- Email: [email protected]
Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR) Juan C. Carrillo-Rodrı´guez a, Ivonne L. Alonso-Lemus b,*, -Ruiz e, A.A. Siller-Ceniceros c, E. Martı´nez G d, Pedro Piza Gregorio Vargas-Gutierrez a, F.J. Rodrı´guez-Varela a,c Sustentabilidad de Los Recursos Naturales y Energı´a, Centro de Investigacion y de Estudios Avanzados Del IPN Unidad Saltillo, Av. Industria Metalu´rgica, Parque Industrial Saltillo Ramos Arizpe, C.P. 25900, Ramos Arizpe, Coah, Mexico b CONACyT, Centro de Investigacion y de Estudios Avanzados Del IPN Unidad Saltillo, Mexico c Nanociencias y Nanotecnologı´a, Centro de Investigacion y de Estudios Avanzados Del IPN Unidad Saltillo, Mexico d Centro de Investigacion en Materiales Avanzados S.C., Alianza Norte 202, PIIT, Carretera Monterrey-Aeropuerto Km. 10, Apodaca, NL, C.P. 66628, Mexico e Centro de Investigacion en Materiales Avanzados S.C., Av. Miguel de Cervantes 120, Parque Industrial Chihuahua, C.P. 31109, Chihuahua, Chih, Mexico a
article info
abstract
Article history:
We report a novel process to synthesize graphene (G) catalyst by mechanical milling using
Received 5 April 2017
graphite flakes as the precursor. G sample has been doped ex situ with hydrazine as ni-
Received in revised form
trogen source via solvothermal procedure to obtain the GD1 catalyst. In a second approach,
9 August 2017
the GD2 sample has been synthesized doping G with uric acid as nitrogen precursor in situ,
Accepted 11 August 2017
i.e., during the milling step. Doping with nitrogen increases the ID/IG Raman spectra ratios
Available online xxx
of GD1 and GD2 to 1.52 and 1.12, respectively, higher than 1.02 of G. XPS analysis shows that Pyridinic, Amine, Pyrrolic, Graphitic and Oxidized nitrogen are formed at GD1, while only
Keywords:
Pyrrolic is present at GD2. Evaluation of catalytic activity for the ORR in 0.5 mol L1 KOH
N-doped graphene
shows an increase in onset potential (Eonset) of the ORR at GD1, compared to G and GD2. GD1
Metal-free electrocatalyst
also generated a higher current density (j) at 0.83 V than G and GD2. The results show that
Mechanical milling
mechanical milling is an efficient method to synthesize G. Even though, the doping can still
Low temperature fuel cells
be improved to form more Nitrogen that promotes the ORR, specifically Pyridinic N and Graphitic N. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Oxygen Reduction Reaction (ORR) is the kinetically limiting part of low-temperature fuel cell devices [1].
Consequently, the use of materials with high catalytic activity is required in order to improve the kinetics at the fuel cell cathode. On this context, Pt is widely applied as cathode catalyst because of its high performance for the ORR. However, it is a costly material with reduced abundance,
* Corresponding author. E-mail addresses: [email protected], [email protected] (I.L. Alonso-Lemus). http://dx.doi.org/10.1016/j.ijhydene.2017.08.084 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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characteristics that have limited the large-scale commercialization of fuel cells [2,3]. Moreover, Pt it is rapidly deactivated by the crossover phenomena in alcohol e fed fuel cells [2,3]. Largely, the research aimed to reduce the cost of fuel cell cathodes by replacing Pt, has been devoted to the development of alternative electrocatalysts based on non-precious metals and metal-free nanostructures [4,5]. Recently, Ndoped graphene nanostructures have shown high catalytic activity for the ORR, in some cases with properties that surpass those of commercial Pt/C catalysts [6]. Chemical Vapour Deposition (CVD) is a well-known method to synthesize graphene [7e9]. Nevertheless, this process has several drawbacks, including high production costs, the need of inert atmosphere, the use of corrosive precursors and very important, lack of scalability [10,11]. Therefore, different routes to produce graphene and doped-graphene must be investigated. One attractive alternative is mechanical milling, which is a process where graphite powder is mixed with an inorganic salt and uses the mechanical forces to produce the displacement of graphite layers. The simplicity of the process and low cost of production makes mechanical milling a promising method to synthesize graphene [11e14]. On the other hand, over the past years there have been mainly two strategies to produce N-doped graphene [15,16]: i) post-treatment, where the carbon material is thermally treated with nitrogen-containing precursors such as ammonia, urea, melamine, cyanamide, dicyandiamide, dicyandiamide, polyaniline and polypyrrole [17,18]; ii) in situ, where the nitrogen precursor is incorporated into the synthesis process while graphene is being produced [17,19]. In this work, the synthesis and characterization of graphene based metal-free catalysts is reported. Graphene is synthesized by mechanical milling and labelled as G. N-doped graphenes are obtained by solvothermal post-treatment using hydrazine as N precursor (GD1) and in situ during mechanical milling using uric acid as N source (GD2). The catalysts are characterized by Xray diffraction (XRD), Raman spectroscopy, BET analysis and Xray photoelectron spectroscopy (XPS). Their electrochemical characterization is focused on the evaluation of catalytic activity for the ORR in alkaline media by Cyclic and Linear Scan Voltammetry (CV and LSV, respectively).
