J. Ledesma-García

Electrocatalytic activity of well-defined and homogeneous cubic-shaped Pd nanoparticles

Well-ordered, homogenous cube-shaped highly ordered Pd, without other geometries, was obtained by chemical reduction in aqueous media by employing ascorbic acid, polyvinylpyrrolidone and sodium bromide as the reducing agent, surfactant and additive, respectively. The X-ray diffraction (XRD) patterns exhibited a face-centred cubic structure with an average crystallite size of 11.5 nm. Transmission electron microscopy (TEM) images showed a homogeneous distribution of cubic Pd nanoparticles (namely Pd nanocubes) with a preferential (200) crystallographic plane and a particle size of approximately 10.6 ± 1.2 nm. The electrocatalytic activity of the Pd nanocubes was evaluated in terms of the current density attributed to the oxidation reaction of the following three common fuels: methanol, ethanol and formic acid. The results obtained showed that the current density achieved with the Pd nanocube electrocatalyst is 4, 3 and 3.5 (12, 10 and 8.7 mA mg−1 cm−2, respectively) times higher than that reached with commercial Pd at similar oxidation potential values. Furthermore, the Pd nanocube catalyst exhibited higher catalytic activity for formic acid oxidation than that reported for Pd-based materials. The oxygen reduction reaction using the Pd nanocubes in basic media was also tested.

Glycerol electro-oxidation in alkaline media using Pt and Pd catalysts electrodeposited on three-dimensional porous carbon electrodes

In this work, Pd and Pt electrocatalysts were electrodeposited on three-dimensional carbon paper and carbon nanofoam with the purpose of increasing the catalytic area to improve the glycerol electro-oxidation. SEM and cross-sectional SEM micrographs showed that Pd and Pt particles were well-distributed over the entire three-dimensional electrode surfaces. Commercial Pd/C and Pt/C catalysts deposited by the spray method were used for comparison, showing lower surface area (SA) utilization than those electrodeposited. The electrodeposition effectiveness to cover the electrode surfaces was evaluated by changes in overall SA and through the calculation of electrochemically active surface area (EASA) and specific surface area (SSA). Despite the larger EASA values found for Pd and Pt on nanofoam, Pt on paper showed the highest utilization of the surface area, obtaining an SSA of 58.1 m2 g−1. Moreover, the electrodeposition of Pd and Pt dramatically increased the EASA versus the geometrical area, improving this ratio 16 (Pd on paper), 151 (Pt on paper), 158 (Pd on nanofoam) and 277-fold (Pt on nanofoam). The electrodeposited porous Pt electrodes showed good activity for glycerol oxidation, exhibiting a more negative potential than Pd-based materials. However, for fuel cell applications operated at intermediate temperatures, Pd on carbon paper is the optimal candidate to be used as an anode because of its high current density and excellent poisoning tolerance.

Full factorial design applied to the synthesis of Pd-Ag nanobars by the polyol method and the perspective for ethanol oxidation

Full factorial design methodology was applied to the synthesis and optimization of Pd–Ag nanobars using the polyol process as the reducer. The concentration of Br− ions, the temperature and the reaction time were selected as factors to study, whereas the yield (% nanobars) was the response to be analyzed. The nanoparticles were characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The nanoparticles were also tested for the ethanol electro-oxidation reaction by cyclic voltammetry in alkaline solution. The three factors had a positive effect on the response: the nanobar yield increased as the level of the variables changed from −1 to +1. The temperature and reaction time were the most determinant variables (main and interacting) on the nanobar yield, whereas the concentration of Br− influenced the yield to a lesser extent. After designing three optimum experiments, a maximum nanobar yield of 47.3% was obtained. The more negative electro-oxidation onset, higher current density and more negative current peak potential show that the incorporation of Ag into Pd nanobars improves the kinetic and thermodynamic behavior towards the ethanol electro-oxidation reaction compared with that obtained with nanometrically pure Pd nanobars. This improvement is the result of surface modification caused by the incorporation of Ag in the formation of Pd–Ag bimetallic nanobars with (200) surfaces.