7.1 Introduction

The future global economy is expected to consume more energy than ever before leading to the energy crisis and detrimental environmental issues. Hence, there is rising interest in energy conversion and storage from sources that do not rely on fossil fuels due to their nonrenewable nature [1]. Potential technologies, such as batteries, supercapacitors, and fuel cells, have been acknowledged to be feasible and efficient, particularly, for stationary and nonstationary energy applications. Fuel cells that convert fuels (methanol or hydrogen) into electricity with zero emission are the most environment‐friendly devices for the future [2]. The working principle of polymer electrolyte membrane (PEM) fuel cell is depicted in Figure 7.1. It consists of two electrodes: a cathode, where oxygen reduction reaction (ORR) occurs, and an anode for fuel (methanol or hydrogen) oxidation, separated by a polymer membrane. ORR is more than six magnitudes slower than the fuel oxidation reaction, which limits the energy conversion efficiency of a fuel cell. In simple terms, ORR represents reaction involving reduction of oxygen molecules into various oxide components depending on the reaction kinetics or electrolyte composition (either aqueous alkaline or acidic electrolyte).

ORR is a multistep reaction that proceeds either through an efficient four‐electron process or via a two‐step, two‐electron pathway. In a four‐electron pathway, either four electrons are inserted into an oxygen molecule (in acid media) or four hydroxyl groups (alkaline media) react at once to form a water molecule [4]. If an oxygen molecule receives only two electrons at first with a delayed supply of two electrons later, then a two‐electron pathway is predominant, which leads to the formation of reactive peroxide species: H₂O₂ (in an acidic medium) and HO₂⁻ (in an alkaline medium). The reduction mechanism becomes more complicated when multiple interactive processes (oxygen adsorption, charge transfer, and product dissociation) come into the picture. Also, there are dissociative and associative mechanisms for both four‐electron and two‐electron reduction pathways. In general, the high‐selectivity four‐electron reduction pathway is preferable, as it is devoid of surface‐damaging peroxidase groups and maximizes the generation of electricity [5]. The typical ORR in acid and alkaline media is given below [6].

Acidic media:

Alkaline media:

Platinum is considered the best ORR electrocatalyst, which has undergone extensive research since the first fuel cell invention in 1839 [7]. However, the rarity and cost of platinum (Pt) have driven researchers to consider alternatives as they pose serious concerns for commercializing fuel cells. Thus, developing robust, low‐cost, and highly active metal‐free ORR electrocatalysts are of utmost importance, if cheap fuel cell technology is to be realized.

7.2 Carbon Materials as ORR Electrocatalysts

At present, low‐temperature PEM fuel cells use Pt nanoparticles on a carbon (C) support (Pt/C) for ORR. Carbon black, which is produced from heavy petroleum product, was investigated as a fuel cell catalytic support, its ORR catalytic activity as compared to the commercially available metal‐based catalyst was meager [8, 9]. In addition, aggregation and detachment of platinum due to carbon corrosion could lead to decrease in the performance in the long run [10]. Even though Pt alloys have been considered an alternative catalyst to lower platinum content, large‐scale production would still require a significant amount of this Pt metal. Also, carbon‐supported metal oxides and other metal oxides have also been considered for ORR catalysts [5]. One major limitation of such electrocatalysts is their dissolution in acidic media, restricting their utility to alkaline media alone. However, novel carbon nanomaterials like carbon nanotubes (CNTs), fullerene, graphene, etc., with tunable physicochemical properties have shown promise as a metal‐free catalyst for ORR [9, 11–15]. So far, such carbons are the best‐suited ORR catalysts developed to replace Pt, not only because of their wide availability but also for their high electrical conductivity, mechanical strength, thermal properties, large surface area, and low cost. Today, they are sought after alternatives to precious metal‐based electrocatalysts.

