Structural Factors for the Chemical Stability of Graft-Type Polymer Electrolyte Membranes Evaluated from the Local Hydration Number

This work presents a strategy to reveal the key structural factor of the local hydration number (λ_local) for improving the chemical stability of hydrocarbon-based polymer electrolyte membranes (PEMs), which is a major challenge for their application in energy conversion devices such as fuel cells and electrolyzers. λ_local is defined as the number of water molecules surrounding ion-conducting ionic groups in nanometer-scale ion channels, determined by partial scattering function analysis. In a series of radiation-grafted PEMs, consisting of poly(styrene sulfonic acid) grafted onto poly(ethylene-co-tetrafluoroethylene) (ETFE-g-PSSA) with ion exchange capacities (IECs) ranging from 0.8 to 2.5 mmol/g, we clarify that the ion-channel structures of the PEM, on a length scale of a few nanometers, undergo a morphological transition from spherical to bicontinuous structure at an approximate IEC of 1.7 mmol/g. Namely, the spherical and bicontinuous structures in these PEMs are the origin of low and high λ_local (∼6.5 and >10), leading to high and low chemical stabilities, respectively. Therefore, it is one of the inevitable strategies to suppress the hydration level to λ_local ∼ 6.5, for high chemical stability of these grafted PEMs, as well as high conductivity.