LETTER pubs.acs.org/NanoLett
Enhancing Oxide Ion Incorporation Kinetics by Nanoscale Yttria-Doped Ceria Interlayers Zeng Fan*,† and Fritz B. Prinz†,‡ †
Department of Materials Science and Engineering, ‡Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States ABSTRACT: Interlayering 17.5 nm of Yttria-doped ceria (YDC) thin films between bulk yttriastabilized-zirconia electrolyte and a porous Pt cathode enhanced the performance of low-temperature solid oxide fuel cells. The added YDC interlayer (14.11% doped Y2O3) was fabricated by atomic layer deposition and reduced the cathode/electrolyte interfacial resistances while increasing the exchange current density j0 by a factor of 4 at operating temperatures between 300500 °C. Tafel plots and the fitted impedance data suggest that the charge transfer coefficient R of interlayered SOFCs was 1.25 times higher, and the electrode/interfacial activation energy was reduced from 0.85 to 0.76 eV. KEYWORDS: Yttria-doped ceria, low-temperature solid oxide fuel cells, interlayer, atomic layer deposition, electrochemical impedance spectroscopy, oxide ion incorporation kinetics
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ecause the thickness of the solid electrolyte can be reduced to tens of nanometers with the help of atomic layer deposition (ALD), electrolyte ohmic resistance is no longer the main factor limiting the performance of solid oxide fuel cells (SOFCs).14 At low operating temperatures (300500 °C), activation loss at the electrode/electrolyte interfaces becomes more evident, and this interfacial activation loss can be reduced by using electrolyte materials with high oxygen surface-exchange coefficient.5 Therefore, yttria-doped ceria (YDC), whose oxygen surface-exchange coefficient is about 10 times higher than that of yttria-stabilized zirconia (YSZ), has been studied as an alternative electrolyte material for low temperature SOFCs.58 Recent observations showed that 1217 M % Y2O3 doped YDC thin films, used as the interlayer between bulk YSZ and sputtered Pt cathode, enhance peak power densities of thin film SOFCs at 300500 °C by a factor of 23.6.9 To investigate the reasons behind the performance enhancement, we conducted electrochemical impedance spectroscopy on YDC interlayered samples and bare YSZ reference samples. By analyzing the impedance data and fitting the Tafel plots, we found that interlayered YDC thin films could actively enhance the oxide ion incorporation kinetics at the cathode/electrolyte interface by decreasing interfacial activation energy Ea. At the same time, the exchange current density j0 and charge transfer coefficient R, two indicators of interfacial reaction kinetics, were both increased by interlaying YDC thin films. SOFCs without YDC interlaying were employed as reference samples. Commercially available YSZ substrates were used as bulk electrolyte (Φ 17 mm 200 um, 8 M % yttria concentration, double-side polished, BEANS International Corporation) and were cleaned with a piranha solution (98% H2SO4:30% H2O2 = 3:1 in volume) for 15 min. The 80 nm porous Pt films were sputtered on both sides of the substrates to use as electrodes. Sputtering was conducted for 150 s at 50 W under the sputtering chamber pressure of 10 Pa. The customized physical mask was used to define the size of the electrodes, which was 0.5 cm2 for both cathode and anode. r 2011 American Chemical Society
Figure 1. TEM image of the fabricated sample structure. Bottom layer: polycrystalline YSZ substrate. Middle layer: ALD YDC interlayer. Top layer: sputtered Pt.
The YDC thin film was uniformly deposited onto the cleaned YSZ substrate with the ALD recipe (6 Ce:1 Y) 35 cycles.6 The resulting film was about 17.5 nm and had 14.11 M % of yttria doping concentration.9 Pt electrodes were then sputtered by the same method described above. Figure 1 shows the sample structure by transmission electron microscopy (TEM) and confirms the thickness of the fabricated YDC film. TEM samples with a thickness ∼80 nm were prepared using a focused ion beam (FIB, FEI Strata 235DB dual-beam FIB/SEM) lift-out Omniprobe technique with a Ga ion beam at 30 keV. Cross-sectional bright-field and high-resolution TEM images were taken by an FEI Tecnai G2 F20 X-TWIN operated at an accelerating voltage of 200 kV. Maximum power densities of this interlayered SOFC were enhanced on average by a factor of 3 compared to SOFCs without the YDC interlayer at operating temperatures of Received: December 17, 2010 Revised: April 19, 2011 Published: May 12, 2011 2202
dx.doi.org/10.1021/nl104417n | Nano Lett. 2011, 11, 2202–2205
Nano Letters
LETTER
Figure 2. EIS spectra for SOFCs with and without the YDC interlayer. The operating temperatures were (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C with a zoom-in view of EIS spectra (
