Templated Pt–Sn electrocatalysts for ethanol, methanol and CO oxidation in alkaline media
Introduction
Recently, there has been increased interest in the development of direct ethanol fuel cells (DEFCs). Ethanol is a renewable resource as it can be produced directly by fermentation of biomass. In addition, building a support infrastructure would be far easier than for other fuels. The kinetics of both anodic oxidation of organic molecules and cathodic oxygen reduction are much more facile in alkaline media in comparison to acid media which presents a key advantage [1], [2], [3], [4]. Certain technical issues of stability such as carbonation of the alkaline electrolyte have prevented the alkaline fuel cell from being a viable commercial power generator. A robust alkaline anion exchange membrane has been developed at Sandia National Laboratories which is capable of meeting chemical stability and service durability challenges. The novel polymer consists of benzyl trimethylammonium groups on a poly(phenylene) backbone and is based on similar poly(phenylene) materials for PEM fuel cell applications [5].
A significant objective in the development of DEFCs is the creation of enhanced catalytic materials for the anodic reaction. The electro-oxidation of ethanol is extremely complicated and the development of efficient anode electrocatalysts presents a difficult challenge. For ethanol electro-oxidation in acid media, platinum-based alloys have shown the greatest activity. These alloys include Pt–Ru, Pt–Sn as well as a variety of other metallic alloys and oxides [6]. Methanol oxidation rates on both Pt/C and Pt–Ru/C catalysts have been reported to be higher in alkaline media than in acid media which has in part been attributed to a higher coverage of OHad at lower potentials [2]. For methanol electro-oxidation in alkali, carbonate and formate have been detected as products formed through several adsorbed intermediates and in part through a COad intermediate. In comparison to methanol electro-oxidation, the electro-oxidation of ethanol is more complex and is the focus of ongoing mechanistic studies. The reaction involves a greater number of electrons exchanged and many adsorbed intermediates and products. In addition, there is a need to tune the catalyst to break the C–C bond effectively for the oxidation of ethanol. In situ infrared spectroscopic studies have shown that adsorbed CO is a primary poisoning species and acetate is the main product for ethanol oxidation by platinum in alkaline media [7]. In general, suitable electrocatalysts for ethanol oxidation in alkali have focused on platinum and palladium-based catalysts. In one particular study, Pt–CeO2/C composites showed better performance for ethanol oxidation than a comparable Pt/C [8]. The improved performance was suggested to be the result of a greater ability of the Pt–CeO2/C catalyst to form oxygen-containing species at lower potentials, functioning in a similar way as Ru addition to Pt does. The addition of ZrO2 and MgO to Pt has also shown to improve ethanol oxidation performance [9], [10].
A significant focus of ethanol electro-oxidation research has been on the superior performance of Pt–Sn catalysts in acid media [6], [11], [12], [13], [14], [15], [16], [17]. The superior performance is attributed to a bifunctional mechanism wherein Sn or Sn oxides provide the surface with oxygen species allowing for improved removal of strongly adsorbed species such as CO. Depending on preparation method, the optimal Pt–Sn composition reportedly varies. In general, the optimal composition of Sn ranges from 10 to 20 at.% for co-impregnated catalysts [16].
In previous work, aerosol-derived templated Pt–Ru electrocatalysts were evaluated for the electro-oxidation of methanol in acid media [18]. Despite these Pt–Ru composites having a lower BET surface area compared to commercial Pt–Ru black, the electrochemical performance suggested a higher metal utilization for the templated materials. In this work, an aerosol-derived templated Pt–Sn electrocatalyst having 80% Pt and 20% Sn composition is compared to a templated Pt catalyst synthesized in an equivalent process. We examine the aerosol-derived templated electrocatalysts in conjunction with the poly(phenylene) ionomer for ethanol, methanol and CO oxidation in alkaline media. The templated catalysts are synthesized in an aerosol-based approach in which mono-disperse silica nanoparticles are used to template the metallic precursors. The resulting nanostructured Pt–Sn material is characterized by TEM, XRD, BET surface area and electrochemical measurements in alkaline media.
Section snippets
Ionomer preparation
A detailed description of the cation exchange form of this polymer synthesis has been published elsewhere [5]. The material synthesis according to the method described previously has been modified to become an anion exchange ionomer. As indicated in Fig. 1, the repeat unit consists of four benzene rings with ambiguous regiochemistry (a mixture of meta and para linkages) on every other ring. In the parent polymer, each of the rings with ambiguous regiochemistry has one pendant phenyl group and
Structural and compositional analysis
The aerosol synthesis technique can produce a range of metallic compositions, although for the purposes of this communication, two particular electrocatalysts were studied and compared. The first was a templated 100% Pt catalyst and the second was a templated Pt–Sn catalyst having an atomic composition of 80% Pt and 20% Sn, determined by EDS. SEM and TEM micrographs of the templated catalysts before and after silica template removal are shown in Fig. 3.
The templated Pt–Sn composite before
Conclusion
The slight benefit of Sn addition to Pt has been shown for the oxidation of small organic molecules in alkaline media. The use of an anion exchange ionomer in these studies provides a unique opportunity to study the electro-oxidation of ethanol in alkaline media. Commonly, it is necessary to use the cation exchange ionomer Nafion to bind powder electrocatalysts catalysts to the glassy carbon working electrode in the RDE configuration, however in these studies, an anion exchange ionomer is used
Acknowledgements
Support for Elise Switzer in the form of a fellowship from Sandia National Laboratories is gratefully acknowledged. This work made use of the characterization facilities at UNM supported by the NSF EPSCOR and NNIN grants and the Department of Earth and Planetary Sciences.
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