A Comparison of Strontium Titanium Iron Oxide Perovskite Oxygen Surface Exchange Coefficients Obtained from Wafer Curvature vs. Optical Relaxation
Authors: Ma Yuxi , Chen Ting , Perry Nicola H. , Nicholas Jason D. 
Published: February 2020

Introduction

The oxygen surface exchange coefficient (kchem
) is a material property determining the speed of oxygen exchange in the mixed ionic electronic conductors (MIEC) used in oxygen sensors, Solid Oxide Fuel Cells, Solid Oxide Electrolysis Cells, mechano-chemical actuators, and other electrochemical devices. Historically, in-situ techniques like electrical impedance spectroscopy (EIS) and electrical conductivity relaxation (ECR) have been used to measure kchem
(1, 2). Unfortunately, these techniques require noble-metal electronic current collectors that may alter the underlying MIEC stress state (potentially altering MIEC kchem
performance through mechano-chemically-induced point defect concentration changes) or interfere with an accurate MIEC kchem
measurement through catalysis of the oxygen exchange reaction (3, 4). Here, the kchem
of identically processed and aged SrTi0.65Fe0.35O3-δ (STF35) thin films were measured by curvature relaxation (κR) and optical transmission relaxation (OTR) to evaluate the consistency of various in-situ, non-contact, current-collector-free kchem
measurement techniques.

Experimental Details

~200-500 micron thick, (100)-oriented, single crystal (Y2O3)0.13(ZrO2)0.87 (YSZ) substrates were coated with 250 nm thick, (110)-oriented, columnar-grained STF35 thin films via pulsed laser deposition (PLD) at 700 oC, in a 5 Pa O2 atmosphere, with a 5 Hz laser repetition rate. The films were then aged with a multi-step thermal ramp to 850 oC to increase their electrochemical stability, and then tested with either κR or OTR upon cooling. Note, κR relaxations were performed by switching between 100% compressed air and 10 times diluted compressed air (i.e. 10% compressed air - 90% nitrogen). In contrast, OTR measurements were performed by switching between desiccated synthetic air (with a composition of 20% oxygen – 80% nitrogen) and 5 times diluted desiccated synthetic air (with a composition of 4% oxygen – 96% nitrogen).

Results and Discussion

Figure 1 shows STF35 kchem
values obtained from κR and OTR. Both techniques yielded kchem
values of similar magnitude and activation energy (2.7 ± 0.1 eV). Additional tests are underway to identify the source of the slight discrepancy observed between the two techniques. However, the similarity of these results is a promising first step in efforts to validate different current-collector-free kchem
measurement techniques and eliminate different measurement techniques as a source of the large kchem
variation observed for “identical materials” tested under “identical conditions”.

References

D. Chen, S. R. Bishop and H. L. Tuller, J Electroceram, 28, 62 (2012).

J. E. ten Elshof, M. Lankhorst and H. J. Bouwmeester, Journal of the Electrochemical Society, 144, 1060 (1997).

Y. Ma and J. D. Nicholas, Physical Chemistry Chemical Physics, 20, 27350 (2018).

N. H. Perry, J. J. Kim and H. L. Tuller, Science and Technology of Advanced Materials, 19, 130 (2018).

Figure 1.

Oxygen surface exchange coefficients measured from curvature relaxation and optical relaxation techniques. Note, as described in the experimental methods, these identically processed samples were tested in slightly different atmospheres.

Figure 1