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TU Berlin

Inhalt des Dokuments

2010
M. Oezaslan, F. Hasché, P. Strasser

Structure-Activity Relationship of Dealloyed PtCo3 and PtCu3 Nanoparticle Electrocatalyst for Oxygen Reduction Reaction in PEMFC

ESC Trans. 33 (1), 333 - 341

doi: 10.1149/1.3484531

We report a synthesis and study on carbon supported PtCo3 and PtCu3 alloy nanoparticle catalyst for ORR. The chemical composition of alloys was carried out with EDS. The electrochemical measurements were conducted using a thin-film RDE method. Recently, we have demonstrated that dealloyed PtCu3 nanoparticle exhibits 3-4 times higher mass activity and 4 times higher specific activity for ORR than Pt. Here, the dealloyed PtCo3 also shows about 4 fold increase in specific activity, but 2-3 fold in mass activity than Pt/C. The in-situ generated Pt rich surface of Co rich Pt alloy nanoparticle catalyst tested for ORR activity. Geometric effects based on Pt surface constitution were assumed for the high activity for ORR. PtCo3 nanoparticle catalyst seems to be an interesting opportunity for further studies for ORR. The thermodynamic instable deposition of Co and the robust Pt ECSA are probably the large advantages to other Pt alloys.
F. Hasché, M. Oezaslan, P. Strasser

Activity, stability and degradation of multi walled carbon nanotube (MWCNT) supported Pt fuel cell electrocatalysts

Phys. Chem. Chem. Phys. 12 (46), 15251 - 15258

DOI: 10.1039/c0cp00609b

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Understanding and improving durability of fuel cell catalysts are currently one of the major goals in fuel cell research. Here, we present a comparative stability study of multi walled carbon nanotube (MWCNT) and conventional carbon supported platinum nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The aim of this study was to obtain insight into the mechanisms controlling degradation, in particular the role of nanoparticle coarsening and support corrosion effects. A MWCNT-supported 20 wt.% Pt catalyst and a Vulcan XC 72R-supported 20 wt.% Pt catalyst with a BET surface area of around 150 m2 g−1 and with a comparable Pt mean particle size were subjected to electrode potential cycling in a “lifetime” stability regime (voltage cycles between 0.5 to 1.0 V vs. RHE) and a “start-up” stability regime (cycles between 0.5 to 1.5 V vs. RHE). Before, during and after potential cycling, the ORR activity and structural/morphological (XRD, TEM) characteristics were recorded and analyzed. Our results did not indicate any activity benefit of MWCNT support for the kinetic rate of ORR. In the “lifetime” regime, the MWCNT supported Pt catalyst showed clearly smaller electrochemically active surface area (ECSA) and mass activity losses compared to the Vulcan XC 72R supported Pt catalyst. In the “start-up” regime, Pt on MWCNT exhibited a reduced relative ECSA loss compared to Pt on Vulcan XC 72R. We directly imaged the trace of a migrating platinum particle inside a MWCNT suggesting enhanced adhesion between Pt atoms and the graphene tube walls. Our data suggests that the ECSA loss differences between the two catalysts are not controlled by particle growth. We rather conclude that over the time scale of our stability tests (10000 potential cycles and beyond), the macroscopic ECSA loss is primarily controlled by carbon corrosion associated with Pt particle detachment and loss of electrical contact.
R. Yang, J. Leisch, P. Strasser, M. F. Toney

Structure of Dealloyed PtCu3 Thin Films and Catalytic Activity for Oxygen Reduction

Chem. Mater. 22 (16), 4712 - 4720

DOI: 10.1021/cm101090p

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The detailed structure and composition (surface and bulk) as well as catalytic activity for oxygen reduction of electrochemically dealloyed PtCu3 thin films have been investigated. Synchrotron-based anomalous X-ray diffraction (AXRD) reveals that a Pt enriched surface region (∼1.0 nm thick) and a Cu depleted interior (atomic ratio different from that of PtCu3) are formed in the dealloyed film, and we directly observe a compressive lattice strain in the Pt surface region. The dealloyed PtCu3 thin films show a ∼2.4 fold increase in the specific oxygen reduction activity over pure Pt thin films as measured by a rotating disk electrode (RDE). Our results show that the enhanced catalytic activity of the dealloyed Pt−Cu film is primarily due to the compressive strain in the surface layer (ligand effect is very weak). We compare our results on thin films to related results on nanoparticles. These studies provide a better understanding of the structure − composition and structure − activity relationships in Pt-skeleton structures prepared by dealloying base-metal-rich alloys.
H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser

The Mechanism of Water Oxidation: From Electrolysis via Homogenous to Biological Catalysis

ChemCatChem 2 (7), 724 - 761

DOI: 10.1002/cctc.201000126

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Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential-determining step. Subsequently, novel cobalt oxide-based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water-oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non-ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox-potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.
P. Strasser, S. Koh, T. Anniyev, J. Greeley, K. More, C. Yu, Z. Liu, S. Kaya, D. Nordlund, H. Ogasawara, M. F. Toney, A. Nilson
 
Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts

Nature Chemistry 2 (6), 454 - 459

DOI: 10.1038/NCHEM.623

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Electrocatalysis will play a key role in future energy conversion and storage technologies, such as water electrolysers, fuel cells and metal–air batteries. Molecular interactions between chemical reactants and the catalytic surface control the activity and efficiency, and hence need to be optimized; however, generalized experimental strategies to do so are scarce. Here we show how lattice strain can be used experimentally to tune the catalytic activity of dealloyed bimetallic nanoparticles for the oxygen-reduction reaction, a key barrier to the application of fuel cells and metal–air batteries. We demonstrate the core–shell structure of the catalyst and clarify the mechanistic origin of its activity. The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and weakening chemisorption of oxygenated species. We combine synthesis, measurements and an understanding of strain from theory to generate a reactivity–strain relationship that provides guidelines for tuning electrocatalytic activity.
K. Yaccato, R. Carhart, A. Hagemeyer, M. Herrmann, A. Lesik, P. Strasser, A. Volpe, H. Turner, H. Weinberg, R. K. Grasselli, C. J. Brooks, J. M. Pigos

High Troughout Discovery of Families of High Activity WGS Catalysts: Part I - History and Methodoloy

Combinatorial Chemistry & High Throughput Screening 13 (4), 318 - 330

DOI: 10.2174/138620710791054286
 
State-of-art water gas shift catalysts (FeCr for high temperature shift and CuZn for low temperature shift) are not active enough to be used in fuel processors for the production of hydrogen from hydrocarbon fuels for fuel cells. The need for drastically lower catalyst volumes has triggered a search for novel WGS catalysts that are an order of magnitude more active than current systems. Novel catalytic materials for the high, medium and low temperature water gas shift reactions have been discovered by application of combinatorial methodologies. Catalyst libraries were synthesized on 4 inch wafers in 16 x 16 arrays and screened in a high throughput scanning mass spectrometer in the temperature range 200 degrees C to 400 degrees C. More than 200 wafers were screened under various conditions and more than 250,000 experiments were conducted to comprehensively examine catalyst performance for various binary, ternary and higher-order compositions.

X. Zhu, J. Greeley, P. Strasser

Adsorption-driven surface segregation effects in core-shell fuel cell nanoparticle electrocatalysts

Abstracts of Papers of the American Chemical Society 239
S. Koh, P. Strasser

Dealloyed Pt nanoparticle fuel cell electrocatalysts: Stability and aging study of catalyst powders, thin films, and inks

J. Electrochem. Soc. 157 (4), B585 - B591

DOI: 10.1149/1.3309729

Dealloyed Pt–Cu alloy nanoparticles are active oxygen reduction electrocatalysts; they are formed from Cu-rich alloy precursors during a selective Cu atom dissolution (dealloying) process. The surface of Cu-rich particle precursors is prone to oxidation under ambient air conditions, which may critically affect the aging behavior of the precursors. Here, we present a systematic stability and aging study of a carbon-supported Pt25Cu75 alloy nanoparticle catalyst precursor. We study the impact of the aging of the catalyst material on its electrocatalytic performance for the oxygen reduction reaction (ORR) after dealloying. We obtain a practical insight into the electrochemical behavior of the materials in the formats of powders, inks, and films. Our studies suggest that the Pt–Cu precursors show a stable catalytic performance when aged as dry powders in air. Ink samples, however, reach their maximum ORR activity of up to 1.3 A/mgPt with aging for 24–48 h after which they deteriorated in performance. Finally, catalyst thin films were the most sensitive to aging in air and generally deteriorated rapidly after just one day. Our results provide practical insights and guidelines regarding the stability and handling of the nanoparticle catalyst powder.
R. Forgie, g. Bugosh, K.C. Neyerlin, Z. Liu, P. Strasser

Bimetallic Ru Electrocatalysts for the OER and Electrolytic Water Splitting in Acidic Media

Electrochem. Solid-State Lett. 13 (4), D36 - D39

DOI: 10.1149/1.3290735

We have explored bimetallic Ru–M oxygen evolution reaction (OER) electrocatalysts for use in water splitting in acidic electrolytes. Using an electrochemical multielectrode cell, we investigated the OER activity of selected compositions of seven binary alloy systems, Ru–M (M = Pd, Ir, Cu, Co, Re, Cr, Ni) . Benchmarked using pure Ru electrocatalysts, Ru–Co, Ru–Ir, and Ru–Cu exhibited improved Ru mass-based catalytic activities. Structural studies of the precursor alloys indicated the presence of hexagonal and cubic mixed metal phases. We hypothesize that the secondary metal component modulates the chemisorption energy of oxygen, which was suggested to be a sensitive rate controlling parameter in the OER catalysis and favors the formation of atomic oxygen Oad and possibly HOOad species rather than species OHad on the oxide catalyst.
T. Anniyev, H. Ogasawara, M. P. Ljungberg, K. T. Wikfeldt, J. B. MacNaughton, L.-A. Näslund, W. Bergmann, S. Koh, P. Strasser, L. G. M. Pettersson, A. Nilsson
 
Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts

Phys. Chem. Chem. Phys. 12, 5694 - 5700

DOI: 10.1039/b926414k

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We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt–Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L2-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.

 

 

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