No abstract

Nanostructured high-surface-area carbon materials are ubiquitous in many applications, including catalysis, energy storage, and separations. [1][2][3] As supports for catalytic nanoparticles, nanostructured carbons can provide conductive substrates with high surface areas and excellent dispersion characteristics, which are important both for optimizing the synergistic nanoparticle-support interactions and for maximizing the mass activity of expensive precious metal catalysts. [4,5] This is particularly important for platinumbased nanoparticle catalysts, the benchmark electrocatalysts for proton-exchange-membrane fuel cells (PEMFC) that facilitate hydrogen oxidation at the anode and the oxygen reduction reaction (ORR) at the cathode.[6] The ORR is critically important for fuel-cell applications because it is the reaction that prevents maximum efficiency from being realized. [7] General strategies for improving ORR catalysis focus on increasing the accessible surface area of the catalyst and enhancing activity by size reduction, nanostructure control, and alloying. [8][9][10] Herein, we describe a significant improvement in apparent platinum mass activity for the ORR in 0.5 m H 2 SO 4 by anchoring platinum nanoparticles (Pt NPs) onto a new type of nano-engineered carbon support: monodisperse spherical nanoshells of graphitic carbon that are intermediate in size between C 60 and most other hollow graphitic nanomaterials. Nanostructured carbon materials are often prepared by carbon replication of sacrificial colloidal and porous templates, [1,2,11] chemical leaching of metals from metal carbides, [12] pyrolysis of carbon-rich organic materials, [1] and a variety of other methods.[13] For all of these methods, hightemperature (> 600 8C) processing is typically required to produce graphitic carbon. [12] Our approach to graphitic carbon nanoshells, shown schematically in Figure 1, is inspired by carbide-derived carbons (CDCs), which are formed by high-temperature/ high-pressure extraction of non-carbon elements from metal carbides.[12] For example, silicon and titanium can be leached from SiC and TiC, respectively, using chlorine gas. The leaching process introduces large numbers of micropores and mesopores, which result in high surface areas. However, the high temperatures and large particle sizes make it difficult to exploit CDCs as highly-dispersible supports for catalytic nanoparticles. Our nanostructure design strategy (Figure 1) yields highly dispersible uniform nanoshells of graphitic carbon with high surface areas by synergistically merging several important materials capabilities from colloidal nanoscience and solid-state chemistry. Recent reports have described the synthesis of nanocrystalline Ni 3 C [14] and NPs of the hcp allotrope of nickel, [15] both of which have similar XRD patterns. The product of a reaction claimed to yield near-monodisperse particles of hcp Ni, [15] when heated, behaves as would be expected for Ni 3 C, decomposing at about 420 8C.[16] In nanoparticle form, this decomposition results i...

Crystalline Ni 3 B with a fused porous nanoparticle network has been synthesized directly in solution using a modified polyol method. KBH 4 serves both as a reducing agent for Ni 2+ and as a boron source, while tetraethylene glycol serves as a solvent capable of achieving reaction temperatures that can crystallize Ni 3 B. The reaction pathway was studied using X-ray diffraction, transmission electron microscopy, and electron diffraction, and a nucleation-aggregation-smoothing mechanism is proposed. Amorphous Ni-B alloy nanoparticles form first and then aggregate into larger networks, which crystallize and smooth to form porous crystalline Ni 3 B nanoparticle networks. Structural characterization by X-ray diffraction indicates that the lattice constants for nanocrystalline Ni 3 B are shifted relative to single-crystal Ni 3 B, likely because of some carbon incorporation based on X-ray photoelectron spectroscopy data. Magnetic measurements suggest the formation of a small amount of nanocrystalline Ni, which provides further insight into the reaction pathway. Scanning electron microscopy and Brunauer-Emmett-Teller surface area measurements indicate a porous morphology, and thermal analysis shows that boronized nickel generated from Ni 3 B is more resistant to oxidation than a similar sample of pure Ni.