Experimental Preparation of graphene Graphene (G) was obtained by mechanical milling. Graphite flakes (Sigma-Aldrich, 99.99%) and aluminium powder (Alfa Aesar, 99.97%) in atomic ratio of 1:1 were exfoliated for 8 h on a planetary mill (Retsch PM100) with agate balls and vials. To avoid shock stress on the graphite crystal-plane, the rotating speed was set at 300 rpm. The ball-to-powders weight ratio was 20:1. After milling and to remove aluminium traces, the powder was washed with 1 M HCl and deionized water, followed by drying under vacuum.
Preparation of N-doped graphene N-doped GD1 was synthesized via solvothermal posttreatment: 1 g of G was dispersed in 30 mL of hydrazine hydrate (Sigma-Aldrich, 65%) as nitrogen precursor. The dispersion was introduced in a Teflon reactor and treated in a sealed stainless-steel autoclave at 180 C for 24 h. Afterwards, the powder was filtered, washed with deionized water and dried under vacuum. N-doped GD2 was obtained from in situ doping of G: a mixture of graphite, aluminium and uric acid as nitrogen precursor (Sigma-Aldrich, 99%), in atomic ratio of 1:1:10 was introduced in the mill. Then, the milling conditions and washing procedure were the same as those described for G.
Physicochemical characterization X-ray diffraction (XRD) patterns were obtained in a Philips radiaX'Pert (PANalaytical) apparatus with Ni-filtered Cu Ka ). BET surface area of the tion, in the range of 10e100 (2e samples was characterized in an Autosorb-1 equipment (Quantachrome Inst.), by degassing the samples at 180 C for 1 h. Structural properties were characterized by Raman spectroscopy in a Horiba LabRam HR VIS-633 (laser He-Ne) and X-ray photoelectron spectroscopy (XPS) in a Thermo Scientific , 20 eV). ESCALAB 250Xi (Al-Ka
Fig. 1 e (a) XRD patterns and (b) Raman spectra of G, GD1 and GD2.
Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Table 1 e Physicochemical properties and electrochemical performance for ORR of graphene and N-doped graphenes. Catalyst
G GD1 GD2
SBET (m2 g1)
427 421 80
Chemical composition (at%)
Nitrogen relative distribution (at%)
C
O
N
Al
N1
N2
N3
N4
N5
90.2 89.3 81.0
7.4 6.8 10.0
0.9 2.0 7.7
1.5 1.9 1.3
e 23.3 e
e 21.6 e
e 32.2 100.0
e 6.0 e
e 16.9 e
Electrochemical measurements Characterization was carried out in a Wavedrive 20 potentiostat, connected to a rotating disk electrode set-up (Pine Inst.). A three-electrode cell was used with a Pt wire as the counter electrode and an Ag/AgCl (sat. KCl) as the reference electrode. A catalytic ink was obtained by ultrasonically mixing 10 mg of the corresponding graphene, 1 mL of propanol and 5 mL of Nafion solution. The working electrodes were prepared by depositing 10 mL of each catalytic ink on a glassy carbon substrate with 5 mm diameter. Cyclic Voltammograms (CVs) of the graphene catalysts were obtained at 20 mV s1, in N2-saturated 0.5 M KOH. The potential range was 0.05e1.2 V vs. RHE. CVs of the ORR were acquired in O2-saturated electrolyte in the same potential window and a scan rate ¼ 5 mV s1. The ORR polarization curves were background current-corrected by subtracting CVs obtained in N2-saturated electrolyte under the same conditions. In this work, only the negative scan of the voltammograms was shown. The rotating rates were u ¼ 400, 800, 1200, 1600 and 2000 rpm.
