Membranes currently used for proton transport in fuel cells are nonporous (e.g. Nafion). In this presentation, we show that the introduction of pores into block copolymer electrolyte membranes provides fine control over water uptake and conductivity. We start with a membrane comprising a mixture of homopolymer polystyrene (hPS) and a polystyrene-b-polyethylene-b-polystyrene (SES) copolymer. Porous membranes are fabricated by rinsing the hPS/SES mixture membranes in tetrahydrofuran and methanol. 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 ϕv, the volume fraction of hPS in the blended membrane. The morphology of the membranes 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 membranes increases with increasing ϕv. Proton conductivity is a non-monotonic function of the water content of the membranes.
Using this novel method, we were able to control the water uptake of block copolymer electrolyte membranes without changing the chemistry or chain architectures. We believe that the novel processing method described above has created microscale pores in the block copolymer electrolyte membranes which allows us to tune the water uptake of these membranes to optimize proton conductivity.
See more of this Group/Topical: Materials Engineering and Sciences Division