Exploring the nitrogen species of nitrogen doped

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Exploring the nitrogen species of nitrogen doped graphene as electrocatalysts for oxygen reduction reaction in Aleair batteries Yisi Liu a, Jie Li a, Wenzhang Li a,*, Yaomin Li b, Faqi Zhan a, Hui Tang a, Qiyuan Chen a a b

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

article info

abstract

Article history:

The relationship of nitrogen species and catalytic activity of nitrogen doped graphene for

Received 8 September 2015

oxygen reduction reaction (ORR) is extensively studied but is still inconclusive. In this

Received in revised form

paper, the specific nitrogen types in N-graphene are controlled by regulating the mass ratio

20 October 2015

of graphene oxide (GO) and urea. The detection of groups on the surface of N-graphene is

Accepted 26 October 2015

carried out by X-ray photoelectron spectroscopy, Raman spectroscopy, and Fourier trans-

Available online 14 November 2015

form infrared spectroscopy. The catalytic activity of N-graphene catalysts with different nitrogen configurations is evaluated by cyclic voltammograms (CV), rotating disk electrode

Keywords:

(RDE), and electrochemical impedance spectroscopy (EIS) measurements in O2-saturated

Nitrogen doped graphene

0.1 M KOH electrolyte. It is found that the N-graphene with the mass ratio of 1:200 (con-

Electrocatalyst

taining graphitic N configuration) shows better oxygen reduction catalytic activity than

Graphitic N

that of other mass ratio (without graphitic N configuration) catalysts. Furthermore, the

Oxygen reduction reaction

Alair battery with the mass ratio of 1:200 N-graphene cathode displays higher open circuit

Aluminumeair battery

voltage and energy density. Density functional theory (DFT) quantum chemical calcula-

Adsorption energy

tions are used to investigate the influence of different nitrogen species on adsorption energy of oxygen atoms. The calculated results indicate graphitic N is more conducive for the adsorption of oxygen atoms. © 2015 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Compared with other chemical-based batteries, metaleair batteries can contribute greatly to relieve the problems involved in the rapid growth of applications, especially in the fields of electric and hybrid electric vehicles [1e3] due to their several advantages including low emissions [4,5], portability,

high energy density [6,7], very low noise and vibrations [7,8]. Among the various metaleair batteries, the aluminumeair battery has been considered as a promising power source and energy storage device [9e11] because of its low cost, environmentally friendly, and high theoretical energy density (4300 Wh g1). However, the industrial application of Al-air batteries is limited because of slow kinetics of the oxygen reduction reaction (ORR) and large overpotential exists in the

* Corresponding author. Tel./fax: þ86 731 8887 9616. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.ijhydene.2015.10.109 0360-3199/© 2015 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.


