Hassan M. Saffarian

Area determination in fractal surfaces of Pt and Pt–Ru catalysts for methanol oxidation

Abstract The surface areas of Pt and Pt–Ru alloys have been estimated by a fractal technique. Fractal measurements were made using a scanning tunneling microscope (STM), which measures the surface corrugations. This STM-based fractal technique is offered as an alternative to gas adsorption technique for area estimation. The usefulness of the fractal approach to area calculation in the Pt–Ru alloy, where the hydrogen adsorption technique cannot be used, is demonstrated using the kinetics of methanol oxidation.

Electrochemical properties of perfluoroalkane disulfonic [HSO[sub 3](CF2)[sub n]SO[sub 3]H] acids relevant to fuel cell technology

The electrochemical properties of a homologous series of perfluoroalkane‐α, ω‐disulfonic acids were measured. These acids are high molecular weight analogs of trifluoromethane sulfonic acid, having the general molecular formula. Properties measured included the solubility and conductivity of aqueous solutions, the extent of anion specific adsorption on platinum, and oxygen reduction kinetics on platinum, for acids having . The following trends in properties with variation in were observed: solubility in water and the conductivity of aqueous solutions decreased dramatically with increasing ; an increase in the extent of anion specific adsorption with increasing ; at constant pH, rate of oxygen reduction on Pt decreased with increasing ; for a given acid (value of ), the rate of oxygen reduction decreased exponentially with increasing acid concentration. These trends in physical properties can be explained in a straightforward manner if it is assumed that there is successively weaker hydration of the anions with increasing ,i.e., size of the anion. Of the acids in this homologous series, the acid appears to have the best combination of properties for a fuel cell electrolyte.

Carbon nanotube coatings for thermal control

Materials based on carbon nanotubes (CNT) with their high thermal conductivity, the high aspect ratios as well as their mechanical strength, provide innovative materials for thermal control applications such as improved thermal interfaces. We demonstrate the feasibility of carbon nanotube based systems for use in thermal control applications, e.g. as a contact layer between two thermally connected materials. Multi-wall carbon nanotube (MWCNT) arrays of different density and length have been grown on silicon and copper surfaces using chemical vapor deposition. Measurements of the heat flow across different surfaces demonstrates that a CNT array as a contact layer will improve the thermal transport in vacuum, with a significant improvement over thermal grease designed for this purpose. An important application for this technology is in a thermal switch, where the contact between the two surfaces is not static and conducting epoxies or thermal grease cannot be used.

Embedded micro-sensor for monitoring pH in concrete structures

Three major causes of corrosion of steel in concrete are chloride ions (Cl-), temperature (T) and acidity (pH). Under normal operating temperatures and with pH above 13, steel does not undergo pitting corrosion. In presence of Cl-, if the pH decreases below 12, the probability of pitting increases. Acid rain and atmospheric carbon dioxide cause the pH to drop in concrete, often leading to corrosion of the structure with the concomitant cost of repair or replacement. Currently, the pH level in concrete is estimated through destructive testing of the structures. Glass ISFET, and other pH sensors that need maintenance and calibration cannot be embedded in concrete. In this paper, we describe an inexpensive solid state pH sensor that can be embedded in concrete, to detect pH changes at the early stages. It employs a chemical reagent, trinitrobenzenesulfonic acid (TNBS) that exhibits changes in optical properties in the 12 - 14 pH range, and is held in a film of a sol-gel/TNBS composite on an optically transparent surface. A simple LED/filter/photodiode transducer monitors pH-induced changes in TNBS. Such a device needs no periodic calibration or maintenance. The optical window, the light-source and sensor can be easily housed and encapsulated in a chemically inert structure, and embedded in concrete.

Acceleration of Oxygen Reduction Rate by Alkyl Derivatives of Uracil on Pt Catalysts Used in Fuel Cells

Alkyl derivatives of uracil, namely, 5,6-dimethyluracil, 5-ethyluracil, and 5-methyluracil accelerate the rate of electrochemical reduction of oxygen on Pt catalysts in aqueous 0.5 M sulfuric acid. Two other uracil derivatives, namely, 1,3-dimethyluracil and 2,4-dimethoxypyrimidine, and uracil itself, also increase the rate of the oxygen reduction reaction, but to a much lesser degree. The organie molecules were adsorbed on the electrode surface from dilute solutions of 0.5 mM in the electrolytes. The organic adsorbates shift the open-circuit potential of the Pt electrode to more positive values and generate higher oxygen reduction currents at lower overpotentials. Replacement of the adsorbed anions from the surface by the organic molecules facilitates oxygen reduction. Partial electron transfer between the electrode and some of the adsorbed molecules also plays a role in their catalytic behavior. The degree of enhancement by the organic adsorbates reported here for the rough Pt electrode is somewhat different from that found on a smooth, polycrystalline electrode. These differences are explained based on the effect of surface roughness and the time-dependent variation of the diffusion layer thickness caused by the diffusion of oxygen to the electrode surface.

Effect of adsorbed uracil and its derivatives on the rate of oxygen reduction on platinum in acid electrolytes

Abstract The rate of oxygen reduction in 0.5 M H 2 SO 4 has been controlled by adsorbing uracil and its alkyl derivatives on a Pt electrode. Addition of an ethyl group to C(5), or methyl groups to the C(5) and C(6) positions of uracil, enhances the reaction rate. 5-Ethyluracil and 5,6-dimethyluracil have such an effect, however, only when their concentrations in the electrolyte are ≤0.1 mM. Substitution of the hydrogen on N(3) with CH 3 , or exocyclic oxygens with OCH 3 groups results in the inhibition of the reaction; 1,3-dimethyluracil and 2,4-dimethoxypyrimidine (0.1 mM) are two systems that cause inhibition. Interaction between the surface Pt atoms and the N(3) and O sites on the organic molecule, especially in the presence of CH 3 or C 2 H 5 groups on the C(5) and/or C(6) centers, are essential for the enhancement. Results of rotating disk electrode experiments suggest that at low overpotentials, 5-ethyluracil and 5,6-dimethyluracil increase the exchange current to produce higher reaction rates.

Electrochemical Evaluation of Bis(trifluoromethylsulfonyl) Methane as a Fuel Cell Electrolyte

This paper reports the kinetics of oxygen reduction on Pt studied in a new type of perfluorinated acid, (CF{sub 3}SO{sub 2}){sub 2}CH{sub 2} (bis(trifluoromethylsulfonyl)methane), containing acidic C{bond}H bonds. This acid appears to be the strongest carbene acid known. At 90{degrees}C, the conductivity of 1.15 {ital M} (CF{sub 3}SO{sub 2}){sub 2}CH{sub 2} is 0.6 {Omega}{sup {minus}1} cm{sup {minus}1}, which is the same as that of 98% phosphoric acid at 170{degrees}C. Even the room temperature conductivity of this acid is higher than that of 85% phosphoric acid at 100{degrees}C. Kinetic measurements were made using the rotating disk electrode technique in {ital p}H = 1 solutions and with porous fuel cell electrodes in solutions at the limit of solubility (1.15 {ital M}). At {ital p}H = 1, the half-wave potential for oxygen reduction is shifted cathodically by 115 mV relative to that for phosphoric acid at the same {ital p}H, representing an approximately one order of magnitude increase in rate. Using standard fuel cell electrodes, the room temperature polarization in 1.15 {ital M} (CF{sub 3}SO{sub 2}){sub 2}CH{sub 2} was 40--60 mV lower than with the same electrodes used in 85% phosphoric acid at 70{degrees}C.