1
Design Principles for Heteroatom-Doped Carbon Materials as Metal-Free Catalysts

Zhenghang Zhao*, Lipeng Zhang*, Chun-Yu Lin*, and Zhenhai Xia

University of North Texas, Department of Materials Science and Engineering, Department of Chemistry, 1155 Union Circle #305310, Denton, TX, 76203-5017, USA

1.1 Introduction

Renewable and clean energy conversion and storage devices are imperative to continuous energy consumption and environmental protection. At the heart of these energy conversion/storage devices, such as fuel cells, metal-air batteries, and water splitting, there are key electrochemical reactions, oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), that generate electric power or hydrogen fuel. As these reactions are naturally sluggish, catalysts are needed to facilitate them. Catalysts are, therefore, one of the critical factors that determine the efficiency and cost of these energy conversion and storage devices. Currently, the state-of-the-art catalysts are noble metals (e.g. Pt) or their compounds, but their limited resources and high price hinder the wide applications of these devices. Therefore, it is necessary to develop metal-free catalysts with high efficiency, low cost, and long durability.

Heteroatom-doped carbon nanomaterials (e.g. graphene and carbon nanotubes (CNTs)) have been shown to have high activities in catalyzing both ORR and OER in fuel cells and Zn-air batteries and HER in hydrogen production [1-3]. In addition to their abundant sources, heteroatom-doped carbon nanomaterials are more efficient, more stable, and more tolerant to crossover/CO-poisoning effects than Pt and RuO2, the most active catalysts so far for ORR and OER, respectively [4, 5]. Although the superior catalytic capabilities of the heteroatom-doped carbon nanomaterials have been clearly demonstrated, it is challenging to find out the best catalyst from numerous possible combinations of dopant elements in the periodic table solely through experimental approach. To rationally design a catalyst, it is critical to understand which intrinsic material characteristics, or descriptors, control the catalytic activity of carbon-based catalysts.

The catalytic activities on a catalyst are correlated with elementary steps of the reactions: for example, adsorption of O2, formation of OOH* and O*, and removal of OH* in ORR, where the star refers to adsorption [6, 7]. Therefore, the activity on catalysts can be described as a function of the adsorption energy of intermediates, which yields a volcano relationship. The best catalyst could be found near the summit of the volcano for the ORR. Recently, new descriptors for describing ORR, OER, and HER activities of p-block-element-doped, carbon-based catalysts have been identified to predict the best catalysts for specific applications. Such descriptors have predictive power to design new metal-free catalysts with enhanced ORR, OER, and HER activities, even better than those reported for platinum-based metal catalysts. These modeling and analyses provide a fundamental understanding of catalytic mechanisms in metal-free, carbon-based material systems and set a theoretical base for the rational design of highly effective, low-cost metal-free catalysts. The fundamental aspects and design principles for heteroatom-doped carbon materials as metal-free catalysts are discussed in this chapter with an emphasis on design principles for heteroatom-doped carbon nanomaterials with enhanced catalytic activities.

1.2 Basic Approaches for Catalyst Design

1.2.1 Origin of Catalytic Activities of Metal-Free Carbon Nanomaterials

Carbon-based nanomaterials have variously tunable structures, large surface area, outstanding thermal stability, excellent mechanical and electrical properties, and high durability in electrochemical environments. These merits ensure the low cost and strong tolerance to large pH range of environments for the materials serving as catalysts. However, pure and perfect carbon nanomaterials exhibit very low catalytic activities. High catalytic activities of metal-free carbon nanomaterials originate from their unique electronic structures of highly active sites induced by modified carbon structures. There are several approaches to tune electronic structures to generate catalytic active sites on metal-free carbon nanomaterials. These approaches are achieved by the localized distribution of the charge density and spin density. Figure 1.1 shows the structures and approaches that are used to generate active sites on metal-free, carbon-based nanomaterials to enhance their catalytic properties.

