Uniform Hollow Carbon Shells: Nanostructured Graphitic Supports for Improved Oxygen‐Reduction Catalysis

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...

Carbon‐Encapsulated Pyrite as Stable and Earth‐Abundant High Energy Cathode Material for Rechargeable Lithium Batteries

“…Transmission electron microscopy (TEM) characterization reveals a uniform coating of thin carbon layers around the whole surface of the porous FeS 2 crystals, as shown in Figure . The selected area electron diffraction (SAED) pattern in the inset of Figure b and high resolution TEM (HRTEM) shown in Figure c and d further indicate the polycrystalline porous texture of FeS 2 and the presence of partly‐graphited carbon thin layers (more TEM and HRTEM images are shown in Figure S3–5 in Supporting Information) . The carbon content of this composite is about 10 wt%, as measured by inductively coupled plasma atomic emission spectroscopy (ICP‐AES).…”

Space‐Confined Synthesis of ZIF‐67 Nanoparticles in Hollow Carbon Nanospheres for CO2 Adsorption

“…Hollow carbon nanospheres (HCSs) are regarded as a promising family of multifunctional advanced materials for many years due to their excellent properties such as high surface‐to‐volume ratios, good electrical conductivity, and remarkable durability . Benefitting from their unique physical and chemical properties, HCSs have many potential applications in catalysis, gas separation, and energy storage and conversion . As demonstrated in previous studies, HCSs coupled with a thin microporous shell are superior to other microporous nanomaterials because of the smaller diffusion resistance .…”

Nitrogen-doped carbon xerogel: A novel carbon-based electrocatalyst for oxygen reduction reaction in proton exchange membrane (PEM) fuel cells

“…Pt/C) have been considered as the most effective elecrocatalysts for ORR in PEM fuel cells due to their high activities and good stabilities. [1][2][3][4][5][6][7][8][9] However, the issues of high cost, scarce sources and long-term durability limit their large-scale production and hinder the commercialization of PEM fuel cells. [10][11][12][13][14] In order to decrease the cost of electrocatalysts and eliminate their dependence on noble metals, various non-noble metal catalysts have been explored recently as alternatives to the Pt-based electrocatalysts, which include chalcogenide catalysts, [15][16][17][18][19] transition metal macrocyclic compounds, [20][21][22] transition metallic oxides [23][24][25][26][27] and carbon-based catalysts.…”