O. Savadogo

Methanol-Tolerant Oxygen Reduction Electrocatalysts Based on Pd-3D Transition Metal Alloys for Direct Methanol Fuel Cells

Palladium-based alloys, such as Pd-Co, Ni, and Cr, have been developed as a novel methanol-tolerant oxygen reduction electrocatalyst for direct methanol fuel cells. The Pd alloy electrocatalysts were fabricated by a rf sputtering method. Their electrochemical characteristics for the oxygen reduction reaction (ORR) were determined in sulfuric acid solution with and without methanol at 30°C. The Pd alloys showed a higher ORR electrocatalytic activity than Pd, although lower than Pt. The Pd alloys also had no electrocatalytic activity for methanol oxidation in the presence of methanol. The maximum electrocatalytic activities for ORR were observed for the alloy composition of ca. 60 atom % Pd in all the Pd alloys. Based on the X-ray photoelectron surface analysis, it was confirmed that the filling of the Pd d-band by alloying decreased the density of states (DOS) at the Fermi level. The decreased DOS inhibited the formation of Pd oxide on the surface of the electrocatalyst. This result should contribute to the improvement of the ORR activity of the Pd alloy electrocatalysts.

Hydrogen/oxygen polymer electrolyte membrane fuel cells (PEMFCs) based on alkaline-doped polybenzimidazole (PBI)

The potential-current characteristics of a H 2 /O 2 polymer electrolyte membrane fuel cell using H 2 SO 4 or H 3 PO 4 doped PBI were studied for the first time. The conditions involved in the doping of PBI with sulphuric acid or phosphoric acid and the preparation of a membrane electrode assembly (MEA) using these membranes were determined. The potential-current fuel-cell characteristics of MEAs using H 2 SO 4 -doped PBI were compared to those of MEAs using Nafion 117. The effects of membrane doping and of drying times on the fuel-cell performances of MEAs based on PBI doped with sulphuric acid in various conditions were determined. It was shown that MEAs based on H 2 SO 4 - doped PBI and non-humidified exhibited higher fuel-cell characteristics than MEAs based on Nafion 17. The fuel-cell characteristics of MEAs based on phosphoric-acid-doped PBI exhibited high performances at 185 oC even with fuelled with hydrogen containing 3% CO.

Parameters of PEM fuel-cells based on new membranes fabricated from Nafion®, silicotungstic acid and thiophene

Abstract Several new cation exchange membranes of different thicknesses (15–500 μm) based on a Nafion® solution and silicotungstic acid with and without thiophene (named NASTATH and NASTA respectively) were synthesized by a simple chemical route for PEM fuel-cell applications. The optimum parameters for the preparation of the membranes have been determined: 10 ml of 5% of the Nafion® 117 solution was reduced by 50% and mixed with 10−3–10−5 MSTA to produce a NASTA membrane. If liquid thiophene (0.5% by volume) is added to the above solution, a NASTATH membrane is produced. The water uptake and ionic conductivity of NASTA and NASTATH were compared with those of Nafion® 117. The effect of membrane thickness and the concentrations of STA and thiophene used during the preparation of NASTA and NASTATH on their water uptake and ionic conductivity were determined. It was shown that the water uptake of the NASTA membrane (60%) was significantly better than that of Nafion® 117 (27%), while the water uptake of NASTATH (40%) was higher than that of Nafion® 117 (27%). The ionic conductivity of both the NASTA (10.10×10−2 Ω−1 cm−1) and the NASTATH (9.5×10−2 Ω−1cm−1) was found to be significantly higher than that of the Nafion® 117 (1.23×10−2 Ω−1 cm−1). The membrane performances were also determined by chemical stability studies. The membranes fabricated with Nafion® and silicotungstic acid with and without thiophene still exhibited good mechanical strength and stability after they had been dipped in an acid or a basic medium for at least 10 months. The voltage-current characteristics of solid polymer electrolyte fuel cells were determined for Nafion® 117, NASTA and NASTATH based membranes. The fuel cell parameters were correlated to the membrane water uptake and ionic conductivity. The current density at 0.600 V of the solid polymer electrolyte fuel cells (SPEFCs) based on NASTATH (810 mA cm−2) membranes was higher than that of SPEFCs based on Nafion® 117 (640 mA cm−2). It was shown that the better fuel cell parameters were not obtained with the modified membranes having the higher water uptake.

