TiO2 Nanotubes with Pt and Pd Nanoparticles as Catalysts for Electro-Oxidation of Formic Acid

Abstract

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).

The Use of Anodic Oxides in Practical and Sustainable Devices for Energy Conversion and Storage

“…NH 4 F and H 2 O dissolved in ethylene glycol (EG) have been the preferred formulae for electrolyte composition [ 52 , 53 , 54 , 55 , 56 , 57 ]. Other studies substituted EG with glycerol and HF by NaF [ 11 , 58 ]. Aqueous solutions, such as CH 3 COOH + HF [ 51 ] and H 3 PO 4 + NH 4 F [ 59 ], were also reported.…”

“…Most of these studies evaluated the potential use of the anodic oxides in the fuel cell electrodes focusing more on the material’s synthesis than the application. In these studies, anodized TiO 2 -NTs are tested as supporting materials for the electrocatalyst in the electrodes for PEMFC, DMFC, and DFAFC applications [ 29 , 30 , 51 , 55 , 58 ]. The evaluation of anodic Y-doped ZrO 2 as the solid electrolyte for SOFC applications was also reported [ 47 ].…”

“…An alternative to methanol as liquid fuel is the formic acid, which is non-toxic in dilute solution and presenting a smaller crossover flux through the ion-conducting membrane than methanol [ 117 ]. Additionally, with a faster oxidation reaction, the formic acid can be produced from biomass or sequestrated CO 2 [ 58 ], reducing CO 2 in the environment. Its primary disadvantage is a volumetric energy density lower than methanol, but it can be compensated for using a high concentration of formic acid.…”

“…Still, it is often poisoned by CO intermediates’ adsorption, a similar effect observed in methanol electrooxidation in DMFCs. The indirect electrooxidation on Pt electrode proceeds according to [ 58 ]: HCOOH → CO ad + H 2 O → CO 2 + 2H + + 2e − …”

“…To promote the direct CO 2 oxidation reducing CO formation, researchers replaced platinum with palladium [ 55 , 58 , 117 ]. Anodic oxide films’ potential as supporting material for the electrocatalyst for DFACF applications was evaluated in some studies [ 55 , 58 , 59 ]. In a preliminary study, Abraham et al [ 55 ] electrodeposited Pd dendrites on TiO 2 -NT to evaluate its catalytic activity in formic acid electro-oxidation.…”