current cathodic catalyst [12,13]. Therefore, one of the critical challenges to widely commercialize is to explore less expensive and highly efficient catalysts with low overpotential for better ORR. Graphene is a novel nanomaterial with a single sheet of carbon atoms packed in a hexagonal lattice. It has been certificated that graphene has many attractive properties such as high specific surface area (2630 m2 g1) [14], high thermal conductivity (~5000 W mK1) [15], and fast charge carrier mobility (~200,000 cm2 V1 s1) [15]. Hence, currently much research effort is trying to apply it to the ORR catalysts in metal-air batteries [12,16,17]. Although chemically reduced graphene exhibits certain catalytic activity, it is also difficult to satisfy the application requirements of metal-air batteries. Therefore, the catalytic performance of graphene need to be further improved. One of the most promising approaches is creating defects [18] or embedding impurities into the graphene lattice using guest atoms, such as boron or nitrogen [19,20]. It has been proved that doping with guest atoms is a suitable method to modify materials intrinsically for using in oxygen reduction reactions [21,22]. Among the numerous candidate dopants, nitrogen is considered to be an excellent element for the chemical doping of graphene because it has similar atomic size to carbon atom, and includes five valence electrons available to form strong valence bonds with carbon atoms [23]. The N-graphene presents different properties compared with the pristine graphene. For example, the spin density and charge distribution of carbon atoms will be affected by the adjacent nitrogen dopants [24,25], which brings about more active sites on the graphene surface. These active sites can participate in the oxygen reduction reaction directly. Recently, several groups have reported studies on metal-free N-graphene with enhanced catalytic activity toward ORR [16,26e31]. Ozaki and co-workers studied the adsorption barriers of O2 on different sites of N-graphene and found several structures that facilitated the adsorption of O2 [32]. Nagaiah et al. [33] reported that oxygen reduction intermediates can be detected on the nitrogen-doped multilayer graphene after ORR, implying that nitrogen close to carbon in N-graphene is involved in the catalysis of the ORR. In the studies about N status in N-graphene used as ORR catalysts [34,35], both quaternary and pyridinic nitrogen have been proved to play a dominant role for the ORR activity. Beyond that, the graphitic N was reported to play a crucial role in oxygen reduction reaction, and pyridinic N and pyrrolic N can serve as metal coordination sites due to their lone-pair electrons [36]. Ouyang et al. [37] explored the active sites of nitrogen-doped graphene as catalysts for oxygen reduction reaction. The valley-N is certificated to play an important effect on ORR. However, the relationship between nitrogen types, nitrogen content and the ORR activity of N-graphene is still unclear. In this paper, we synthesized nitrogen doped graphene as an active electrocatalyst for Aleair battery by a facile hydrothermal method with urea as nitrogen source. The relationship of nitrogen types, nitrogen content and ORR catalytic activity was further discussed by a series of electrochemical measurements. Additionally, DFT calculations were carried out to evaluate the catalytic activity through calculating the adsorption energy of oxygen atoms (O). To the best of our

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knowledge, we provide experimental evidences revealing the significance of different bonding characters for the embedded N atoms, and analyze the influences of different N types on the catalytic activity using electrochemical tests and quantum chemical calculations.

Experimental Preparation of nitrogen doped graphene (N-rGO) Graphene oxide (GO) was synthesized from natural graphite by modified Hummer's method [38]. The N-rGO catalysts were prepared by a hydrothermal method. In the first step of synthesis, 50 mg of graphene oxide was dispersed in 100 mL DI water and sonicated for at least 1 h to form a homogeneous suspension of 0.5 mg mL1. The pH of the suspension was regulated to 8.0 by using concentrated ammonia. 30 mL of GO suspension was measured into a beaker, then a certain mass of urea was slowly added into the GO suspension, making the mass ratios of GO and urea were 1:100, 1:200 and 1:300, respectively. Corresponding products were labeled as NG-1, NG-2, and NG-3. Subsequently, the mixed solution was sonicated for 2 h to form a homogeneous dispersed mixture and transferred into an autoclave, sealed and heated at 170 C for 12 h. The autoclave was naturally cooled to room temperature, then the precipitate was collected and centrifuged several times with DI water and ethanol to remove any impurities. The obtained black slurry dried at 60 C in an oven for overnight.

Materials characterization The morphology and microstructure of the synthesized samples were investigated by a high resolution transmission electron microscope (HRTEM, FEI TECNAI G2 F20) operated at 120 kV. Raman spectra were measured by a LabRAM Hr800 confocal Raman microscopic system with a 532 nm excitation laser. Transmission electron microscopy (TEM) measurements were carried out using a JEM-2011EM microscope operated at 120 kV. Fourier transform infrared (FTIR) transmission spectra for the catalysts were recorded by a Thermo Fisher Nicolet 6700 FTIR spectrometer with a resolution of 4 cm1 in the 400e4000 cm1 region. The surface elemental analysis of the samples were performed by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific) spectrometer using monochromatized Al Ka (1486 eV) radiation with 0.1 eV energy increments at 20 eV pass energy.

