In Situ NMR Spectroscopic Investigations of the Electrocatalytic Ethanol Oxidation Reaction

Abstract

During the past decades, the development of alternative energy sources became increasingly important because the growing consumption of non-regenerative fossil energy poses a threat to the environment. Hence, the development of fuel cells plays an important role for E-mobility and further portable and stationary applications. Hydrogen is highly efficient as a fuel, in particular in a polymer electrolyte fuel cell. It can, however, only be used to a limited extent because of its expensive and hazardous production and storage. Therefore, the use of hydrogen carriers, such as alcohols, in direct alcohol fuel cells is an approach to tackle this problem. [1] The electrochemical oxidation of such small organic molecules in alkaline media has been much less studied, because membranes, which are stable under these conditions, have not been developed. [2] In the last 20 years the alkaline fuel cells has been studied extensively, because high performance membranes are being developed. Additionally, the faster kinetics of the Oxidation reaction in alkaline medium allows the use of non-noble metal catalysts. [3] Methanol is the smallest alcohol and has already been intensively studied as a fuel due to its less complex oxidation mechanism. [4] In contrast, ethanol shows a significantly more complex reaction at the catalyst surface. Nevertheless, the higher energy density and availability in large quantities by fermentation from renewable resources makes it a highly promising alternative. Breaking the carbon-carbon-bond is the most challenging part of the complete oxidation of ethanol. If this bond cleavage is not complete, a considerable amount of by-products must be expected, depending on the catalyst. Thus, it is essential to elucidate the adsorption and reaction mechanisms on the surface of the catalyst for the development of direct ethanol fuel cells with different catalysts. [5] Identification and quantification of intermediates and products of the electrocatalytic oxidation of ethanol in alkaline medium are desirable in terms of understanding the underlying mechanism. In situ NMR spectroscopic measurements allow to detect and quantify the entire range of oxidation products non-invasively and with high chemical specificity. [6] We have developed methods to follow the electrochemical oxidation of ethanol in situ by NMR spectroscopy. For the implementation of these in situ measurements, special Pouch Cells were constructed and optimized. These Pouch Cells consist of carbon-based electrodes, which have been coated for example with a commercial platinum catalyst. The main products of the ethanol oxidation were detected by in situ NMR spectroscopy. [1] F. Vigier, C. Coutanceau, F. Hahn, E. M. Belgsir, C. Lamy, J. Electroanal. Chem. 2004, 563, 81. [2] S. C. S. Lai, M. T. M. Koper, Phys. Chem. Chem. Phys. 2009, 11, 10446. [3] G. F. McLean, T. Niet, S. Prince-Richard, N. Djilali, Int. J. Hydrogen Energ. 2002, 27, 507. [4] J. W. Gosselink, Int. J. Hydrogen Energ. 2002, 27, 1125. [5] B. Pierozynski, Int. J. Electrochem. Sci. 2012, 7, 4261. [6] E. G. Sorte, Y. Y. J. Tong, J. Electroanal. Chem-, 2016, 769, 1.