Electrocatalytic oxygen reduction on nitrogen-doped graphene in alkaline media

https://doi.org/10.1016/j.apcatb.2013.09.011Get rights and content

Highlights

  • Pyrolysis in the presence of nitrogen precursors was used for N-doping of graphene.

  • Melamine, urea and dicyandiamide were pyrolysed with graphene oxide at 800 °C.

  • N-doped graphene nanosheets were used as catalysts for oxygen reduction reaction.

  • N-doped graphene showed a high electrocatalytic activity for ORR in alkaline media.

  • N-containing graphene is a promising cathode catalyst for alkaline membrane fuel cells.

Abstract

Nitrogen-doped graphene nanosheets were prepared from nitrogen precursor and graphene oxide (GO), which was synthesised from graphite by modified Hummers’ method. Melamine, urea and dicyandiamide (DCDA) were used as nitrogen precursors and the doping was achieved by pyrolysing GO in the presence of these nitrogen-containing compounds at 800 °C. The N-doped graphene (NG) samples were characterised by scanning electron microscopy and X-ray photoelectron spectroscopy, the latter method revealed successful nitrogen doping. The oxygen reduction reaction (ORR) was examined on NG-modified glassy carbon (GC) electrodes in alkaline media using the rotating disk electrode (RDE) method. It was found on the basis of the RDE results that nitrogen-containing catalysts possess higher electrocatalytic activity towards the ORR than the annealed GO. Oxygen reduction on this GO material and on NG catalysts prepared by pyrolysis of GO-melamine and GO-urea followed a two-electron pathway at low overpotentials, but at higher cathodic potentials the desirable four-electron pathway occurred. For NG catalyst prepared from GO-DCDA a four-electron O2 reduction pathway dominated in a wide range of potentials. The half-wave potential of O2 reduction on this NG catalyst was close to that of Pt/C catalyst in 0.1 M KOH. These results are important for the development of alkaline membrane fuel cells based on non-platinum cathode catalysts.

Introduction

Nowadays mainly Pt and Pt-based catalysts have been employed as cathode catalysts in low-temperature fuel cells. Because of the high price and scarcity of Pt, a great deal of work has been made to develop new electrocatalysts for oxygen reduction reaction (ORR) in alkaline and acidic media. In order to replace platinum, different carbon-based materials that possess lower price, better availability and improved chemical stability have been studied [1], [2], [3], [4]. It is well known that the greater the extent of graphitisation of the carbon material, the better its durability [5]. Few-layer graphene, its characteristics and production is attracting as much attention as carbon nanotubes (CNTs) received during the last decade [6]. Despite the fact that CNTs have unique structure and high electrical conductivity, their electrochemical performance is still suffered due to their low stability against oxidation by peroxide intermediates leading to fuel cell catalyst degradation [7]. In addition, comparing to alkaline media, CNT-based catalysts show lower activity towards the ORR in acids [8], [9], [10], [11]. These might be few reasons why there has been an immense research activity in the search for alternative carbon materials for ORR [4].

Graphene is a flexible and expandable two-dimensional (2D) monolayer carbon material consisting of sp2 carbon atoms [12], [13]. It is an attractive material thanks to several features: the unique chemistry of the edges of a graphene sheet, the mechanical strength, high thermal and chemical stability, high electrical conductivity and large surface area [6], [12], [13], [14], [15], [16]. Graphene has been used in various electrochemical applications like batteries, fuel cells, electrochemical energy storage and supercapacitor electrodes due to its high electrical conductivity and remarkable electronic properties [17], [18]. Because of these promising applications a facile, environmentally friendly and cost-effective method for mass production of graphene is needed [19]. The most promising graphene synthesis method is based on the reduction of graphene oxide (GO), where graphite is chemically exfoliated into GO, followed by chemical or thermal elimination of oxygen groups [20].

Recently, Zhu and Dong reviewed the application of graphene-based electrocatalysts for ORR [21]. Nitrogen-doped carbon materials have been considered as promising cathode catalysts for low-temperature fuel cells [22], [23]. For the chemical doping of carbon materials, nitrogen is considered to be an excellent choice, because it has comparable atomic size with carbon and it forms strong bonds with carbon atoms. The lone electron pairs of nitrogen atoms can form a delocalised conjugated system with the sp2-hybridised carbon frameworks, which improves the reactivity and electrocatalytic performance of graphene [24]. Sheng et al. reached nitrogen level up to 10% in the N-doped graphene (NG) material and the obtained catalyst exhibited an outstanding performance for O2 reduction in fuel cell [25]. Nitrogen atoms can donate electrons to the conjugated π orbital in carbon, polarising the C atoms into C (δ+), making it easy to adsorb the O2 molecule and also helping to form a strong chemical bond between C and O, which splits the O–O bond [14], [26]. Introducing abundant defects is another possible approach to enhance the ORR activity [7]. It is well known that the electrocatalytic activity of nitrogen-doped graphene is related not only to the nitrogen level, but also to the type of nitrogen [21]. Typically, the NG catalysts consist of different types of N-containing groups. This makes it difficult to interpret the electrocatalytic behaviour of NG towards the ORR, because one cannot unequivocally relate the ORR activity to a certain type of N sites. According to XPS analysis, the following types of nitrogen have been detected in N-doped carbon materials: pyridinic N, quaternary N (graphitic N), pyrrolic N and pyridine-N-oxide [22], [23]. Some research groups have claimed that the graphitic nitrogen is responsible for the high ORR activity and the other N functionalities that mainly exist on the edges of NG have smaller impact [27]. Others state that pyridinic N and the small pores of the catalyst are responsible for electrocatalysis of the ORR [28]. Which type of C–N bonding configuration is responsible for improved electrocatalytic activity is still under the debate [21].

