Résumé
Many electrochemical reactions of technical interest (fuel cells, electrolyzers, etc.) need the development of efficient catalysts to increase the reaction rate and selectivity. In order to find new electrocatalysts it is of prime importance to elucidate the reaction mechanisms by identifying the different intermediates involved and elucidating their role in the rate determining step (r.d.s.). Apart from the hydrogen oxidation and evolution reactions, the mechanisms of which are thoroughly established, the oxygen reduction (ORR) and evolution reactions (OER), or the oxidation of low weight alcohols in a Direct Alcohol Fuel Cell (DAFC), are relatively complex since they involve multi electron transfer with the formation of several adsorbed intermediates and reaction by-products. The reaction rate of ORR is very low and it is the main cause of energy efficiency limitation in a Polymer Electrolyte Fuel Cell (PEFC) [1-3]. Alcohols (particularly methanol and ethanol) are of great interest as liquid fuels in a DAFC [4–10], but a rather poor kinetics of their electroxidation is observed with platinum, the only catalyst activating the C–H bond cleavage at ambient temperatures. The adsorption and oxidation of methanol and ethanol on a Pt electrode lead to the formation of poisoning species (mainly adsorbed CO), as observed by in situinfrared reflectance spectroscopy [11-12]. In both cases, the formation of such poisoning species leads to a poor electrocatalytic activity, and the challenge is to enhance the activity of platinum by avoiding the formation of poisoning species through the development of bimetallic or multi-metallic platinum-based electrocatalysts. The elucidation of the reaction mechanisms needs the use of different spectroscopic and analytical techniques under electrochemical control, i.e. the combination of electrochemical methods with other physicochemical techniques. This allows identifying the nature of adsorbed intermediates, the structure of adsorbed layers, the nature of the reaction products and by-products, etc., and determining the amount of these species, as a function of the electrode potential and experimental conditions. This will lead to a deep understanding of the reaction mechanisms, which are depending on the nature of the catalyst, the composition and structure of the electrode. In this communication we will present spectro-electrochemical methods (Infrared Reflectance Spectroscopy [13], Electron Spin Resonance Spectroscopy [14] and UV-visible Reflectance Spectroscopy [15]) able to identify in situthe adsorbed species and reaction products involved in electrochemical reactions of great interest for energy conversion. For the ORR, the comparison of electrochemical data from rotating disc electrodes with results from spectroscopic methods gives information on the reaction mechanism. For alcohol electroxidation reactions, the coupling of electrochemical measurements with Infrared Reflectance Spectroscopy allows the identification of the intermediate species and reaction products as a function of the electrode potential and of its structure. This allowed proposing mechanisms for the reactions involved in electrochemical reactions encountered in the development of Fuel Cells or Electrolyzers: the Oxygen Reduction Reaction and the electroxidation of some alcohols (methanol, ethanol, ethylene glycol, and glycerol). References : [1] G. J. K. Acres, J. C. Frost, G. A. Hards, R. J. Potter, T. R. Ralph, D. Thompsett, G. T. Burstein, G. J. Hutchings, Catal. Today, 38 (1997) 393-400. [2] T.R. Ralph, M.P. Hogarth, Platinum Metals Rev., 46 (2002) 3-14. [3] C.-C. Yang, Int. J. Hydrogen Energy, 29 (2004) 135-143. [4] C. Lamy, A. Lima, V. Le Rhun, F. Delime, C. Coutanceau, J.-M. Léger, J. Power Source,s 105 (2002) 283-296. [5] E. Peled, T. Duvdevani, A. Aharon, A. Melman, Electrochem. Solid State Lett., 4 (2001) A38-A41. [6] C. Lamy, J.-M. Léger, S. Srinivasan, Direct Methanol Fuel Cells: From a twentieth century electrochemist’s dream to twenty-first century emerging technology, in "Modern Aspects of Electrochemistry", J.O’M. Bockris and B.E. Conway (Eds.), Vol. 34, Plenum Press, New York, 2000, Chapter 3, pp.53-118. [7] C. Lamy, E.M. Belgsir, J.-M. Léger, J. Appl. Electrochem., 31 (2001) 799-809. [8] T. Iwasita-Vielstich, in Advances in "Electrochemical Science and Engineering", H. Gerischer and C.W. Tobias (Eds.), Vol.1, VCH Verlag, Weinheim, 1990, p. 127. [9] A. Hamnett, Catal. Today 38 (1997) 445-457. [10] K. Y. Chan, J. Ding, J. W. Ren, S. A. Cheng, K.Y. Tsang, J. Mater. Chem., 14 (2004) 505-516. [11] B. Beden, F. Hahn, S. Juanto, J.M. Léger, C. Lamy J. Electroanal. Chem., 225 (1987) 215. [12] J.M. Perez, B. Beden, F. Hahn, A. Aldaz, C. Lamy, J. Electroanal. Chem., 262 (1989) 251. [13] B. Beden, C. Lamy, Infrared reflectance spectroscopy, in "Spectroelectrochemistry - Theory and Practice", R.J. Gale (Ed.), Plenum Press, New York, 1988, Chapter 5, pp. 189-261 [14] P. He, C. Cha, P. Crouigneau, J.M. Léger, C. Lamy, J. Electroanal. Chem., 290 (1990) 203. [15] O. El Mouahid, A. Rakotondrainibe, P. Crouigneau, J.M. Léger, C. Lamy, J. Electroanal. Chem., 455 (1998) 209.