Measurement of Oxygen Potential Distribution on the Surface of a Mixed Conducting Electrode
Authors: Takasu Toshiki , Yashiro Keiji , Kawada Tatsuya 
Published: February 2020

Introduction

Mixed ionic and electronic conductors (MIEC) are widely used as cathodes of intermediate temperature SOFCs. With a mixed conducting cathode, electrode reaction proceeds mainly through oxygen incorporation reaction at gas/electrode double phase boundary (DPB) followed by diffusion of oxide ion in the electrode particles. However, in certain conditions, the reaction through TPB (gas / electrode / electrolyte triple phase boundary) via surface diffusion may not be negligible. The contributions of TPB and DTB paths to the total process remain unclarified, but should be understood in a quantitative way to optimize the electrode design. An experimental method to evaluate the behavior of oxygen “on” and “in” the MIEC electrode is required. Kawada et al. proposed a potentiometric method to measure oxygen potential “on” the surface of a polarized electrode using a porous electrolyte that was made softly in contact with the surface of an electrode [1]. They explored the method to a position-resolved measurement to evaluate the surface diffusion of oxygen. Although it appeared promising, the reliability and reproducibility of the method was left unestablished. On the other hand, Hendriksen et al. used the similar technique to measure the local oxygen potential “in” the solid, by tightly pushing a pointed cone of an electrolyte on the surface of a MIEC [2]. The difference of those results could be from the way how the probe touches the sample surface, which makes the uncertainty of this technique. This paper revisits the experiments of oxide ion probes aiming at establishing the method and its interpretation. Discussion will be made on possibility of using this technique for studying surface reaction mechanism in detail.

Methods

2.1 Oxide ion probe

We made an oxide ion probe by coating a Pt wire with the porous Ce0.9Gd0.1O1.95 (GDC). The probe was made in contact with the MIEC electrode La0.6Sr0.4CoO3-δ (LSC64). In order to make the contact force as constant as possible, the probe was slowly brought into contact with the electrode surface and pressed until the error of the potentiostat disappeared and the measurement could be started.

2.2. Measurement condition

The test cell was composed of GDC electrolyte, LSC64 thin film as Working Electrode (WE), LSC64 porous as Counter Electrode (CE), and Pt paste as Reference Electrode (RE). Measurements were made by three terminal electrochemical impedance spectroscopy method. Temperature dependence was measured from 723 K to 873 K. Oxygen partial pressure dependence was measured from 100ppm to 100%. The frequency response of the probe was evaluated by dividing the probe potential by the cell current.

Results and Discussion

Temperature and partial pressure dependences of the WE impedance and probe response are shown in Fig.1. At 873 K, the frequency response of the probe and WE were close to each other although the probe response did not include high frequency resistance which could be assigned to the contribution of the electrolyte. The WE and probe responses similarly increased as the oxygen partial pressure became lower. On the other hand, temperature dependence was different between WE and probe responses. WE resistance increased more remarkably than probe response when the temperature was decreased. A possible cause for the difference in the responses of the probe and WE is the resistance of oxide ion diffusion through the film. The oxide ion conductivity of LSC64 was estimated from the vacancy diffusion coefficient reported in [3] and vacancy concentration obtained from the chemical capacitance of the film. The roughly estimated diffusion resistance was 0.14 Ω at 873 K, 8.81 Ω at 773 K, and 32.7 Ω at 723 K, which are much smaller than the reaction resistance of WE. Additionally, the high frequency response of the impedance did not show 45° line in the Nyquist plot. So Thus, it can be said that the difference between the probe and WE is not due to the contribution of diffusion. Another possible factor is the oxygen incorporation resistance. Budiman et al. suggested the existence of a barrier layer just below the surface of the LaNi0.6Fe0.4O3-δ electrode [4]. A similar but less critical barrier layer could exist on LSC64. This also suggests the oxide ion probe measures at least the above the barrier layer, that is, almost the surface of the electrode.

References

[1] T. Kawada, et al, Electrochemical Society Proceedings Volume 2007-07, 470-477, (2003)
[2] B. T. Dalslet, M. Søgaard, and P. V. Hendriksen, Electrochemical Society, 154 (12) B1276-B1287 (2007)
[3] A. V. Berenov, A. Atkinson, J. A. Kilner, E. Bucher, W. Sitte, Solid State Ionics, 181, 819 (2010)
[4] R. Budiman, MS Thesis, Tohoku University (2013)

Figure 1