Pt monolayer on porous Pd-Cu alloys as oxygen reduction electrocatalysts

We demonstrate the synthesis of a core-shell catalyst consisting of a Pt monolayer as the shell and porous/hollow Pd-Cu alloy nanoparticles as the core. The porous/hollow Pd-Cu nanoparticles were fabricated by selectively dissolving a less noble metal, Cu, using an electrochemical dealloying process. The Pt mass activity for the oxygen reduction reaction of a Pt monolayer deposited on such a porous core is 3.5 times higher than that of a Pt monolayer deposited on bulk Pd nanoparticles and 14 timeshigherthanthat of state-of-the-artPt/ Celectrocatalysts. © 2010 American Chemical Society

Oxidation resistance of bare and pt-coated electrically conducting diamond powder as assessed by thermogravimetric analysis

A corrosion-resistant electrocatalyst support was prepared by overcoating high surface-area diamond powder (3-6 nm diameter, 250 m2 /g) with a thin layer of boron-doped ultrananocrystalline diamond (B-UNCD) by microwave plasma-assisted chemical vapor deposition. This core-shell approach produces electrically conducting (0.4-0.5 S/cm) and high surface-area (150-170 m 2/g) diamond powder (B-UNCD-D). Accelerated degradation testing was performed by thermogravimetric analysis (TGA) to assess the oxidation resistance (i.e., corrosion resistance) of powder in the absence and presence of nanoscale Pt. The oxidation onset temperature for B-UNCD-D powder decreased with the Pt loading from 0 to 30 wt % (Pt/C). However, compared with the bare powder, the rate of carbon consumption was significantly greater for Pt-(XC-72) as compared to the platinized diamond powder. For example, the temperature of the maximum carbon consumption rate, Td, occurred at 426°C for Pt-(XC-72) (20% Pt/C), which was 295°C lower than the Td for bare XC-72. In contrast, Td for Pt-(B-UNCD-D, 20% Pt/C) was 558°C; a temperature that was only 62°C lower than that for bare diamond. Isothermal oxidation at 300°C for 5 h produced negligible weight loss for Pt-UNCD-D (20% Pt/C) while a 75% weight loss was observed for Pt-(XC-72) (20% Pt/C). The results clearly demonstrate that platinized diamond is more resistant to gas phase oxidation than is platinized Vulcan at elevated temperatures. © 2009 The Electrochemical Society

Tungsten carbide modified high surface area carbon as fuel cell catalyst support

Phase pure WC nanoparticles were synthesized on high surface area carbon black (800 m2 g-1) by a temperature programmed reaction (TPR) method. The particle size of WC can be controlled under 30 nm with a relatively high coverage on the carbon surface. The electrochemical testing results demonstrated that the corrosion resistance of carbon black was improved by 2-fold with a surface modification by phase pure WC particles. However, the WC itself showed some dissolution under potential cycling. Based on the X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis, most of the WC on the surface was lost or transformed to oxides after 5000 potential cycles in the potential range of 0.65-1.2 V. The Pt catalyst supported on WC/C showed a slightly better ORR activity than that of Pt/C, with the Pt activity loss rate for Pt/WC/C being slightly slower compared to that of Pt/C. The performance and decay rate of Pt/WC/C were also evaluated in a fuel cell. © 2010 Elsevier B.V. All rights reserved

Thermal treatment of PtNiCo electrocatalysts: Effects of nanoscale strain and structure on the activity and stability for the oxygen reduction reaction

