Improved oxygen surface exchange kinetics at grain boundaries in nanocrystalline yttria-stabilized zirconia
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esearch Letters
Improved oxygen surface exchange kinetics at grain boundaries in nanocrystalline yttria-stabilized zirconia Joong Sun Park, Department of Mechanical Engineering, Stanford University, Stanford, California 94305; Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Timothy P. Holme, Department of Mechanical Engineering, Stanford University, Stanford, California 94305 Joon Hyung Shim, Department of Mechanical Engineering, Stanford University, Stanford, California 94305; Department of Mechanical Engineering, Korea University, Seoul, Korea Fritz B. Prinz, Department of Mechanical Engineering, Stanford University, Stanford, California 94305; Department of Material Science and Engineering, Stanford University, Stanford, California 94305 Address all correspondence to Joong Sun Park at [email protected] (Received 28 March 2012; accepted 30 July 2012)
Abstract Quantum simulations of oxygen incorporation at a Σ5 grain boundary in yttria-stabilized zirconia (YSZ), a common solid oxide fuel cells (SOFCs) electrolyte, show that the incorporation energy is reduced compared with YSZ with no grain boundaries. The simulation results are supported by electrochemical impedance spectroscopy (EIS) measurements conducted on a single crystalline YSZ substrate with nanogranular interlayered YSZ. EIS results showed that single crystalline YSZ membranes with nanogranular surface (i.e., high grain boundary densities) exhibit small electrode impedances than the reference single crystalline YSZ. The 20-nm-thick nanogranular YSZ interlayer was fabricated by atomic layer deposition and the performance for SOFCs with nanograined interlayer was increased by factor of 2 at operating temperatures between 350 and 450 °C.
Acceptor-doped cubic fluorite structures such as doped zirconia and ceria have been widely investigated as oxide ion conductors, which can be applicable to oxygen sensors or electrolytes for solid oxide fuel cells (SOFCs). Among those materials, yttria-stabilized zirconia (YSZ) is of most interest because of its high ionic conductivity combined with chemical stability. Recently, those materials have shown superior properties when they are synthesized at the nanoscale.[1–2] Knoner et al.[1] reported that ionic conductivity is significantly enhanced in nanocrystalline YSZ. The conductivity seems related to structural characteristics, i.e., increased grain boundaries that are loosely packed where vacancy-type free volume forms. Huang et al.[3] has also demonstrated enhanced ionic conduction in gadolinia-doped ceria (GDC) when it is fabricated at the nanoscale due to reduction of number of blocking grain boundaries perpendicular to ion motion, which greatly impede ion transport across them. Our group has recently succeeded in fabrication of lowtemperature SOFCs equipped with 65-nm-thick YSZ electrolytes by atomic layer deposition (ALD) techniques.[4] The performance of those fuel cells has been improved at low temperatures due to reduction of electrolyte thickness. I
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