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Lupe
Lupe
2014
Lin Gan, Chunhua Cui, Marc Heggen, Fabio Dionigi, Stefan Rudi and Peter Strasser

Element-specific anisotropic growth of shaped platinum alloy nanocrystals

Science 346, 1502-1506

DOI:10.1126/science.1261212

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Morphological shape in chemistry and biology owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-corrected scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chemical degradation pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.
Arindam Indra, Prashanth W. Menezes, Nastaran Ranjbar Sahraie, Arno Bergmann, Chittaranjan Das, Massimo Tallarida, Dieter Schmeißer, Peter Strasser and Matthias Driess
 
Unification of Catalytic Water Oxidation and Oxygen Reduction Reactions: Amorphous Beat Crystalline Cobalt Iron Oxides
 
J. Am. Chem. Soc. 136 (50), 17530 – 17536
 
DOI:10.1021/ja509348t
 
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Catalytic water splitting to hydrogen and oxygen is considered as one of the convenient routes for the sustainable energy conversion. Bifunctional catalysts for the electrocatalytic oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are pivotal for the energy conversion and storage, and alternatively, the photochemical water oxidation in biomimetic fashion is also considered as the most useful way to convert solar energy into chemical energy. Here we present a facile solvothermal route to control the synthesis of amorphous and crystalline cobalt iron oxides by controlling the crystallinity of the materials with changing solvent and reaction time and further utilize these materials as multifunctional catalysts for the unification of photochemical and electrochemical water oxidation as well as for the oxygen reduction reaction. Notably, the amorphous cobalt iron oxide produces superior catalytic activity over the crystalline one under photochemical and electrochemical water oxidation and oxygen reduction conditions.
Serhiy Cherevko, Tobias Reier, Aleksandar R. Zeradjanin, Zarina Pawolek, Peter Strasser and Karl J. J. Mayrhofer

Stability of nanostructured iridium oxide electrocatalysts during oxygen evolution reaction in acidic environment

Electrochemistry Communications 48, 81 - 85

Doi:10.1016/j.elecom.2014.08.027

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The electrochemical stability of thermally prepared Ir oxide films is investigated using a scanning flow cell (SFC)–inductively coupled plasma mass-spectrometer (ICP-MS) setup under transient and stationary potential and/or current conditions. Time-resolved dissolution rates provide important insights into critical conditions for material breakdown and a fully quantitative in-situ assessment of the electrochemical stability during oxygen evolution reaction (OER) conditions. In particular, the results demonstrate that stability and OER activity of the IrOx catalysts strongly depend on the chemical and structural nature of Ir oxide species and their synthesis conditions.
Nastaran Ranjbar Sahraie, Jens Peter Paraknowitsch, Caren Göbel, Arne Thomas, Peter Strasser
 
Noble-Metal-Free Electrocatalysts with Enhanced ORR Performance by Task-Specific Functionalization of Carbon using Ionic Liquid Precursor Systems
 
J. Am. Chem. Soc. 136 (41), 14486 - 14497  
 
DOI: 10.1021/ja506553r

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The synthesis and characterization of functionalized carbon using variable doping profiles are presented. The hybrids were obtained from nitrile-functionalized ionic precursors and a ferric chloride mediator. This way, novel nitrogen doped and nitrogen‑sulfur, nitrogen‑phosphorus, and nitrogen‑boron codoped carbon hybrids with a morphology containing microporous nanometer-sized particles were obtained. As-prepared heteroatom doped carbons exhibited superior electrocatalytic activity toward the oxygen reduction reaction (ORR) in alkaline and acid electrolytes. In particular, both the heteroatom type and iron were found to play crucial roles in improving the catalytic activity of functionalized carbon. It is worth noting that sulfur−nitrogen codoped functionalized materials synthesized in the presence of ferric chloride showed higher activity and stability in comparison to those of the commercial state-of-the-art Pt catalyst in alkaline electrolyte. Moreover, in acid electrolyte, sulfur‑nitrogen codoped catalyst rivaled the activity of Pt with a stability outperforming that of Pt. Our X-ray photoelectron spectroscopy (XPS) investigation revealed a distinctive atomic structure in nitrogen−sulfur codoped material in comparison to other codoped catalysts, most likely explaining its superior electrocatalytic activity. This work presents a novel toolbox for designing advanced carbon hybrids with variable heteroatom doping profiles which presents tunable and enhanced ORR performance.
Jiao Wu, Zhenrong Yang, Qijun Sun, Xiaowei Li, Peter Strasser, Ruizhi Yang
 
Synthesis and electrocatalytic activity of phosphorus-doped carbon xerogel for oxygen reduction
 
