Nitrogen-doped graphene-supported Co/CoNx nanohybrid as a highly efficient electrocatalyst for oxygen reduction reaction in an alkaline medium

Abstract

In this work, we utilize a one-step pyrolysis method to thermally synthesize a non-precious cobalt-based nitrogen-doped graphene (Co-NG) using graphene oxide (GO) and guanidine hydrochloride (GuHCl) with a small amount of CoCl₂ precursor as a low-cost and highly efficient catalyst for the oxygen reduction reaction (ORR). The synthesized cobalt-based nitrogen-doped graphene (Co-NG) was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activity of the Co-NG composite towards the ORR was evaluated using linear sweep voltammetry method. Electrochemical measurements reveal that the obtained Co-NG 850 composite has excellent catalytic activity towards the ORR in an alkaline electrolyte, including a large kinetic-limiting current density and good stability, as well as it exhibits the desirable four-electron pathway for the formation of water. These superior properties make the Co-NG 850 a promising cathode catalyst for alkaline fuel cells.

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1. Introduction

Fuel cells can directly convert chemical energy into electrical energy with a high conversion efficiency, high power density, and easy operation, which are considered to be a promising green energy generation technology.¹ Oxygen reduction reaction (ORR) on the cathode is the critical pivot in fuel cells. Platinum (Pt) is regarded as the best ORR catalyst for fuel cells because it provides the lowest overpotential and the highest current response towards a direct 4-electron reduction of oxygen to water.² However, the use of electrocatalysts with a high loading of Pt or Pt-alloys in the electrode, leads to high cost, which hinders the further commercialization and market demands of fuel cells.^1,3 Thus, a massive improvement in fuel cells requires not only the development of electrocatalysts based on sustainable and abundant elements (e.g. non-noble metal catalysts) but also the rational design and synthesis of electrocatalysts that are capable of performing ORR as efficiently as, if not better than platinum.⁴ Some alternative ORR catalysts have been studied in fuel cells, including transition metal alloys,⁵ chalcogenides,⁶ metal-coordinating polymers,^7–9 metal-N4 complexes,^10–12 metal oxides,^13–15 and heteroatom-doped carbon materials.^16–20 However, the limitations associated with these alternatives, such as the high manufacturing cost, complex preparing procedures, poor durability, and low performance, prevent them from being ideal candidates for large-scale fuel cell applications.

Currently, N-doped carbon materials are considered to be potential substitutes for Pt to reduce the cost and enhance the stability of ORR catalysts.^21–23 The incorporation of the N element into carbon materials endows them with unique electronic properties due to the conjugation between the lone-pair of electrons on N and the graphene p system, which significantly enhances the number of active sites for ORR.^24,25 It is proposed that the inherent active sites for ORR in N-doped carbon materials might include pyridinic N, pyrrolic N, and graphitic N species.²⁶ Among them, graphitic N is regarded as one of the most efficient ORR active sites because the graphitic N atoms in the carbon lattice facilitate electron transfer from the electronic bands of carbon to the antibonding orbitals of O₂.^27,28

As is well known, graphene, one-atom-thick layers of sp²-hybridized carbon atoms packed in a honeycomb lattice,²⁹ has attracted widespread attention in the next generation electronic devices for the application in nanoelectronics,³⁰ energy storage,^31,32 biosensing³³ and catalysts³⁴ owing to its large specific surface area, high thermal conductivity, and excellent chemical and thermal stability. Therefore, graphene can provide an effective platform for the fundamental investigations of the nature of metal-like reactivity in all carbon-based materials. Some carbon atoms on pristine graphene can be substituted by heteroatoms, such as N, B, P and S, and the obtained heteroatom-doped graphene materials are considered to be highly efficient electrocatalysts for ORR.^18–20,35

