In Situ Fourier Transform Infrared Spectroscopy (FTIR) Studies on Pt-Rh-Sn Nanoparticle Electrocatalysts for Ethanol Oxidation in Direct Liquid Fuel Cells

Alcohol fuel cells are of great interest in academic and industrial fuel cell research due to their high volumetric energy density compared to hydrogen/air fuel cells, and have high potential as an environmental friendly power source.¹ As a liquid fuel, ethanol has higher energy densities (8 kWh kg^-1), better efficiency, low toxicity, biocompatibility and abundant availability. However, ethanol oxidation suffers from slow kinetics, sluggish adsorption and C-C bond cleavage as well as poisoning of the active sites on Pt catalysts by CO-intermediates. The desired reaction involves the cleavage of the C-C bond, i.e. the total ethanol oxidation to CO₂, while partial oxidation leads to by-products like acetic acid or acetaldehyde, which reduce the faradaic efficiency of the anodic reaction of DEFC:

C₂H₅OH + 3 H₂O -->  2 CO₂ + 12 H⁺ + 12 e^-

To improve the performance for the ethanol oxidation reaction (EOR) the development of novel efficient electrocatalysts is necessary. Research has been widely focused on the addition of co-catalysts to platinum. Recently, ternary metal systems like Pt-Rh-Sn have shown enhanced EOR activity.² However, the elucidation of the mechanism and the role of single metal atoms is still not completely understood and therefore a pressing subject of current research.³ Our aim is to increase the understanding of the relation between catalyst structure and composition and the EOR activity. Therefore, we tested different carbon supported nanoparticle catalysts, prepared by simultaneous reduction or consecutive reduction of Pt, Rh and Sn precursor salts, using oleic acid and oleylamine as surfactants and tetradecanediol as the reducing agent in dioctyl ether. A uniquely comprehensive set of experimental techniques was employed to determine the composition, morphology and atomic-scale structural coherence of the synthesized ternary nanoalloy electrocatalysts, including inductively coupled plasma optical emission spectroscopy (ICP-OES), Cu Kα based and synchrotron-based X-ray diffraction (XRD), transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-Ray photoelectron spectroscopy, etc. Cu Kα and synchrotron XRD studies coupled to atomic pair distribution function analysis showed that PtRhSn/C nanoalloy had PtSn niggliite type structure with Rh occupying random Pt sites. Electrochemical experiments showed that the catalytic activity towards EOR of chemically homogeneous PtRhSn/C is the highest when compared to those of the other catalysts. The results showed that while Sn is more important than Rh as a component in nanoalloys used for oxidation of ethanol, the role of Rh is definitely not negligible since PtRhSn/C nanoalloy showed best catalytic property as compared to the corresponding catalyst without Rh/Sn. Achieving a very good degree of chemical homogeneity and the underlying atomic arrangement in the PtRhSn/C ternary alloy is a crucial condition in producing a catalyst with improved properties for oxidation of ethanol.

Here, we present results of an in situ FTIR study of Pt-Rh-Sn based nanoparticle for the ethanol oxidation reaction (EOR). While the investigation of the different prepared catalysts revealed structural changes in morphology and chemical composition and electrochemical behaviour and the products /intermediates distributions on the electrode surface could provide valuable conclusions, we are also able to give an insight into significant results for the different materials and their part in the electrocatalysis of ethanol to CO₂. In-situ FTIR data showing formation of CO₂during the EOR as a result of C-C bond splitting in ethanol at an early potential is will help in further understanding of the mechanistic aspects of these electrocatalysts towards EOR and has the potential to help in the future design of DLFC catalysts.

References

1.   De Souza, R. F. B.; Parreira, L. S.; Rascio, D. C.; Silva, J. C. M.; Teixeira-Neto, E.; Calegaro, M. L.; Spinace, E. V.; Neto, A. O.; Santos, M. C., Journal of Power Sources 2010, 195 (6), 1589-1593.

2.   Kowal, A., Nature Materials 2009, 8, 325 - 330.

3.  García-Rodríguez, S., Rojas, S., Pena, M.A., Fierro, J.L.G., Baranton, S., Leger, J.M, Applied Catalysis B: Environmental 2011, 106, 520– 528