Highly stable Pt monolayer on PdAu nanoparticle electrocatalysts for the oxygen reduction reaction

Supersaturation-controlled surface structure evolution of Pd@Pt core-shell nanocrystals: enhancement of the ORR activity at a sub-10 nm scale

Here, we designed and implemented a facile strategy for controlling the surface evolution of Pd@Pt core-shell nanostructures by simply adjusting the volume of OH(-) to control the reducing ability of ascorbic acid and finally manipulating the supersaturation in the reaction system. The surface structure of the obtained Pd@Pt bimetallic nanocrystals transformed from a Pt {111} facet-exposed island shell to a conformal Pt {100} facet-exposed shell by increasing the pH value. The as-prepared well aligned Pd@Pt core-island shell nanocubes present both significantly enhanced electrocatalytic activity and favorable long-term stability toward the oxygen reduction reaction in alkaline media.

Synthesis of core/shell structured Pd3Au@Pt/C with enhanced electrocatalytic activity by regioselective atomic layer deposition combined with a wet chemical method

Core/shell structured Pd₃Au@Pt/C created by regioselective atomic layer deposition combined with a wet chemical method demonstrates improved electrocatalytic activity toward formic acid oxidation and oxygen reduction compared with commercial Pt/C.

Surfactant free synthesis of high surface area Pt@PdM3 (M = Mn, Fe, Co, Ni, Cu) core/shell electrocatalysts with enhanced electrocatalytic activity and durability for PEM fuel cell applications

High surface area core/shell nanostructures of Pt covered Pd alloys were synthesized and they exhibited enhanced electrocatalytic activity in oxygen reduction reactions.

Three-dimensional hyperbranched PdCu nanostructures with high electrocatalytic activity

In this study, three-dimensional (3D) PdCu alloyed nanostructures, consisting of one-dimensional (1D) branches, were successfully synthesized through a facile wet-chemical method without using any seeds or organic solvent.

One-pot facile synthesis of PtCu coated nanoporous gold with unique catalytic activity toward the oxygen reduction reaction

PtCu@NPG prepared by a one-pot protocol preserved the 3D nanostructure of NPG and presented unique catalytic activity and durability towards the ORR.

Multifunctional Ultrathin PdxCu(1-x) and Pt∼PdxCu(1-x) One-Dimensional Nanowire Motifs for Various Small Molecule Oxidation Reactions

Developing novel electrocatalysts for small molecule oxidation processes, including formic acid oxidation (FAOR), methanol oxidation reaction (MOR), and ethanol oxidation reaction (EOR), denoting the key anodic reactions for their respective fuel cell configurations, is a significant and relevant theme of recent efforts in the field. Herein, in this report, we demonstrated a concerted effort to couple and combine the benefits of small size, anisotropic morphology, and tunable chemical composition in order to devise a novel "family" of functional architectures. In particular, we have fabricated not only ultrathin 1-D Pd(1-x)Cu(x) alloys but also Pt-coated Pd(1-x)Cu(x) (i.e., Pt∼Pd(1-x)Cu(x); herein the ∼ indicates an intimate association, but not necessarily actual bond formation, between the inner bimetallic core and the Pt outer shell) core-shell hierarchical nanostructures with readily tunable chemical compositions by utilizing a facile, surfactant-based, wet chemical synthesis coupled with a Cu underpotential deposition technique. Our main finding is that our series of as-prepared nanowires are functionally flexible. More precisely, we demonstrate that various examples within this "family" of structural motifs can be tailored for exceptional activity with all 3 of these important electrocatalytic reactions. In particular, we note that our series of Pd(1-x)Cu(x) nanowires all exhibit enhanced FAOR activities as compared with not only analogous Pd ultrathin nanowires but also commercial Pt and Pd standards, with Pd9Cu representing the "optimal" composition. Moreover, our group of Pt∼Pd(1-x)Cu(x) nanowires consistently outperformed not only commercial Pt NPs but also ultrathin Pt nanowires by several fold orders of magnitude for both the MOR and EOR reactions in alkaline media. The variation of the MOR and EOR performance with the chemical composition of our ultrathin Pt∼Pd(1-x)Cu(x) nanowires was also discussed.

