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TU Berlin

Inhalt des Dokuments

2009
J. Christoph, T.-G. Noha, G. Lee, P. Strasser, M. Eiswirth

Spatiotemporal self-organization in the oscillatory HCOOH oxidation on a Pt ribbon electrode - Theory and Experiments

Surface Science 603 (10-12), 1652-1661

DOI: 10.1016/j.susc.2008.11.054

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Since the current density near the edges of ribbon and disk electrodes is enhanced, the resulting stationary and non-stationary double layer potential is generally inhomogeneous in all electrochemical reactions. We investigate the impact of this edge effect induced spatial inhomogeneity on the pattern formation of the oscillatory formic acid oxidation on thin Pt ribbon electrodes. In order to be able to theoretically describe the spatiotemporal behavior of the double layer potential distribution, we derive and discuss the properties of the electrochemical ribbon coupling function for various distances of the reference electrode. The resulting reaction–migration equation is analyzed in connection with a chemical model accounting for the specific reaction mechanism of the formic acid oxidation. The interaction of structural inhomogeneity, chemically induced temporal instability and nonlocal spatial coupling due to ion migration gives rise to novel types of spatiotemporal behavior. The results compare favorably with experiments conducted so far, which are presented as well and can be explained within the framework of reaction–migration equations.
P. Strasser, F. Hasché, M. Özaslan

Brennstoffzellen in der Lehre

HZwei - Zeitschrift für Wasserstofftechnologie 9 (4), 26-27
P. Strasser

80% weniger Platin - Kern-Schale Katalysatorkonzepte

HZwei - Zeitschrift für Wasserstofftechnologie 9 (1), 8-12
P. Strasser

Nanostrukturierte Kern-Schale Katalysatoren für PEM Brennstoffzellen - Hochaktive Materialien durch partielles "Dealloying"

Chemie Ingenieur Technik 81 (5), 573-580

DOI: 10.1002/cite.200800506

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Die elektrochemische Umwandlung von Sauerstoff zu Wasser sowie ihre Umkehrreaktion gehören im Moment zu den größten Herausforderungen der Wissenschaft und Forschung zum Zwecke einer effizienten und nachhaltigen Nutzung elektrochemischer Energieumwandlungsprozesse basierend auf Brennstoffzellen oder der Elektrolyse von Wasser. Reines Platin – der am häufigsten verwendete Kathodenkatalysator für die Sauerstoffreduktion in Brennstoffzellen – ist teuer und nicht ausreichend aktiv und stabil. Vielversprechend ist ein neues Katalysatorkonzept basierend auf einem sehr Pt armen nanostrukturierten Kern-Schale-Prinzip. Durch selektives partielles Entlegieren, d. h. elektrochemische Entfernung von unedlen Metallatomen aus der Oberfläche einer nichtedelmetallreichen Pt-Legierung („Dealloying”), wird eine Pt reiche Schale auf einem nichtedelmetallreichen Kern von definierbarer Dicke gebildet, die eine gezielte Steuerbarkeit von katalytischer Aktivität ermöglicht. Eine Reduzierung der Pt-Menge in der Zellkathode um mehr als 80 % ist dabei möglich.
R. Srivastava, P. Mani, P Strasser

In situ voltammetric de-alloying of fuel cell catalyst electrode layer: A combined scanning electron microscope/electron probe micro-analysis study

Journal of Power Sources 190 (1), 40-47

DOI: 10.1016/j.jpowsour.2008.09038

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In situ voltammetric de-alloying, i.e. partial selective dissolution of less noble alloy components, is a recently proposed, effective strategy to prepare active electrocatalysts for the oxygen reduction reaction (ORR) [S. Koh, P. Strasser, J. Am. Chem. Soc. 129 (2007) 12624–12625; R. Srivastava, P. Mani, N. Hahn, P. Strasser, Angew. Chem. Int. Ed. 46 (2007) 8988–8991]. However, in situ de-alloying of bimetallics inside electrode layers of membrane-electrode-assemblies (MEAs) seems to defy the requirement of keeping the membrane free of cationic contaminants; yet, when followed by ion exchange, de-alloyed cathodes result in previously unachieved single cell activities of polymer electrolyte membrane fuel cell cathode layers of up to 0.4 A mgPt−1 at 900 mV cell voltage. The effects of voltammetric Cu de-alloying on the MEA have never been studied before. In the present study, we therefore address this issue and report detailed scanning electron microscope (SEM) imaging of the morphology and electron probe micro-analysis (EPMA) mapping of a MEA at various stages of the de-alloying and ion-exchange process. We investigate the significant loss of Cu from the cathode particle catalyst after de-alloying, demonstrate how the membrane can be cleaned from Cu-ion contamination using ion exchange with protons from liquid inorganic acids, and show that Cu ion exchange does ultimately not affect the activated catalyst particles inside the cathode layer. We correlate the microscopic study of the MEA with its cyclic voltammetric response curves as well as the single cell polarization data.
K.C. Neyerlin, R. Srivastava, C. Yu, P. Strasser

