Dongsheng Geng

High oxygen-reduction activity and durability of nitrogen-doped graphene

Nitrogen-doped graphene as a metal-free catalyst for oxygen reduction was synthesized by heat-treatment of graphene using ammonia. It was found that the optimum temperature was 900 °C. The resulting catalyst had a very high oxygen reduction reaction (ORR) activity through a four-electron transfer process in oxygen-saturated 0.1 M KOH. Most importantly, the electrocatalytic activity and durability of this material are comparable or better than the commercial Pt/C (loading: 4.85 µgPt cm−2). XPS characterization of these catalysts was tested to identify the active N species for ORR.

A Highly Durable Platinum Nanocatalyst for Proton Exchange Membrane Fuel Cells: Multiarmed Starlike Nanowire Single Crystal

Despite significant recent advances, the long-term durability of Pt catalyst at the cathode is still being recognized as one of the key challenges that must be addressed before the commercialization of proton exchange membrane fuel cells (PEMFCs). 2] The loss of Pt electrochemical surface area (ECSA) over time, because of corrosion of the carbon support and Pt dissolution/aggregation/Oswald ripening, is considered one of the major contributors to the degradation of fuel cell performance. Up to now, highly dispersed Pt nanoparticles (NPs, 2–5 nm) on carbon supports are still being widely used as the state-of-the-art commercial catalysts, and most reported studies are focused on nanoparticles of Pt. However, Pt with nanosized particle morphologies has high surface energies, thereby inducing severe Oswald ripening and/or grain growth during fuel cell operation. One-dimensional (1D) nanostructures of Pt, such as nanowires (NWs) and nanotubes (NTs), have been demonstrated to overcome the drawbacks of NPs in fuel cells, owning to their unique 1D morphologies. Yan et al. reported that unsupported Pt nanotubes exhibit much enhanced catalytic activity and durability as fuel cell cathode catalyst. Sun et al. and Zhou et al. reported the improved oxygen reduction reaction (ORR) activities of Pt NWs at the cathode under fuel cell operating conditions. However, up to now, the durability of Pt NW-based electrocatalysts has never been reported in the literature. Here we describe a new approach to address, for the first time, both the activity and durability issues by using carbonsupported multiarmed starlike Pt nanowires (starlike PtNW/ C) as electrocatalysts. Interestingly, the durability can be further improved by eliminating the carbon support, that is, using unsupported Pt nanowires as the cathode catalyst. As a result of their unique 1D morphology, the starlike Pt nanowire electrocatalyst can provide various advantages. First, the interconnected network consists of multiarmed, star-shaped 1D NWs with arm lengths of tens of nanometers which makes the Pt less vulnerable to dissolution, Ostwald ripening, and aggregation during fuel cell operation compared to Pt nanoparticles. Second, this network structure reduces the number of embedded electrocatalyst sites in the micropores of the carbon supports relative to those in nanogrannular Pt. Third, the mass transfer within the electrode can be effectively facilitated by networking the anisotropic morphology. Carbon-supported multiarmed starlike platinum nanowires were synthesized by the chemical reduction of a Pt precursor with formic acid in aqueous solution at room temperature and under ambient atmosphere. No surfactant, which is usually harmful for catalytic activities, was used in the experiments. Figure 1A and B show the representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, respectively, of carbon-supported Pt nanowires at 40 wt % loading of Pt. It can be seen that the assynthesized Pt is nanostar-shaped, being composed of several short arms of Pt nanowires. The number of arms of each nanostar is found to vary ranging from several to over ten. Occasionally, single-armed nanowires standing on the carbon surface can also be observed. Diameter and length of the arms of starlike Pt nanowires are about 4 nm and 15 nm, respectively. More interestingly, from the connected atomic arrangement shown in the high-resolution TEM (HRTEM) images (see Figure S1 in the Supporting Information and the inset in Figure 1B), the nanostar is found to be a single crystal. The fast Fourier transform (FFT; see inset in Figure S1) of the original HRTEM image shows a dotted pattern, further proving that the nanostar is a single crystal. This indicates that the formation mechanism of the nanostar involves seeded growth rather than an aggregation of seeded particles or an assembly process of the nanowires. The X-ray diffraction (XRD) pattern (Figure S2) confirms that the carbon-supported Pt nanowires are crystallized in a face-centered-cubic (fcc) structure similar to bulk Pt, which is consistent with the HRTEM investigations. We believe that the growth of the multiarmed starlike PtNWs on carbon black supports follows a similar process to that for Pt NWs on other supports. Typically, Pt nuclei are first formed in solution through the reduction of H2PtCl6 by HCOOH, and they deposit on the surface of carbon spheres. The freshly formed nuclei act as the sites for further nucleation through the continual absorption and reduction of Pt(IV) ions leading to the formation of particle seeds. For fcc structures, the sequence of surface energies is g{111} < g{100} [*] Dr. S. Sun, Dr. G. Zhang, Dr. D. Geng, Y. Chen, R. Li, Prof. X. Sun Department of Mechanical and Materials Engineering The University of Western Ontario London, Ontario N6A 5B9 (Canada) Fax: (+ 1)519-661-3020 E-mail: xsun@eng.uwo.ca

Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition

Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.

