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TU Berlin

Inhalt des Dokuments

2007
S. Koh, P. Strasser

Electrocatalysis on Bimetallic Surfaces:  Modifying Catalytic Reactivity for Oxygen Reduction by Voltammetric Surface Dealloying

J. Am. Chem. Soc. 129 (42), 12625-12626

DOI: 10.1021/ja0742784

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We report a synthetic electrochemical strategy to deliberately modify the catalytic reactivity of Pt bimetallic surfaces. The strategy consists of voltammetric surface dealloying of the non-noble constituent from Pt-poor bimetallic precursor compounds. We exemplify this method by dealloying carbon-supported Pt25Cu75 alloy nanoparticle precursors and testing the resulting active catalyst phase for the oxygen reduction reaction (ORR). We show that dealloyed Pt−Cu electrocatalysts exhibit an extraordinary increase in intrinsic reactivity of 4−6 times as compared to pure Pt electrocatalysts. Our results indicate that electrochemical treatment of the alloy precursors selectively dissolves Cu near the particle surface. The partially dealloyed particles constitute the active catalyst phase. While Cu is retained in the core of the particles after dealloying, the essentially pure Pt surface suggests a core−shell structure of the active catalyst. Geometric effects, such as exposure of more active crystallographic facets or a more favorable Pt−Pt surface interatomic distance are proposed to play a key role in the enhancement mechanism. This work suggests that the selective electrochemical dissolution (dealloying) of non-noble components from noble metal bimetallics can serve as a general strategy toward tuning surface electrocatalytic properties.
S. Koh, C. Yu, P. Mani, R. Srivastava, P. Strasser

Activity of ordered and disordered Pt-Co alloy phases for the electroreduction of oxygen in catalysts with multiple coexisting phases

J. Power Sources 172 (1), 50-56

DOI: 10.1016/j.jpowsour.2007.01.002

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This study investigates the relative electrochemical activity of ordered and disordered Pt-Co alloy phases coexisting in multi-phase catalyst materials. Of particular interest is the effect of the relative distribution between ordered and disordered Pt-Co alloy phases on the observed electrocatalytic activity for the oxygen reduction reaction (ORR). Three Pt-Co catalysts with identical overall composition, Pt50Co50, but with distinct distributions between two disordered face centered cubic (fcc) Pt-Co alloy phases and one ordered face centered tetragonal (fct) alloy phase are considered. Comparing the structure of the catalysts with their electrocatalytic activity for ORR suggests that the Co-rich (60–80 at% Co) disordered phase is linked to the observed 3× activity enhancement compared to a pure Pt catalyst. If the ordered fct phase outweighs the Co-rich disordered phase the activity drops drastically. It is concluded that Co-rich disordered phases are the preferred Pt-Co alloy phases with respect to catalyst activity.
S. Koh, C. Yu, J. Leisch, M.F. Toney, P. Strasser

Size and composition distribution dynamics of Pt alloy nanoparticle electrocatalysts probed by anomalous small angle X-ray scattering

Abstracts of Papers of the American Chemical Society 233
S. Koh, J. Leisch, M.F. Toney, P. Strasser

Structure-Activity-Stability Relationships of Pt−Co Alloy Electrocatalysts in Gas-Diffusion Electrode Layers