Recently, nitrogen-doped graphene (NG) with transition metals has attracted considerable attention as non-precious catalysts for ORR because of their low price, immunity to CO and rather good stability.^36–39 Among the different types of non-precious metal-graphene-based catalysts, nitrogen-coordinated transition metals (Fe, Co, Ni, Mn and V) in graphene have been extensively studied for ORR, and considerable improvements in catalytic performance have been achieved.^40–44 Previous reports showed that Co-incorporated NG (Co-NG) possessed a higher ORR activity than Co-graphene-based catalysts in alkaline media.^15,45 It is generally believed that Co-NG can provide more active sites for oxygen reduction and generate a synergistic effect between NG and cobalt nitride (cobalt oxide), leading to an enhanced electrocatalytic performance towards ORR.^45–48

Here, we obtained a Co-NG electrocatalyst by introducing highly efficient CoNx moieties into graphene via the direct pyrolysis of the mixture of CoCl₂, guanidine hydrochloride, and graphene oxide (GO) under an argon atmosphere. The synthesized Co-NG was characterized by TEM, Raman spectroscopy, XRD, and XPS. The electrocatalytic activity of the Co-NG composite towards the ORR was evaluated using linear sweep voltammetry methods. The fabricated Co-NG 850 catalyst for the ORR showed high activity and good stability, which could be used as a promising Pt-free catalyst in alkaline fuel cells.

2. Experimental section

2.1 Preparation of Co-NG electrocatalysts

GO was synthesized from graphite powder by a modified Hummers method.⁴⁹ The Co-NG composite was prepared as follows: graphene oxide was dispersed into distilled water by ultrasonic vibration for 2 h at a concentration of 2 mg mL⁻¹. Then, 100 mg of GuHCl was added to 50 mL of the above GO solution with stirring for 30 min at room temperature. Subsequently, 10 mg of cobalt dichloride was added to the above suspension. The mixture was stirred for 12 hours and dried overnight at 30 °C in an oven. Annealing of the composite was carried out in a tube furnace. The Co-NG catalysts were prepared by pyrolysing the obtained mixture of cobalt dichloride, GuHCl and GO at 650, 750, 850 and 950 °C for 1 h in an Ar atmosphere, which were abbreviated as Co-NG 650, Co-NG 750, Co-NG 850 and Co-NG 950, respectively. The heating ramp rate was 10 °C min⁻¹. N-doped graphene pyrolysed at 850 °C (NG 850) was also synthesized using GuHCl/GO composite with the same synthesis route as that of Co-NG 850. The synthesis route of Co-NG is illustrated in Scheme 1.

Scheme 1 The synthesis route of Co-NG composites.

2.2 Preparation of modified electrode

Prior to modification, the working electrode was successively polished with 1.0, 0.3 and 0.05 mm aluminum oxide slurry, then thoroughly rinsed with distilled water, absolute ethanol and distilled water for 5 min. Subsequently, the cleaned GC electrode was blow-dried with N₂ at ambient temperature. To modify GC, 1.0 mg Co-NG and 1.0 mL ethanol were ultrasonically mixed to obtain a homogeneous ink. A specific amount of the catalyst ink at a concentration of 1 mg mL⁻¹ was dropped onto a freshly polished electrode surface to prepare a catalyst film. The catalyst loading per area on the GC electrode was maintained to be 283 μg cm⁻². The same amount of commercial Pt/C catalyst was also loaded onto the GC electrode for comparison.

2.3 Characterization

The morphology and structure of the as-prepared samples was observed by transmission electron microscopy (TEM, JEOL-2010 transmission electron microscope operating at 200 kV) and X-ray diffraction studies (RIGAK, D/MAX2550 VB/PC, Japan). Raman spectra were obtained on a TriVista™555CRS Raman spectrometer at 785 nm. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCLAB 250 spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV photons) to identify surface chemical composition and bonding state.

