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TU Berlin

Inhalt des Dokuments

2008
P. Strasser, M. Özaslan, F. Hasché, S. Koh, C. Yu, R. Srivastava

Nanoparticulate bimetal core shell catalysts for fuel cells

Chemie Ingenieur Technik 80, 1267
Z. Lui, C. Yu, I. A. Rusakova, D. Huang, P. Strasser

Synthesis of Pt3Co Alloy Nanocatalyst via Reverse Micelle for Oxygen Reduction Reaction in PEMFCs

Topics in Catalysis 49 (3-4), 241-250

DOI: 10.1007/s11244-008-9083-2

Monodispersed, uniformly alloyed Pt3Co alloy nanoparticle electrocatalysts were synthesized via reduction of metallic precursors by sodium borohydride in heptane/polyethylene glycol dodecylether (Brij)/water reverse micelles. These particles were further adsorbed on XC-72R carbon powder, separated from micelles, and characterized using X-ray diffraction (XRD), transmission electronic microscopy (TEM). The electrochemical activity for the oxygen reduction reaction (ORR) was characterized using a Rotating Disk Electrode (RDE) technique. Even though residual surfactants on the metallic nanoparticle reduced the active surface area of the electrocatalytic particles, the catalytic activity of the prepared Pt3Co nanoparticles exhibited higher Pt mass and Pt surface area specific activities compared to pure Pt. The impact of heat treatment on the mean particle size, the electrochemical surface area (ESA), and on the activity was investigated and correlated to the residual surfactant coverage. Intermediate annealing temperatures resulted in larger ESA, despite particle growth pointing to lower surfactant coverage. Higher annealing temperatures caused large particle growth and reduced ESA, yet significant activity gains. A surface segregation mechanism resulting in a catalytically active Pt skin structure is hypothesized.
P. Strasser

Combinatorial Optimization of Ternary Pt Alloy Catalysts for the Electrooxidation of Methanol

J. Comb. Chem. 10 (2), 216-224

DOI: 10.1021/cc700166p

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We report the combinatorial and high-throughput optimization of improved ternary Pt alloy electrocatalysts for the oxidation of methanol for use in direct methanol fuel cell (DMFC) anodes. Following up on the discovery of a ternary Pt20Co60Ru20 catalyst(1) with significantly improved electrocatalytic activity for methanol oxidation over standard Pt−Ru catalysts, we optimize the electrocatalytic activity of this composition using a closely sampled Pt−Co−Ru “optimization library”. We also screen for compositional and structural stability using high-throughput methods. Composition−activity maps confirmed improved activity in compositional neighborhood of the Pt20Co60Ru20 catalyst. Activity trends of Pt−Ru binary alloys were in excellent agreement with fundamental surface electrochemical studies. Structural and compositional catalyst stability was probed using X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). Combination of the stability−composition and activity−composition maps resulted in “consensus maps” pointing to a new optimized ternary alloy electrocatalyst for methanol electrooxidation with an overall composition of Pt18Co62Ru20.
P. Mani, R. Srivastava, P. Strasser

Dealloyed Pt−Cu Core−Shell Nanoparticle Electrocatalysts for Use in PEM Fuel Cell Cathodes

J. Phys. Chem. C 112 (7), 2770-2778

DOI: 10.1021/jp0776412

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We report the synthesis, characterization, and single fuel cell testing of a novel class of nanostructured Pt−Cu alloy particle materials for use as oxygen reduction electrocatalyst in polymer electrolyte membrane fuel cells. The active phase of the Pt alloy nanoparticle catalysts is prepared by electrochemical dissolution (voltammetric dealloying) of Cu surface atoms from Cu-rich Pt−Cu alloy precursors. Bulk and surface structural and compositional characterization suggests that the dealloyed active catalyst phase consists of a core−shell structure in which a multilayer Pt rich shell is surrounding a Pt-poor alloy particle core. The electrocatalytic Pt mass activity of the dealloyed core−shell particles for the oxygen reduction reaction (ORR) exceeds that of state-of-the art Pt electrocatalyst by more than a factor of 4 and thus meets performance targets for fuel cell cathodes [Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B:  Environ. 2005, 56, 9−35].1 It is hypothesized that a reduced Pt−Pt distance near the particle surface, stabilized by the lattice-contracted alloy core, may explain the modification of the surface catalytic reactivity. Dealloying of base-metal-rich noble metal alloy precursors is proposed as a more general strategy toward modifying the surface catalytic properties of noble metal surfaces.
K.C. Neyerlin, R. Srivastava, P. Strasser

Electrochemical Stability of PtCu and PtCuCo Core-Shell Oxygen Reduction Reaction Electrocatalysts in Liquid Electrolyte