Layer by layer assembly of sandwiched graphene/SnO2 nanorod/carbon nanostructures with ultrahigh lithium ion storage properties

Sandwiched structures consisting of carbon coated SnO2 nanorod grafted on graphene have been synthesized based on a seed assisted hydrothermal growth to form graphene supported SnO2 nanorods, followed by a nanocarbon coating. As a potential anode for high power and energy applications, the hierarchical nanostructures exhibit a greatly enhanced synergic effect with an extremely high lithium storage capability of up to 1419 mA h g−1 benefiting from the advanced structural features.

Direct Growth of Single‐Crystal Pt Nanowires on Sn@CNT Nanocable: 3D Electrodes for Highly Active Electrocatalysts

A newly designed and fabricated novel three dimensional (3D) nanocomposite composed of single-crystal Pt nanowires (PtNW) and a coaxial nanocable support consisting of a tin nanowire and a carbon nanotube (Sn@CNT) is reported. This nanocomposite is fabricated by the synthesis of Sn@CNT nanocables by means of a thermal evaporation method, followed by the direct growth with PtNWs through a facile aqueous solution approach at room temperature. Electrochemical measurements demonstrate that the PtNW--Sn@CNT 3D electrode exhibits enhanced electrocatalytic performance in oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells (PEMFCs), methanol oxidation (MOR) for direct methanol fuel cells (DMFCs), and CO tolerance compared with commercial ETEK Pt/C catalyst made of Pt nanoparticles.

Hierarchically porous LiFePO4/nitrogen-doped carbon nanotubes composite as a cathode for lithium ion batteries

A porous composite of LiFePO4/nitrogen-doped carbon nanotubes (N-CNTs) with hierarchical structure was prepared by a sol–gel method without templates or surfactants. Highly conductive and uniformly dispersed N-CNTs incorporated into three dimensional interlaced porous LiFePO4 can facilitate the electronic and lithium ion diffusion rate. The LiFePO4/N-CNTs composites deliver a reversible discharge capacity of 138 mA h g−1 at a current density of 17 mA g−1 while the LiFePO4/CNTs composites only deliver 113 mA h g−1, demonstrating N-CNTs modified composites can act as a promising cathode for high-performance lithium-ion batteries.

Hierarchical nanostructured core-shell Sn@C nanoparticles embedded in graphene nanosheets: spectroscopic view and their application in lithium ion batteries.

Hierarchical carbon encapsulated tin (Sn@C) embedded graphene nanosheet (GN) composites (Sn@C-GNs) have been successfully fabricated via a simple and scalable one-step chemical vapor deposition (CVD) procedure. The GN supported Sn@C core-shell structures consist of a crystalline tin core, which is thoroughly covered by a carbon shell and more interestingly, extra voids are present between the carbon shell and the tin core. Synchrotron spectroscopy confirms that the metallic tin core is free of oxidation and the existence of charge redistribution transfer from tin to the carbonaceous materials of the shell, facilitating their intimate contact by chemical bonding and resultant lattice variation. The hybrid electrodes of this material exhibit a highly stable and reversible capacity together with an excellent rate capability, which benefits from the improved electrochemical properties of tin provided by the protective carbon matrix, voids and the flexible GN matrices.

Discharge product morphology and increased charge performance of lithium–oxygen batteries with graphene nanosheet electrodes: the effect of sulphur doping

Sulphur-doped graphene was successfully fabricated and its influence on the discharge product formation in lithium–oxygen batteries was demonstrated. The growth and distribution of the discharge products were studied and a mechanism was proposed. This will have significant implication for cathode catalysts and rechargeable battery performance.

Potential of metal-free "graphene alloy" as electrocatalysts for oxygen reduction reaction

Extensive research and development on theoretical calculation and synthetic methods over the past few years have made doped graphene one of the most promising candidates for metal-free oxygen reduction reaction (ORR) catalysts. However, from the performance point of view, there is still a long way to go for these doped graphene-based catalysts to meet the requirements needed for commercial applications. What is the key to further improve the catalytic activity of doped graphene toward ORR to make them commercially viable? In this review, we will try to answer this question by fundamentally giving a detailed analysis based on the theoretical calculations to reveal the origin of ORR activity of doped graphene and the structure–performance relationship of such materials. Thereafter, we will provide an overview on the recent advances in the catalytic activity improvement of doped graphene, including major works using approaches of increasing the number of active sites, controlling the doping types (particularly for nitrogen doped graphene), developing co-doped graphene, and extending the surface area of doped graphene. Finally, in this perspective, we discuss some development opportunities and pathways that can lead to more efficient doped-graphene based ORR electrocatalysts approaching the practical use for fuel cells and metal–air batteries.

Controllable synthesis of graphene-based titanium dioxide nanocomposites by atomic layer deposition

Atomic layer deposition (ALD) was used to synthesize graphene-based metal oxide nanocomposites. This strategy was fulfilled on the preparation of TiO(2)-graphene nanosheet (TiO(2)-GNS) nanocomposites using titanium isopropoxide and water as precursors. The synthesized nanocomposites demonstrated that ALD exhibited many benefits in a controllable means. It was found that the as-deposited TiO(2) was tunable not only in its morphologies but also in its structural phases. As for the former, TiO(2) was transferable from nanoparticles to nanofilms with increased cycles. With regard to the latter, TiO(2) was changeable from amorphous to crystalline phase, and even a mixture of the two with increased growth temperatures (up to 250 °C). The underlying growth mechanisms were discussed and the resultant TiO(2)-GNS nanocomposites have great potentials for many applications, such as photocatalysis, lithium-ion batteries, fuel cells, and sensors.