Ion-conducting polymers are ideal solid electrolytes for most energy storage and conversion devices where ion transport is a critical functionality. The system performance and stability are related to the transport and mechanical properties of the ionomers, which are correlated through physiochemical interactions and morphology. Thus, there exists a balance between the chemical and mechanical energies which controls the structure−function relationship of the ionomer. In this paper, it is reported how and why thermal treatments result in different water uptakes and nanostructures for a perfluorinated sulfonic acid (PFSA) membrane. The nanostructure of the PFSA membrane is characterized using small-and wide-angle Xray scattering experiments. These changes are correlated with water content and mechanical properties and result in fundamental relationships to characterize the membrane with different thermal histories. Moreover, quasi-equilibrium water uptake and domain spacing both decrease with predrying or preconstraining the membrane, thereby suggesting that similar mechanical energies govern the structural changes via internal and external constraints, respectively. The findings suggest that heat treatments alter the balance between the chemical−mechanical energies where the interplay of the morphology and mechanical properties controls the structure−function relationship of the membrane. Finally, a model is developed using an energy-balance approach with inputs of the mechanical and structural properties; the dependence of water uptake on pretreatment is successfully predicted.

The objective of this effort was to correlate the local surface ionic conductance of a Nafion ® 212 proton-exchange membrane with its bulk and interfacial transport properties as a function of water content. Both macroscopic and microscopic proton conductivities were investigated at different relative humidity levels, using electrochemical impedance spectroscopy and currentsensing atomic force microscopy (CSAFM). We were able to identify small ion-conducting domains that grew with humidity at the surface of the membrane. Numerical analysis of the surface ionic conductance images recorded at various relative humidity levels helped determine the fractional area of ion-conducting active sites. A simple square-root relationship between the fractional conducting area and observed interfacial mass-transport resistance was established. Furthermore, the relationship between the bulk ionic conductivity and surface ionic conductance pattern of the Nafion ® membrane was examined.

Catalyst layers (CLs) in proton-exchange-membrane fuel cells (PEMFCs) facilitate electrochemical reactions and therefore play a critical role in cell performance. Absorption and desorption of water into both the CL ionomer and the CL pore structure are integral aspects of PEMFC water management and performance. In this work, the water uptake from both the vapor and liquid phases is examined experimentally. Specifically, the dynamic water-uptake behavior of the CL ionomer is investigated as a function of relative humidity, temperature, Pt-loading and pretreatment. The water content of the ionomer in the CL, even after pretreatment, is found to be significantly lower than that for the bulk ionomer membrane, yet with similar sorption time constants. Thus, there is probably substantially slower transport into the ionomer which is likely due to its interfacial character. From the liquid phase, measured capillary pressure -saturation relationships show that the CL has an appreciably hydrophilicity that is strongly dependent on the existence of cracks. These findings are critical to the understanding and optimization of water management and transport phenomena within PEMFCs.

Transition metal porphyrin complexes have been mounted in a three dimensional homogenous distribution inside the ionomer of catalyst layers in MEAs to achieve competitive fuel cell catalysis activity. The effect of electrode components including ionomer, carbon, catalyst, and mediator, and ionomer film thickness, is investigated in fuel cell molecular catalysis system. Membrane electrode assembly (MEA) durability testing has been conducted. SEM and TEM techniques are employed to investigate molecular catalysis electrode micro- and nano- structure and morphology. To date, surprisingly, the best fuel cell performance, i.e. 1280 mA/cm2 of maximum/short-circuit current density is achieved, approaching that of Pt-based electrode, indicating higher turnover frequencies than Pt although with poorer voltages.