J. Phys. Chem C 111 (9), 3744-3752

DOI: 10.1021/jp067269a

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We establish relationships between the atomic structure, composition, electrocatalytic activity, and electrochemical corrosion stability of carbon-supported Pt−Co alloy nanoparticles in electrode catalyst layers. These Pt−Co catalysts have received much attention for use as cathode layers in polymer electrolyte membrane fuel cells (PEMFCs) because of their favorable oxygen-reduction-reaction (ORR) activity and suspected corrosion stability. We reported an enhancement of activity of low-temperature Pt50Co50 of 3 times that of pure carbon supported Pt catalysts. The use of synchrotron X-ray diffraction has enabled structural characterization of the alloy nanoparticles both before and, importantly, after electrocatalysis under fuel cell like conditions. From this, a detailed picture of the relative activity and stability of Pt−Co alloy phases as a function of synthesis conditions has emerged. We have investigated the structure, composition, chemical ordering, and concentration of Pt−Co alloy phases in (i) a dry, freshly synthesized nanoparticle catalyst, (ii) the catalytic electrode layer in a proton-conducting polymer electrolyte before electrocatalytic activity, and (iii) the same electrode layer after electrocatalytic activity. We find that Pt50Co50 catalysts annealed at 600 °C consist of multiple phases:  a chemically ordered face-centered tetragonal (fct) and two chemically disordered face-centered cubic (fcc) phases with differing stoichiometries. The Co-rich fcc phase suffers from corrosive Co loss during the preparation of conducting polymer electrode layers and, more significantly, during the ORR electrocatalysis. Most importantly, these fcc phases exhibit high catalytic activities for ORR (about 3× compared to a pure Pt electrocatalyst). Pt50Co50 catalysts annealed at 950 °C consist mainly of the fct Pt50Co50 phase. This phase shows favorable stability to corrosion in the conducting polymer electrode and during electrocatalysis, as the relative intensities of fcc(111)/fct(101) peak ratio remained consistently around 0.5 before and after preparation of conducting polymer electrode layers and before and after electrochemical measurements; however, it exhibits a lower catalytic ORR activity compared to the low-temperature fcc alloy phases (about 2.5× compared to a pure Pt electrocatalyst). Our results demonstrate the complexity in these multiphase materials with respect to catalyst activity and degradation. By understanding of the relationships between crystallographic phase, chemical ordering, composition, and the resulting electrochemical activity and corrosion stability of fuel cell catalysts within polymer-electrolyte/catalyst composites, we can move toward the rational design of active and durable catalyst materials for PEMFC electrodes.
S. Koh, M.F. Toney, P. Strasser

Activity–stability relationships of ordered and disordered alloy phases of Pt3Co electrocatalysts for the oxygen reduction reaction (ORR)

Electrochim. Acta 52 (8), 2765-2774

DOI: 10.1016/j.electacta.200608039

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We report on synthesis–structure–activity–stability relationships of Pt3Co nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). We have synthesized Pt3Co alloy electrocatalysts using liquid impregnation techniques followed by reductive annealing at high and low temperatures. We have performed detailed structural X-ray diffraction (XRD)-based structural characterization (symmetry, lattice parameters and composition) of individual Pt–Co alloy phases before and, importantly, after electrochemical rotating disk electrode (RDE) measurements. This enables us to directly evaluate the corrosion stability of various Pt–Co alloy phases under typical fuel cell cathode conditions.
Pt3Co prepared at low annealing temperatures (600 °C) resulted in multiple phases including (i) a disordered face-centered cubic (fcc) Pt95Co5 phase and (ii) an ordered face-centered tetragonal (L10) Pt50Co50 phase; high temperature annealing (950 C) resulted in a single ordered primitive cubic (L12) Pt3Co phase. The ordered alloy phases in both catalysts were not stable under electrochemical treatment: The ordered face-centered tetragonal (fct) phase showed corrosion and dissolution, while the ordered primitive cubic (L12) Pt3Co phase transformed into a disordered structure. The ordered primitive cubic structure exhibited higher resistance to sintering.
Low annealing temperatures resulted in higher Pt surface-area specific activities for ORR. Kinetic Tafel analysis confirmed a general shift in the formation potential of oxygenated surface species, such as Pt–OH, for both alloy catalysts. Reduced OH coverage alone proved insufficient to account for the observed activity trends of the two alloy catalysts.
R. Srivastava, P. Mani, N. Hahn, P. Strasser

Efficient Oxygen Reduction Fuel Cell Electrocatalysis on Voltammetrically Dealloyed Pt–Cu–Co Nanoparticles

Angew. Chem. Int. Ed. 46 (47), 8988-8991

DOI: 10.1002/anie.200703331

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Getting rid of copper: A class of ternary Pt–Cu–Co electrocatalysts for the reduction of oxygen in polymer electrolyte membrane fuel cells shows unprecedented activity improvements over state-of-the-art Pt catalysts. The active phase of the catalysts is synthesized by selective electrochemical dissolution (dealloying, see picture) of Cu-rich alloy-particle precursors, resulting in Pt-enriched core–shell particles.
Z. Liu, S. Koh, C. Yu, P. Strasser

