The Role of Sulfur-Related Species in Oxygen Reduction Reactions The Role of Sulfur-Related Species in Oxygen Reduction Reactions

Heteroatom (metal and nonmetal) doping is essential to achieve excellent oxygen reduc- tion reaction (ORR) activity of carbon materials. Among the heteroatoms that have been studied to date, sulfur (S) doping , including metal sulfides and sulfur atoms, has attracted tremendous attention. Since S-doping can modify spin density distributions around the metal centers as well as the synergistic effect between S and other doped heteroatoms, the S - C bond and metal sulfides can function as important ORR active sites. Furthermore, the S-doped hybrid sample shows a small charge-transfer resistance. Therefore, S-doping contributes to the superior ORR performance. This chapter describes the recent advance-ments of S-doped carbon materials, and their development in the area of ORR with regard to components, structures, and their ORR activities of S-related species.


Introduction
Fuel cells are considered as promising energy conversion and storage devices. In such a device, fuels (such as hydrogen, methanol, ethanol, or formic acid) react with oxygen at the anode, while oxygen molecules are reduced to water molecules at the cathode [1][2][3][4]. However, the oxygen reduction reaction (ORR) rate is ~5 orders of magnitude slower than the reaction on the anode due to its high overpotential [5]. The search for catalysts that can conquer these huge activation energy barriers has attracted much attention. Although Pt-based electrocatalysts have been commercialized, the high cost of Pt and their poor tolerance to methanol significantly hamper their large-scale commercialization. Thus, great effort has been devoted to developing low cost, non-precious-metal, and metal-free catalysts with improved electrocatalytic efficiency [6][7][8][9].
Excellent electrocatalysts for ORR should possess a high specific surface area, finely tuned pore structure, and good electron conductivity. The former two facilitate easy accessibility to the active sites and ion diffusion, and the latter is beneficial for electron transfer. Much attention has been focused on the carbonaceous materials due to their remarkable advantages, such as low cost, facile preparation strategy, and high conductivity. For constructing ORR catalysts with promising electrocatalytic activity, single atom doping or co-doping of two or multiple heteroatoms are essential. Metal/nitrogen/carbon (M/N/C) catalysts have been regarded as the most promising alternative for precious metal catalysts. For example, Fe species not only facilitate the formation of catalytically active N-C sites, but Fe atoms also contribute to the graphitization of carbon. More importantly, Fe atoms and related nanoparticles are generally suggested as the active site of ORR catalysts. Recently, the introduction of nonmetal heteroatoms such as N, P, S, or B into carbon materials is generally effective in enhancing ORR activities of catalysts. In N-doped carbon, the N atom with higher electronegativity (3.04) than that of carbon (2.55) leads to more charged adjacent C atoms. With respect to S, the electronegativity of S (2.58) is similar to that of carbon; however, S can easily change the band gap of carbon due to its two lone pair electrons [2]. P with an electronegativity of 2.19 and B with an electronegativity of 2.04 can also induce imbalanced charge distribution in carbon materials, thus forming positively polarized C-P and C-B more active sites to ORR [10,11]. Furthermore, N/B, N/P, N/S, and N/S/P co-doped carbons also show excellent catalytic activity due to their synergistic effects on spin or charge density of carbon matrix. Notably, designing a carbon matrix with different morphologies combined with hierarchical porous structures, such as micro-, meso-, and macroporosity, can further optimize ORR activity.
Recently, the S atom has attracted particular interest and especially its high synergetic effect with N dopants and metal dopants [12][13][14][15][16][17][18][19]. In this chapter, we briefly summarize the S-related species as active sites in the ORR, such as S-M/N/C, metal chalcogenides, N/S, and N/S/P. We then discuss the S-containing electrocatalysts including their carbon sources, heteroatom dopants, and preparation methods as well as the nanostructure of the supports.

