Matthew Fayette

From Au to Pt via Surface Limited Redox Replacement of Pb UPD in One-Cell Configuration

This work is aimed at developing a protocol based on surface limited redox replacement (SLRR) of underpotentially deposited (UPD) Pb layers for the growth of epitaxial and continuous Pt thin films on polycrystalline and single crystalline Au surfaces. Different from previously reported papers using SLRR in multiple immersion or flow cell setups, this work explores the one-cell configuration setup as an alternative to improve the efficiency and quality of the growth. Open circuit chronopotentiometry and quartz-crystal microbalance experiments demonstrate steady displacement kinetics and a yield that is higher than the stoichiometric Pt(II)-Pb exchange ratio (1:1). This high yield is attributed to oxidative adsorption of OH(ad) taking place on Pt along with the displacement process. Also, ex situ scanning tunneling microscopy surface characterization reveals after the first replacement event the formation of a dense Pt cluster network that homogenously covers the Au surface. The Pt films grow homogenously with no significant changes in the cluster distribution and surface roughness observed up to 10 successive replacement events. X-ray diffraction analysis shows distinct (111) crystallographic orientation of thicker Pt films deposited on (111) textured Au thin films. Coarse energy dispersive spectroscopy measurements and finer X-ray photoelectron spectroscopy suggest at least 4 atom % Pb incorporating into the Pt layer compared to 13 atom % alloyed Cu when the growth is carried out by SLRR of Cu UPD.

All electrochemical fabrication of a platinized nanoporous Au thin-film catalyst.

In an effort to decrease the high cost associated with the design, testing, and production of electrocatalysts, a completely electrochemical scheme has been developed to deposit and platinize a nanoporous Au (NPG) based catalyst for formic acid oxidation. The proposed route enables synthesis of an alternative to the most established, nanoparticles based catalysts and addresses issues of the latter associated with either contamination inherent from the synthetic route or poor adhesion to the supporting electrode. The synthetic protocol includes as a first step, electrochemical codeposition of a Au((1-x))Ag(x) alloy in a thiosulfate based electrolyte followed by selective electrochemical dissolution (dealloying) of Ag as the less noble metal, that generates an ultrathin and preferably continuous porous structure featuring thickness of less than 20 nm. NPG is then functionalized with Pt (no thicker than 1 nm) by surface limited redox replacement (SLRR) of underpotentially deposited Pb layer to form Pt-NPG. SLRR ensures complete coverage of the surface with Pt, believed to spread evenly over the NPG matrix. Testing of the catalyst at a proof-of-concept level demonstrates its high catalytic activity toward formic acid oxidation. Current densities of 40-50 mA cm(-2) and mass activities of 1-3 A.mg(-1) (of combined Pt-Au catalyst) have been observed and the Pt-NPG thin films have lasted over 2600 cycles in standard formic acid oxidation testing.

Catalytic Performance Comparison of Shape-Dependent Nanocrystals and Oriented Ultrathin Films of Pt4Cu Alloy in the Formic Acid Oxidation Process

Research efforts continue to focus on the development of viable and cost-effective fuel cell catalysts with minimized Pt content. This work presents a comparative study between Pt4Cu nanocubes and nano-octahedra as well as Pt4Cu (100) and (111) thin films used as catalysts for formic acid oxidation. This paper introduces a novel synthetic method for Pt4Cu nano-octahedra, and it also demonstrates for the first time the use of surface limited redox replacement of Pb underpotentially deposited layer for epitaxial growth of thin alloy films. Overall, the nanoparticle catalysts exhibit superior performance in terms of durability when compared to their thin film counterparts but feature nearly fivefold lower activity. As a result, it was determined that both types of catalysts accumulate nearly equal charge density in their lifespan. In terms of crystallographic orientation, the results indicate that the nanocubes and Pt4Cu (100) thin films outperform the nano-octahedra and Pt4Cu (111) thin films in terms of durability but feature equal to slightly lower activity. This significant difference in durability of catalysts with different crystallographic orientation is attributed to interplay of passivation (from CO poisoning and Pt oxidation) and dissolution of Pt. When compared to pure Pt catalysts (nanoparticles and thin films), all of the Pt4Cu catalysts in this work exhibit superior performance toward formic acid oxidation in terms of activity and durability.