Influence of binder properties on kinetic and transport processes in polymer electrolyte fuelcellelectrodes

Abstract

The objectives of this study are to estimate the contributions of kinetic, ohmic and mass transport overpotentials to the overall voltage loss in polymer electrolyte membrane fuel cell (PEMFC) electrodes and to relate these overpotentials to electrode binder properties such as ionic conductivity, ion exchange capacity (IEC) and O₂ permeability. The model electrode binders studied were 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). 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. The electrodes prepared were characterized by cyclic voltammetry to estimate electrochemically active surface area (ECA) of platinum. Steady state polarization (V–I) experiments were performed with hydrogen as fuel and oxidants including O₂, 21% O₂/N₂ (air), 21% O₂/He (Helox) and 4% O₂/N₂. The V–I data obtained was analyzed to determine the relative contributions of the different sources of polarization in the electrode. Electrodes prepared with PFSA binders had similar ECAs of 28 m² 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. Based on this analysis, it is suggested that the SPEEK and SPSU bound electrodes suffered from binder phase diffusion limitations in addition to gas phase diffusion limitation, while the PFSA bound electrodes predominantly exhibited gas-phase diffusion limitations.