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TU Berlin

Inhalt des Dokuments

2011
M. Oezaslan, F. Hasché, P. Strasser

Oxygen Electroreduction on PtxCo1-x and PtxCu1-x Alloy Nanoparticles for Basic and Acidic PEM Fuel Cell

ECS Trans. 41 (1), 1659 - 1668

doi: 10.1149/1.3635697

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Our study presents the electrochemical characterization of PtxCo1-x and PtxCu1-x nanoparticle electrocatalysts after the voltammetric treatment in basic and acidic electrolytes at room temperature. The chemical composition and mean particle size of the Pt alloy nanoparticles were determined before and after the voltage cycling using TEM and EDS. The electrochemical experiments were conducted with the RDE technique. We show that the electrochemical conditioning is a critical step for the formation of highly active Pt alloy nanoparticle electrocatalysts for ORR. The voltage cycling in acid leads to the leaching of less noble metal to generate a reactive Pt enriched particle surface, while in basic stable metal hydroxide/oxide are primarily formed on the surface of the Pt alloy particles. In particular, in basic voltammetric pretreated PtM3 shows the lowest Pt mass based activity for ORR. In contrast, in acid dealloyed PtCu3 and PtCo3 exhibit 3 - 4 fold increase in jmass compared with Pt/HSAC.
F. Hasché, M. Oezaslan, P. Strasser

Activity and Structure of Dealloyed PtNi3 Nanoparticle Electrocatalyt for Oxygen Reduction Reaction in PEMFC

ECS Trans. 41 (1), 1079 - 1088

doi: 10.1149/1.3635640

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Here, we report a synthesis and activity study of the dealloyed, highly active PtNi3 alloy nanoparticle catalyst for the oxygen reduction reaction (ORR). The dealloyed PtNi3 exhibits 7 - 8 times higher Pt mass based activity and 6 - 7 times higher Pt surface area specific based activity for ORR than pure Pt by similar mean particle size. Further, we have tested the long-term durability of the dealloyed PtNi3 for the typical and corrosive operating fuel cell conditions. After the voltage testing with 10000 voltage cycles between 0.5 - 1.0 V vs. RHE and a scan rate of 50 mV s-1 in deaerated 0.1 M HClO4 the activated PtNi3 catalyst still shows 4 - 5 fold increase in Pt surface area specific based activity compared with that for pure Pt.
F. Muench, M. Oezaslan, T. Seidl, S. Lauterbach, P. Strasser, H.J. Kleebe, W. Ensinger

Multiple activation of ion track etched polycarbonate for the electroless synthesis of metal nanotubes

Appl. Phys. A 105 (4), 847 - 854 

DOI: 10.1007/s00339-011-6646-z

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In our study, we examined the formation of thin films of silver nanoparticles on polycarbonate and the influence of the silver loading on the electroless synthesis of metal nanotubes. Control of the silver film thickness occurred by consecutive dipping of the polymer template in tin(II) and silver(I) solutions. The deposition progress was studied using UV-Vis spectroscopy. The reaction mechanism relies on the adsorption of reactive ions on the polymer template as well as on the silver nanoparticles. The initial catalytic activity of silver-covered ion track etched polycarbonate is an important governing factor for the electroless synthesis of metal nanotubes with desired thickness and shape. Therefore, the  presented method allows specific template preparation according to given synthetic demands. High aspect ratio copper, gold, and platinum nanotubes were produced by the combination of sufficiently activated templates with optimized electroless plating procedures.
F. Hasché, M. Oezaslan, P. Strasser

Activity, Stability, and Degradation Mechanisms of Dealloyed PtCu3 and PtCo3 Nanoparticle Fuel Cell Catalysts

