Abstract
Modified point defect chemistry at grain boundaries can be leveraged to turn a predominantly ionic conductor or a predominantly electronic conductor into a mixed conductor, by changing the grain boundary density [1,2]. There are reports indicating that introducing some electronic conductivity into the ionically conducting electrolyte of a solid oxide electrolysis cell could help to alleviate the problem of delamination at or near the electrolyte-oxygen electrode interface [3]. For this reason, we revisited the possible role of grain boundaries in tailoring mixed conduction in the fast oxide-ion conductor La0.9Sr0.1Ga0.9Mg0.1O3-x (LSGM). LSGM is a candidate electrolyte as it exhibits good thermo-chemo-mechanical compatibility with adjacent perovskite-structured electrodes. Previous work has indicated enhanced p-type electronic transference numbers at the grain boundaries of LSGM compared to the bulk, attributed to space charge effects on the basis of the observed non-linear current-voltage behavior of the grain boundaries at sufficiently high voltages [4]. However, the interplay of processing route, grain size, and dopant concentration in tailoring the extent of this effect had not been investigated.
In this work [5] we fabricated dense LSGM pellets with non-dilute doping levels typical of electrolytes, with a range of average grain sizes from ~100 nm to ~6 μm, using field- and pressure- assisted sintering with various post-annealing steps as well as conventional sintering. The electrical transport behavior of grain cores and grain boundaries (GBs) was subsequently studied by impedance spectroscopy as a function of temperature, oxygen partial pressure, and dc bias. The brick layer model and the nano-grain composite model were applied to determine specific GB conductivities and electrical widths from the impedance data. Structure and composition were evaluated by X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, scanning transmission microscopy, and energy dispersive X-ray spectroscopy.
With increasing sintering temperature (and grain size), the following trends were observed: increasing pO2-dependence of the GB conductivity indicating higher local electronic transference numbers, higher apparent GB potentials, lower specific GB conductivity, lower pre-exponential factor for specific GB conductivity, increasing electrical GB widths, and greater dc bias dependence of GB conductivity. On the other hand, as expected, grain core electrical behavior was independent of processing route, grain size, and pO2. The results are suggestive of an increasing grain boundary space charge effect with increasing sintering temperature (and grain size), as has been observed for other ionic conductors where grain size and sintering temperature/time were coupled. Additionally, in all cases, the GB electronic transference numbers in these highly doped samples appeared to be smaller than those reported for lightly doped LSGM. Microscopy and EDS revealed that while grain boundaries appeared to be well-crystalline and "clean" in all samples, there was increasing microstructural and compositional homogeneity in samples that were sintered at progressively higher temperatures. The significance of the nanoscale compositional and microstructural fluctuations in the small-grained samples is not yet clear, as the apparent phase purity and density of all the samples was very similar macroscopically. Nonetheless, it seems possible that such inhomogeneity (pores and Sr- rich and Mg-rich areas not limited to GB regions) might limit the space charge contribution at GBs, and that some of the pores could have arisen from residual volatile impurities trapped during the pressure-assisted sintering. Overall, the results confirm that 1) grain boundaries can be sites of mixed conduction in an otherwise predominantly ionically conducting electrolyte and 2) processing route (thermal, pressure, and electrical history) is an important variable for tailoring the magnitude of the local ionic and electronic conductivities and transference numbers.
[1] Y.M. Chiang et al., Applied Physics Letters, 69(2), 185-187 (1996).
[2] A.M. Saranya et al., Advanced Energy Materials, 5(11), 1500377 (2015).
[3] A.V. Virkar et al., International Journal of Hydrogen Energy, 40, 5561-5577 (2015).
[4] C.T. Chen et al., Physical chemistry chemical physics, 14, 9047-9049 (2012).
[5] T. Chen et al., "Modifying grain boundary ionic/electronic transport in nano-Sr- and Mg- doped LaGaO3-δ by sintering variations," (under review)