Tuning graphene for energy and environmental applications: Oxygen reduction reaction and greenhouse gas mitigation

Journal of Power Sources 328 (2016) 472e481

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Tuning graphene for energy and environmental applications: Oxygen reduction reaction and greenhouse gas mitigation Enamul Haque a, c, Shuranjan Sarkar b, Mahbub Hassan a, Md. Shahriar Hossain c, Andrew I. Minett d, Shi Xue Dou c, Vincent G. Gomes a, * a

Integrated Polymer and Systems Engineering Group, School of Chemical & Biomolecular Engineering, The University of Sydney, NSW, Australia Department of Chemistry, Kyungpook National University, Daegu 702-701, South Korea Institute for Superconducting and Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, NSW, Australia d Laboratory for Sustainable Technology, School of Chemical & Biomolecular Engineering, The University of Sydney, NSW, Australia b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Tuning of solid precursor based nitrogen doped graphene for multiple applications.
Nitrogen doping and thermal treatment enabled efficient carbon dioxide capture.
Quaternary nitrogen enabled high efficiency oxygen reduction reaction.
N-graphene electrode had superior stability and tolerance to crossover effect.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 February 2016 Received in revised form 3 August 2016 Accepted 8 August 2016 Available online 17 August 2016

Porous nitrogen-doped graphene samples were synthesized and tuned via pyrolysis of solid nitrogen precursor dimethyl-aminoterephthalate with graphene oxide as template. Our investigations show that the extent of thermal treatment, total concentration of nitrogen and the nature of nitrogen moieties play important roles in enhancing oxygen reduction reaction (ORR) and CO2 uptake. N-doped graphene synthesized at 650  C (NG-650) with specific BET surface area of 278 m2/g, exhibits enhanced CO2 sorption capacity of 4.43 mmol/g (at 298 K, 1 bar) with exceptional selectivity (CO2:N2 ¼ 42) and cyclic regeneration stability. In contrast, nitrogen-doped graphene synthesized at 750  C (NG-750) demonstrated excellent catalytic activity for ORR via favourable 4e transfer, performance stability with tests conducted up to 5000 cycles, and is unaffected by methanol cross-over effect. Thus, NG-750 shows potential to replace metal-based electrodes for fuel cell application. The comparative results for ORR with non-doped and nitrogen-doped graphene electrodes showed that graphitic nitrogen sites play vital role in enhancing catalytic activity. © 2016 Elsevier B.V. All rights reserved.

Keywords: Graphene Doping Oxygen reduction reaction Adsorption Carbon dioxide

1. Introduction Energy generation and CO2 capture are among the key challenges facing humanity [1]. On the one hand, the energy demand is * Corresponding author. E-mail address: [email protected] (V.G. Gomes). http://dx.doi.org/10.1016/j.jpowsour.2016.08.042 0378-7753/© 2016 Elsevier B.V. All rights reserved.

on the rise, while environmental challenges from greenhouse gases remain unabated [1c]. The task of tackling both problems in the face of climate change is an important goal for industry [2]. Apart from atmospheric pollution, high concentrations of CO2 are lethal in space-limited chambers such as submarines and space ships [3]. The selective capture and storage of CO2 in a cost effective manner

