2013

Combinatorial discovery of Ni-based binary and ternary catalysts for hydrazine electrooxidation for use in anion exchange membrane fuel cells
 
Journal of Power Sources 247, 605 - 611
 
DOI: 10.1016/j.jpowsour.2013.08.107
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Ni-based catalysts, binary Ni-M (with M = Mn, Fe, Zn, La) and ternary Ni-Mn-Fe and Ni-Zn-La were investigated for hydrazine oxidation in direct hydrazine hydrate fuel cell anodes by a temperature controlled 16-channel electrochemical combinatorial array. The binary Ni_0.8Zn_0.2 and Ni_0.9La_0.1 catalysts are significantly more active than the Ni reference catalyst for hydrazine oxidation. While the best Ni_0.8Zn_0.1La_0.1 ternary catalyst is close to the high active binary catalysts in composition. Additionally, Ni_0.6Fe_0.2Mn_0.2 catalysts also showed high catalytic activity for hydrazine oxidation in alkaline media over standard Ni catalyst. The X-ray diffraction (XRD) analysis indicated that the alloying effect between Ni and added elements improves the catalytic activity for hydrazine oxidation. As a result of the screening tests and our previous research, unsupported binary Ni_0.87Zn_0.13 and Ni_0.9La_0.1 catalysts were synthesized by spray pyrolysis and tested in a direct hydrazine hydrate fuel cell MEA (DHFC) producing 486 mW cm^-2 and 459 mW cm^-2, respectively. 

Lin Gan, Stefan Rudi and Peter Strasser

Core-Shell and Nanoporous Particle Architectures and Their Effect on the Activity and Stability of Pt ORR Electrocatalysts

Topics in Catalysis 57 (1-4), 236 - 244
 
DOI: 10.1007/s11244-013-0178-z

 
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We review our recent progress in the development of Pt-Ni bimetallic electrocatalysts with both high sustained activity and sustained stability for oxygen reduction reaction (ORR). This was achieved by an atomic understanding and rational control of the core-shell compositional patterns and size-related nanoporosity within the bimetallic nanoparticles formed during chemical and electrochemical pretreatment and electrocatalysis. In particular, we reveal how the size of the nanoparticle directly influences the nanoporosity formation and thereby the near surface composition, catalytic activity and stability. Our atomic insights provide a clearer picture on how bimetallic nanoparticles should be tailored for optimal ORR performance.

Weiyong Bian, Zhenrong Yang, Peter Strasser, Ruizhi Yang

A CoFe₂O₄/graphene nanohybrid as an efficient bi-functional electrocatalyst for oxygen reduction and oxygen evolution

Journal of Power Sources 250, 196 - 203

DOI: 10.1016/j.jpowsour.2013.11.024

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Development of efficient electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remain key issues for the commercialization of fuel cells and metal-air batteries. In this study, A CoFe₂O₄/graphene nanohybrid is facilely synthesized via a two-step process and applied as an electrocatalyst for the ORR and the OER. The as-prepared CoFe₂O₄/graphene nanohybrid demonstrates excellent catalytic activity for the ORR. At the same mass loading, the Tafel slope of CoFe₂O₄/graphene electrocatalyst for the ORR is comparable to that of the commercial Pt/C (20 wt% Pt on Vulcan XC-72, Johnson Matthey). The ORR on CoFe₂O₄/graphene mainly favours a direct 4e^- reaction pathway. The CoFe₂O₄/graphene nanohybrid also affords high catalytic activity for the OER. The chronoamperometric tests show that CoFe₂O₄/graphene catalyst exhibits excellent stability for both the ORR and the OER, outperforming the commercial Pt/C. The high electrocatalytic activity and durability of CoFe₂O₄/graphene nanohybrid are attributed to the strong coupling between CoFe₂O₄ nanoparticles and graphene.

Mehtap Oezaslan, Frederic Hasche and Peter Strasser

Pt-Based Core-Shell Catalyst Architectures for Oxygen Fuel Cell Electrodes

J. Phys. Chem. Lett. 4 (19), 3273 - 3291

DOI: 10.1021/jz4014135
 
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Pt-based core−shell nanoparticles have emerged as a promising generation of highly active electrocatalysts to accelerate the sluggish kinetics of oxygen reduction

reaction (ORR) in fuel cell systems. Their electronic and structural properties can be easily tailored by modifying the Pt shell thickness, core composition, diameter, and shape; this results in significant improvements of activity and durability over state-of-the-art pure Pt catalysts. Prompted by the relevance of efficient and robust ORR catalysts for electrochemical energy conversion, this Perspective reviews several concepts and selected recent developments in the exploration of the structure and composition of core−shell nanoparticles. Addressing current achievements and challenges in the preparation as well as microscopic and spectroscopic characterization of core−shell nanocatalysts, a concise account of our understanding is provided on how the surface and subsurface structure of multimetallic core−shell nanoparticles affect their reactivity. Finally, perspectives for thelarge-scale implementation of core−shell catalysts in polymer exchange membrane fuel cells are discussed.

B. Johnson, F. Girgsdies, G. Weinberg, D. Rosenthal, A. Knop-Gericke,T. Reier and P. Strasser and R. Schlögl
 
Suitability of Simplified (Ir,Ti)O_x Films for Characterization during Electrocatalytic Oxygen Evolution Reaction
 
J. Phys. Chem. C 117 (48), 25443 - 25450
 

DOI: 10.1021/jp4048117
 
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Simplified IrOx electrodes lacking typical mud crack structure have been produced on polycrystalline Ti cylinders with spin-coating using an iridium acetate solution and are compared to thicker samples in terms of stability, composition, and suitability as a model system. The spin-coating process forms smooth, thin islands of IrOx with limited cracking and decreases the surface-to-bulk ratio to allow a more intimate study of the growth, composition, and stability of the layer without the complications of the mud crack morphology. XPS and XRD measurements show a resulting (Ir,Ti)O_x surface (x near 2) with OH^- groups and H₂O. Cyclic voltammetry measurements indicate the expected high catalytic activity for the oxygen evolution reaction as well as a dry IrO_x phase resulting from the thermal manufacturing process, although evidence of hydrous phases are found in XPS. Both films required only small overpotentials for the oxygen evolution reaction, with the spin-coated sample showing a slightly lower activity. CO temperature desorption spectroscopy analysis showed CO → CO₂ oxidation and, in combination with XPS, an unstable surface. The oxidation of CO was not due to the TiO_x, and the absence of any evidence of an Ir-suboxide phase indicates the presence of near-surface active species present after synthesis or an active surface termination.