Electrochemical activity and stability of dealloyed Pt-Cu and Pt-Cu-Co electrocatalysts for the oxygen reduction reaction (ORR)

Journal of Power Sources 186 (2), 261-267

DOI: 10.1016/j.jpowsour.2008.10.062

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A comparative study of the electrochemical stability of Pt25Cu75 and Pt20Cu20Co60 alloy nanoparticle electrocatalysts in liquid electrolyte half-cell environment was conducted. The aforementioned catalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss during voltage cycling relative to commercially available pure Pt electrocatalysts. The difference in ECSA loss was attributed to their initial mean particle size, which varied depending on the temperature at which the alloy catalysts were prepared (e.g. 600, 800 and 950 °C). Higher preparation temperatures resulted in larger particles and lead to lower ECSA loss. Liquid electrolyte environment short-term durability testing (5000 voltages cycles) revealed the addition of cobalt to be beneficial as ternary compositions exhibited stability advantages over binary catalysts.
Oxygen reduction reaction (ORR) activity and catalyst stability tests were then performed for both Pt25Cu75 and Pt20Cu20Co60 alloy catalysts in membrane electrode assemblies (MEA). ORR activity data, taken both prior to and at the conclusion of 30,000 voltage cycles from 0.5 to 1.0 V vs. reversible hydrogen electrode (RHE), revealed that both Pt25Cu75 and Pt20Cu20Co60 were able to retain both their mass and Pt surface area-based activity advantage relative to Pt/C [R. Srivastava, P. Mani, N. Hahn, P. Strasser, Angew. Chem. Int. Ed. 46 (2007), 8988; P. Mani, R. Srivastava, P. Strasser, J. Phys. Chem. C 112 (2008), 2770; S. Koh, P. Strasser, J. Am. Chem. Soc. 129 (2007), 12624]. Further analysis revealed that the Pt surface area-based activity, measured at 0.9 V vs. RHE, of commercially available Pt catalysts, as well as that for both Pt25Cu75 and Pt20Cu20Co60 increased on the order of tens of μA⋅cmPt-2per 1000 voltage cycles. This increase in specific activity combined with a reduced ECSA loss resulted in a negligible change for the Pt mass-based activity of Pt25Cu75 alloys annealed at 950 °C.
P. Strasser

Dealloyed Core Shell Fuel Cell Electrocatalysts

Reviews in Chemical Engineering 25 (4), 255-295

DOI: 10.1515/REVCE.2009.25.4.255

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We review our recent work on dealloyed nanoparticle electrocatalysts and address their synthesis, structural characterization and surface catalytic performance in low-temperature Polymer Electrolyte Membrane fuel cells (PEMFCs). The active form of the catalyst is obtained by voltammetric dealloying of non-noble metal rich Pt alloy precursors. In the dealloying process, the less noble precursor component, here Cu, is selectively removed from the surface of the precursor alloy particles and hence a Pt enriched particle shell is formed. Single fuel cell tests showed that, when used on the cathode of PEMFCs, dealloyed Pt catalysts show reactivities for the oxygen
reduction reaction (ORR) which are up to 6 times higher than those of conventional pure Pt fuel cell catalysts. Similarly, the stability of dealloyed nanoparticle catalysts is superior to that of pure Pt particles. X-ray based structural and compositional studies suggested a core—shell particle structure as the active form of the catalyst consisting of a Pt enriched particle shell surrounding a Pt alloy core. At the present time, this catalyst system constitutes one of the most active fuel cell catalyst system reported in the literature.
K.C. Neyerlin, G. Bugosh, R. Forgie, Z. Liu, P. Strasser

Combinatorial Study of High Surface-Area Binary and Ternary Electrocatalysts for the Oxygen Evolution Reaction