E. Haque et al. / Journal of Power Sources 328 (2016) 472e481

at the emission level are crucial in certain applications [4]. Zeolites [5], mesoporous silica [6], activated carbons [7], metal-organic frameworks (MOFs) [8], aqueous amines [9], carbon nanotubes [10] microporous polymers (MOPs) [11], and zeolitic imidazolate frameworks (ZIFs) [12] have been widely explored for CO2 capture via absorption, adsorption and membrane separation [13]. However, the use of nitrogen doped graphene is yet to be reported. In industry, chemisorptive formation of NeC bonded carbamate using ammonia solution and aqueous amine technologies are widely used for absorbing CO2, however, they suffer from corrosion and high regeneration and recycling costs [9,14]. For efficient capture of CO2 from flue gases, solid adsorbents [15] with their desirable characteristics of chemical inertness, high surface area and selectivity, and carbon based adsorbents in particular are promising for CO2 capture [10,16]. In addition, the decorated basic N functionalities on carbon framework provide active basic centres and favourable surface chemistry for capturing acidic guest molecules such as CO2 [17,18]. Zeolitic framework and microporous polymers synthesized with nitrogen containing organic compounds as crosslinkers have limitations of structural stability, selectivity and CO2 sorption/desorption efficiency in the presence of water [19,94]. The main limitation of amine modified solids is loss of ammonia with thermal energy input during regeneration [20]. Similarly, amine impregnated or surface modified porous silica such as SBA-15 [21], MCM-48 [22], MCM-41 [23], and zeolite 13X fail to capture CO2 effectively, and require high temperature and long regeneration times, while structural stability weakens considerably after multiple cycles [24]. In addition, post-synthetic incorporation of amine groups causes pore blockage [25], requiring costly, time consuming procedures with the use of corrosive, toxic reagents. Therefore, it is desirable to develop permanently doped porous carbon material for superior selective CO2 uptake. The dual challenges of a lack of renewable energy source and pollution caused by fossil fuels can potentially be overcome with fuel cells [26]. However, two major challenges need to be overcome. First, electrodes in conventional fuel cells contain Pt or Pt alloys as electrocatalysts [27] for oxygen reduction reaction (ORR); however, the scarcity, high cost and low stability limit their commercial viability [28,71]. Second, the poor efficiency of conventional ORR means just one of two redox (half) reactions occurs in fuel cells [29]. Metal-containing catalysts are not favourable with ORR for long term usage because in acidic and basic media, metals leach from electrode surfaces to the detriment of electrocatalytic activity and catalyst life [30]. Thus, the wider acceptance of fuel cells for large scale commercial applications requires not only the development of electrocatalysts based on abundant, non-precious and non-metallic electrocatalysts but also efficient, catalytically active and inexpensive materials having long term stability. The principal non-metallic electrodes under investigation for ORR are: porous carbon [31], carbon nanotubes [32], graphene [33,77] and graphite [34,97]. Recent substitution of carbon atoms in carbon structural materials with electronegative heteroatoms N, P, S and B have been attracting attention [35]. As the electronegativity of N (3.04) is greater than that of carbon (2.55), N-doping of carbon material is able to create positively charged sites which are more favourable towards oxygen adsorption for enhancing catalytic activity in ORR [36]. In general, tailoring the electronic arrangement of carbon-based material is a viable option to design and fabricate heteroatom-doped catalysts for improving electrocatalytic activity towards ORR. Graphene, a two dimensional single-layer nanostructure of sp2-hybridized conjugated carbon [37,38], is a suitable electrode material due to its superior electrical, mechanical and thermal properties [38e40] as well as its high surface area, chemical stability and low cost [41e48,51]. Nitrogen doping of

473

graphene has the potential of tuning its local electronic structure [49] and enhance electron mobility [50] for superior performance. N-graphene is typically synthesized by chemical vapour deposition (CVD) [52], thermal conversion with nitrogen precursors [42,43,53,54,65], hydrothermal treatment [55], plasma [56,82], electrical arc discharge [57], and flame [58]. However, most of these techniques use gaseous or liquid sources of nitrogen to synthesize N-graphene, which suffers from contamination due to metal deposition and toxicity of precursors. For example, doping graphene via CVD with N-precursors is slow, tedious and expensive [48,51,59]. Further, toxic precursors use special devices or severe conditions that limit the scale-up of N-doped graphene via annealing with NH3 [60,77], electric arc discharge [61] or nitrogen plasma [62]. Thus, new non-toxic, cost-effective methods are required to synthesize N-graphene for commercial applications [63,65]. Direct co-pyrolysis of graphene oxide with solid nitrogen source is attractive because the method can be scaled without using any catalyst or other potential contaminant [64]. A few solid nitrogen precursors reported include melamine [65], cyanamide [66], urea [67] sugar [67] aminoterephthalic acid [42], and methylimidazole [43]. In this work, we report a facile and scalable synthesis step using a solid nitrogen precursor dimethyl-amino-terephthalate with graphene oxide via pyrolysis to produce porous N-doped graphene that exhibits not only superior CO2 adsorbance due to the presence of basic N-sites but also excellent ORR catalytic activities. The facile tuning of N-graphene via doping and thermal treatment and the mechanism of CO2 capture and ORR activity for targeted applications are discussed. 2. Experimental 2.1. Synthesis of graphene oxide (GO) Graphene oxide (GO) was synthesized [42,43,68] to obtain thermally expanded graphite; graphite flakes (Asbury Carbons, Grade: 3772, carbon content: 99%) on a ceramic boat were placed in a tube furnace at 1050  C for 15 s. Then 500 ± 2 mg of expandable graphite and 200 ± 4 mL of H2SO4 (98%, Sigma Aldrich) were mixed in a three neck round bottom flask and stirred for 24 h at room temperature. Next, 5 ± 0.02 g of KMnO4 (99%, Sigma Aldrich) was added gradually and was kept under continuous agitation for 24 h. The mixture was cooled in an ice bath. A solution of 200 ± 3 mL deionized water and 50 ± 2 mL of H2O2 (30%, Sigma Aldrich) was prepared and added dropwise to GO under agitation making a slurry. The resulting mixture changed colour from dark to a light brown colour. Finally, the GO dispersion was washed five times with HCl solution (9H2O:1HCl vol%) and subjected to centrifugation at 5000 rpm. The GO precipitate was washed 10 times with water till a pH of about 6 was obtained. Subsequently, GO was collected and dried at 80  C in a vacuum oven for 24 h. 2.2. Synthesis of N-doped graphene To prepare pristine graphene (PG), 30 mg GO was placed on a quartz boat inside a tube furnace (OTF-1200X), and heated to 750  C for 2 h under argon flow. N-doped graphene samples were synthesized from GO-dimethyl-aminoterephthalate composite materials. In preparation of this composite, 30 ± 2 mg GO and 30 ± 3 mg 2-methylimidazole (99%, Sigma-Aldrich) were mixed in 50 ± 2 mL H2O, and dispersed in a sonication bath (Soniclean 80T, 60 W) for 1 h. The mixture was subsequently heated to 90  C and stirred for 12 h while removing water. The resulting solid composite (GO-dimethylterephthalate) was taken in a ceramic boat and