Maria Wuithschick, Benjamin Paul, Ralf Bienert, Adnan Sarfraz, Ulla Vainio, Michael Sztucki, Ralph Kraehnert, Peter Strasser, Klaus Rademann, Franziska Emmerling and Jorg Polte
 

Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on Mechanistic Understanding
 

Chem. Mater. 25 (23), 4679 - 4689

DOI: 10.1021/cm401851g
 
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Metal nanoparticles have attracted much attention due to their unique properties. Size control provides an effective key to an accurate adjustment of colloidal properties. The common approach to size control is testing different sets of parameters via trial and error. The actual particle growth mechanisms, and in particular the influences of synthesis parameters on the growth process, remain a black box. As a result, precise size control is rarely achieved for most metal nanoparticles. This contribution presents an approach to size control that is based on mechanistic knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the reduction of AgClO₄ with NaBH₄. Conducting this approach allowed a well-directed modification of this synthesis that enables, for the first time, the size-controlled production of silver nanoparticles 4−8 nm in radius without addition of any stabilization agent.

Amandine Guiet, Tobias Reier, Nina Heidary, Diana Felkel ,Benjamin Johnson, Ulla Vainio, Helmut Schlaad, Yilmaz Aksu, Matthias Driess, Peter Strasser, Arne Thomas, Jörg Polte and Anna Fischer

A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film Electrodes Bearing One Metal Nanoparticle per Pore with Enhanced Electrocatalytic Properties

Chem. Mater. 25 (23), 4645 - 4652

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The controlled incorporation of single metal nanoparticles within the pores of mesostructured conducting metal oxide ultrathinfilms is demonstrated, taking advantage of the controlled metal precursor loading capacities of PS-b-P4VP inverse micellar templates. The presented one-pot approach denoted as Evaporation-Induced Hydrophobic Nanoreactor Templating (EIHNT) unusually involves the nanostructuration of the metal oxide via the hydrophobic shell of the micellar template, while the concomitant nanostructuration of the metal is achieved via its confinement in the hydrophilic micellar core. This approach is applied to tin-rich ITO and gold, to yield unique mesoporous tin-rich ITO ultrathinfilm electrodes remarkably loaded with one size-controlled gold nanoparticle per pore. Interestingly, the resulting tin-rich ITO-supported gold nanoparticles exhibit improved catalytic activity and durability in electrocatalytic CO oxidation compared to similarly sized gold nanoparticles supported on conventional ITO coatings.

Chunhua Cui, Lin Gan, Marc Heggen and Peter Strasser

Structural and Compositional Behaviors of Shaped Pt Alloy Nanoparticle Electrocatalysts

ECS Transactions 58 (1), 575 - 579

doi: 10.1149/05801.0575ecst

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We present the design and synthesis of phase segregated Pt alloy nanoparticle electrocatalysts through a facile solvothermal route in a dimethylformamide solvent. The compositional segregation near the alloyed PtNi nanoparticle surface was achieved by controlling the reaction kinetics and site-dependent compositional segregation in the facet centers of the shaped PtxNi1-x nanoparticles was controlled by the bulk Pt/Ni composition ratio, respectively. Based on this unique compositional segregation, we study their atomic-scale evolution of surface microstructure and composition by using high-resolution scanning transmission electron microscopy with high angle annular dark field and electron energy loss spectroscopy line scans and investigate their structural and compositional behaviors under reactive environments. The surface evolution of these materials helps ones understand the reactivity and degradation of a catalyst and understand the interactions between reactants and catalysts by introducing environmental impact factors.

Jiao Wu, Zhenrong Yang, Xiaowei Li, Qijun Sun, Chao Jin, Peter Strasser, Ruiz Yang

Phosphorus-doped porous carbons as efficient electrocatalysts for oxygen reduction

J. Mater. Chem. A. 1 (34), 9889 - 9896

DOI: 10.1039/c3ta11849e

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Efficient electrocatalysts for the oxygen reduction reaction (ORR) play a critical role in the performance of fuel cells and metal-air batteries. In this study, we report a facile synthesis of phosphorus (P)-doped porous carbon as a highly active electrocatalyst for the ORR. Phosphorus-doped porous carbon was prepared by simultaneous doping and activation of carbon with phosphoric acid (H₃PO₄) in the presence of Co. Both phosphorus and cobalt were found to play significant roles in improving the catalytic activity of carbon for the ORR. The as-prepared phosphorus-doped porous carbon exhibited considerable catalytic activity for the ORR as evidenced by rotating ring-disk electrode studies. At the same mass loading, the Tafel slope of phosphorus-doped porous carbon electrocatalysts is comparable to that of the commercial Pt/C catalysts (20 wt% Pt on Vulcan XC-72, Johnson Matthey) with stability superior to Pt/C in alkaline solutions.

Mahdi Ahmadi, Farzad Behafarid, Chunhua Cui, Peter Strasser and Beatriz Roldan Cuenya

Long-Range Segregation Phenomena in Shape-Selected Bimetallic Nanoparticles: Chemical State Effects

ACS Nano 7 (10), 9195 - 9204

DOI:10.1021/nn403793a

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A study of the morphological and chemical stability of shape-selected octahedral Pt_0.5Ni_0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt_0.5Ni_0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H₂, and O₂ environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtO_x species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (<200-270  °C), while PtO_x species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established.

Rulle Reske, Matteo Duca, Mehtap Oezaslan, Klaas Jan P. Schouten, Marc T. M. Koper and Peter Strasser

Controlling Catalytic Selectivities during CO2 Electroreduction on Thin Cu Metal Overlayers

J. Phys. Chem. Lett. 4 (15), 2410 - 2413

DOI:10.1021/jz401087q

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The catalytic activity and selectivity of the electrochemical CO₂ reduction on Cu overlayers with varying atomic-scale thickness on Pt was investigated. Hydrogen, methane, and ethylene were the main products. Beyond an activity improvement with increasing copper layer thickness, we observed that the thickest 15 nm Cu layer behaved bulk-like and resulted in high relative faradaic selectivities for hydrocarbons. With decreasing Cu layer thickness, the formation of methane decreased much faster than that of ethylene. As a result, the relative faradaic selectivity of the technologically useful product ethylene increased sharply. The selectivity ratios between methane and ethylene were independent of electrode potential on a Cu monolayer. A combination of geometric tensile strain effects and electronic effects is believed to control the surface reactivity and product distribution on the copper surfaces. This study highlights the general strategy to tune product distributions on thin metal overlayers.

Gan, Lin Gan, Marc Heggen and Peter Strasser

Subsurface Enrichment of Highly Active Dealloyed Pt-Ni Catalyst Nanoparticles for Oxygen Reduction Reaction

ECS Transactions 50 (2), 1627 - 1631

doi: 10.1149/05002.1627ecst

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We present the synthesis of homogenous PtNi and PtNi₃ alloy nanoparticles by organic solution approach, which showed 4-5 fold increases in Pt-mass normalized activity of oxygen reduction reaction (ORR) compared to conventional Pt catalyst after electrochemical dealloying. Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we found that there is a strong tendency of subsurface enrichment of Ni in the dealloyed Pt-Ni nanoparticles at a higher initial Ni composition (PtNi₃), which therefore resulted in unusual core-shell fine structures. Based on this, a correlation of near-surface compositions to the ORR activities of the dealloyed Pt-Ni catalyst nanoparticles was established.