Several groups have reported that NG possesses excellent electrocatalytic activity towards the ORR in alkaline media, but in acid the activities comparing to Pt/C catalyst are much lower [25], [29], [30], [31]. It also needs to be stressed that NG is more stable than Pt/C during accelerated degradation test and it shows significantly better tolerance towards methanol and CO. This is particularly attractive for low-temperature fuel cell application [21].

For nitrogen doping, there are different opportunities including chemical and physical synthesis and each method creates doped graphene with different characteristics. Chemical vapour deposition (CVD) is capable of producing high-quality N-doped graphene sheets, but suffers from complicated equipment, high cost and is not suitable for mass production [32]. Nitrogen species can be incorporated into graphene by annealing of GO in ammonia [32]. Thermal decomposition method has been frequently employed for the preparation of nitrogen-doped graphene [22]. Various nitrogen precursors were used for the high temperature pyrolysis in the presence of GO. Urea has also served as an expansion–reduction agent for graphene generation [6], [16]. The use of an expansion-reducing agent will not only create volatile species that mechanically expand GO, but will also allow the volatile gases to reduce oxygen-containing groups on the surface of GO, which is favourable for incorporating nitrogen into graphene structure [6], [16].

Recently, we have studied the reduction of oxygen on N-doped CNTs prepared by CVD [33] and by post-deposition treatment of CNTs with urea [34]. Both materials showed high ORR activity in alkaline media. In current work, we synthesised nitrogen-doped graphene via reducing GO with three different nitrogen-containing compounds (dicyandiamide, urea and melamine) by high-temperature pyrolysis in an inert atmosphere. By controlling the experimental conditions, including pyrolysis temperature and mass ratio between GO and nitrogen precursor, the overall content and type of nitrogen could be modified. The electrochemical reduction of oxygen has been studied on NG materials in alkaline media using linear sweep voltammetry and the rotating disk electrode method.

Section snippets

Preparation of graphene oxide (GO)

GO is a highly oxidised form of graphene and contains many carbon–oxygen functionalities on the surface, for example carboxylic, carbonyl, epoxy and hydroxyl groups [35]. GO is well dispersed in water, because the oxygen-containing groups are hydrophilic and carboxyls bring negative charges to the GO surface [35]. The GO material used in this work was synthesised from graphite powder (Graphite Trading Company) by a modified Hummers’ method [15], [36]. Firstly, 50 ml of concentrated sulphuric

Surface morphology and XPS analysis of NG samples

The surface morphology of NG and GO samples was characterised by scanning electron microscopy (SEM). A drop of aqueous suspension of these materials was applied onto the Si substrate and the SEM micrographs of the NG catalyst and undoped GO are shown in Fig. 1. The NG layer contains micron-sized flakes and three dimensional (3D) particles that have size of hundreds of nanometers. Typical graphene structure (crumpled sheet-like morphology and porous architecture) revealing a high exfoliation

Conclusions

In this work the electrocatalysis of O2 reduction on annealed graphene oxide and nitrogen-doped graphene catalysts has been explored. Nitrogen doping was achieved by pyrolysis of GO at 800 °C in the presence of melamine, urea or dicyandiamide. According to the XPS results the nitrogen doping level up to 5 at% was observed. The synthesised NG materials were electrochemically characterised in alkaline media and the RDE and LSV results revealed that these catalysts possess a high electrocatalytic

Acknowledgements

This research was financially supported by the Estonian Science Foundation (Grant No. 9323) and by the Estonian Nanotechnology Competence Center. We also acknowledge support from the Archimedes Foundation (Project No. 3.2.0501.10-0011).

References (50)

  • R. Othman et al.

    Int. J. Hydrogen Energy

    (2012)
  • Y.Y. Shao et al.

    Appl. Catal., B

    (2008)
  • S. Wakeland et al.

    Carbon

    (2010)
  • I. Kruusenberg et al.

    Carbon

    (2009)
  • I. Kruusenberg et al.

    Carbon

    (2011)
  • G. Jürmann et al.

    J. Electroanal. Chem.

    (2006)
  • Q. Liu et al.

    Electrochim. Acta

    (2012)
  • E. Antolini

    Appl. Catal., B

    (2012)
  • C. Soldano et al.

    Carbon

    (2010)
  • S. Stankovich et al.

    Carbon

    (2007)
  • Z. Yang et al.

    J. Power Sources

    (2013)
  • K.R. Lee et al.

    Electrochem. Commun.

    (2010)
  • N. Alexeyeva et al.

    J. Electroanal. Chem.

    (2010)
  • M. Vikkisk et al.

    Electrochim. Acta

    (2013)
  • V. Datsyuk et al.

    Carbon

    (2008)
  • S. Chandra et al.

    Mater. Sci. Eng., B Solid

    (2010)
  • I. Kruusenberg et al.

    Electrochem. Commun.

    (2013)
  • R.E. Davis et al.

    Electrochim. Acta

    (1967)
  • J.J. Wu et al.

    J. Power Sources

    (2013)
  • C.H. Choi et al.

    Appl. Catal. B

    (2014)
  • W.Y. Wong et al.

    Int. J. Hydrogen Energy

    (2013)
  • A. Rabis et al.

    ACS Catal.

    (2012)
  • Z.W. Chen et al.

    Energy Environ. Sci.

    (2011)
  • D.A. Stevens et al.

    J. Electrochem. Soc.

    (2005)
  • Y. Li et al.

    Nat. Nanotechnol.

    (2012)
  • Cited by (219)

    View all citing articles on Scopus
    View full text