Showing papers on "Platinum published in 2005"
TL;DR: In this article, the authors quantified the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state of the art platinum electrocatalysts.
Abstract: The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.
4,298 citations
TL;DR: In this article, a short-stack of proton exchange membrane fuel cells PEMFCs operated at open-circuit voltage 0.95 V was shown to have higher surface area loss than another operated under load 0.75 V.
Abstract: Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H2SO4 at 80°C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells PEMFCs operated at open-circuit voltage 0.95 V was shown to be higher than another operated under load 0.75 V. Both findings suggest that the formation of soluble platinum species such as Pt 2+ plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode from 63 to 23 m 2 /gPt in 100 h was observed upon potential cycling, a cycled membrane electrode assembly MEA cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy TEM to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: i Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from 3t o6 nm on carbon, and ii migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H2 molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to 50% of the overall platinum area loss of the potential cycled electrode.
1,322 citations
1,061 citations
TL;DR: These new air-stable complexes using the bis(trifluoromethanesulfonyl)imidate moiety as a weakly coordinating counter-anion are described and are exceedingly active for the catalysis of a wide range of enynes cycloisomerizations.
594 citations
TL;DR: The results demonstrated that high-activity electrocatalysts can be devised that contain only a fractional amount of Pt and a very small amount of another noble metal.
Abstract: We synthesized a new class of O2 electrocatalysts with a high activity and very low noble metal content. They consist of Pt monolayers deposited on the surfaces of carbon-supported nonnoble metal−noble metal core−shell nanoparticles. These core−shell nanoparticles were formed by segregating the atoms of the noble metal on to the nanoparticles' surfaces at elevated temperatures. A Pt monolayer was deposited by galvanic displacement of a Cu monolayer deposited at underpotentials. The mass activity of all the three Pt monolayer electrocatalysts investigated, viz., Pt/Au/Ni, Pt/Pd/Co, and Pt/Pt/Co, is more than order of magnitude higher than that of a state-of-the-art commercial Pt/C electrocatalyst. Geometric effects in the Pt monolayer and the effects of PtOH coverage, revealed by electrochemical data, X-ray diffraction, and X-ray absorption spectroscopy data, appear to be the source of the enhanced catalytic activity. Our results demonstrated that high-activity electrocatalysts can be devised that contain ...
561 citations
429 citations
TL;DR: In this paper, a variety of synthetic approaches are employed to shed light on the influence of nanoparticle agglomeration on their electrocatalytic properties, showing that the reaction is strongly size sensitive, exhibiting an increase of the reaction overpotential as the particle size decreases below ca. 3 nm.
Abstract: Fuel cell electrocatalysts usually feature high noble metal contents, and these favour particle agglomeration. In this paper a variety of synthetic approaches (wet chemical deposition, electrodeposition and electrodeposition on chemically preformed Pt nuclei) is employed to shed light on the influence of nanoparticle agglomeration on their electrocatalytic properties. Pt loading on model glassy carbon (GC) support is increased systematically from 1.8 to 10.6 μg Pt cm−2 and changes in the catalyst structure are followed by transmission electron microscopy. At low metal loadings (≤5.4 μg Pt cm−2) isolated single crystalline Pt nanoparticles are formed on the support surface by wet chemical deposition from H2PtCl4 precursor. An increase in the metal loading results, first, in a systematic increase of the average diameter of isolated Pt nanoparticles and, second, in coalescence of nanoparticles and formation of particle agglomerates. This behaviour is in line with the previous observations on carbon-supported noble metal fuel cell electrocatalysts. The catalytic activity of Pt/GC electrodes is tested in CO monolayer oxidation. In agreement with the previous studies (F. Maillard, M. Eikerling, O. V. Cherstiouk, S. Schreier, E. Savinova and U. Stimming, Faraday Discuss., 2004, 125, 357), we find that the reaction is strongly size sensitive, exhibiting an increase of the reaction overpotential as the particle size decreases below ca. 3 nm. At larger particle sizes the dependence levels off, the catalytic activity of particles with diameters above 3 nm approaching that of polycrystalline Pt. Meanwhile, Pt agglomerates show remarkably enhanced catalytic activity in comparison to either isolated Pt nanopraticles or polycrystalline Pt foil, catalysing CO monolayer oxidation at ca. 90 mV lower overpotential. Enhanced catalytic activity of Pt agglomerates is ascribed to high concentration of surface defects. CO stripping voltammograms from Pt/GC electrodes, comprising Pt agglomerates along with isolated single crystalline Pt nanoparticles from 2 to 6 nm size, feature double voltammetric peaks, the more negative corresponding to CO oxidation on Pt agglomerates, while the more positive to CO oxidation on isolated Pt nanoparticles. It is shown that CO stripping voltammetry provides a fingerprint of the particle size distribution and the extent of particle agglomeration in carbon-supported Pt catalysts.
