Solvothermal Synthesis of Nitrogen-Containing Graphene for Electrochemical Oxygen Reduction in Acid Media∗

Graphene is ideally suited to electrochemistry by virtue of its high surface area and impressive electronic properties. Nitrogen incorporation can be used to tailor the properties of graphene. Here we present a simple solvothermal technique to produce a nitrogen-containing foam-like macroporous graphene powder doped with up to 15 wt% nitrogen. This is applied as an effective non-precious, metal-free electrochemical catalyst for oxygen reduction in acid media. [DOI: 10.1380/ejssnt.2012.29]

Tailoring the properties of graphene will be crucial for many of these applications.A practical and controllable method of doing so is by doping or functionalizing with heteroatoms, such as nitrogen.There have been many theoretical studies of nitrogen-containing graphene (NG), predicting increased conductivity, tunable spin polarization, negative differential resistance, ferromagnetism, semiconductor behavior, enhanced gas adsorption, and enhanced Pt binding [7].There have been various attempts to synthesize NG by graphene oxide reduction [8], arc discharge [9], chemical vapor deposition [10], plasma irradiation [11], and organic synthesis [12].
NG has been applied to the electrochemical oxygen reduction reaction (ORR) in several instances, mainly in alkaline solution [13].The excellent ORR activity of nitrogen-doped carbons in alkaline media is wellknown [14].However, alkaline fuel cells are not practical due to the limited availability/performance of negative ion conduction membranes, and the poor stability of fuel cell materials in alkaline media [15].It is currently more desirable to measure ORR in acid media [16], since fuel cells based on Nafion R ⃝ are well-established.For example, NG has been applied as a catalyst for ORR in HClO 4 [17].Additionally, NG may provide a simple platinum cata-lyst support in acid-based fuel cells, as a replacement for carbon black [18].A promising method of NG production is by chemical synthesis, which is inherently bottom-up, tunable, and scalable [19].In this communication we fabricate NG via a solvothermal technique, in which a nitrogen-containing alcohol is reacted with sodium in a sealed vessel, at high temperature and pressure.

II. EXPERIMENTAL
3.05 g of 2-aminoethanol and 1.05 g sodium were added to a PTFE melting pot, under inert gas.The vessel was sealed and heated to 200 • C under solvothermal conditions for three days, yielding a clathrate-like precursor powder.This comprises an alkoxide matrix entrapping liquid 2aminoethanol inside the cells [19].The residual alcohol inside the cells of this precursor was ignited in air by careful insertion into a tube furnace at 600 • C. The carbonized product was collected, mixed with deionized water, sonified, washed, and dried, yielding a black powder.Subsequently, this was pyrolysed at various temperatures for one hour under flowing nitrogen, in an attempt to modify the conductivity, surface area, and nitrogen content, by carbonization and decomposition.Undoped graphene was also synthesized, according to the literature [19].The product was characterized using transmission electron microscopy (TEM) (JEOL, JEM2010), atomic force microscopy (AFM) (SII SPA-400), CHN elemental analysis (Yanaco, CHN Corder MT-6), X-ray photoelectron spectroscopy (XPS) (JPS-9010MC, JEOL), and nitrogen adsorption (Belsorp II mini, Bel Japan, Inc.).The ORR activity was measured via rotating ring-disk electrode (RRDE) voltammetry at room temperature in a 0.5 M H 2 SO 4 electrolyte solution, as previously described [20].

