Ethanol is an ideal fuel for direct oxidation fuel cells for portable and mobile application as it offers multiple advantages over hydrogen and methanol. The most important of these are ease of transportation, storage and handling, and high energy density [1]. Platinum and palladium based catalyst are commonly used for ethanol oxidation. The major issue with current catalysts is their inability to cleave the C-C bond in ethanol [2] resulting in incomplete oxidation of ethanol [3]. As a result the fuel cell operates at low efficiency. Hence, bimetallic or trimetallic catalyst are developed to both promote the C-C bond cleavage and to stabilize the hydroxyl group for reducing CO poisoning [4]. In particular, the Pt-Rh-SnO₂ catalyst showed predominant oxidation of ethanol to CO₂. SnO₂ provides hydroxyl species to oxidize the dissociated CO at Rh sites and Pt facilitate ethanol dehydrogenation. The electronic structure of Rh is modified to afford moderate bonding to ethanol, intermediates and products, promoting the C-C bond cleavage.[5,6]

Preliminary results of Pt-Rh-SnO₂ (3:1:3) catalyst synthesis via reactive spray deposition technology (RSDT) shows promising activity for ethanol oxidation (Figure 1a) and good fuel cell performance (Figure 1b). This can be attributed to the porous electrode morphology that provide high surface area for the gas diffusion electrode (Figure 1c). However, in-situ infrared reflection-adsorption spectroscopy (IRRAS) in the electrochemical cell suggests mixed products of carbon dioxide, acetic acid, and acetaldehyde, as shown in Figure 1d. This implies that the mixing between Pt, Rh, and SnO₂ could be inhomogeneous and the interplay between the three components could be hindered.[6]

This work focus on the optimization of the interface between three components by varying the relative atomic ratio of Pt, Rh, and Sn. Furthermore, the effect of RSDT process parameters, such as fuel and gas flow rate, precursor solubility, on the precursor mixing are investigated. The surface elemental components and chemical environment are characterized using X-ray photoelectron spectroscopy (XPS). In addition, high-resolution transmission electron microscopy (HRTEM) is employed to study the elemental distribution of the catalyst particles and investigate the phase separation of Pt, Rh, and SnO₂.

Figure 1. (a) Electrochemical measurement of ethanol oxidation with rotating disk electrode; (b) fuel cell performance Anode: RSDT deposited gas diffusion electrode of Pt-Rh-SnO₂, Cathode 45 wt% Pt/C, 2 mg/cm². Membrane: Nafion 115; (c) Surface morphology of Pt-Rh-SnO₂ on gas diffusion electrode; (d) In situ IRRAS of ethanol oxidation at various potentials form -0.2 V to 0.55 V vs. Ag/AgCl.

Acknowledgment

This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

References

[1] L. An, T.S. Zhao, Y.S. Li, Renewable and Sustainable Energy Reviews. 50 (2015) 1462-1468.

[2] M.A.F. Akhairi, S.K. Kamarudin, International Journal of Hydrogen Energy. 41 (2016) 4214-4228.

[3] S.Y. Shen, T.S. Zhao, Q.X. Wu, International Journal of Hydrogen Energy. 37 (2012) 575-582.

[4] H. Wang, Z. Liu, J.Am.Chem.Soc. 130 (2008) 10996-11004.

[5] A. Kowal, M. Li, M. Shao, K. Sasaki, M.B. Vukmirovic, J. Zhang, N.S. Marinkovic, P. Liu, A.I. Frenkel, R.R. Adzic, Nature Materials. 8 (2009) 325.

[6] M. Li, W.-. Zhou, N.S. Marinkovic, K. Sasaki, R.R. Adzic, Electrochimica Acta. 104 (2013) 454-461.