Synthesis, Dealloying, and ORR Electrocatalysis of PDDA-Stabilized Cu-Rich Pt Alloy Nanoparticles

J. Electrochem. Soc. 154 (11), B1192-B1199

DOI: 10.1149/1.2778856

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We report on the polymeric surfactant-assisted synthesis and characterization of highly dispersed, uniformly alloyed Cu-rich PtCu bimetallic nanoparticles with Cu contents up to 75atom% Cu for use as an oxygen reduction reaction (ORR) electrocatalyst at polymer electrolyte membrane fuel cells. Base-metal-rich Pt alloy particles are generally very difficult to prepare as a well-alloyed single-phase material with high dispersion using conventional liquid impregnation/reductive annealing routes. A comparison between the characteristics of PtCu alloy particle obtained from a poly(dimethyl-diallyl ammonium) chloride (PDDA) assisted polyol process and a conventional impregnation method shows that the polyol process is able to form single-phase nanoparticles with a very narrow particle size at temperatures below 200°C. We further investigated the electrochemical behavior of Cu-rich electrocatalysts. We demonstrate the formation of highly active ORR PtCu catalyst phases from a PtCu3 precursor by selective electrochemical dissolution (dealloying) of Cu. The dealloyed catalyst yields ORR surface-area-specific activities rivaling those of the Pt-rich state-of-the-art Pt3Co electrocatalysts. Based on the severe depletion in Cu near the surface combined with moderate surface area increases, we propose geometric rather than electronic or surface-area effects as the origin of the observed activity enhancement.
P. Strasser

Combinatorial development of ternary electrocatalysts, for methanol oxidation

Proceedings of the 2nd Energy Nanotechnology International Conference, 21-26
vor 2007
S. Koh, N. Hahn, P. Strasser,
Corrosion and ORR activity of Pt alloy electrocatalysts during voltammetric pretreatment
,
ECS Transactions, 3 (1), 139-149 (2006
P. Strasser, S. Koh,
Fuel 221-Lattice-strained Pt shell nanoparticle catalysts for the electroreduction of oxygen at PEMFC cathodes,
Abstracts of Papers of the American Chemical Society, 232, (2006)
K. Yaccato, R. Carthart, A. Hagemeyer, A. Lesik, P. Strasser, A. Volpe, H. Turner, H. Weinberg, R. Grassselli, C. Brooks,
Competitive CO and CO2 methanation over supported noble metal catalysts in high troughput scanning mass spectrometer,
Applied Catalysis A, 296 (1), 30-48 (2005), DOI: 10.1016/j.apcata.2005.07.052
P. Strasser, Q. Fan, M. Devenney, W.H. Weinberg, P. Liu, J.K. Norskov,
High Throughput Experimental and Theoretical Predictive Screening of Materials – A Comparative Study of Search Strategies for New Fuel Cell Anode Catalysts
,
J. Phys. Chem. B, 107 (40), 11013-11021 (2003), DOI: 10.1021/jp030508z
M. Chatterjee, A. K. Chava, G. Kohla, S. Pal, A. Merling, S. Hinderlich, U. Unger, P. Strasser, G. J. Gerwig, J. P. Kamerling, R. Vlasak, P. R. Crocker, R. Schauer, R. Schwartz-Albiez, C. Mandal,
Identification and characterization of adsorbed serum sialoglycans on Leishmania donovani promastigotes,
Glycobiology, 13 (5), 351-361 (2003), DOI: 10.1093/glycob/cwg027
J. Lee, J. Christoph, P. Strasser, M. Eiswirth, G. Ertl,
Existence regions of spatiotemporal patterns in the electro-oxidation of formic acid,
Phys. Chem. Chem. Phys., 5 (5), 935-938 (2003), DOI: 10.1039/b209434
J. Lee, P. Strasser, M. Eiswirth, G. Ertl,
On the origin of oscillations in the electrocatalytic oxidation of HCOOH on a Pt electrode modified by Bi deposition,
Electrochim. Acta, 47 (3), 501-508 (2001), DOI: 10.1016/S0013-4686(01)00744-7
J. Lee, J. Christoph, P. Strasser, M. Eiswirth, G. Ertl,
Spatio-temporal interfacial potential patterns during the electrocatalyzed oxidation of formic acid on Bi-modified Pt,
J. Chem. Phys., 115 (3), 1485-1492 (2001), DOI: 10.1063/1.1379535
P. Strasser,
Electrochemistry in Self-Organized States,
Interface, 9, 46 (2000)
P. Strasser, J. Christoph, W.F. Lin, M. Eiswirth, J.L. Hudson,
Standing Wave oscillations in an electrocatalytic reaction,

