A PEM fuel cell with the Nafion ionomer phase of the cathode catalyst layer (CL) that was exposed to hot dry gas during the hot-pressing process showed improved performance over the whole current density range and $ 220% peak power increase with humidified air at 80 C. This enhanced performance is attributed to the modified structure of the perfluorosulfonic acid (PFSA) ionomer layer in the CL during the MEA's hot-pressing process. The dry gas exposure above the glass transition temperature (T g ) results in the aggregation of the ionic groups to retain the residue water molecules. This process separates the ionomer into ionic-group-rich domains and ionic-group-sparse domains. The ionic-group-sparse domains create hydrophobic interface and reactant transport channels with lower water content and thus higher oxygen solubility in the ionomer. Accordingly, the water-unsaturated ionomer and its surface hydrophobicity enhance the kinetic-controlled and concentration-polarized regions of the fuel cell polarization curve, respectively. The surface hydrophobicity of the ionomer layer is analyzed by the contact angle measurement and XPS. The durability of the hydrophobic effect below T g is demonstrated by boiling the treated material. Re-treating the hydrophobic sample with humidified gas exposure above T g eventually exhibits hydrophilic features, further proving the manipulability of the ionic group distribution.nanostructure of PFSA, PEM fuel cell, PFSA polymer, water flooding of cathode | INTRODUCTIONH 2 -Air PEM (Proton Exchange Membrane) fuel cells offer high-energy density and conversion efficiency with the added environmental benefits of zero carbon emission and water as a benign final product. 1 Water management in these PEM fuel cells plays a crucial role in their performance and durability. [2][3][4] The water in the cathode catalyst layer (CL) comes from the oxygen reduction reaction (ORR) and the net effect of the electro-osmotic drag of water from the anode to the cathode. The back-transport of water from the cathode to the anode is driven by both the water concentration gradient and capillary pressure gradient. The water flooding in the cathode CL obstructs the oxygen transport in two directions, along (1) the direction of the gas pores from the gas diffusion layer (GDL) to the CL and (2) the direction of the gas pores to the reactive sites in the CL. 5,6 The two directions of oxygen transport are described in Figure 1A, B.