Multinuclear Solid-State NMR Studies of Ionic Conduction Mechanisms in Low-Cost and Rare-Earth-Free Superior Fast Oxide-Ion Conductor Sr3-3xNa3xSi3O9-1.5x

One of the most promising next-generation energy conversion technologies is the solid oxide fuel cell (SOFC), which is more efficient and less polluting than conventional internal combustion engines (ICEs). However, in order to commercially compete with ICEs, SOFCs must be cost-competitive and operation-reliable. To achieve this goal, lowering the operating temperature of a SOFC from the current 700-1000^oC to ≤ 600^oC is vitally important. The realization of such intermediate-temperature SOFCs (IT-SOFCs) depends critically upon the availability of high-conductivity electrolytes and high-activity electrode catalysts. In recent years, significant progress has been made in the area of cathode catalysts through new material discovery and microstructural optimization, which has demonstrated a polarization resistance (area-specific resistance) as low as 0.1 W×cm² even at 500^oC. ^{1 2} In contrast, the development of high-conductivity oxide-ion conductors significantly lags behind that of IT-SOFC cathodes. Therefore, identification of new IT solid-state fast oxide-ion conductors is in high demand.

A practical criterion for an electrolyte to be suitable for an IT-SOFC is its oxide-ion conductivity s_o ≥0.01 S/cm at an operating temperature < 600^oC. None of the existing chemically stable oxide-ion conductors can satisfy this requirement at a temperature ≤500^oC. Recently, the Goodenough group at the University of Texas at Austin discovered a new class of oxide-ion conductors that can meet this conductivity-temperature requirement. These new oxide-ion conductors have a generic formula Sr_3-3xNa_3xSi₃O_9-1.5x (or SNS hereinafter) and layered structure as shown in Fig.1 (a). ^{3 4} The oxide-ion conductivity of SNS is s_o»0.01 S/cm at 500^oC, Fig.1 (b), the highest among all known chemically stable oxide-ion conductors. More importantly, it is stable over a broad pO₂ range of approximately from 10^-30 to 1 atm and an extended lifetime, making it an ideal electrolyte for IT-SOFCs. ⁵

Despite the exciting discovery of this superior ion conductor, so far very little is known about the transport mechanisms governing the high oxide-ion conductivity in the SNS. This study investigates the origin of high oxide-ion conductivity by probing the local structure and dynamics of ionic motion with high-temperature multinuclear (¹⁷O, ²³Na, and ²⁹Si) solid-state NMR techniques. It reveals the inhomegeneity of chemical phases in this electrolyte. The increased level of Na-doping has led to increased degree of amorphorization in structure, which may be correlated to the enhancement in ionic conductivity. The foundational knowledge gained from this research may have significant impact on understanding the critical structure-conductivity relationship and designing new solid-state fast ion conductors for advanced electrochemical energy conversion and storage systems.

References

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