Huyen N. Dinh

Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized

Effect of Substrate on Polyaniline Film Properties A Cyclic Voltammetry and Impedance Study

“© The Electrochemical Society, Inc. 2000. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of the Electrochemical Society, 2000, Vol. 147(10).”

Nitrogen: unraveling the secret to stable carbon-supported Pt-alloy electrocatalysts

Nitrogen functionalities significantly improve performance for metal-based carbon-supported catalysts, yet their specific role is not well understood. In this work, a direct observation of the nanoscale spatial relationship between surface nitrogen and metal catalyst nanoparticles on a carbon support is established through principal component analysis (PCA) of electron energy loss spectral (EELS) imaging datasets acquired on an aberration-corrected scanning transmission electron microscope (STEM). Improved catalyst–support interactions correlated to high substrate nitrogen content in immediate proximity to stabilized nanoparticles are first demonstrated using model substrates. These insights are applied in direct methanol fuel cell prototypes to achieve substantial improvements in performance and long-term stability using both in-house and commercial catalysts doped with nitrogen. These results have immediate impact in advanced design and optimization of next generation high performance catalyst materials.

The Effect of Film Thickness and Growth Method on Polyaniline Film Properties

The properties of polyaniline (PANI) films, grown electrochemically on Au using the potential sweep method to two different upper potential limits (type I and II) to various thicknesses in sulfuric acid solution, were studied systematically using cyclic voltammetry, ac impedance, and for the first time, by tracking the rate of the hydrogen evolution reaction (HER). The HER results show that both films follow the nucleation and growth mechanism initially and that continuing growth occurs primarily at the outer PANI surface. Because type II PANI films are formed at a much more rapid rate than type I films, a greater amount of anodic degradation products are incorporated into type II films. Impedance and HER measurements show that type II films, compared for the first time in this work with type I films, have a more open PANI film/Au interfacial structure. A new observation is that the cyclic voltammograms for type I films show a prepeak which is absent for type II films. Furthermore, constant PANI redox peak potentials are observed for type I films, whereas these shift with film thickness for type II films. This unique observation implies that type II films undergo structural changes during their growth, while type I films remain more uniform.

UV-Vis Spectroscopy

In a UV-Vis (ultraviolet-visible light) spectroscopic measurement, light absorption as a function of wavelength provides information about electronic transitions occurring in the material. For semiconductors, UV-Vis spectroscopy offers a convenient method of estimating the optical band gap, since it probes electronic transitions between the valence band and the conduction band. Transmission UV-Vis, Diffuse Reflectance UV-Vis, and Absorption UV-Vis configurations are discussed.

Characterizing Polymeric Leachants for Potential System Contaminants of Fuel Cells

The role of system contaminants in the performance degradation of Proton Exchange Membrane (PEM) fuel cells has been underappreciated to date. This work seeks to identify potential contaminants of system components with the ultimate goal of tying contaminant exposure to performance and durability After aging of select polymers in solution, leachant samples were qualitatively identified via GCMS and FTIR-ATR. Total Organic Carbon (TOC) content quantitatively provided information relative to contaminant level extracted from polymeric samples. Results will be presented focusing on SBR rubber and neoprene. Qualitative concentration vs. time charts elucidate leachant evolution, showing among other things potential chemical degradation in solution.

Effect of Halide-Modified Model Carbon Supports on Catalyst Stability

Modification of physiochemical and structural properties of carbon-based materials through targeted functionalization is a useful way to improve the properties and performance of such catalyst materials. This work explores the incorporation of dopants, including nitrogen, iodine, and fluorine, into the carbon structure of highly-oriented pyrolytic graphite (HOPG) and its potential benefits on the stability of PtRu catalyst nanoparticles. Evaluation of the changes in the catalyst nanoparticle coverage and size as a function of implantation parameters reveals that carbon supports functionalized with a combination of nitrogen and fluorine provide the most beneficial interactions, resulting in suppressed particle coarsening and dissolution. Benefits of a carefully tuned support system modified with fluorine and nitrogen surpass those obtained with nitrogen (no fluorine) modification. Ion implantation of iodine into HOPG results in a consistent amount of structural damage to the carbon matrix, regardless of dose. For this modification, improvements in stability are similar to nitrogen modification; however, the benefit is only observed at higher dose conditions. This indicates that a mechanism different than the one associated with nitrogen may be responsible for the improved durability.

Controlled Synthesis of Nanoscale Icosahedral Gold Particles at Room Temperature

The shape of nanocrystals determines surface atomic arrangement and coordination, influencing their chemical and physical properties. We present a novel and facile approach to synthesize gold icosahedra by employing glucose as reducing reagent and sodium dodecyl sulfate as directing agent in the environmentally benign medium of water at room temperature. The size of the icosahedra can be controlled in the range of 30–250 nm by altering reaction conditions. High‐resolution microscopy and diffraction studies indicate the icosahedra are composed of rotational twins that owe likely to assemblage of tetrahedral units. The gold icosahedra particles catalytic properties are probed in the borohydride reduction of p‐nitrophenols and exhibit a size‐dependence reaction property. Comparison studies with spherical particles prepared by the Turkevich method, coupled with poisoning experiments, infer that the shape has a strong influence in the abundance of active surface sites as well as their activities. The properties of nanoscale icosahedra particles has promising applications for further catalytic processes, surface enhancement spectroscopic methods, chemical or biological sensing, and the fabrication of nanoscale devices.

Evaluating the Influence of PEMFC System Contaminants on the Performance of Pt Catalyst via Cyclic Voltammetry

Using cyclic voltammetry as a quick ex situ screening tool, the impact of the extracted solution and the individual leachable constituents from prospective balance of plant component materials on the performance and recoverability of the platinum catalyst were evaluated. Taking an extract from Zytel HTN51G35HSLR (polyphthalamide) as an example, the major leachable organic components are caprolactam and 1,6-hexanediol. While these organic compounds by themselves do poison the Pt catalyst to some extent, such influence is mostly recoverable by means of potential holding and potential cycling. The extracted solution, however, shows a more drastic poisoning effect and it was not recoverable. Therefore, the non-recoverable poisoning effect observed for the extracted solution is not from the two organic species studied. This demonstrates the complexity of such a contaminant study. Inorganic compounds that are known poisons (like sulfur) even in very low concentration may have a more dominant effect on the Pt catalyst and the recoverability.

Flat-Band Potential Techniques

It is important to determine the conductivity and flat-band potential (Efb) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. The three different techniques that can estimate the Efb are: Illuminated OCP, Mott–Schottky and Photocurrent Onset. The Efb should be independent of the technique used to determine it. Due to the inherent shortcomings of each technique, there is often a lack of agreement of the values determined by the various analyses. Researchers should be aware of these limitations in interpreting results.