J. Electrochem. Soc. 156 (3), B363-B369

DOI: 10.1149/1.3049820

We present the design, fabrication, and utilization of a high-throughput combinatorial electrochemical screening platform consisting of 16 individually addressable, three-electrode chambers for rapid evaluation of supported high-surface-area nanoparticle electrocatalysts. Repeatable hydrogen adsorption/desorption surface area measurements along with oxygen evolution kinetics for carbon supported pure platinum catalysts (PtC) illustrate practical reproducibility (±10%) across the 16 glassy carbon working electrodes. Automated liquid-precursor impregnation using robotic liquid dispensing is shown to produce catalysts in precisely controllable atomic ratios as confirmed via energy dispersive spectroscopy. The combinatorial array demonstrates the ability to produce reliable trends in electrocatalytic activity data for the oxygen evolution reaction (OER) on both binary Pt–M catalysts (with M=Ir, Re, Ru, Pd), as well as ternary Pt–Ru–M catalysts (with M=Ir, Pd)), when compared to a more rigorous kinetic analysis on a rotating disk electrode. It was shown that Ru-rich materials are better suited for use as OER catalysts relative to Pt-, Ir-, Pd-, and Re-rich materials. The OER catalyst materials discovered are expected to significantly lower the necessary input power of electrochemical water splitting devices in acidic environments (proton exchange membrane fuel cell electrolyzers).
Z. Liu, C. Yu, P. Strasser

Phase Stabilized and Enhanced PtCu3/C Oxygen Reduction Electrocatalysts via Au Galvanic Displacement

Clean Technology 2009: Bioenergy, Renewables, Storage, Grid, Waste and Sustainability, 171-174

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We reported here that the enhanced electrocatalytic oxygen reduction activity of PtCu3/C catalysts was stabilized using Au galvanic displacement of Cu in PtCu3/C. The results showed that mass activities decreased by 24 and 29%, and specific activities decreased by 6 and 26% for Pt/C and PtCu3/C catalysts, respectively, while PtCu3/C modified by Au can retain both mass and specific activities after 1000 potential cycles between 0.6 and 1.0V vs RHE. XRD patterns showed that additional peaks related to Au-riched phase appeared, while the peaks related to PtCu alloy phase didn’t change much. In combination with XPS analysis, Cu in PtCu3/C catalysts was displaced spontaneously by Au3+. EDS analysis combined with HRTEM showed big particles were enriched with Au, while small particles were enriched with Pt. STEM showed Pt-Au shell Pt-Cu core structures. Au atoms in PtCu3/C catalysts affected electronic structure of Pt-enriched shell, and therefore improved the ORR activity compared to the dealloyed PtCu3/C catalysts. On the other hand, Au atoms inhibited Pt oxidation and helped to keep favorable Pt-Pt distances created by dealloying, thus improving the stability of ORR activity on PtCu3/C catalysts.
Z. Liu, C. Yu, P. Strasser

Phase Stabilized and Enhanced PtCu3/C Oxygen Reduction Electrocatalysts via Au Galvanic Displacement

Nanotech Conference & Expo 2009 3, 66-69

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We reported here that the enhanced electrocatalytic oxygen reduction activity of PtCu3/C catalysts was stabilized using Au galvanic displacement of Cu in PtCu3/C. The results showed that mass activities decreased by 24 and 29%, and specific activities decreased by 6 and 26% for Pt/C and PtCu3/C catalysts, respectively, while PtCu3/C modified by Au can retain both mass and specific activities after 1000 potential cycles between 0.6 and 1.0V vs RHE. XRD patterns showed that additional peaks related to Au-riched phase appeared, while the peaks related to PtCu alloy phase didn’t change much. In combination with XPS analysis, Cu in PtCu3/C catalysts was displaced spontaneously by Au3+. EDS analysis combined with HRTEM showed big particles were enriched with Au, while small particles were enriched with Pt. STEM showed Pt-Au shell Pt-Cu core structures. Au atoms in PtCu3/C catalysts affected electronic structure of Pt-enriched shell, and therefore improved the ORR activity compared to the dealloyed PtCu3/C catalysts. On the other hand, Au atoms inhibited Pt oxidation and helped to keep favorable Pt-Pt distances created by dealloying, thus improving the stability of ORR activity on PtCu3/C catalysts.

 

 

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