ChemCatChem 3 (11), 1805  - 1813

DOI: 10.1002/cctc.201100169

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A key challenge in today’s fuel cell research is the understanding and maintaining the durability of the structure and performance of initially highly active Pt fuel cell electrocatalysts, such as dealloyed Pt or Pt monolayer catalysts. Here, we present a comparative long-term stability and activity study of supported dealloyed PtCu3 and PtCo3 nanoparticle fuel cell catalysts for the oxygen reduction reaction (ORR) and benchmark them to a commercial Pt catalyst. PtCu3 and PtCo3 were subjected to two distinctly different voltage cycling tests: the “lifetime” regime [10 000 cycles, 0.5–1.0 V vs. RHE (reversible hydrogen electrode), 50 mV s−1] and the corrosive “start-up” regime (2000 cycles, 0.5–1.5 V vs. RHE, 50 mV s−1). Our results highlight significant activity and stability benefits of dealloyed PtCu3 and PtCo3 for the ORR compared with those of pure Pt. In particular, after testing in the “lifetime” regime, the Pt-surface-area-based activity of the Pt alloy catalysts is still two times higher than that of pure Pt. From our electrochemical, morphological, and compositional results, we provide a general picture of the temporal sequence of dominant degradation mechanisms of a Pt alloy catalyst during its life cycle.
E. Ortel, T. Reier, P. Strasser, and R. Kraehnert

Mesoporous IrO2 Films Templated by PEO-PB-PEO Block-Copolymers: Self-Assembly, Crystallization Behavior, and Electrocatalytic Performance

Chem. Mater. 23, 3201 - 3209

DOI: 10.1021/cm200761f

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Synthesis of mesoporous iridium oxide films via soft templating and evaporation-induced self-assembly is demonstrated employing an amphiphilic triblock-copolymer PEO-PB-PEO. Films possess nanocrystalline walls and feature locally ordered pores of about 16 nm diameter. Analysis of the film properties by SEM, TEM, EDX, XPS, SAXS, XRD, and BET along the thermal treatment that succeeds dipcoating shows that the polymer template is removed by calcination between 200 and 300 °C, accompanied by uniaxial shrinkage of film and pore system perpendicular to the substrate. Treating the film in excess of 450 °C leads to further growth of crystallite size and loss of surface area progressing gradually with increasing calcination temperature. Templated IrO2 films conditioned at 450 °C show substantially reduced electrocatalytic overpotentials (efficiency increases) for the oxygen evolution reaction (OER) compared to those of untemplated coatings. Pore templating thus enables direct control over surface catalytic properties of iridium oxide.
M. Oezaslan, M. Heggen, P. Strasser

Dealloyed Pt nanoparticle electrocatalysts for PEMFC cathodes: Core-shell fine structure and size-dependent morphology

Abstracts of Papers of the American Chemical Society 242

De-alloyed Pt nanoparticle electrocatalysts show unique catalytic activity for the electroreduction of oxygen, both in RDE and single cell Membrane Electrode Assembly (MEA) experiments. MEA tests in scaled-up industrial-size single cells showed that the beginning-of-life (BOL) activity of dealloyed Pt cathode catalysts meets the Department of Energy 2015 activity goals. Maintaining Pt mass activity is currently the biggest challenge associated with this catalyst class. Based on recent scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) studies, we have investigated the fine structure of the active dealloyed core-shell bimetallic nanoparticle catalysts at the subnanometer level. We find that a two-phase single core shell structure - often proposed as the structurally and catalytically dominant feature - appears to be an oversimplified model for dealloyed particles. We further report on recent studies on how the catalyst particle morphology after dealloying depends on its initial size.
M. Oezaslan and P. Strasser

Activity of dealloyed PtCo3 and PtCu3 nanoparticle electrocatalyst for oxygen reduction reaction in polymer electrolyte membrane fuel cell

