The presence of water in polymer membranes is considered essential for ion transport and in this respect the membrane must be sufficiently hydrated during operation. Water transport to, from, and through, the membrane involves a complex interplay of processes. In this aspect of the group's work, we perform water permeability measurements ex-situ and compare the data with water balance measurements obtained in-situ. We compare and study water transport data obtained when membranes are exposed to either liquid or vapour phases of water, and wherein a chemical potential gradient is developed across the membrane by either controlling the differential humidity in the case of water vapour or hydraulic pressure in the case of liquid water.
An example of this project appears in “Correlation of in-situ and ex-situ measurements of water permeation through Nafion NRE211 Proton Exchange Membranes”, J. Electrochem. Soc., 156 (2009) B782-B790. Water permeability at 70 °C is determined for Nafion NRE211 membrane exposed to either liquid or vapour phases of water. Chemical potential gradients of water across the membrane are controlled through use of differential humidity (38-100% RH) in the case of water vapour and hydraulic pressure (0-1.2 atm) in the case of liquid water. Accordingly, three types of water permeation are defined: vapour-vapour permeation (VVP), liquid-vapour permeation (LVP) and liquid-liquid permeation (LLP).
LLP, LVP and VVP fluxes for Nafion membranes versus wet membrane thickness. Temp., 70 . LLP, Delta P = 1.0 bar