Arno Bergmann, Ivelina Zaharieva, Holger Dau and Peter Strasser

Electrochemical water splitting by layered and 3D cross-linked manganese oxides: correlating structural motifs and catalytic activity

Energy Environ. Sci. 6, 2745 - 2755

DOI: 10.1039/C3EE41194J

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Manganese based precious metal-free electrocatalysts for the oxygen evolution reaction (OER) are promising materials for energy storage systems based on dark or photo-coupled water electrolysis, because they are active, inexpensive and of low toxicity. In this work, atomic scale structure–activity relationships of two different nano-structured manganese oxides, MnO_x, are established using a combination of X-ray absorption, diffraction and electrochemistry. Prepared by chemical symproportionation (s-MnO_x) and impregnation (i-MnO_x), the s-MnO_x catalyst consisted of a layered structure similar to δ-MnO₂ while the i-MnO_x catalyst displayed a mixture of tunnelled, 3D cross-linked β- and defective γ-MnO₂ structures. During electrocatalytic oxygen evolution the structural motifs of both MnOx remain largely unchanged, but the oxidation state of Mn increases from 3.5 to 3.9–4. Kinetic parameters of the electrocatalytic oxygen evolution reaction were extracted using Tafel slope analysis and pH titration experiment, and the role of the protons abstracted was analyzed. The study reveals fundamental differences of general importance in the catalytic activity between layered and cross-linked structures. The exclusive presence of di-μ-oxo-bridged Mn ions in the layered structure is coupled to a pronounced redox and charge capacity behaviour. This ensured efficient use of surface and bulk active sites, and resulted in a relatively large Tafel slope. Consequently, the intrinsic OER activity is especially high in s-MnO_x. In contrast, 3D cross-linked structures with both mono- and di-μ-oxo-bridged Mn ions resulted in lower intrinsic activity but smaller Tafel slope, and thus favourable activity at technological water-splitting rates. The insights from this comparative study will provide guidance in the structural design and optimization of other non precious metal oxide OER catalysts.

Lin Gan, Stefan Rudi, Chunhua Cui and Peter Strasser

Ni-Catalyzed Growth of Graphene layers during Thermal Annealing: Implications for the Synthesis of Carbon-Supported Pt-Ni Fuel-Cell Catalysts

ChemCatChem 5, 2691 – 2694

DOI: 10.1002/cctc.201300235

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Thermal annealing is an important and widely adopted step during the synthesis of Pt bimetallic fuel-cell catalysts, although it faces the inevitable drawback of particle sintering. Understanding this sintering mechanism is important for the future development of highly active and robust fuel-cell catalysts. Herein, we studied the particle sintering during the thermal annealing of carbon-supported Pt_1–xNi_x (PtNi, PtNi₃, and PtNi₅) nanoparticles, a reported recently class of highly active fuel-cell catalysts. By using high-resolution transmission electron microscopy, we found that annealing at an intermediate temperature (400 °C) effectively increased the extent of alloying without particle sintering; however, high-temperature annealing (800 °C) caused severe particle sintering, which, unexpectedly, was strongly dependent on the composition of the alloy, thus showing that a higher Ni content resulted in a higher extent of particle sintering. This result can be ascribed to the solid-state transformation of the carbon support into graphene layers, catalyzed by Ni-richer catalyst, which, in turn, promoted particle migration/coalescence and, hence, more-significant sintering. Therefore, our results provide important insight for the synthesis of carbon-supported Pt-alloy fuel-cell catalysts.

Nadine Menzel, Erik Ortel, Katharina Mette, Ralph Kraehnert and Peter Strasser

Dimensionally Stable Ru/Ir/TiO₂-Anodes with Tailored Mesoporosity for Efficient Electrochemical Chlorine Evolution

ACS Catal. 3 (6), 1324 - 1333

DOI: 10.1021/cs4000238

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Chlorine evolution is one of the most important electrochemical reactions applied in industry. We present a method for the synthesis of chlorine evolution catalysts with improved performance. The performance increase results from the introduction of controlled mesoporosity into the pore system of Ru- and Ir-containing TiO₂ catalysts by pore templating with micelles of amphiphilic block-copolymers. Micelle-templated TiO₂-based catalysts were synthesized with loadings up to 15 wt % of either Ru, Ir, or a combination of both active metals. The catalysts' walls are composed of nanocrystalline mixed oxides with rutile structure. The templated mesopores are about 10 nm in size and form an ordered cubic pore system with good pore connectivity. All studied catalysts are active in chlorine evolution. Adding templated mesoporosity doubles the catalyst performance at identical catalyst composition. The influences of film thickness, composition, and porosity of the developed catalytic coatings on the catalytic performance are discussed.

Björn Eckhardt, Erik Ortel, Denis Bernsmeier, Jörg Polte, Peter Strasser, Ulla Vainio, Franziska Emmerling and Ralph Kraehnert

Micelle-Templated Oxides and Carbonates of Zinc, Cobalt, and Aluminum and a Generalized Strategy for Their Synthesis

Chem. Mater. 25 (14), 2749 – 2758

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Catalysis, energy storage, and light harvesting require functional materials with tailored porosity and nanostructure. However, common synthesis methods that employ polymer micelles as structure-directing agents fail for zinc oxide, for cobalt oxide, and for metal carbonates in general. We report the synthesis of the oxides and carbonates of zinc, cobalt, and aluminum with micelle-templated structure. The synthesis relies on poly(ethylene oxide)-block-poly(butadiene)-block-poly(ethylene oxide) triblock copolymers and a new type of precursor formed by chemical complexation of a metal nitrate with citric acid. A general synthesis mechanism is deduced. Mechanistic insights allow for the prediction of optimal processing conditions for different oxides and carbonates based on simple thermogravimetric analysis. Employing this synthesis, films of ZnO and Co₃O₄ with micelle-controlled mesoporosity become accessible for the first time. It is the only soft-templating method reported so far that also yields mesoporous metal carbonates. The developed synthesis is generic in nature and can be applied to many other metal oxides and carbonates.

Lin Gan, Marc Heggen, Rachel O'Malley, Brian Theobald and Peter Strasser

Understanding and Controlling Nanoporosity Formation for Improving the Stability of Bimetallic Fuel Cell Catalysts

Nano Lett. 13 (3),1131 - 1138

DOI: 10.1021/nl304488q

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Nanoporosity is a frequently reported phenomenon in bimetallic particle ensembles used as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. It is generally considered a favorable characteristic, because it increases the catalytically active surface area. However, the effect of nanoporosity on the intrinsic activity and stability of a nanoparticle electrocatalyst has remained unclear. Here, we present a facile atmosphere-controlled acid leaching technique to control the formation of nanoporosity in Pt-Ni bimetallic nanoparticles. By statistical analysis of particle size, composition, nanoporosity, and atomic-scale core?shell fine structures before and after electrochemical stability test, we uncover that nanoporosity formation in particles larger than ca. 10 nm is intrinsically tied to a drastic dissolution of Ni and, as a result of this, a rapid drop in intrinsic catalytic activity during ORR testing, translating into severe catalyst performance degradation. In contrast, O₂-free acid leaching enabled the suppression of nanoporosity resulting in more solid core-shell particle architectures with thin Pt-enriched shells; surprisingly, such particles maintained high intrinsic activity and improved catalytic durability under otherwise identical ORR tests. On the basis of these findings, we suggest that catalytic stability could further improve by controlling the particle size below ca. 10 nm to avoid nanoporosity. Our findings provide an explanation for the degradation of bimetallic particle ensembles and show an easy to implement pathway toward more durable fuel cell cathode catalysts.