As a typical first example, Dai et al. prepared vertically aligned nitrogen‐doped CNTs as a metal‐free catalyst for ORR, which exhibited more activity and operational stability than commercial Pt/C electrodes in alkaline fuel cells (Figure 7.2a–d) [16]. In such systems, crossover effects from methanol were nil. Also, theoretical and experimental studies have demonstrated that nitrogen doping improves the catalytic effect. The presence of nitrogen could create a net positive charge on the nearby carbon atom in the carbon plane, to alter the mode of oxygen absorption and thereby attract electrons from anode for better ORR [16]. The vertically aligned nitrogen‐doped CNT electrode was established to be unresponsive to CO poisoning even after adding a significant amount (10%) of CO in oxygen (grey line in Figure 7.2b). The commercial Pt/C on glassy carbon (GC) electrode was, however, poisoned under the same conditions (black line in Figure 7.2b). This result shows the high tolerance of the vertically aligned nitrogen‐doped CNTs/GC electrode for ORR. The metal‐free, heteroatom‐doped, carbon‐based catalyst is one of the most effective materials for overcoming the problems that Pt catalysts are facing.

Confirming the significant ORR activity of such carbon materials, researchers have ventured into developing new strategies to engineer efficient metal‐free ORR catalysts, which include graphene [17], heteroatom‐doped graphene [5], heteroatom‐doped graphite [18], and other carbon nanomaterials [19–24] with superior performance, excellent durability, and methanol tolerance in fuel cells. Especially, graphene‐based, metal‐free catalysts have been demonstrated to show exceptional stability and high efficiency in fuel cells [11]. With well‐established and scalable synthesis protocols that are available today, utilizing graphene‐based electrocatalysts can help in cost‐cutting and contribute to maintaining economic viability for fuel cells and other energy devices, leading to substantial impact on the energy community. Forthcoming sections are aimed at providing an outlook of the significant developments that took place in utilizing metal‐free, graphene‐based electrocatalysts for ORR in fuel cells. In particular, the following sections will shed light on the surface engineering of graphene, mainly discussing the restoration of π conjugations and the importance of tailoring graphene and heteroatom doping, as a new class of promising metal‐free electrocatalysts for the ORR. The chapter takes the readers through the future perspectives and design considerations for a high‐performance, scalable ORR catalyst using metal‐free graphene.

7.3 Structurally Engineered Graphene as Metal‐Free Catalysts for ORR

Graphene is a vast carpet of the aromatic framework with tremendous opportunities for surface design. It is a one‐atom‐thick sheet with a densely packed honeycomb crystal lattice. Single‐layer graphene isolation was first reported by Geim and Novoselov in 2004 using a scotch tape method, which splits graphite galleries into thinner layers until a single layer was obtained [25]. Although mechanical peeling from graphite produces graphene, its yield is inherently low. In general, the preparation of graphene can be achieved either by the top‐down or by the bottom‐up approach. The bottom‐up approach uses chemical vapor deposition (CVD) of hydrocarbons or another chemical synthesis [26]. A top‐down method for preparation of graphene involves exfoliation of graphite by chemical means to form graphene oxide (GO) [27] followed by its reduction. GO itself possesses excellent activity toward many important transformations as an electrocatalyst. GO offers a wide range of carbonyl and epoxy groups, which can be selectively transformed to other functionalities to serve as an efficient catalyst. The alternation of carboxyl groups into other functionalities requires activation, which can then form covalent linkages with nucleophiles. In general, carboxyl groups are transformed into amide or ester groups by reaction with an amine or hydroxyl containing nucleophiles. Similarly, alteration of GO through epoxy is believed to happen via a ring‐opening reaction [26]. Such alternations restore the π conjugations of such reduced graphene oxide (rGO) with properties similar to those of graphene.

