Synthesis of High Pd-Loaded Pd/C Catalyst Using Ultrasonic Dispersions for the Direct Formic Acid Fuel Cells

Abstract

In recent years, direct formic acid fuel cells, DFAFCs, have received considerable attention for the power sources of the small electronic devices due to their high power density. Considering to the easy handling of the liquid fuel, we have considered that DFAFC can be promising power sources for stationary or transportation as well as the PEFC. However, the current power density of DFAFC has still far from that of PEFC. Therefore, the improvement of the power density is required. Moreover, improvement of the stability of the Pd catalyst is also required since Pd catalyst is poisoned by CO or CO species. In order to achieve high power density and good stability, a numerous catalyst supports, which contribute to the dispersion of the Pd catalyst and inhibit the Pd poisoning due to the interaction between Od and the supports, have been presented. This approach is quite available to solve the disadvantage of the DFAFC, however, it causes the increase of thickness of the anode catalyst layer since the Pd is usually loaded to the support around 20 wt% or less. Thick catalyst layer caused a mass transport limitation of the anode which significantly affects a performance of the DFAFC, therefore, the high Pd-loaded supported catalyst is required. When high Pd-loaded catalyst is prepared, the Pd nanoparticles is easy to agglomerate on the support resulting in the decrease of the electrochemical surface area. In this study, in order to fabricate the high Pd-loaded supported catalyst with well dispersions, we focused on the ultrasonic dispersions of the precursor and the catalyst supports before the chemical reduction. PdCl2 and the Vulcan XC-72 were used as the precursor and the supports, and the effects of the intensity and the time of the ultrasonication on the formic acid oxidation reaction, FAOR, activity, the BET surface area, ECSA and the crystallite diameter measured by XRD were investigated by fabricating Pd/C contained 20 wt % of Pd. From these results, the higher FAOR is obtained when the ultrasonication was conducted at appropriate intensity and time comparing the conventional Pd/C fabricated without ultrasonication. Based on the optimum ultrasonication conditions, Pd/C catalyst contained 50 wt% and 80 wt% of Pd catalyst were prepared. It was found that the Pd/Cs fabricated with ultrasonic dispersions showed much superior FAOR activity comparing to the Pd/Cs fabricated without ultrasonication although they showed a slight decrease of the FAOR activity per unit weight of Pd comparing to that of 20wt% Pd/C fabricated with ultrasonication. Form these results, the ultrasonic dispersion is quite important and the effective for the fabrication of high Pd-laded Pd/C catalyst.