Samira Siahrostami, Mårten E. Björketun, Peter Strasser, Jeff Greeleyand Jan Rossmeisl

Tandem cathode for proton exchange membrane fuel cells

Phys. Chem. Chem. Phys. 15 (23), 9326 - 9334

DOI: 10.1039/c3cp51479j

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The efficiency of proton exchange membrane fuel cells is limited mainly by the oxygen reduction reaction at the cathode. The large cathodic overpotential is caused by correlations between binding energies of reaction intermediates in the reduction of oxygen to water. This work introduces a novel tandem cathode design where the full oxygen reduction, involving four electron-transfer steps, is divided into formation (equilibrium potential 0.70 V) followed by reduction (equilibrium potential 1.76 V) of hydrogen peroxide. The two part reactions contain only two electron-transfer steps and one reaction intermediate each, and they occur on different catalyst surfaces. As a result they can be optimized independently and the fundamental problem associated with the four-electron catalysis is avoided. A combination of density functional theory calculations and published experimental data is used to identify potentially active and selective materials for both catalysts. Co-porphyrin is recommended for the first step, formation of hydrogen peroxide, and three different metal oxides - SrTiO₃(100), CaTiO₃(100) and WO₃(100) - are suggested for the subsequent reduction step.

Chunhua Cui, Mahdi Ahmadi, Farzad Behafarid, Lin Gan, Maximilian Neumann, Marc Heggen, Beatriz R. Cuenya and Peter. Strasser

Shape-selected bimetallic nanoparticle electrocatalysts: evolution of their atomic-scale structure, chemical composition, and electrochemical reactivity under various chemical environments

Faraday Discuss. 162 , 91 - 112

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Solid surfaces generally respond sensitively to their environment. Gas phase or liquid phase species may adsorb and react with individual surface atoms altering the solid-gas and solid-liquid electronic and chemical properties of the interface. A comprehensive understanding of chemical and electrochemical interfaces with respect to their responses to external stimuli is still missing. The evolution of the structure and composition of shape-selected octahedral PtNi nanoparticles (NPs) in response to chemical (gas-phase) and electrochemical (liquid-phase) environments was studied, and contrasted to that of pure Pt and spherical PtNi NPs. The NPs were exposed to thermal annealing in hydrogen, oxygen, and vacuum, and the resulting NP surface composition was analyzed using X-ray photoelectron spectroscopy (XPS). In gaseous environments, the presence of O₂ during annealing (300 °C) lead to a strong segregation of Ni species to the NP surface, the formation of NiO, and a Pt-rich NP core, while a similar treatment in H₂ lead to a more homogenous Pt-Ni alloy core, and a thinner NiO shell. Further, the initial presence of NiO species on the as-prepared samples was found to influence the atomic segregation trends upon low temperature annealing (300 °C). This is due to the fact that at this temperature nickel is only partially reduced, and NiO favors surface segregation. The effect of electrochemical cycling in acid and alkaline electrolytes on the structure and composition of the octahedral PtNi NPs was monitored using image-corrected high resolution transmission electron microscopy (TEM) and high-angle annular dark field scanning TEM (HAADF-STEM). Sample pretreatments in surface active oxygenates, such as oxygen and hydroxide anions, resulted in oxygen-enriched Ni surfaces (Ni oxides and/or hydroxides). Acid treatments were found to strongly reduce the content of Ni species on the NP surface, via its dissolution in the electrolyte, leading to a Pt-skeleton structure, with a thick Pt shell and a Pt-Ni core. The presence of Ni hydroxides on the NP surface was shown to improve the kinetics of the electrooxidation of CO and the electrocatalytic hydrogen evolution reactions. The affinity to water and the oxophilicity of Ni hydroxides are proposed as likely origin of the observed effects.

Chunhua Cui, Lin Gan, Marc Heggen, Stefan Rudi and Peter Strasser

Compositional Segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis

Nature Materials 12 (8), 765  - 771

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Shape-selective monometallic nanocatalysts offer activity benefits based on structural sensitivity and high surface area. In bimetallic nanoalloys with well-defined shape, site-dependent metal surface segregation additionally affects the catalytic activity and stability. However, segregation on shaped alloy nanocatalysts and their atomic-scale evolution is largely unexplored. Exemplified by three octahedral Pt_xNi_1−x alloy nanoparticle electrocatalysts with unique activity for the oxygen reduction reaction at fuel cell cathodes, we reveal an unexpected compositional segregation structure across the {111} facets using aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy. In contrast to theoretical predictions, the pristine Pt_xNi_1−x nano-octahedra feature a Pt-rich frame along their edges and corners, whereas their Ni atoms are preferentially segregated in their {111} facet region. We follow their morphological and compositional evolution in electrochemical environments and correlate this with their exceptional catalytic activity. The octahedra preferentially leach in their facet centres and evolve into ‘concave octahedra’. More generally, the segregation and leaching mechanisms revealed here highlight the complexity with which shape-selective nanoalloys form and evolve under reactive conditions.

Xenia Tuaev, Stefan Rudi, Valeri Petkov, Armin Hoell and Peter Strasser

In Situ Study of Atomic Structure Transformations of Pt-Ni Nanoparticle Catalysts during Electrochemical Potential Cycling

ACS Nano 7 (7), 5666 – 5674

DOI: 10.1021/nn402406k

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When exposed to corrosive anodic electrochemical environments, Pt alloy nanoparticles (NPs) undergo selective dissolution of the less noble component, resulting in catalytically active bimetallic Pt-rich core-shell structures. Structural evolution of PtNi₆ and PtNi₃ NP catalysts during their electrochemical activation and catalysis was studied by in situ anomalous small-angle X-ray scattering to obtain insight in element-specific particle size evolution and time-resolved insight in the intraparticle structure evolution. Ex situ high-energy X-ray diffraction coupled with pair distribution function analysis was employed to obtain detailed information on the atomic-scale ordering, particle phases, structural coherence lengths, and particle segregation. Our studies reveal a spontaneous electrochemically induced formation of PtNi particles of ordered Au₃Cu-type alloy structures from disordered alloy phases (solid solutions) concomitant with surface Ni dissolution, which is coupled to spontaneous residual Ni metal segregation during the activation of PtNi6. Pt-enriched core-shell structures were not formed using the studied Ni-rich nanoparticle precursors. In contrast, disordered PtNi₃ alloy nanoparticles lose Ni more rapidly, forming Pt-enriched core-shell structures with superior catalytic activity. Our X-ray scattering results are confirmed by STEM/EELS results on similar nanoparticles.