As discussed earlier, there is a predominant interest in the development of metal‐free catalysts for ORR due to their enhanced stability in alkaline media. Given the unique electrical and thermal properties, resource availability, corrosion resistance, and large specific surface area, graphene‐based materials are believed to be best suited as a metal‐free catalyst for ORR applications [2]. Graphene or its derivatives by virtue of itself can exhibit excellent catalytic properties [28]. Functionalizing graphene with other nitrogen‐containing groups like amine can significantly enhance the electrical conductivity. Hou and coworkers [29] used ammonia to functionalize graphene from GO, with the presence of 10.8 at.% nitrogen. The role of nitrogen was investigated for its superior catalytic performance. It was suggested that the graphitic and pyridinic nitrogen compositions were key factors in enhancing the current density. Following a similar strategy, Chen and coworkers [30] synthesized an amine‐functionalized holey graphene using GO and ammonia via hydrothermal method followed by etching. Interestingly, this functionalized graphene showed superior performance than nitrogen‐doped graphene, possibly due to the porous structure and the presence of electron‐donating group (e.g. amine) as well as large number of holes in its sheet plate, thus providing higher electrical conductivity and more active edge nitrogen atoms. Although a small overpotential was observed toward ORR, it was more favorable than the commercial Pt/C catalyst. Also, the functionalized graphene was immune toward methanol crossover and CO poisoning, making it desirable as a metal‐free catalyst. Later, Rezaei's group reported a pyridine‐functionalized rGO. Pyridine was attached to the epoxy group or hydroxyl group at the surface of GO through a thermal reduction strategy. The catalyst showed superior performance for oxygen reduction at lower overpotential with improved current density; with performance comparable with that of Pt catalyst [31]. In another instance, a layer‐by‐layer assembly technique was used to prepare a graphene film [32]. An unprecedented catalytic activity for ORR in alkaline media was observed. This was credited to the synergistic effect of poly(diallyl dimethyl ammonium chloride) and rGO used in the film preparation.

Baek's group reported a large‐scale production of edge‐selective functionalized graphene nanoplatelets (EFGnPs) in the presence of hydrogen, carbon dioxide, sulfur trioxide, or carbon dioxide/sulfur trioxide mixture through a simple ball‐milling technique to obtain hydrogen GnP (HGnP), carboxylic acid GnP (CGnP), sulfonic acid GnP (SGnP), and carboxylic acid/sulfonic acid functionalized GnP (CSGnP) (Figure 7.3) [33]. The functionalization in the mechanochemical ball‐milling process involves the mechanical cleavage of carbon bonds and gases in the ball mill equipment.

Some unreacted group in the graphite is terminated by exposure to air, leading to the formation of hydroxyl and carboxyl groups at the broken edges of the functionalized graphene nanoplatelets (GnPs) (Figure 7.3a). The reaction kinetics of edge‐functionalized graphene electrodes were studied using a rotating disk electrode (RDE). In comparison with that of the onset potentials of pristine graphite (∼−0.25 V), those with edge functionalization gradually decreased according to their polarity order of HGnP (−0.24 V) < CGnP (−0.22 V) < CSGnP (−0.16 V) < SGnP (−0.16 V) (Figure 7.3b–e), indicating that ORR mechanism was a mixture of a two‐electron ORR process at lower potential and a four‐electron ORR process at a higher potential with respect to functionality. The overall electrocatalytic activity is in the following order: SGnP > CSGnP > CGnP > HGnP > the pristine graphite. Among all the functionalized GnPs, the sulfonic‐acid‐functionalized GnP exhibited superior ORR performance when compared to the commercially available platinum‐based electrocatalyst.

Using similar same ball‐milling technique, the same group reported edge‐functionalized GnPs using halogens (Cl, Br, and I) [34]. A large specific surface area of 471–662 m² g⁻¹ was obtained, indicating successful exfoliation of XGnPs (X = Cl, Br, or I), with easy processability in various solvents. The samples showed high electrocatalytic activity toward ORR with a superior tolerance to methanol crossover/CO poisoning effects and high shelf life.

Edge/surface functionalization of GnPs is sought after as a promising method to tailor carbon materials. Still, there is plenty of room to improve producibility and performance depending on the tailoring technique. In addition, much care should be taken to differentiate functionalized and doped GO, as sometimes both can coexist. Therefore, region‐selective functionalization and/or doping (heteroatoms) seems to be a more efficient strategy that can provide clear outlines as to how a heteroatom can affect the ORR catalytic performance in fuel cells.

7.3.1 Heteroatom‐Doped Graphene

Heteroatom‐doped graphene materials are a promising class of metal‐free electrocatalysts for the ORR in fuel cell. In general, a carbon material is said to be doped, when a heteroatom replaces some of the carbon atoms in the graphitic lattice [11]. Heteroatom doping in carbon nanomaterials can be done either during material synthesis or post‐synthesis through heteroatom‐containing precursors. Post‐synthesis doping only alters the surface properties without affecting the bulk properties. In contrast, a homogeneous incorporation of dopants into the carbon sample could be achieved through in situ heteroatom doping. Surface engineering of graphene with heteroatoms, such as nitrogen, sulfur, phosphorous, and boron enables better tuning of the electron donor properties and consequently enhances the catalytic activity (Figure 7.4). Recently, investigations have revealed that the electrochemical performance of such metal‐free, carbon‐based catalysts arise from change in the charge distribution caused by the differences in the electron negativities between carbon and heteroatoms [23].

