Gaixia Zhang

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.

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.

A facile synthesis of Fe3O4 nanoparticles/graphene for high-performance lithium/sodium-ion batteries

In this study, a facile, simple, and inexpensive co-precipitation method is used to fabricate diamond-like Fe3O4 nanoparticle/graphene composites for use as lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) electrode materials. In our synthesis, high-temperature treatment and complicated procedures and apparatus are avoided. Physical characterizations reveal that the as-prepared product is composed of a large fraction of diamond-like Fe3O4 nanoparticles uniformly distributed on thin graphene nanosheets. Compared to bare Fe3O4 and most of the previously reported studies, the as-obtained Fe3O4/graphene composite exhibits greatly enhanced electrochemical properties for both LIBs and SIBs, including excellent reversible capacity, superior cyclability and good rate performance. Specifically, when tested as an anode for LIBs, the Fe3O4/graphene composite shows specific capacity of 1430 mA h g−1 after 100 cycles at 200 mA g−1. The initial discharge capacity tested in SIBs is 855 mA h g−1, and after 40 cycles, the discharge capacity stabilizes at ∼210 mA h g−1 for 250 cycles. The excellent performance can be attributed to the greatly improved electrical conductivity, large surface area and excellent stability of the electrode material.

Highly Stable and Active Pt/Nb-TiO2 Carbon-Free Electrocatalyst for Proton Exchange Membrane Fuel Cells

The current materials used in proton exchange membrane fuel cells (PEMFCs) are not sufficiently durable for commercial deployment. One of the major challenges lies in the development of an inexpensive, efficient, and highly durable and active electrocatalyst. Here a new type of carbon-free Pt/Nb-TiO2 electrocatalyst has been reported. Mesoporous Nb-TiO2 hollow spheres were synthesized by the sol-gel method using polystyrene (PS) sphere templates. Pt nanoparticles (NPs) were then deposited onto mesoporous Nb-TiO2 hollow spheres via a simple wet-chemical route in aqueous solution, without the need for surfactants or potentiostats. The growth densities of Pt NPs on Nb-TiO2 supports could be easily modulated by simply adjusting the experimental parameters. Electrochemical studies of Pt/Nb-TiO2 show much enhanced activity and stability than commercial E-TEK Pt/C catalyst. PtNP/Nb-TiO2 is a promising new cathode catalyst for PEMFC applications.

Chemical and morphological characterizations of CoNi alloy nanoparticles formed by co-evaporation onto highly oriented pyrolytic graphite

CoNi alloy nanoparticles, formed by co-evaporation onto freshly cleaved highly oriented pyrolytic graphite (HOPG) surfaces, have been studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and scanning electron (SEM) and atomic force (AFM) microscopies. ToF-SIMS detected Co(x)Ni(y) fragments, indicating alloy formation. Even under ultra-high vacuum, the nanoparticles reacted with residual C- and O-containing gases to form surface contaminants (carbides, oxides, etc.) as revealed by both XPS and ToF-SIMS. On prolonged exposure to air, both the zerovalent metal and carbide peaks of each component decreased with time, as each metal reacted with atmospheric oxygen; as with the pure metals, the Co component of the alloy was the more reactive.

A specific demetalation of Fe–N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells

In this study, we explored the behavior of NC_Ar + NH3, an initially highly active catalyst for oxygen electroreduction, in H2/air fuel cells from 0.8 to 0.2 V at 80 °C and 25 °C, in order to find the causes of its instability. We discovered that the decay of the current density always involves the superposition of fast and slow first order kinetics, for which half-lives were obtained. The half-life of the fast decay was practically the same at all potentials and temperatures with a value of around 138 ± 55 min, while the half-life of the slow decay greatly varied from a minimum of ≈2400 min (40 h) to infinity. From the adsorption–desorption isotherm of NC_Ar + NH3, it was deduced that the Fe/N/C carbonaceous catalyst is characterized by interconnected open-end slit-shaped micropores, in which water (with dissolved H+ and O2) quickly flows in the fuel cells if their width is ≥0.7 nm as it has no interaction with the hydrophobic walls of the micropores. The driving force of this quick water flow is the humidified air streaming through the working cathode. Fe–N4-like active sites are thermodynamically stable in stagnant acidic conditions, but according to the Le Chatelier principle, they demetalate in the flux of water running into the micropores. This specific demetalation is the cause of the initial loss of ORR activity of NC_Ar + NH3 catalysts assigned to the fast current decay in fuel cells.

Controlled growth/patterning of Ni nanohoneycombs on various desired substrates.

