In polymer electrolyte fuel cells (PEFCs), it is important to secure proximate diffusion paths of reactants and electrons.  One approach is to optimize the boundary between polymer electrolyte and Pt nanoparticle surface.  Based on synchrotron X-ray absorption fine structure to monitor directly the status of catalysts in PEFCs, it was found that Pt sites were reduced to Pt0 by alcohols contained in polymer electrolyte dispersion solution during the preparation of cathode of PEFC.  As in membrane electrolyte assembly, only the Pt sites not covered by polymer electrolyte reoxidized to Pt2+/4+.  Thus, the interface between Pt and polymer electrolyte was evaluated.  The other approach is to functionalize carbon surface with sulfonate/sulfate group to conduct protons.  Similar level of proton conductivity was observed in current-voltage dependence compared to using polymer electrolyte, but polymer electrolyte was advantageous to lose less voltage for activation.  Based on this comparison, optimum catalyst on cathode is proposed comprising surface sulfonate/sulfate group on carbon mixed with polymer electrolyte.  Further optimization of cathode catalyst is proposed to functionalize carbon with sulfonate group linked to fluorocarbon branch.