Co(1-x)S-graphene hybrid: a high-performance metal chalcogenide electrocatalyst for oxygen reduction.

Owing to their high energy-conversion efficiency, low or even zero emission, and high energy and power density, fuel cells such as proton-exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) have drawn tremendous attention as potential clean and efficient power sources for both electric vehicles and portable electronics. A major limiting factor of energy-conversion efficiency for present fuel cells is the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. Platinum and its alloys have been so far the most active ORR catalysts. However, the prohibitive cost, scarcity, and declining activity of Pt-based catalysts have hindered widespread application and commercialization of fuel cells. Consequently, alternative electrocatalysts based on nonprecious metals have been actively pursued. Cobalt sulfides have been investigated as ORR catalyst with the highest activity among all chalcogenides of nonprecious metals in acidic solution. Theoretical studies predicted electrocatalytic activity of Co9S8 similar to that of Pt via a four-electron ORR pathway. However, current ORR catalysts based on cobalt sulfides and cobalt selenides have exhibited activities far lower than that of Pt. Moreover, it has been shown that two-electron reduction is the dominant reaction pathway for cobalt sulfide catalysts, especially at potentials higher than 0.5 V versus the reversible hydrogen electrode (RHE). It is thus highly desirable to design and synthesize high-performance cobalt chalcogenide based electrocatalyst materials capable of catalyzing fourelectron ORR. Here we describe a novel cobalt sulfide–graphene hybrid electrocatalyst for ORR, obtained by controlled two-step synthesis of Co1 xS nanoparticles on reduced graphene oxide (RGO). The RGO sheets underlying the Co1 xS nanoparticles provide an electrically conducting support for the catalyst, control the size of the catalyst particles, and enhance the ORR catalytic activity of the Co1 xS nanoparticles through strong electrochemical coupling. In 0.5m H2SO4, our Co1 xS/ RGO hybrid catalyst shows an ORR current onset at about 0.8 V versus RHE.When the electrode is rotated at 1600 rpm, the ORR current density is as high as about 1.1 mAcm 2 at 0.7 V versus RHE with a loading of about 100 mgcm . Measurements with both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) show that nearly fourelectron ORR can be achieved with the Co1 xS/RGO hybrid catalyst. The Co1 xS/RGO hybrid is the highest performance ORR catalyst among all cobalt chalcogenide based materials reported in the literature. Cobalt sulfide nanoparticles were synthesized on RGO sheets by a low-temperature solution-phase reaction followed by a high-temperature annealing step. Graphene oxide (GO) was made by a modified Hummers method (see Supporting Information), in which a six times lower concentration of KMnO4 was used (see Supporting Information) to give GO sheets with lower oxygen content than Hummers GO (ca. 15 vs. ca. 30%, measured by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy). In the first reaction step, nanoparticles of cobalt sulfide precursor were selectively and uniformly coated onto GO surface by treating Co(OAc)2 with thioacetamide (TAA) in a GO/water suspension at 80 8C for 12 h (Figure S1, Supporting Information). The intermediate product was then annealed at 500 8C in 1 atm of Ar for 1 h to give the final hybrid material of cobalt sulfide nanoparticles on graphene (see Supporting Information for experimental details). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) clearly revealed that cobalt sulfide nanoparticles were selectively grown on RGO sheets without free growth of unattached nanoparticles in solution (Figure 1a, b, and d). The cobalt sulfide nanoparticles on RGO were crystalline with an average particle size of about 10–20 nm (Figure 1b and d). X-ray diffraction (XRD, Figure 1c) showed that the cobalt sulfide nanoparticles grown on RGO are a Co1 xS phase (ICDD PDF #00-0420826) with hexagonal structure in space group P63/mmc (no. 194). The lattice fringes of the Co1 xS nanocrystals (Figure 1d) and the electron diffraction pattern (Figure 1d inset) are consistent with the crystal structure. The Co1 xS/ RGO hybrid contains about 30 wt% of RGO with a designed Co/C ratio of 1/4. The ORR catalytic activity of Co1 xS/RGO was first characterized by loading the hybrid material onto glassy carbon electrode for cyclic voltammetry (CV) measurements in 0.5m H2SO4 at 25 8C (see Supporting Information for experimental details). Comparison of CV curves in O2versus Ar-saturated electrolyte clearly revealed the ORR catalytic activity of Co1 xS/RGO. The hybrid showed an ORR onset potential at about 0.8 V versus RHE, and the peak current was reached at about 0.73 V versus RHE (Figure 2a). Measurements with an RDE were carried out to reveal the ORR kinetics of the Co1 xS/RGO hybrid in 0.5m H2SO4 (Figure 2b). At 1600 rpm, the Co1 xS/RGO hybrid catalyst showed a current density of about 1.1 mAcm 2 at 0.7 V versus [*] H. Wang, Y. Liang, Y. Li, Prof. H. Dai Department of Chemistry, Stanford University, Keck Building 380 Roth Way, Stanford, CA 94305 (USA) E-mail: [email protected]