Preparation and characterization of nafionTM/polyaniline composite membrane for direct methanol fuel cell

Abstract

Nafion/Polyaniline composite membranes in different intrinsic oxidation states for DMFC, including leucoemeraldine base (LEB), emeraldine salt (ES), and pernigraniline base (PB), were prepared by chemical in-situ oxidation-controlled polymerization under different experimental conditions. The molar ratio of oxidant/aniline, polymerization time, and reducing agent were controlled to obtain the distinct oxidation states of PANI in Nafion membarne. In particular, the influence of oxidation states of polyaniline (PANI) was extensively examined on physicochemical and transport properties of Nafion/PANI composite membrane by using several instruments. After polymerization of aniline monomers in Nafion matrix, the presence of PANI particles in three different types of composite membranes was observed by SEM and AFM, displaying that polymerization occurred at the surface of membrane as well as the inside. In addition, the composite membranes in different oxidation states had the uniform distribution of polyaniline particles over the cross section of the membrane irrespective of the oxidation states of PANI, as observed by EDAX. The microstructure changes of Nafion depending upon the oxidation forms of the embedded PANI in Nafion matrix were analyzed by FT-IR ATR, SAXS, and WAXD. LEB and ES forms of Nafion/PANI membranes had swelled the ionic clusters related to the transport pathway, increasing Bragg spacing more strongly than PB of Nafion/PANI composite membrane by a favorable hydrogen bonding interactions between the sulfonic groups and PANI. In contrast, the lamellar ordering region of backbone of Nafion was strongly disturbed by the interactions of backbone group with PANI in the fully oxidized state. Consequently, The thermal properties of the composite membranes were influenced by the morphological changes. As given by the DSC data, the cluster transition temperature and the melting temperature of Nafion were shifted by means of conformational changes of back...