Measurement Of Methanol Permeability Of Polymer Electrolyte Membranes Using Cyclic Voltammetry Method

Growing interest has been paid to direct methanol fuel cells (DMFCs) due to their development. The methanol permeability of the proton exchange membrane electrolyte affects the performance of the DMFCs, It is essential to evaluate the methanol permeability of the membranes with a suitable procedure, which is important to illustrate the mechanism of methanol crossover in the membrane as well as to promote the application and commercialization of the DMFCs. The methanol permeation of Nafion membrane is usually measured by determining the methanol permeation via the membrane with a gas chromatography method. However, this method is only available to pure polybenzimidazole (PBI) membrane. It is known that the PBI membrane needs to be doped with phosphoric acid for using as electrolyte in fuel cells. Nevertheless no effective methods to determine the methanol permeation of the acid doped PBI membrane due to the loss of the doped acid during the measurement. The aim of this work is to propose a method to measure the methanol permeability of the acid-alkali composite membrane like phosphoric acid doped PBI.This measurement of the methanol permeability was proposed based on cyclic voltammetry with a self-made testing device. The methanol permeability of Nafion11membrane, as reference, was first studied using sulfuric acid as electrolyte and its methanol permeability was4.49×10-6cm2/s at room temperature (RT). This result was in good agreement with that of the literature value (1.3×10-cm2/s). This method was followed to be developed to measure the methanol permeability of the phosphoric acid doped PBI membrane using phosphoric acid as the electrolyte. The results indicated that the methanol permeabilities of the acid doped PBI membranes increased with the increase of the acid doping level. For example, the methanol permeability of the phosphoric acid doped PBI membrane at RT increased from3.19×10-6to1.40×10-5cm2/s as the acid doping level changed from2.5to3.3. Efforts were also made to measure the methanol permeability of the membrane at elevated temperatures. However, a lower methanol permeability, i.e.3.4×10-6cm2/s at35℃than that of at RT was obtained for Nafion115membrane.