III. RESULTS AND DISCUSSION
The morphology of the product was investigated by TEM, AFM and BET/BHJ surface area analysis.Fig- ure 1 shows a TEM image of the NG product, comparing favorably with the literature [19].A three-dimensional macroporous foam-type structure is observed, made up of graphene-like walls encapsulating cells of ∼200 nm diameter.The size of this particle is around 2.5 µm in diameter.The image is slightly defocused in the center, reflecting the three-dimensional nature of the material.The thickness of the graphene walls was investigated by AFM. Figure 2 shows an AFM cross-section and a smoothed topology map (inset) of a piece of NG, presumably originating from one of the walls of the graphene foam.This sample is ∼100 nm across and has a height of ∼1.7 nm, which is thicker than pristine graphene (∼0.67 nm) [21].This increased thickness may be due to gas adsorption, surface functionalization, or the presence of several graphene layers.BET nitrogen adsorption measurements were used to probe the surface area and porosity of the NG samples.The BET surface areas for NG samples pyrolysed at 600, 800, and 1000 • C were 60, 161, and 170 m 2 /g, respectively.The BJH porosities were 14.8, 75.1 and 54.7 m 2 /g, respectively.Evidently, decomposition by pyrolysis has a significant effect on the surface area.These numbers are lower than predicted for an isolated graphene sheet [2], which may be due to sample impurity, greater thickness as measured by AFM, and/or enclosed pores.Despite this, the surface area numbers compare favorably with the literature [8][9][10][11][12] and with commercially available graphene powder.
The elemental composition and chemical environment of atoms in the NG samples after pyrolysis was measured by CHN elemental analysis and XPS.CHN elemental analysis gave nitrogen contents of 15.1, 8.8, and 4.8 wt.%, after pyrolysis at 600, 800, and 1000 • C, respectively.This demonstrates that the nitrogen content can be readily tuned over a wide range, by post-pyrolysis.The initial content is extremely high, and a large amount of nitrogen remains even after pyrolysis at 1000 • C, indicating high stability of nitrogen in the sample.The XPS N 1s signal (Fig. 3) was used to glean information about pyridinic (∼398.5 eV), pyrrolic (∼399.9eV), graphite-like http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology (∼400.5 eV), amine (∼401.4eV), and NO x (∼402.5 eV) nitrogen species, but it must be noted that meaningful deconvolution of such small signals is difficult, and bond binding energy assignments vary wildly throughout the literature.Pyridinic and graphite-like bonds are the major components, with roughly equal intensities, observed as two major peaks.There is a large amount of graphitelike nitrogen in all the samples, suggesting that nitrogen is doped throughout the basal-plane and not just at edges and defects (as is often observed) [8,9,11], reflecting the bottom-up nature of this synthesis technique.These graphite-like nitrogen atoms should be relatively stable, by virtue of their bonding to three carbon atoms.This may explain the large nitrogen content observed in these samples by CHN elemental analysis even after high temperature pyrolysis.
Linear sweep voltammograms (LSVs) give information about the ORR activity of a catalyst.Figure 4 shows LSVs of the NG product pyrolysed at 600, 800, and 1000 • C. The data for undoped graphene derived from ethanol is also shown for reference.The onset potential is defined here at a current density of −2 µA/cm 2 .Undoped graphene has an onset potential of 0.78 V and a current density at 0 V of −0.5 mA/cm 2 .NG pyrolysed at 600 • C has a much higher onset potential of 0.87 V, and a slightly better current density of −0.75 mA/cm 2 .The improved onset potential reflects an increase in the inherent ORR activity of this catalyst as a result of nitrogen doping; graphite-like nitrogen bonding at zigzag edges of graphene sheets has been calculated to be active for ORR [22], and significant graphite-like bonds are observed in these samples via XPS (Fig. 3).However, the current density is rather low, and this may reflect the low BET surface area (just 60.3 m 2 /g), or low electrical conductivity due to the extremely high nitrogen content (15.1 wt.% ).After pyrolysis at 800 • C, the onset potential is 0.78 V and the current density is −2.0 mA/cm 2 .Finally, after pyrolysis at 1000 • C, the onset potential is 0.84 V and the current density is −2.3 mA/cm 2 .The current density is much improved by pyrolysis at elevated temperature, possibly due to the increase in surface area, and/or the increase in electronic conductivity associated with the decrease in nitrogen and other impurities.The current measured by the ring electrode in RRDE measurements can be used to estimate the proportion of H 2 O 2 produced during ORR (Fig. 5).This in turn can be used to gain information about the electron pathway, specifically, the number of electrons transferred per ORR event (n).For a Pt/carbon electrode, n ≈ 4.0, and this is the target for fuel cell applications.The expected value for pure carbon is n ≈ 2.0.The highest value measured here is n ≈ 2.9, after pyrolysis of NG at 1000 • C.This indicates an approximately equal mix of 2-and 4-electron pathways in NG, providing further evidence that iron does not necessarily form part of the 4-electron oxygen reduction active site in Fe/C/N-based catalysts.This has also previously been shown in pyrolysed carbon nitride and polyimide systems [16].However, this value still fall short of that required for fuel cell applications.With further improvements, the high onset potential and current density show that NG materials could potentially be used as non-precious catalysts for ORR in fuel cells.

IV. CONCLUSIONS
Nitrogen doping offers a facile method of tailoring of the properties of graphene.In this work, a macroporous NG powder with up to 15% nitrogen was synthesized using a solvothermal method with simple precursors.The nitrogen content and surface area were tailored by pyrolysis.The electrochemical behavior of the material was tested, and good ORR activity in acid media was observed for pyrolysed samples.This work shows that nitrogen-doped graphene powder has potential for use as a non-precious catalyst in fuel cells, even in acid media, although further work clearly needs to be done.

FIG. 5 :
FIG.5:Estimated electrochemical H2O2 production during the oxygen reduction reaction, obtained from the ring current.