Svitlana Pylypenko

Bifunctional Oxygen Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins

A study on the oxygen reduction reaction (ORR) mechanism that occurs on non-platinum electrocatalysts, specifically materials derived from pyrolyzed cobalt tetramethoxyphenyl porphyrin in acidic media, is presented here. Reactant and product flux analysis is performed on rotating ring-disk electrode (RRDE) data to evaluate the non-platinum-based materials. An in-depth X-ray photelectron spectroscopy surface characterization analysis is performed and discussed in the context of structure-to-property correlations that are established using a multivariant analysis technique. Pyrolyzed cobalt porphyrin catalysts are highly heterogeneous materials that include both Co species that are associated with nitrogen (CoN x ) and Co nanoparticles coated by native Co oxides. This study proposes an ORR mechanism that occurs on this class of non-Pt electrocatalysts based on structure-to-property correlations and qualitative analysis of the RRDE flux data. The combined flux analysis and structural characterization suggests that the series type, 2 × 2 peroxide ORR pathway is supported on the bifunctional catalyst materials. In this model, two distinct active sites are involved following a bifunctional catalysis scheme. It is suggested that oxygen is initially adsorbed and reduced to peroxide on a CoN x -type site. The intermediate product, peroxide, can be further reduced to water in a series reaction step on a decorating active cobalt oxide species on the catalyst surface.

Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cmPt–2 at 0.9 V) and high surface areas (>90 m2 gPt–1). Platinum–nickel (Pt—Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt–1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt—Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.

Functional DMFC Cathode Catalysts and Supports Based on Niobium Oxide Phase

Composite electrocatalysts consisting of platinum nanophase supported on carbon black decorated with niobium oxide phase were prepared by a two step precipitation/reduction procedure. The intrinsic catalytic activity of these materials was evaluated for the oxygen electroreduction reaction, measured in the absence and presence of methanol. It was found that the Nb x O y -composite materials are more hydrophilic and thus initially have lower intrinsic catalytic activity for oxygen reduction. However, materials containing Nb x Oy offer better overall platinum mass utilization, achieved through much better dispersion of the platinum nanophase. Such advantageous dispersion is observed even for high metal loadings. Another advantage of these materials is minimal effect of poisoning with the intermediates formed during methanol oxidation that results in the enhanced methanol tolerance in direct methanol fuel cells (DMFC). .

Atomic Layer Deposition of Platinum onto Functionalized Aligned MWNT Arrays for Fuel Cell Application

High aspect ratio materials, such as carbon nanotubes (CNTs), provide unique opportunities and advantages as catalyst support materials in fuel cells. In particular, CNTs are highly conductive and corrosion resistant; properties which represent limitations for current carbon supports. While most advanced catalysts research focuses on the production of small nanoparticles to increase the percent of surface accessible Pt; here, we specifically attempt to conformally coat Pt in thin layers onto CNT arrays. We present our work on modifying CNT surfaces inside high-density, surface-bound aligned CNT arrays (aspect ratio ~1:750) with non-toxic gas phase chemistries. The number of nucleation sites and the onset of growth of Pt by ALD can be tuned by using Ar plasma, O2 plasma and chemical functionalization. This, in turn, affects the uniformity of the Pt ALD coating down the length of the tubes within the CNT array.

Hierarchically Structured Pt–Alloy Ethanol Oxidation Electrocatalysts

Bimodal-sized Pt–Sn and Ru–alloy catalysts for the electro-oxidation of ethanol were synthesized using a novel templating approach and evaluated for ethanol oxidation in alkaline media. This templating approach leads to trimodal-sized catalyst particles embedded in and on bimodal-pore carbon support. Electrochemical evaluation suggested that Pt–Sn phases enhance dehydrogenation and/or C–C bond splitting, while Pt–Ru phases facilitates complete oxidation of the intermediate reaction product CO. The criteria for best-performing catalyst are derived from studies and found to be 2–5xa0nm Pt(Sn) and the addition of Ru is conjectured to be beneficial. Mass transport effects observed demonstrates that it is possible to effect catalytic performance using the hierarchically structured templating approach used.

Ion Beam Alignment of Nematic Liquid Crystal on MEH-PPV-Layers

Ion Beam alignment of nematic liquid crystal on the surface of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] was studied. Our results demonstrate that ion beam treatment provides uniform alignment of LC at the polymer surface, which is characterized by a weak anchoring. Application of polarization microscopy, magneto-optical measurements, Confocal Microscopy and X-ray Photoelectron Spectroscopy, allows us to propose a model of ion beam alignment. Oblique ion bombardment results in an angular anisotropic destruction of side chains and conjugated fragments of the polymer. This results in anisotropic angular distribution of polymer fragments, producing an axis of easy orientation for LC in the direction parallel to the projection of the ion beam.

Droplet Based Microfluidics for Synthesis of Mesoporous Silica Microspheres

Herein we present methods for synthesizing monodisperse mesoporous silica particles and silica particles with bimodal porosity by templating with surfactant micelle and microemulsion phases. The fabrication of monodisperse mesoporous silica particles is based on the formation of well-defined equally sized emulsion droplets using a microfluidic approach. The droplets contain the silica precursor/surfactant solution and are suspended in hexadecane as the continuous oil phase. The solvent is then expelled from the droplets, leading to concentration and micellization of the surfactant. At the same time, the silica solidifies around the surfactant structures, forming equally sized mesoporous particles. We show that hierarchically bimodal porous structures can be obtained by templating silica microparticles with a specially designed surfactant micelle/microemulsion mixture. Oil, water, and surfactant liquid mixtures exhibit very complex phase behavior. Depending on the conditions, such mixtures give rise to highly organized structures. A proper selection of the type and concentration of surfactants determines the structuring at the nanoscale level. Tuning the phase state by adjusting the surfactant composition and concentration allows for the controlled design of a system where microemulsion droplets coexist with smaller surfactant micellar structures. The microemulsion droplet and micellar dimensions determine the two types of pore sizes.

The Influence of Surfaces and Deposition Processes on Pt Structure and Properties

Transparent conductive oxides (TCOs), In-Zn-O (IZO) and Ga-Zn-O (GZO), on glass are used as model substrates to study the effect of surface treatments and deposition processes on Pt growth and nanostructure. The TCO type and surface treatments appear to affect Pt nucleation and growth. Ar and O2 plasma surface treatments significantly lowered the contact angle of water measured on TCOs compared to trimethylaluminum surface treatment and samples without surface treatment. Annealing TCO samples in oxygen resulted in lower IZO conductivity and higher contact angle; while annealing in vacuum or hydrogen resulted in increased carbon on the surface, which appears to be related to higher water contact angles and higher conductivity. Higher amounts of zinc and carbon (probably due to contamination from the annealing chamber) on the IZO surface seem to correlate with lower water contact angles and lower conductivity.

Effect of Alloying Pd with Oxophillic Metals on Electro-Oxidation of Alcohols in Alkaline Media

The effect of alloying Pd with Zn for use in electro-oxidation of methanol and ethanol in alkaline media was studied. Pd, Zn, and PdZn supported on Vulcan XC72R were synthesized, tested for alcohol electro-oxidation and compared to unsupported PdZn. It was found that alloying Pd with Zn had a synergistic effect on the oxidation of ethanol but not of methanol.