Timothy J. Fuller

Investigation of Low-Temperature Proton Transport in Nafion Using Direct Current Conductivity and Differential Scanning Calorimetry

The proton conductance of Nafion 117 was measured as a function of water content and temperature and compared to changes in the phase state of water. Conductance was measured using a direct current four-point probe technique, while the water phase was determined from differential scanning calorimetry of the melting transitions. Arrhenius plots of conductance show a crossover in the activation energy for proton transport for temperatures coinciding with the melting and freezing of water. This crossover temperature depends on the membranes water content per acid group, λ, and displays hysteresis between heating and cooling. Using calorimetry to estimate the fraction of the frozen water phase, both the crossover temperature and the hysteresis are found to correlate with the phase state of the water. For membranes starting with water contents above λ ∼ 8, the calorimetry and conductivity curves merge at low temperature, suggesting the formation of a common acid hydrate with similar network connectivity; for lower starting water contents, the low-temperature conductivity drops rapidly with λ. Based on Poisson-Boltzmann models, differences between the conductivity and calorimetry are attributed to gradients in the proton concentration that result in a proton-depleted core in the hydrated pores, which freezes first and contributes minimally to conductivity.

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Membranes: Design and Characterization

A series of ex situ and in situ diagnostic tests have been developed to quantitatively screen proton-exchange membranes (PEMs) for automotive fuel cell applications with respect to performance and mechanical and chemical durability. A comparison of the measured lifetimes of perfluorosulfonic acid (PFSA) and sulfonated aromatic hydrocarbon membranes under accelerated test conditions reveals the inherent differences between the two membrane chemistries. Upon subjecting membranes to deep hydration–dehydration cycles, the mechanical durability of PFSA membranes is more robust compared to that of aromatic hydrocarbon membranes, which have higher modulus and lower elasticity. By contrast, under in situ conditions promoting chemical degradation, aromatic hydrocarbon membranes can display improved stability. The next generation of alternative PEMs receiving a lot of attention are low-cost, sulfonated hydrocarbon polymers having controlled molecular architectures. Improved aromatic hydrocarbon PEM performance under conditions of low relative humidity can be facilitated by mimicking the positive attributes of PFSA membranes. New design tools allow for the optimization of nanophase separation of structurally reinforced hydrophobic domains and concentrated hydrophilic domains, thereby improving the performance of aromatic hydrocarbon membranes. A design guideline for polymer scientists is presented outlining the methodology to develop new PEMs for automotive fuel cell applications, including new metrics such as the membrane humidity stability factor and the hydrophilic volume ion-exchange capacity.