Design of Cluster-free Polymer Electrolyte Membranes and Implications on Proton Conductivity

Nanoscale ionic aggregates are ubiquitous in copolymers containing charged and uncharged monomers. In most cases, these clusters persist when these polymers are hydrated and ion-conducting channels percolate through the sample. We argue that these clusters impede ion motion due to (1) the requirement that ions must hop across ion-free regions in the channels as they are transported from one cluster to the next, and (2) increased counterion condensation due to proximity of fixed acid groups in the clusters. Block copolymers wherein the size of the ion-containing microphase is 6 nm or less provides one approach for eliminating the clusters.