S-related active sites in the ORR
The development of novel strategies for the design and synthesis of carbon-based high performance electrocatalysts is a hot topic. Therefore, efforts have been made toward the design and synthesis of extraordinary ORR catalytic carbon materials with different morphologies and single or multiple active sites from diverse sources. S-related active sites have been extensively investigated due to their excellent performance in ORR. S atom mono-doping can induce structural defects in the carbon matrix. The resulting charge dislocation can improve the oxygen adsorption. Furthermore, protonation of S is not as severe as that of N [20]. More importantly, dual doping of N and S or multiple doping of N, S, and M [21] can dramatically enhance ORR activity due to the synergistic effects. Wu et al. prepared Fe, N, and S decorated hierarchical carbon layers (S, N-Fe/N/C-CNT) from pyrolysis of 2,2-bipyridine and Fe(SCN) 3 -coated CNTs [22]. Adding S salts not only contributed to the formation of atomically dispersed Fe-N x species, but also improved the surface area of the carbon matrix. The half-wave potential (E 1/2 ) of the S,N-Fe/N/C-CNT catalyst is about 0.85 V, which is higher than that of commercial Pt/C (0.82 V). The catalyst also exhibited superior durability in alkaline medium. Theoretical calculations predicted that atomically dispersed Fe-N x species function as highly active sites, while co-doping of N and S improved the electrical conductivity. Furthermore, Wan et al. fabricated a sandwich-like graphene/carbon hybrid from graphene oxide (GO) and nontoxic starch (Figure 1) [23]. Graphene/carbon nanosheets decorated by N, S, and Fe (Fe, S/NGC) were obtained via treatment with FeCl 3 and KSCN. Fe,S/NGC showed outstanding ORR performance in alkaline medium (E 1/2 of 0.83 V vs. RHE, surpassing E 1/2 of NGC (0.76 V) and the Pt/C catalyst (0.81 V)), due to the simultaneous introduction of Fe and S. The Fe 3 N and S were considered major active centers in this hybrid. Furthermore, Fe,S/NGC also displayed a high ORR activity in the acidic solution. In addition, an S and N dual-doped Fe-N-S electrocatalyst (Fe-M-LA/C) was obtained via pyrolysis of the mixture of melamine, lipoic acid, FeCl 3 , and carbon black [24]. FeS and Fe 3 C formed in the Fe-M-LA/C. It has been suggested that Fe 2+ has high catalytic activity in ORR and that Fe 3 C is the active site for the ORR. Combined with the N and S-doping, Fe-M-LA/C showed promising ORR activity. Interestingly, sewage sludge itself can be used as "all-inone" precursor for ORR catalysts [25]. The innate N, Fe, and S compounds in the sewage sludge function as N, Fe, and S dopants. The N, Fe, and S self-doped nanoporous carbon material exhibited favorable electrocatalytic activity in both alkaline and acidic environments. The nanostructure of the carbon matrix also played an important role in ORR. Wan et al.  and encapsulated iron nanoparticles combined with hierarchical porous structures contributed to the excellent ORR performance [2].

M-N-S-based active sites
Bimetal-based N and S doped catalysts have also been reported. Li et al. synthesized PdW alloy nanoparticles decorated S-doped graphene via a microwave irradiation method [26]. S-doping contributed to the formation of small particles and the uniform distribution of alloy particles. The as-prepared catalyst was highly active for ORR due to the specific electronic structure of the alloy. CoFe 2 O 4 nanoparticles decorated rGO designed by Yang et al. demonstrated high ORR activity due to the existence of defects resulting from the doping of N and S and the covalent coupling between the CoFe 2 O 4 and rGO matrix [27]. Moreover, Ren et al. synthesized PdNi decorated N and S co-doped three-dimensional ordered carbon derived from acrylonitrile telomere (C, N, and S sources) using silica as template [28]. Due to the codoping of N and S, the strong electronic interaction between Pd and Ni, and three-dimensional honeycomb-ordered structure, the electrocatalyst exhibited superior performance compared to commercial Pd/C in alkaline solution. Cobalt chalcogenides as active sites have also attracted significant attention. For example, Li et al. successfully anchored Co 9 S 8 nanoparticles on N and S dual doped carbon nanosheets (Co 9 S 8 /N,S-CNS) via facile pyrolysis of CoCl 2 , citric acid, and thiourea as carbon source and cubic NaCl crystals were used as template (Figure 3) [31]. Due to the highly dispersed nanoparticle and the synergistic catalytic effect between Co 9 S 8 nanoparticles and the doped N and S in the carbon nanosheets, Co 9 S 8 /N,S-CNS showed high catalytic activity and stability. Moreover, Liao et al. prepared S co-doped graphene nanoholes with cobalt sulfide hollow nanospheres decorated in (Co 1−x S/N-S-G) using GO (graphene oxide), phen, and S [32]. The catalyst presented high ORR catalytic activity with an E 1/2 of 30 mV, which was more positive than that of a commercial Pt/C catalyst. Similarly, Zhang et al. prepared CoS decorated N, S co-doped reduced GO aerogel showing highly efficient activity for ORR [33].

