Phase separation and ion conductivity in the bulk and at the surface of anion exchange membranes with different ion exchange capacities at different humidities

For higher performances of anion exchange membrane (AEM) fuel cells, understanding the phase separated structures inside AEMs is essential, as well as those at the catalyst layer/membrane interfaces. The AEMs based on quaternized aromatic semi-block copolymers with different ion exchange capacities (IECs) were systematically investigated. With IECs of 1.23 and 1.95 mequiv g(-1), the water uptakes at room temperature were 37% and 98%, and the anion conductivities 23.6 and 71.4 mS cm(-1), respectively. The increases were not proportional to the IEC. Images obtained by transmission electron microscopy in vacuum were similar with both IEC values, but the development of a clear phase separation in humidified nitrogen was observed in the profiles only with 1.95 mequiv g(-1) obtained by small angle X-ray scattering. At the temperature of 40 degrees C and the relative humidity (RH) of 30%, the average currents observed at the tip apex by current-sensing atomic force microscopy were <0.5 and 10 pA with 1.23 and 1.95 mequiv g(-1), respectively, and those at 70% RH were 10 and 15 pA, respectively. The humidity gave a larger influence on the bulk structure with 1.95 mequiv g(-1), whereas a larger influence on the surface conductivity with 1.23 mequiv g-1. (C) 2017 Elsevier B.V. All rights reserved.