Precipitation of metallic platinum into Nafion ionomer membranes. I: Experimental results

• Published in: Journal of the Electrochemical Society 1993, Vol.140 (5), p.1373-1380
• Main Authors: MILLET, P, DURAND, R, DARTYGE, E, TOURILLON, G, FONTAINE, A
• Format: Article
• Language: English
• Subjects: Adsorption; Chemistry; Colloidal state and disperse state; Dissolution; Exact sciences and technology; General and physical chemistry; Hydrodynamics; Interfaces (materials); Ion exchange; Membranes; Molecular structure; Platinum; Polymeric membranes; Precipitation (chemical); Reaction kinetics; Reduction
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.2221563

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 PtL sub(III) 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 PtL sub(III) 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.