A major limitation for the development of reversible solid oxide cells (R-SOCs) is the delamination between the oxygen electrode and electrolyte in electrolysis mode. Previously, Virkar reported that higher electronic conductivity within the predominantly ionically conducting electrolyte led to a lower tendency for the formation of high internal pressure ^[1]. Therefore an electrolyte having a small amount of electronic conduction may be beneficial for relieving the delamination between the oxygen electrode and electrolyte in R-SOCs. Strontium- and magnesium-doped lanthanum gallate (LSGM) has been widely investigated as an electrolyte for medium-low temperature (400-650 ºC) R-SOCs due to its high ionic conductivity and compatibility with some perovskite electrodes. Compared to well-established yttria stabilized zirconia electrolytes, which have ionic transference numbers close to 1 across a wide range of oxygen partial pressures, LSGM can exhibit non-negligible p-type electronic conductivity under oxidizing conditions ^{[2, 3]}. Furthermore, limited studies have demonstrated enhanced electronic transference numbers at grain boundaries compared to the bulk, attributed to space charge effects ^{[3, 4]}. Therefore, it is of interest to understand the impact of nanostructuring and grain boundaries on the ionic and electronic transport properties of LSGM for relieving the delamination issue in R-SOCs.

In this work, two methods - field assisted sintering technique/spark plasma sintering (FAST/SPS) and traditional pressureless sintering were employed to prepare dense La_0.9Sr_0.1Ga_0.9Mg_0.1O_3-δ (LSGM9191) pellets from nanopowders synthesized by the Pechini method. Post-sintering anneals were applied to the FAST/SPS pellets to oxidize them and control grain size. Microstructure was evaluated by scanning and transmission electron microscopy with EELS/EDS line scans at grain boundaries, and transport properties were measured by 2-point ac-impedance spectroscopy over the temperature range using porous Pt/Ag electrodes. Equivalent circuit fitting with application of microstructural models was applied to evaluate local conductivities. The impact of grain size on grain core (gc), grain boundary (gb) and total conductivity were studied. Grain size did not affect the gc conductivity, and the gb and total conductivity increased with grain growth. Specific gb conductivity decreased with increasing grain size.

Additionally, the dependence of the conductivities on oxygen partial pressure (pO2) and applied dc bias was measured to assess any electronic contribution to the conductivity. We found that the gb conductivities of LSGM9191 pellets prepared by FAST/SPS with different grain sizes (ranging from ~100nm to 800 nm) did not exhibit pO₂ dependence, showing different behavior than some previous research ^[5]. The dependence of gb conductivity on dc bias (both in N₂ or 21% O₂) was also smaller than previous reports ^{[6, 7]}. Therefore, the electronic conductivity and space charge effect was limited, which might be caused by the high dopant content in the present work compared to previous studies and/or by impurities/ amorphous material/ organic residue trapped at the GBs as a result of the FAST/SPS process. However, the gb conductivity of a LSGM9191 pellet prepared by pressureless sintering did show a small pO₂ dependence, indicating some p-type conductivity, albeit lower than in previous reports. The results suggest that factors such as grain size, doping level, grain boundary chemistry, and processing routes could be modified to tailor mixed conduction in LSGM for R-SOC applications.

Acknowledgements

Support from WPI-I²CNER to NHP and a JSPS Fellowship (201702103) to TC are gratefully acknowledged.

Reference

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[4] N.H. Perry, Local electrical and dielectric properties of nanocrystalline solid oxide fuel cell electrolytes, Northwestern University, Ph.D. thesis, 2009

[5] H.J. Park et al., Mixed conduction behavior in nanostructured lanthanum gallate, Electrochemistry Communications 11(2009) 962-964

[6] C.T. Chen et al., Current–voltage characteristics of grain boundaries in polycrystalline Sr-doped LaGaO₃ Phys. Chem. Chem. Phys., 2012, 14, 9047–9049

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