7.3.1.1 Nitrogen‐Doped Graphene Structures as Metal‐Free Catalysts for ORR

Nitrogen‐doped graphene has received significant interest in the recent years for applications including photocatalysis and ORR. The introduction of nitrogen into the carbon backbone can alter the electrochemical and thermomechanical properties of graphene [36]. Doping heteroatoms like nitrogen into the carbon architecture could be achieved during graphene fabrication, either by chemical vapor deposition method or using hydrothermal process. Also, posttreatment of graphene using reducing agents like hydrazine and ammonia can enrich the graphene with nitrogen [37, 38]. The nitrogen configuration in graphene falls under the following categories: graphitic, quaternary, pyrrolic, and pyridinic, which is mainly evaluated by X‐ray photoelectron spectroscopy (XPS) (Figure 7.5) [5]. Graphitic nitrogen represents the nitrogen atom within the backbone of a hexagonal ring. Pyrrolic and pyridinic nitrogen form sp³‐ and sp²‐hybridized bonds, respectively, by donating electrons to the π conjugation.

Dai and coworkers first reported nitrogen‐doped graphene as a useful catalyst for ORR [40]. A nitrogen‐containing gas mixture was used to dope graphene film that was grown on a nickel support. The nitrogen to carbon atomic ratio was maintained at 4%. The resulting flexible and transparent film (Figure 7.6A) showed exceptional performance as an electrocatalyst for ORR. In comparison to the natural graphite (exhibits a two‐electron process), the nitrogen‐doped graphene showed a single‐step four‐electron process. The steady‐state catalytic current density of the nitrogen‐doped graphene showed about three times higher current density than that of the commercial Pt/C electrode over a large potential range (Figure 7.6B). The number of electrons transferred per oxygen molecule at the graphene electrode was calculated to be 3.6–4 (potentials range −0.4 to −0.8 V). These results indicate that the nitrogen‐doped graphene electrode is as promising as a metal‐free catalyst for the ORR in an alkaline solution. In addition, the nitrogen‐doped graphene remained unchanged with the addition of methanol, was insensitive to CO, and exhibited stable performance over cycling even after 200 000 cycles (potential range: −1.0 and 0 V) in air‐saturated potassium hydroxide electrolyte. This material showed high stability during operation, superior selectivity, crossover effect tolerance, and posed the potential for other electrocatalytic applications.

In another work by Chen et al. [41], an interesting strategy was followed to obtain nitrogen‐doped graphene nanoribbons. Here, a CNT was longitudinally unzipped to form graphene nanoribbons, which were doped with nitrogen using a hydrothermal treatment method followed by a freeze‐drying process. The obtained 3D macroscopic aerogel was fire‐resistant with ultralow density, large surface area, and superior conductivity. The nitrogen‐doped nanoribbons exhibited excellent ORR activity, which was much better than the commercial Pt/C electrodes. Also, the material showed superior performance in both alkaline and acidic media. Zhou et al. [42] used a hard templating (silica template) method to produce a foam of nitrogen‐doped graphene, which also performed well as superior ORR catalyst in both acidic and alkaline media. Sun and coworkers [43] pyrolyzed graphene in the presence of ammonia to dope 2.8% of nitrogen. The as‐prepared nitrogen‐doped graphene showed a 4e⁻ process with a very high electrocatalytic activity in alkaline media. The performance of the material was comparable or even better than the commercially available Pt/C electrodes. The XPS studies indicated the presence of graphitic nitrogen, which was attributed to the superior ORR performance. However, in another study, pyridinic and pyrrolic nitrogen groups were associated with this performance, which was contradictory to the previous results [44]. This study used a variety of nitrogen precursors to dope graphene with different configurations of nitrogen (Figure 7.7a).