Eonset (V/RHE)
J (mA cm2 at 0.4 V/RHE)
0.86 0.87 0.85
2.07 2.16 1.92
The BET specific surface area (SBET) of the catalysts is shown in Table 1. The solvothermal procedure has no significant effect on the surface properties of graphene, since the SBET of GD1 is similar to that of G (421 and 427 m2 g1,
Results and discussion The XRD patterns of G, GD1 and GD2 are shown in Fig. 1a. All of the catalysts have diffraction peaks at 2q ¼ 26.4, 43.1 and 77.4, which correspond to the (002), (101) and (110) planes of carbon materials, indicating their graphitic structure. GD1 and GD2 have broader reflections, which can be attributed to the incorporation of nitrogen in their structure and/or amorphization caused by milling. Fig. 1b shows the Raman spectra of the three catalysts. The D band at about 1328 cm1 is ascribed to the disorder or defects in the lattice of carbon materials. The G (1580 cm1) and 2D (2631 cm1) bands, both related to the sp2 hybridization of graphite, i.e., the ordered structure of carbon materials, can also be observed. The position, shape and relative intensity of 2D band can be correlated with the number of graphene layers on the material [14]. Intense and narrow 2D bands emerge for graphene with 5 layers or less. On the contrary, broad and lowintensity 2D bands are observed for multilayer graphene. In the case of Fig. 1b, the features of the 2D bands suggest the presence of multilayer graphene and/or exfoliated graphite. The ID/IG band intensity ratio is used to evaluate the degree of disorder, indicating defects in carbon structures. The ID/IG values for G, GD1 and GD2 are 1.02, 1.52 and 1.12, respectively. The higher ID/IG ratio determined for GD1 and GD2 is attributed to the structural defects promoted by the doping with N.
Fig. 2 e N 1s spectrum of (a) G, (b) GD1 and (c) GD2.
Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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respectively). On the other hand, in situ doping show a reduction in the SBET of GD2, which is considerably smaller (80 m2 g1) compared to GD1 metal-free catalysts. The reduction in the GD2 SBET could be caused by of uric acid blocking sites in graphene structure. XPS analysis has been carried out to determine the surface elemental composition and electronic configuration of the catalysts. G, GD1 and GD2 contain C, O, N and Al traces as shown in Table 1. It is observed that the two doping methods have an effect in the surface chemical composition compared with G. Metal-free electrocatalyst GD1 contain nearly 90 at% C, while the value for GD2 is 81 at%. GD2 also shows higher O which comes from the uric acid used as nitrogen precursor. Furthermore, nitrogen was detected in 2.0 and 7.7 at% for GD1 and GD2, respectively. A detailed analysis of N 1s peak was carried to determine the nitrogen species formed into the graphene. Fig. 2(a) shows the high-resolution N 1s spectrum of G. The data are highly dispersed and of low intensity. A peak is shown at 399.1 eV after deconvolution, but it is not conclusive of the presence of N on the graphene structure. Fig. 2(b) shows the N 1s spectrum of GD1. The deconvolution shows five peaks at 398.8, 399.4, 400.3, 402.5 and 405.3 eV, ascribed to pyridinic
(N1), amine (N2), pyrrolic (N3), graphitic (N4) and oxidized (N5) species of N, respectively. This analysis confirms the incorporation of nitrogen into graphene to form N-doped GD1 [17,20]. The relative distribution of the five N species is given in Table 1, where at least three species that promote the ORR were detected (N1, N3 and N4). Meanwhile, Fig. 2(c) is the spectrum corresponding to GD2. Only one peak at 400.3 eV which can be ascribed to N3 has been obtained after deconvolution for this catalyst [21]. Thus, there is a significant effect of the doping procedure on the N-species formed at the graphene structure. Solvothermal doping is an effective method to produce N-doped graphene, due that develops 5 different species, among them those that have been identified as the promoters of catalytic activity for the ORR of carbon nanostructures [18]. Even though, GD2 synthetized by in situ method has high nitrogen content (7.7 at%) is an ineffective method to produce N-doped graphene. The N3 species detected in this material may be attributed to the nitrogen-carbon bonds of the uric acid. On the other hand, these features may have influenced the lower SBET of GD2. Fig. 3(a) shows the CVs of the graphene-based catalysts exhibiting an almost rectangular shape and high capacitive
Fig. 3 e (a) CVs of G, GD1 and GD2 in N2-saturated 0.5 M KOH, with scan rate ¼ 20 mV s¡1. (b) LSVs of the ORR at 2000 rpm of G, GD1 and GD2, (c) GD1 and (d) GD2 LSVs in O2-saturated 0.5 M KOH, with scan rate ¼ 5 mV s¡1. The rotation rates are indicated.
Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
behaviour. It is known that the area within a CV due to capacitive currents is proportional to the electrochemically active surface area (ECSA) of carbonaceous catalysts [22,23]. There is a correlation between ECSA and SBET of the catalysts in the sense that GD1 and G have higher current densities over the potential scanned than GD2. On the other hand, the capacitive current density of GD2 is the smallest, this agrees with the smallest value of SBET for this electrocatalyst (Table 1). However, although the SBET values of G and GD1 are very similar, there is a significant increase in the capacitive current of GD1 compared to G, due to an effect attributed to enhanced adsorption/desorption processes on the surface of the Ndoped catalyst in the alkaline electrolyte. Fig. 3(b) shows a comparative linear sweep voltammetry curves (LSV) of G, GD1 and GD2. The performance is very similar in all catalist showing a two-step process for ORR. The first sharp step over 0.83e0.87 V vs. RHE and the second sharp step over 0.25e0.31 V vs. RHE is attributed to the two-electron reduction O2 to HO 2 [24]. The two-step process has been commonly reported in carbon nanostructures doped with nitrogen or doped with some other heteroatom [25e29]. Fig. 3(c) and (d) shows the background-current corrected polarization curves of the ORR at GD1 and GD2, respectively (negative scan only). The kinetic, mixed and diffusioncontrolled regions can be identified. The current density of the ORR at the two metal-free electrocatalysts increases as the rotating rate is augmented. The onset potential (Eonset) and j at 0.4 V/RHE are higher at GD1 relative to G and GD2: 0.87 V/RHE and 2.16 mA cm2 (Table 1). The increased performance of GD1 is attributed to the N species on its structure. GD1 has N1, N2, N3, N4 and N5 nitrogen species, while GD2 only has N3 groups. Several reports attribute the electrocatalytic activity to N1 and N4 nitrogen species, however, it has also been reported that N2 groups exhibit catalytic activity [30]. Even though the solvothermal post-treatment of graphene resulted in the formation of these species (Table 1), the graphitic N content at GD1 is relatively low (6 at%). It is expected that by modifying the treatment, the composition of N-species can be changed, aiming to increase the graphitic percent and enhance Eonset and j from the ORR at such sample.
Conclusions An easy approach to synthesize graphene by mechanical milling with ORR activity was developed. Two nitrogen doping processes were implemented: solvothermal posttreatment and in situ. The GD1 catalysts exhibited five Nspecies after post-treatment doping with hydrazine. It also showed higher capacitive current densities and catalytic activity for the ORR in terms of Eonset and j at 0.83 V/RHE. By modifying the post-treatment conditions, the composition can be optimized aiming to increase the graphitic N-species content and enhance the performance of GD1 for the ORR. Ndoped graphene synthetized by mechanical milling and doped by solvothermal treatment is a promising large-scale method to obtain low-cost electrocatalyst for ORR in alkaline media.
