Investigations On Proton Conductivity And Transport Number Of ProtonExchange Membranes

Proton exchange membrane fuel cell (PEMFC) has been attracted many attentions because of its outstanding advantages. Proton exchange membrane (PEM) is one of the the key components of PEMFC. The ability of proton conduction of PEM, which can be evaluated by proton transport number and conductivity, directly affects the performance of the PEMFC.From the point of view of both theoretical research and practical application, proton transport number and conductivity are the important parameters for evaluating the ion conducting ability of membranes. The proton conductivity of the polymer electrolyte affects the performance of the PEMFC. The proton transport number represents the ion mobility.Studies on the proton transport number as well as the properties of the PEM can provide scientific basis for exploring the proton conducting mechanism of the membrane. In this paper, proton transport numbers of sulfonated polyetheretherketone (SPEEK), SPEEK/TFAPA, and SPEEK/BMIMPF6membranes were measured with a homemade-equipment according to the method of Hittorffs. It was found that the proton transport number increased with the increase of the sulfonation degree of the SPEEK membrane. The proton transport number of the SPEEK composite membranes with ionic liquids TFAPA and BMIMPF6were higher than that of the pure SPEEK membrane and the high content of the ionic liquids resulted in high proton transport number of the membranes. However, an obvious deviation was observed between the results calculated from the variations of the acid concentration of the cathode and anode due to the adsorption of acids by the SPEEK/BMIMPF6membrane and the loss of ionic liquid of the SPEEK/TFAPA membrane, respectively, when the ionic liquid content was high.Although the studies on conductivity of proton exchange membranes have been conducted for many years, deviations among different measurements exist because the proton conductivity of solid phase electrolyte, which is affected by many factors, is different from that of the liquid one. The most used method for conductivity measurements of the solid electrolyte are coaxial probe method and the four-probe AC impedance method. It was found that the resistance of in-plane and out-plane was different to the membranes. The most measurements are based on the in-plane resistance of the membranes. In this study, the conductivity measurement cell was designed by simulation the working situation of the PEMFC. The out-plane conductivity was measured by controlling the humidity with a micro flow injection pump. The conductivity of Nafion(?)115was measured at50-90℃and at water flow rates of0.10-0.30mL/min and the results were compared with that of the reports.