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placed in a tube furnace (OTF-1200X). These samples were heated separately at 650  C and 750  C under argon flow (150 mL/min) for 2 h to obtain the nitrogen doped graphene samples NG-650 and NG-750, respectively. 2.3. Characterization techniques The products PG, NG-650 and NG-750 were characterized using several techniques. Diffraction measurements were conducted using an X-ray diffractometer (XRD, GBC MMA, Cuka radiation, l ¼ 0.154 nm) to understand the phase structure of the synthesized graphene materials. X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo scientific, UK; X-ray source: monochromated Al K alpha, Power: 164 W (10.8 mA and 15.2 kV); binding energy reference: C1s ¼ 285.0 eV for adventitious hydrocarbon) was used to examine surface chemical composition of the synthesized graphene materials, and also to detect the types of nitrogen on doped graphene structure. Transmission electron microscopy (TEM) studies were conducted using a JEOL ARM200F instrument equipped with a spherical aberration corrector in STEM mode. The TEM images were acquired using a Gatan UltraScan CCD camera. The microscope, equipped with a Centurio EDS (Electron dispersive spectroscopy) detector, enables a collection angle close to ~1 sr ¼ (180/p)2, thereby significantly reducing the time needed to acquire a low noise mapping. Nitrogen physisorption was conducted using an Autosorb-iQ (Quantachrome Instruments, USA) adsorption unit at liquid nitrogen temperature (196  C). Samples were evacuated at 120  C for 12 h prior to analysis. 2.4. CO2 adsorption measurement Gas adsorption measurements were carried out using a Belsorp mini II (Japan) apparatus. Before each measurement, the samples were heat treated at 100  C in vacuum for 24 h. CO2 adsorption isotherms were measured at 298 K for up to 1 bar pressure. To estimate the selectivity of CO2 over N2 using Henry's Law, the initial slopes of the CO2 and N2 adsorption isotherms were measured. Using these initial slopes, the selectivity of CO2 was computed:

 S ¼ ðqCO2 =qN2 Þ ðpCO2 =pN2 Þ where q is the amount adsorbed and p is the partial pressure. 2.5. Electrochemical measurement Electrochemical measurements were carried out at room temperature using a three-electrode system (Biologic digital potentiostat/galvanostat, Science Instruments, SP-300) with glassycarbon (GC) working electrode, Pt wire counter electrode and an Ag/AgCl reference electrode filled with saturated KCl solution. Cyclic voltammometry (CV) was performed at a scan rate of 10 mV s1 in an alkaline solution (0.1 M KOH) purged with nitrogen (10 min) prior to electrochemical tests. To prepare the working electrode, Ndoped carbon samples were dispersed in a mixture of 0.04 wt% nafion, 5:1 wt% isopropanol:water with 30 min sonication. A dispersion of 12 mL (1 ± 0.01 mg N-doped carbon in 1 ± 0.01 mL1 nafion) was dropped on the GC electrode of 5 mm diameter and 0.196 cm2 surface area. The electrode was allowed to dry at room temperature for 20 min before testing. The CV and linear-scan voltammetry (LSV) measurements were conducted using a VersaSTAT 2-channel system (Princeton Applied Research, USA). The same procedures were repeated for electrode preparation and experiments to determine ORR activities. Kinetic parameters were determined with the Koutecky-Levich plots using the following relation:

