Oxygen quenching of [Ru(Ph2phen)3]Cl2 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline) has been studied in a diverse series of polymers, most with a common poly-(dimethylsiloxane) (PDMS) component. Systematic variations in the polymer properties have been made in order to delineate the structural features important for satisfactory use of supports for oxygen sensors. Most measurements were made using homo- or copolymers containing a PDMS region, although some measurements were made on small ring siloxane polymers. In particular, quenching behavior was examined as a function of polymer structure as well as the type of and amount of polar copolymer cross-linkers. Cross-linkers were added to enhance the solubility of oxygen probes in an otherwise nonpolar polymer. In addition, hydrophobic silica was added to alter quenching properties. Domain models are used to explain the variations in oxygen quenching properties as a function of additives and cross-linkers. These considerations have led to the most sensitive ruthenium-based sensor reported to date. The relative affinity of the different domains for the complex and the efficacy of the domains for oxygen quenching control the overall behavior of the sensing response. Guidelines for design of suitable polymer supports for oxygen sensors are proposed.

The electrochemical characterization of thin films of the ionically functionalized polyacetylene analogues poly(tetramethylammonium 2-cyclooctatetraenylethanesulfonate) (P(A)) and poly[(2-cyclooctatetraenylethyl)trimethylammonium trifluoromethanesulfonate] (P(C)) is reported along with an electrochemical approach to the fabrication of interfaces between dissimilarly doped conjugated polymers. Such interfaces are of interest because of the central role analogous interfaces based on silicon play in conventional microelectronics. The cationically functionalized P(C) can be both oxidatively (p-type) and reductively (n-type) doped to a conductive state, whereas the anionically functionalized P(A) can only be p-type doped. The voltammetry of P(C) displays relatively sharp waves with minimal history or relaxation effects. In contrast, the voltammetry of P(A) exhibits broader doping waves and a dependence on electrochemical history. The apparent formal potentials reported in 0.075 M Me4NBF4/CH3CN were -1.04 V versus SCE for the n-doping of P(C) and 0.40 and 0.30 V versus SCE for the p-doping of P(C) and P(A), respectively. These values depend on electrolyte concentration consistent with a Donnan potential due to the selective partitioning of ions between the electrolyte and polymer. Electrochemical quartz crystal microbalance data demonstrate that the p-type doping of P(A) and the n-type doping of P(C) proceed with the loss of ions from the polymer film and the formation of the internally compensated state. Voltammetry in tetrabutylammonium poly(styrenesulfonate)/CH3CN supporting electrolyte is also reported. It is demonstrated how a polyanion supporting electrolyte in concert with a conjugated ionomer can be used to control redox chemistry by governing the sign of ions available for charge compensation. In particular, we demonstrate the self-limiting oxidation of P(A) to inhibit deleterious overoxidation and prepare the precisely internally compensated state; the selective oxidation of P(A) over P(C), despite their similar apparent formal potentials; and the inhibition of the reoxidation of the n-doped form of P(C). The use of such polyelectrolyte-mediated electrochemistry in the fabrication of interfaces between dissimilarly doped conjugated polymers is discussed.

Commercial perfluorosulphonic acid membranes ͑Nafion͒ have been impregnated with polypyrrole by in situ polymerization to decrease the crossover of methanol in direct methanol fuel cells ͑DMFCs͒. Modified membranes produced by polymerization of the pyrrole with hydrogen peroxide and iron͑III͒ have been evaluated in a DMFC. Both methods produce membranes that can provide enhanced cell performance, although membranes produced with iron͑III͒ as the oxidizing agent for the polymerization require additional treatments to restore their conductivity and promote bonding to the electrodes. Performance gains result from substantial reductions of the cathode overpotential, while anode overpotentials increase due to the lower conductivities of the modified membranes. Part of the beneficial effect at the cathode appears to be due to lower water crossover from the anode to the cathode.Direct methanol proton exchange membrane fuel cells ͑DMFCs͒ 1-3 are currently being developed by many companies for commercialization over the next few years. 4,5 Initial applications will be in consumer electronics such as cell phones and computers, while transportation and utility applications will become increasingly important toward the end of this decade.Organizations that are involved in the development of the DMFC face a number of significant obstacles that currently limit the potential market. 4 The crossover of methanol through the membrane from the anode to the cathode is a major problem. 3 It decreases both the performance and the fuel efficiency of the cell, and limits its lifetime.Conjugated polymer/Nafion composite membranes show excellent potential for DMFC applications 6,7 because they are less permeable to methanol and therefore efficiency losses from methanol crossover are significantly less. Preliminary results have demonstrated that composite membranes prepared using poly͑1-methylpyrrole͒ reduce methanol crossover by as much as 50% without a significant increase in the resistance of the composite membrane. 6 The work described here characterizes the properties of polypyrrole/Nafion composite membranes in an operational DMFC. Pyrrole was selected as the polymer precursor in this work because its greater solubility in water simplifies the membrane modification process and preliminary results in fuel cells were better than those achieved with a poly͑1-methylpyrrole͒/Nafion composite. Methanol crossover and DMFC performance ͑including the separate performances of the anode and cathode͒ have been evaluated and results are compared with results obtained with unmodified Nafion membranes. ExperimentalMembrane modification.-Membranes were cleaned in 15% H 2 O 2 (aq), 1 M H 2 SO 4 (aq) ͑1 h at 60-80°C in each solution͒, and water. They were then immersed in a pyrrole solution ͑0.08-0.10 M for 80-120 min for Fe͑III͒ as the oxidant ͑except F5E͒, 0.20 M for 5 min ͑15 min for H5B͒ for H 2 O 2 ), and then rinsed well with water. The pyrrole within the membrane was then polymerized by immersion in a 0.03-0.05 M Fe͑NO 3 ) 3 (aq) solution for 8...

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Details of Nafion/polypyrrole composite formation have been obtained using electronic absorption spectroscopy. Three distinct processmonomer loading, polymerization, and removal of unreacted monomerhave been studied. Pyrrole loading displays a square root dependence on time, indicative of a diffusion controlled partitioning process. Partitioning of pyrrole into Nafion, however, is complicated by protonation of pyrrole and acid-catalyzed oxidation to give oligomeric and polymeric species. These processes are affected by the presence of oxygen and are photosensitive. A variety of oxidizing agents have been used to effect polymerization including Fe3+, H2O2, ammonium persulfate, and UV irradiation.