Journal Power Sources 196 (12), 5240 - 5249

DOI: 10.1016/j.jpowsour.2010.11.016

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We report a comparative study of the alloy formation and electrochemical activity of dealloyed PtCo3 and PtCu3 nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). For the Pt–Co system the maximum annealing temperatures were 650 °C, 800 °C and 900 °C for 7 h to drive the Pt–Co alloy formation and the particle growth. EDS and XRD were employed for the characterization of catalyst powders. The RDE and RRDE experiments were conducted in 0.1 M HClO4 at room temperature.                                         We demonstrate that the mass and surface area specific ORR activities of Pt–Co and Pt–Cu alloys after voltammetric activation exhibit a considerable improvement compared to those of pure Pt/C. The dealloyed PtCo3 (800 °C/7 h) electrocatalyst performs 3 times higher in terms of Pt-based mass activity and 4–5 times higher in terms of ECSA-based specific activity than a 28.2 wt.% Pt/C. Dealloyed Pt–Co catalysts (800 °C/7 h) show the most favorable balance between mass and specific ORR activity with a particle size of 2.2 ± 0.1 nm. We hypothesize that geometric strain effects of the dealloyed Pt–Co nanoparticles, similar to those found in dealloyed PtCu3 nanoparticles, are responsible for the improvement in ORR activity.
R. Yang, P. Strasser, M. F. Toney

Dealloying of Cu3Pt (111) Studied by Surface X-ray Scattering

J. Phys. Chem. C 115 (18), 9074  - 9080

DOI: 10.1021/jp111978m

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The structural evolution during dissolution of Cu from Cu3Pt (111) single crystal surfaces under potential control has been studied by X-ray scattering. An epitaxial, compressively strained, Pt-rich overlayer is formed upon Cu dissolution and thickens as the potential increases (more anodic). The compressive lattice strain in the Pt-rich overlayers decreases as the potential and overlayer thickness increase. The Pt-rich overlayers exhibit same fcc stacking sequence as the substrate. We compare and contrast the behavior of the dealloyed single crystals with similarly dealloyed Cu3Pt thin films and nanoparticles.
M. Oezaslan, F. Hasché, P. Strasser

In Situ Observation of Bimetallic Alloy Nanoparticle Formation and Growth Using High-Temperature XRD

Chem. Mater. 23 (8), 2159 - 2165

DOI: 10.1021/cm103661q

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Bimetallic alloy nanoparticles exhibit a complex, (for the most part) poorly understood, crystallographic phase behavior, rarely following their macroscopic counterparts. We have studied formation kinetics, time scales of individual processes, compositional changes, and particle growth rates of Pt bimetallic alloy nanoparticles. We chose the Pt−Cu system, because of its technological importance as a precursor for core−shell nanoparticle electrocatalysts. We provide correlation of annealing control parameters, such as heating rate, temperature, and time, with microscopic alloy structure, composition, and particle size. We have clarified the roles of annealing temperature and time in the alloy formation process and traced out entire Vegard-type structure composition relationships over a wide temperature range. We have found that, during heating ramps, the annealing temperature essentially controls the Cu content of the resulting disordered Pt−Cu lattices. Increasing annealing times, in contrast, leads primarily to particle growth. Phase ordering occurs only during cooling. Our insight offers practical synthetic guidelines toward single-phase ordered and disordered PtCu3 alloy nanoparticles with optimized particle dispersion.
J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka, P. Strasser

Noble-metal free hydrazine fuel cell catalysts: EPOC effect in competing chemical and electrochemical reaction pathways

