New approach for the evaluation of membranes transport properties for polymer electrolyte membrane fuel cells

The two major hurdles obstructing the large scale commercialization of PEMFCs (polymer electrolyte membrane fuel cells) are the cost and durability under a wide range of operating conditions. Among the main factors affecting the PEMFC, the durability of the polymeric membrane is one of the most determinant. Ideally, the membrane present in the MEA should be impermeable to gases such as H-2, O-2, N-2, letting past only the H+ from the anode to the cathode side. However, gases permeate the membrane giving the so-called "crossover". The crossover cannot be considered negligible specifically when the membrane thickness is lower and lower. Nevertheless, the rates of hydrogen and air/oxygen permeation to the opposite side of the membrane are relatively slow, the crossover leads to a depletion of PEMFC efficiency and accelerates the degradation of the polymeric membrane. The evaluation of the permeation of gases through the membranes usually used in the PEMFC becomes, thus, fundamental in the estimation of the overall PEMFC performance. In this work, a new approach for the systematic evaluation of the mass transport properties of a PEM is proposed. A protocol for permeation measurements has been elaborated to compare the transport properties of different membranes as a function of the same operating conditions due to the strict dependence of the membrane performance on the relative humidity (RH) and temperature. The method has been proven on two different membranes: a Nafion 117 and a cross linked home-made SPEEK membrane. The proton conductivity represents the other transport property of the membrane to be considered together with the gas permeation properties. The ideal/desired membrane must exhibit low gas permeability and high proton conductivity. Moving in this logic, in the present work a new parameter called Transport Performance Index has been defined as the ratio of hydrogen permeability and proton conductivity. This leads to an immediate idea of the whole mass transport performance of the membrane. The mass transport properties, proton conductivity and Transport Performance Index were measured for Nafion 117 and SPEEK membranes as a function of temperature, pressure, and relative humidity feeding different gaseous streams. The properties of the different membranes were also critically compared.