Characterization of Nanoscale Electrolytes for Solid Oxide Fuel Cell Membranes

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1 Characterization of Nanoscale Electrolytes for Solid Oxide Fuel Cell Membranes Cynthia N. Ginestra 1 Michael Shandalov 1 Ann F. Marshall 1 Changhyun Ko 2 Shriram Ramanathan 2 Paul C. McIntyre 1 1 Department of Materials Science and Engineering, Stanford University 2 School of Engineering and Applied Sciences, Harvard University 1

2 Introduction Solid oxide fuel cells (SOFCs) are interesting as an energy conversion technology with potential applications in transportation systems. A major disadvantage of current SOFC technology, however, is the relatively high operating temperatures ( ºC), limiting their range of practical use to large power systems. Reduced operating temperatures may be achieved by making the SOFC electrolyte membrane thinner, thereby making it easier for oxygen ions to move across the membrane at lower temperatures. Solid Oxide Fuel Cell (SOFC) Yttria-Stabilized Zirconia (YSZ) Yttria-stabilized zirconia (YSZ) is the most widely used material for SOFC electrolytes and is a well-studied alloy, known for its thermal stability, fast oxygen ion conduction, and electronic insulation. The YSZ SOFC electrolyte is typically tens of microns in thickness, and reducing its thickness to the nanometer scale may lead to improved SOFC performance at lower operating temperatures. We report on the microstructure and electrochemical properties of nanoscale (20-50nm) YSZ synthesized using a laminate approach via atomic layer deposition (ALD). 2

3 Nanolaminate Technique Purge Purge Atomic Layer Deposition (ALD) 4 As a route to preparing nanoscale YSZ electrolytes, zirconia/yttria nanolaminates having either long (~10nm) or short (~1nm) bilayer periods and various average compositions were deposited via atomic layer deposition (ALD). ALD is a CVD-type film growth technique noted for its efficacy in deposition of thin pin-hole-free metal oxide films. Different yttria contents were achieved by varying the number of Zr and Y precursor pulses during alloy film growth. Post-deposition annealing at 950ºC for 2.5 hours enabled layer interdiffusion in the nanolaminate structures. 3

4 Nanoscale ZrO 2 with TDEAZr Nanoscale zirconia (ZrO 2 ) is typically deposited via ALD at 250ºC onto 3 Si(100) wafers with a native 1.5nm surface chemical oxide. In 2007 we changed the ZrO 2 process precursor from zirconium tetrachloride to tetrakis(diethylamino)zirconium, or TDEAZr. TDEAZr is a liquid precursor that provides increased species volatility and eliminates the common clogging problem of chloride-based chemistries. We established reproducible ZrO 2 growth ( Å/cycle) and verified film chemistry with x-ray photoelectron spectroscopy. Surface roughness studies suggest that higher precursor source temperatures resulted in a rougher ZrO 2 surface (see below, left figure), which could be attributed to higher species concentration. Furthermore, higher substrate temperatures result in a smoother ZrO 2 surface (see below, right figure), which could be attributed to more complete chemical reactions and higher species mobility at the film surface. Our studies suggest that a higher TDEAZr temperature coupled with a higher substrate temperatures yield smoother films with expected growth rates. Effect of Precursor Temperature Effect of Substrate Temperature TDEAZr 70 C Substrate 150 C 0.41nm TDEAZr 80 C Substrate 150 C 0.67nm TDEAZr 90 C Substrate 150 C 1.26nm Surface roughness of 6nm-thick ZrO2 increases with TDEAZr source temperature, all other processing conditions identical.(note: vertical scale is 20nm, scan area is 1um x 1um) TDEAZr 90 C Substrate 150 C 1.16nm TDEAZr 90 C Substrate 350 C 0.59nm Surface roughness of 6nm-thick ZrO2 decreases with substrate temperature, all other processing conditions identical. (Note: vertical scale is 20nm, scan area is 500nm x 500nm) 4

5 Oxide Interdiffusion As-grown Annealed 20 nm total film thickness Polycrystalline tetragonal ZrO 2 layers Amorphous Y 2 O 3 layers Total laminate thickness decreased from ~20.8 nm to ~19.7 nm Significant SiO 2 interfacial growth from ~1 nm to ~14.5 nm Initially, thick zirconia/yttria layers (bilayer period ~10 nm) were grown to examine interdiffusion of the two oxides upon annealing. Cation diffusion calculations based on published Y, Zr, and YSZ diffusion studies 1,2 and Darken s flux equation suggested that post-deposition anneals at 950ºC for 2.5 hours would result in an effective diffusion distance of ~0.6nm. This distance is similar to the bilayer periodicity of short-period nanolaminates (intended for YSZ growth) but quite small compared to that of thick-layer laminates. In the above images of thick ZrO 2 /Y 2 O 3 layers, interdiffusion after annealing (950ºC, 2.5hrs) is evident, but discernible Y 2 O 3 layers suggest incomplete intermixing, as expected for this bilayer periodicity. 1. Kilo, M., et al, J Appl Phys 94, (2003). 2. Lee, T., Navrotsky, A., J. Mater. Res. 18, (2003). 5