Ruizhi Yang, Weiyong Bian, Peter Strasser, Michael F. Toney

Dealloyed PdCu₃ thin film electrocatalysts for oxygen reduction reaction

Journal of Power Sources 222, 169 - 176 

DOI: 10.1016/j.jpowsour.2012.08.064

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The catalytic activity of electrochemically dealloyed PdCu₃ thin films for oxygen reduction reaction (ORR) in acidic media has been studied by using a rotating disk electrode (RDE). The dealloyed PdCu₃ thin films show a ∼2.0 fold increase in the specific oxygen reduction activity over pure Pd thin films. The structure of electrochemically dealloyed PdCu₃ thin films has been investigated at an atomic scale by synchrotron-based anomalous X-ray diffraction (AXRD). AXRD reveals that a Pd enriched surface layer is formed in the dealloyed film and a compressive lattice strain exists in this Pd surface layer. The enhanced catalytic activity of dealloyed Pd-Cu films for the ORR is primarily due to the compressive strain in the surface layer. We compare the structure-composition-catalytic activity relationships in dealloyed Pd-Cu thin films to related results on dealloyed Pt-Cu thin films. These studies show that dealloying and the resulting structure and the ORR activity are dependent on the nature of the noble component of alloy.

F. Münch, M. Özaslan, M. Rauber, S. Kaserer, A. Fuchs, E. Mankel, J. Brotz, P. Strasser, C. Roth, W. Ensinger

Electroless synthesis of nanostructured nickel and nickel-boron tubes and their performance as unsupported ethanol electrooxidation catalysts

Journal of Power Sources 222, 243 - 252 

DOI: 10.1016/j.jpowsour.2012.08.067

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Considering the low abundance of platinum group metals and the high catalytic performance of nickel for the oxidation of small organic molecules, nickel catalysts are promising substitutional materials for direct alcohol fuel cells. Despite the simplicity, good scalability and flexibility of electroless plating, reports on the fabrication of nickel-based catalysts with this method are rare, in particular regarding the deposition of pure nickel. To expand the existing synthetic repertoire, we developed an electroless plating bath allowing the homogeneous deposition of spiky nickel films on very complex shaped substrates. Nanostructured nickel and nickel-boron tubes were obtained by combination of the new and a borane-based plating reaction polymer templates, respectively. The composition, morphology and crystallinity of the products was comprehensively investigated with X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Finally, the nickel and nickel-boron tubes were applied as unsupported electrocatalysts for the oxidation of ethanol (EtOH) in alkaline environment. Compared to a macroscopic reference, both of the nanostructured catalysts showed improved utilization of high EtOH concentrations and considerably increased oxidation activities, rendering the applied deposition reactions promising routes towards novel catalysts for direct alcohol fuel cells.

2012

Koteswara R. Vuyyuru, Peter Strasser

Oxidation of biomass derived 5-hydroxymethylfurfural using heterogeneous and electrochemical catalysis

Catalysis Today 195 (1), 144 - 154

DOI: 10.1016/j.cattod.2012.05.008

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The concept of biorefineries involves multiple catalytic processes to convert biomass feedstock into valuable chemical products. While conventional catalytic biomass-related processes are thermally activated, the catalytic activation of biomass conversion reactions using an electrical interfacial potential (electrocatalysis) is new and poorly explored to date. Here, we report a comparative study of biomass catalysis using thermal heterogeneous and electrocatalysis in liquid-phase. First, the oxidation of the biomass-derived model molecule 5-hydroxymethyl-2-furfural (HMF) was studied using aqueous-phase heterogeneous catalysis under mild temperature (50 °C) and pressure (10 bar O₂) conditions. Oxidation reactions were carried out in a semi-batch reactor to test catalytic activity of Pt/C catalyst and compared with Au/TiO₂, Ru/C, Rh/C and Pd/C. The important reaction parameters such as influence of pH, effect of pressure and type of catalytic metal surface were explored. HMF degradation at higher pH in the absence of metal catalyst was discussed using in situ NMR study. At lower pH (≤7), the alcoholic group of HMF oxidizes faster than aldehyde on Pt surface, whereas at higher pH (≤13), oxidation of alcoholic group appears as the rate-limiting step. At given reaction conditions, Au shows better catalytic activity than Pt, Pd, Ru and Rh at pH 13. The heterogeneously catalyzed oxidation of HMF was then compared to the corresponding electrochemical oxidation catalysis. Electrochemical catalysis offers an added advantage by providing the electrode potential and the faradaic current as two additional external control parameters. These are helpful to tune the thermodynamic driving force, activation energy and thus the reaction rate and selectivity of complex reaction processes. The electrochemical activation of water at anodic electrode potentials results in the in situ generation of reactive oxygenated surface species from the aqueous solvent and thus eliminates the use of molecular oxygen. The electrocatalytic oxidation of HMF was found very selective for the formation of 2,5-furandicarbaldehyde.

A.R. Zeradjanin, N. Menzel, P. Strasser, W. Schuhmann

Role of Water in the Chlorine Evolution at RuO₂-Based Electrodes - Understanding Electrocatalysis as a Resonance Phenomenon

ChemSusChem 5 (10), 1897 - 1904

DOI: 10.1002/cssc.201200193

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The reaction path of the Cl₂ evolution reaction (CER) was investigated by combining electrochemical and spectroscopic methods. It is shown that oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO₂ and water. The state of the RuO₂ surface during the electrochemical reaction was analyzed in situ by using Raman spectroscopy to monitor vibrations of the crystal lattice of RuO₂ and changes in the surface concentration of the adsorbed species as a function of the electrode potential. The role of the solvent was recognized as being crucial in the formation of an oxygen-containing hydrophilic layer, which is a key prerequisite for electrocatalytic Cl₂ formation. Water (more precisely the OH adlayer) is understood not just as a medium that allows adsorption of intermediates, but also as an integral part of the intermediate formed during the electrochemical reaction. New insights into the general understanding of electrocatalysis were obtained by utilizing the vibration frequencies of the crystal lattice as a dynamic catalytic descriptor instead of thermodynamic descriptors, such as the adsorption energy of intermediates. Interpretation of the derived "volcano" curve suggests that electrocatalysis is governed by a resonance phenomenon.

M. Heggen, M. Özaslan, L. Houben, P. Strasser

Formation and Analysis of Core-Shell Fine Structures in Pt Bimetallic Nanoparticle Fuel Cell Electrocatalysts

J. Phys. Chem. C 116 (36), 19073 - 19083

DOI: 10.1021/jp306426a

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An Ångstrom-scale structural and compositional investigation of a dealloyed Pt−Co core−shell nanoparticle fuel cell catalyst with characteristic diameter of 10−15 nm in an early stage of its life cycle reveals unusual selforganized compositional subsurface fine structure, that is, subsequent shells of Co depletion and enrichment. The origin of the unusual structure is rationalized by interplay of Co dissolution, Pt surface diffusion, and an inverse Kirkendall effect. A detailed picture about the chemical composition of the surface and subsurface provides a fundamental insight into the catalytically active structure of bimetallic electrocatalysts.

