Nanoneedle-Covered Pd-Ag Nanotubes: High Electrocatalytic Activity for Formic Acid Oxidation

Nanoneedle-covered palladium-silver nanotubes were synthesized through a galvanic displacement reaction with Ag nanorods at 100 degrees C (PdAg-100) and room temperature (PdAg-25). Transmission and scanning electron microscopic measurements displayed that the synthesized PdAg nanotubes exhibit a hollow structure with a nanoneedle-covered surface, providing the perfect large surface area for catalytic reactions. The PdAg nanotubes formed at 100 degrees C exhibit a more uniform surface morphology than those obtained at room temperature. The high-resolution TEM, energy-dispersive X-ray analysis, and powder X-ray diffraction measurements indicated that the surface of the nanotubes is decorated with crystalline Pd nanoparticles with Pd(111) planes, and meanwhile, Ag and AgCl particles are dispersed in the inner space of the nanotubes. The electrocatalytic activity of the synthesized PdAg nanotubes toward formic acid oxidation was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). With the same loading on a glassy carbon electrode, the PdAg-100 nanotubes show high catalytic activity and stability from the CV and chronoamperometric analyses, which may be ascribed to the annealing process of the nanotube surface structures at 100 degrees C. The reaction kinetics of the HCOOH oxidation on the PdAg nanotubes was then examined by EIS measurements. It was found that the impedance responses are strongly dependent on the electrode potentials. With the potential increasing, the reaction kinetics evolve from resistive to pseudoinductive and then to inductive behaviors. On the basis of the proposed equivalent circuits, the synthesized PdAg nanotubes exhibit a much lower (almost 3 orders of magnitude smaller) charge-transfer resistance (R-CT, a characteristic quantity for the rate of charge transfer for the electrooxidation of formic acid) than that obtained at the Pt-based nanoparticles reported previously. It was also found that the R-CT at the PdAg-100 nanorods is much smaller than that at the PdAg-25 nanorods, indicating the electron-transfer kinetics for formic acid oxidation at the PdAg-100 nanorods is much better facilitated. The present work highlights the application of the nanoneedle-covered PdAg nanotubes with high surface areas as anode electrocatalysts in fuel cells and the influence of surface structure on their catalytic activity.