Metal chalcogenide-based active sites
There are other metal sulfides as active sites in ORR. Suh et al. prepared nano-CuS@Cu-BTC composites using Cu-MOF as a sacrificial template and thioacetamide as sulfide source [34].
With the increasing amount of nano-CuS in the composite, electrical conductivity increased, thus contributing to the positive shifts of E onset . MoS 2 -embedded nitrogen-doped porous carbon nanosheets were prepared using MoS 2 nanosheets as templates and conjugated microporous polymers as N and C sources [35]. The novel electrocatalysts showed enhanced performance for ORR, due to their strong interaction between MoS 2 and carbon layer, high conductivity, and high specific surface area. Recently, it has been reported that Ni 3 S 2 [36] and WS 3−x [37] are also potential catalyst for ORR. Furthermore, metallic double sulfides as an ORR catalyst were investigated in recent years. Li et al. prepared NiCo 2 S 4 and N, S-doped graphene aerogel hybrid for application in ORR [38].

N-, S-, B-, and P-based active sites
Recently, metal-free catalysts have received much attention, and intensive research efforts have been made. For example, Sun et al. synthesized N,S-co-doped nanocarbon polyhedral morphology using a metal organic framework (MOF) as precursor followed by thermal treatment with ammonia gas (NH 3 ) and further co-doping of S (Figure 4) [39]. The obtained catalyst showed improved electrocatalytic efficiency, comparable to that of the Pt/C catalyst.
Mu et al. synthesized N and S dual-doped 3D porous graphene from waste biomass and GO ( Figure 5) [40]. The resultant catalyst showed high ORR performance and stability comparable to commercial Pt/C in both alkaline and acidic media due to their unique porous structure and synergistic effects of N and S doping. Furthermore, Wang et al. prepared N and S co-doped 3D hollow structured carbon spheres based on a soft template method [41]. Aniline and pyrrole function as carbon source and N dopant and Na 2 S serve as S dopant. The obtained hollow carbon spheres with uniform size, mesoporous structure, and high number of active sites exhibited high ORR activity comparable to that of Pt/C. In contrast, Liao et al.   prepared N and S co-doped hollow carbon nanospheres from polyacrylonitrile and S via a hard template method [42]. They reported that S-doping facilitated the formation of pyridinic N, which is more active than other N species in ORR. The catalyst exhibits excellent ORR performance with high stability and selectivity.
Dai et al. reported the development of N, S co-doped graphitic sheets from melamine (as carbon precursor and nitrogen dopant), Ni 2 SO 4 (as S dopant and template), and KCl (as template) [43]. The unique hierarchical porous structure renders active sites easily accessible and facilitates electron and mass transfer. Therefore, this catalyst was not only effective in ORR, but also demonstrated excellent activities in OER/HER. In addition, Zhi et al. reported that atomic S doping in mesoporous carbon-supported C 3 N 4 can remarkably enhance ORR activity [44]. In this work, thiourea was selected as S dopant and C 3 N 4 serve as N source. XPS analysis showed the formation of C 3 N 4−x S x , indicating the atomic modification over the C 3 N 4 .
Song et al. prepared S-N dual doped ordered mesoporous carbon based on a hard template method [45]. In their work, polythiophene (PTh) and polypyrrole (PPy) were used as precursors and ordered mesoporous silica (SBA-15) was selected as template. Based on this method, N and S contents can be easily adjusted. Furthermore, the mesoscopic morphology provided more accessible active sites. Therefore, this catalyst showed excellent ORR performance.
Recently, P/S binary-doped carbon materials have also been reported. P, with higher electrondonating ability, heavily affects neighboring carbon atoms, thus tending to induce more active sites than N. For instance, Cao et al. prepared P/S co-doped porous carbon derived from resorcinol, furaldehyde, and phosphorus pentasulfide. Due to the synergetic merits of P and S, the onset potential positively shifted for ORR in alkaline media [46].

