We report polymer electrolyte membrane fuel cells (PEMFCs) in which poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) copolymer was added to the existing sPEEK binder in cathode catalyst layers (CCLs). Compared to a control case with no such copolymer, the cell with the copolymer exhibits improved performance, particularly in the oxygen mass transport. The improved mass transport behavior is attributed to the copolymer that makes CCLs more hydrophobic and thus suppresses water flooding significantly. Contact angle measurements and various electrochemical characterizations consistently support the copolymer effect for the improved oxygen mass transport. In addition, the introduction of P(VdF-co-HFP) lowers the glass transition temperature of the binder, which contributes to enhancing the adhesion properties between the CCLs and membranes.