Doping-Controlled Ion Diffusion in Polyelectrolyte Multilayers: Mass Transport in Reluctant Exchangers

A new paradigm for nonlinear doping-controlled ion transport in soft condensed matter is presented, where the mobility of a minority “probe” ion is controlled by majority “salt” ion. The class of materials to which this paradigm applies is represented by ultrathin films of polyelectrolyte complexes, or multilayers. Intersite hopping of probe ions of charge ν occurs only when the charge of the destination site, produced by clustering of monovalent salt ions, is at least −ν, conserving electroneutrality. Salt ions are reversibly “doped” into the multilayer under the influence of external salt concentration. In situ ATR-FTIR reveals that the doping level, y, is proportional to salt concentration. Because hopping requires coincidence, or clustering, of salt, a strongly nonlinear dependence of flux, J, on salt concentration is observed:  J ∼ [NaCl]ν ∼ yν. This scaling was reproduced both by Monte Carlo simulations of ion hopping and by continuum probability expressions. The theory also predicts the observed scaling, though it underestimates the magnitude, of the strong selectivity of multilayers for ions of different charge

Engineering Ionic and Electronic Conductivity in Polymer Catalytic Electrodes Using the Layer-By-Layer Technique

The platinum loading, electronic and ionic conductivity, tuned porosity, and electrode potential of layer-by-layer (LBL) conducting polymer films for thin film catalytic electrodes are presented. Films of polyaniline (PANi)/poly(acrylic acid) (PAA) or PANi/poly(acrylic acid)-co-polyacrylamide (PAA-co-PAAm) of 3.0-μm thickness were pH-tuned to induce porosity as they were assembled. Three different techniques were used to dose the LBL PANi films with platinum. The first method used reductive precipitation of platinum and ruthenium salts adsorbed within LBL films of PANi/PAA-co-PAAm. The second method, termed polyelectrolyte colloidal platinum stabilization, was applied to load platinum nanoclusters into LBL films of either PANi/PAA or PANi/poly(styrene sulfonate) films. The third method used a PANi/platinum powder dispersion to load platinum crystals into LBL films of PANi/PAA-co-PAAm or poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAA-co-PAMPS). The first method yielded the best metal loadings with maximum platinum loadings of 0.3 mg cm-2, and the resulting Pt-containing PANi/PAA-co-PAAm films were further examined for their electrochemical characteristics. The electrode potential and chronopotentiometric current control in the resulting electrodes were examined for the best-performing LBL PANi film assembled in this study. The catalyzed PANi/PAA-co-PAAm electrodes exhibited an electrode potential similar to that of pure platinum, a relatively high and stable electrical conductivity of 2.3 S cm-1, and an ionic conductivity of up to 10-5 S cm-1