Microfluidic and high temperaturetechniques for aqueous electrochemistry

Abstract

Liquid organic fuels are highly desired energy carriers as they have a high energy density, simple production, and can benefit directly from existing infrastructure. However, the electrochemical conversion efficiency is low due to slow kinetics. An improved understanding of the reaction mechanism is paramount to make these types of fuels a viable alternative in PEM based fuel cells. In this thesis, we have identified and developed two techniques that allowed us to expand the current knowledge. These were (1) to make a method that could study aqueous electrochemistry at temperatures above the normal boiling point of water, and (2) to find a way to make reproducible microfluidic fuel cells that ultimately can be used as an electroanalytical tool. Both of these methods can potentially be combined with spectroscopic techniques and dynamic electrochemical impedance spectroscopy (dEIS) to expand the scope of a study. Based on current knowledge of reaction kinetics, the main objectives of this thesis were: 1. Develop a robust electrochemical method to study alcohol oxidation and other processes in aqueous solutions at high temperatures. 2. Get a better understanding of the temperature dependence of alcohol oxidation by studying these processes at elevated temperatures. 3. Use dynamic electrochemical impedance spectroscopy to investigate the surface processes at the electrode. 4. Develop numerical and analytical methods for describing microfluidic channel electrodes and employ these for characterization purposes as a step on the way to utilizing microfluidic flow cells as a rotating ring-disc analog for kinetic investigations. A method for running aqueous temperatures in a self-pressurized autoclave was developed. After solving problems related to solution resistance, seal quality, and temperature expansion, the method was successfully used to do conventional electrochemical experiments at temperatures up to 140◦C in aqueous electrolytes. The self-pressurized autoclave was used to study methanol and glycerol oxidation at platinum electrode for temperatures from room temperature up to 140◦C. This allowed for efficient acquisition of kinetic parameters such as onset potentials, Tafel slopes, and activation energies. For methanol oxidation, the dissociative adsorption of either methanol or water was suggested to be the rate-determining step at the relevant potentials for fuel cells. For glycerol oxidation, an apparent change in mechanism was observed at 110◦C. At temperatures below this, either dissociative water adsorption or succeeding reaction step is suggested to be rate-determining. At temperatures above 110◦C, dissociative glycerol adsorption was found to be rate-determining, suggesting that the role of water as an oxygen donor was reduced. This indicated that the glycerol reaction to glyceraldehyde was the main reaction occurring, especially at low overpotentials. The high temperature autoclave method was used to study platinum oxidation as a function of temperature combined with dynamic electrochemical impedance spectroscopy. The oxidation mechanism was found to be similar at all temperatures, while the reduction mechanism was dependent on the thickness of the platinum oxide layer. Little temperature dependence on the fitted impedance parameters were found and it was evident that the surface oxide charge density was a more important factor than the temperature for the value of the fitted parameters. A semi-analytical method that solved the governing convective-diffusion equation for channel electrodes was developed by using an eigenvalue method implemented in Maple. This method resulted in a semi-analytical solution by assuming that axial diffusion was negligible. This was in contrast to previous derivations assuming both axial diffusion and a linear flow regime. The semi-analytical method showed excellent results when compared to the numerical result for a larger parameter space than the analytical solutions reported in literature. A method for making microfluidic electrochemical cells of high quality was developed. These microfluidic cells were characterized by using electrochemical impedance spectroscopy at a Pt electrode in an electrolyte with a reversible redox couple. The results were numerically modelled and the model was used to define the zones of validity of previous analytical solutions, where no axial diffusion and L´evˆeque approximation were assumed. A new method for performing impedance in a two subsequent channel electrode setup using only a single potentiostat was demonstrated. This method was named downstream impedance, and it gives beautiful spirals. Through numerical modelling and normalization of the experimental data, the qualitative features of the resulting impedance were investigated and potential use of this method was discussed.