Alkanethiol-protected silver clusters of average diameter 4.0 ( 0.5 nm form single-phase superlattice solids, and their X-ray powder diffractograms have been fully indexed to single cubic unit cells. Whereas alkanethiols with five or more carbon atoms form superlattices, the corresponding cluster with four carbons yield only separated clusters. The superlattice solids can be recrystallized from nonpolar solvents. No such superlattices are seen for the corresponding gold clusters. The superlattice collapses upon heating, but the solid retains the structure even at 398 K, much above the melting point of crystalline alkanes and the corresponding self-assembled monolayer. In situ variable-temperature X-ray diffraction investigations did not show any solid-state phase transitions in the superlattice. Temperature-dependent infrared spectroscopy reveals the melting of the alkyl chain, and it is seen that the chain as a whole achieves rotational freedom prior to the collapse of the superlattice. Calorimetric investigations show distinct monolayer and superlattice melting transitions. The chemical nature of the cluster-molecule interaction is similar to that of the previously investigated gold and silver systems, as revealed by NMR, mass, infrared, and X-ray photoelectron spectroscopies and thermogravimetry analyses. Conductivity measurements clearly manifest the superlattice melting transition. Diffusion constants in solution measured by NMR show that the relative decrease in the diffusion constant with increasing monolayer chain length is smaller for silver than for gold, suggested to be a signature of intercluster interaction even in solution. Corroborative evidence is provided by the variable-temperature UV/vis investigations of the clusters.

A membrane with interpenetrating networks between poly͑vinyl alcohol͒ ͑PVA͒ and poly͑styrene sulfonic acid͒ ͑PSSA͒ coupled with a high proton conductivity is realized and evaluated as a proton exchange membrane electrolyte for a direct methanol fuel cell ͑DMFC͒. Its reduced methanol permeability and improved performance in DMFCs suggest the new blend as an alternative membrane to Nafion membranes. The membrane has been characterized by powder X-ray diffraction, scanning electron microscopy, time-modulated differential scanning calorimetry, and thermogravimetric analysis in conjunction with its mechanical strength. The maximum proton conductivity of 3.3 ϫ 10 −2 S/cm for the PVA-PSSA blend membrane is observed at 373 K. From nuclear magnetic resonance imaging and volume localized spectroscopy experiments, the PVA-PSSA membrane has been found to exhibit a promising methanol impermeability, in DMFCs. On evaluating its utility in a DMFC, it has been found that a peak power density of 90 mW/cm 2 at a load current density of 320 mA/cm 2 is achieved with the PVA-PSSA membrane compared to a peak power density of 75 mW/cm 2 at a load current density of 250 mA/cm 2 achievable for a DMFC employing Nafion membrane electrolyte while operating under identical conditions; this is attributed primarily to the methanol crossover mitigating property of the PVA-PSSA membrane. Direct methanol fuel cells ͑DMFCs͒ using a proton exchange membrane have been identified as one of the most promising candidates for portable power applications.1,2 Unlike hydrogen-air polymer electrolyte fuel cells, DMFCs do not require a fuel reformer or a high-volume hydrogen storage system. The membrane electrolyte employed with the DMFC, besides exhibiting a good proton conductivity, should act as a physical separator to prevent fuel crossover from the anode to the cathode. At present, Nafion a perfluorosulfonated membrane with a hydrophobic fluorocarbon backbone and hydrophilic sulfonic acid pendant side chains, happens to be the only commercially available and widely used membrane electrolyte in the DMFC. It has been documented that proton conduction in Nafion occurs through the ionic channels formed by micro-or nanophase separation between the hydrophilic proton exchange sites and the hydrophobic domains.3 However, the methanol crossover from anode to cathode across the Nafion membrane brings about a mixed potential at the cathode causing both the loss of fuel and cell polarization impeding their commercial realization. [4][5][6] It has been reported that even over 40% of methanol could be lost in a DMFC due to crossover across the membrane.7 Methanol crossover across the Nafion membrane can be kept to a minimum by controlling the methanol-feed concentration. Alternatively, membranes that are relatively impermeable to methanol have been employed for this purpose. [8][9][10][11][12] Membranes with a lower methanol permeability allow a higher methanol-feed concentration, enhancing the performance of the DMFC. To optimize fuel cell performance, it is neces...