1 1 1 ¼ þ i ik B$u1=2 where, i is the measured current, ik the kinetic current and u the electrode rotation rate (rpm). The theoretical value of the Levich slope (B) was evaluated from:

B ¼ 0:2nFAC02 D02 v1=6 2=3

where n is the overall number of electrons transferred by ORR, 0.2 is a constant when the rotation speed is expressed in rpm, F (Faradaic constant) ¼ 96485 C/mol, A (electrode area) ¼ 0.196 cm2; CO2 is the oxygen concentration in 0.1 M KOH (1.2  106 mol cm3), DO2 is the oxygen diffusion coefficient in 0.1 M KOH (1.9  105 cm2 s1) and v is the kinematic viscosity of electrolyte (0.01 ± cm2 s1) [69]. 3. Results and discussion The synthesized graphene based materials were characterized by powder X-ray diffraction to identify phase crystallinity. Fig. 1 shows a comparison of results for synthesized GO with prepared graphene (PG via pyrolysis of GO at 750  C) and N-doped graphene (NG-650 and NG-750), prepared by pyrolysis of GO-dimethylamino-terephthalate at 650  C and 750  C, respectively. GO shows a peak at 2q ¼ 10.9 with its corresponding dspacing ¼ 0.812 which is the typical GO peak position [42,43,70]. After pyrolysis of GO and GO-dimethylamino-terephthalate solid composite under argon flow, the XRD peak position for PG, NG-650 and NG-750 shifted dramatically at 2q ¼ 26.56 , 26.32 and 26.24 , respectively. A comparison of dspacing of GO (dspacing ¼ 0.812), PG, NG-650 and NG-750 shows significant decrease occurs after pyrolysis when graphene (dspacing ¼ 0.336) and N-doped graphene (dsapcing of 0.338 and 0.339 for NG-650 and NG-750, respectively) are produced. These results indicate the occurrence of significant deoxygenation when GO is reduced to produce graphene [42,43]. To confirm the removal of oxygen from GO surface on pyrolysis and to determine the chemical composition of synthesized graphene materials, XPS analyses were conducted. The XPS spectra for GO, PG, NG-650 and NG-750 (Fig. 2), show that strong O1s and relatively weak C1s peaks appear for GO, but after thermal treatment, strong C1s and weak O1s peaks appear for PG, NG-650 and NG-750. These demonstrate the reduction of oxygen functionalities [42], and are noted from changes in the C and O content for the synthesized materials (Table 1). GO has C and O elemental composition of 66.45 and 33.55 at%, respectively,

NG-750 Intensity / a.u

474

NG-650 PG GO 5

10

15

20

25

30

35

40

2 theta / deg Fig. 1. X-ray diffraction profiles for GO, PG, NG-650 and NG-750.

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(b)

Intensity / a.u

C1s

1200

1000

800

600

400

C1s

Intensity / a.u

O1s

(a)

200

O1s

1200

1000

Binding Energy / eV

1000

800

600

600

400

(d)

200

N1s

400

C1s

Intensity / au

C1s

Intensity / a.u 1200

800

Binding Energy / eV

(c)

O1s

475

O1s N1s

1200

200

1000

800

600

400

200

Binding Energy / eV

Binding Energy / eV

Fig. 2. XPS spectra for: (a) GO, (b) PG, (c) NG-650 and (d) NG-750.

whereas the converse trend is noted for PG (C ¼ 96.26, O ¼ 3.74%), NG-650 (C ¼ 86.82% O ¼ 5.55%) and NG-750 (C ¼ 92.51%, O ¼ 3.16%). Following thermal treatment, dramatic changes in C/O ratio are noted, increasing from 1.98 for GO to 25.72, 15.64 and 29.27 for PG, NG-650 and N-750, respectively. These indicate the removal of oxygen from GO during its transformation to graphene [42,43]. The N1s spectra for NG-650 and NG-750 show the presence of nitrogen species of both NG-650 and NG-750. The types of nitrogen bonding configurations in N-doped graphene depend on the nitrogen source as well as the synthesis method [42]. Nitrogen is found primarily in three bonding configurations [71]: quaternary-N (or graphitic-N), pyridinic-N and pyrrolic-N (Scheme 3b). N atoms that replace C atoms in hexagonal