The RDE voltammograms were used to investigate the electrocatalytic properties of nitrogen‐doped rGO, polyaniline rGO, and polypyrrole rGO composites and were compared to bare platinum, GC, and commercial Pt/C catalysts. The linear voltammetric scans of different catalysts were taken in an oxygen‐saturated 0.1 M aq. KOH electrolyte with a rotation rate of 2500 rpm. With regard to the limiting current density, nitrogen‐doped rGO had the highest value followed by others. The polarization curves of different potentials and rotation speeds are shown in the (Figure 7.7b,c). Furthermore, it was found that the total atomic percentage of nitrogen had no effect on the electrocatalytic activity for ORR in alkaline media. However, each nitrogen group improves a portion of activity in ORR. For instance, the increased content of pyridinic nitrogen directed the 2e⁻ pathway to a 4e⁻ one. It is still a matter of consideration as to how the configuration of nitrogen in graphene addresses the ORR activity.

Xia and coworkers used melamine as a nitrogen precursor to dope graphene, which showed superior performance in ORR in alkaline media, which was accredited to the pyridinic nitrogen [45]. Woo and coworkers [46] confirmed that pyridinic group also shows higher ORR catalytic activity rGO even in acidic media. To further decipher the arguments on the nitrogen configuration, simulations were carried out by calculating the energetics of nitrogen‐doped graphene by changing the nitrogen content [47, 48]. The presence of graphitic nitrogen was found to significantly lower the energy costs of oxygen reduction [49]. A graphene with pyridinic nitrogen doping in acidic media for ORR activity was investigated by Xia and coworkers using B3LYP hybrid density functional theory [50, 51]. It was found that doping with nitrogen delivered high catalytic activity due to a high atomic charge and spin density. Also, the catalytic process was spontaneous and corroborated with the experimental results, leading to a reversible potential of 1.5 V (versus standard hydrogen electrode).

7.3.1.2 Other Heteroatom‐Doped Graphene for ORR

Apart from nitrogen, sulfur, phosphorous, and boron are commonly sought heteroatom dopants to dope graphene/graphitic lattice. Doping graphene with sulfur has been widely reported to increase the ORR activity [6, 11, 36]. According to calculations, graphene could be either metallic or semiconducting in nature depending on the extent of doping [52]. Using DFT calculations, it was found that sulfur‐doped graphene could show ORR catalytic properties comparable to those of platinum [53]. Experimentally, sulfur can be doped to a graphene lattice using GO with sulfur‐containing gases and also through chemical means. Recently, many chemical modification strategies to dope sulfur have been reported as a potential metal‐free alternative electrocatalyst [54–58]. Among the many possible strategies to dope sulfur, Baek et al. [59], produced edge‐selectively sulfurized GnPs using a simple ball‐milling technique (Figure 7.8). The ORR catalytic activity of the product was found to be efficient. In addition, the authors also found that sulfur‐doped graphene has a much slower rate of attenuation and no fuel crossover. Also, it was found that the ORR activity can in fact surpass the activity of commercially available platinum when the sample was oxidized. Sulfur‐oxide‐doped edges (OSO bonds) were found to promote ORR activity.

Very recently, tremendous progress was made in the field. For example, Huang and coworkers [60] reported a three‐dimensional (3D) porous graphene‐based nanosphere frameworks an efficient metal‐free electrocatalyst for ORR. The catalyst showed a four‐electron transfer reaction in alkaline media with better methanol tolerance and shelf life. Such a performance was attributed to the positive effect of sulfur doping and the presence of pores, thereby aiding diffusion of ions and electron transfer more feasible. In another instance, sulfur‐doped graphene was synthesized by thermally treating an exfoliated graphene with CS₂ gas [54]. Depicting a higher current density than commercially available Pt/C catalyst (4.7 versus 6.99 mA cm⁻² at −1.0 V), the sulfur‐doped graphene was very durable. To further understand the contribution of sulfur toward ORR, theoretical studies using density functional theory by various groups [53, 57]. Although it was found that sulfur could replace carbon atoms as sulfur or sulfur oxide or form a cluster of sulfur [53], it was also predicted that sulfur‐doped monovacancy graphene was stable and the sulfur dopant itself was probably the active center [57]. In most cases, calculations corroborated experimental results, suggesting that sulfur‐doped graphene can show superior performance as a metal‐free ORR catalyst.