T. Reier, M. Özaslan, P. Strasser

Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt catalysts: A Comparative Study of Nanoparticles and Bulk Materials

ACS Catalysis 2 (8), 1765 - 1772

DOI: 10.1021/cs3003098

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A comparative investigation was performed to examine the intrinsic catalytic activity and durability of carbon supported Ru, Ir, and Pt nanoparticles and corresponding bulk materials for the electrocatalytic oxygen evolution reaction (OER). The electrochemical surface characteristics of nanoparticles and bulk materials were studied by surface-sensitive cyclic voltammetry. Although basically similar voltammetric features were observed for nanoparticles and bulk materials of each metal, some differences were uncovered highlighting the changes in oxidation chemistry. On the basis of the electrochemical results, we demonstrated that Ru nanoparticles show lower passivation potentials compared to bulk Ru material. Ir nanoparticles completely lost their voltammetric metallic features during the voltage cycling, in contrast to the corresponding bulk material. Finally, Pt nanoparticles show an increased oxophilic nature compared to bulk Pt. With regard to the OER performance, the most pronounced effects of nanoscaling were identified for Ru and Pt catalysts. In particular, the Ru nanoparticles suffered from strong corrosion at applied OER potentials and were therefore unable to sustain the OER. The Pt nanoparticles exhibited a lower OER activity from the beginning on and were completely deactivated during the applied OER stability protocol, in contrast to the corresponding bulk Pt catalyst. We highlight that the OER activity and durability were comparable for Ir nanoparticles and bulk materials. Thus, Ir nanoparticles provide a high potential as nanoscaled OER catalyst.

J. Rossmeisl, P. Ferrin, G.A. Tritsaris, A.U. Nilekar, S. Koh, S.E. Bae, S.R. Brankovic, P. Strasser, M. Mavrikakis

Bifunctional anode catalysts for direct methanol fuel cells

Energy & Environmental Science 5 (8), 8335 - 8342 

DOI: 10.1039/c2ee21455e

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Using the binding energy of OH* and CO* on close-packed surfaces as reactivity descriptors, we screen bulk and surface alloy catalysts for methanol electro-oxidation activity. Using these two descriptors, we illustrate that a good methanol electro-oxidation catalyst must have three key properties: (1) the ability to activate methanol, (2) the ability to activate water, and (3) the ability to react off surface intermediates (such as CO* and OH*). Based on this analysis, an alloy catalyst made up of Cu and Pt should have a synergistic effect facilitating the activity towards methanol electro-oxidation. Using these two reactivity descriptors, a surface PtCu3 alloy is proposed to have the best catalytic properties of the Pt–Cu model catalysts tested, similar to those of a Pt–Ru bulk alloy. To validate the model, experiments on a Pt(111) surface modified with different amounts of Cu adatoms are performed. Adding Cu to a Pt(111) surface increases the methanol oxidation current by more than a factor of three, supporting our theoretical predictions for improved electrocatalysts.

C. Yu, E.F. Holby, R. Yang, M.F. Toney, D. Morgan, P. Strasser

Growth Trajectories and Coarsening Mechanisms of Metal Nanoparticle Electrocatalysts

ChemCatChem 4 (6), 766 - 770

DOI: 10.1002/cctc.201200090

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In situ small angle X-ray scattering (SAXS) combined with an electrochemical rate model yield insight in the evolution of the particle size distributions (PSDs) of carbon-supported Pt nanoparticle ensembles. A critical 3–5 nm size region where particles remained structurally stable was identified. A dominant role for a surface energy-driven Ostwald growth mechanism was deduced.

K. Mette, A. Bergmann, J.-P. Tessonnier, M. Havecker, L. Yao, T. Ressler, R. Schlögl, P. Strasser, M. Behrens

Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting

ChemCatChem 4 (6), 851 - 862

DOI: 101002/cctc.201100434

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Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnO_x/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as-prepared state of a 6.5 wt. % MnO_x/CNT sample could be comprehensively characterized by comparison to an unsupported MnO_x reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as-prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnO_x compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post-treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5 wt. % MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state-of-art MnO_x catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions.

B. Eckhardt, E. Ortel, J. Polte, D. Bernsmeier, O. Görke, P. Strasser, R. Kraehnert

Micelle-templated Mesoporous Films of Magnesium Carbonate and Magnesium Oxide

Advanced Materials 24 (23), 3115 – 3119

DOI: 10.1002/adma.201104984

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Ordered mesoporous MgO films are synthesized via micelle-templating for the first time. The problems of low melting points, insolubility and excessive crystallization of the metal oxide precursors are overcome by synthesizing in-situ a magnesium nitrate-citric acid complex. The stepwise thermal transformation into magnesium carbonate and then MgO was studied as well as the evolution of the mesopore structure.

L. Gan, M. Heggen, S. Rudi, P. Strasser

Core−Shell Compositional Fine Structures of Dealloyed Pt_xNi_1−x Nanoparticles and Their Impact on Oxygen Reduction Catalysis

Nano Lett. 12 (10), 5423 - 5430

DOI: 10.1021/nl302995z

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Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy line profiles with Ångstrom resolution, we uncover novel core–shell fine structures in a series of catalytically active dealloyed Pt_xNi_1–x core–shell nanoparticles, showing the formation of unusual near-surface Ni-enriched inner shells. The radial location and the composition of the Ni-enriched inner shells were sensitively dependent on the initial alloy compositions. We further discuss how these self-organized Ni-enriched inner shells play a key role in maintaining surface lattice strain and thus control the surface catalytic activity for oxygen reduction.

C. Cui, L. Gan, H.-H. Li, S.-H. Yu, M. Heggen, P. Strasser

Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition

Nano Lett. 12 (11), 5885 - 5889

DOI: 10.1021/nl3032795

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We demonstrate how shape selectivity and optimized surface composition result in exceptional oxygen reduction activity of octahedral PtNi nanoparticles (NPs). The alloy octahedra were obtained by utilizing a facile, completely surfactant-free solvothermal synthesis. We show that the choice of precursor ligands controls the shape, while the reaction time tunes the surface Pt:Ni composition. The 9.5 nm sized PtNi octahedra reached a 10-fold surface area-specific (∼3.14 mA/cm_Pt²) as well as an unprecedented 10-fold Pt mass based (∼1.45 A/mg_Pt) activity gain over the state-of-art Pt electrocatalyst, approaching the theoretically predicted limits.