370 citations
TL;DR: Data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles, and could be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol.
Abstract: We present a study on the catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes that act as carriers. The spherical polyelectrolyte brushes consist of a solid core of poly(styrene) onto which long chains of poly(2-methylpropenoyloxyethyl) trimethylammonium chloride are grafted. These positively charged chains form a dense layer of polyelectrolytes on the surface of the core particles ("spherical polyelectrolyte brush") that tightly binds divalent PtCl6-(2) ions. The reduction of these ions within the brush layer leads to nearly monodisperse nanoparticles of metallic platinum. The average size of the particles is approximately 2 nm. The composite particles exhibit excellent colloidal stability. The catalytic activity is investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4 in the presence of the nanoparticles. The kinetic data could be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. In all cases, a delay time t0 has been observed, after which the reactions start. This time is shorter when the catalyst has already been used. All data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles.
345 citations
TL;DR: In this paper, a potential cycling test between 0.87 and 1.2 V versus RHE was applied to the system to illustrate how cobalt or platinum dissolution might affect the cell performance and the results indicated that cobalt dissolution neither detrimentally reduces the cell voltage nor dramatically affects the membrane conductance.
329 citations
TL;DR: This work demonstrates that Pt-CNT nanocomposites synthesized in supercritical carbon dioxide are effective electrocatalysts for low-temperature fuel cells.
Abstract: Carbon nanotube (CNT)-supported Pt nanoparticle catalysts have been synthesized in supercritical carbon dioxide (scCO2) using platinum(II) acetylacetonate as metal precursor. The structure of the catalysts has been characterized with transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images show that the platinum particles' size is in the range of 5−10 nm. XPS analysis indicates the presence of zero-valence platinum. The Pt−CNT exhibited high catalytic activity both for methanol oxidation and oxygen reduction reaction. The higher catalytic activity has been attributed to the large surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. Cyclic voltammetric measurements at different scan rates showed that the oxygen reduction reaction at the Pt−CNT electrode is a diffusion-controlled process. Analysis of the electrode kinetics using Tafel plot suggests that Pt−CNT from scCO2 provides a strong electrocata...
307 citations
TL;DR: It was found that the morphology, composition, and crystal phase of the resultant CoPt alloy nanomaterials could be controlled by changing the concentration and molar ratio of platinum and cobalt reactant precursors.
Abstract: Cobalt platinum nanorods, hyperbranched nanorods, and nanoparticles were synthesized in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BMIM][Tf2N], ionic liquid though the thermal reduction of platinum acetylacetonate, and cobalt acetylacetonate at 350 °C in the presence of cetyltrimethylammonium bromide (CTAB). It was found that the morphology, composition, and crystal phase of the resultant CoPt alloy nanomaterials could be controlled by changing the concentration and molar ratio of platinum and cobalt reactant precursors.
TL;DR: In this paper, the potentials and limits of sulfonic acid and alternatively phosphonic acid as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures and low humidification are discussed.
Abstract: Traditionally, sulfonated polymers are used as separator materials in PEM fuel cells. Based on recent experimental results on model compounds this paper critically discusses the potentials and limits of sulfonic acid and alternatively phosphonic acid and heterocycles (imidazole) as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures (T > 100 °C) and low humidification. Apart from transport properties, the stability and reactivity of mono-functionalized model compounds (1-heptylsulfonic acid (S-C7), 1-heptylphosphonic acid (P-C7) and 2-heptyl-imidazole (I-C7)) and a few diphosphonic acids are examined under wet and dry conditions. These are characterized with respect to their proton conductivity (ac impedance spectroscopy), proton diffusion coefficient (pulsed-field gradient NMR), thermo-oxidative stability (TGA under air), electrochemical stability (cyclic voltammetry) and their hydration behavior (TGA under water vapor). The sulfonic acid functionalized compound shows reasonable properties only when a minimum hydration level is guaranteed, while phosphonic acid functionalized compounds combine satisfactory proton conductivity even in the water-free state at intermediate temperatures (T < 200 °C), comparatively high thermo-oxidative and electrochemical stability and electrochemical reactivity (hydrogen oxidation and oxygen reduction at platinum surfaces). The presence of water leads to moderate water uptake allowing for reasonable conductivities even at room temperature and prevents condensation reactions at higher temperature. The imidazole based system shows the largest electrochemical stability window, but its moderate proton conductivity and thermo-oxidative stability and the very high overpotential for oxygen reduction on platinum turn out to be severe disadvantages for the envisaged application.
