Fuel cell cathode catalyst layers from “green” catalyst inks

Abstract

Fuel cell cathode catalyst layers deposited from a water-based catalyst ink formulation, using high water content and minimum volatile organic compounds, are investigated. Cathodes fabricated from a dispersion medium containing 96 wt% water are compared with cathodes fabricated from conventional alcohol-based inks containing 1-propanol–water 3 : 1 (w/w). The morphology of the two catalyst layers are similar, as are electrochemically-active surface areas at relative humidities of 100, 70 and 30% RH. Oxygen reduction kinetics obtained under fully humidified H₂/O₂ conditions exhibit similar Tafel slopes, 67 ± 3 mV per dec. However, cathodes prepared from water-based inks exhibit a lower H₂/air fuel cell performance under 100, 70 and 30% RH while its porosity, obtained using mercury porosimetry, is slightly higher. EIS measurements obtained under high current density indicate that the mass transport resistance of the water-based catalyst layer is lower, which is consistent with porosimetric data, and suggests that factors other than mass transport limit the performance of the water-based cathode. The protonic resistance of the catalyst layers was found to be 105 and 145 mΩ cm² for the propanol- and water-based catalyst layers, respectively. The differences are more pronounced when RH is decreased from 100 to 30%. This trend is consistent with the observed decrease in fuel cell performance under conditions of lower RH, and indicates that the higher proton resistance of the water-based catalyst layer is the cause of its lower fuel cell performance.