Electro-oxidation of small organic molecules

Abstract

An alternative to fossil fuels is to utilize biofuels like methanol and ethanol directly in fuel cells. One drawback of this system is the sluggish reaction kinetics in methanol and ethanol oxidation where strongly bound intermediates, like CO(ads), block the electrode surface. One way to enhance the kinetics is to increase the operating temperature of the fuel cell.

The main objectives of the thesis were: i) model CO oxidation on clean and oxidized platinum electrodes and compare the modelling results with experimental results, ii) employ dynamic electrochemical impedance spectroscopy to study the reaction mechanisms during oxidation of small organic molecules, and iii) study oxidation of methanol and ethanol at elevated temperatures using an autoclave setup.

CO oxidation was modelled by assuming a set of elementary reaction steps and numerically solve the corresponding set of differential equations describing the net formation rate of each adsorbed species. Two different approaches were used to model CO oxidation on clean and oxidized platinum. In the first approach, platinum oxide formation and reduction were modelled with elementary reaction steps while an empirical rate equation was used in the second approach. The second approach resulted in good agreement with experimental results. The modelling showed that the rate constants on the negative-going sweep are different than on the positive-going sweep, specifically, the rate of CO adsorption decreased and the rate of PtOH(ads) formation increased.

Bulk oxidation of CO on a platinum rotating disk electrode (RDE) was studied by dynamic electrochemical impedance spectroscopy (dEIS). The effect of rotation on the impedance spectra was found and the spectra were fitted to equivalent circuits to extract information about the charge-transfer resistance and the double-layer capacitance. Increasing the rotation rate resulted in a positive shift for the onset potential for platinum oxide formation together with a reduction in the growth rate. Also the rate of platinum oxide reduction was increased when increasing the rotation rate. It was suggested that the oxygen-containing species consumed during the CO oxidation reaction is related to the platinum oxide formation and reduction reactions.

The methanol and ethanol oxidation reactions were, for the first time, studied with electrochemical techniques in an autoclave setup with temperatures ranging from room temperature and up to 140 ◦C. Cyclic voltammetry results showed that the onset potentials for the oxidation reactions are significantly shifted to more negative potentials while the peak potentials are shifted to more positive potentials. At high temperatures, linear slopes in the cyclic voltammograms were observed for certain experimental conditions, contrary to the more typical exponential shape. As an explanation, limitations by a chemical reaction step is proposed.

Also electrochemical impedance spectroscopy (EIS), both steady state and dynamic, was employed to study oxidation of methanol and ethanol. The spectra were interpreted in terms of equivalent circuits corresponding to none, one, or two independently adsorbed intermediates present on the surface. At low temperatures, evidence for two adsorbed intermediates was found for both molecules. This was reduced to only one adsorbed intermediate at higher temperatures. For methanol oxidation, this was interpreted as a reduction in the contribution from the direct pathway when increasing the temperature. The species present on the surface were discussed with basis in the number of adsorbed species. The impedance spectra also revealed potential regions with hidden negative differential resistance (HNDR) and the possible presence of Hopf bifurcation. Interestingly, the HNDR signature diminished and eventually disappeared with increasing temperature.

The adsorption time constants were calculated from the fitted circuit elements. These remained always positive for the entire potential scan during oxidation of dissolved CO, methanol, and ethanol. This was interpreted as no observed, or experimental evidence of, nucleation-growth-collision (NGC) behaviour in the oxidation processes with the given experimental conditions.