Proton Exchange Membran Fuel Cells 8, PTS 1 and 2 16 (2), 509-514

DOI: 10.1149/1.2981885

Pt25Cu75 and Pt20Cu20Co60 electrochemically dealloyed electrocatalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss relative to commercially available Pt/HSC and Pt/Vu catalysts. The difference in ECSA loss was shown to be directly related to the temperature at which the alloy catalysts were prepared (i.e. reduced/annealed), with higher temperatures resulting in greater resistance to ECSA loss. To normalize for preparation conditions, commercially available catalysts were exposed to a heat treatment process mimicking the annealing/reducing procedure for the Pt alloys.
P. Strasser, Z. Liu, S. Koh

Aging studies of Voltammetrically dealloyed Pt-Cu nanoparticle ORR electrocatalysts

Proton Exchange Membran Fuel Cells 8, PTS 1 and 2 16 (2), 515-522

DOI: 10.1149/1.2981886

Voltammetrically dealloyed Pt-Cu alloy nanoparticles show significantly improved Pt-mass and specific activities for the oxygen reduction reaction in acidic media in both RDE and MEA configurations. The active phase of the catalyst is prepared from very Cu-rich precursor materials which are formulated into catalyst inks and electrode layers. The high Cu content of the precursor materials raises fundamental and practical questions as to the molecular stability / oxidation state of the Cu rich catalyst surface and the shelf-life of precursor powders, inks and electrode layers. Here we present aging studies of a carbon-supported Pt25Cu75 nanoparticle catalyst precursor. We monitored how the surface area (ECSA) and the oxygen electroreduction (ORR) reactivity of dealloyed precursors (active form of catalyst) change as a function of the exposure time to air of the Pt25Cu75 precursor at room temperature. Similarly, we aimed to investigate how ECSA and ORR of the active catalyst changed with the time the liquid catalyst ink was exposed to air. We find little detrimental effect of air aging of the precursor on ORR activity over the time scales investigated. The effect of air aging of the catalyst inks is more complex, however. Initially, aged inks show increased ORR reactivity, that is, aged inks result in more active catalyst layers. After about 4-5 days of aging of the inks in air, however, we observed a drop in ORR activity of the resulting catalyst layers. This complex aging behavior of the inks seems independent of prior air aging of the dry powder precursor.
M.F. Toney, S. Koh, C. Yu, P. Strasser

Use of Anomalous X-ray Scattering for Probing the Structure, Composition and Size of Binary Alloy Nanoparticle Electrocatalysts

Proton Exchange Membran Fuel Cells 8, PTS 1 and 2 16 (2), 595-601

DOI: 10.1149/1.2981894

X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) are methods that are used to characterize the atomic structure and particle size of nanoparticle electrocatalysts. These methods do not inherently provide element specific information on structure and particle size. However, with the use of an anomalous scattering approach, one can obtain element specificity. This contribution describes these methods and applies these to fuel cell nanoparticle catalysts.
S. Koh, N. Hahn, C. Yu, P. Strasser

Effects of Annealing Conditions on Catalytic Activities of Pt-Cu Nanoparticle Electrocatalysts for PEM Fuel Cells

Proton Exchange Membran Fuel Cells 8, PTS 1 and 2 16 (2), 1093-1103

DOI: 10.1149/1.2981950

Dealloyed Pt25Cu75 bimetallic nanoparticle electrocatalysts exhibit up to 6 times higher oxygen reduction reaction activities than Pt-C catalysts. The effects of annealing temperature and duration on the catalyst activity are studied, with annealing temperature varied from 600 °C to 950 °C and for 7h and 14 h. XRD and electrochemical analyses is used to obtain insight to the structural details of the catalyst samples. Information regarding extend of alloying, Pt and Cu compositions and distributions on the nanoparticles and particle (crystallite) sizes is correlated with the trends observed from mass and specific activities of the catalysts. It is found that annealing duration of 14 h offers little or no benefit to catalytic activities compared to 7 h. Pt25Cu75 annealed for 7 h, at 800 °C was found as an optimal compromise between the extend of alloying and particle size growth to give the highest catalytic activity.
S. Koh, N. Hahn, C. Yu, P. Strasser

Effects of Compositions and Annealing Conditions on the Catalytic Activities of Pt-Cu Nanoparticle Electrocatalysts for PEMFCs