Effect of surface treatment and sterilization processes on the corrosion behavior of NiTi shape memory alloy

Nickel-titanium (NiTi) alloy derives its biocompatibility and good corrosion resistance from a homogeneous oxide layer mainly composed of TiO(2), with a very low concentration of nickel. In this article, we described the corrosion behavior of NiTi alloys after mechanical polishing, electropolishing, and sterilization processes using cyclic polarization and atomic absorption. As a preparative surface treatment, electropolishing decreased the amount of nickel on the surface and remarkably improved the corrosion behavior of the alloy by increasing the mean breakdown potential value and the reproducibility of the results (0.99 +/- 0.05 V/SCE vs. 0.53 +/- 0. 42). Ethylene oxide and Sterrad(R) sterilization techniques did not modify the corrosion resistance of electropolished NiTi, whereas a steam autoclave and, to a lesser extent, peracetic acid sterilization produced scattered breakdown potential. In comparing the corrosion resistance of common biomaterials, NiTi ranked between 316L stainless steel and Ti6A14V even after sterilization. Electropolished NiTi and 316L stainless-steel alloys released similar amounts of nickel after a few days of immersion in Hanks solution. Measurements by atomic absorption have shown that the amount of released nickel from passive dissolution was below the expected toxic level in the human body. Auger electron spectroscopy analyses indicated surface contamination by Ca and P on NiTi during immersion, but no significant modification in oxide thickness was observed.

Studies on new chemically deposited photoconducting antimony trisulphide thin films

A new room temperature chemical deposition technique has been developed to deposit semiconducting antimony trisulphide thin films on conducting and ordinary glass substrates. The method is based on aqueous ammonia bath containing potassium antimonyl tartarate (PAT), triethanolamine (TEA) and thioacetamide (TAM). It has been found that the proper control of deposition bath compasitions significantly influences the quality and the thickness of the Sb2S3 films. Moreover, addition of a small amount (10−5 M) of silicotungstic acid (STA) in the deposition bath enhanced the rate of deposition and improves significantly the photoactivity of the films. The deposited films were characterized by X-ray, SEM and neutronic activation analysis. The effect of the annealing and STA on the change of the optical bandgap (Eg) of the films was determined at 300 K. The influence of the STA on the resistivity, carrier concentrations and mobility of the films was determined by the resistivity and Hall effect measurements on the annealed samples. A very small (≈ 5%) change in the electrical resistivity has been observed for the films in which STA is incorporated. X-ray photoelectron spectroscopic (XPS) studies were carried out to determine the surface and the bulk compositions of the films. The photoconductivity studies revealed that the deposited films were highly photoconducting in nature.

Chemically and electrochemically deposited thin films for solar energy materials

Direct energy gap materials, e.g. CdTe, CuInSe2, CuInGaSe2, CdSe, ZnP2 and Zn3P2, are the most interesting for thin-film solar cell applications. Among the various methods of preparation of these films, chemical bath deposition and electrodeposition deserve special attention because they have been shown to be inexpensive, low-temperature and non-polluting methods. Based on Pourbaix diagrams of CdS, CdTe, CuInSe2, CdSe, etc., drawn from basic considerations, the best parameters for their electrodeposition are deduced. Theoretical considerations on the chemical-bath deposition of CdS, CdSe and Sb2S3 are also indicated. In particular, the role of the complexing agent and of the ligands in chemical bath deposition quality is discussed, as are the uniformity and stability of the films. The photoelectrochemical, Schottky barrier and heterojunction solar cell properties based on chemically and electrochemically deposited thin films with heteropolyacids are shown. Future trends for chemically and electrochemically deposited polycrystalline thin films are addressed. Results from very recent work done in the improvement of chemically and electrochemically deposited thin films are presented. Significant results obtained on advanced CdS/CdTe, CdS/CIS and CdS/CIGS solar cells developed by industry and by laboratory groups worldwide are indicated. Emerging low cost materials or/and less environmental hazards materials which may introduce solar cells into worldwide market are considered in the conclusion.

