Short CommunicationCatalytic enhancement of formic acid electro-oxidation through surface modifications with gold on supported Pt nanoparticles
Introduction
As liquid fuel, Formic Acid (FA) in low temperature polymeric electrolyte membrane fuel cells (PEMFC) it is considered an optimal fuel–cell system for portable electronic applications [1], since it combines the advantage of high-power density and simplicity of fuel handling [2], [3].
During the last years, several materials composed by Pt/Au were used as catalysts in fuel cells [4], [5], [6], [7]. Excellent results have been reported for FAEO, including Pt NPs decorated with Au, Pt electrodeposited on Au, and those of PtAu NPs alloys [8], [9]. The improvements are evident since a remarkable current density increase in the useful range of operation of PEMFC is observed [10]. Thus, all studies concludes that bimetallic Pt/Au surfaces are more active for the FAEO with respect to bare Pt [11], [12], [13].
The most studied Pt/Au catalyst supports are flat electrodes, which are useful from a fundamental point of view [14], [15]. However, an optimum DFAFC anode requires a high dispersion of small electrocatalyst nanoparticles on an electrically conductive surface. In addition, towards commercialization of fuel cell systems, lowering the amount of expensive catalysts by increasing its utilization ratio is an important issue. As a support material, porous carbons are generally used, due to their advantages like high electrical conductivity, good chemical stability and low cost [16], [17], [18]. Moreover, the fuel access towards electrocatalyst active sites is partially hindered (or blocked) by the support in supported catalysts. This last fact severely affects the mass transfer rate and, therefore, this effect acts in detriment of anode performance. Particularly, on the FAEO case, only two electrons per FA molecule are transferred during oxidation, according to:
For this reason, a high turnover frequency (TOF) is required to give an acceptable current density [19]. In other words, the support should provide appropriated mass transfer rates, followed by adequate elimination of reaction products.
Based on the results previously obtained for flat Pt surfaces modified by spontaneous deposition of Au [20], we evaluated the feasibility of its implementation in a carbon-supported catalyst. To this purpose, in this work we use a well reproducible hierarchical porous carbon structure (HPC) modified with Pt NPs as the starting material (HPC-Pt) [21]. The HPC-Pt substrate was modified with Au by spontaneous deposition. The materials morphology and composition was characterized using SEM-EDS. Then, the catalytic material obtained (HPC-PtAu) was evaluated towards FAEO reaction using cyclic voltammetry and chronoamperometry.
Section snippets
Preparation HPC-PtAu catalysts
The porous carbon with a hierarchical structure was obtained by using SiO2 nanoparticles as hard template. In a typical synthesis, a porous SiO2 solid was obtained by sintering of SiO2 NPs, made by the Stöber method [22], at 1000 °C. Next, the solid was impregnated with a formaldehyde/resorcinol/sodium carbonate precursor solution (1:0.6:0.0025 M ratio). Then, the sample was kept in an oven at 110 °C for 24 h to let the monomers polymerize. The resulting SiO2-Resin composite was carbonized at
HPC support
The porous carbon obtained by using SiO2 nanoparticles as hard template displays a hierarchical structure of porous (see Fig. 1A). The origin of the hierarchical porous structure has been discussed elsewhere [21]. In brief, the contraction that should take place during polymer densification at T < 350 °C, it is no allowed by the existence of the hard template “skeleton” of SiO2, giving rise to additional fractures and defects on carbon surface. BET analysis give a surface area of 535 m2g-1 (see
Conclusion
Based on these results, it could be established that surface modification of the HPC-Pt NP substrate with spontaneous Au deposits was possible. By means of CV, two successive Au deposition steps are tested. On HPC-Pt NP an Au surface coverage of 8% and 15% was obtained. The catalytic activity, studied by cyclic voltammetry and chronoamperometry, confirm that the Au presence on HPC-PtAu catalysts surface improves the catalyst performance for FAEO when compared with unmodified HPC-Pt NP. Our
Acknowledgments
The authors thank financial support from Agencia Nacional de Promoción Científica y Tecnológica (Argentina, Project PICT 2010-2421), CONICET (Argentina, Project PIP 112-201101-01035). CAB, and GAP are permanent research fellows of CONICET. PSC and AMBM thank to CONICET for a graduate fellowship.
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