A. Marcu, G. Toth, R. Srivastava, P. Strasser

Preparation, characterization and degradation mechanisms of PtCu alloy nanoparticles for automotive fuel cells

Journal Power Sources 208, 288 - 295

DOI: 10.1016/j.jpowsour.2012.02.065

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Electrochemically dealloyed PtCu alloy nanoparticles successfully meet the automotive technology target of having four times higher Pt mass activity for the electroreduction of molecular oxygen compared to current state-of-the-art platinum catalysts. However, the catalysts must also maintain their activity throughout the aggressive automotive drive-cycles in order to be implemented in fuel cells cars. Here, the durability of dealloyed PtCu catalysts was systematically evaluated under various voltage-cycles using a rotating ring disk electrode. The stability of the non-noble metal alloy component was proven at electrode potentials below 0.6 V. The platinum stability was evaluated at potentials up to 1.1 V to avoid carbon corrosion and then up to 1.2 V to be closer to the more aggressive cycles developed in startup/shutdown events of the fuel cells. The major known failure modes such as non-noble metal dissolution, platinum dissolution, and particle growth/agglomeration were monitored in order to understand closely the PtCu nanoparticles behavior under different potential cycles and to provide a degradation fingerprint.

F. Hasché, T.-P. Fellinger, M. Oezaslan, J.P. Paraknowitsch, M. Antonietti, P. Strasser

Mesoporous Nitrogen Doped Carbon Supported Platinum PEM Fuel Cell Electrocatalyst Made From Ionic Liquids

ChemCatChem 4 (4), 479 - 483 

DOI: 10.1002/cctc.201100408

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An ionic liquid based mesoporous nitrogen doped carbon supported Pt nanoparticle fuel cell electrocatalyst (Pt/meso-BMP) as a new material class was synthesized and examined for the oxygen reduction reaction (ORR). The structural and electrochemical characterization of this new material class shows a high electrochemical active surface area (ECSA) and the suitability for the ORR.

N. Menzel, E. Ortel, R. Kraehnert, P. Strasser

Electrocatalysis Using Porous Nanostructured Materials

ChemPhysChem 13 (6), 1385  -1394

DOI: 10.1002/cphc.201100984

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The performance of electrochemical reactions depends strongly on the morphology and structure of the employed catalytic electrodes. Nanostructuring of the electrode surface represents a powerful tool to increase the electrochemically active surface area of the electrodes. Moreover, it can also facilitate faster diffusive mass transport inside three-dimensional electrodes. This minireview describes recent trends in the development of synthesis routes for porous nanostructured electrode materials and discusses the respective important electrocatalytic applications. The use of structure-directing agents will play a decisive role in the design and synthesis of improved catalysts.

F. Hasché, M. Oezaslan, P. Strasser

In Situ Observation of the Thermally Induced Growth of Platinum-Nanoparticle Catalysts Using High-Temperature X-ray Diffraction

ChemPhysChem 13 (3), 828 - 834

DOI: 10.1002/cphc.201100857

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Fundamental understanding about the thermal stability of nanoparticles and deliberate control of structural and morphological changes under reactive conditions is of general importance for a wide range of reaction processes in heterogeneous and electrochemical catalysis. Herein, we present a parametric study of the thermal stability of carbon-supported Pt nanoparticles at 80 °C and 160 °C, with an initial particle size below 3 nm, using in situ high-temperature X-ray diffraction (HT-XRD). The effects on the thermal stability of carbon-supported Pt nanoparticles are investigated with control parameters such as Brunauer–Emmet–Teller (BET) surface area, metal loading, temperature, and gas environment. We demonstrate that the growth rate exhibits a complex, nonlinear behavior and is largely controlled by the temperature, the initial particle size, and the interparticle distance. In addition, an ex situ transmission electron microscopy study was performed to verify our results obtained from the in situ HT-XRD study.

T.-P. Fellinger, F. Hasché, P. Strasser, M. Antonietti

Mesoporous Nitrogen-Doped Carbon for the Electrocatalytic Synthesis of Hydrogen Peroxide

J. Am. Chem. Soc. 134 (9), 4072  - 4075

DOI:10.1021/ja300038p

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Mesoporous nitrogen-doped carbon derived from the ionic liquid N-butyl-3-methylpyridinium dicyanamide is a highly active, cheap, and selective metal-free catalyst for the electrochemical synthesis of hydrogen peroxide that has the potential for use in a safe, sustainable, and cheap flow-reactor-based method for H₂O₂ production.

P. Mathew, J.P. Meyers, R. Srivastava, P. Strasser

Analysis of Surface Oxidation on Pt and Pt Core-Shell Electrocatalysts for PEFCs

J. Electrochem. Soc. 159 (5), B1 - B10

doi: 10.1149/2.066205jes

Studies on the formation and reduction of surface oxide on Pt-Cu/C core-shell electrocatalysts have been carried out with reference to the nature and type of the adsorbed intermediates formed on the surface as a function of applied potential. In-situ measurements were made using EQCM coupled with cyclic voltammetry to monitor the surface changes. Comparisons between the novel “core-shell” electrocatalysts, conventional Pt/C electrocatalysts and unsupported Pt black electrocatalysts have been made. We find that interfacial mass of the adsorbed species increases in a continuous linear manner as the potential is increased during the anodic oxidation. We also find evidence to suggest that the oxide formation on the core-shell electrocatalyst occurs via the formation of adsorbed hydroxyl species involving one electron per surface site and that on pure Pt proceeds via adsorbed oxide involving two electrons per site. A different mechanism for oxide growth on these catalysts was confirmed by the change in the Tafel slopes for anodic oxidation of the surface. The results showed that the core-shell catalyst surface is less oxidized and that the surface strain imposes a barrier to surface oxidation. This also explains the shift in the oxide stripping peak that has been observed for the Pt binary alloy catalysts.

M. Oezaslan, M. Heggen, P. Strasser

Size-dependent morphology of dealloyed bimetallic catalysts: Linking the nano to the macro scale

J. Am. Chem. Soc. 134, 514 - 524

DOI: 10.1021/ja2088162

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Chemical dealloying of Pt binary alloy precursors has emerged as a novel and important preparation process for highly active fuel cell catalysts. Dealloying is a selective (electro)chemical leaching of a less noble metal M from a M rich Pt alloy precursor material and has been a familiar subject of macroscale corrosion technology for decades. The atomic processes occurring during the dealloying of nanoscale materials, however, are virtually unexplored and hence poorly understood. Here, we have investigated how the morphology and intraparticle composition depend on the particle size of dealloyed Pt–Co and Pt–Cu alloy nanoparticle precursor catalysts. To examine the size–morphology–composition relation, we used a combination of high-resolutionscanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), electron energy loss (EEL) spectroscopy, energy-dispersive X-ray spectroscopy (EDS), and surface-sensitive cycling voltammetry. Our results indicate the existence of three distinctly different size-dependent morphology regimes in dealloyed Pt–Co and Pt–Cu particle ensembles: (i) The arrangement of Pt shell surrounding a single alloy core (“single core–shell nanoparticles”) is exclusively formed by dealloying of particles below a characteristic diameter d_{multiple cores} of 10–15 nm. (ii) Above d_{multiple cores}, nonporous bimetallic core–shell particles dominate and show structures with irregular shaped multiple Co/Cu rich cores (“multiple cores–shell nanoparticles”). (iii) Above the second characteristic diameter d_pores of about 30 nm, the dealloyed Pt–Co and Pt–Cu particles start to show surface pits and nanoscale pores next to multiple Co/Cu rich cores. This structure prevails up to macroscopic bulklike dealloyed particles with diameter of more than 100 nm. The size–morphology–composition relationships link the nano to the macro scale and provide an insight into the existing material gap of dealloyed nanoparticles and highly porous bulklike bimetallic particles in corrosion science.