5
Acknowledgments The authors would like to thank the Mexican Council for Science and Technology (CONACyT) for financial support (grants INFR-2015-251603, CB-2015-250632, 241526 and 252079).
references
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Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR) Juan C. Carrillo-Rodrı´guez a, Ivonne L. Alonso-Lemus b,*, -Ruiz e, A.A. Siller-Ceniceros c, E. Martı´nez G d, Pedro Piza Gregorio Vargas-Gutierrez a, F.J. Rodrı´guez-Varela a,c Sustentabilidad de Los Recursos Naturales y Energı´a, Centro de Investigacion y de Estudios Avanzados Del IPN Unidad Saltillo, Av. Industria Metalu´rgica, Parque Industrial Saltillo Ramos Arizpe, C.P. 25900, Ramos Arizpe, Coah, Mexico b CONACyT, Centro de Investigacion y de Estudios Avanzados Del IPN Unidad Saltillo, Mexico c Nanociencias y Nanotecnologı´a, Centro de Investigacion y de Estudios Avanzados Del IPN Unidad Saltillo, Mexico d Centro de Investigacion en Materiales Avanzados S.C., Alianza Norte 202, PIIT, Carretera Monterrey-Aeropuerto Km. 10, Apodaca, NL, C.P. 66628, Mexico e Centro de Investigacion en Materiales Avanzados S.C., Av. Miguel de Cervantes 120, Parque Industrial Chihuahua, C.P. 31109, Chihuahua, Chih, Mexico a
article info
abstract
Article history:
We report a novel process to synthesize graphene (G) catalyst by mechanical milling using
Received 5 April 2017
graphite flakes as the precursor. G sample has been doped ex situ with hydrazine as ni-
Received in revised form
trogen source via solvothermal procedure to obtain the GD1 catalyst. In a second approach,
9 August 2017
the GD2 sample has been synthesized doping G with uric acid as nitrogen precursor in situ,
Accepted 11 August 2017
i.e., during the milling step. Doping with nitrogen increases the ID/IG Raman spectra ratios
Available online xxx
of GD1 and GD2 to 1.52 and 1.12, respectively, higher than 1.02 of G. XPS analysis shows that Pyridinic, Amine, Pyrrolic, Graphitic and Oxidized nitrogen are formed at GD1, while only
Keywords:
Pyrrolic is present at GD2. Evaluation of catalytic activity for the ORR in 0.5 mol L1 KOH
N-doped graphene
shows an increase in onset potential (Eonset) of the ORR at GD1, compared to G and GD2. GD1
Metal-free electrocatalyst
also generated a higher current density (j) at 0.83 V than G and GD2. The results show that
Mechanical milling
mechanical milling is an efficient method to synthesize G. Even though, the doping can still
Low temperature fuel cells
be improved to form more Nitrogen that promotes the ORR, specifically Pyridinic N and Graphitic N. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Oxygen Reduction Reaction (ORR) is the kinetically limiting part of low-temperature fuel cell devices [1].
Consequently, the use of materials with high catalytic activity is required in order to improve the kinetics at the fuel cell cathode. On this context, Pt is widely applied as cathode catalyst because of its high performance for the ORR. However, it is a costly material with reduced abundance,
* Corresponding author. E-mail addresses: [email protected], [email protected] (I.L. Alonso-Lemus). http://dx.doi.org/10.1016/j.ijhydene.2017.08.084 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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characteristics that have limited the large-scale commercialization of fuel cells [2,3]. Moreover, Pt it is rapidly deactivated by the crossover phenomena in alcohol e fed fuel cells [2,3]. Largely, the research aimed to reduce the cost of fuel cell cathodes by replacing Pt, has been devoted to the development of alternative electrocatalysts based on non-precious metals and metal-free nanostructures [4,5]. Recently, Ndoped graphene nanostructures have shown high catalytic activity for the ORR, in some cases with properties that surpass those of commercial Pt/C catalysts [6]. Chemical Vapour Deposition (CVD) is a well-known method to synthesize graphene [7e9]. Nevertheless, this process has several drawbacks, including high production costs, the need of inert atmosphere, the use of corrosive precursors and very important, lack of scalability [10,11]. Therefore, different routes to produce graphene and doped-graphene must be investigated. One attractive alternative is mechanical milling, which is a process where graphite powder is mixed with an inorganic salt and uses the mechanical forces to produce the displacement of graphite layers. The simplicity of the process and low cost of production makes mechanical milling a promising method to synthesize graphene [11e14]. On the other hand, over the past years there have been mainly two strategies to produce N-doped graphene [15,16]: i) post-treatment, where the carbon material is thermally treated with nitrogen-containing precursors such as ammonia, urea, melamine, cyanamide, dicyandiamide, dicyandiamide, polyaniline and polypyrrole [17,18]; ii) in situ, where the nitrogen precursor is incorporated into the synthesis process while graphene is being produced [17,19]. In this work, the synthesis and characterization of graphene based metal-free catalysts is reported. Graphene is synthesized by mechanical milling and labelled as G. N-doped graphenes are obtained by solvothermal post-treatment using hydrazine as N precursor (GD1) and in situ during mechanical milling using uric acid as N source (GD2). The catalysts are characterized by Xray diffraction (XRD), Raman spectroscopy, BET analysis and Xray photoelectron spectroscopy (XPS). Their electrochemical characterization is focused on the evaluation of catalytic activity for the ORR in alkaline media by Cyclic and Linear Scan Voltammetry (CV and LSV, respectively).