Being adjacent to carbon in the periodic table, boron and nitrogen show positive catalytic effects when doped to graphene. Considerable efforts have been made in the recent past to dope boron to graphene as boron being deficient with one electron than that of carbon. Boron generally acts as a p‐type dopant forming sp² hybridization within the lattice. In 1974, pyrolytic graphite enriched with boron showed an increased space charge capacitance [61]. The methodology of doping boron significantly affects the electrochemical performance. For instance, boron‐doped in‐plane substituting carbon in the graphene is more stable than bonding out of plane. Such a boron‐rich graphene could be synthesized by simply annealing graphene in the presence of boron oxide. Several research groups have recently attempted to synthesize boron‐doped graphene or even graphene quantum dots [62–66]. In general, the electronegativity of boron is lesser compared with that of nitrogen. Nevertheless, they exhibited excellent catalytic activity for ORR in alkaline electrolytes, with better CO tolerance and long‐term stability [36]. In an interesting approach, inspired by a Chinese food named “Fried Ice,” a 3D graphene framework with hierarchical pores were synthesized using boric acid as the dopant (Figure 7.9) [66]. By controlling the reaction temperature, the boron content, specific surface area, and the morphology of the samples could be tailored. As a metal‐free catalyst with high ORR performance in alkaline media, this material showed a better onset potential (−0.12 V versus saturated calomel electrode (SCE)) than the commercial Pt/C catalyst. A four‐electron transfer process for oxygen reduction was deduced (Figure 7.9d). Its tolerance to methanol and carbon monoxide was tested against a commercial Pt/C catalyst. Although Pt/C depicted a distinct decrease in stability on the addition of 1 wt% methanol, the boron‐doped graphene, however, was highly stable (Figure 7.9e).

Phosphorous shares the same group in the periodic table and displays chemical properties similar to those of nitrogen despite of its larger atomic size. When doped to graphene, this causes distortion to the structure by transforming sp² to sp³ carbons, thereby creating a defect‐based active site [67]. The defect caused by phosphorus substitution in graphene has a little energy of formation and can increase the energy bandgap. Although, phosphorous‐doped graphitic layers were found to be efficient ORR catalysts [68], only a handful reports are available as metal‐free ORR catalysts. Recently, a thermal annealing approach with triphenylphosphine as the reductant has been used to fabricate phosphorous‐doped graphene [69]. The phosphorus doping introduces a significant number of oxygen functional groups to the graphene skeleton. They show superior performance as a metal‐free catalyst for ORR as shown in Figure 7.10.

The cyclic voltammograms (Figure 7.10a) reveal a reduction process at ∼0.6 V in an oxygen‐saturated 0.1 M aq. KOH solution, whereas no peaks were found when nitrogen‐saturated electrolyte was used in the same potential range. The linear sweep voltammetry curves recorded in an oxygen‐saturated 0.1 M aq. KOH showed a much higher current for the phosphorous‐doped graphene (1.4 times at −0.2 V) in comparison to that of graphene over the entire potential range. A positive ORR onset potential of ∼0.92 V than that of graphene and commercial Pt/C was observed. When methanol was introduced in the electrolyte, no traceable change was observed in the ORR current as opposed to Pt/C, indicating its negligible crossover effect and methanol tolerance. In addition, a highly stable cycling was obtained for phosphorous‐doped graphene, indicating their superior performance as an ORR electrocatalyst.

Very recently, Xu and coauthors [70] reported the synthesis of a metal‐free, phosphorus‐doped graphene nanosheets with a surface area of 496.67 m² g⁻¹ (phosphorous content = 1.16 at.%). GO and 1‐butyl‐3‐methlyimidazolium hexafluorophosphate were thermally annealed in an inert atmosphere. It was found that the phosphorous atoms were covalently bonded to the carbon atoms in graphene and the carbon framework was partially oxidized, which acted as active sites for the ORR. The performance was comparable to that of Pt/C in an alkaline medium.

7.3.1.3 Co‐doped Graphene Structures as Metal‐Free Catalysts for ORR

Doping multiple heteroatoms with different electronegativities (various combinations of nitrogen, boron, phosphorous, and sulfur) into graphitic carbons provides synergistic coupling effects, thereby enhancing the catalytic activity in ORR [71]. This effect makes such multiple atoms‐doped graphene more catalytically favorable than single‐variety heteroatom‐doped graphene. Also, first‐principles calculations predicted that the symbiotic effects of boron‐ and nitrogen‐doped graphene were excellent electrocatalysts for ORR [72, 73]. Hence, co‐doping strategies are being increasingly used for tuning the physical and electrochemical properties of graphene for various applications [12].