The ability to control the nanoscale size, composition, phase, and facet of multimetallic catalysts is important for advancing the design and preparation of advanced catalysts. This report describes the results of an investigation of the thermal treatment temperature on nanoengineered platinum-nickel-cobalt catalysts for oxygen reduction reaction, focusing on understanding the effects of lattice strain and surface properties on activity and stability. The thermal treatment temperatures ranged from 400 to 926 °C. The catalysts were characterized by microscopic, spectroscopic, and electrochemical techniques for establishing the correlation between the electrocatalytic properties and the catalyst structures. The composition, size, and phase properties of the trimetallic nanoparticles were controllable by our synthesis and processing approach. The increase in the thermal treatment temperature of the carbon-supported catalysts was shown to lead to a gradual shrinkage of the lattice constants of the alloys and an enhanced population of facets on the nanoparticle catalysts. A combination of the lattice shrinkage and the surface enrichment of nanocrystal facets on the nanoparticle catalysts as a result of the increased temperature was shown to play a major role in enhancing the electrocatalytic activity for catalysts. Detailed analyses of the oxidation states, atomic distributions, and interatomic distances revealed a certain degree of changes in Co enrichment and surface Co oxides as a function of the thermal treatment temperature. These findings provided important insights into the correlation between the electrocatalytic activity/stability and the nanostructural parameters (lattice strain, surface oxidation state, and distribution) of the nanoengineered trimetallic catalysts. © 2010 American Chemical Society

Core-shell catalysts consisting of nanoporous cores for oxygen reduction reaction

A comprehensive experimental study was conducted on the dealloying of PdNi6 nanoparticles under various conditions. A two-stage dealloying protocol was developed to leach >95% of Ni while minimizing the dissolution of Pd. The final structure of the dealloyed particle was strongly dependent on the acid used and temperature. When H2SO4 and HNO 3 solutions were used in the first stage of dealloying, solid and porous particles were generated, respectively. The porous particles have a 3-fold higher electrochemical surface area per Pd mass than the solid ones. The dealloyed PdNi6 nanoparticles were then used as a core material for the synthesis of core-shell catalysts. These catalysts were synthesized in gram-size batches and involved Pt displacement of an underpotentially deposited (UPD) Cu monolayer. The resulting materials were characterized by scanning transmission electron microscopy (STEM) and in situ X-ray diffraction (XRD). The oxygen reduction reaction (ORR) activity of the core-shell catalysts is 7-fold higher than the state-of-the-art Pt/C. The high activity was confirmed by a more than 40 mV improvement in fuel cell performance with a Pt loading of 0.1 mg cm-2 by using the core-shell catalysts. © the Owner Societies 2013

Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high activity of 3.3 A/mgPt for the oxygen reduction reaction

Nanoscale Pt-Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt-Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt -1 at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt -1). Our analysis suggests that this great enhancement of ORR activity could be attributed to the presence of a clean, well-preserved (111) surface for the Pt-Ni octahedra. © 2013 American Chemical Society

Evolution of Structure and Activity of Alloy Electrocatalysts during Electrochemical Cycles: Combined Activity, Stability, and Modeling Analysis of PtIrCo(7:1:7) and Comparison with PtCo(1:1)

This study explores the changes in bulk composition/structure and oxygen reduction activity of two alloys, Pt₇IrCo₇ and PtCo, caused by Co leaching during electrochemical cycles and as a result of membrane electrode assembly (MEA) fabrication procedures. Exposure to liquid electrolyte and electrochemical cycles in a rotating disc electrode (RDE) environment resulted in substantial Co loss and no stabilization from the low levels of Ir used in the ternary material. The true composition of the ternary material was determined as Pt₈IrCo₃ following initial exposure to 0.1 M HClO₄ (before cycling) and Pt₁₁IrCo₄ after 5000 cycles. Density functional theory (DFT) modeling of the cycled catalyst compositions indicated that structures with Pt-rich upper layers would show the highest stability; however, addition of 0.25 ML oxygen adsorption favored Co segregation from second and third atomic layers. The high initial activities (>0.44A/mgPt) achieved in the RDE environment decreased with cycles and were not reproduced in MEAs. X-ray diffraction (XRD) analysis revealed a measurable increase in lattice parameter caused by the MEA preparation procedure, consistent with Co (and some Ir) leaching into the ionomer phase and relaxation of the lattice. MEA fabrication procedures and cycling in 1 M H₂SO₄ at 80^◦C showed greater changes to catalyst structure and increased Ir and Co loss compared to exposing the catalyst to RDE like conditions (0.1 M HClO₄, RT) explaining the observed discrepancy in activity between RDE and MEA