Electrochimica Acta  127, 53 – 60
 
doi:10.1016/j.electacta.2014.02.016

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The electrocatalyst for oxygen reduction reaction (ORR) plays an important role in determining the per-formance, cost and durability of fuel cells and metal–air batteries. In this study, low-cost and highly active phosphorus (P)-doped carbon xerogel electrocatalyst for the ORR was facilely synthesized. The catalytic activity of P-doped carbon xerogel for the ORR in 0.1 M KOH solution has been studied by using a rotating ring-disk electrode (RRDE) technique. The RRDE results show that P-doped carbon xerogel exhibits excellent catalytic activity for the ORR and long-term stability in basic media. The ORR on P-doped carbon xerogel with optimized amount of P mainly favors a direct four electron pathway. The high electrocatalytic activity and durability of P-doped carbon xerogel are primarily attributed to the P-doping in the carbon lattice. Furthermore, the amount of P incorporated into carbon instead of the specific surface area of the P-doped carbon xerogel is found to play a critical role in the ORR activity enhancement and the ORR pathway modification.
Rameshwori Loukrakpam, Qiuyi Yuan, Valeri Petkov, Lin Gan, Stefan Rudi, Ruizhi Yang, Yunhui Huang, Stanko R. Brankovic and Peter Strasser

Efficient C–C bond splitting on Pt monolayer and sub-monolayer catalysts during ethanol electrooxidation: Pt layer strain and morphology effects

Phys. Chem. Chem. Phys. 16, 18866 - 18876

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Efficient catalytic C–C bond splitting coupled with complete 12-electron oxidation of the ethanol molecule to CO2 is reported on nanoscale electrocatalysts comprised of a Pt monolayer (ML) and sub-monolayer (sML) deposited on Au nanoparticles (Au@Pt ML/sML). The Au@Pt electrocatalysts were synthesized using surface limited redox replacement (SLRR) of an underpotentially deposited (UPD) Cu monolayer in an electrochemical cell reactor. Au@Pt ML showed improved catalytic activity for ethanol oxidation reaction (EOR) and, unlike their Pt bulk and Pt sML counterparts, was able to generate CO2 at very low electrode potentials owing to efficient C–C bond splitting. To explain this, we explore the hypothesis that competing strain effects due to the Pt layer coverage/morphology (compressive) and the Pt–Au lattice mismatch (tensile) control surface chemisorption and overall activity. Control experiments on well-defined model Pt monolayer systems are carried out involving a wide array of methods such as high-energy X-ray diffraction, pair-distribution function (PDF) analysis, in situ electrochemical FTIR spectroscopy, and in situ scanning tunneling microscopy. The vibrational fingerprints of adsorbed CO provide compelling evidence on the relation between surface bond strength, layer strain and morphology, and catalytic activity.
Hemma Mistry, Rulle Reske, Zhenhua Zeng, Zhi-Jian Zhao, Jeffrey Greeley, Peter Strasser and Beatriz Roldan Cuenya

Exceptional Size-Dependent Activity Enhancement in the Electroreduction of CO2 over Au Nanoparticles
 
J. Am. Chem. Soc. 136 (47), 16473 – 16476

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The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ~ 1−8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles.  
Zhen Li,Yan Jiang,Lixia Yuan,Ziqi Yi,Chao Wu,Yang Liu,Peter Strasser, and Yunhui Huang

A Highly Ordered Meso@Microporous Carbon-Supported Sulfur@Smaller Sulfur Core-Shell Structured Cathode for Li-S Batteries

ACS Nano 8 (9), 9295 – 9303

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For lithiumsulfur batteries, commercial application is hindered by the insulating nature of sulfur and the dissolution of the reaction intermediates of polysulfides. Here, we present an ordered meso-microporous coreshell carbon (MMCS) as a sulfur container, which combines the advantages of both mesoporous and microporous carbon. With large pore volume and highly ordered porous structure, the“core”promises a sufficient sulfur loading and a high utilization of the active material, while the“shell”containing microporous carbon and smaller sulfur acts as a physical barrier and stabilizes the cycle capability of the entire S/C composite. Such a S/MMCS composite exhibits a capacity as high as 837 mAh g 1 at 0.5 C after 200 cycles witha capacity retention of 80% vs the second cycle (a decay of only 0.1% per cycle), demonstrating that the diffusion of the polysulfides into the bulk electrolyte can be greatly reduced. We believe that the tailored highly ordered meso-microporous coreshell structured carbon can also be applicable for designing some other electrode materials for energy storage.
Stefan Rudi,Chunhua Cui, Lin Gan and Peter Strasser

Comparative Study of the Electrocatalytically Active Surface Areas (ECSAs) of Pt Alloy Nanoparticles Evaluated by Hupd and CO-stripping voltammetry