Cyclic voltammetry and impedance spectroscopy measurements All the electrochemical measurements were carried out via an electrochemical analyzer (Zennium, Zahner, Germany) in a standard three-electrode cell at room temperature. A threeelectrode cell configuration was consisted of a glassy carbon electrode of 3 mm in diameter as working electrode, a platinum foil as counter electrode, and an Ag/AgCl in 3 M KCl as reference electrode. The electrolyte was 0.1 M KOH aqueous solution for all measurements. 4 mg of catalytic material and

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13 mL 5 wt.% Nafion solution was dispersed in 1 mL of 1:4 (v/v) water/isopropanol mixed solvent as catalyst ink, then ultrasonicated for at least 2 h to form a homogeneous suspension. 5 mL of the prepared catalytic ink was dropped onto a glassy carbon electrode and dried at ambient temperature in air, resulting in a catalyst loading of 0.238 mg cm2. The electrolyte was deaerated by purging high-purity O2 for at least 30 min before each electrochemical measurement and a flow of O2 was maintained over the electrolyte during electrochemical measurements. Cyclic voltammograms were conducted from 1.6 to 0.2 V (vs. Ag/AgCl) at a scan rate of 50 mV s1. The EIS measurement was performed with the alternative current (AC) voltage amplitude of 5 mV, the voltage frequency ranging from 100 KHz to 0.01 Hz and the applied potential of 0.8 V (vs. Ag/AgCl).

Rotating disk electrode measurements The RDE measurements were also carried out by a threeelectrode cell configuration. The working electrode is glassy carbon rotating disk electrode (RDE) of 5 mm in diameter. The catalyst ink was prepared by mixing 4 mg of catalytic material and 15 mL of 5 wt.% Nafion (D-521, Alfar Aesar) solution dispersed in 300 mL isopropanol, then ultrasonicated for about 40 min to form a homogeneous suspension. 5 mL of the prepared catalytic ink was dropped onto a glassy carbon RDE and dried in air, giving a catalyst loading of around 0.45 mg cm2. Other experimental conditions were the same as that of cyclic voltammograms measurements. The linear sweep voltammetry (LSV) were recorded from 0.8 to 0.2 V (vs. Ag/AgCl) at a scan rate of 10 mV s1 with various rotation rates (400, 900, 1600, and 2500 rpm).

Fabrication of air cathode and home-made Al-air battery An air cathode consists of a gas-diffusion layer on the air side and a catalyst layer on the electrolyte side. The catalyst paste was consisted of 10 mg of catalytic materials and 67 mL of 5 wt.% Nafion (D-521, Alfar Aesar.) solution dispersed in 1.0 mL of isopropyl alcohol. The air electrode was prepared by coating catalyst paste on a 2 cm 2 cm carbon paper (Toray carbon paper, thickness ¼ 0.28 mm), and dried in an vacuum oven at 70 C for 3 h to achieve a loading of 1.5 mg cm2. The catalytically active surface area was 1.0 cm2. The Al-air battery was fabricated with a polished aluminum foil (2.5 cm 2 cm) as anode. The gap between the cathode and anode was 2.0 cm and no separator was used. A 6 mol L1 KOH aqueous solution was used as the electrolyte.

Aluminumeair battery performance testing The catalytic activity of the catalysts was tested by fabricating a home-made Aleair battery. The discharge testing of batteries for NG-1, NG-2, and NG-3 were carried out by using a multichannel potentiostat (LANHE CT2001A, Wuhan) at room temperature with a constant discharge current (10 mA) mode under ambient condition, until the voltages of batteries dropped to 0 V.

DFT calculations Density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulation Package (VASP) to compute the adsorption energy of oxygen atom on nitrogen doping graphene [39e41]. The generalized gradient approximation (GGA) method with PBE functional for the exchangecorrelation term was used [42,43]. A kinetic energy cutoff of 400 eV was used with a plane-wave basis set. A 2 2 1 Monkhorst-Pack k-point sampling was used for the graphene unit cell of 6 6 1. The supercells were in a hexagonal lattice with the unit cell vectors a and b in the surface plane and c vertical to the graphene plane. The convergence of energy and A1, respectively. To forces were set to 1 104 eV and 0.03 eV study the adsorption energy of oxygen atoms on N-doping configurations, five different nitrogen distributed models were built.

