This work focuses on the effect of Nation ionomer aggregation within the Pd catalytic electrode on electro-catalytic oxidation of formic acid. By a simple heat-treatment, the particle sizes of both Nafion ionomers in Nation solution and congeries formed between Pd nanoparticles and Nation ionomers in the catalyst ink decrease and their size distribution becomes narrow. Heat treatment of the catalyst ink leads to a significantly enhanced catalytic activity for formic acid oxidation on the Pd catalytic electrode. Such an enhancement is ascribed to the improvement in catalyst utilization verified by CO stripping voltammo-grams and to the decrease in charge-transfer resistance of oxidation reaction confirmed by impedance analysis. Typical XPS analysis shows that there are at least two kinds of Pd and S surface states within the catalytic electrode with the ink pre-heated at 25 degrees C and only one kind of Pd and S surface state at 80 degrees C, indicative of a better dispersion between Pd nanoparticles and smaller Nation ionomers at a higher heat treatment temperature. Furthermore, the decrease in congeries size within the anode catalyst ink leads to a significant decrease in Nation loading within the catalytic layer and a remarkable improvement in direct formic acid fuel cell's performance. (C) 2010 Elsevier Ltd. All rights reserved.