In Situ Probing of the Active Site Geometry of Ultrathin Nanowires for the Oxygen Reduction Reaction

To create truly effective electrocatalysts for the cathodic reaction governing proton exchange membrane fuel cells (PEMFC), namely the oxygen reduction reaction (ORR), necessitates an accurate and detailed structural understanding of these electrocatalysts, especially at the nanoscale, and to precisely correlate that structure with demonstrable performance enhancement. To address this key issue, we have combined and interwoven theoretical calculations with experimental, spectroscopic observations in order to acquire useful structural insights into the active site geometry with implications for designing optimized nanoscale electrocatalysts with rationally predicted properties. Specifically, we have probed ultrathin (∼2 nm) core-shell Pt∼Pd9Au nanowires, which have been previously shown to be excellent candidates for ORR in terms of both activity and long-term stability, from the complementary perspectives of both DFT calculations and X-ray absorption spectroscopy (XAS). The combination and correlation of data from both experimental and theoretical studies has revealed for the first time that the catalytically active structure of our ternary nanowires can actually be ascribed to a PtAu∼Pd configuration, comprising a PtAu binary shell and a pure inner Pd core. Moreover, we have plausibly attributed the resulting structure to a specific synthesis step, namely the Cu underpotential deposition (UPD) followed by galvanic replacement with Pt. Hence, the fundamental insights gained into the performance of our ultrathin nanowires from our demonstrated approach will likely guide future directed efforts aimed at broadly improving upon the durability and stability of nanoscale electrocatalysts in general.

Direct Transformation from Graphitic C3N4 to Nitrogen-Doped Graphene: An Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction

Carbon-based nanomaterials provide an attractive perspective to replace precious Pt-based electrocatalysts for oxygen reduction reaction (ORR) to enhance the practical applications of fuel cells. Herein, we demonstrate a one-pot direct transformation from graphitic-phase C3N4 (g-C3N4) to nitrogen-doped graphene. g-C3N4, containing only C and N elements, acts as a self-sacrificing template to construct the framework of nitrogen-doped graphene. The relative contents of graphitic and pyridinic-N can be well-tuned by the controlled annealing process. The resulting nitrogen-doped graphene materials show excellent electrocatalytic activity toward ORR, and much enhanced durability and tolerance to methanol in contrast to the conventional Pt/C electrocatalyst in alkaline medium. It is determined that a higher content of N does not necessarily lead to enhanced electrocatalytic activity; rather, at a relatively low N content and a high ratio of graphitic-N/pyridinic-N, the nitrogen-doped graphene obtained by annealing at 900 °C (NGA900) provides the most promising activity for ORR. This study may provide further useful insights on the nature of ORR catalysis of carbon-based materials.

Pt monolayer coating on complex network substrate with high catalytic activity for the hydrogen evolution reaction

A deposition process has been developed to fabricate a complete-monolayer Pt coating on a large-surface-area three-dimensional (3D) Ni foam substrate using a buffer layer (Ag or Au) strategy. The quartz crystal microbalance, current density analysis, cyclic voltammetry integration, and X-ray photoelectron spectroscopy results show that the monolayer deposition process accomplishes full coverage on the substrate and the deposition can be controlled to a single atomic layer thickness. To our knowledge, this is the first report on a complete-monolayer Pt coating on a 3D bulk substrate with complex fine structures; all prior literature reported on submonolayer or incomplete-monolayer coating. A thin underlayer of Ag or Au is found to be necessary to cover a very reactive Ni substrate to ensure complete-monolayer Pt coverage; otherwise, only an incomplete monolayer is formed. Moreover, the Pt monolayer is found to work as well as a thick Pt film for catalytic reactions. This development may pave a way to fabricating a high-activity Pt catalyst with minimal Pt usage.

Beneficial Role of Copper in the Enhancement of Durability of Ordered Intermetallic PtFeCu Catalyst for Electrocatalytic Oxygen Reduction

