Using neutron methods SANS and PGAA to study evolution of structure and composition of Alkali-doped Polybenzimidazole membranes

Potassium hydroxide (KOH) doped polybenzimidazole (PBI) membranes are investigated as compelling candidates for water electrolysis applications, drastically reducing the ohmic losses in contrast to thick ZrO2 based diaphragms. Using small angle neutron scattering (SANS) we have found that the structure of the (KOH doped) PBI changes with doping time on a minute time scale, and that the development of the structure is highly dependent on the KOH concentration. This data is correlated with macroscopic measurements of membrane swelling resulting from the doping process which also occurs on a minute time scale. Then, using prompt gamma activation analysis (PGAA) to follow the changes in time of the chemical composition, we have found that the K concentration of these samples only increases slightly with doping times after a very rapid initial uptake, reaching a saturation value that is relatively independent of KOH concentration for long doping times of up to 24 h. However measurements of similarly doped samples show increases in ion-conductivity of nearly 3 fold, and resistivity reductions of over 2 fold on the same time scales. These measurements prove that PGAA is a sensitive method to follow changes in the chemical compositions during doping, while SANS can give information on the sub-micro structural changes of polymer electrolyte membranes. Since these methods can be correlated with ex-situ measurements of composition, resistance, ion-conductivity and macro-structure, the combined use of PGAA and SANS provides a promising means for in-operando study in order to elucidate changes in membrane performance due to electrochemical cycling, as well as to help characterize and optimize doping parameters though in-situ doping measurements, by enabling real-time study of such membrane systems

Effect of Catalyst Layer Ionomer Content on Performance of Intermediate Temperature Proton Exchange Membrane Fuel Cells (IT-PEMFCs) under Reduced Humidity Conditions

For intermediate-temperature polymer electrolyte membrane fuel cells (IT-PEMFCs), the effects of the cathode gas flow rate and the ionomer content are experimentally examined at 120 °C under conditions of low relative humidity (RH). The IT-PEMFC operation at low RH should be beneficial in developing compact systems with smaller humidifiers. First, analysis of the effect of gas flow rate at various current densities confirms that drying of the membrane electrode assembly (MEA) is an important factor in IT-PEMFC operation, whereas cathode flooding becomes significant in regions of high current density with low flow rates. Then, MEAs with various contents of Aquivion™ ionomer are fabricated, and the combined effect of drying and flooding is further investigated by IT-PEMFC tests at various RH conditions and current densities. The optimum ionomer content increases with decreasing current density at 20% RH or below, indicating that MEA drying becomes dominant over cathode flooding. © 2016 Elsevier Lt6