NANO LETTERS
Surface Modification of Yttria-Stabilized Zirconia Electrolyte by Atomic Layer Deposition
2009 Vol. 9, No. 10 3626-3628
Cheng-Chieh Chao,†,* Young Beom Kim,† and Fritz B. Prinz†,‡ Department of Mechanical Engineering, Department of Materials Science and Engineering, Stanford UniVersity, Stanford, California 94305 Received June 10, 2009; Revised Manuscript Received August 4, 2009
ABSTRACT Yttria-stabilized zirconia (YSZ) electrolyte membranes were surface modified by adding a 1 nm thin, high-yttria concentration YSZ film with the help of atomic layer deposition. The addition of the 1 nm film led to an increase of the maximum power density of a low-temperature solid oxide fuel cell (LT-SOFC) by a factor of 1.50 at 400 °C. The enhanced performance can be attributed to an increased oxide ion incorporation rate on the surface of the modified electrolyte.
In a previous study, we developed a method for fabricating a low-temperature solid oxide fuel cell (LT-SOFC) electrolyte with the help of atomic layer deposition (ALD).1,2 Our results showed that the LT-SOFCs with these ultrathin ALD yttria-stabilized zirconia (YSZ) electrolytes yielded high power densities between 350 and 450 °C. I-V performances of these LT-SOFCs demonstrated not only a reduction of Ohmic loss, but also a reduction of activation loss that can be attributed to the enhancement of oxygen incorporation kinetics at the ALD YSZ surface. To investigate possible enhancements through the ALD YSZ surface, we modified the surface of the YSZ electrolyte with different levels of yttria concentrations, thereby changing the kinetics of oxygen incorporation. Surface-engineering of the electrolyte was done by depositing a 1 nm surface modification layer (SML) composed of YSZ with varying concentrations of yttria, controlled by a specific deposition sequence of ALD zirconia and yttria. To avoid substantial changes in electrolyte resistance, thickness of the SML was kept below 1 nm. In the membrane electrode assembly (MEA) structure, commercially available YSZ substrates were used as electrolytes to maintain a common comparison reference. With operating temperature between 350 and 450 °C, we fabricated nanostructured platinum electrodes due to their well-defined electrode geometries and superior catalytic abilities in this temperature range comparing to typical electrode materials such as lanthanum strontium manganite * To whom correspondence should be addressed. E-mail: ccchao1@ stanford.edu. † Department of Mechanical Engineering. ‡ Department of Materials Science and Engineering. 10.1021/nl901724j CCC: $40.75 Published on Web 08/12/2009
2009 American Chemical Society
Table 1. ALD Recipes of the SMLa 0Y8Zr 1Y7Zr 2Y6Zr 3Y5Zr 4Y4Zr 5Y3Zr 6Y2Zr 7Y1Zr 8Y0Zr Zr Y Y Y Y Y Y Y Y Zr Zr Zr Zr Zr Y Y Y Y Zr Zr Zr Zr Y Zr Y Y Y Zr Zr Zr Y Zr Y Zr Y Y Zr Zr Y Zr Y Y Y Y Y Zr Zr Zr Zr Zr Zr Y Y Y Zr Zr Zr Y Y Y Y Y Y Zr Zr Zr Zr Zr Zr Zr Zr Y a Each column represents a total of eight cycles for the SML with a different ratio between zirconia and yttria.
and Ni-YSZ.3,4 Fuel cell performances of the MEAs were characterized as a function of the ALD sequences in the SMLs. A 50% performance enhancement was observed with an addition of a merely one-nanometer SML to the YSZ electrolyte at 400 °C. Two types of commercially available standard YSZ substrates were tested, polycrystalline YSZ (1 cm × 1 cm × 100 µm, 8 M % of yttria concentration, one-side polished, Market Tech, Inc.) and single-crystalline YSZ (1 cm × 1 cm × 500 µm, 〈100〉, 8 M % of yttria concentration, twosides polished, MTI Corporation). The substrates were cleaned with a Piranha solution at 100 °C before the deposition of SMLs. The SMLs were fabricated by sequential ALD of zirconia and yttria, following the recipes listed in Table 1. Each recipe consists of 8 ALD cycles, with the yttria concentration tuned by the ratios of zirconia cycles to yttria cycles. The metal precursors used for the zirconia and yttria deposition were tetrakis(dimethylamino)zirconium(IV) (99.99%, Sigma-Aldrich) and tris(methylcyclopentadienyl)yttrium(III) (99.9%, Strem Chemicals), respectively.5,6 The oxidant used was
Figure 3. Relationship between number of yttria cycles in ALD sequence vs yttria molar percent in the SML. Figure 1. (a) Synthesized silica nanospheres. (b) Nanosphere mask after dry etching. (c,d) Nanostructured platinum electrode after removing mask.
