**1. 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 [1–4]. 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

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to developing low cost, non-precious-metal, and metal-free catalysts with improved electrocatalytic efficiency [6–9].

**2.1. M-N-S-based active sites**

dispersed Fe-N<sup>x</sup>

and that Fe<sup>3</sup>

aqueous solution of FeCl<sup>3</sup>

pyrolysis of 2,2-bipyridine and Fe(SCN)<sup>3</sup>

ous introduction of Fe and S. The Fe<sup>3</sup>

uted to the formation of atomically dispersed Fe-N<sup>x</sup>

pyrolysis of the mixture of melamine, lipoic acid, FeCl<sup>3</sup>

Wu et al. prepared Fe, N, and S decorated hierarchical carbon layers (S, N-Fe/N/C-CNT) from

area of the carbon matrix. The half-wave potential (E1/2) 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

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<sup>3</sup>

KSCN. Fe,S/NGC showed outstanding ORR performance in alkaline medium (E1/2 of 0.83 V vs. RHE, surpassing E1/2 of NGC (0.76 V) and the Pt/C catalyst (0.81 V)), due to the simultane-

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

formed in the Fe-M-LA/C. It has been suggested that Fe2+ has high catalytic activity in ORR

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. synthesized Fe/N/S-doped carbon from glucose, thiourea, and iron nitrate based on a dualtemplate method. Multiple active sites such as graphitic-N, pyridinic-N, thiophene-S, FeN<sup>x</sup>

**Figure 1.** (a) The raw materials of synthesis of NGC nanosheets used as the precursor of Fe,S/NGC-900. (b) Mixed

as-obtained catalyst (above) and illustration of nitrogen and sulfur atoms in carbon skeleton (below) of Fe,S/NGC-900.

and KSCN (above) and NGC nanosheets prepared by hydrothermal reaction (below). (c) The

C is the active site for the ORR. Combined with the N and S-doping, Fe-M-LA/C

species function as highly active sites, while co-doping of N and S improved


The Role of Sulfur-Related Species in Oxygen Reduction Reactions

N and S were considered major active centers in this

species, but also improved the surface

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

, and carbon black [24]. FeS and Fe<sup>3</sup>

and

83

C

,

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 [12–19]. 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.
