In this study, the nanostructures of the carbon-free cathode catalyst layers using connected platinum-iron nanoparticle catalysts with porous hollow capsule structures are controlled by the ionomer morphologies (aggregated or not) formed by different treatments (ultrasonication or autoclaving) to the catalyst inks, and the relationship between the catalyst-layer nanostructures and the fuel-cell performances is investigated. Structural and electrochemical analyses of the capsule catalyst layers reveal that the nanosized ionomer morphologies on the capsules strongly influence the oxygen mass-transport resistance; the fully-swollen and thick ionomer layer would cause blocking of the interspaces between the capsules and slow oxygen-diffusion to the capsule surfaces. Thus, the uniform and thin ionomer coating formed by autoclaving produces a higher cell performance due to lower oxygen mass-transport resistance, compared with the nonuniform and partially thick coating formed by ultrasonication. Moreover, the thinner ionomer layer with a very small ionomer/capsule ratio reduces cell performance at high current densities when the humidity at the capsule catalyst layer increases, probably due to the increased oxygen mass-transport resistance through liquid water inside the catalyst layer. Therefore, in addition to the ionomer morphologies, water management in the capsule catalyst layers is important for further improving cell performance. (C) 2016 The Electrochemical Society. All rights reserved.