J. Phys .Chem. A, 104 (9), 1854-1860 (2000), DOI: 10.1021/jp993061w
S. Krömker, A.F. Münster, J. Christoph, P. Strasser, M. Eiswirth,
Diffusion-induced instabilities in stoichiometric networks,
Communicaciones de la Universidad de Cuernavaca, 35 (2000)
P. Strasser, M. Eiswirth, M.T.M. Koper,
Mechanistic Classification of electrochemical oscillators - operational experimental strategy,
J. Electroanal. Chem., 478 (1-2), 50-66 (1999), DOI: 10.1016/S0022-0728(99)00412-X
F. Fechner, P. Strasser, M. Eiswirth, F.W. Schneider, A.F. Münster,
Spatial entrainment during the polymerization of acrylamid in the presence of the methylene blue-sulfide chemical oscillator,
Chem. Phys. Lett., 313 (1-2), 205-210 (1999), DOI: 10.1016/S0009-2614(99)01033-7
J. Christoph, P. Strasser,
Waving in the distance,
Abstracts of Papers of the American Chemical Society, 218, U277-U277 (1999)
J. Christoph, P. Strasser, M. Eiswirth, G. Ertl,
Remote triggering of waves in an electrocatalytic system,
Science, 284 (5412), 291-293 (1999), DOI: 10.1126/science.284.5412.291
P. Strasser, M. Luebke, C. Eickes, M. Eiswirth,
Modeling galvanostatic potential oscillations in the electrocatalytic iodate reduction system,
J. Electroanal. Chem., 462 (1), 19-33 (1999), DOI: 10.1016/S0022-0728(98)00386-6
P. Strasser, M. Ata,
Electrochemical Synthesis of polymerized LiC60 films,
J. Phys. Chem. B, 102 (21), 4131-4134 (1998), DOI: 10.1021/jp980431s
P. Strasser, M. Luebke, P. Parmananda, M. Eiswirth, G. Ertl,
Mechanistic Analysis of electrochemical oscillators using derivative feedback control techniques,
J. Phys. Chem. B, 102 (17), 3227-3237 (1998), DOI: 10.1021/jp9801572
R. Alessio, D. Belli-Dell`Amico, F. Calderazzo, U. Englert, A. Guarini, L. Labella, P. Strasser,
N,N-Dialkylecarbamato Complexes of the d(10) cations of Copper, Silver and Gold,
Helv. Chim. Acta, 81 (2), 219-230 (1998), DOI: 10.1002/hlca.19980810204 
P. Strasser, M. Eiswirth, G. Ertl,
Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentostatic control, II. Model Calculations,
J. Chem. Phys., 107 (3), 991-1003 (1997), DOI: 10.1063/1.474451
P. Strasser, M. Luebke, F. Raspel, M. Eiswirth, G. Ertl,
Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentostatic control, I. Experimental,
J. Chem. Phys., 107 (3), 979-990 (1997), DOI: 10.1063/1.474450 
M. Eiswirth, J. Burger, P. Strasser, G. Ertl,
Oscillating Langmuir-Hinshelwood Mechanisms,
J. Phys. Chem., 100 (49), 19118-19123 (1996), DOI:10.1021/jp961688y 
P. Strasser, O.E. Roessler, G. Baier,
Hyperchaos and chemical turbulence in enzymatic reaction-diffusion systems,
J. Chem. Phys., 104 (24), 9974-9982 (1996), DOI: 10.1063/1.471725
G. Baier, P. Strasser, U. Kummer,
Instabilities in a simple enzyme reaction caused by pH dependence,
Z. Naturforsch, 50a (12), 1147-1150 (1995)
P. Strasser, J. Stemwedel, J. Ross,
Analysis of a mechanism of the chlorite-iodide reaction,
J. Phys. Chem., 97 (12), 2851-2862 (1993), DOI: 10.1021/j100114a006

 

 

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