Ruichun Jiang

Through-Plane Proton Transport Resistance of Membrane and Ohmic Resistance Distribution in Fuel Cells

Ohmic losses in fuel cells contain both ionic (mainly protonic) and electronic contributions. In this work, we developed a method to distinguish the resistance contributions from individual components. Proton transport resistance of proton exchange membranes (PEMs) in the through-plane direction, the major proton transport direction in in situ fuel cells, was measured using electrochemical impedance spectroscopy combined with compression-controlled fuel cell hardware. Membrane resistance was obtained by subtracting the nonmembrane contributions from the total resistance. Proton conductivities of PEMs with different equivalent weights, calculated from through-plane resistance measurements at various relative humidity conditions, were compared with good agreement to the in-plane measurements. Fuel cell electronic resistance and membrane-electrode interfacial resistance were also evaluated using both ex situ and in situ methods. The membrane-electrode interface resistance was nearly constant over a range of relative humidity conditions. The effects of test procedure and cell build strategy were investigated and had a significant impact on the membrane resistance measurements.

NMR studies of proton transport in fuel cell membranes at sub-freezing conditions

Water uptake activities and transport properties are critical for water management in fuel cell membranes. In this work, three perflourosulfonic acid (PFSA) fuel cell membranes, including Nafion®-117 and two Gore membranes, were evaluated at different relative humidity controlled conditions. These fuel cell membranes were studied using variable temperature 1H spin–lattice relaxation times (T1) and pulsed field gradient (PFG) NMR techniques in the temperature range of 298 to 239 K. Water self-diffusion coefficients and proton transport activation energies in the fuel cell membranes were obtained from the PFG-NMR experiments. The results show that the water self-diffusion coefficients increase with increasing hydration level, and decrease with decreasing temperature. The water molecular motion is significantly slowed at low temperatures; however, the water molecules in these membranes are not frozen, even at 239 K. The water uptake activity and diffusivity in these membranes were compared as a function of temperature and hydration level. At the same temperature and hydration level, the water self-diffusion coefficients of two Gore fuel cell membranes are higher than that of Nafion®-117. This is attributed to the lower EW of the Gore membranes. The presence of an expanded polytetrafluoroethylene (ePTFE) reinforcing layer in the membrane also has an impact on water diffusivity.

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.