XAS-DAFC · Core-shell catalysts for Direct Alcohol Fuel Cells (DAFCs): Study of the structure and stability under electrochemical conditions by X-ray structural techniques

Core-shell Catalysts for Direct Alcohol Fuel Cells (DAFCs): Study of the structure and stability under electrochemical conditions by X-ray structural techniques

Currently, direct alcohol fuel cells (DAFCs) are amongst the most promising alternative energy systems for portable applications such as mobile phones and transport, having the advantages of ease of handling of the fuel (compared to hydrogen polymer electrolyte membrane (PEM) fuel cells) and flexibility in cell operation. However, full commercialisation of this technology for such demanding applications requires a reduction of the platinum content and an increase of the electrocatalyst stability. Binary and ternary platinum-based catalysts have been tested as electrode materials, but the efficiency and/or durability of systems operating with these catalysts is still insufficient for more demanding practical applications. Therefore, further optimisation of the catalysts is necessary in order to maximise utilisation of platinum (Pt). Recently, the use of core-shell catalysts has been proposed as one of the most feasible solutions for reducing the platinum content and, therefore, the fuel cell cost. This nanostructure involves the replacement of the core of the nanoparticles with less expensive metals. Furthermore, using this nanostructure, the utilisation efficiency (fraction of metal atoms available to take part in the catalytic reaction) is increased, and an enhancement of the platinum activity can be achieved due to an electronic, structural or morphological effect that depends on the metal used as core. However, it is necessary to understand how these materials work and determine their stability in operating conditions, since a better understanding of the structure and mechanism of Pt degradation can provide the necessary information for the design of more durable catalysts and for definition of operating conditions. In this context, the main objective of the project is to confirm the structure and stability of these core-shell materials and understand how they work in comparison with the conventional alloy catalysts. In order to achieve this objective, bi-metallic (Pt-X/C, X = Pd, Ru, Sn) and tri-metallic (PtRu/Pd/C and Ru/Pt/Pd/C) catalysts with different nanostructures have been prepared and well characterised. Various physicochemical techniques, such as SEM-EDX, XRD, XPS, TEM, STEM-EDS, and ex situ XANES /E XAFS, have been used in order to determine the structure of the materials and study the effect of the second metal on the structural and electronic properties of Pt. In addition, in order to establish the relationship between the properties and the activity, electrochemical techniques, such as cyclic voltammetry and CO-stripping, have been used to determine the activity of the different materials in the methanol and ethanol electrooxidation reactions. Finally, the catalysts have been characterised in situ during the alcohol electrooxidation by FTIR and EXAFS to determine the selectivity to carbon dioxide (CO2) and the stability of the structure under electrochemical conditions, respectively. The results indicate that the methods used for the synthesis of the materials lead to the desired alloy and core-shell structures and that the second metal has a different effect on the Pt properties depending on this structure. For example, when tin (Sn) is alloyed with Pt, Sn modifies the electronic structure of Pt through an electron transfer from Sn to Pt, and causes the stretching of the Pt lattice parameter. However, when Sn is deposited on the Pt surface, it does not perturb either the electronic or structural properties of Pt. Regarding the electrochemical behaviour, the alloy nanostructure was found to be more active than the core-shell one in terms of specific activity (mA cmPt-2), but less active in terms of mass activity (mA gPt-2). For the methanol electrooxidation reaction, the geometric effects are shown to be more important than the electronic ones, whereas the opposite was found to be the case for ethanol oxidation. The most significant results obtained during this project come from the in situ characterisation of the catalysts during the electrooxidation of methanol and ethanol. The in situ EXAFS measurements allowed us to demonstrate, on one hand, that these materials are stable under electrochemical conditions (potential range = 0.2 - 0.6 V versus RHE), that is, the nanostructure does not change when a potential is applied and on the other hand, that the presence of alcohol does not affect the structure of the materials. The in situ FTIR measurements allowed us to determine the dependence of the selectivity to CO2 in the ethanol electrooxidation on the nanostructure of the materials. In the literature, it is stated that Pt-Sn alloys are more active in the electrooxidation of ethanol due to the stretching of the lattice parameter of Pt, which could catalyse the cleavage of the C-C bond. Nevertheless, we have found that Pt-X alloys (with X = Sn or Ru) did not result in the total oxidation of ethanol to CO2, but rather to the production of acetic acid, whereas the Sn or Ru modified Pt surface nanostructure were more selective to CO2. We attribute this result to the effect that the second metal has on the electronic properties of Pt. In addition, the catalysts have been subjected to an accelerated aging treatment for the fuel cell environment, in order to study their stability and durability. Electrochemical cycling of the catalyst materials indicated a slight loss of the surface area (measured by CO-stripping) for the Pt-Ru materials (< 20 %) but for the Pt-Sn materials this loss was significant (45 - 50 %). However, after the aging treatments, the Pt-Sn/C materials were shown to retain stable activity, whereas the Pt-Ru/C materials were shown to have a significant loss of activity that cannot be explained by the loss in surface area alone, but must also be accounted for by a change in the metal nanostructure. In order to further study the stability and durability of the catalysts, the cycled samples will be characterised by in situ EXAFS at ALBA (Spain) in October 2013. These in situ XAS measurements are crucial to understanding the structure / property relationships and the degradation mechanisms that limit the stability of the materials under operation conditions. The project has no direct socio-economic impacts, however, the results will assist those who are developing direct alcohol fuel cells to design more active and stable electrocatalytsts. This represents a long-term economic advantage and the use of such 'greener' power sources will eventually provide a sociological impact. Contact details: Prof. Andrea E. Russell School of Chemistry University of Southampton Highfield Campus Southampton SO17 1BJ United Kingdom (UK) E-mail: A.E.Russell@soton.ac.uk Dr Laura Calvillo Department of Chemical Sciences University of Padova Via Marzolo 1 35131 Padova Italy E-mail address: laura.calvillolamana@unipd.it or L.Calvillo@soton.ac.uk

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