Water Uptake and Proton Conductivity in Porous Block Copolymer Electrolyte Membranes

Abstract

We demonstrate that the water uptake and conductivity of proton-conducting block copolymer electrolyte membranes can be controlled systematically by the introduction of pores in the conducting domains. We start with a membrane comprising a mixture of homopolymer polystyrene (hPS) and a polystyrene-<i>b</i>-polyethylene-<i>b</i>-polystyrene (SES) copolymer. Rinsing the membranes in tetrahydrofuran and methanol results in the dissolution of hPS, leaving behind a porous membrane. The polystyrene domains in the porous SES membranes are then sulfonated to give a porous membrane with hydrophilic and hydrophobic domains. The porosity is controlled by controlling ϕ_v, the volume fraction of hPS in the blended membrane. The morphology of the membranes before and after sulfonation was studied by scanning transmission electron microscopy (STEM), electron tomography, and resonance soft X-ray scattering (RSoXS). The porous structures before and after sulfonation are qualitatively different. Water uptake of the sulfonated membranes increased with increasing ϕ_v. Proton conductivity is a nonmonotonic function of ϕ_v with a maximum at ϕ_v = 0.1. The introduction of microscopic pores in the conducting domain provides an additional handle for tuning water uptake and ion transport in proton-conducting membranes