Experimental Preparation of graphene Graphene (G) was obtained by mechanical milling. Graphite flakes (Sigma-Aldrich, 99.99%) and aluminium powder (Alfa Aesar, 99.97%) in atomic ratio of 1:1 were exfoliated for 8 h on a planetary mill (Retsch PM100) with agate balls and vials. To avoid shock stress on the graphite crystal-plane, the rotating speed was set at 300 rpm. The ball-to-powders weight ratio was 20:1. After milling and to remove aluminium traces, the powder was washed with 1 M HCl and deionized water, followed by drying under vacuum.
Preparation of N-doped graphene N-doped GD1 was synthesized via solvothermal posttreatment: 1 g of G was dispersed in 30 mL of hydrazine hydrate (Sigma-Aldrich, 65%) as nitrogen precursor. The dispersion was introduced in a Teflon reactor and treated in a sealed stainless-steel autoclave at 180 C for 24 h. Afterwards, the powder was filtered, washed with deionized water and dried under vacuum. N-doped GD2 was obtained from in situ doping of G: a mixture of graphite, aluminium and uric acid as nitrogen precursor (Sigma-Aldrich, 99%), in atomic ratio of 1:1:10 was introduced in the mill. Then, the milling conditions and washing procedure were the same as those described for G.
Physicochemical characterization X-ray diffraction (XRD) patterns were obtained in a Philips radiaX'Pert (PANalaytical) apparatus with Ni-filtered Cu Ka ). BET surface area of the tion, in the range of 10e100 (2e samples was characterized in an Autosorb-1 equipment (Quantachrome Inst.), by degassing the samples at 180 C for 1 h. Structural properties were characterized by Raman spectroscopy in a Horiba LabRam HR VIS-633 (laser He-Ne) and X-ray photoelectron spectroscopy (XPS) in a Thermo Scientific , 20 eV). ESCALAB 250Xi (Al-Ka
Fig. 1 e (a) XRD patterns and (b) Raman spectra of G, GD1 and GD2.
Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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Table 1 e Physicochemical properties and electrochemical performance for ORR of graphene and N-doped graphenes. Catalyst
G GD1 GD2
SBET (m2 g1)
427 421 80
Chemical composition (at%)
Nitrogen relative distribution (at%)
C
O
N
Al
N1
N2
N3
N4
N5
90.2 89.3 81.0
7.4 6.8 10.0
0.9 2.0 7.7
1.5 1.9 1.3
e 23.3 e
e 21.6 e
e 32.2 100.0
e 6.0 e
e 16.9 e
Electrochemical measurements Characterization was carried out in a Wavedrive 20 potentiostat, connected to a rotating disk electrode set-up (Pine Inst.). A three-electrode cell was used with a Pt wire as the counter electrode and an Ag/AgCl (sat. KCl) as the reference electrode. A catalytic ink was obtained by ultrasonically mixing 10 mg of the corresponding graphene, 1 mL of propanol and 5 mL of Nafion solution. The working electrodes were prepared by depositing 10 mL of each catalytic ink on a glassy carbon substrate with 5 mm diameter. Cyclic Voltammograms (CVs) of the graphene catalysts were obtained at 20 mV s1, in N2-saturated 0.5 M KOH. The potential range was 0.05e1.2 V vs. RHE. CVs of the ORR were acquired in O2-saturated electrolyte in the same potential window and a scan rate ¼ 5 mV s1. The ORR polarization curves were background current-corrected by subtracting CVs obtained in N2-saturated electrolyte under the same conditions. In this work, only the negative scan of the voltammograms was shown. The rotating rates were u ¼ 400, 800, 1200, 1600 and 2000 rpm.
