Nafion Sheet Research Articles

Electromechanical response of ionic polymer-metal composites

An ionic polymer-metal composite (IPMC) consisting of a thin Nafion sheet, platinum plated on both faces, undergoes large bending motion when an electric field is applied across its thickness. Conversely, a voltage is produced across its faces when it is suddenly bent. A micromechanical model is developed which accounts for the coupled ion transport, electric field, and elastic deformation to predict the response of the IPMC, qualitatively and quantitatively. First, the basic three-dimensional coupled field equations are presented, and then the results are applied to predict the response of a thin sheet of an IPMC. Central to the theory is the recognition that the interaction between an imbalanced charge density and the backbone polymer can be presented by an eigenstress field (Nemat-Nasser and Hori, Micromechanics, Overall Properties of Heterogeneous Materials, 2nd Ed., Elsevier, Amsterdam, 1999). The constitutive parameter connecting the eigenstress to the charge density is calculated directly using a simple microstructural model for Nafion. The results are applied to predict the response of samples of IPMC, and good correlation with experimental data is obtained. Experiments show that the voltage induced by a sudden imposition of a curvature, is two orders of magnitude less than that required to produce the same curvature. The theory accurately predicts this result. The theory also shows the relative effects of different counter ions, e.g., sodium versus lithium, on the response of the composite to an applied voltage or a curvature.

Precipitation of Metallic Platinum into Nafion Ionomer Membranes: I . Experimental Results

Extended x‐ray absorption fine structure (EXAFS) spectroscopy was used to investigate the precipitation of metallic platinum into Nafion ionomer membranes. First, x‐ray absorption spectra above the edge were obtained to compare the structure of platinum tetramine (i) solid, (ii) dissolved in water, and (iii) incorporated to Nafion sheets. The four amine groups remain linked to the platinum atom even after dissolution and incorporation into the membrane. EXAFS collected with dispersive optics was used to follow in situ the incorporation and the precipitation of the salt at edge. Time dependent concentrations of platinum salt in the membrane were obtained by measuring the time variation of the absorbance of the sample. The kinetics of incorporation were proportional to the salt concentration in the solution and depended on the hydrodynamic conditions at the interfaces. The precipitation was performed using a reducing solution of sodium borohydride. The chemical reduction of platinum tetramine into metallic platinum occurred directly without the formation of any stable intermediates. The kinetics of precipitation were proportional to the reducer concentration in the solution. After precipitation, concentration profiles of the metallic platinum across the membrane thickness were obtained by electron microprobe analysis. The concentration profiles in metallic platinum depend markedly upon (i) the sodium borohydride concentration in the solution, and (ii) the number of successive cycles of loading‐precipitation. The experimental data presented in this paper are employed to check the validity of two models developed for describing the two steps of ion‐exchange and precipitation.