2.4 Electrochemical measurements

Rotating disk electrode (RDE) measurements were performed on a CHI660E electrochemical workstation (CH Instruments, USA) in a conventional three-electrode cell using the coated GC electrode (5 mm in diameter) as the working electrode, a platinum wire as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. RDE measurements were performed in an O₂-saturated solution at a scan rate of 10 mV s⁻¹. Linear sweep voltammetry measurements were performed at a GC rotating disk electrode having a diameter of 5 mm. The rotating ring disk electrode (RRDE) experiments were performed using a Pine Instrument Company's AF-MSRCE modulator speed rotator on a CHI660E electrochemical workstation (CH Instruments, USA). For RRDE experiments, the working electrode was a glassy carbon disk (5.61 mm diameter)and a platinum ring (collection efficiency N = 0.37). These experiments were performed at 1600 rpm in an O₂-saturated solution. The disk potential was swept at 10 mV s⁻¹. The Pt ring electrode was polarized at 0.1 V vs. SCE for oxidizing the hydrogen peroxide generated during oxygen reduction at the modified GC disk electrode. During the test, a trachea was placed on the solution surface. All the experiments were carried out in 0.1 M KOH solution at room temperature.

3. Results and discussions

3.1 Characterization of Co-NG composite

The morphological and structural features of the synthesized Co-NG 850 hybrids were obtained by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). TEM image reveals that the Co-NG 850 hybrid has a wrinkled structure with nanocrystal decorated on the surface of the nitrogen-doped graphene (Fig. 1a). HRTEM image and selected area electron diffraction (SAED) pattern confirm that these nanocrystals loaded on Co-NG 850 have a good crystalline structure (Fig. 1b). There are two different lattice spacings of these nanocrystals, in which 0.239 nm of the lattice spacing corresponds to that of CoNx, whereas 0.205 nm of the lattice spacing is assigned to that of Co, demonstrating that Co and CoNx crystalline structures coexist in these nanocrystals. For nitrogen-doped graphene, the interlayer spacings of the graphitic layers are revealed to be ca. 0.34 nm (Fig. 1c). X-ray diffraction (XRD) was carried out to further study the crystallographic structure of the Co-NG hybrid (Fig. 1d). The XRD spectrum of the Co-NG 850 hybrid shows good crystalline structures of Co and CoNx, as evidenced by sharp diffraction peaks corresponding to the (111), (200) and (220) crystal facets of cubic Co, and the (111), (200) and (220) crystal facets of cubic CoNx. The broad peak, which appeared around 23.9°, illustrates the successful reduction of GO and formation of graphitic structures.⁴⁸ These results are in agreement with those of the TEM.

Fig. 1 (a) The TEM image of Co-NG 850; (b) and (c) the HRTEM images of Co-NG; inset shows the corresponding SAED pattern; (d) XRD patterns of Co-NG 850; (e) Raman spectra of Co/GuHCl/GO, Co-NG 650, Co-NG 750, Co-NG 850 and Co-NG 950; (f) the corresponding I_D/I_G ratio.

Raman spectroscopy is the most direct and nondestructive technique to characterize the structure and quality of carbon materials,⁵⁰ particularly to examine the defects, the ordered and disordered structures, and the layers of graphene. The Raman spectra of Co-NG exhibit two remarkable peaks at around 1350 and 1585 cm⁻¹ (Fig. 1e) corresponding to the well-defined D band and G band, respectively. The I_D/I_G ratio (the intensity of the G band divided by the intensity of the D band) is widely used to assess the density of defects in carbon materials.⁵¹ As shown in Fig. 1f, the I_D/I_G ratio of Co-NG first decreases and then slowly increases with the increase of pyrolysis temperature, but it is obviously lower than that of GO. The decrease of I_D/I_G ratio at low pyrolysis temperatures (650–750 °C) should be attributed to the recovery of the partly conjugated structure of graphene through thermal reduction. However, the increase of I_D/I_G ratio at high pyrolysis temperatures (850–950 °C) is probably due to the fact that most of the epoxy and hydroxyl groups begin to decompose and give rise to an in-plane C=C crack at higher temperatures.⁵²

X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental composition of Co-NG. As shown in Fig. 2a, the XPS survey spectra of Co-NG samples provide evidence for the presence of C and O, as well as limited nitrogen and cobalt without any other impurities, confirming that Co and N were successfully incorporated into rGO. The analysis of N 1s spectra reveals the presence of pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic N⁺-O⁻, corresponding to the binding energies of 398.4, 400.1, 401.0 eV, and 402.0–404.0 eV, respectively (Fig. 2b).^53–55 It can be seen from Fig. 2c that the content of nitrogen atoms significantly decreases with increasing pyrolysis temperature from 650 to 850 °C, but remains virtually unchanged when the pyrolysis temperature is increased from 850 to 950 °C.