Eonset (V/RHE)
J (mA cm2 at 0.4 V/RHE)
0.86 0.87 0.85
2.07 2.16 1.92
The BET specific surface area (SBET) of the catalysts is shown in Table 1. The solvothermal procedure has no significant effect on the surface properties of graphene, since the SBET of GD1 is similar to that of G (421 and 427 m2 g1,
Results and discussion The XRD patterns of G, GD1 and GD2 are shown in Fig. 1a. All of the catalysts have diffraction peaks at 2q ¼ 26.4, 43.1 and 77.4, which correspond to the (002), (101) and (110) planes of carbon materials, indicating their graphitic structure. GD1 and GD2 have broader reflections, which can be attributed to the incorporation of nitrogen in their structure and/or amorphization caused by milling. Fig. 1b shows the Raman spectra of the three catalysts. The D band at about 1328 cm1 is ascribed to the disorder or defects in the lattice of carbon materials. The G (1580 cm1) and 2D (2631 cm1) bands, both related to the sp2 hybridization of graphite, i.e., the ordered structure of carbon materials, can also be observed. The position, shape and relative intensity of 2D band can be correlated with the number of graphene layers on the material [14]. Intense and narrow 2D bands emerge for graphene with 5 layers or less. On the contrary, broad and lowintensity 2D bands are observed for multilayer graphene. In the case of Fig. 1b, the features of the 2D bands suggest the presence of multilayer graphene and/or exfoliated graphite. The ID/IG band intensity ratio is used to evaluate the degree of disorder, indicating defects in carbon structures. The ID/IG values for G, GD1 and GD2 are 1.02, 1.52 and 1.12, respectively. The higher ID/IG ratio determined for GD1 and GD2 is attributed to the structural defects promoted by the doping with N.
Fig. 2 e N 1s spectrum of (a) G, (b) GD1 and (c) GD2.
Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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respectively). On the other hand, in situ doping show a reduction in the SBET of GD2, which is considerably smaller (80 m2 g1) compared to GD1 metal-free catalysts. The reduction in the GD2 SBET could be caused by of uric acid blocking sites in graphene structure. XPS analysis has been carried out to determine the surface elemental composition and electronic configuration of the catalysts. G, GD1 and GD2 contain C, O, N and Al traces as shown in Table 1. It is observed that the two doping methods have an effect in the surface chemical composition compared with G. Metal-free electrocatalyst GD1 contain nearly 90 at% C, while the value for GD2 is 81 at%. GD2 also shows higher O which comes from the uric acid used as nitrogen precursor. Furthermore, nitrogen was detected in 2.0 and 7.7 at% for GD1 and GD2, respectively. A detailed analysis of N 1s peak was carried to determine the nitrogen species formed into the graphene. Fig. 2(a) shows the high-resolution N 1s spectrum of G. The data are highly dispersed and of low intensity. A peak is shown at 399.1 eV after deconvolution, but it is not conclusive of the presence of N on the graphene structure. Fig. 2(b) shows the N 1s spectrum of GD1. The deconvolution shows five peaks at 398.8, 399.4, 400.3, 402.5 and 405.3 eV, ascribed to pyridinic
(N1), amine (N2), pyrrolic (N3), graphitic (N4) and oxidized (N5) species of N, respectively. This analysis confirms the incorporation of nitrogen into graphene to form N-doped GD1 [17,20]. The relative distribution of the five N species is given in Table 1, where at least three species that promote the ORR were detected (N1, N3 and N4). Meanwhile, Fig. 2(c) is the spectrum corresponding to GD2. Only one peak at 400.3 eV which can be ascribed to N3 has been obtained after deconvolution for this catalyst [21]. Thus, there is a significant effect of the doping procedure on the N-species formed at the graphene structure. Solvothermal doping is an effective method to produce N-doped graphene, due that develops 5 different species, among them those that have been identified as the promoters of catalytic activity for the ORR of carbon nanostructures [18]. Even though, GD2 synthetized by in situ method has high nitrogen content (7.7 at%) is an ineffective method to produce N-doped graphene. The N3 species detected in this material may be attributed to the nitrogen-carbon bonds of the uric acid. On the other hand, these features may have influenced the lower SBET of GD2. Fig. 3(a) shows the CVs of the graphene-based catalysts exhibiting an almost rectangular shape and high capacitive
Fig. 3 e (a) CVs of G, GD1 and GD2 in N2-saturated 0.5 M KOH, with scan rate ¼ 20 mV s¡1. (b) LSVs of the ORR at 2000 rpm of G, GD1 and GD2, (c) GD1 and (d) GD2 LSVs in O2-saturated 0.5 M KOH, with scan rate ¼ 5 mV s¡1. The rotation rates are indicated.
Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084
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behaviour. It is known that the area within a CV due to capacitive currents is proportional to the electrochemically active surface area (ECSA) of carbonaceous catalysts [22,23]. There is a correlation between ECSA and SBET of the catalysts in the sense that GD1 and G have higher current densities over the potential scanned than GD2. On the other hand, the capacitive current density of GD2 is the smallest, this agrees with the smallest value of SBET for this electrocatalyst (Table 1). However, although the SBET values of G and GD1 are very similar, there is a significant increase in the capacitive current of GD1 compared to G, due to an effect attributed to enhanced adsorption/desorption processes on the surface of the Ndoped catalyst in the alkaline electrolyte. Fig. 3(b) shows a comparative linear sweep voltammetry curves (LSV) of G, GD1 and GD2. The performance is very similar in all catalist showing a two-step process for ORR. The first sharp step over 0.83e0.87 V vs. RHE and the second sharp step over 0.25e0.31 V vs. RHE is attributed to the two-electron reduction O2 to HO 2 [24]. The two-step process has been commonly reported in carbon nanostructures doped with nitrogen or doped with some other heteroatom [25e29]. Fig. 3(c) and (d) shows the background-current corrected polarization curves of the ORR at GD1 and GD2, respectively (negative scan only). The kinetic, mixed and diffusioncontrolled regions can be identified. The current density of the ORR at the two metal-free electrocatalysts increases as the rotating rate is augmented. The onset potential (Eonset) and j at 0.4 V/RHE are higher at GD1 relative to G and GD2: 0.87 V/RHE and 2.16 mA cm2 (Table 1). The increased performance of GD1 is attributed to the N species on its structure. GD1 has N1, N2, N3, N4 and N5 nitrogen species, while GD2 only has N3 groups. Several reports attribute the electrocatalytic activity to N1 and N4 nitrogen species, however, it has also been reported that N2 groups exhibit catalytic activity [30]. Even though the solvothermal post-treatment of graphene resulted in the formation of these species (Table 1), the graphitic N content at GD1 is relatively low (6 at%). It is expected that by modifying the treatment, the composition of N-species can be changed, aiming to increase the graphitic percent and enhance Eonset and j from the ORR at such sample.
Conclusions An easy approach to synthesize graphene by mechanical milling with ORR activity was developed. Two nitrogen doping processes were implemented: solvothermal posttreatment and in situ. The GD1 catalysts exhibited five Nspecies after post-treatment doping with hydrazine. It also showed higher capacitive current densities and catalytic activity for the ORR in terms of Eonset and j at 0.83 V/RHE. By modifying the post-treatment conditions, the composition can be optimized aiming to increase the graphitic N-species content and enhance the performance of GD1 for the ORR. Ndoped graphene synthetized by mechanical milling and doped by solvothermal treatment is a promising large-scale method to obtain low-cost electrocatalyst for ORR in alkaline media.
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Acknowledgments The authors would like to thank the Mexican Council for Science and Technology (CONACyT) for financial support (grants INFR-2015-251603, CB-2015-250632, 241526 and 252079).
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Please cite this article in press as: Carrillo-Rodrı´guez JC, et al., Easy synthesis of N-doped graphene by milling exfoliation with electrocatalytic activity towards the Oxygen Reduction Reaction (ORR), International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.08.084