Keith M. Beers

Effect of Morphology of Nanoscale Hydrated Channels on Proton Conductivity in Block Copolymer Electrolyte Membranes

Hydrated membranes with cocontinuous hydrophilic and hydrophobic phases are needed to transport protons in hydrogen fuel cells. Herein we study the water uptake and proton conductivity of a model fuel cell membrane comprising a triblock copolymer, polystyrenesulfonate-block-polyethylene-block-polystyrenesulfonate (S-SES), as a function of water activity in both humid air and liquid water. We demonstrate that the water uptake and proton conductivity of S-SES membranes equilibrated in liquid water are fundamentally different from values obtained when they were equilibrated in humid air. The morphological underpinnings of our observations were determined by synchrotron small-angle X-ray scattering and cryogenic scanning transmission electron microscopy. A discontinuous increase in conductivity when nearly saturated humid air is replaced with liquid water coincides with the emergence of heterogeneity in the hydrated channels: a water-rich layer is sandwiched between two polymer-rich brushes. While the possibility of obtaining heterogeneous hydrated channels in polymer electrolyte membranes has been discussed extensively, to our knowledge, this is the first time that direct evidence for the formation of water-rich subdomains is presented.

Design of a humidity controlled sample stage for simultaneous conductivity and synchrotron X-ray scattering measurements.

We report on the design and operation of a novel sample stage, used to simultaneously measure X-ray scattering profiles and conductivity of a polymer electrolyte membrane (PEM) surrounded by humid air as a function of temperature and relative humidity. We present data obtained at the Advanced Light Source and Stanford Synchrotron Radiation Laboratory. We demonstrate precise humidity control and accurate determination of morphology and conductivity over a wide range of temperatures. The sample stage is used to study structure-property relationships of a semi-crystalline block copolymer PEM, sulfonated polystyrene-block-polyethylene.

Conductivity and water uptake in block copolymers containing protonated polystyrene sulfonate and their imidazolium salts

There is considerable interest in the properties of polymer electrolyte membranes due to their presence in fuel cells. In this paper, we report on the ionic conductivity and degree of hydration, λ, of model membranes composed of polystyrene sulfonate-b-polymethylbutylene (PSS–PMB) copolymers and their imidazolium salts (PSI–PMB). The membranes were in intimate contact with humid air, and their properties were studied as a function of temperature and relative humidity of the air (RH = 50 and 98%). All of the samples have a lamellar structure in the dry state, and λ = 14 ± 2 for PSS–PMB and PSI–PMB at RH = 98%. However, the conductivity behaviors of PSS–PMB and PSI–PMB are very different. The normalized conductivity, σn (the normalization accounts for small differences in the ion concentrations in the different samples), of PSS–PMB is highly history-dependent and equilibrated behavior is only seen when the samples are annealed at high temperature (80 °C) for long times (about 24 h). In contrast, the equilibrated behavior is obtained rapidly in PSI–PMB samples over the entire temperature window (25–90 °C). At RH = 98%, the equilibrated conductivities of the PSI–PMB samples at RH = 98% were independent of sample molecular weight and within the experimental error of that obtained from the high molecular weight PSS–PMB sample. The low molecular weight PSS–PMB sample exhibited higher conductivity than the three samples described above. At RH = 50% both PSS–PMB and PSI–PMB samples were relatively dry with λ < 5 over the accessible temperature window. In the dry state (1) PSS–PMB samples exhibited slow kinetics while PSI–PMB samples equilibrated rapidly, (2) molecular weight had no effect on conductivity in both PSS–PMB and PSI–PMB samples, and (3) the conductivities of PSI–PMB were significantly lower than those of PSS–PMB.

Absence of Schroeder's Paradox in a Nanostructured Block Copolymer Electrolyte Membrane

This is a study of morphology, water uptake, and proton conductivity of a sulfonated polystyrene-block-polyethylene (PSS-PE) copolymer equilibrated in humid air with controlled relative humidity (RH), and in liquid water. Extrapolation of the domain size, water uptake, and conductivity obtained in humid air to RH = 100% allowed for an accurate comparison between the properties of PSS-PE hydrated in saturated vapor and in liquid water. We demonstrate that extrapolations of domain size and water uptake on samples equilibrated in humid air are consistent with measurements on samples equilibrated in liquid water. Small (5%) differences in proton conductivity were found in samples equilibrated in humid air and liquid water. We argue that differences in transport coefficients in disordered heterogeneous systems, particularly small differences, present no paradox whatsoever. Schroeders Paradox, wherein properties of polymers measured in saturated water vapor are different from those obtained in liquid water, is thus not observed in the PSS-PE sample.