TL;DR: In this article, a sub-stoichiometric titanium oxide, Ti4O7-supported Pt electrocatalysts for polymer electrolyte fuel cells (PEFCs) were prepared, and the electrochemical properties under PEFC operating conditions were examined.
TL;DR: In this article, carbon-supported catalysts were held at elevated temperatures under dry air conditions, and the carbon support was evaluated with a first-order kinetic model to describe the results.
TL;DR: A highly effective synthesis of platinum multipods from platinum 2,4-pentanedionate in organic solvents is reported, resembling those observed for II-VI semiconducting materials, such as CdS and CdSe.
Abstract: This paper reports a highly effective synthesis of platinum multipods from platinum 2,4-pentanedionate in organic solvents. A trace amount of silver acetylacetonate is used to trigger the nucleation and the anisotropic growth of Pt nanocrystals. The morphologies of Pt multipods made include I- and V-shaped bipods, various types of tripods, and planar and three-dimensional (3D) tetrapods. The 3D Pt tetrapods can be well-defined, resembling those observed for II-VI semiconducting materials, such as CdS and CdSe. Control of morphology and the multipod-to-sphere transitions under various conditions have been systematically studied. A mode of formation based on the induced kinetically controlled growth has been discussed.
TL;DR: In this article, the impact of Pt promoter loading on catalyst structural-property relationships was explored, and it was shown that Pt may not only serve to facilitate the generation of the bridging OH group active sites at low temperature, but may also be involved in accelerating surface formate decomposition.
TL;DR: The trans influence of boryl ligands has been investigated theoretically via density functional theory (DFT) calculations on a series of square-planar platinum(II) complexes of the form trans-[PtL(Cl)(PMe3)2].
Abstract: In this paper, the trans influence of boryl ligands, together with that of other ligands commonly believed to have a strong trans influence, has been investigated theoretically via density functional theory (DFT) calculations on a series of square-planar platinum(II) complexes of the form trans-[PtL(Cl)(PMe3)2]. The following order of trans influence has been obtained: −BMe2 > −SiMe3 > −BH2 > −SnMe3 ∼ > −Bpin > > ∼ −Bcat ∼ −BCl2 ∼ −BBr2 ∼ −SiH3 > −CH2CH3 > −CHCH2 > −H ∼ −Me > −C6H5 > −SiCl3 > −SnCl3 > −C⋮CH. Natural bond order analyses have been used to understand how the substituents at the boron center affect the trans-influence properties of the boryl ligands. The major factor is the σ-donor strength of the boryl ligand. However, surprisingly, very strong π acceptors also enhance the trans influence.
TL;DR: To gain insights into the controlling surface chemistry, density functional theory calculations for the energy of dehydrogenation were used to approximate the potential-dependent methanol dehydration pathways over aqueous-solvated platinum interfaces.
Abstract: The dual path mechanism for methanol decomposition on well-defined low Miller index platinum single crystal planes, Pt(111), Pt(110), and Pt(100), was studied using a combination of chronoamperometry, fast scan cyclic voltammetry, and theoretical methods. The main focus was on the electrode potential range when the adsorbed intermediate, CO(ad), is stable. At such "CO stability" potentials, the decomposition proceeds through a pure dehydrogenation reaction, and the dual path mechanism is then independent of the electrode-substrate surface structure. However, the threshold potential where the decomposition of methanol proceeds via parallel pathways, forming other than CO(ad) products, depends on the surface structure. This is rationalized theoretically. To gain insights into the controlling surface chemistry, density functional theory calculations for the energy of dehydrogenation were used to approximate the potential-dependent methanol dehydrogenation pathways over aqueous-solvated platinum interfaces.
TL;DR: A number of gamma- and delta-amino olefins underwent intramolecular hydroamination to form the corresponding pyrrolidine derivatives in moderate to good yield and displayed excellent functional group compatibility and low moisture sensitivity.
Abstract: Reaction of benzyl-2,2-diphenyl-4-pentenylamine with a catalytic 1:2 mixture of [PtCl2(H2CCH2)]2 (2.5 mol %) and PPh3 in dioxane at 120 °C for 16 h led to isolation of 1-benzyl-2-methyl-4,4-diphenylpyrrolidine in 75% yield. A number of γ- and δ-amino olefins underwent intramolecular hydroamination to form the corresponding pyrrolidine derivatives in moderate to good yield. The reaction displayed excellent functional group compatibility and low moisture sensitivity.
TL;DR: Results showed that the ordered macroporous materials with enhanced selectivity and sensitivity are promising for fabrication of nonenzymatic glucose biosensors.