Design of Pt alloy catalysts with enhanced activity and durability is a key challenge for polymer electrolyte membrane fuel cells. In the present work, we compare the durability of the ordered intermetallic face-centered tetragonal (fct) PtFeCu catalyst for the oxygen reduction reaction (ORR) relative to its counterpart bimetallic catalysts, i.e., the ordered intermetallic fct-PtFe catalyst and the commercial catalyst from Tanaka Kikinzoku Kogyo, TKK-PtC. Although both fct catalysts initially exhibited an ordered structure and mass activity approximately 2.5 times higher than that of TKK-Pt/C, the presence of Cu at the ordered intermetallic fct-PtFeCu catalyst led to a significant enhancement in durability compared to that of the ordered intermetallic fct-PtFe catalyst. The ordered intermetallic fct-PtFeCu catalyst retained more than 70% of its mass activity and electrochemically active surface area (ECSA) over 10 000 durability cycles carried out at 60 °C. In contrast, the ordered intermetallic fct-PtFe catalyst maintained only about 40% of its activity. The temperature of the durability experiment is also shown to be important: the catalyst was more severely degraded at 60 °C than at room temperature. To obtain insight into the observed enhancement in durability of fct-PtFeCu catalyst, a postmortem analysis of the ordered intermetallic fct-PtFeCu catalyst was carried out using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) line scan. The STEM-EDX line scans of the ordered intermetallic fct-PtFeCu catalyst over 10 000 durability cycles showed a smaller degree of Fe and Cu dissolution from the catalyst. Conversely, large dissolution of Fe was identified in the ordered intermetallic fct-PtFe catalyst, indicating a lesser retention of Fe that causes the destruction of ordered structure and gives rise to poor durability. The enhancement in the durability of the ordered intermetallic fct-PtFeCu catalyst is ascribed to the synergistic effects of Cu presence and the ordered structure of catalyst.

2D ultrathin core-shell Pd@Pt(monolayer) nanosheets: defect-mediated thin film growth and enhanced oxygen reduction performance

An operational strategy for the synthesis of atomically smooth Pt skin by a defect-mediated thin film growth method is reported. Extended ultrathin core-shell structured d@Pt(monolayer) nanosheets (thickness below 5 nm) exhibit nearly seven-fold enhancement in mass-activity and surprisingly good durability toward oxygen reduction reaction as compared with the commercial Pt/C catalyst.

Unusual Activity Trend for CO Oxidation on Pd(x)Au(140-x)@Pt Core@Shell Nanoparticle Electrocatalysts

A theoretical and experimental study of the electrocatalytic oxidation of CO on PdxAu140-x@Pt dendrimer-encapsulated nanoparticle (DEN) catalysts is presented. These nanoparticles are comprised of a core having an average of 140 atoms and a Pt monolayer shell. The CO oxidation activity trend exhibits an unusual koppa shape as the number of Pd atoms in the core is varied from 0 to 140. Calculations based on density functional theory suggest that the koppa-shaped trend is driven primarily by structural changes that affect the CO binding energy on the surface. Specifically, a pure Au core leads to deformation of the Pt shell and a compression of the Pt lattice. In contrast, Pd, from the pure Pd cores, tends to segregate on the DEN surface, forming an inverted configuration having Pt within the core and Pd in the shell. With a small addition of Au, however, the alloy PdAu cores stabilize the core@shell structures by preventing Au and Pd from escaping to the particle surface.

Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction

Conformal deposition of platinum as ultrathin shells on facet-controlled palladium nanocrystals offers a great opportunity to enhance the catalytic performance while reducing its loading. Here we report such a system based on palladium icosahedra. Owing to lateral confinement imposed by twin boundaries and thus vertical relaxation only, the platinum overlayers evolve into a corrugated structure under compressive strain. For the core-shell nanocrystals with an average of 2.7 platinum overlayers, their specific and platinum mass activities towards oxygen reduction are enhanced by eight- and sevenfold, respectively, relative to a commercial catalyst. Density functional theory calculations indicate that the enhancement can be attributed to the weakened binding of hydroxyl to the compressed platinum surface supported on palladium. After 10,000 testing cycles, the mass activity of the core-shell nanocrystals is still four times higher than the commercial catalyst. These results demonstrate an effective approach to the development of electrocatalysts with greatly enhanced activity and durability.

Core-shell catalysts can enhance activity while reducing the loading of expensive catalyst materials. Here, the authors report a palladium@platinum system in which the platinum shells evolve into a corrugated structure with compressive strains, with subsequent enhancement of oxygen reduction activity.

Strain-Induced Segregation in Bimetallic Multiply Twinned Particles

We analyze the possibility of strain-induced segregation in bimetallic multiply twinned particles by an analytic first-order expansion within a continuum model. The results indicate that while the change in free energy may be small, there will be a noticeable segregation of larger atoms to the external surface and smaller ones to the core, which could have interesting effects when such nanoparticles are used as heterogeneous catalysts.

