INSITU ELECTRODE FORMATION ON A NAFION MEMBRANE BY CHEMICAL PLATINIZATION

Three chemical techniques to platinize the surface of a Nafion polymer electrolyte membrane (PEM) to form a Pt/PEM electrode have been studied. The resulting Pt/PEM composites were characterized visually by transmission electron microscopy and scanning electron microscopy and electrochemically by (i) hydrogen adsorption to determine surface area, and (ii) the polarization characteristics for the oxidation of hydrogen. The Takenaka-Torikai method was used in which a reductant diffuses through the membrane to contact and reduce platinic acid at the opposite face, and two variations of an impregnation-reduction procedure were employed in which a cationic salt is first impregnated into the Nafion and subsequently the membrane is exposed to a reductant. A previously reported equilibrium impregnation method was modified such that the impregnation step was not carried to equilibrium and the concentration of the anionic reductant was significantly lowered. Upon judicious choice of the fabrication conditions, platinum deposited by the nonequilibrium impregnation-reduction procedure is concentrated in a thin (submicron), porous layer located predominantly within the membrane but near the surface where it is accessible to both gaseous reactants and a current collector. The metal distribution forms an electrode with a larger active surface area per unit weight of platinum and a better platinum utilization (mA/mg Pt for the hydrogen oxidation reaction) compared to Pt/PEM electrode prepared by the equilibrium impregnation-reduction process or the Takenaka-Torikai method. The chemical and physical processes which occur to affect the metal film formed in all three techniques are discussed.