The high temperature oxygen surface exchange kinetics of mixed conducting oxides play a critical role in the efficiency of solid oxide fuel/electrolysis cells (SOCs). Recently, an in situ Optical Transmission Relaxation (OTR) approach has been applied to quantify the oxygen surface exchange coefficients (k_chem) of thin films, with the advantage of providing contact-free, in situ and continuous measurements of native surfaces. The technique relies upon the application of the Beer-Lambert law, where optical absorption is proportional to the concentration of absorbing species, e.g. oxidized Pr (~Pr⁴⁺) in Pr_xCe_1-xO_2-δ (PCO) ^[1,2] or oxidized Fe (~Fe⁴⁺) in SrTi_1-xFe_xO_3-δ (STF), and to the oxygen stoichiometry via electroneutrality. For example in STF, during reduction (oxygen evolution), absorbing Fe⁴⁺ is replaced by Fe³⁺, resulting in an increase in measured optical transmission through the STF with time, which can be described by the surface exchange-limited kinetics equation to determine k_{chem .} ^[3].

As we know, the grain size and degree of crystallinity could affect the thin films’ electrical, optical and catalytic properties. In our previous work, we examined the impact of crystallinity, grain boundaries, orientation, and surface chemistry on k_chem for STF (x=0.35, STF35) by preparing films of different structures by pulsed laser deposition under different conditions. We found that fast oxygen surface exchange needs both crystallinity (typically obtained at high growth temperatures) and low Sr surface concentration (typically obtained at low growth temperatures). Amorphous STF thin films were not able to exhibit optically measurable oxygen exchange, while excellent crystallinity, obtained at high growth temperatures, coexisted with high Sr segregation and therefore non-optimal k_chem. However, crystallization at intermediate temperatures was used to obtain much faster surface exchange kinetics ^[4].

In this work, we further studied the effect of crystallization on oxygen surface exchange kinetics, by post-annealing as-grown amorphous STF35 thin films and observing the evolution of their oxygen exchange behavior in situ during crystallization. The impact of annealing temperature (500-800 °C) and time (2-100 h) on the crystallinity and microstructure was observed by scanning probe microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and in situ optical transmission changes. The consistent appearance of a significant decrease in optical transmission during heating suggested that crystallization began around 545-550 ºC. The best k_chem was found for annealing at 600 ºC for 2 h. The thin film with lower (or perhaps no) crystallinity annealed at 500 °C was not able to exhibit detectable oxygen surface exchange, while higher annealing temperatures and/or times led to non-optimal k_chem, which is consistent with our previous research.

In order to assess whether the benefits of crystallization under mild conditions for rapid surface exchange also applied to other compositions and structures, we investigated the surface exchange kinetics during in situ crystallization of other materials, such as perovskite SrTi_0.65Co_0.35O_3-δ (STC35), fluorite Pr_0.1Ce_0.9O_2-δ (PCO), and Ruddlesden-Popper Sr₂Ti_0.65Fe_0.35O_4±δ (RP-STF35). Optical relaxations were observed in these materials, enabling the quantification of their k_chem. For the perovskite STC35 and RP-STF35 thin films, in-situ crystallization was observed to benefit the oxygen exchange kinetics. On the other hand, for a fluorite PCO thin film, we found that it was already crystalline after deposition at 25 ºC. Therefore, the annealing process for PCO thin film did not show a positive effect on k_chem. Implications for use of thin film electrodes in intermediate temperature devices will be addressed.

Acknowledgements

Support from WPI-I²CNER and a JSPS Kakenhi Grant-in-aid for Young Scientists (B) project number JP15K18213 (to N. H. Perry) and JSPS Fellowship (201702103) are gratefully acknowledged.

Reference

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[2] J. J. Kim, S. R. Bishop, N. J. Thompson and H. L. Tuller, “Investigation of redox kinetics by simultaneous in situ optical absorption relaxation and electrode impedance measurements: pr doped ceria thin films”, ECS Transactions, 57 (1) 1979-1984 (2013)

[3] I. Denk, F. Noll and J. Maier, “In situ profiles of oxygen diffusion in SrTiO3: bulk behavior and boundary effects”, Journal of the American Ceramic Society, 1997, 80, 279-285.

[4] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach”, Journal of Materials Chemistry A, 2017, 5, 23006-23019