We report a two-step process for the growth/patterning of Ni honeycomb nanostructures on various substrates, such as carbon paper, carbon nanotubes (CNTs), silicon wafers, and copper grids, via the combination of a sputter-coating/patterning technique and a replacement reaction solution method. The morphology, crystallinity, and chemical composition of the honeycombs were analyzed by SEM, TEM, high-resolution TEM, and EDX. These honeycombs are composed of numerous nanocells, several tens of nanometers in diameter and with cell wall thickness of approximately 10 nm, randomly connecting to each other. The growth process of honeycomb nanostructures has been systematically studied. Interestingly, the diameter and wall thickness of the cells could be easily tuned by simply adjusting the experimental parameters, such as the concentrations and cations of metal salts. Additionally, this simple method has been successfully extended to synthesize Co nanostructures with well-controlled morphologies, which indicates the great potential of this strategy in the synthesis of other metal nanostructures on various desired substrates. These metal-substrate composites, especially with desired patterns, are expected to be ideal candidates for wide application in modern electronic and optoelectronic devices, sensors, fuel cells, and energy storage systems.

New Insight into the Conventional Replacement Reaction for the Large-Scale Synthesis of Various Metal Nanostructures and their Formation Mechanism

Metal nanomaterials have attracted considerable interest, because of their unique sizeand shape-dependent chemical and physical properties, as well as their potential applications in catalysis, information storage, electrochemical devices, and biological and chemical sensing. Most methods reported so far for synthesizing such materials have focused on template or surfactant processes, electrochemical depositions and sol–gel approaches. However, such methods require that either the template/surfactant/ substrate be thoroughly removed for purifying the product or the reaction be conducted at elevated temperatures. Thus, there still remains much interest in exploring simpler and more versatile synthetic routes with a more wise control of nanomaterial morphology and structure. Galvanic replacement (transmetalation) reactions have a long history. However, it is only recently that approaches for synthesizing nanomaterials based on this reaction, involving sacrificial metals and suitable metal ions, have been employed and developed. Xia et al. and Sastry et al. 18] synthesized Pt, Au, and AuPt hollow structures by using pre-synthesized Ag nanostructures as templates by means of replacement reactions. Bai et al. synthesized Pt and AuPt bimetallic hollow nanostructures, exploiting pre-synthesized Co nanoparticles as sacrificial templates. Ag and Co are expensive metals, and the pre-synthesizing processes of these nanostructure templates further increase the cost; in addition, AgCl precipitated in the solution during reaction, which complicated the procedure and also influenced the yield of hollow products. Bhargava et al. used a galvanic replacement reaction to create Cu nanoscale pores in Ni foil. By employing the galvanic reaction between aqueous [Ag ACHTUNGTRENNUNG(NH3)2]OH and a copper plate, Yao et al. obtained a superhydrophobic pure silver film with flower-like microstructures. These porous structures were synthesized on bulk foil substrates, which may limit their applications. Here, we report a simple, cost effective and versatile strategy, which we call the commercial sacrificial metal-based replacement reaction (CSMRR), for synthesizing various metals (including magnetic, rare-earth, noble, etc.) with a series of novel nanostructures. The key difference between our method and those reported previously is that this is the first time very cheap, commercially available Mg/Al powders have been used, rather than any pre-synthesized nanostructures or bulk materials, to reduce the desired metal salt precursors. Specifically, the use of such Mg/Al powders as sacrificial metals has the following main advantages:

3D Porous Fe/N/C Spherical Nanostructures As High-Performance Electrocatalysts for Oxygen Reduction in Both Alkaline and Acidic Media

Exploring inexpensive and high-performance nonprecious metal catalysts (NPMCs) to replace the rare and expensive Pt-based catalyst for the oxygen reduction reaction (ORR) is crucial for future low-temperature fuel cell devices. Herein, we developed a new type of highly efficient 3D porous Fe/N/C electrocatalyst through a simple pyrolysis approach. Our systematic study revealed that the pyrolysis temperature, the surface area, and the Fe content in the catalysts largely affect the ORR performance of the Fe/N/C catalysts, and the optimized parameters have been identified. The optimized Fe/N/C catalyst, with an interconnected hollow and open structure, exhibits one of the highest ORR activity, stability and selectivity in both alkaline and acidic conditions. In 0.1 M KOH, compared to the commercial Pt/C catalyst, the 3D porous Fe/N/C catalyst exhibits ∼6 times better activity (e.g., 1.91 mA cm-2 for Fe/N/C vs 0.32 mA cm-2 for Pt/C, at 0.9 V) and excellent stability (e.g., no any decay for Fe/N/C vs 35 mV negative half-wave potential shift for Pt/C, after 10000 cycles test). In 0.5 M H2SO4, this catalyst also exhibits comparable activity and better stability comparing to Pt/C catalyst. More importantly, in both alkaline and acidic media (RRDE environment), the as-synthesized Fe/N/C catalyst shows much better stability and methanol tolerance than those of the state-of-the-art commercial Pt/C catalyst. All these make the 3D porous Fe/N/C nanostructure an excellent candidate for non-precious-metal ORR catalyst in metal-air batteries and fuel cells.