**2.4. Mechanism**

**Figure 4.** Schematic illustration of the fabrication of the N,S-co-doped nanocarbon as the electrocatalyst toward ORR. The ZIF-8 precursor and thiourea are used as C/N and S precursors, respectively. Reproduced with permission from Ref.

**Figure 5.** Schematic illustration of the formation of NSG: (stage 1) homogeneous mixture of graphene oxide and horn, (stage 2) disintegration/release of cysteine moieties and coverage of GO surface leading to reaction of functional groups,

of N and S into the graphene carbon network. Reproduced with permission from Ref. 40. Copyright 2016, American

S, NH<sup>3</sup>

, etc.), and (stage 3) doping

eviction of gaseous species, and the formation of S and N containing moieties (e.g., H<sup>2</sup>

Chemical Society.

[39]. Copyright 2017, Royal Society of Chemistry.

86 Chalcogen Chemistry

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<sup>x</sup> -. 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.

Particularly, tuning the mode of heteroatom-doping and the underlying the role of active

The Role of Sulfur-Related Species in Oxygen Reduction Reactions

http://dx.doi.org/10.5772/intechopen.78647

89

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

This work was supported by the National Nature Science Foundation of China (No. 21603156),

 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

sites in ORR catalysis still remains challenging.

Pt/C catalysts, especially those with hierarchical porous structures.

Jiangsu Province Science Foundation for Youths (No. BK20170331).

CS2

**Acknowledgements**

**Acronyms and abbreviations**

CB carbon black

CNT carbon nanotube

FeCl<sup>3</sup> iron(III) chloride

NH<sup>3</sup> ammonia gas

P phosphorus

KSCN potassium sulfocyanide

OER oxygen evolution reaction

rGO reduced graphene oxide

(M/N/C) metal/nitrogen/carbon

HER hydrogen evolution reaction

ORR oxygen reduction reaction

PEM proton exchange membrane

Pt/C platinum/carbon black catalyst

XPS X-ray photoelectron spectroscopy

**Figure 6.** Optimized structure of pristine g-C3 N4 as (a) top view and (b) side view, in which the N atoms are numbered from 1 to 8, while C atoms are numbered from 9 to 14. Top views of the optimized structures of the energetically most favorable (c) B-CN, (d) P-CN, and (e) SCN, in which the B and P atoms substitute the bay carbon C13, while S atom replaces the pyridinic nitrogen N7. In each structure, the largest value of charge and spin densities on carbon atoms are indicated by black and red colors, respectively; additionally, the related carbon atoms are illustrated by green arrows. Reproduced with permission from Ref. [50]. Copyright 2017, American Chemical Society.

investigated the componential influences of heteroatoms doping (B, P, and S) in graphitic C3 N4 (g-C3 N4 )-based electrocatalysts (**Figure 6**) [50]. They found that S-doped C3 N4 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<sup>3</sup> , a Friedel-Crafts reaction catalyst, acts as both a metal dopant and a porogen. Interestingly, Fe1−xS and Fe<sup>3</sup> O4 nanoparticles formed in the S-doped and N-doped carbon, respectively. In fact, N/Fe<sup>3</sup> O4 acts as a higher catalytic active site than S/Fe1−xS. 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 Fe1−xS/Fe<sup>3</sup> O4 and the N, S-doped carbon showed superior ORR performance.

## **3. 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 CS2 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.