Recognizing the importance of co‐doping heteroatoms to graphene lattice, nitrogen‐ and sulfur‐dual‐doped graphene was prepared in a one‐pot synthesis procedure without the formation of any side products [74]. Later, the same group used a two‐step procedure to produce boron‐ and nitrogen‐doped graphene [75]. In both cases, it was found that the synergistic effect originating from the dopants was prime for the high‐performance ORR activity. In another report, Dai and coworkers [63] prepared a metal‐free BCN graphene with tunable boron and nitrogen levels as electrocatalyst for ORR in alkaline media. Modest boron and nitrogen levels in the graphene were found to be sufficient to obtain the best ORR activity. Later, Ajayan and coworkers demonstrated that nanoribbons of boron‐ and nitrogen‐substituted graphene can be used as efficient electrocatalysts for ORR [76]. Using GO as building blocks and boric acid and ammonia as boron‐ and nitrogen‐doping sources, porous 3D architectures with abundant edges and thin walls were prepared (Figure 7.11a). Rotating ring disk electrode (RRDE) voltammetry was performed in an O₂‐saturated 0.1 M aq. KOH solution at a scanning rate of 10 mV s⁻¹. It was found that the onset potential, half‐wave potential, saturated current density, and electron transfer number were strongly dependent on the doping concentrations in the nanoribbons and optimal doping was sufficient. The reported metal‐free catalysts showed superior performance compared to commercial Pt/C catalyst. In particular, the half‐wave potential (−0.03 V versus Ag/AgCl) was higher than other reported metal‐free catalysts in alkaline media and also higher than the commercial Pt/C catalyst (Figure 7.11b).

Apart from doping boron and nitrogen, researchers have also investigated co‐doping sulfur or phosphorus with nitrogen [22, 77–80]. Using a covalent functionalization strategy, a nitrogen–sulfur‐co‐doped graphene was synthesized using GO [81]. In addition to possessing superior electrocatalytic performance, the material also showed potential as anodes for lithium‐ion batteries. In another report, Cui and coworkers [82] produced a porous network of nitrogen–sulfur‐co‐doped graphene, maintaining a significant amount of catalytic active site. The pyrolysis temperature dictated the specific surface area and pore, with the optimum temperature at 900 °C. The doped graphene obtained at this temperature showed a four‐electron transfer pathway.

Another interesting variety of doping is triple doping where three different heteroatoms are doped into graphene [83, 84]. Thiele and coworkers [85] reported a fluorine‐, nitrogen‐, and sulfur‐doped graphene inexpensively synthesized by a simple route via pyrolysis of sulfur‐doped rGO at 600 °C in the presence of nafion and dimethyl formamide. The co‐doping of GO with fluorine and nitrogen created active sites due to the polarization of charge at CF and CN bonds. The presence of sulfur also provides additional catalytic active sites because of the unpaired electrons present at the defect sites of graphene. The synergistic combination of the three heteroatoms was credited to the enhanced performance in ORR. In an acidic and alkaline media, power densities of 14 and 46 mW cm⁻² were obtained, respectively, in a PEM fuel cell, with the as‐prepared material as the cathode catalyst layer. In another report, Wang and coworkers developed a nitrogen‐, sulfur‐, and phosphorous‐tri‐doped graphene by annealing GO with acephate (O,S‐dimethyl acetylphosphoramidothioate) [86]. The tri‐doped graphene exhibited superior ORR catalytic activity along with excellent selectivity and durability as compared to commercial Pt/C fuel cells.

7.3.1.4 Graphene‐Based Composites as Metal‐Free Catalysts for ORR

Graphene‐based composites have been well established to provide superior performance in a variety of applications ranging from medicine to electronics. The combinations of two materials can provide synergistic effects to overcome the deficit property of the other. One significant problem associated with graphene synthesis is its aggregation, which lowers the effective surface area, thereby limiting the active sites and hampering catalytic ORR activity. A general approach followed is to sandwich other carbon materials such as carbon nanoparticles, CNTs, etc., between graphitic layers, so as to obtain superior surface area [87]. In this case, the carbon material acts as a spacer to limit the agglomeration and provide more active sites for ORR.