Figure 2. MEA structure of an YSZ-electrolyte LT-SOFC with a SML (not drawn to scale).
distilled water. The deposition temperature was 250 °C. Thicker layers with a repeated ALD sequence were deposited on silicon substrates for characterization of yttria concentration and film thicknesses by means of X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XRR). Nanostructured platinum electrodes were fabricated with the help of nanosphere lithography (NSL) following the procedure described below. Silica nanospheres with diameters of about 450 nm, which were used as a mask, were created by chemical synthesis.7,8 These nanospheres were transferred onto the YSZ substrate by a LangmuirBlodgett trough (KSV Instruments, Ltd.), forming a closepacked monolayer. Spacing between the close-packed silica nanospheres was then introduced by dry etching with trifluoromethane and oxygen (AMT 8100 Plasma Etcher). Eighty nanometers of platinum was sputtered on top of the nanosphere mask and the YSZ substrate, and the mask was subsequently removed by sonication in methanol, leaving nanostructured platinum electrodes on the substrate. Figure 1 shows that the nanostructured platinum electrodes fabricated through NSL consists of a close-packed and interconnected platinum structure, which facilitates the diffusion of fuel/oxidant to the well-defined triple-phase boundaries. These electrodes and thick YSZ electrolytes serve as a common reference for comparing the effect of SMLs with different yttria concentrations on fuel cell performances. The structure of the MEA with an SML is illustrated in Figure 2. The assembly was operated at temperatures between 350 and 450 °C, with the anode exposed to pure Nano Lett., Vol. 9, No. 10, 2009
hydrogen and the cathode exposed to air. The cathode temperature was calibrated by taking advantage of melting points of tin and zinc shots (99.999%, 3 mm, Sigma-Aldrich) at 232 and 420 °C, respectively. The flow rate of hydrogen was 10 sccm. Electrochemical measurements were performed with Gamry/FAS-32. The yttria concentration in the SML as a function of the ALD sequence is illustrated in Figure 3. The nonlinearity between the number of yttria cycles and the yttria concentration stems from the nucleation of the oxides growth.9,10 The growth rates of ALD zirconia and ALD yttria, extrapolated from the correlation between the film thickness and the number of deposition cycles, are 1 and 1.5 Å/cycle, respectively. The thickness of the SML is estimated to be about 1 nm. For an MEA with an electrolyte thicker than 100 µm, the impact of the additional SML (1 nm) on the ohmic resistance is negligible; however, the interface kinetics of oxygen incorporation changes significantly depending on the yttria concentration. It was difficult to characterize yttria diffussion in the electrolyte, since the SML was too thin. However, we were able to demonstrated that yttria concentration in this onenanometer SML (without nanostructured platinum electrodes) was stable for 12 h at 650 °C using angle-resolved XPS in our previous experiment.11 Figure 4 shows the performances of MEAs with different SML recipes on polycrystalline YSZ substrates operated at 400 °C. The maximum power density of each sample has been extracted and normalized against a blank sample reference. The normalized maximum power density is plotted as a function of the SML recipe and operating temperature, as illustrated in Figure 5. The reference sample with no SML had maximum power densities of 0.8 mW/cm2, 3.3 mW/ cm2, 6.5 mW/cm2 at 350 °C, 400 °C, 450 °C, respectively. Similarly, we investigated MEAs with single-crystalline YSZ substrates, and they demonstrated characteristics similar to MEAs with polycrystalline YSZ substrates. Figure 6 shows the normalized maximum power density of these MEAs as a function of the SML recipe and operating temperature. The reference sample with no SML had maximum power densities of 0.15, 0.55, 1.49 mW/cm2 at 350, 400, 450 °C, respectively. 3627
Figure 4. Current density vs voltage and power density of MEA with different SMLs on polycrystalline YSZ substrates operated at 400 °C.