Fig. 2 (a) XPS broad scan spectra of Co-NG at different temperatures; (b) high-resolution N1s XPS spectra of Co-NG at different pyrolysis temperatures; (c) The Nitrogen percentage of three nitrogen species in Co-NG sheets at different pyrolysis temperatures; (d) the percentage of the three nitrogen species in the nitrogen content of Co-NG at different pyrolysis temperatures; (e) high-resolution Co2p XPS spectra of Co-NG.

Fig. 2d plots the percentages of different N species in the sample versus the pyrolysis temperature. With increasing pyrolysis temperature from 650 to 950 °C, graphitic-N becomes dominant, while the content of pyridinic-N, pyrrolic-N and pyridinic N⁺-O⁻ decreases. The results suggest that the pyrolysis temperature has an obvious effect on the total content of nitrogen, whereas the high pyrolysis temperatures (750–950 °C) can promote the formation of graphitic-N and prohibit the generation of pyridinic-N, pyrrolic-N and pyridinic N⁺-O⁻, which is beneficial for nitrogen to form highly efficient active sites for ORR. The analysis of Co 2p spectra reveals the presence of Co and CoNx, corresponding to the binding energies of 778.5 eV and 781.6 eV, respectively, (Fig. 2e),⁵⁶ which is in accordance with the TEM and XRD results.

3.2 Electrocatalytic activity in an alkaline medium

To test the electrocatalytic behaviour of our obtained catalysts, both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements were carried out. Fig. 3a and c show the experimental RDE data of O₂ reduction on Co-NG 850 and commercial Pt/C in O₂-saturated 0.1 M KOH. Moreover, the RDE measurements of NG and other Co-NG materials were also performed, which are displayed in ESI Fig. S1a, c, e and g.† Obviously, the limiting current densities of these catalysts increase with increasing rotation speed. To gain better insight into the electron transfer process for ORR on each sample, the Koutecky–Levich (K–L) plots were obtained for these catalysts at reaction potentials from −0.4 to −0.8 V vs. SCE based on LSVs at various rotation speeds (Fig. 3b and d and ESI Fig. S1b, d, f and h†). The Koutecky–Levich (K–L) equation is given as follows:⁵⁷

B = 0.62nFC₀(D₀)^2/3υ^−1/6 (2)

where J is the measured current density, J_K and J_L are the kinetic and diffusion limiting current densities, respectively, n is the overall number of electrons transferred, F is the Faraday constant, C₀ is the bulk concentration of O₂ dissolved in the electrolyte, D₀ is the O₂ diffusion coefficient, υ is the kinematic viscosity of the electrolyte, K is the electron transfer rate constant, and ω is the angular velocity of the disk (ω = 2πN, N is the linear rotation speed). The number of electrons transferred in the O₂ reduction process can be calculated from the slope of the K–L equation.

Fig. 3 LSVs for ORR on (a) Co-NG 850, (c) Pt/C in O₂-saturated 0.1 M KOH under different rotation speeds at a scan rate of 10 mV s⁻¹. The corresponding K–L plots for (b) Co-NG 850, (d) Pt/C at fixed potentials of −0.4, −0.5, −0.6, −0.7 and −0.8 V vs. SCE, respectively; (e) LSVs of Co-NG 650, Co-NG 750, Co-NG 850 and Co-NG 950, NG and Pt/C in O₂-saturated 0.1 M KOH at a scan rate of 10 mV s⁻¹ with a rotation speed of 1600 rpm; (f) electron transfer number of Co-NG 650, Co-NG 750, Co-NG 850 and Co-NG 950, NG and Pt/C at fixed potentials of −0.4, −0.5, −0.6, −0.7 and −0.8 V vs. SCE.

Fig. 3e shows the ORR polarization curves of Co-NG 650, Co-NG 750, Co-NG 850, Co-NG 950, NG and commercial Pt/C at a rotation rate of 1600 rpm. The Tafel plots of LSVs are derived and shown in ESI Fig. S2.† To comprehensively evaluate the catalytic efficiency, some electrochemical parameters for ORR on these catalysts are compared according to the polarization curves, as shown in Table 1.