Abstract: A three-dimensionally ordered, macroporous, inverse-opal platinum film was synthesized electrochemically by the inverted colloidal-crystal template technique. The inverse-opal film that contains platinum nanoparticles showed improved electrocatalytic activity toward glucose oxidation with respect to the directly deposited platinum; this improvement is due to the interconnected porous structure and the greatly enhanced effective surface area. In addition, the inverse-opal Pt-film electrode responds more sensitively to glucose than to common interfering species of ascorbic acid, uric acid, and p-acetamidophenol due to their different electrochemical reaction mechanisms. Results showed that the ordered macroporous materials with enhanced selectivity and sensitivity are promising for fabrication of nonenzymatic glucose biosensors.
TL;DR: Improved electrochemical behavior of L-cysteine (CySH) on platinum (Pt)/carbon nanotube (CNT) electrode was investigated by cyclic voltammetry and a sensitive CySH sensor was developed based on Pt/CNT/graphite electrode.
TL;DR: In this paper, the electrocatalytic performance of iron phthalocyanine, FePc, dispersed on a high surface area carbon substrate (Vulcan XC 72) was investigated in acid medium.
TL;DR: A review of carbon fluorine bond activation taking place at platinum group metal (PGM) coordination compounds is presented in this article, where the relevant examples in the area of fluorocarbon coordination to PGM leading to, at least, one C F bond cleavage by surveying the work reported since 1994, covering the literature up to August 2004.
TL;DR: Electrochemical impedance spectroscopy revealed that the carbon nanotube-based electrodes exhibited an order of magnitude lower charge-transfer reaction resistance (R(ct)) for the hydrogen evolution reaction (HER) than did the commercial carbon black (CB)-based electrodes.
Abstract: A membrane electrode assembly (MEA) for hydrogen fuel cells has been fabricated using single-walled carbon nanotubes (SWCNTs) support and platinum catalyst. Films of SWCNTs and commercial platinum (Pt) black were sequentially cast on a carbon fiber electrode (CFE) using a simple electrophoretic deposition procedure. Scanning electron microscopy and Raman spectroscopy showed that the nanotubes and the platinum retained their nanostructure morphology on the carbon fiber surface. Electrochemical impedance spectroscopy (EIS) revealed that the carbon nanotube-based electrodes exhibited an order of magnitude lower charge-transfer reaction resistance (Rct) for the hydrogen evolution reaction (HER) than did the commercial carbon black (CB)-based electrodes. The proton exchange membrane (PEM) assembly fabricated using the CFE/SWCNT/Pt electrodes was evaluated using a fuel cell testing unit operating with H2 and O2 as input fuels at 25 and 60 °C. The maximum power density obtained using CFE/SWCNT/Pt electrodes as b...
TL;DR: PtRu/C nanocatalysts were prepared by changing the molar ratio of citric acid to platinum and ruthenium metal salts from 1: 1, 2:1, 3:1 to 4:1 using sodium borohydride as a reducing agent as mentioned in this paper.
TL;DR: The results demonstrated in this paper open up methods to determine the atomic distribution of bimetallic NPs, which is an extremely important parameter that strongly influences the physicochemical properties of NPs and their applications.
Abstract: In this report, we describe a general methodology to determine the extent of alloying or atomic distribution quantitatively in bimetallic nanoparticles (NPs) by X-ray absorption spectroscopy (XAS). The structural parameters determined in these studies serve as a quantitative index and provide a general route to determine the structural aspects of the bimetallic NPs. We have derived various types of possible structural models based on the extent of alloying and coordination number parameters of bimetallic NPs. We also discussed the nature of homo- and heterometallic interactions in bimetallic NPs based on the extent of alloying. Herein, we use carbon-supported platinum-ruthenium bimetallic nanoparticles to demonstrate the proposed methodology, and this can be extended further to get more insights into the alloying extent or atomic distribution of other bimetallic systems. The results demonstrated in this paper open up methods to determine the atomic distribution of bimetallic NPs, which is an extremely important parameter that strongly influences the physicochemical properties of NPs and their applications.
TL;DR: In this paper, a high ordered meso-porous carbon, denoted CMK-3 was synthesized by using mesoporous silicates, SBA-15 as the starting templating materials.
TL;DR: In this article, a mathematical model of platinum dissolution and movement through the layers of a polymer electrolyte membrane (PEM) fuel cell is presented based on dilute-solution theory.
Abstract: This paper presents a mathematical model of platinum dissolution and movement through the layers of a polymer electrolyte membrane (PEM) fuel cell. The model is based on dilute-solution theory. Dilute solution equations are used to describe the movement of soluble platinum through the PEM.
TL;DR: In this paper, the performance of Pt/CNT electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs) is analyzed with respect to catalyst synthesis and Pt loading.