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Study of reversible electrode reaction and mixed ionic and electronic conduction of lithium phosphate electrolyte for an electrolchemical co2 gas sensor

Lee, Chong-Hoon
http://rave.ohiolink.edu/etdc/view?acc_num=osu1073047249

Abstract Details

2004, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
An electrochemical CO2 gas sensor with lithium ion conductor was developed and characterized in order to examine the potential for real-life applications and understand its sensing mechanism. Li2CO3 and Li2TiO3+TiO2 mixture were used as a sensing and a reference auxiliary phase, respectively. This electrochemical cell with a solid state Li3PO4 electrolyte has shown good selectivity, sensitivity and linear response in laboratory and automobile exhaust tests. However, the sensor response to CO2 gas showed a systematic deviation from the Nernst equation. Measured EMF did not agree with that calculated from the Nernst equation, even though it followed logarithmic behavior. Moreover, high sensitivity was observed for high CO2 concentrations (5~50%), compared to that for concentrations (500~5000 ppm). Two possible reasons for this deviation are: (1) reversibility of electrode reaction and (2) mixed ionic and electronic conduction of the electrolyte. Unless electrode reaction is fast enough, electrode polarization can easily induce overpotential. Pure ionic conduction of electrolyte is also necessary to avoid EMF loss during open circuit potential measurement. EIS (Electrochemical Impedance Spectroscopy) was used to study electrode kinetics. We found that Li2TiO3+TiO2 mixture reference electrode reaction is sluggish showing large electrode impedance. This impedance, however, was not affected by gas concentration change. On the other hand, that at the Li2CO3 sensing electrode is relatively small and it increased with decreased CO2 and O2 concentration. It was also observed that these electrode impedances induced the overpotential when the current flowed through the sensor. This electrode overpotential problem was minimized by mixing gold powder or porous sputtered gold electrode increasing effective reaction sites of the electrode. New electrode design improved the sensor EMF closer to the Nernstian values, however, the discrepancy still remained. Moreover, at higher sensor operating temperatures (T>500°C), the sensitivity deviated even further from the Nernstian value. Therefore, the temperature dependence of the current sensor clearly indicates that the non-Nernstian behavior is not just due to non-reversible electrode reaction. More significant effect on the non-Nernstain behavior is due to mixed ionic and electronic conduction of Li3PO4 electrolyte. Based on the EMF measurement and a modified Nernst equation, the transference number was estimated and the conduction domain boundary separating the n-type from the ionic conduction was constructed. This calculation predicted that the sensing side Li activity would be such that the electrolyte would be a mixed conduction (electronic and ionic) domain. Hebb-Wagner (HW) DC polarization measurement also confirmed a significant n-type electronic conduction of Li3PO4 electrolyte. The transference numbers obtained from the EMF measurement and the HW DC polarization measurement were compared and the results confirmed that the origin of the non-Nernstian sensor behavior is mainly due to the mixed conduction of Li3PO4 electrolyte at high temperatures (>500°C).
Sheikh Akbar (Advisor)
Engineering, Materials Science
Electrochemical sensor; Mixed Ionic and Electronic conductor; CO2; Reversible electrodes; Gas sensors; Ceramic

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Citations

  • Lee, C.-H. (2004). Study of reversible electrode reaction and mixed ionic and electronic conduction of lithium phosphate electrolyte for an electrolchemical co2 gas sensor [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1073047249

    APA Style (7th edition)

  • Lee, Chong-Hoon. Study of reversible electrode reaction and mixed ionic and electronic conduction of lithium phosphate electrolyte for an electrolchemical co2 gas sensor. 2004. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1073047249.

    MLA Style (8th edition)

  • Lee, Chong-Hoon. "Study of reversible electrode reaction and mixed ionic and electronic conduction of lithium phosphate electrolyte for an electrolchemical co2 gas sensor." Doctoral dissertation, Ohio State University, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=osu1073047249

    Chicago Manual of Style (17th edition)

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© 2004, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.