J. Electrochem. Soc. 155 (12), B1281-B1288

DOI: 10.1149/1.2988741

Dealloyed Pt25Cu75 bimetallic nanoparticle electrocatalysts exhibit up to six times higher oxygen reduction reaction activities than pure nanoparticle Pt catalysts at 0.9V / reversible hydrogen electrode (RHE). The active form of the catalyst is formed in situ from Pt–Cu precursor material using voltammetric dealloying. The effects of composition of precursors as well as effects of the annealing temperature and duration on the catalyst activity are studied. We vary the composition between Pt25Cu75 and Pt75Cu25 and change the annealing conditions from 600 to 950°C and for 7 and 14. X-ray diffraction and electrochemical analyses are used to obtain insight on the structural details of the catalyst samples. Information regarding the extent of alloying, atomic ordering, the Pt and Cu compositions, and distributions on the nanoparticles and particle (crystallite) sizes is correlated with the trends observed from mass and specific activities of the catalysts. It was found that an annealing duration of 14h offers little or no benefit to catalytic activities compared to 7h. Dealloyed Pt25Cu75 annealed for 7h, at 800°C yielded an optimal active material with respect to the extent of alloying and particle size growth and exhibited the highest Pt mass-based and favorable specific catalytic oxygen reduction reaction (ORR) activity. The occurrence and role of a noncubic Pt50Cu50 Hongshiite phase is discussed.
C. Yu, S. Koh, J. Leisch, M. Toney, P. Strasser

Size and composition distribution dynamics of alloy nanoparticle electrocatalysts probed by anomalous Small Angle X-Ray Scattering (ASAXS)

Faraday Discussions 140, 283-296

DOI: 10.1039/b801586d

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Anomalous small angle X-ray scattering (ASAXS) is shown to be an ideal technique to investigate the particle size and particle composition dynamics of carbon-supported alloy nanoparticle electrocatalysts at the atomic scale. In this technique, SAXS data are obtained at different X-ray energies close to a metal absorption edge, where the metal scattering strength changes, providing element specificity. ASAXS is used to, first, establish relationships between annealing temperature and the resulting particle size distribution for Pt25Cu75 alloy nanoparticle electrocatalyst precursors. The Pt specific ASAXS profiles were fitted with log-normal distributions. High annealing temperatures during alloy synthesis caused a significant shift in the alloy particle size distribution towards larger particle diameters. Second, ASAXS was used to characterize electrochemical Cu dissolution and dealloying processes of a carbon-supported Pt25Cu75 electrocatalyst precursor in acidic electrolytes. By performing ASAXS at both the Pt and Cu absorption edges, the unique power of this technique is demonstrated for probing composition dynamics at the atomic scale. These ASAXS measurements provided detailed information on the changes in the size distribution function of the Pt atoms and Cu atoms. A shift in the Cu scattering profile towards larger scattering vectors indicated the removal of Cu atoms from the alloy particle surface suggesting the formation of a Pt enriched Pt shell surrounding a Pt-Cu core. Together with XRD and TEM, ASAXS is proposed to play an increasingly important role in the mechanistic study of degradation phenomena of alloy nanoparticle electrocatalysts at the atomic scale.
P. Strasser, S. Koh, J. Greeley

Voltammetric Surface Dealloying of Pt bimetallic nanoparticles: An experimental and DFT computational analysis

Phys. Chem. Chem. Phys. 10 (25), 3670-3683

DOI: 10.1039/b803717e

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Voltammetric dealloying of bimetallic platinum–copper (Pt–Cu) alloys has been shown to be an effective strategy to modify the surface electrocatalytic reactivity of Pt bimetallic nanoparticles (S. Koh and P. Strasser, J. Am. Chem. Soc., 2007, 129, 12624). Using cyclic voltammetry and structural XRD studies, we systematically characterize the Pt–Cu precursor compounds as well as the early stages of the selective Cu surface dissolution (dealloying) process for Pt25Cu75, Pt50Cu50, and Pt75Cu25 alloy nanoparticles annealed at both low and high temperature. We also assess the impact of the synthesis conditions on the electrocatalytic reactivity for the oxygen reduction
reaction (ORR). To gain atomistic insight into the observed voltammetric profiles, we compare our experimental results with periodic DFT calculations of trends in the thermodynamics of surface Cu dissolution potentials from highly stepped and kinked Pt(854) single crystal surfaces. The modeling suggests a dependence of the electrochemical Cu dissolution potentials on the detailed atomic environment (coordination number, nature of coordinating atoms) of the bimetallic Pt–Cu surfaces. The DFT-predicted shifts in electrochemical Cu dissolution potentials are shown to qualitatively account for the observed voltammetric profiles during Cu dealloying. Our study suggests that metal-specific energetics have to be taken into account to explain the detailed dealloying behavior of bimetallic surfaces.

P. Strasser

Fuel cell catalyst particles have platinum-rich shell, copper core

Advanced Materials and Processes 166 (1), 13-13

 

 

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