Stabilization of Pt monolayer catalysts under harsh conditions of fuel cells

We employed density functional theory to explore the stability of core (M = Cu, Ru, Rh, Pd, Ag, Os, Ir, Au)-shell (Pt) catalysts under harsh conditions, including solutions and reaction intermediates involved in the oxygen reduction reaction (ORR) in fuel cells. A pseudomorphic surface alloy (PSA) with a Pt monolayer (Pt(1ML)) supported on an M surface, Pt(1ML)/M(111) or (001), was considered as a model system. Different sets of candidate M cores were identified to achieve a stable Pt(1ML) shell depending on the conditions. In vacuum conditions, the Pt1ML shell can be stabilized on the most of M cores except Cu, Ag, and Au. The situation varies under various electrochemical conditions. Depending on the solutions and the operating reaction pathways of the ORR, different M should be considered. Pd and Ir are the only core metals studied, being able to keep the Pt(ML) shell intact in perchloric acid, sulfuric acid, phosphoric acid, and alkaline solutions as well as under the ORR conditions via different pathways. Ru and Os cores should also be paid attention, which only fall during the ORR via the *OOH intermediate. Rh core works well as long as the ORR does not undergo the pathway via *O intermediate. Our results show that PSAs can behave differently from the near surface alloy, Pt(1ML)/M(1ML)/Pt(111), highlighting the importance of considering both chemical environments and the atomic structures in rational design of highly stable core-shell nanocatalysts. Finally, the roles that d-band center of a core M played in determining the stability of supported Pt(1ML) shell were also discussed.

A theoretical and experimental approach for correlating nanoparticle structure and electrocatalytic activity

The objective of the research described in this Account is the development of high-throughput computational-based screening methods for discovery of catalyst candidates and subsequent experimental validation using appropriate catalytic nanoparticles. Dendrimer-encapsulated nanoparticles (DENs), which are well-defined 1-2 nm diameter metal nanoparticles, fulfill the role of model electrocatalysts. Effective comparison of theory and experiment requires that the theoretical and experimental models map onto one another perfectly. We use novel synthetic methods, advanced characterization techniques, and density functional theory (DFT) calculations to approach this ideal. For example, well-defined core@shell DENs can be synthesized by electrochemical underpotential deposition (UPD), and the observed deposition potentials can be compared to those calculated by DFT. Theory is also used to learn more about structure than can be determined by analytical characterization alone. For example, density functional theory molecular dynamics (DFT-MD) was used to show that the core@shell configuration of Au@Pt DENs undergoes a surface reconstruction that dramatically affects its electrocatalytic properties. A separate Pd@Pt DENs study also revealed reorganization, in this case a core-shell inversion to a Pt@Pd structure. Understanding these types of structural changes is critical to building correlations between structure and catalytic function. Indeed, the second principal focus of the work described here is correlating structure and catalytic function through the combined use of theory and experiment. For example, the Au@Pt DENs system described earlier is used for the oxygen reduction reaction (ORR) as well as for the electro-oxidation of formic acid. The surface reorganization predicted by theory enhances our understanding of the catalytic measurements. In the case of formic acid oxidation, the deformed nanoparticle structure leads to reduced CO binding energy and therefore improved oxidation activity. The final catalytic study we present is an instance of theory correctly predicting (in advance of the experiments) the structure of an effective DEN electrocatalyst. Specifically, DFT was used to determine the optimal composition of the alloy-core in AuPd@Pt DENs for the ORR. This prediction was subsequently confirmed experimentally. This study highlights the major theme of our research: the progression of using theory to rationalize experimental results to the more advanced goal of using theory to predict catalyst function a priori. We still have a long way to go before theory will be the principal means of catalyst discovery, but this Account begins to shed some light on the path that may lead in that direction.

New approach to fully ordered fct-FePt nanoparticles for much enhanced electrocatalysis in acid

Fully ordered face-centered tetragonal (fct) FePt nanoparticles (NPs) are synthesized by thermal annealing of the MgO-coated dumbbell-like FePt-Fe3O4 NPs followed by acid washing to remove MgO. These fct-FePt NPs show strong ferromagnetism with room temperature coercivity reaching 33 kOe. They serve as a robust electrocatalyst for the oxygen reduction reaction (ORR) in 0.1 M HClO4 and hydrogen evolution reaction (HER) in 0.5 M H2SO4 with much enhanced activity (the most active fct-structured alloy NP catalyst ever reported) and stability (no obvious Fe loss and NP degradation after 20 000 cycles between 0.6 and 1.0 V (vs RHE)). Our work demonstrates a reliable approach to FePt NPs with much improved fct-ordering and catalytic efficiency for ORR and HER.

