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In the present work, the magnetron sputtering technique was used to prepare new catalysts of formic acid electrooxidation based on TiO 2 nanotubes decorated with Pt (platinum), Pd (palladium) or Pd + Pt nanoparticles. TiO 2 nanotubes (TiO 2 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 TiO 2 NTs, forming 'rings' and agglomerates. A part of metal nanoparticles decorated also TiO 2 NTs walls, thus providing sufficient electronic conductivity for electron transportation between the metal nanoparticle rings and Ti current collector. The electrocatalytic activity of the TiO 2 NTs/Ti foil, decorated by Pt, Pd and/or Pd + Pt NPs was investigated by cyclic voltammetry (CV) and new Pd/TiO 2 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 TiO 2 NTs decorated with Pd was considerably higher than that one decorated with Pt. Moreover, for TiO 2 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/TiO 2 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)., Competing Interests: The authors declare no conflict of interest.

Characterizing materials at small scales presents major challenges in the engineering of nanocomposite materials having a high specific surface area. Here, we show the application of electron tomography to describe the three-dimensional structure of highly ordered TiO 2 nanotube arrays modified with Pt nanoparticles. The titanium oxide nanotubes were prepared by the electrochemical anodization of a Ti substrate after which Pt was deposited by magnetron sputtering. Such a composite shows high electrochemical activity that depends on the amount of the metal and the morphological parameters of the microstructure. However, a TiO 2 structure modified with metallic nanoparticles has never been visualized in 3D, making it very difficult to understand the relationship between electrocatalytic activity and morphology. In this paper, TiO 2 nanotubes of different sizes and different amounts of Pt were analyzed using the electron microscopy technique. Electrocatalytic activity was studied using the cyclic voltammetry (CV) method. For selected samples, electron tomography 3D structure reconstruction was performed to describe their fine microstructure. The highest activity was detected in the sample having bigger nanotubes (25 V) where the porosity of the structure was high and the Pt content was 0.1 mg cm -2 . 3D imaging using electron tomography opens up new possibilities in the design of electrocatalytic materials.