Nanostructured Pt(Cu) pseudo core-shell catalysts for alcohols electrooxidation

R.D. CASTAGNA; JUAN MANUEL SIEBEN; A.E. ALVAREZ; MARTA M.E. DUARTE

Santa Fé

Conferencia; VI San Luis School and Conference on Surfaces, Interfaces and Catalysis; 2018

Universidad Nacional del Litoral, CONICET, Instituto de Desarrollo Tecnológico para la Industria Química y Ministerio de Ciencia, Tecnología e Innovación de Santa Fé

Direct alcohol fuel cells (DAFCs) have been considered as the most appropriate substitutes for rechargeable batteries in portable, mobile and stationary applications. Nevertheless, the widespread manufacture and massive commercialization of low temperature fuel cell stacks is limited by the high cost of Pt, high fuel crossover through the ionomer membrane, deficient selectivity and low stability of anode electrocatalysts at normal operating temperatures. The new strategies to prepare catalysts with low Pt, extended service life and excellent electrochemical activity involve the preparation of multimetallic core-shell nanoparticles. Hence, the main objective of this work is evaluate the catalytic properties of different Pt(Cu) pseudo core-shell nanoparticles supported on a pretreated carbon powder for methanol, ethanol and glycerol electro-oxidation in acid and alkaline media. A series of bimetallic Cu-rich core and Pt-rich shell nanoparticles supported over a pretreated graphitized carbon powder (oxidized Vulcan XC-72R) were synthesized by a two-step route. Briefly, Cu nanoparticles (about 3 nm in size) supported on carbon were synthesized by reduction of CuSO4 with N2H4 in ethylene glycol at pH 10 and at a temperature of 80 ºC. Afterwards, the core-shell particles were formed by the partial galvanic replacement of Cu with Pt. The bulk composition of the as-prepared electrocatalysts was determined by energy dispersive X-ray (EDX) microanalysis, while the amount of the metals deposited on modified Vulcan carbon substrate was estimated using ICP-AES analysis. In addition, the valence state and the surface composition of the as-prepared materials were determined by X-ray photoelectron spectroscopy (XPS). The particle size and morphology were analyzed using transmission electronic microscopy (TEM), while the structure of the as-prepared materials was investigated by X-ray diffraction (XRD). Furthermore, the electrocatalytic properties of the different as-prepared electrodes were evaluated by cyclic voltammetry and chronoamperometry, and compared with the activity of a commercial PtRu/C catalyst (20 wt.% Pt and 10 wt.% Ru). Four different carbon-supported Pt(Cu) pseudo core-shell catalysts with Pt:Cu ratios of 1:3, 1:2.2, 1.6:1 and 3:1 have been successfully synthesized. The noble metal loading of the as-synthesized materials on carbon was determined to be in the range of 6-11 wt.%. TEM micrographs revealed the presence of a large amount of irregular particles with diameters between 2.4 and 3.2 nm, which appear regularly distributed over the carbon support. The formation of nanoparticles comprised of a Cu-rich core surrounded by a Pt-rich shell was confirmed by the combination of XRD, XPS and EDX analyses. The electrochemical surface area per unit mass (ECSA) of the as-synthesized electrodes was determined to be in the range of 80-120 m2 g-1, indicating high degree of Pt utilization. Cyclic voltammetry and chronoamperometric experiments showed that the Pt62(Cu)38/C catalyst presented the best performance for the electro-oxidation of the alcohols, followed by Pt(Cu)/C electrode. In addition, the as-prepared materials exhibited enhanced activity towards the alcohols oxidation when compared with the commercial PtRu/C electrocatalysts. For instance, the catalytic activity of the most active Pt(Cu)/C electrode towards ethanol oxidation in acid media in long-term potentiostatic experiments (E = 0.6 V vs. SCE and t = 1800 s) was ten times higher than that of the commercial system (61 mA mg-1 vs. 6 mA mg-1). On the other hand, the long-term poisoning rate of the as-prepared catalysts is lower than that of the commercial PtRu/C. A similar behavior was observed for the electro-oxidation of the other alcohols in acid and alkaline media. The results of this study suggests that the two-step process is a viable method to making highly active catalysts with good stability and low platinum content for application in DAFCs. The enhanced electrocatalytic properties of the as-prepared materials can be attributed to the high ECSA (i.e., high Pt utilization) and the modification of the surface electronic structure due to lattice strain in the Pt-rich shell and ligand effects.