X. Tuaev, J. P. Paraknowitsch, R. Illgen, A. Thomas, P. Strasser

Nitrogen-doped coatings on carbon nanotubes and their stabilizing effect on Pt nanoparticles

Phys. Chem. Chem. Phys. 14 (18), 6444 - 6447

DOI: 10.1039/c2cp40760d

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A homogeneous coating of nitrogen-doped carbon on carbon nanotubes is performed using ionic liquids. The N-doped material is employed as a support for nanoparticles. Electrochemical degradation behavior is monitored in situ and compared to an unmodified material. The strongly enhanced stability is explained on the basis of a Pt–nitrogen interaction.

F. Hasché, M. Oezaslan, P. Strasser

Activity, Structure, and Degradation of Dealloyed PtNi₃ Nanoparticle Electrocatalyst for the Oxygen Reduction Reaction in PEMFC

J. Electrochem. Soc. 159 (1), B25 - B29

DOI: 10.1149/2.030201jes

We report a synthesis, activity and stability study of a dealloyed, highly active PtNi₃ alloy nanoparticle catalyst for the oxygen reduction reaction (ORR) in acidic media. After activation by electrochemical dealloying of a PtNi₃ precursor, the Pt-Ni nanoparticle catalyst (referred to as “dealloyed PtNi₃”) exhibits a 7–8 times higher Pt mass based activity and a 6–7 times higher Pt surface area specific based activity for ORR than pure Pt at comparable mean particle size. In addition, the long-term stability of the dealloyed PtNi₃ was tested for typical fuel cell operating as well as more corrosive fuel cell start-up conditions. After 10000 voltage cycles between 0.5–1.0 V vs. RHE at 50 mV s⁻¹ the dealloyed PtNi₃ catalyst still shows 4–5 fold increase in Pt surface area specific based activity compared with that for pure Pt.

M. Oezaslan, F. Hasché, P. Strasser

Oxygen Electroreduction on PtCo₃, PtCo and Pt₃Co Alloy Nanoparticles for Alkaline and Acidic PEM Fuel Cells

J. Electrochem. Soc. 159 (4), B394 - B405

DOI: 10.1149/2.075204jes

Pt-Co alloy nanoparticles have emerged as one of the most promising electrocatalysts for the oxygen reduction reaction (ORR) in hydrogen fuel cells. Our study presents a comprehensive structural, compositional and electrochemical characterization linked with ORR activity for carbon supported PtCo₃, PtCo, and Pt₃Co alloy nanoparticle catalysts in 0.1 M HClO₄ and 0.1 M KOH. Surface-sensitive cyclic voltammetry was used to investigate the changes of composition of outermost atomic layers of Pt-Co alloys. Our electrochemical results in alkaline media clearly show the stability and voltage-induced accumulation of Co on the particle surface, whereas in 0.1 M HClO₄ the voltage cycling initiates the rapid dissolution of Co to form a Pt-enriched surface surrounding by alloy core. We correlated the ECSA and ORR activity with the as-synthesized chemical composition of Pt-Co alloys. In results, after electrochemical treatment in 0.1 M HClO₄ the Pt mass based activities (j_mass) increase according: Pt(HT) < PtCo < Pt₃Co < PtCo₃ at comparable particle size. Unlike to acid, after voltage cycling in 0.1 M KOH j_mass increase according: PtCo₃ < Pt(HT) < PtCo < Pt₃Co. However, in 0.1 M KOH activated PtCo₃ core-shell catalyst shows 4–5 fold higher mass activity compared to pure Pt and Pt(HT).

M. Oezaslan, F. Hasché, P. Strasser

PtCu₃, PtCu and Pt₃Cu Alloy Nanoparticle Electrocatalysts for Oxygen Reduction Reaction in Alkaline and Acidic Media

J. Electrochem. Soc. 159 (4), B444 - B454

DOI: 10.1149/2.106204jes

Dealloying of Pt bimetallic nanoparticles is a promising synthesis method to prepare highly active electrocatalysts for oxygen reduction reaction (ORR) in alkaline and acidic PEM fuel cells. We present here a structural, compositional and electrochemical characterization linked with ORR activity for carbon supported PtCu₃, PtCu, and Pt₃Cu alloy nanoparticles in different electrolytes and pH values. The effects of electrolyte and pH are systematically examined on the ECSA and Pt mass based activity (j_mass) for various Pt-Cu alloys. We observed the formation of Cu oxide species and redissolution/redeposition of Cu species during the voltage cycling up to 1.0 V/RHE in 0.1 M KOH. In contrast, the voltage cycling in 0.1 M HClO₄ immediately causes the dissolution of Cu and results in Pt-enriched particle surface. We have correlated the ECSA and mass activity with the as-synthesized composition in dependence on both electrolytes. In summary, after voltage cycling in 0.1 M HClO₄ the values of j_mass increase according: Pt₃Cu < PtCu < PtCu₃. However, after voltage cycling in 0.1 M KOH the values of j_mass increase in the following trend: PtCu₃ < PtCu < Pt₃Cu. Only after activation process, PtCu₃ core-shell catalyst shows significantly enhanced ORR activity in 0.1 M KOH compared to pure Pt.

L. Borchardt, F. Hasché, M. Oschatz, F. Schmidt, E. Kockrick, Ch. Ziegler, T. Lescouet, A. Bachmatiuk, B. Buechner, D. Farrusseng, P. Strasser, S. Kaskel

Transition metal loaded silicon carbide-derived carbons with enhanced catalytic properties

Carbon 50 (5), 1861 - 1870

DOI: 10.1016/j.carbon.2011.12.036

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Carbide-derived carbons (CDC) with incorporated transition metal nanoparticles (∼2.5 nm) were prepared using a microemulsion approach. Time-consuming post synthesis functionalization of the carbon support material can thus be avoided and nanoparticle sizes can be controlled by changing the microemulsion composition. This synthesis strategy is a technique for the preparation of highly porous carbon materials with a catalytically active component. In particular we investigated the integration of ruthenium, palladium, and platinum in a concentration ranging from 4.45 to 12 wt.%. It was found that the transition metal has a considerable influence on sorption properties of resulting nanoparticle-CDC composite materials. Depending on the used metal salt additive the surface area and the pore volume ranges from 1480 m²/g and 1.25 cm³/g for Pt to 2480 m²/g and 2.0 cm³/g for Ru doped carbons. Moreover, members of this material class show impressive properties as heterogeneous catalysts. The liquid phase oxidation of tetralin and the partial oxidation of methane were studied, and electrochemical applications were also investigated. Primarily Pt doped CDCs are highly active in the oxygen reduction reaction, which is of great importance in present day fuel cell research.