In Situ Diagnosis of Electrolytic and Fuel Cells using Electrochemical Impedance Spectroscopy
Open Access
- Author:
- Khurana, Sanchit
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 11, 2014
- Committee Members:
- Serguei Lvov, Dissertation Advisor/Co-Advisor
Serguei Lvov, Committee Chair/Co-Chair
Derek Elsworth, Committee Member
Michael Anthony Hickner, Committee Member
Jeremy Michael Gernand, Committee Member - Keywords:
- Fuel cells
batteries
degradation
CuCl electrolyzer
Electrochemical Impedance Spectroscopy - Abstract:
- In situ EIS data are presented for the CuCl/HCl electrolyzer and Liquid Metal Anode Solid Oxide Fuel Cells (LMA-SOFC) operating under different conditions. The durability of the CuCl electrolyzer operating for 168 hours while maintaining the high current density of 0.3 A cm-2 under 0.7 V is reported for the first time. The impedance spectra of the cell, along with the polarization curves (V-I data), show the effect of operating temperature and contact pressure at the end plates of the membrane electrode assembly (MEA). After the durability test, the potential required to maintain the cell at 0.3 A cm-2 increased from 630 mV of the fresh membrane electrode assembly (MEA) to 710 mV. The increase in the ohmic resistance of the membrane by 41 % was observed to be the primary cause of degradation. Starting from 0.6 Ω cm2, there was a small change in the ohmic resistance of the cell during the first 90 hours, followed by a significant increase of 25 %, and then attained a steady value of 0.85 Ω cm2. Simultaneously, a change in the decomposition potential was observed as it increased from 0.18 V in the beginning to 0.25 V at the end of testing. This is mainly due to the decrease in efficiency of the regeneration column and dilution of the anolyte solution over time. Further, the use of EIS in the through-plane conductivity cell proved to be a reliable and time-efficient method for evaluation new and existing membranes before testing in the electrolyzer. A liquid metal anode solid oxide fuel cell (LMA-SOFC) is constructed to study the kinetics and transport properties of the system. The behavior of the system is investigated by operating the cell as a metal-air battery while operating under argon, and as a fuel cell with hydrogen and coal as the fuel feeds. EIS signatures and OCP analysis provides insight into the reaction mechanism and indicate the formation of a SnO2 layer at the electrolyte/anode interface. The OCP of 0.885 and 1.117 V was observed for the coal and hydrogen powered fuel cell, respectively. The results show the gradual increase in efficiency of the reduction of SnO2 by using carbon and hydrogen as the fuels. The EIS spectra obtained for the hydrogen fed cell was a characteristic of the diffusion controlled systems and equivalent circuit modeling was used to calculate the oxygen diffusion coefficients. The effective oxygen diffusion coefficients of 1.9 10-3 cm2 s-1 at 700 ºC, 2.3 10-3 cm2 s-1 at 800 ºC and 3.5 10-3 cm2 s-1 at 900 ºC are similar to the published results. The resistance added by the SnO2 layer was the primary cause of degradation and further improvements in performance rely heavily on minimizing losses in the liquid Sn layer.