J. Am. Chem. Soc. 133 (14), 5425 - 5431

DOI: 10.1021/ja111160r

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We report the discovery of a highly active Ni−Co alloy electrocatalyst for the oxidation of hydrazine (N2H4) and provide evidence for competing electrochemical (faradaic) and chemical (nonfaradaic) reaction pathways. The electrochemical conversion of hydrazine on catalytic surfaces in fuel cells is of great scientific and technological interest, because it offers multiple redox states, complex reaction pathways, and significantly more favorable energy and power densities compared to hydrogen fuel. Structure−reactivity relations of a Ni60Co40 alloy electrocatalyst are presented with a 6-fold increase in catalytic N2H4 oxidation activity over today’s benchmark catalysts. We further study the mechanistic pathways of the catalytic N2H4 conversion as function of the applied electrode potential using differentially pumped electrochemical mass spectrometry (DEMS). At positive overpotentials, N2H4 is electrooxidized into nitrogen consuming hydroxide ions, which is the fuel cell-relevant faradaic reaction pathway. In parallel, N2H4 decomposes chemically into molecular nitrogen and hydrogen over a broad range of electrode potentials. The electroless chemical decomposition rate was controlled by the electrode potential, suggesting a rare example of a liquid-phase electrochemical promotion effect of a chemical catalytic reaction (“EPOC”). The coexisting electrocatalytic (faradaic) and heterogeneous catalytic (electroless, nonfaradaic) reaction pathways have important implications for the efficiency of hydrazine fuel cells.
R. Yang, P. Strasser, M.F. Toney

Surface X-ray scattering studies of Cu3Pt (111) model electrocatalysts

Abstracts of Papers of the American Chemical Society 241

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P. Mani, R. Srivastava, P. Strasser

Dealloyed binary PtM3 (M=Cu,Co,Ni) and ternary PtNi3M (M=Cu,Co,Fe,Cr) electrocatalysts for the oxygen reduction rection: Performance in polymer electrolyte membrane fuel cells

Journal Power Sources 196 (2) , 666 - 673

DOI: 10.1016/j.jpowsour.201007.047

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Dealloyed Pt bimetallic nanoparticles are highly active electrocatalysts for the electroreduction of molecular oxygen (ORR), the key barrier to more efficient polymer electrolyte membrane fuel cells (PEMFCs). Most previous studies of dealloyed Pt alloys focused on the structure and mechanism of dealloyed Pt–Cu bimetallic materials. Also, stability concerns related to Cu prompted the search for alternative non-noble metal components for dealloying.Here, we report on a comparative study of dealloyed binary PtM3 (M = Co, Cu, Ni) electrocatalyst for use in PEMFC cathodes. We also study synergistic effects of a third metal in ternary PtNi3M (M = Co, Cu, Fe, Cr) cathode electrocatalysts. All catalyst precursor materials were prepared by an impregnation, freeze-drying, annealing route. After deployment of the catalyst precursor in single PEM cells, the active dealloyed form of the catalysts was obtained through a voltammetric dealloying protocol. Dealloyed binary PtM3 catalysts showed more than a threefold activity improvement for ORR for M = Co, Cu, and close to a threefold improvement for M = Ni in terms of the Pt-mass activity (A mgPt−1) of the single fuel cell, compared to a 45 wt% Pt/C reference cathode catalyst. Improvements in specific surface area normalized activities (A cmPt−2) followed those in Pt-mass activity. All ternary catalysts, except the Fe containing one, showed clearly improved catalytic ORR performance compared to PtNi3, in particular PtNi3Co and PtNi3Cu. A previously unachieved four- to fivefold activity improvement in real single MEAs was observed.Near-surface (XPS) and bulk (EDS/ICP) compositional characterizations suggested that the degree of dealloying of Pt–Co and Pt–Ni binary precursors is lower than that of Pt–Cu compounds. Pt–Co and Pt–Ni still showed 15–20 at.% non-noble metal near the surface and in the bulk of the dealloyed particles, whereas, under the chosen dealloying conditions, Pt–Cu formed core–shell structures with a Pt-rich surface and a Pt–Cu core. Of the selectively characterized Pt–Ni–Co and Pt–Ni–Cu ternaries, the near-surface composition of dealloyed Pt–Ni compounds showed an atomic ratio of about 1:1, compared to about 5:1 in the bulk, pointing to a Ni enrichment at the surface with only small residual amounts of Co or Cu.Our study highlights a number of novel active cathode catalyst compositions and underscores the sensitive dependence of the ORR activity of dealloyed Pt binary and ternary nanoparticle electrocatalysts on the nature and initial composition of the non-noble alloy component.

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