Description
Historically, graphitic and semi-graphitic materials have always played a major role in a wide range of electrical and electrochemical systems. The past two decades have brought some of the most interesting synthetic and processing advances in the area of advanced carbon materials, including the discovery and/or isolation of several sp2 allotropes (fullerenes, carbon nanotubes, graphene, etc.). In addition, a variety of top-down [1–3] and bottom-up [4] synthetic approaches emerged as a way to control the architecture and chemical functionality. This, in turn, has led to the development of nanostructured carbons for advanced applications such as supercapacitors, fuel cells, batteries, water-splitting systems, sensors, and gas chemisorbents [5, 6].
In the past, the focus of the field of advanced carbons has been gradually shifting from control over the nanostructure to control over the chemical functionality. One of the most important driving forces in the quest for such “chemical nanocarbons” has been the growing understanding of the electronic properties of graphene. The next breakthroughs in this area can be expected to involve more precise control of the edge states of graphenic domains, including the incorporation of heteroatoms and more complex functionalities [5, 7]. Nitrogen is a particularly attractive heteroatom because of its relative ease of incorporation and abundance in various carbon precursors. Viewed simplistically, nitrogen doping (N-doping) introduces basicity into the carbon structure that can be utilized for a variety of electrochemical and electrocatalytic systems; however, the detailed chemistry behind its effect is still poorly understood.This is largely due to the complexities arising fromthe heterogeneous nature and, still, relatively ill-defined structure of many of thematerials studied.As shown in Figure 1.1, the edge functionalities can be introduced in graphene either through the attachment of pendant groups (e.g., amine, Figure 1.1a) or through edge substitution with heteroatoms (Figure 1.1b,c). Basal plane substitution (quaternary nitrogen, Figure 1.1d) should not significantly impart electrochemical properties [5].
Pyridinic nitrogens (Figure 1.1b) incorporated into the graphitic network are often thought to be the most reactive and beneficial nitrogen-containing functionality for electrochemical systems [5]. While there are many synthetic routes to N-doped carbon materials, two key requirements are necessary to fully realize the advantages afforded through graphitic edge N-doping: (i) efficient formation of pyridinic species and (ii) assurance of their (electro)chemical accessibility. The latter could be accomplished by designing material with high-surface area nanoporous structure with pyridinic species preferentially exposed on pore wall surfaces. In this chapter, we demonstrate how these two requirements can be satisfied simultaneously through the general approach developed in recent years in our laboratories, in which the nanostructure of carbon is templated by the self-assembly of block copolymer or hybrid precursors comprised of a carbon source and a sacrificial block/element [9–23]. In these materials, which will be referred to as copolymer templated nitrogen-rich carbons (CTNCs), high nitrogen content polyacrylonitrile (PAN) is the carbon precursor of choice since its carbonization results in high content of pyridinic functionalities [24]. For block copolymer templating, the sacrificial block needs to be immiscible with the carbon source to assure nanoscale phase separation and formation of well-defined template morphology. Heat treatment results in an N-doped carbon material with morphology replicating the copolymer precursor [9, 11, 13]. As discussed below, copolymer templates of certain compositions afford materials with high surface area and efficient nitrogen exposure to the pore walls, presumably through preferential orientation of PAN chains at the interface with the sacrificial block.
Following a more detailed discussion on the motivation for this choice of templating approach when compared with other methods, this chapter will discuss the structural aspects CTNCs and their performance in applications such as supercapacitors, oxygen-reduction reactions (ORRs), and CO2 adsorption. Since the main goal of this contribution is to discuss the merits of CTNCs with respect to other approaches in the synthesis of nitrogen-rich electroactive carbons, a particularly strong emphasis will be placed on the critical overview of the other strategies.