Spectroscopic methods fuel cells

The aim of this chapter is to show that the choice of a catalyst formulation leading to increase the activity and the selectivity of a given electrochemical reaction involved in a fuel cell can only be achieved when the mechanism of the electrocatalytic reaction is sufficiently understood. The elucidation of the mechanism caimot be obtained by using only electrochemical techniques (e.g. cyclic voltammetry, chronopotentiometry, chrono-amperometiy, coulo-metry, etc.), and usually needs a combination of such techniques with spectroscopic and analytical techniques. A detailed study of the reaction mechanism has thus to be carried out with spectroscopic and analytical techniques under electrochemical control. In short, the combination of electrochemical methods with other physicochemical methods cannot be disputed to determine some key reaction steps. For this purpose, it is then necessary to be able to identify the nature of adsorbed intermediates, the stractuie of adsorbed layers, the natirre of the reaction products and byproducts, etc., and to determine the amormt of these species, as a fimction of the electrode potential and experimental conditions. 

In order to improve the fuel utilization in a Direct Alcohol Fuel Cell (DAFC) it is important to investigate the reaction mechanism and to develop active electrocatalysts able to activate each reaction path. The elncidation of the reaction mechanism, thus, needs to combine pnre electrochemical methods (cyclic voltammetry, rotating disc electrodes, etc.) with other physicochemical methods, such as in situ spectroscopic methods (infrared and UV-VIS" reflectance spectroscopy, or mass spectroscopy such as EQCM, DEMS " ), or radiochemical methods to monitor the adsorbed intermediates and on line chromatographic techniques"" to analyze qnantitatively the reaction products and by-products. 

Study the kineties of fuel cell electrode reactions on well-characterized model eleetrodes and high surface area fuel eell electrocatalysts using modem eleetroanalytieal methods. Study the meehanisms of the reactions using state-of-the art in-situ spectroscopes. 

The list of spectroscopic methods used in in situ characterization of supported metal particles is expanding continuously, new cell designs being proposed to improve the sensitivity and the signal-to-noise ratio and to provide real-time monitoring of fuel cell catalysts. 

The electrooxidation of methanol occupies, since many years, the center of interest in electrocatalysis of fuel cell reactions. Some of the most important findings on this issue are described in Chapter 5.2. We do not intend to review here all of the spectroscopic work done on methanol but to illustrate some highlights of the FTIRS method when applied to study the electrocatalysis of methanol oxidation. 

W. Jenseit, O. Bdhme, F. U. Leidich, and H. Wendt [1993] Impedance Spectroscop a Method for in situ Characterization of Experimental Fuel Cells, Electrochim. Acta, 38, 2115-2120. 

CO adsorption and oxidation have been studied for many years, but a greater understanding was achieved by the development of ex situ and in situ spectroscopic and microscopic methods for application in electrochemistry [9, 143-146], together with the use of well-defined nanocrystalline electrode surfaces [147]. The opportunity to study in situ electrooxidation of carbon monoxide [148-157] under fuel cell reaction conditions has brought significant progress in understanding interfacial electrochemistry on metallic surfaces, hi combination with conventional electrochemical methods these techniques have been used to find connections between the microscopic surface structures and the macroscopic kinetic rates of the reactions. 

In relative terms, the largest single source of ohmic losses is from the membrane. A simple way to determine the ohmic resistance is employ the impedance spectroscopic method. In a fuel cell impedance spectrum, the ohmic resistance is the real value of the impedance of the point for which the imaginary impedance is zero at the maximum frequency. The effects of these losses are most pronounced at intermediate current densities. Minimizing the ohmic losses requires effective water management in the membrane, excellent electron conductive materials, and minimal contact resistance. 

Off-line techniques. These are for measurement of signals from which physicochemical properties can be extracted, and with some of these techniques it is also possible to examine the possible relationship between these properties. The panel of current electrochemical techniques belongs to this group, together with all methods to be used for determination of the chemical composition of hquids emitted by the cell, and all spectroscopic or microscopic techniques for the material evaluation of a fuel cell component spectroscopic and microscopic techniques are post mortem techniques, contrary to the other off-line techniques, which are carried out under real operation of the fuel cell. 

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