Effect of cathode binder IEC on kinetic and transport losses in all-SPEEK MEAs

The effect of ion exchange capacity (IEC) and loading of sulfonated polyether ether ketone (SPEEK) binder on PEFC cathode performance was studied. MEAs were prepared by decal transfer onto a SPEEK membrane (IEC-1.75 mequiv./g). The IEC of SPEEK binder in the MEA cathode was varied between 1.3 and 2.1 mequiv./g. Cathodes prepared with 30 wt.% SPEEK loading had an electrochemically active surface area (ECA) that was 25% lower than a Nafion^® bonded electrode with similar loading. Polarization curves were obtained at 80 °C and 75% RH with hydrogen as fuel and air and oxygen (O₂) as oxidants. Polarization data was analyzed to determine the relative contributions of different sources of polarization, namely membrane ohmic losses, electrode ohmic losses, and mass transport losses in the gas diffusion layer, binder film and electrodes. The electrode ohmic and mass transport losses decreased with increase in SPEEK IEC. However, even for the highest SPEEK IEC, these losses were higher than those obtained in a Nafion^® bonded electrode. This was attributed to the lower proton activity and O₂ permeability in SPEEK. The loading of SPEEK in the electrode was found to influence performance in the activation controlled region, with a loading of 7.5 wt.% giving the highest performance. However, gains in this region were negated at higher current densities due to enhanced ohmic and transport losses and MEAs with all binder loadings between 7.5 and 30 wt.% had similar limiting currents.