High-Performance Proton Exchange Membrane of Sulfonated Polystyrene-b-polyisobutylene-b-polystyrene

Sulfonated polystyrene-b-polyisobutylene-b-polystyrene copolymers (SSIBS) with various average molar percentages of sulfonic acid side groups based on all the structural units along SSIBS polymer chains (SP) were synthesized via sulfonation of SIBS with acetyl sulfate. A transmission electron microscopy (TEM) method was developed to directly observe the dark phase of –(SO3)2Pb ionic channels of sulfonic acid ion clusters by the ion exchange of H+ with Pb2+. The size of hydrophilic ionic channels in SSIBS enlarged with increasing SP and the desired interconnected ionic transport channels could be formed in SSIBS with SP of more than 6.9 mol %. Effects of copolymer composition and functionality on micromorphology and finally on electrochemical properties of the SSIBS membranes show that the SSIBS membranes with bicontinuous phase separation micromorphology exhibit high through-plane proton conductivity, low fuel permeability, and high selectivity. The optimized SSIBS-3160–13.4 membrane was selected for assembling the single direct methanol fuel cell (DMFC) and the DMFC exhibits high performances of open circuit voltage (OCV) of 560 mV, limiting current density (Jlim) of 650 mA·cm–2, and peak power density (Pmax) of 32.24 mW·cm–2. These fuel cell performances are similar to those (OCV = 510 mV, Jlim = 600 mA·cm–2, and Pmax = 35.56 mW·cm–2) by using the commercial Nafion 117 membrane under the same conditions. To the best of our knowledge, this is the first example of SSIBS-based DMFC exhibiting comparable performances with Nafion 117 under the same test conditions.