Results and discussions Characterizations of cathode materials Fig. 1aef exhibit the transmission electron microscopy (TEM) images of the synthesized N-doped graphene with the GO/ urea mass ratio of 1:100, 1:200, and 1:300 at different magnifications, respectively. The graphene planar sheets with some folds are observed clearly in N-graphene, indicating that the features of high specific surface area and two-dimensional structure of graphene morphology are well maintained. The samples exhibit chiffon shapes, consisting of several layers of graphene. There is no apparent morphologic difference among the three samples, which illustrates nitrogen doping amount could not affect the morphology of graphene. Raman spectroscopy is a very powerful tool to characterize carbon materials. In this work, it was used to investigate the doping effects in nitrogen doped graphene. The D, G, and 2D bands are the conspicuous features in the spectrum of nitrogen doped graphene. They are manifested by the peaks at around 1320e1360, 1570e1590, and 2650e2720 cm1, respectively [44]. Fig. 2 presents the Raman spectra of pristine graphene and N-doped graphene (NG-1, NG-2, and NG-3). The inset shows the zoom-in view around the 2D band. The G band corresponds to the doubly degenerate E2g phonons at the Brillouin zone center [45]. The D band is attributed to the breathing mode of sp2-rings and requires a defect for its activation by an intervalley double-resonance (DR) Raman process [46]. The 2D band is due to the same second-order, intervalley DR process, but no defects are required for its activation, in contrast to that resulting in the D band [46]. For all the samples, high intensity of D band is easily observable, indicating the existence of significant defects. The intensity ratio of D and G bands (ID/IG) is a measurement of the amount of defects leading to intervalley scattering [47,48]. Apart from ID/IG, the intensity ratio of 2D and G bands (I2D/IG) has been used to estimate the nitrogen doping level [49], and is related to the thickness of the graphene [50]. These ratios of all the samples are listed in Table 1. As shown in Table 1, the values of ID/IG have no obvious changes, which indicate that the


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Fig. 1 e TEM images of (a, b) NG-1, (c, d) NG-2, and (e, f) NG-3 at different magnifications.

amount of defects and disorder is similar. Through calculating the I2D/IG, which are pyrrolic N > pyridinic N > quaternary N > pyridinic N oxide. Our calculated results clearly indicate that the adsorption energy of O on the N doped graphene with graphitic N is more negative than that of other nitrogen species. The DFT investigation of five models suggests that graphitic N is conducive for the ORR process and the adjacent carbons should be the main active sites, in good agreement with some of the previous experimental studies. The adjacent carbon atoms of graphitic N favor atomic charges that can induce the ORR process: the formation of a CeO bond, and the disassociation of an OeO bond [69]. The enhancement of ORR performance should be ascribed to the increased electron density and electron donating properties resulting from the valence electrons donated by the N atoms.

Conclusions In summary, there is no linear relationship between the content of nitrogen dopant and catalytic performance, so an optimal nitrogen content is critical to obtain a high ORR activity. The experimental and quantum chemical calculational examinations of nitrogen configurations in nitrogen doped graphene were used to detect the correlation between nitrogen species and catalytic activity. The results suggest that the graphitic N configuration can facilitate electrons transfer and electrocatalytic activity in the ORR process, and the adjacent carbons should be the main active sites. Moreover, the graphitic N configuration possesses higher adsorption energy of oxygen atoms to enhance the catalytic activity. This work implies the N doping offers an effective way to adjust the properties of graphene, and the N content and species can regulate the catalytic activity, thus promoting the application of N-graphene in the Alair battery.

Acknowledgments This study was supported by the National Nature Science Foundation of China (No. 51474255), the Hunan Provincial Natural Science Foundation of China (No. 13JJ6003).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.10.109.

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