6 Oxide Interdiffusion SIMS Chemical Depth Profile (raw data by EAG Labs, Sunnyvale, CA) SIMS chemical depth profiling confirmed our TEM observation of partial layer interdiffusion in thick ZrO 2 /Y 2 O 3 layers. These annealing experiments strongly suggest complete species interdiffusion should occur for zirconia/yttria films with considerably thinner laminate layers under the same annealing conditions, a relatively low temperature (950 C) compared to the melting points of ZrO 2 and Y 2 O 3. Y:Zr content depth profiles. Extracted from SIMS data for as-grown (in blue) and annealed (in red) long-period ZrO 2 - Y 2 O 3 nanolaminate. Making nanoscale YSZ. Subsequently, in order to promote thorough interdiffusion of homogeneous zirconia/yttria alloys, short-period (bilayer period < 1nm) zirconia/yttria laminates were deposited and annealed under the same conditions as the thick ZrO 2 /Y 2 O 3. 6

7 Annealing Nanolaminates Zirconia/yttria (short bilayer period) nanolaminates were annealed at 950ºC in flowing O 2 or Ar gas for 2.5 hours to promote layer interdiffusion, induce crystallization, and remove excess H 2 O oxidant incorporated into the laminates during the ALD process. The AFM images below show decreases in surface roughness as a result of annealing the nanolaminates, consistent with our previous work. 3YSZ 6YSZ 10YSZ As-Deposited < 1.8 nm <1.00nm <0.72nm Annealed (950ºC, 2.5 hrs) <0.37nm <0.96nm <0.52nm Lam #1, 30nm thick Lam #37, 30nm thick Lam #41, 20nm thick 7

8 Nanoscale YSZ Structure The ionic conductivity of bulk YSZ is closely related to its microstructure. Bulk YSZ is fully stabilized into the cubic phase for compositions near and above 8mol% yttria (8YSZ). At the threshold composition of 8mol% Y 2 O 3, bulk YSZ exhibits a peak in ionic conductivity, which is key for SOFC electrolytic performance. In our study of nanoscale YSZ, we are investigating phase changes related to composition in order to determine whether (and how) the nanostructure of ultrathin YSZ affects its ionic conductivity and ultimately, its potential as an SOFC electrolyte. As deposited, nanoscale yttria-stabilized zirconia (YSZ) contains mixed amorphous/ polycrystalline material that becomes fully crystalline (see TEM diffraction images below) upon annealing (950ºC, 2.5hrs). Columnar, hexagonal grains (diameters 5-25nm) span the entire thickness of the nanoscale YSZ films, as indicated by distinct boundaries between crystallites. A tetragonal-to-cubic phase transition occurs for a composition between 6 and 10mol% Y 2 O 3. 3 YSZ - Tetragonal 6YSZ - Tetragonal 10YSZ - Cubic 50nm 2nm 50nm 50nm Lam #1, 30nm thick, annealed (950C, 2.5hrs) Lam #37, 30nm thick, annealed (950C, 2.5hrs) Lam #41, 20nm thick, annealed (950C, 2.5hrs)

9 Nanoscale YSZ Ionic Conductivity 6YSZ (50nm on MgO) 8.6YSZ (27nm on Si 3 N 4 ) 3YSZ (30nm on Si) 2YSZ (30nm on Si) Measurements performed by Ramanathan group at Harvard University Solartron electrochemical system, 1Hz-300kHz Bulk 8-10YSZ Strickler &Carlson, JAmCerSoc (1964) After additional annealing treatment (900ºC, 6hrs) to ensure equilibration of point defects, detailed in-plane electrical conductivity measurements were performed on nanoscale YSZ films using the van der Pauw technique. Nanoscale YSZ films show high total conductivity even with low yttria content, comparable to the total conductivity of bulk 8YSZ at higher temperatures. Further investigations of yttria-doping effects, phase transformation kinetics, and grain size effects are needed to understand the origin of these observed increases in electrical conductivity of nanoscale YSZ. 9

10 Future Work Electrical characterization of nanoscale YSZ films exhibit enhanced total electrical conductivity compared to bulk YSZ. It is possible that small grain sizes in nanoscale YSZ are responsible for the measured increase in conductivity. There is similar evidence that other fluorite-structured materials exhibit increased ionic conductivity which may be attributed to grain boundary and interface effects that become more pronounced with decreased film thickness. In the context of solid oxide fuel cells, an increase in electrolyte conductivity may lead to a decrease in the overall operating temperature and open up new applications for this energy technology. Phase-composition Identify composition threshold for tetragonal-cubic phase transformation Phase-thickness Investigate thickness effects for select compositions In-plane conductivity Identify conductivity trends related to composition Through-plane conductivity Identify conductivity trends related to composition Acknowledgements Andy Lin Yasuhiro Oshima 10