rings are termed quaternary nitrogens. Pyridinic N refers to a nitrogen atom bonded to two C atoms at an edge that contributes one p-electron to the p system. Pyrrolic N exists in a five-membered pyrrole-like ring, and contributes two p-electrons into the p system [72]. For other types of nitrogen, the N atom is bonded to two C atoms and one O atom as N-oxides of pyridinic N. The types of nitrogen were observed from the deconvolution of N1s in XPS spectra (Fig. 3a). The N1s spectrum of NG-650 shows three peaks at binding energies of 398.51, 400.18 and 401.44 eV, while NG-750 showed peaks at 398.43, 400.14 and 401.46 eV which are attributed to pyridinic-N, pyrrolic-N and quaternary-N, respectively [42,71]. These results confirm the types of nitrogen atoms in the

Table 1 Textural properties, composition, CO2 capture and catalytic activity of graphene based materials. Materials

Textural properties BET surface areaa (m2/g)

GO PG NG-650 NG-750 Pt/C nd

h

e 76.5 278 296 eh

Chemical compositiond (at.%) Pore sizeb (nm) h

e 1.75e4.55 2.48e6.97 3.52e8.01 eh

Pore volumec (cm3/g) h

e 0.29 0.68 0.89 eh

C 66.45 96.26 86.82 92.51 eh

O 33.55 3.74 5.51 3.16 eh

CO2 uptakee (mmol/g)

Catalytic activity Onset potentialf (V)

N nd

e end 7.67 4.33 eh

h

e 0.83 4.43 3.12 eh

h

e 0.214 0.157 0.139 0.117

Not detected. a Calculated from adsorption branch over P/Po ¼ 0.05e0.25 (Fig. S1). Calculated from the adsorption branch of the N2 adsorption-desorption isotherm (acquired at 196  C) using BJH method. c Calculated over P/Po ¼ 0.99 (Fig. S1). d Determined from detection-sensitivity-adjusted XPS peaks (Fig. 2). e CO2 adsorption calculated at 298 K and 1 bar pressure, (Fig. 6). f Calculated from Fig. 8a. g Average ± standard deviation calculated from potential 0.4 ~ 0.8 V (Figs. S5, S7, Fig. 8b). h Not measured. b

Electron transferredg (n) eh 1.52 3.48 3.87 4.18

± ± ± ±

0.83 0.51 0.62 0.29

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Fig. 3. (a) Deconvoluted N1s region of X-ray photoelectron spectra for NG-650 and NG-750; (b) Schematic diagram of types of nitrogen in N-doped graphene: Green, blue and red spheres represent quaternary N, pyridinic N and pyrrolic N; gray spheres represent carbon. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

synthesized N-doped graphene structure. The XPS measurements (total chemical composition) indicated that the total N contents were 7.67 and 4.33 at% for NG-650 and NG-750 respectively. Based on total N content, NG-650 occupied 38.58% pyridinic N, 37.34% pyrrolic N and 24.08% graphitic N. On the other hand, NG-750 occupied 35.03% pyridinic N, 34.27% pyrrolic N and 30.60% graphitic N. These results show that total content of N, pyridinic N and pyrrolic N were decreased, but graphitic N content increased by increase in synthesis temperature. The high resolution transmission electron microscopy (HRTEM) images (Fig. 4) of N-doped graphene clearly show the nature of the graphene sheet. The corresponding FFT diffraction pattern with six fold symmetry and ring mask illustrates the features of a few layer graphene structure [43], and that the pyrolysis did not damage the structure. In addition, the multilayer graphene structure is also evident in the TEM image of Fig 4a. Homogeneous doping was confirmed from the elemental mapping based on EDS (electron dispersive spectroscopy). The EDS map (Fig. 5) illustrates the wide distribution of C and the sparse homogeneous presence of N and O in comparison, which also confirms the compositional data of XPS analysis qualitatively. Nitrogen sorption (Fig. S1) measured at 196  C enabled us to examine the textural properties of the graphene materials, and the results are summarized in Table 1. The BrunauerEmmettTeller

