Miomir B. Vukmirovic

Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes

More than skin deep: Platinum monolayers can act as shells for palladium nanoparticles to lead to electrocatalysts with high activities and an ultralow platinum content, but high platinum utilization. The stability derives from the core protecting the shell from dissolution. In fuel-cell tests, no loss of platinum was observed in 200?000 potential cycles, whereas loss of palladium was significant.

Enhanced Electrocatalytic Performance of Processed, Ultrathin, Supported Pd–Pt Core–Shell Nanowire Catalysts for the Oxygen Reduction Reaction

We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell-Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of 2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core-Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm(2) and 1.83 A/mg(Pt), respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core-shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ~1.5 fold after a simulated catalyst lifetime.

Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt−Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy

We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd(3)Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd(3)Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surfaces electronic properties by underlying Fe. The Pd(3)Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.

Intermetallics as Novel Supports for Pt Monolayer O2 Reduction Electrocatalysts: Potential for Significantly Improving Properties

We report on a new class of core-shell electrocatalysts for the oxygen-reduction reaction. These electrocatalysts comprise a Pt monolayer shell and ordered intermetallic compounds cores and have enhanced activity and stability compared with conventional ones. These advantages are derived from combining the unique properties of Pt monolayer catalysts (high activity, low metal content) and of the intermetallic compounds (high stability and, possibly, low price). This method holds excellent potential for creating efficient fuel cell electrocatalysts.

Increasing Stability and Activity of Core–Shell Catalysts by Preferential Segregation of Oxide on Edges and Vertexes: Oxygen Reduction on Ti–Au@Pt/C

We describe a new class of core-shell nanoparticle catalysts having edges and vertexes covered by refractory metal oxide that preferentially segregates onto these catalyst sites. The monolayer shell is deposited on the oxide-free core atoms. The oxide on edges and vertexes induces high catalyst stability and activity. The catalyst and synthesis are exemplified by fabrication of Au nanoparticles doped by Ti atoms that segregate as oxide onto low-coordination sites of edges and vertexes. Pt monolayer shell deposited on Au sites has the mass and specific activities for the oxygen reduction reaction about 13 and 5 times higher than those of commercial Pt/C catalysts. The durability tests show no activity loss after 10 000 potential cycles from 0.6 to 1.0 V. The superior activity and durability of the Ti-Au@Pt catalyst originate from protective titanium oxide located at the most dissolution-prone edge and vertex sites and Au-supported active and stable Pt shell.

Tetrahedral Palladium Nanocrystals: A New Support for Platinum Monolayer Electrocatalysts with High Activity and Stability in the Oxygen Reduction Reaction

Abstract The recent availability of tetrahedral palladium (PdTH) nanocrystals with cleaned surfaces allowed us to evaluate their facet-specific electrochemical properties as a new support of platinum monolayer (PtML) catalysts. The Pd–PtML core-shell electrocatalyst was examined by combining structural analyses and Density Functional Theory (DFT) with electrochemical techniques. The surfaces of the PdTH core are composed of (111) facets wherein the Pd atoms are highly coordinated and have low surface energy. Our results revealed that in comparison with sphere Pd (PdSP)-supported PtML or pure Pt, the PdTH-supported PtML features more surface contraction and a downshift of d-band relative to the Fermi level. These geometric- and electronic-effects determine the higher activity of PtML/PdTH/C for the oxygen reduction reaction (ORR) compared to that of PtML/PdSP/C. This shape-property interdependence illuminated new approaches to basic- and applied- research on Pt-based ORR electrocatalysts of significant importance to the widespread use of fuel cells.

Controlling Bimetallic Nanostructures by the Microemulsion Method with Subnanometer Resolution Using a Prediction Model.

We present a theoretical model to predict the atomic structure of Au/Pt nanoparticles synthesized in microemulsions. Excellent concordance with the experimental results shows that the structure of the nanoparticles can be controlled at subnanometer resolution simply by changing the reactant concentration. The results of this study not only offer a better understanding of the complex mechanisms governing reactions in microemulsions, but open up a simple new way to synthesize bimetallic nanoparticles with ad hoc controlled nanostructures.

Some Recent Studies in Ruthenium Electrochemistry and Electrocatalysis

Ruthenium is a metal of a considerable importance in electrochemical science and technology. It is a catalyst or co-catalyst material in Pt-Ru alloys for methanoland reformate hydrogenoxidation in fuel cells, while ruthenium oxide, a component in chlorine-evolution catalysts, represents an attractive material for electrochemical supercapacitors. Its facile surface oxidation generates an oxygen-containing species that provides active oxygen in some reactions. Ru sites in Pt-Ru catalysts increase the “CO tolerance” of Pt in the catalytic oxidation-reaction in direct methanol fuel cells (DMFC) and in reformate hydrogen-oxidation in proton exchange membrane fuel cells (PEMFC). The mechanism of Ru action is not completely understood, although the current consensus revolves around the so-called “bifunctional mechanism” wherein Ru provides oxygenated species to oxidize CO that blocks Pt sites, and has an electronic effect on Pt-CO interaction.

Correlating the chemical composition and size of various metal oxide substrates with the catalytic activity and stability of as-deposited Pt nanoparticles for the methanol oxidation reaction

The performance of electrode materials in conventional direct alcohol fuel cells (DAFC) is constrained by (i) the low activity of the catalyst materials relative to their overall cost, (ii) the poisoning of the active sites due to the presence of partially oxidized carbon species (such as but not limited to CO, formate, and acetate) produced during small molecule oxidation, and (iii) the lack of catalytic stability and durability on the underlying commercial carbon support. Therefore, as a viable alternative, we have synthesized various metal oxide and perovskite materials of different sizes and chemical compositions as supports for Pt nanoparticles (NPs). Our results including unique mechanistic studies demonstrate that the SrRuO3 substrate with immobilized Pt NPs at its surface evinces the best methanol oxidation performance as compared with all of the other substrate materials tested herein, including commercial carbon itself. Additionally, data from electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of electron transfer from bound Pt NPs to surface Ru species within the SrRuO3 substrate itself, thereby suggesting that favorable metal support interactions are responsible for the increased methanol oxidation reaction (MOR) activity of Pt species with respect to the underlying SrRuO3 composite catalyst material.

Chapter 6: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.