Electric response of anion-conducting membranes

The development of new ion-conducting membranes able to transport the desired charged species is one of the most important aspects of the research for red-ox flow battery and fuel cells. The complex interplay between the chemical composition, structure and conductivity mechanism of these innovative membranes is still poorly understood. In this work a family of ion-conducting membranes based on poly (vinyl benzyl trimethyl ammonium)-b-poly (methylbutylene) [PVBTMA-b-PMB] block copolymers is considered. The copolymers include p-tolyl side groups; the latter are further functionalized with trimethyl ammonium groups neutralized by OH anions. Three different OH-membranes are studied, labeled I, II and III. Each is characterized by a different degree of functionalization (DF) of the p-tolyl groups, as follows: I – DF = 42%; II – DF = 63%; III – DF = 81%. The electric response of the membranes in the completely hydrated form is studied by broadband electrical spectroscopy (BES). Results highlight a number of events, indicated as σ1, σ2, σ3 and σ4 in Figure 1, associated to different polarization phenomena occurring within the materials. This behaviour is typical of systems characterized by a complex multiphasic nanostructure, including more than one domain with different dielectric constants. The electric response also evidences some step changes as the temperature is varied, which point to significant reorganizations in the domain structure of the materials. These evidences are studied in detail, allowing to: (a) propose a comprehensive model for the long-range charge transfer in this family of materials; and (b) elucidate how the latter is correlated to the chemical composition and nanostructure of the samples.