Figure 1.1 Metal-free, carbon-based nanomaterials with the catalytic activities to ORR, OER, or HER. (a) Defects. (b) Heteroatom doping. (c) Adsorptions. (d) N-doped graphene carbon nanotube hybrid. (e) Adsorption of ammonia borane at N-doped carbon nanotube. (f) Adsorption of oxygen molecule at N-doped carbon nanotube.

Source: Gong et al. 2009 [12]. Reproduced with permission from AAA.

Source: Chen et al. 2014 [11]. Reproduced with permission from Elsevier.

Source: Tian et al. 2014 [10]. Reproduced with permission from John Wiley & Sons.

Source: Shen et al. 2016 [9]. Reproduced with permission from IOP Publishing.

Source: Zhao et al. 2015 [6]. Reproduced with permission from John Wiley & Sons.

Source: Zhang et al. 2015 [8]. Reproduced with permission from Royal Society of Chemistry.

1.2.1.1 Intrinsic Defects and the Edge Topological Structures

The first approach to tune the electronic properties of carbon nanomaterials is to introduce intrinsic defects and the edge topological structures (Figure 1.1a). The defects, such as vacancies, Stone-Wales defects, pentagon, heptagon, and the pentagon-heptagon (57)/pentagon-pentagon-octagon (558) grain boundaries, would induce the rotation of honeycomb lattice to change the electronic states around the vacancies. The edges of graphene or CNTs show the different electronic states from their internal regions. For example, along a zigzag edge of graphene, there is one localized state per three lattice units and the local charge is a function of the distance to the edge. If the edges are combined with pentagon carbon rings or 57/558 grain boundaries, they enhance ORR catalytic activity on graphene clusters; the active sites locate at the zigzag edges with higher spin density or charge density [8].

1.2.1.2 Heteroatom Doping in Carbon Nanomaterials

The second approach is to dope atoms in carbon nanomaterials (graphene or CNTs), in which carbon atoms are substituted by heteroatoms, such as N, P, and B [13-16], or bonded with heteroatoms, e.g. O, S, Se, Cl, Br, and I [17-20], on carbon surfaces or edges (Figure 1.1b). Because of the different electronegativity between carbon atoms and these heteroatoms, the charge and spin densities would also be localized, which enhance catalytic activities. The active sites are the carbon atoms locating near the dopant or the carbon atoms locating at edges, or dopant atom itself. It was demonstrated through density functional theory (DFT) calculations that doping heteroatom (N, N, B, S, Cl, P, Cl, Br, and I) in graphene significantly altered charge density and spin density distributions on the doped carbon materials, consequently enhancing their ORR/OER catalytic properties [14, 21]. Oxygen functional group ?COOH, ?C?O, and ?C?OH with the synergistic effect of edge defects on graphene was also discovered to show ORR catalytic activities [18].

1.2.1.3 Adsorption of Organic Molecules

The third approach is the physisorption of organic molecules on CNTs and graphene or hybrid structure of N-doped graphene and graphite structural C3N4(C3N4@NC) [22]. The catalytic activity of this kind of carbon materials stems from the electron transfer between the organic molecule and the graphene or between the C3N4 and the doped graphene. DFT calculation shows that there is an electron transfer from graphene sheets to the adsorbed tetracyanoethylene (TCNE) molecules (Figure 1.1c) [9, 23]. The catalytic active sites are generated at the carbon atoms with higher charge density due to the electron transfer from the graphene to TCNE molecule. On C3N4@NC, it allows one *H bonding with two pyridinic nitrogen in one tri-s-triazine periodic unit to form a C2N3H heteroring; a large number of electrons transferred from N-graphene to catalytically active g-C2N4 layer can significantly reduce these adsorbed H* species to the final molecular hydrogen, showing high HER catalytic activity [22]. In addition, N-doped graphene CNT hybrid (Figure 1.1d), and the adsorptions of oxygen molecule, ammonia borane, on N-doped CNT (Figure 1.1e and f) were also studied [10-12].

1.2.2 Charge Transfer in Carbon Due to Defects, Doping, and Adsorption

Defects on carbon materials could induce the electron transfer, as shown in Figure 1.2b [25]. The electron transfer is beneficial to the conductivity and...