Electrocatalysis 5, 408 - 418

DOI 10.1007/s12678-014-0205-2

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This study intends to provide some insight in the up-to-date elusive assessment of a correct choice of method for estimating the active surface area of Pt alloy nanoparticle catalysts. Taking PtNi3nanoparticles as an example, we have compared three types of electrochemically active surface area (ECSA) data, CO-ECSA, Hupd-ECSA, and Hupd/CO-ECSA, which were evaluated from CO stripping and underpotentially deposited hydrogen stripping steps applied at different times along a reference catalyst activity test protocol. Considering a total of six different detailed voltammetric test protocols, we address Pt alloy particle size effects, analyze the effect of the time of application of CO and hydrogen stripping, and study their effect on the Pt mass and Pt surface-specific activities for the oxygen reduction reaction (ORR). In a discussion of theratio of CO charge to hydrogen charge, it is shown that this quantity is more complex than previously thought and not associated with a specific surface structure. The Hupd/COECSA data are found to be a reasonable balance for the estimate of surface area normalized, so-called specific catalytic ORR activities.
T. Reier, D. Teschner, T. Lunkenbein, A. Bergmann,S. Selve,R. Kraehnert,R. Schlögl and P. Strasser

Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species


J. Electrochem. Soc. 161 (9), 876 - 882

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The morphology, crystallinity, and chemical state of well-defined Ir oxide nanoscale thin-film catalysts prepared on Ti substrates at various calcination temperatures were investigated. Special emphasis was placed on the calcination temperature-dependent interaction between Ir oxide film and Ti substrate and its impact on the electrocatalytic oxygen evolution reaction (OER) activity. The Ir oxide films were characterized by scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and cyclic voltammetry. Furthermore, temperature programmed reduction was applied to study the Ir oxide species formed as a function of calcination temperature and its interaction with the Ti substrate. A previously unachieved correlation between the electrocatalytic OER activity and the nature and structural properties of the Ir oxide film was established. We find that the crystalline high temperature Ir oxide species is detrimental, whereas low temperature amorphous Ir oxy-hydroxides are highly active and efficient catalysts for the OER. Moreover, at the highest applied calcination temperature (550°C), Ti oxides, originating from the substrate, strongly affect chemical state and electrocatalytic OER activity of the Ir oxide film.
Hong Nhan Nong, Lin Gan, Elena Willinger, Detre Teschner and Peter Strasser

IrOx core–shell nanocatalysts for cost- and energy-efficient electrochemical water splitting

Chem. Sci. 5, 2955 - 2963

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A family of dealloyed metal–oxide hybrid (M1M2@M1Ox) core@shell nanoparticle catalysts is demonstrated to provide substantial advances toward more efficient and less expensive electrolytic water splitting. IrNi@IrOx nanoparticles were synthesized from IrNi precursor alloys through selective surface Ni dealloying and controlled surface oxidation of Ir. Detailed depth-resolved insight into chemical structure,composition, morphology, oxidation state was obtained using spectroscopic, diffraction, and scanning microscopic techniques (XANES, XRD, STEM-EDX, XPS), which confirmed our structural hypotheses at the outset. A 3-fold catalytic activity enhancement for the electrochemical oxygen evolution reaction (OER) over IrO2 and RuO2 benchmark catalysts was observed for the core–shell catalysts on a noble metal mass basis. Also, the active site-based intrinsic turnover frequency (TOF) was greatly enhanced for the most active IrNi@IrOx catalyst. This study documents the successful use of synthetic dealloying for the preparation of metal–oxide hybrid core–shell catalysts. The concept is quite general, can be applied to other noble metal nanoparticles, and points out a path forward to nanostructured proton exchange-electrolyzer electrodes with dramatically reduced noble metal content.
Rulle Reske, Hemma Mistry, Farzad Behafarid, Beatriz Roldan Cuenya and Peter Strasser

Particle Size Effects in the Catalytic Electroreduction of CO2 on Cu Nanoparticles


J. Am. Chem. Soc. 136, 6978 − 6986

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A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2−15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity−selectivity− size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (∼2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification.
Aleksandar R. Zeradjanin, Nadine Menzel, Wolfgang Schuhmann and Peter Strasser

On the faradaic selectivity and the role of surface inhomogeneity during the chlorine evolution reaction on ternary Ti–Ru–Ir mixed metal oxide electrocatalysts

Phys. Chem. Chem. Phys 16 (27), 13741 - 13747

DOI: 10.1039/c4cp00896

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The faradaic selectivity of the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) on the industrially important Ti–Ru–Ir mixed metal oxide is discussed. Absolute evolution rates as well as volume fractions of Cl2 and O2 were quantified using differential electrochemical mass spectrometry (DEMS), while the catalyst surface redox behavior was analyzed using cyclic voltammetry. The spatial inhomogeneity of the surface catalytic reaction rate was probed using Scanning Electrochemical Microscopy (SECM). Although the nature of the competition between electrochemical discharging of chloride ions and water molecules remains elusive on a molecular scale, new insights into the spatial reactivity
distribution of the CER and OER were obtained. Oxidation of water is the initial step in corrosion and concomitant deactivation of the oxide electrodes; however, at the same time the nature of interaction between the oxide surface and water is used as a rational indicator of selectivity and catalytic activity. An experimental procedure was established that would allow the study of selectivity of a variety of differentcatalyst materials using polycrystalline electrode surfaces.
Nina Erini, Rameshwori Loukrakpam, Valeri Petkov, Elena A. Baranova, Ruizhi Yang, Detre Teschner, Yunhui Huang, Stanko R. Brankovic and Peter Strasser

