SUBJECT: Ph.D. Dissertation Defense
BY: Pablo Salazar Zarzosa
TIME: Friday, August 1, 2014, 2:30 p.m.
PLACE: Marcus Nanotechnology building, 1116
TITLE: Modeling and Experiments to Develop Thermo-electrochemical Cells
COMMITTEE: Dr. Baratunde Cola, Chair (ME)
Dr. Thomas Fuller (ChBE)
Dr. Peter Hesketh (ME)
Dr. Satish Kumar (ME)
Dr. Sankar Nair (ChBE)
Dr. Todd Sulchek (ME)


Low-temperature waste heat recovery is an important goal in order to generate a more efficient, cost-effective and environmental-friendly energy source. To this goal, thermo-electrochemical cells (TECs) are electrochemical devices that produce a steady electric current under an applied temperature difference between the electrodes. Besides having zero gas emissions, TECs can be flexible and relatively inexpensive. However, current TECs have low conversion efficiencies. As a starting point, I developed a comprehensive multiscale model that couples the governing equations in TECs (mass and heat transfer, electro-kinetics and fluid dynamics). The model was used to understand the fundamental principles and limitations in TECs, and to find the optimum cell thickness, aspect ratio and number of cells in a series stack. Doped multiwall carbon nanotubes (MWCNTs) are then explored as alternative electrodes for TECs. The results show that nitrogen and boron doping of MWCNT buckypaper, through plasma-enhanced chemical vapor deposition, increases the electrochemically active surface area of the electrodes. However, electrostatic interactions with potassium ions, in potassium ferri/ferrocyanide electrolytes, reduces the charge transfer kinetics of doped electrodes; resulting in lower power of TECs. One of the main objectives of this dissertation is the study of multi-wall carbon nanotube/ionic liquid (MWCNT/IL) mixtures as alternative electrolytes for TECs. Previous authors showed that the addition of carbon nanotubes (CNTs) to a solvent-free IL electrolyte improves the efficiency of dye solar cells by 300%. My research plan involved a spectroscopy analysis of imidazolium-based ionic liquids (IILs) mixed with MWCNTs using impedance spectroscopy and nuclear magnetic resonance. The results show a percolation threshold below 1 wt% of MWCNTs due to their high dispersibility in IILs. The addition of MWCNTs increases the diffusion coefficient of the anions up to 35%, which is likely due to weak van der Waals interactions between the MWCNT walls and the cations. The MWCNTs also appear to polarize their interface with IILs, yielding a 3 to 5-fold increase in electrical conductivity of the mixture at MWCNT concentrations that are under the threshold for percolation. The results show that the combination of these effects (interfacial polarization and ion pair dissociation) reduces mass transfer resistances and enhances the power of TECs at low wt% of MWCNTs, in spite of reduction of open circuit voltage due to percolated networks.