Ion Transport and Surface Kinetics in Oxygen Ion Conducting Nanocrystalline Oxide Thin Films

Nanocrystalline oxide thin films, e.g., doped oxides of zirconia and ceria films, exhibit extraordinary electrical and electrochemical properties that are of great interest for applications such as solid oxide fuel cells (SOFCs). In nanocrystalline doped oxide thin films fabricated by atomic layer deposition (ALD) or pulsed laser deposition (PLD), interfacial properties are often found to dominate the electrochemical properties. For example, grain boundary plays a crucial role in ionic transport and surface exchange reactions. Ionic transport across grain-boundaries(GBs) is known to be hindered due to an electrical potential barrier built up by defect segregation, while exchange reaction is enhanced near the GBs due to the higher concentration of oxide ion vacancies. Such exotic electrochemical properties of oxide thin films are closely related to the distribution of defects near GBs.

Along these lines, we investigated the defect structure near GBs in doped oxide thin films as well as the surface exchange and the ionic transport inside the films associated with grain size and the grain boundary density. We report on the atomic scale study of the defect structure near a single GB of yttria-stabilized zirconia (YSZ) [1]. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width of 0.6 nm, which corroborates well with electron energy loss spectroscopy (EELS) measurements. Additionally, we recently conducted near-atomic-scale EELS study on gadolinia-doped ceria (GDC) thin films [2]: Homogeneous distribution of dopants was observed in as-deposited films, while strong segregation of dopant was shown in the annealed (1100°C) film a width of 1.2 nm to 1.5 nm and an enhancement factor  of 1.9.

It has been reported that grain boundaries at the electrode(cathode)/electrolyte interface are of great importance in enhancing the oxygen reduction kinetics in SOFCs. Oxide-ion incorporation is greatly facilitated by these surface grain boundaries [3]. Thin-film electrolytes with high surface grain-boundary density consistently show higher exchange current density, and therefore, smaller activation loss. These experimental observations are supported by secondary ion mass spectroscopy (SIMS) and electrochemical impedance spectroscopy (EIS) studies on doped zirconia and doped ceria thin films [3,4]. Polycrystalline YSZ and YDC were annealed in an ¹⁸O₂ environment so that the exchange reaction between ¹⁶O₂ inside YSZ and YDC and ¹⁸O in the environment can take place. Surface mapping using NanoSIMS clearly indicates the preferential enrichment of ¹⁸O along grain boundaries. The activation resistance at the cathode interface is significantly reduced by an electrolyte surface featuring a high density of grain boundaries that facilitates high surface exchange rates. In parallel, surfaces modified by compositionally graded cation doping layers of ~1 nm facilitate enhanced oxide-ion vacancy concentration and improve the power density of the thin-film SOFC by lowering the activation loss [5].

It is known that grain boundaries consisting of grain boundary core and the adjacent space charge layer pose a blocking effect on cross-boundary transport of ions. We show that thermal annealing, which in turn facilitates defect segregation and redistribution, plays an important role on the ionic conductivity of oxide thin films [2,6]. Nanocrystalline thin films of YDC and GDC were fabricated and then post-annealed for grain growth. Interestingly, as the grain size increases, ionic transport is indeed hindered, showing higher activation energy and lower ionic conductivity than the as-deposited films, which have the smallest grains. We speculate that the uniform distribution of dopants may attenuate the space charge layer barrier height and maintain high ionic conductivity, while the annealed films have fully developed space charge layer, which impedes ionic transport.

In summary, the grain size and the grain boundary effects have a huge impact on surface exchange and ionic transport. We show that fundamental understanding and surface engineering of grain structures in oxide thin films may have significant implications in improving the electrochemical performance of thin film SOFCs, and therefore, widening their applications.

References

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