Table 1 Electrochemical parameters for ORR estimated from RDE polarization curves in 0.1 M KOH solution^a

Electrocatalysts	Eonset V vs. SCE	E1/2 V vs. SCE	JL (mA cm^−2) at −0.8 V vs. SCE	Tafel plot slopes (mV dec^−1)
a Obtained from Fig. 3e.
Co-NG 650	−0.003	−0.222	3.42	128
Co-NG 750	−0.007	−0.245	4.34	104
Co-NG 850	0.005	−0.164	5.08	71
Co-NG 950	−0.079	−0.224	4.52	74
NG	−0.009	−0.229	4.49	127
Pt/C	0.004	−0.181	5.28	65

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It can be seen from Table 1 that the performance of Co-NG 850 catalyst is better than other Co-NG materials in terms of the onset potential, half-wave potential and limiting current density. Furthermore, the catalytic activity of Co-NG 850 is also higher than that of NG, which should be attributed to the synergistic effect between NG and CoNx. More importantly, the limiting current density of Co-NG 850 is comparable with that of Pt/C, whereas the onset potential and half-wave potential of O₂ reduction on the Co-NG 850 catalyst are more positive than those of Pt/C, demonstrating the superior electrocatalytic activity of Co-NG 850 catalyst. The excellent ORR activity of Co-NG 850 catalyst are also gleaned from the much smaller Tafel slope of 71 mV dec⁻¹ in the low overpotential region than those measured with NG and other Co-NG hybrids. The Tafel slope of 71 mV dec⁻¹ is similar to that of Pt/C (65 mV dec⁻¹), indicating that the ORR on these two catalysts has a similar catalytic mechanism.⁵⁸

Fig. 3f presents the number of electrons transferred at different potentials in the ORR process for all the samples. The value of the electron transfer number (n) for ORR on Co-NG 850 varies between 3.76 and 3.97, suggesting that the Co-NG 850 hybrid favors a 4-electron process for ORR, and the final reduced products is almost H₂O. The electron transfer number of Pt/C is determined to be 3.86, agreeing well with a previous report.⁴⁸

The electrocatalytic properties of our Co-NG 850 catalyst are also compared with those previously reported in the literature (Table 2). It is worth noting that the onset potential and limiting current density of our catalyst is comparable to or even better than those of the previous reports (as shown in Table 2).

Table 2 The bench mark of our Co-NG 850 with values obtained from some other independent literature. All in alkaline conditions with 0.1 M KOH

Electrocatalyst	ΔEonset^a^,^c (V)	ΔE1/2^a^,^c (V)	JL^b^,^c (mA cm^−2)	Reference electrode	Ref.
a Represents the difference of onset potentials or half-wave potentials between various catalysts and Pt/C.b Represents the difference of diffusion-limited current densities of various catalysts between various catalysts and Pt/C at rotation speed of 1600 rpm.c The onset (Eonset) and half-wave (E1/2) potential are reported from the corresponding literatures and the corresponding figures in the present studies.
Co-NG 850	0.001	0.017	5.08	SCE	In this work
Co–N-GN	−0.036	−0.023	4.12	SCE	48
Co10-NMCV	−0.008	−0.020	4.60	SCE	52
NG/Fe5.0	−0.040	−0.058	4.76	SCE	50
Fe-NG-30-b	−0.028	−0.023	3.74	SCE	59
Co/N/rGO	−0.011	−0.010	5.40	SCE	46
PDMC/Fe	−0.052	−0.061	4.12	RHE	4
PDMC/Co	−0.050	−0.063	4.15	RHE	4

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An additional rotating ring disk electrode (RRDE) experiment can be used to estimate n and further verify the ORR pathways, in which the peroxide species produced during the ORR process at the disk electrode can be detected by the ring electrode. The electron transfer number and the percentage of hydrogen peroxide species can be determined by the following equations:^60,61

where I_D is the disk current, I_R is the ring current, and N is the current collection efficiency of the Pt ring.