(BET) surface areas were 76, 278 and 296 m2/g, and pore volumes were 0.29, 0.68 and 0.89 cm3/g for PG, NG-650 and NG-750, respectively. Our synthesized graphene materials display type-I nitrogen isotherms with steep uptake at low relative pressures, thereby suggesting that micropores are dominant [25]. The hysteresis of the desorption curves at higher temperatures, reveal the presence of meso- and macro-pores [25]. The pore sizes calculated from the adsorption branch of the isotherm using the BarretteJoynereHalenda (BJH) equation gave: 1.75e4.55 nm, 2.48e6.97 nm and 3.52e8.01 nm for PG, NG-650 and NG-750, respectively. We note that the BET surface areas, pore volumes and pore diameters of N-doped graphenes are greater than that of non-doped graphene. This is due to the increase in defects during nitrogen doping of graphene which enhances the porous structure. CO2 sorption was measured using a Belsorp II (Japan) device at 298 K and 1 bar. The synthesized PG, NG-650 and NG-750 show CO2 adsorption capacities of 0.83, 4.43 and 3.12 mmol/g, respectively (Fig. 6). These results illustrate two key characteristics: (a) typically materials with higher surface areas and pore volumes (PG-650 BET ¼ 278 m2/g; PG-750 BET ¼ 296 m2/g), tend to capture larger amounts of CO2 than those having of lower surface area and pore volume (e.g., PG BET ¼ 76 m2/g); (b) nitrogen doped graphenes have higher CO2 capture efficiency than that of non-doped graphene.

Fig. 4. HRTEM images of N-doped graphene with insets showing corresponding FFT diffraction patterns for: (a) NG-650 and (b) NG-750.

E. Haque et al. / Journal of Power Sources 328 (2016) 472e481

477

Fig. 5. TEM image of (a) NG-750, and corresponding elemental mapping of (b) carbon, (c) nitrogen and (d) oxygen.

5

5

4 3

NG-750 2

PG

1

Gas uptake (mmol/g)

CO uptake (mmol/g)

NG-650

CO2

4 3 2 1

N2 0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (atm)

0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (atm)

Fig. 6. (a) CO2 sorption isotherms of PG, NG-650 and NG-750 at 298 K (symbols, filled: adsorption; open: desorption); (b) Sorption-desorption curves of CO2 and N2 at 298 K using NG-650.

NG-650 recorded higher CO2 adsorption (4.43 mmol/g) relative to NG-750 (3.12 mmol/g) even though the surface area of NG-750 is higher than that of NG-650. This demonstrates that the efficiency of CO2 capture depends not only on textural properties (BET surface area, pore volume and pore size), but also additional factors. According to the XPS analysis, NG-650 contains higher nitrogen

concentration (7.67 at%) than that of NG-750 (4.33 at%), and helps enhance adsorption capacity of CO2. This agrees with previous reports, which note that both the surface area and N content play important roles in determining the CO2 adsorption capacity [25]. The high CO2 uptake by NG-650 confirms stronger interaction of CO2 molecules with the more basic N decorated graphene

