Highly Active Pt-Sn/C Catalysts for Ethanol Electro-Oxidation Prepared By a Polyol-Alcohol Reduction Process

Introduction.

Fuel cells are very attractive devices as power source for mobile, stationary or portable applications. In a Direct Ethanol Fuel Cell, the fuel is directly feed into the cell anode compartment without any previous chemical modification. However, alcohols are very difficult and complex to electrooxidize completely to CO₂, because of the need to break C-C bonds, which results in the formation of intermediates that poison the Pt active sites. In order to overcome these issues, the efforts are focused on developing new bimetallic catalysts with higher catalytic activity and tolerance to the presence of the intermediates, mainly CO, than Pt-alone. The most efficient alternative has been to add a second metal to the composition of the catalysts used as anodes for the electrooxidation of organic molecules. Therefore, this work presents the results of a study carried out to evaluate how different synthesis variables can have an effect on the chemical reduction process of the salts, and thus, on the chemical composition of PtSn/C catalysts. Then, the chemical composition has an effect on the performance of the PtSn/C materials for the ethanol oxidation reaction in acid media.

Methodology.

PtSn/C anodes with Pt:Sn atomic ratio of 1:1 were prepared by the polyol method, in the presence different concentrations of ethanol and water, which were used to dissolve the Sn precursor. The structural characteristics of the catalysts were studied by XRD analysis in a Philips X’Pert diffractometer using Cu Kα Kα (λ= 1.5406 Å) radiation operating at 40 kV. The chemical composition of the catalysts was analyzed by SEM-EDS and ICP-AES.

Results.

Through XRD analysis, the degree of alloying of the Pt-Sn/C catalysts is evaluated with Vegard’s law. Also, the average particle size is obtained with the Scherrer equation. The results show the formation of nanocatalysts having a particle size of 2-4 nm. Chemical analysis using SEM-EDS and ICP-AES indicate the actual Pt:Sn ratio, confirming the formation of Sn oxides as a function of the concentration of ethanol and water.

The electrochemical characterization indicates that the Pt-Sn/C with high degree of alloying and less Sn oxides is more active for the EOR than Pt-Sn/C anodes with a high Sn oxides formation and low degree of alloying. Moreover, the Pt-Sn/C anodes demonstrated a higher catalytic activity than Pt/C synthesized by the same method.