Andrzej Borodzinski

Direct Formic Acid Fuel Cell (DFAFC) is considered as a promising liquid fuel cell. Here, the effect of palladium particle size (d_Pd) of anodic DFAFC catalysts on initial power density was studied. The number of electrochemically available Pd atoms on the surface of Pd nanocrystallites in the anodes were determined experimentally. The cathode potential as a function of the current density has been measured. The mathematical model describing the effect of d_Pd on DFAFC performance has been developed. It was assumed that: (i) Pd crystallites have shape of cuboctahedron, (ii) Pd atoms lying on edges and corners are inactive, (iii) The activity of Pd atoms lying on crystallite (111) and (100) facets do not change with d_Pd. The mathematical model describes well the effect of current density and d_Pd on fuel cell voltage and power density. The model predicts that maximum power density is reached at d_Pd=2.4 nm.

In the present work, the magnetron sputtering technique was used to prepare new catalysts of formic acid electrooxidation based on TiO₂ nanotubes decorated with Pt (platinum), Pd (palladium) or Pd + Pt nanoparticles. TiO₂ nanotubes (TiO₂ NTs) with strictly defined geometry were produced by anodization of Ti foil and Ti mesh in a mixture of glycerol and water with ammonium fluoride electrolyte. The above mentioned catalytically active metal nanoparticles (NPs) were located mainly on the top of the TiO₂ NTs, forming ‘rings’ and agglomerates. A part of metal nanoparticles decorated also TiO₂ NTs walls, thus providing sufficient electronic conductivity for electron transportation between the metal nanoparticle rings and Ti current collector. The electrocatalytic activity of the TiO₂ NTs/Ti foil, decorated by Pt, Pd and/or Pd + Pt NPs was investigated by cyclic voltammetry (CV) and new Pd/TiO₂ NTs/Ti mesh catalyst was additionally tested in a direct formic acid fuel cell (DFAFC). The results so obtained were compared with commercial catalyst—Pd/Vulcan. CV tests have shown for carbon supported catalysts, that the activity of TiO₂ NTs decorated with Pd was considerably higher than that one decorated with Pt. Moreover, for TiO₂ NTs supported Pd catalyst specific activity (per mg of metal) was higher than that for well dispersed carbon supported commercial catalyst. The tests at DFAFC have revealed also that the maximum of specific power for 0.2 Pd/TiO₂ catalyst was 70% higher than that of the commercial one, Pd/Vulcan. Morphological features, and/or peculiarities, as well as surface composition of the resulting catalysts have been studied by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and chemical surface analytical methods (X-ray photoelectron spectroscopy—XPS; Auger electron spectroscopy—AES).