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framework. Also, the slight hysteresis in CO2 sorption curves for Ndoped graphene materials reflects greater affinity for CO2 [73]. N2 adsorption on NG-650 shows a much smaller capacity (0.245 mmol/g at 298 K) relative to CO2, indicating that NG-650 can be used for selective separation of CO2 from flue gases (Fig. 6b). The selectivity of CO2 from the N2 and CO2 isotherms (Henry's law region) is 42 which indicates suitability for industrial applications [74]. The high selectivity is due to the electron-rich graphene network and high charge density at the N sites which facilitate favourable interaction with the polarizable acidic CO2 molecules through local dipole-quadrupole interactions [73]. Cyclic stability with high adsorption capacity is important for large scale CO2 sequestration. The synthesized N-doped graphene (NG-650) show excellent capacity and stability during cyclic adsorptionedesorption processes (Fig. S2). CO2 adsorption-desorption cycles measured at 298 K show 5.9% and 13.7% decrease in CO2 sorption capacity after 2nd and 3rd cycles, respectively. A comparison of various N-doped graphenes (Fig. S3, Table S1) shows improved CO2 selectivity and uptake for NG-650 relative to samples prepared by CVD [76], thermal treatment [77], arc discharge [78], flame ionization [79,80], solvothermal [81], and plasma treatment processes [82], as well as N-doped carbon synthesized by pyrolysis [83,84], grafting [85,86], carbonization [75], calcination [87], hydrothermal [88] and thermal activation [89]. For example, templating to produce N-enriched carbon from melamineeformaldehyde yielded a maximum capacity of 2.25 mmol/g [90] which is lower by a factor of two compared to NG-650. Ndoped porous carbon monoliths prepared by co-polymerization of resorcinol and formaldehyde in the presence of L-lysine had a capacity of 3.13 mmol/g for CO2 [91], while, 2-[2-(3-trimethoxysilylpropylamino)-ethylamino]-ethylamine grafted MCM-41 [85] and 3-amino-propyl-triethoxysilane grafted MCM-48 [86] had capacities of 2.05 and 0.639 mmol/g, respectively; these are lower by factors of 2.2 and 6.9. The calcination of pyrrole and triblock copolymer Pluronic P-123 to synthesize N-doped porous carbon had adsorption capacity of 3.10 mmol/g [87], while 3.4 mmol/g was recorded for N-doped carbon fibre prepared by carbonization of pitch and ureaformaldehyde resin [75]. NG-650 shows 60% higher CO2 sorption capacity than N-graphene-poly(aniline) composite (2.7 mmol/g [89]). Hydrothermal synthesis of N-doped carbon spheres prepared from ethylene-diamine and resorcinol/formaldehyde exhibit CO2 uptake of 4.1 mmol/g [88], which is close to that of NG-650. These illustrate that our synthesized N-doped graphene (NG-650) shows high capacity for CO2 uptake (4.43 mmol/g) compared to previously reported doped carbon materials. Table S2 shows that NG-650 has much higher CO2 sorption capacity than non-doped carbon materials with enhancement factors of 1.98e2.6, 2.2 and 2.8e7.4 times greater than activated carbon [92,93], single-walled carbon nanotube [10], and multi-walled carbon nanotube [92,93]; respectively. In comparison with other porous frameworks, the enhancements are about 3 times over that of zeolite [93] and 1.3e7.3 times than that of metal-organic frameworks (MOF-2, MOF-5, Cu-BTC) [94]. In terms of selectivity (Table S1, Fig. S3), RGO/PANI composite [89] had a selectivity of 17.9 which is 2.3 times lower than that of NG-650. NG-650 shows selectivity enhancements by factors of 1.3e2.3 compared to zeolitic MOFs (ZIF-68,69,70,79,81) [99] and Cu-Ln-MOFs (Cu-BTTri, [Ln2(TPO)2(HCOO)]$(Me2NH2), H3[(Cu4Cl)3(BTTri)8]) [73,100]. Porous carbon [101], N-doped carbon fibre [75] and N-doped porous carbon [83] show lower performance by factors of 1.2e1.6, while N-doped porous carbon prepared by nanocasting using mesoporous silica IBN-9 [102] show similar selectivity as NG-650. Notably, NG-650 has selectivity greater than activated carbons [103,104] and zeolites [105,106] by factors of 1.2e1.6 and 2.5e2.7.

The effect of doping on ORR was investigated by cyclic voltammetry using O2-saturated KOH electrolyte (Fig. 7a). N-doped graphene (NG-650 and NG-750) displayed a large cathodic current at ~0.4 V, which indicates significant reduction. However, nonnitrogen containing graphene (PG) did not display similar cathodic currents. This illustrates the remarkable response in catalytic activity of N-doped graphene. In addition, NG-750 showed larger cathodic current response than that of NG-650 which illustrates the superior catalytic potential of NG-750. When NG-750 was tested in Ar-saturated environment, no reduction peak was observed, confirming that O2 reduction occurs only in O2 saturated environment (Fig. S6). ORR electrocatalysis occurs via two-electron transfer process to produce H2O2, or through a four-electron transfer pathway to produce H2O. The four-electron transfer pathway is the desirable one due to its higher efficiency and is typically obtained with Ptbased electrodes. ORR performance of PG, NG-650 and NG-750 were evaluated via linear sweep voltammograms (LSVs), recorded by a rotating-disk-electrode (RDE) at variable spinning rates as shown in Fig. 7b, S4a, S5a. The number of electron transfer by PG, NG-650 and NG-750 were calculated from RDE voltammograms (Fig. 7b, S4a and S5a) by constructing Koutecky-Levich plots (Fig. 7c,S4b, S5b) and applying the Koutecky-Levich equation [68,95]. The number of electron transfer for PG (n ¼ 1.52 ± 0.83) is negligible, whereas NG-650 (n ¼ 3.48 ± 0.51) and NG-750 (n ¼ 3.87 ± 0.62) perform close to the favourable four-electron transfer process for producing water. The comparison of these results with the commercial Pt/C catalyst (Fig. 8, Table 1) based on RDE voltamomograms at 2000 rpm shows that the cathodic current density, the onset potential, as well as the average electron transfer number for NG-750 are close to those for Pt/C. NG-650 shows lower ORR efficiency compared to NG-750, although it contains more nitrogen. Thus, nitrogen concentration is not the only factor for 4e transfer efficiency, the type of nitrogen is also important. According to XPS analysis (Table 1, Fig. 3a), both NG-650 and 750 contain three types of nitrogen (pyridinic, pyrrolic and graphitic N) but differ in specific concentrations. NG-750 contains relatively more graphitic N which is responsible for the superior catalytic activity of NG-750; this agrees with reports indicating the vital role played by quaternary nitrogen for ORR and that the most desirable 4e transfer pathway is due to the presence of quaternary nitrogen [33,77,96]. Further, increasing quaternary nitrogen has been correlated with higher ORR activity than those with larger pyridinic N in N-doped mesoporous carbon through experimental [97] and computational studies [98]. Thus, our findings indicate that suitable doping coupled with thermal treatment is able to tune carbon electrodes such as NG-750 which has potential for application in ORR. Apart from catalytic performance with respect to electron transfer (4e), NG-750 also showed superior stability and tolerance to crossover effect (Table S5). After 5000 consecutive CV cycles in O2-saturated KOH electrolyte (Fig. S6), the cathodic current decreased by 12.3% for N-doped RGO compared to 46% reduction for Pt/C electrode. To examine the performance of N-doped graphene for crossover effect, electrocatalytic selectivity was measured against electro-oxidation of methanol (a typical fuel in fuel cells) in O2-saturated 0.1 M KOH. The cyclic voltammogram (Fig. S7a) of N-doped reduced graphene in the presence of 3 M methanol showed no noticeable change in oxygen-reduction current, indicating that methanol does not interfere with the ORR reaction. In contrast, methanol oxidation occurred with Pt/C based electrode (Fig. S7b). Thus, methanol cross-over results show that Ndoped graphene is promising for direct methanol and alkali-based fuel cells.

