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.
Has parts
Holm, Thomas; Sunde, Svein; Harrington, David A.;Seland, Frode. Using a self-pressurized autoclave for aqueous electrochemical experimentsHolm, Thomas; Dahlstrøm, Per Kristina; Bruheim, Odine; Sunde, Svein; Harrington, David A..; Seland, Frode. Method for studying high temperature aqueous electrochemical systems: Methanol and glycerol oxidation. © 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Holm, Thomas;Sacci, Robert L.; Sunde, Svein; Seland, Frode; Harrington, David A.. Dynamic Electrochemical Impedance Spectroscopy Study of Platinum Oxidation at High Temperatures
Holm, Thomas; Sunde, Svein; Seland, Frode; Harrington, David A.. A semianalytical method for simulating mass transport at channel electrodes. Journal of Electroanalytical Chemistry 2015 ;Volum 745. s. 72-79 http://dx.doi.org/10.1016/j.jelechem.2015.03.019 © 2015. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Holm, Thomas; Ingdal,; Fanavoll, Espen Vinge; Sunde, Svein; Seland, Frode; Harrington, David A. Mass-transport impedance at channel electrodes: Accurate and approximate solutions Electrochimica Acta, Volume 202, 1 June 2016, Pages 84–89 http://dx.doi.org/10.1016/j.electacta.2016.03.096 © 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Holm, Thomas; Ingdal, Mats; Strobl, Jonathan; Fanavoll, Espen Vinge; Sunde, Svein; Seland, Frode; Harrington, David A. Mass transport impedance at channel electrodes: Using a double electrode setup