Fig. 4 shows the disk current and ring current for the Co-NG 850 and Pt/C catalysts in an O₂-saturated 0.1 M KOH solution. The potential of the Pt ring electrode in the RRDE system was set to 0.1 V vs. SCE for detecting peroxide species formed at the disk electrode. As can be seen from Fig. 4b, the percentage of hydrogen peroxide ions produced by Co-NG 850 is varied from 9.97% to 12.03% at potentials ranging from −0.4 to −0.7 V vs. SCE. The electron transfer number calculated is in the range of 3.76–3.80, which is in agreement with the results calculated from the K–L equation.

Fig. 4 (a) Rotating ring disk electrode (RRDE) linear sweep voltammograms of Co-NG 850 and Pt/C in O₂-saturated 0.1 M KOH at a scan rate of 10 mV s⁻¹ with a rotation speed of 1600 rpm; (b) peroxide percentage and electron transfer number (inset) of Co-NG 850 and Pt/C at fixed potentials of −0.4, −0.5, −0.6 and −0.7 V vs. SCE.

In addition, chronoamperometric experiments were carried out to evaluate the stability of the Co-NG 850 composite and Pt/C electrocatalysts for ORR. The test was performed at a constant potential of −0.3 V vs. SCE in 0.1 M KOH solution saturated with O₂. As shown in Fig. 5, the corresponding current–time chronoamperometric curve of Pt/C exhibits a faster attenuation loss of 49.77% in its initial current after chronoamperometric measurements for 10 000 s. However, the Co-NG 850 composite displays a very slow degradation of 32.79% in its initial current. This indicates that the Co-NG 850 catalyst has better stability than Pt/C.

Fig. 5 Stability evaluation of Co-NG 850 and Pt/C for 10 000 s in an O₂-saturated 0.1 M KOH solution at −0.3 V vs. SCE with a rotation speed of 1600 rpm.

Previous studies on N-doped graphene suggest that graphitic-N can exist in any part of the graphene layer, while pyridinic-N and pyrrolic-N locate only at the edge and defect sites of the graphene layers.⁶² It is believed that graphitic-N possesses much higher catalytic activity than pyridinic-N and pyrrolic-N for ORR.⁶³ In addition, CoNx is more active than graphitic-N in catalyzing oxygen reduction.⁶⁴ For our fabricated Co–N G850 catalyst, XPS measurements show that the atomic percentage of N is only 0.76%; nevertheless, most of the N exist in the form of graphitic-N (50.13%) and pyridinic-N (35.32%). The TEM and XRD results demonstrate that the Co and CoNx crystalline structures coexist in the nanocrystals loaded on the N-doped graphene. RS analysis proves Co-NG 850 has a high degree of graphitization. Therefore, compared with Pt/C catalyst, the higher catalytic activity of Co-NG 850 catalyst for ORR should be attributed to more active sites of highly efficient graphitic-N and CoNx, faster electron transfer kinetics due to the existence of Co in the nanocrystals and the high degree of graphitization for N-doped graphene, as well as the synergistic effect between graphitic-N and CoNx.

4. Conclusion

In summary, this work reports the preparation of a novel nitrogen-doped graphene-supported Co/CoNx nanohybrid catalyst for ORR. Compared with Pt/C, the obtained Co-NG 850 catalyst for ORR exhibited higher catalytic activity in terms of a more positive onset potential and half-wave potential. Further studies revealed that the performance enhancement can be associated with the presence of more active sites of highly efficient graphitic-N and CoNx, faster electron transfer kinetics due to the existence of Co in the nanocrystals and the high degree of graphitization for N-doped graphene, as well as the synergistic effect between CoNx and N-doped graphene. The Co-NG catalyst also showed excellent durability. Furthermore, this Co-NG material was prepared via a facile procedure with low-cost precursors. All these advantages make the Co-NG 850 catalyst an ideal candidate for the replacement of Pt/C in alkaline fuel cells and metal-air batteries.

Acknowledgements

This research has been financed by the National Natural Science Foundation of China (no. 21273024) and the Natural Science Foundation of Jilin Province, China (no. 201215135).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10133b

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