K. Kusakabe, J. Phys. Soc. Jpn., 65 (1996) 1920. [32] D.E. Ramaker, M. Teliska, Y. Zhang, A.Yu. Stakheev, D.C. Koningsberger, Phys. Chem. Chem. Phys., 5 (2003) 4492. [33] D. Deng, X. Pan, L. Yu, Y. Cui, Y. Jiang, J. Qi, W.-X. Li, Q. Fu, X. Ma, Q. Xue, Chem. Mater., 23 (2011) 1188.
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Figure captions Fig. 1. Structure characterization, a. TEM images of N-MC-1000, b. TEM images of N-MC-1200, c. TEM images of N-MC-1400, d. EDX mapping of C and N in N-MC-1000, e. EDX mapping of C and N in N-MC-1200, f. EDX mapping of C and N in N-MC-1400, g. SEM images of N-MC-1000 cathode, h. SEM images of N-MC-1200 cathode, i. SEM images of N-MC-1400 cathode. Fig.2. XRD pattern of N-MC-1000, N-MC-1200 andN-MC-1400. Fig. 3. Pore structure of samples treated at different temperature, a. N2 adsorption/desorption isotherms, b. PSD curves. Fig. 4. XPS analysis of N1s signals, a. N-MC-1000, b. N-MC-1200, c. N-MC-1400. Fig. 5. Electrochemical performance of N-MC-x, a. the first cycle discharge curves at 300 mA g-1 in 1.2 atm O2, b. the first cycle CV curves for the reduction of O2-saturated 1.0 M LiTFSI/TEGDME on GC electrode at sweep rate 10 mV s-1 .
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Table 1 Porosity parameters of N-MC-x.
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Table 2 Detail breakdown of N1s signal showing the peak position and relative composition of different surface nitrogen groups.
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N-MC-1000
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Pyrrolic-N
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Peak position(eV ) 398.5
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Peak position(eV ) 397.9
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