Abstract
Efficient metal-free electrocatalysts for oxygen reduction reaction (ORR) are highly expected in future low-cost energy systems. We have successfully prepared crumpled, sheet-like, sulfur-doped graphene by magnesiothermic reduction of easily available, low-cost, nontoxic CO2 (in the form of Na2CO3) and Na2SO4 as the carbon and sulfur sources, respectively. At high temperature, Mg can reduce not only carbon in the oxidation state of +4 in CO32− to form graphene, but also sulfur in SO42− from its highest (+6) to lowest valence which was hybridized into the carbon sp2 framework. Various characterization results show that sulfur-doped graphene with only few layers has an appropriate sulfur content, hierarchically robust porous structure, large surface area/pore volume, and highly graphitized textures. The S-doped graphene samples exhibit not only a high activity for ORR with a four-electron pathway, but also superior durability and tolerance to MeOH crossover to 40% Pt/C. This is mainly ascribed to the combination of sulfur-related active sites and hierarchical porous textures, facilitating fast diffusion of oxygen molecules and electrolyte to catalytic sites and release of products from the sites.
Highlights
Efficient metal-free electrocatalysts for oxygen reduction reaction (ORR) are highly expected in future low-cost energy systems
Compared to N- doped graphenes, little attention has been paid to the doping of S atoms into the graphene framework[33,34,35], Sdoped graphene has great potential as the electrocatalyst for oxygen reduction reaction (ORR), which is of high industrial importance in fuel cells
We showed that the as-formed S-doped graphene materials demonstrate high electrocatalytic activity for ORR with a dominant four-electron reaction pathway, as well as excellent durability and MeOH crossover compared to commercial Pt/C catalyst
Summary
Efficient metal-free electrocatalysts for oxygen reduction reaction (ORR) are highly expected in future low-cost energy systems. Since the seminal work of Geim and co-workers on the preparation of single layer graphene (sp2-hybridized carbon)[19], immense excitement has ensued, with graphene and modified graphene structures being shown to have notable and potentially exploitable electronic[18,19], optical[20], catalytic[21,22], physical properties23,24,, and others[25,26] Both theoretical and experimental studies have shown that the doping of heteroatoms (e.g. nitrogen (N, electrowww.nature.com/scientificreports negativity: 3.04), phosphorus (P, electronegativity: 2.19), sulfur (S, electronegativity: 2.58), boron (B, electronegativity: 2.04), etc) with distinct electronegativity with respect to carbon atoms (electronegativity: 2.55) into sp2-hybridized carbon frameworks can remarkably modify their electronic structures and chemical activities as well as the Fermi level of the host. Compared to N- (or B-) doped graphenes, little attention has been paid to the doping of S atoms into the graphene framework[33,34,35], Sdoped graphene has great potential as the electrocatalyst for oxygen reduction reaction (ORR), which is of high industrial importance in fuel cells
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