Effects of sterilization processes on NiTi alloy: Surface characterization

Sterilization is required for using any device in contact with the human body. Numerous authors have studied device properties after sterilization and reported on bulk and surface modifications of many materials after processing. These surface modifications may in turn influence device biocompatibility. Still, data are missing on the effect of sterilization procedures on new biomaterials such as nickel-titanium (NiTi). Herein we report on the effect of dry heat, steam autoclaving, ethylene oxide, peracetic acid, and plasma-based sterilization techniques on the surface properties of NiTi. After processing electropolished NiTi disks with these techniques, surface analyses were performed by Auger electron spectroscopy (AES), atomic force microscopy (AFM), and contact angle measurements. AES analyses revealed a higher Ni concentration (6-7 vs. 1%) and a slightly thicker oxide layer on the surface for heat and ethylene oxide processed materials. Studies of surface topography by AFM showed up to a threefold increase of the surface roughness when disks were dry heat sterilized. An increase of the surface energy of up to 100% was calculated for plasma treated surfaces. Our results point out that some surface modifications are induced by sterilization procedures. Further work is required to assess the effect of these modifications on biocompatibility, and to determine the most appropriate methods to sterilize NiTi.

A methodological concept for material selection of highly sensitive components based on multiple criteria decision analysis

Material selection of highly sensitive components is one of the most challenging issues in the design and development of structural elements in aerospace and nuclear industry. This work compares some of the most widely potential multi-criteria decision making models for addressing all the stages in solving a material selection problem of highly sensitive components involving conflicting as well as multiple design objectives. For the first step, the compensatory models are discussed and employed to solve a multi-criteria material selection for a thermal loaded conductor in the presence of its required multi-functional characteristics. For the next step, using different versions of the non-compensatory methods examine the outranking approach to solve the same problem. The results are compared to each other to verify the effect of compensations and non-compensations in the methods and their sensitivity to ranking stability. It is of particular interest to see how different approaches of the Multiple Attribute Decision Making (MADM) models differ from each other when criterion of cost is a critical factor in the problem. The effect of individual attributes of cost criterion has been studied to ensure the reliability of the chosen candidate material by MADM models.

Comparison of Methanol Crossover among Four Types of Nafion Membranes

The crossover behavior of methanol through Nafion 112, 117, 1035, and 1135 membranes using cyclic voltammetry (CV) was studied at 25°C and the results compared. The multiple CV technique was used to study the electrochemical characteristics of the crossed methanol during this crossover. Meanwhile, a method established in our laboratory was introduced to determine the concentrations of methanol crossover. The use of various types of Nafion membranes resulted in significant differences in the anodic current responses of the CV curves under similar experimental conditions. The concentration of crossed methanol decreases with time for all four types of Nafion membrane. The mathematical expression of the variation of the methanol crossover changes with the initial methanol concentrationt and the thickness of the membrane. The type of the expression depends on the membrane thickness and the initial methanol concentration. By contrast, the concentration of crossed methanol decreases with an increase in the thickness of the Nation membrane. It was observed that, for membranes of the same thickness, the crossed methanol increases with a decrease in the equivalent weight of the membrane.

Five Percent Platinum‐Tungsten Oxide‐Based Electrocatalysts for Phosphoric Acid Fuel Cell Cathodes

A Pt-tungsten oxide-based electrocatalyst has been fabricated by an inexpensive chemical route for use as an oxygen cathode in 99% phosphoric acid at 180 C. The effect of %WO{sub 3} (wt/wt) on the Pt-tungsten oxide/C-based electrode performance was studied. The electrocatalytic properties for the oxygen reduction reaction (ORR), e.g., exchange current density and mass activity of a 5% Pt-40% WO{sub 3}-based electrode were found to be twice as high as those of 10% Pt, which contains double the amount of platinum. The Tafel slope and specific activity of the two electrodes are similar. It was shown that an increase in its electrochemically active surface area was the only reason for the performance of the 5% Pt-40% WO{sub 3}-based electrode. The electrocatalytic parameters of the 5% Pt-40% WO{sub 3}-based electrode for the ORR were compared to those of the 2% Pt-1% H{sub 2}WO{sub 4}-based electrode.