No abstract

Geometries of monomers through hexamers of cylopentadiene, pyrrole, furan, silole, phosphole, thiophene, selenophene and tellurophene, and monomers through nonamers of borole were optimized employing density functional theory with a slightly modified B3P86 hybrid functional. Bandgaps and bandwidths were obtained by extrapolating the appropriate energy levels of trimers through hexamers (hexamers through nonamers for borole) to infinity. Bandgaps increase with increasing ~-donor strengths of the heteroatom. In general, second period heteroatoms lead to larger bandgaps than their higher period analogs. Polyborole is predicted to have a very small or no energy gap between the occupied and the unoccupied w-levels. Due to its electron deficient nature polyborole differs significantly from the other polymers. It has a quinoid structure and a large electron affinity. The bandgaps of heterocycles with weak donors (CH2, Sill2 and PH) are close to that of polyacetylene. For polyphosphole this is due to the pyramidal geometry at the phosphorous which prevents interaction of the phosphorus lone pair with the or-system. The bandgap of polypyrrole is the largest of all polymers studied. This can be attributed to the large w-donor strength of nitrogen. Polythiophene has the third largest bandgap. The valence bandwidths differ considerably for the various polymers since the avoided crossing between the flat HOMO-1 band and the wide HOMO band occurs at different positions. The widths of the wide HOMO bands are similar for all systems studied. All of the polymers studied have strongly delocalized or-systems.

A detailed investigation of the electrochemistry of Ir in 0.5 M H 2 S0 4 has been used as an experimental basis for a model for oxide growth at Ir. It appears that a compact oxide (probably IrO,) is formed initially. At potentials above +1.2 V vs. RHE, the outer monolayer of this compact oxide is oxidised and becomes hydrated. The hydrated surface layer inhibits further oxidation of the compact oxide and therefore only one monolayer of hydrous oxide can be formed at constant potential. To obtain more hydrous oxide than this, the compact oxide must be continually reduced to Ir metal and reformed, by cycling of the potential. On each cycle, the hydrated surface layer of the compact oxide remains after reduction of the compact oxide. Thus, this material accumulates as a hydrous oxide layer.

No abstract

The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell ͑PEMFC͒ electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization, determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed ͑55-76%͒ cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder.