Palladium/Nafion composite membranes were synthesized by supercritical impregnation method to reduce methanol crossover in direct methanol fuel cells. The palladium complexes used in this study were palladium(II) acetylacetonate, palladium(II) hexafluoroacetylacetonate, and palladium (II) bis(2,2,6,6-tetramethyl-3,5-heptane-dionato). The palladium complexes with various loading amounts from 0.010 to 0.050 g in a high-pressure vessel were dissolved in supercritical CO2, and impregnated into Nafion membranes. The SEM images indicated that the palladium complexes were successfully deposited into Nafion membrane, and there were no problems such as cracking and pinhole. The EDX analysis showed that the palladium particles were distributed both at the membrane surface and also extended deeper into the membrane. The TEM images indicated that thin dense band of agglomerated Pd particles can be observed near the membrane surface, and a significant number of isolated Pd particles can be seen deeper into the membrane, when Pd(II) acetylacetonate was used as palladium complex. When palladium(II) hexafluoroacetylacetonate and palladium (II) bis(2,2,6,6-tetramethyl-3,5-heptane-dionato) were used, dense band of agglomerated Pd particles cannot be observed near the membrane surface, and small Pd particles were observed inside the membranes. The XRD analysis indicated that the crystalline peak of Nafion membrane at 2 theta=170 degrees increased with the supercritical CO2 treatment. It means that the degree of crystallinity for Nafion membrane increased by supercritical CO2. The metal Pd peak at 2 theta=4 degrees was observed for the Pd/Nafion membranes. The methanol crossover was reduced and the DMFC performance was improved for the Pd/Nafion membranes compared with Nafion membrane at 40 degrees C. The successful preparation of Pd/Nafion membranes by supercritical CO2 demonstrated an effective alternative way for modifying membranes and for depositing electrode catalytic nanoparticles onto electrolyte. (C) 2014 Elsevier B.V. All rights reserved.