Poly(3,4-ethylenedioxythiophene)-modified nafion membrane for direct methanol fuel cells

Abstract

Membranes Nafion 117 are modified with poly(3,4-ethylenedioxythiophene) (PEDT) by chemical polymerization of EDT with H2O2 or FeCl3 as the oxidants in a two-compartment cell. Depending on the oxidant and polymerization conditions, PEDT is deposited either as a thin film on the membrane surface or inside the Nafion membrane depending on whether FeCl3 or H2O2 is used as the oxidant. The decrease in the ionic conductivity and methanol permeability is studied as a function of the polymerization time. A linear dependence is found with H2O2 and a t −1/2 dependence, with FeCl3. The contributions of PEDT and Nafion to the overall conductivity of the composite membranes are separated by impedance measurements. The modified membranes (FeCl3) are also tested in direct methanol fuel cells (DMFC). The methanol permeation through the membranes is measured by operating the fuel cell in an electrolysis mode. The influence of the modified membranes on the DMFC current-voltage characteristics is studied with 2 M CH3OH and O2 at 1.2 barabs and 80°C. Membrane electrode assemblies (MEAs) are prepared by hot pressing the modified membrane between two commercial gas diffusion electrodes with 1 mg cm−2 of Pt loading. A decrease of the methanol permeation of 25% is observed at MEA with the modified membrane (1 h polymerization time), compared with that of MEA with a Nafion membrane. However, the overall DMFC performance decreases in the same relation: a maximal power density of 36 W cm−2 is measured at MEA with a PEDT-modified membrane compared with 45 W cm−2 for MEA with a Nafion membrane.