Studies on interfacial stability of MEA for long-term performance of direct methanol fuel cell

Abstract

Polymer electrolyte membranes (PEMs) for direct methanol fuel cells (DMFCs) require high protonic conductivity and low methanol permeability among a variety of other attributes. The performance limitation of currently used perfluorinated sulfonic acid ionomers such as Nafion arises from their inherently high methanol permeability. This leads to high methanol crossover which adversely affects the cell performance due to a mixed potential at the cathode and lowers fuel efficiency. As a result, much research effort has been focused on developing modified Nafion membrane that have lower methanol permeability with minimal loss of proton conductivity. However, most reported DMFCs studied using modified Nafion membrane exhibited rather poor performance regarding their higher relative selectivity, i.e., the ratio of proton conductivity to methanol permeability compared with the Nafion membrane. One of the possible reasons for this is the incompatibility and high interfacial resistance of these membranes with the perfluorinated Nafion-bonded electrode generally employed for DMFC applications. In the intial stage of this study, the proton conducting membrane based on semi-IPNs of Nafion and crosslinked poly(AMPS) was prepared and characterized. The modification of Nafion with crosslinked poly(AMPS) such as hydrocarbon polymer changed the state of water in membranes. Without a significant increase of the membrane resistance, the semi- IPNs demonstrated a reduction of the methanol permeability, comparing to the native Nafion. And the initial maximum power density of AMPS60 increased. However, the semi-IPNs had the chemical instability and interfacial instability between electrode and membrane. To overcome the chemical instability of semi-IPNs, the composite membrane with Nafion and sulfonated silica was introduced. The proton conductivity of the composite membrane and the adhesion between electrod and membrane were enhance compared with the Nafion. So, the cell perf...