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Cobalt thin films composed of a large number of nanopetals were fabricated on the glassy carbon (GC) substrate by cyclic voltammetric deposition of Co 2+ ions on a glass carbon electrode (GCE). The hierarchical Co nanostructures were further used as the sacrificial template to acquire Pd (or Pt) thin film electrocatalysts with hierarchical architectures through the galvanic replacement reaction between Co nanopetals and chloropalladite (or tetrachloroplatinate). The as-prepared Pd (or Pt) thin films contain quantities of nanoparticles and many hollow Pd aggregates in the range of submicrometer to micrometer scale. The hollow Pd aggregates were found to burst in acidic solutions at potentials more negative than the hydrogen evolution potential since Pd absorbed too much hydrogen. As an electrocatalyst for the formic acid oxidation, the Pd thin films presented much higher catalytic activity than the Pt thin films with a similar architecture. An important reason is that the formic acid oxidation at the Pd nanostructures proceeds via a non-CO reaction pathway, while the reaction at the Pt nanostructures involves formation of CO-adsorbed species, which has been confirmed by the CO stripping voltammetric curves on Pd or Pt thin films. The as-prepared Pd thin films with hierarchical architectures are expected to be a promising electrocatalyst in direct formic acid fuel cells (DFAFCs).

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