Chapter 1
Heteroatom-Doped Carbon Nanotubes as Advanced Electrocatalysts for Oxygen Reduction Reaction

Jintao Zhang, Sheng Zhang, Quanbin Dai, Qiuhong Zhang and Liming Dai

1.1 Introduction

With the diminishing fossil fuels and even increasing demand on energy resources as well as the growing environmental concerns, the development of clean and sustainable energy conversion and storage systems with high efficiency at low cost has attracted intense research interests [1-3]. Fuel cells and metal-air batteries are promising energy devices with unique properties, such as large theoretical specific energy (up to 3600 W h kg-1 for Li-O2 battery). However, implementing these energy technologies in our daily life requires highly effective, but low-cost, electrocatalysts to efficiently reduce O2 [4]. Specifically, both fuel cells and metal-air batteries involve oxygen reduction reaction (ORR) at the cathode [5-11]. Pt/C catalysts are generally used as electrocatalysts for the sluggish ORR. However, the scarcity and high cost of platinum pose one of the major concerns that have precluded fuel cells from commercial applications [12].

Carbon materials with unique structures, including zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes (CNTs), two-dimensional (2D) graphene, and three-dimensional (3D) graphite, are of particular interest because of their desirable properties, including excellent electrical conductivity, controllable porosity, and electrocatalytic activity, and high mechanical strength [13]. Owing to their wide availability, environmental acceptability, corrosion resistance, and unique surface and bulk properties, CNTs are ideal candidates as efficient ORR catalysts [2, 14-17]. In this regard, doping CNTs with nitrogen has been demonstrated to transfer the inert carbon surface to more active electrocatalytic sites for ORR [10, 11]. Recently, substantial progress has been made to understand the doping process associated with doped CNTs. As a result, a number of significant breakthroughs have been witnessed in the development of metal-free carbon-based ORR electrocatalysts [2]. This chapter begins with a brief description of the ORR principles, which is followed by a summary of recent work on the rational preparation of carbon-based ORR electrocatalysts for potential applications. Finally, various heteroatom (N, B, P, S)-doped CNTs are discussed in order to correlate their ORR activities with the syntheses and structures.

1.2 Experimental Evaluation of Electrocatalytic Activity toward ORR

In an aqueous solution, the complete oxygen reduction goes through either a two-electron transfer process with hydrogen peroxide as the intermediate, followed by further reduction to OH-/water or a more efficient four-electron transfer to produce water directly. As shown below, the two-electron and four-electron reduction processes can occur in both alkaline and acid media [18]:

To evaluate the electrocatalytic activity of catalysts, especially for ORR in aqueous electrolytes, the most commonly used techniques are rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) coupling with linear scan voltammetry (Figure 1.1) [19]. The current of ORR is dependent on the kinetic current (Jk) and diffusion-limiting current (Jd). Kinetic current is determined by the reaction kinetic process. However, the slow mass transport of the reactants (e.g., O2) from the bulk of electrolyte solution to the electrode surface results in the formation of a concentration profile of the reactants in front of the electrode surface. If the applied overpotential is high enough, every atom/ion reaching the electrode reacts immediately, resulting in nearly zero concentration at the surface, leading to a diffusion-limiting current density, which is only determined by the rate of diffusion. RDE is used to separate the diffusion and the kinetic currents of ORR. In a RDE, rotating movement leads to convection, and subsequently determines the thickness of the diffusion layer. As the diffusion rate is dependent on the rotating speed, the diffusion-limiting current is determined by the rate at which the reactant diffuses to the surface of the electrode, and hence also depends on the rotating speed. When the small effect of a Nafion film on diffusion is neglected for the rotating electrode, the overall measured current, J, can be expressed as being dependent on the kinetic current (Jk) and the diffusion-limiting current (Jd), which can be expressed in terms of the Koutecky-Levich equation as follows [20]:

where ? is the electrode rotating rate. B is determined from the slope of the Koutecky-Levich plot based on the Levich equation as given below:

in which n represents the transferred electron number per oxygen molecule, F is Faraday constant , is the diffusion coefficient of O2 in electrolyte, ? is the kinetic viscosity, is the bulk concentration or solubility of O2. These parameters are listed in Table 1.1. The constant 0.2 is adopted when the rotation speed is expressed in revolutions per minute.

Figure 1.1 Configuration of RDE (a) and RRDE (c). Linear sweep voltammogram (LSV) curves of electrocatalysts in oxygen-saturated electrolyte with different rotating speeds (b). Typical oxygen reduction curves on the disc and ring electrodes, respectively (d).

Table 1.1 The parameters of commonly used electrolytes

Figure 1.1b shows the typical linear sweep voltammogram (LSV) curves of ORR tested on RDE at various rotating speeds. The diffusion-limiting current increases with increasing rotating speed, which is associated with the increase of oxygen diffusion to and reduction at the electrode surface. At high overpotentials, the oxygen reduction is fast enough that a flat limiting plateau is achieved (Figure 1.1b). It is explained that the current plateau could be associated to the distribution of the electrocatalytic sites on the electrode surfaces. Typically, the uniform distribution of active sites leads to the well-defined current plateau at the diffusion-limiting region. By contrast, the distribution of active sites is less uniform and the electrocatalytic reaction is slower, the current plateau is more inclined [26]. The transferred electron number and kinetically controlled currents can be obtained by Koutecky-Levich plots, in which the diffusion limitations can be eliminated. For the Tafel plot, the kinetic current is calculated from the mass transport correction of RDE by .

Alternatively, the RRDE enables to determine the kinetics and the mechanism of ORR. Accordingly, the dominated reaction on the central disc electrode is the direct reduction of O2 to H2O (with a four-electron charge transfer) and the potential intermediate species, hydrogen peroxide (H2O2), is either oxidized or reduced on the concentric ring electrode, depending on the potential of this electrode (Figure 1.1c, d). Thus, this technology can be used to quantitatively evaluate the molar proportion of produced HO2- on ring electrode (platinum or gold). The disc and ring currents (ID and IR, respectively) are recorded as a function of the disc electrode potential (Figure 1.1d). The total disk current, ID, is the sum of the O2 reduction currents to water, , and intermediate (H2O2), . The is related to IR through a collection efficiency (N) as follows:

The H2O2 yield (H2O2%) and the electron transfer number (n) are determined by the following equations [19, 27, 28], respectively:

where N is current collection efficiency of the ring electrode. The collection efficiency is defined as and is usually determined by using [Fe(CN)6]4-/[Fe(CN)6]3- redox couple [25].

1.3 Doped Carbon Nanotubes for ORR

1.3.1 Carbon Nanotubes Doped with Nitrogen

Carbon-based ORR electrocatalysts are usually obtained by doping sp2 carbon materials with different dopants [29-32]. As a representative nanocarbon material that has been studied for more than two decades, CNTs offer several notable advantages over carbon blacks as supports for fuel cell electrocatalysts. Those advantages include, but not limited to, the improved mass transfer of reagents/products, enhanced electronic conductivity, and higher resistance to corrosion. The explosion of interest in the carbon-based metal-free ORR catalysts [33-45] started in 2009 when Dai's group [10] reported excellent four-electron ORR performance of vertically aligned nitrogen-doped carbon nanotube (VA-NCNT) arrays (Figure 1.2), which showed comparable onset potential to that...