2011

M. Oezaslan, F. Hasché, P. Strasser

Oxygen Electroreduction on Pt_xCo_1-x and Pt_xCu_1-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 mg_Pt⁻¹) of the single fuel cell, compared to a 45 wt% Pt/C reference cathode catalyst. Improvements in specific surface area normalized activities (A cm_Pt⁻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.

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/mg_Pt 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 O_ad and possibly HOO_ad species rather than species OH_ad 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.