Ethanol Electro-Oxidation on Ternary Platinum−Rhodium−Tin Nanocatalysts: Insights in the Atomic 3D Structure of the Active Catalytic Phase

ACS Catal. 4 (6), 1859 − 1867

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Novel insights in the synthesis−structure−catalytic activity relationships of nanostructured trimetallic Pt−Rh−Sn electrocatalysts for the electrocatalytic oxidation of ethanol are reported. In particular, we identify a novel single-phase Rh-doped Pt−Sn Niggliite mineral phase as the source of catalytically active sites for ethanol oxidation; we discuss its morphology, composition, chemical surface state, and the detailed 3D atomic arrangement using high-energy (HE-XRD), atomic pair distribution function (PDF) analysis, and X-ray photoelectron spectroscopy (XPS). The intrinsic ethanol oxidation activity of the active Niggliite phase exceeded those of earlier reports, lending support to the notion that the atomic-scale neighborhood of Pt, Rh, and Sn is conducive to the emergence of active surface catalytic sites under reaction conditions. In situ mechanistic Fourier transform infrared (in situ FTIR) analysis confirms an active 12 electron oxidation reaction channel to CO2 at electrode potentials as low as 450 mV/RHE, demonstrating the favorable efficiency of the PtRhSn Niggliite phase for C−C bond splitting.
Chunhua Cui, Lin Gan, Maximilian Neumann, Marc Heggen, Beatriz Roldan Cuenya and Peter Strasser

Carbon Monoxide-Assisted Size Confinement of Bimetallic Alloy Nanoparticles

J. Am. Chem. Soc. 136 (13), 4813 − 4816

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Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 degrees C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of similar to 4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations.
Panagiotis Trogadas, Thomas F. Fuller and Peter Strasser

Carbon as Catalyst and Support for Electrochemical Energy Conversion

Carbon 75, 5 - 42

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Carbon has unique characteristics that make it an ideal material for use in a wide variety of electrochemical applications ranging from metal refining to electrocatalysis and fuel cells. In polymer electrolyte fuel cells (PEFCs), carbon is used as a gas diffusion layer, electrocatalyst support and oxygen reduction reaction (ORR) electrocatalyst. When used as electrocatalyst support, amorphous carbonaceous materials suffer from enhanced oxidation rates at high potentials over time. This drawback has prompted an extensive effort to improve the properties of amorphous carbon and to identify alternate carbon-based materials to replace carbon blacks. Alternate support materials are classified in carbon nanotubes and fibers, mesoporous carbon, multi-layer graphene (undoped and doped with metal nanoparticles) and reduced graphene oxide. A comparative review of all these supports is provided. Work on catalytically active carbon hybrids is focused on the development of non-precious metal electrocatalysts that will significantly reduce the cost without sacrificing catalytic activity. Of the newer electrocatalysts, nitrogen/metal-functionalized carbons and composites are emerging as possible contenders for commercial PEFCs. Nitrogen-doped carbon hybrids with transition metals and their polymer composites exhibit high ORR activity and selectivity and these catalytic properties are presented in detail in this review.
Tomokazu Sakamoto, Koichiro Asazawa, Jean Sanabria-Chinchilla, Ulises Martinez, Barr Halevi, Plamen Atanassov, Peter Strasser  and Hirohisa Tanaka

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 Ni0.8Zn0.2 and Ni0.9La0.1 catalysts are significantly more active than the Ni reference catalyst for hydrazine oxidation. While the best Ni0.8Zn0.1La0.1 ternary catalyst is close to the high active binary catalysts in composition. Additionally, Ni0.6Fe0.2Mn0.2 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 Ni0.87Zn0.13 and Ni0.9La0.1 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.
2013
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)Ox 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)Ox 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 IrOx 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 TiOx, 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 Pt0.5Ni0.5 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 Pt0.5Ni0.5 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 PtOx 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 PtOx 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, MnOx, are established using a combination of X-ray absorption, diffraction and electrochemistry. Prepared by chemical symproportionation (s-MnOx) and impregnation (i-MnOx), the s-MnOx catalyst consisted of a layered structure similar to δ-MnO2 while the i-MnOx 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-MnOx. 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 Pt1–xNix (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 Greeley and 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 PtxNi1−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 PtxNi1−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.

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