Highly porous non-precious bimetallic electrocatalysts for efficient hydrogen evolution

A robust and efficient non-precious metal catalyst for hydrogen evolution reaction is one of the key components for carbon dioxide-free hydrogen production. Here we report that a hierarchical nanoporous copper-titanium bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at more than twice the rate of state-of-the-art carbon-supported platinum catalyst. Although both copper and titanium are known to be poor hydrogen evolution catalysts, the combination of these two elements creates unique copper-copper-titanium hollow sites, which have a hydrogen-binding energy very similar to that of platinum, resulting in an exceptional hydrogen evolution activity. In addition, the hierarchical porosity of the nanoporous copper-titanium catalyst also contributes to its high hydrogen evolution activity, because it provides a large-surface area for electrocatalytic hydrogen evolution, and improves the mass transport properties. Moreover, the catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.

Investigations into non-precious metal catalysts for hydrogen evolution are ongoing. Here, the authors report a hierarchical, nanoporous copper-titanium electrocatalyst, and demonstrate that it catalyses hydrogen production at twice the over-all rate of commercial platinum-based catalysts.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

Based on the understanding of the ORR catalytic mechanism, advanced Pt-based and Pt-free catalysts have been explored.

Noble metal alloy complex nanostructures: controllable synthesis and their electrochemical property

From the perspective of noble metal alloy nanocrystals with complex structures, we highlight their controllable synthesis and improved electrochemical property.

Oxygen reduction reaction activity and structural stability of Pt–Au nanoparticles prepared by arc-plasma deposition

Sequential arc-plasma deposited Pt–Au alloy nanoparticles show superior electrochemical structural durability compared with arc-plasma deposited Pt nanoparticles before and after electrochemical potential cycles.

Interface-controlled synthesis of heterodimeric silver-carbon nanoparticles derived from polysaccharides

Hybrid nanoparticles composed of multiple components can offer unique opportunities for understanding the nanoscale mechanism and advanced material applications. Here, we report the synthesis of heterodimeric silver-carbon dot nanoparticles (Ag-CD NPs) where the Ag NP is grown on the surface of CDs derived from polysaccharides, such as chitosan and alginate, through the photoelectron transfer reaction between CD and Ag(+) ions. The nanoscale interface between the Ag NPs and the CDs is highly tunable depending on the precursor of the CDs and the amount of additives, resulting in fine modification of photoluminescence of the CDs as well as the related surface plasmon resonance of the Ag NPs. This result demonstrates the critical role of the interface between the hybrid nanoparticles in governing the electrical and optical properties of respective nanoparticles.

Highly alloyed PtRu nanoparticles confined in porous carbon structure as a durable electrocatalyst for methanol oxidation

The state-of-the-art carbon-supported PtRu catalysts are widely used as the anode catalysts in polymer electrolyte fuel cells (PEMFCs) but suffer from instability issues. Severe ruthenium dissolution occurring at potentials higher than 0.5 V vs NHE would result in a loss of catalytic activity of PtRu hence a worse performance of the fuel cell. In this work, we report an ultrastable PtRu electrocatalyst for methanol oxidation by confining highly alloyed PtRu nanoparticles in a hierarchical porous carbon structure. The structural characteristics, e.g., the surface composition and the morphology evolution, of the catalyst during the accelerated degradation test were characterized by the Cu-stripping voltammetry and the TEM/SEM observations. From the various characterization results, it is revealed that both the high alloying degree and the pore confinement of PtRu nanoalloys play significant roles in suppressing the degradation processes, including Ru dissolution and particle agglomeration/migration. This report provides an opportunity for efficient design and fabrication of highly stable bimetallic or trimetallic electrocatalysts in a large variety of applications.