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Fig. 7. (a) Cyclic voltammograms (CVs) of PG, NG-650 and NG-750; (b) RDE voltammograms of NG-750 in oxygen-saturated 0.1 M KOH at a scan rate of 10 mV/s; (c) KouteckyeLevich plots of NG-750 calculated from the RDE voltammograms of (b).

Fig. 8. (a) Comparison of RDE voltammograms of PG, NG-650, NG-750 and Pt/C at 2000 rpm rotation in oxygen-saturated 0.1 M KOH at a scan rate of 10 mV/s; (b) Comparison of electron transfer numbers of PG, NG-650, NG-750 and Pt/C as functions of applied potential.

4. Conclusions Nitrogen-doped graphene materials with nitrogen concentrations of 7.67e4.33% were synthesized within 1 h via thermal treatments at 650  C and 750  C, respectively. The method involved the pyrolysis of solid nitrogen precursor dimethylaminoterephthalate with graphene oxide as template. The synthesized NG-650 and NG-750 had BET surface areas of 278 and 296 m2/g, respectively. Both nitrogen doped graphene samples showed three types of nitrogen moieties: pyridinic, pyrrolic and graphitic N. But they differed in total nitrogen concentration (NG-

650 had higher N content) and concentration of specific nitrogen sites (NG-750 had higher graphitic N sites). NG-650 demonstrated superior CO2 adsorption capacity of 4.43 mmol/g (at 298 K, 1 bar), which is greater than those of previously reported doped carbon materials, e.g., porous carbons, carbon fibre, carbon spheres, MCM41, MCM-48, graphene composite materials, as well as non-doped carbon materials (activated carbon, carbon nanotubes), zeolites and metal-organic frameworks. NG-650 also exhibited superior selectivity for CO2 over N2 with excellent regeneration ability. The comparative CO2 uptake of non-doped graphene and N-doped graphene demonstrate that N-doping as well as total content of

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nitrogen are responsible for efficient CO2 adsorption. In contrast, NG-750 showed excellent catalytic activity for ORR via favourable 4e transfer, similar to commercial electrodes. In addition, NG-750 displayed improved cyclic stability with 12% decrease in efficiency compared to a Pt/C electrode (46% decrease) after 5000 cycles, and showed no sensitivity towards methanol cross-over effect. These performance metrics illustrate that NG-750 has improved characteristics and has potential for application in fuel cells. These results demonstrate the versatility of N-graphene and their facile tunability for critical multiple applications related to greenhouse gas mitigation and energy generation.

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Acknowledgements

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Authors are grateful for the financial support from The University of Sydney and the Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong for in-kind support. The authors also acknowledge Dr. Gilberto Casillas for the technical assistance with TEM and the Electron Microscopy Centre, University of Wollongong.

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