Facile and rapid synthesis of spherical porous palladium nanostructures with high catalytic activity for formic acid electro-oxidation.

Highly uniform, spherical porous palladium nanostructures (SPPNs) with rough surfaces were prepared by a facile and rapid ultrasound assisted reduction. The synthesis involves sonicating a solution of K(2)PdCl(4) and ascorbic acid for only 7 min at 40 °C without any additives. The products are isolated structures with a narrow size distribution, and their average diameters are controllable in a range from 40 to 100 nm via the K(2)PdCl(4) concentration. Typical products have a diameter of 52 nm and consist of loosely packed grains of 2-3 nm. They are thus very porous, with a specific surface area of 47 m(2) g(-1). The growth mechanism of SPPNs is discussed on the basis of varying relevant reaction parameters and characterizations from different microscopy techniques, nitrogen absorption analysis, and time-dependent UV-vis spectra. The electrocatalytic performance of the SPPNs was evaluated by electro-oxidation of formic acid. The mass current density per mass of SPPNs (1.88 A mg(-1)) exceeds that of commercial Pd black (1.69 A mg(-1)) and is more than twice that of commercial Pd/C catalyst (0.79 A mg(-1)). Long-term stability of the activity makes this material a promising anode catalyst for direct formic acid fuel cells.