Kinetic and transport processes in polymer electrolyte fuel cells

The objectives of this dissertation were (a) to correlate fundamental polymer properties to measurable kinetic, ohmic and transport losses in a fuel cell electrode and (b) to study the effect of inorganic additives on polymer properties such as proton conductivity and durability. In the first part of the study, SPEEK composite membranes with additives (1) silica and (2) silica functionalized with sulfonic acid (-SO3H) were tested to determine the proton conductivity and fuel cell performance. At 80°C and 75% RH the measured conductivity was 0.05 S/cm for SPEEK (IEC-1.75 meq/g) containing 10% sulfonic acid functionalized silica and 0.02 S/cm for the pristine SPEEK membrane.;In the second part of the study, PFSA, SPEEK and SPSU were used as model electrode binders to study the influence of IEC, O2 permeability and loading on the kinetic, ohmic and transport overpotentials of O2 reduction. The focus was on the fuel cell cathode as it contributes to a major portion of the overall cell overpotential in a catalyst-coated membrane. IEC of the electrode binder governs proton conductivity and reactant permeability while the binder loading influences ohmic losses, transport flux and Pt utilization. The ohmic and transport losses in the SPEEK based electrodes were always higher than PFSA bound electrodes because of the lower proton activity and O2 permeability in SPEEK compared to PFSA, even at the highest IEC.;On the lines of this study, additional electrode binders such as perfluorosulfonic acid ionomers (PFSA; of IECs 1.35 meq.g-1 and 0.95 meq.g -1), sulfonated poly ether ether ketone (SPEEK; of IECs 1.35, 1.75 and 2.1 meq.g-1) and sulfonated poly sulfone (SPSU; of IEC 1.5 meq.g-1) were studied. The objective here was to relate the measured O2 permeability to O2 transport losses in electrodes. The O2 permeability of these binders varied from 0.15·10-12 mol.cm-1.s -1 for SPSU to 6·10-12 mol.cm -1.s-1 for PFSA IEC 0.95 meq.g-1 at 80°C and 75% RH.;Electrodes prepared with PFSA binders had similar ECAs of 28m 2.g-1-Pt, while those prepared using hydrocarbon binders had an ECA of 10 to 14 m2.g-1-Pt at 80°C and 75%RH. The same trend was seen in mass activity. At optimized binder loadings, a semi-quantitative relationship was obtained relating binder O2 permeability to the mass transport losses within the electrode. Furthermore, a novel semi-quantitative method of plotting helox-air voltage gain against O2-air gain was employed to probe the O2 transport limitations in the electrodes.;Binder properties such as IEC, O2 permeability, proton conductivity and density were observed to be crucial in the choice of binders for PEMFC electrodes.