Electrochemical Atomic-level Controlled Syntheses of Electrocatalysts for the Oxygen Reduction Reaction

It is becoming apparent that the electrocatalysts consisting of a platinum (Pt) monolayer (ML) shell on a metal, or alloy nanoparticle cores are one of the most promising classes of fuel cell catalysts offering ultra-low Pt content, complete Pt utilization, very high activity and excellent performance stability. In this chapter, the electrochemical strategies for depositing a Pt ML-shell on various nanostructured cores are discussed. The advantages of the electrodeposition techniques over the conventional chemical methods for synthesis of electrocatalysts for the oxygen reduction reaction are described. Illustrations include the electrodeposition of Pt ML on mono- and bi-metallic (Pd, PdAu, PdIr, NiW) nanostructures on functionalized carbons that creates highly efficient cathode electrocatalysts for proton exchange membrane fuel cells. These features, and a simple scale-up of this syntheses, make the electrodeposition strategies a viable way of solving the remaining obstacles hindering the fuel cell commercialization.

Multimetallic core/interlayer/shell nanostructures as advanced electrocatalysts

The fine balance between activity and durability is crucial for the development of high performance electrocatalysts. The importance of atomic structure and compositional gradients is a guiding principle in exploiting the knowledge from well-defined materials in the design of novel class of core-shell electrocatalysts comprising Ni core, Au interlayer, and PtNi shell (Ni@Au@PtNi). This multimetallic system is found to have the optimal balance of activity and durability due to the synergy between the stabilizing effect of subsurface Au and modified electronic structure of surface Pt through interaction with subsurface Ni atoms. The electrocatalysts with Ni@Au@PtNi core-interlayer-shell structure exhibit high intrinsic and mass activities as well as superior durability for the oxygen reduction reaction with less than 10% activity loss after 10,000 potential cycles between 0.6 and 1.1 V vs the reversible hydrogen electrode.

Gold-promoted structurally ordered intermetallic palladium cobalt nanoparticles for the oxygen reduction reaction

Considerable efforts to make palladium and palladium alloys active catalysts and a possible replacement for platinum have had a marginal success. Here we report on a structurally ordered Au10Pd₄₀Co₅₀ catalyst that exhibits comparable activity to conventional platinum catalysts in both acid and alkaline media. Electron microscopic techniques demonstrate that, at elevated temperatures, palladium cobalt nanoparticles undergo an atomic structural transition from core-shell to a rare intermetallic ordered structure with twin boundaries forming stable {111}, {110} and {100} facets via addition of gold atoms. The superior stability of this catalyst compared with platinum after 10,000 potential cycles in alkaline media is attributed to the atomic structural order of PdCo nanoparticles along with protective effect of clusters of gold atoms on the surface. This strategy of making ordered palladium intermetallic alloy nanoparticles can be used in diverse heterogeneous catalysis where particle size and structural stability matter.

Confining Pt nanoparticles in porous carbon structures for achieving durable electrochemical performance

Carbon-supported Pt catalysts have been widely employed as electrocatalysts for energy storage/conversion applications, but have encountered challenging instability issues. In this work, we investigated the degradation behaviors of pore-confined and surface-located Pt nanocatalysts, employing hollow porous carbon spheres with precisely controlled structure as catalyst supports. It is found that by uniformly confining Pt nanoparticles in porous carbon structures, remarkably improved stability and long-term performance of Pt electrocatalysts can be achieved. The nanopore-confined Pt exhibits high retention ratios of both ECSA (54%) and electrocatalytic activity after accelerated degradation tests (ADTs), both of which are almost two times higher than those of the surface-located ones. TEM analysis of the degraded electrocatalysts further revealed that the pore-confinement effect can significantly suppress the Pt degradation processes, including particle migration/agglomeration and detachment from the carbon support.

Electrochemically induced surface metal migration in well-defined core-shell nanoparticles and its general influence on electrocatalytic reactions

Bimetallic nanoparticle catalysts provide enhanced activity, as combining metals allows tuning of electronic and geometric structure, but the enhancement may vary during the reaction because the nanoparticles can undergo metal migration under catalytic reaction conditions. Using cyclic voltammetry to track the surface composition over time, we carried out a detailed study of metal migration in a well-defined model Au-Pd core-shell nanocatalyst. When subjected to electrochemical conditions, Au migration from the core to the shell was observed. The effect of Pd shell thickness and electrolyte identity on the extent of migration was studied. Migration of metals during catalytic ethanol oxidation was found to alter the particle's surface composition and electronic structure, enhancing the core-shell particles' activity. We show that metal migration in core-shell nanoparticles is a phenomenon common to numerous electrochemical systems and must be considered when studying electrochemical catalysis.

Mass-selected nanoparticles of PtxY as model catalysts for oxygen electroreduction

Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy PtxY is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt(-1) at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.