Figure 6. Normalized maximum power density as a function of the SML recipe and operating temperature for MEAs with singlecrystalline YSZ substrates. The number of yttria cycles contained in the SML is indicated on the x-axis.
the rate of forward reaction of oxygen incorporation into the electrolyte. This reduces activation loss of fuel cell operations. Conversely, a high yttria concentration in YSZ reduces ionic conductivity, which increases ohmic loss. The optimal yttria concentration in the SML minimizes the combination of both losses. By using ALD, we were able to control the yttria concentration in the SML between the YSZ electrolyte and the cathode to reduce activation losses. A 50% improvement in performance was observed with an addition of a merely 1 nm SML with 14-19 M % of yttria. With such of a surface modification, it is possible to lower the interfacial resistance of an electrolyte without affecting its electrolyte resistance notably. Figure 5. Normalized maximum power density as a function of the SML recipe and operating temperature for MEAs with polycrystalline YSZ substrates. The number of yttria cycles contained in the SML is indicated on the x-axis.
For MEAs with both polycrystalline YSZ substrates and single-crystalline YSZ substrates, the maximum power density increased by 50% at 400 °C with 2-3 yttria cycles in the SML, corresponding to 14-19 M % of yttria concentration. Other samples with either higher zirconia or yttria concentrations showed reductions in the performances. The performance of MEA with all yttria in the SML dropped about 30%. Bulk YSZ with 8 M % of yttria has been reported to have optimal oxide ion conductivity.12-14 However, both Figure 5 and Figure 6 show that the MEA with a near-interface yttria concentration between 14-19 molar percent demonstrates the highest power density, indicating that the optimal platinum-YSZ interface condition for oxygen incorporation occurs at higher yttria concentrations. A high yttria concentration in the SML increases the vacancy density near the electrode-electrolyte interface, which is likely to enhance 3628
Acknowledgment. The authors would like to thank Swagelok for providing high-temperature ALD valves and Dr. Xirong Jiang for valuable discussions. References (1) Shim, J. H.; Chao, C. C.; Huang, H.; Prinz, F. B. Chem. Mater. 2007, 19, 3850–3854. (2) Su, P. C.; Chao, C. C.; Shim, J. H.; Fasching, R.; Prinz, F. B. Nano Lett. 2008, 8, 2289–2292. (3) Kim, Y. B.; Chao, C. C.; Gu¨r, T.; Prinz, F. B. ECS Trans. 2009, to be published. (4) Mogensen, M.; Jensen, K. V.; Jøgensen, M. J.; Primdahl, S. Solid State Ionics 2002, 150, 123. (5) Hausmann, D. M.; Kim, E.; Becker, J.; Gordon, R. G. Chem. Mater. 2002, 14, 4350. (6) Niinisto, J.; Putkonen, M.; Niinisto, L. Chem. Mater. 2004, 16, 2953. (7) Bogush, G. H.; Zukoski, C. F. J. Colloid Interface Sci. 1991, 142, 1. (8) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (9) Watanabe, T.; Hoffmann-Eifert, S.; Mi, S.; Jia, C.; Waser, R.; Hwang, C. S. J. Appl. Phys. 2007, 101, 014114. (10) Riikka, L. P. J. Appl. Phys. 2005, 97, 121301. (11) Chao, C. C.; Park, J. S.; Prinz, F. B. ECS Trans. 2009, to be published. (12) Kuwabara, M.; Murakami, T.; Ashizuka, M.; Kubota, Y.; Tsukidate, T. J. Mater. Sci. Lett. 1985, 4, 467. (13) Rojana, P.; Panchapakesan, R.; Charles, B. M.; Fritz, B. P. J. Appl. Phys. 2005, 98, 103513. (14) Steele, B. C. H. Solid State Ionics 1995, 75, 157.
NL901724J Nano Lett., Vol. 9, No. 10, 2009
