Influence of morphology on the electrochemical properties of proton exchange membranes

This work investigated four different series of proton exchange membranes. The series consist of graft copolymers containing poly(styrenesulfonic acid) (PSSA) side chains attached onto polymer backbones of polystyrene (PS), polyacrylonitrile (PAN), poly(ethylenetetrafluoroethylene) (ETFE), and poly(vinylidenefluoride) (PVDF). Water sorption behavior, proton conductivity, and oxygen electrochemistry were studied in relation to the chemical structure, morphology, and ion exchange capacity.; PS-g-PSSA and PAN-g-PSSA were prepared by an emulsion copolymerization of styrene or acrylonitrile with poly(sodium styrene sulfonate) macromonomers. Stable free radical polymerization (SFRP) was used to prepare pseudo-living chains of poly(sodium styrene sulfonate). With this method, the length and the number density of ionic graft chains can be controlled and in this case, influenced the degree of phase separation in the membranes. Transmission electron microscopy (TEM) was used to observe the effect of variables such as: ionic content, polymer backbone, and graft chain length on the morphology of these membranes. It was found that as the ion content increased, the ionic domains coalesced to form a continuous network of nanochannels. This has a strong effect on the water content and conductive properties of the membranes.; ETFE-g-PSSA and PVDF-g-PSSA membranes were prepared by the radiation-grafting method. Styrene was copolymerized onto preformed ETFE and PVDF films and subsequently sulfonated. These membranes exhibit high water contents and high proton conductivities. Kinetics and mass transport parameters for the oxygen reduction reaction were determined using microelectrode techniques in a solid-state electrochemical cell at 100% relative humidity and over a range of temperatures. The oxygen diffusion coefficients and permeabilities were found to increase with increasing water content, while the oxygen solubility slightly decreased. For all the membranes studied, comparison of the oxygen mass transport properties shows that the water content has a marked influence on the diffusion properties, while the oxygen solubility is primarily dependent on the structure of the polymer backbone.