Mechanism
In fact, the mechanism of S-related active sites in ORR is still debated. Suib et al. prepared S-doped carbon nanotube-graphene nanolobes via sequential bidoping strategy, in which the nature of S functionalization can be finely tuned [47]. First, thiourea functioned as the S source. To further stabilize and enhance the content of S, the second doping of benzyl disulfide was introduced. Different doping types of S were detected, such as C-S, C-S-C, and -SO x -. The S-doped CNT showed high catalytic activity and good stability for ORR. Furthermore, Guo et al. studied the effect of Fe/N/C and C-S-C active sites in alkaline and acidic media [48]. It is worth noting that no Fe-S bond formed in the catalyst. They found that no synergistic effects between Fe/N/C and C-S-C were observed in alkaline solution as the two active centers are separated. In contrast, synergistic effects between Fe/N/C and C-S-C sites remarkably enhanced ORR activity in acidic media because the C-S-C active sites facilitated the 4e-ORR pathway.
Furthermore, S can function as platinum nanowire catalyst anchoring sites. Chen et al. studied the influence of S content on the ORR activity of S-doped graphene supported platinum nanowires (PtNW/SGs) [49]. S doping increased the band gap, while the electrical conductivity decreased. PtNW/SGs with 1.40 at% S showed the best ORR performance. Zhi et al.
investigated the componential influences of heteroatoms doping (B, P, and S) in graphitic C 3 N 4 (g-C 3 N 4 )-based electrocatalysts (Figure 6) [50]. They found that S-doped C 3 N 4 with the smallest charge-transfer resistance dramatically boosted the reaction kinetics and activities of ORR.
Recently, Xu et al. designed Fe-N-, Fe-S-, and Fe-N-S-based model catalysts to investigate heteroatom induced performance differences in ORR [51]. Pyrrole-derived and thiophenederived hypercrosslinked polymers were selected as carbon precursors. FeCl 3 , a Friedel-Crafts reaction catalyst, acts as both a metal dopant and a porogen. Interestingly, Fe 1−x S and Fe 3 O 4 nanoparticles formed in the S-doped and N-doped carbon, respectively. In fact, N/Fe 3 O 4 acts as a higher catalytic active site than S/Fe 1−x S. The possible reason is that the strong electronegativity of N generates more charged active sites, while the electronegativity of S is similar to that of carbon. However, the synergistic effect between Fe 1−x S/Fe 3 O 4 and the N, S-doped carbon showed superior ORR performance.

Conclusions
Although state-of-the-art Pt-based ORR catalysts are applicable in fuel cell vehicles, source scarcity limits their mass application. M-N-C materials are still far from satisfaction for commercialization. Presently, design and synthesis of novel ORR catalysts with various structures were at the center of research. Furthermore, to experimentally and theoretically explore the relationship between component structure-properties has attracted extensive interest. Particularly, tuning the mode of heteroatom-doping and the underlying the role of active sites in ORR catalysis still remains challenging.
Currently, S-related species represent promising active sites for ORR catalysis. S doping can lead to a higher degree of graphitization because S can react with imperfect carbon to form CS 2 gas [52]. Furthermore, S-doping can modify the spin density distributions around the carbon framework. More importantly, the synergistic effect between the metal center and the N, S-codoped carbon contributes to the superior ORR performance. With regard to metal free catalysts, first-principle calculations indicate that N and S atoms close to each other were more active than isolated N and S sites, indicating a synergistic effect of N and S. Therefore, S-related active sites containing ORR catalyst will be promising alternatives for commercial Pt/C catalysts, especially those with hierarchical porous structures.