Influence of Equivalent Weight of Ionomer on Local Oxygen Transport Resistance in Cathode Catalyst Layers

Local oxygen transport resistances around the surface of Pt particles were examined by analyzing varying equivalent weight (EW) of ionomer in cathode catalyst layers (CCLs) of polymer electrolyte fuel cells. Traditional limiting current method was performed to estimate total oxygen transport resistances in CCLs. The local oxygen transport resistances were determined analytically from the total oxygen transport resistances in CCLs with different Pt loadings. Analysis applying a macro homogeneous model in CCLs revealed that lowering ionomer EW in CCLs caused significant increase in not only the oxygen transport resistance in thickness direction of a CCL (rO(2,macro)), but also the local oxygen transport resistance (RO2,local). Additionally, the obtained oxygen transport resistances were reexamined by simple dissolution/diffusion models considering the results of measurement of water sorption isotherm. Results revealed rO(2,macro) were within the expected range. However, experimental RO2,local values were much higher than values calculated by use of both the oxygen transport property of Nafion membrane and bulk PTFE/water approximated model. This indicates that unconsidered parameters related to interfacial phenomena of ionomer such as the coverage of absorbed water on the surface, oxygen dissolution rate in ionomer or blockage of Pt surface by sulfonic acid group should be employed. (C) 2013 The Electrochemical Society. All rights reserved.