Yu and coworkers [88] reported the synthesis of nitrogen‐doped graphene and CNT composite via a hydrothermal approach using GO, CNTs, and ammonia as precursors. The composite delivered superior ORR catalytic activity via a four‐electron pathway. In another instance, Liu and coworkers [89] fabricated a nitrogen‐ and sulfur‐co‐doped mesoporous carbon/graphene composite using amino acid precursors. Excellent electrocatalytic activity, selectivity, and stability were obtained for ORR in alkaline media. Recently, Dai and coworkers [90] used a ball‐milling approach to prepare a composite of graphitic carbon nitride with nitrogen‐doped graphene. The composite showed excellent ORR activity with an onset potential of −0.002 V (versus SCE) and half‐wave potential at about −0.22 V (versus SCE). The performance was attributed to the combination of nitrogen‐rich carbon nitride and nitrogen‐doped graphene. To scale up the carbon material production for ORR, sustainable biomass precursors have been used. Pan and coworkers [87] recently reported a porous graphene and carbon composite using glucose as a precursor. A specific surface area of 1510.83 m² g⁻¹ was obtained. The presence of pyridinic and graphitic nitrogen helped in boosting the performance as an efficient metal‐free electrocatalyst, with an onset potential of 0.91 V (versus reversible hydrogen electrode (RHE)). A nearly four‐electron pathway for ORR in alkaline solution was observed. Although carbon/graphene composites are being reported, there are still some hurdles to overcome, such as weak interaction between the carbon material and graphene and feasibility of large‐scale productions owing to the cost of the other CNTs, especially, pure single‐walled CNTs. In addition to cost, CNTs can always have metal impurities, thus making the metal‐free catalyst topic less valid. Overall, almost all the reported studies have demonstrated excellent ORR activity with respect to onset potential, which might be directly and/or indirectly attributed to metallic residues, when compared to that of commercially available Pt/C such as Vulcan.

7.4 Conclusions and Perspectives

A paradigm shift from the use of commercial Pt/C catalyst to efficient and low‐cost, carbon‐based electrocatalyst for ORR seems indispensable. Therefore, the interest in developing metal‐free catalysts for ORR has multitudinously increased among the research community. Over the past decade, ample number of research groups have demonstrated the use of graphene‐based, metal‐free ORR catalysts in fuel cells. This chapter highlighted and summarized recently reported important works and breakthroughs carried out in this emerging energy arena. It is clear that functionalization of graphene and/or doping graphene sheets are considered to be effective strategies to tailor the properties of materials to obtain the best catalytic activity. Up to day, doping nitrogen atom to graphitic frameworks has been extensively studied. Despite the several advances made and the cost of the catalyst dramatically reduced compared to that of Pt/C, there are still unanswered questions and not all the reported works are on par with that of the commercially available Pt/C catalyst in terms of onset potential and reaction current. In addition to the difficulty in controlling the heteroatom configuration in graphitic structure, there is an ambiguity in deciphering the origin of ORR activity. This critically hampers the understanding of the clear catalytic mechanism. Therefore, steps to control the heteroatom‐doping level and configuration are paramount.

Although steps to dope multiple atoms to graphitic lattice are in its nascent stage, some progress has been achieved, the active centers and mechanism of catalytic activities still remain vague. Theoretical modeling and experimental validation would be beneficial to realize the full potential of these metal‐free catalysts. There has been considerable debate in research community about trace metal impurities that could contribute toward the ORR activity (more significantly to onset potential). Researchers working on making composites of CNTs and graphene should take utmost care to remove impurities in the nanotubes. Similarly, due to the harsh oxidation treatments, impurities can also originate in GO materials and thus they can directly affect the catalytic activity. An important aspect to note is that most of the metal‐free, graphene‐based catalysts are found to be less effective in acidic media. Therefore, more fundamental research toward developing such metal‐free catalyst would revolutionize the future energy technology.

Acknowledgements

This research was supported by the Creative Research Initiative (CRI, 2014R1A3A2069102), BK21 Plus (10Z20130011057), Science Research Center (SRC, 2016R1A5A1009405), and Climate Change (2016M1A2940910) programs through the National Research Foundation (NRF) of Korea.

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