Facile, scalable synthesis of edge-halogenated graphene nanoplatelets as efficient metal-free eletrocatalysts for oxygen reduction reaction (original) (raw)

A series of edge-selectively halogenated (X 5 Cl, Br, I) graphene nanoplatelets (XGnPs 5 ClGnP, BrGnP, IGnP) were prepared simply by ball-milling graphite in the presence of Cl 2 , Br 2 and I 2 , respectively. High BET surface areas of 471, 579 and 662 m 2 /g were observed for ClGnP, BrGnP and IGnP, respectively, indicating a significant extent of delamination during the ball-milling and subsequent workup processes. The newly-developed XGnPs can be well dispersed in various solvents, and hence are solution processable. Furthermore, XGnPs showed remarkable electrocatalytic activities toward oxygen reduction reaction (ORR) with a high selectivity, good tolerance to methanol crossover/CO poisoning effects, and excellent long-term cycle stability. First-principle density-functional calculations revealed that halogenated graphene edges could provide decent adsorption sites for oxygen molecules, in a good agreement with the experimental observations. O ne of the major hurdles for commercialization of the fuel cell technology is the sluggish oxygen reduction reaction (ORR) at cathode 1-3. So far, high cost and scarce precious platinum (Pt) and its alloys have been considered to be the most reliable cathodic ORR electrocatalysts in fuel cells 4-8. In addition to the high cost, however, Pt and its alloys are also suffered from methanol crossover/carbon monoxide (CO) poisoning effects and poor operation stability. Therefore, it is essential to search for non-precious metal 9-11 or metal-free 12-17 electrocatalysts with a high catalytic activity and long-term operation stability to reduce or replace Pt-based ORR electrocatalysts in fuel cells. Although extensive efforts have been devoted to the development of non-precious metal-based electrocatalysts, their practical application is still out of sight due largely to their limited electrocatalytic activity, poor cycle stability and sometimes environmental hazard. Recently, carbon-based materials doped with heteroatoms, such as boron (B) 14,18 , halogen (Cl, Br, I) 19,20 , nitrogen (N) 12,15,21-25 , phosphorus (P) 26 , sulfur (S) 27 , and their mixtures 28-30 , have attracted tremendous attentions as metal-free ORR eletrocatalysts. The difference in electronegativity (x) between the heteroatom dopants (B 5 2.04, I 5 2.66, N 5 3.04, P 5 2.19 and S 5 2.58) and carbon atom (2.55) 31 in covalently doped graphitic carbon frameworks can polarize adjacent carbon atoms. Indeed, quantum mechanics calculations revealed that the electron accepting/donating ability of the heteroatom dopants created net positive/negative charges on adjacent carbon atoms in graphitic lattice to facilitate the oxygen reduction process 12. Thus, both the vertically-aligned nitrogen-doped carbon nanotubes (VA-NCNTs) 12 and nitrogen-doped graphene (N-graphene) 25 catalyzed an efficient four-electron ORR process with a higher electrocatalytic activity and better operation stability than the commercially available Pt/C-based electrocatalyst (Pt: 20 wt%, Vulcan XC-72R). Furthermore, the excellent stability over the methanol crossover/CO poisoning effects is additional advantage of these carbon-based metal-free catalysts. Although the basic catalytic mechanism has been established, the full potential of these