Formation of yttria-stabilized zirconia nanotubes by atomic layer deposition toward efficient solid electrolytes

We describe a fabrication strategy for preparing yttria-stabilized zirconia nanotube (YSZ-NT) arrays embedded in porous alumina membranes by means of template-directed atomic layer deposition (ALD) technique. The individual YSZ-NTs have a high aspect-ratio of well over 120, about ~ 110 nm in diameter, and ~ 14 µm in length. Interfacing the tube arrays with porous Pt was also introduced on the basis of partial etching technique in order to construct Pt/YSZ-NTs/Pt membrane electrode assembly (MEA) structures. The resulting YSZ-NTs MEAs show a 7 mm in diameter with a roughness factor of ~ 2. Area specific resistance was measured up to 1.84 Ω cm2 at 400 °C using H2 as fuel.


Introduction
Owing to high diffusivity for oxygen ions [1], exceptional resistance against mechanical and thermal stress [2], electrical insulation [3] and high biocompatibility [4], zirconia-based materials have many industrial applications. Notable examples are solid oxide fuel cells [5][6][7], catalysts [8], thermal barrier coatings [9], jet engines [10], alternative gate-oxides in microelectronics [11,12], and implantable biomaterials for a hip joint [13]. Bulk ZrO 2 has three well-known polymorphisms at normal atmospheric pressure [14]. The monoclinic baddeleyite structure (m-ZrO 2 ) is thermodynamically stable under ambient conditions, where Zr atoms are in a distorted sevenfold coordination, and O atoms have four-or threefold coordination. m-ZrO 2 transforms reversibly to the tetragonally distorted fluorite structure (t-ZrO 2 ) above ~ 1175 °C, with Zr in an eightfold coordination. This phase transformation is known to be accompanied by a substantial volume change of ~ 5 vol.%. Cubic fluorite structured ZrO 2 (c-ZrO 2 ) is the most stable one and is stabilized upon ~ 2370 °C. Stabilization of t-ZrO 2 and c-ZrO 2 , which is required to be technologically viable over m-ZrO 2 , is of significant importance in many applications. In order to stabilize the cubic structure even down to room temperatures, adding aliovalent oxides such as CaO, Y 2 O 3 and Gd 2 O 3 has been suggested [15].
In principle, formation of the oxygen vacancies plays a major role in applications using zirconia-based ceramics. The vacancy diffusivity depends not only on the phase itself [15], but also on the strain [16], space-charge [17,18], and defect localization [19] effects. Therefore, microstructures such as grain boundaries and defect densities are essential in determining the materials properties [20]. For example, thin film/layer electrolytes of doped-ZrO 2 exhibited enhanced performance at lower operation temperatures in application for solid oxide fuel cells (SOFCs) [21,22]. Currently, YSZ is the most common solid-electrolyte materials for SOFCs.
To accomplish adequate ionic conductivity in conventional ZrO 2 -based electrolytes, SOFCs generally require an operating temperature above 850 °C. Such high operating temperatures require severe demands on the materials used to satisfy chemical as well as thermal stability [23]. Quite long start-up and shutdown time is also limited to many applications, such as portable power and transportations [24]. There are considerable interests in bringing the operating temperature down to intermediate range (600-800 °C) and even to lower (< 500 °C) temperature. At even lower temperature, system costs can significantly reduce due to wider range of materials used for components. On the other hand, low operating temperature could cause reducing significantly the ionic conductivity of the solid electrolyte and thus leads to higher ohmic losses. Ohmic loss is governed by the ionic conductivity and the layer thickness of electrolytes. The first approach is to introduce novel electrolytes with higher ionic conductivity at lower temperatures such as La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-δ (LSGM) [25], and Sm 0.075 Nd 0.075 Ce 0.85 O 2-δ (SNDC) [26]. These have generally shown lower chemical stability than that observed in YSZ electrolytes. The second approach is to extremely reduce the layer thickness of electrolyte less than 100 nm, i.e. to decrease the diffusion path of oxygen ions [10]. Typical MEA structure for thin-film SOFCs is a planar thin-film membrane with two porous electrodes separated by a thin and at the same time dense, air-tight, oxygen-ion conducting electrolyte. Micrometer-thick MEAs have been fabricated using micro-electro-mechanical systems (MEMS) processing based on silicon wafers using chemical etching as mechanical support for free-standing ultrathin MEA [27][28][29][30][31][32]. Several substrates have been used as alternate support materials for MEA, including nickel foil [33], porous nickel cermet [34], glass-ceramic [35], and AAO substrates [36][37][38][39]. Moreover, three-dimensional (3D) nanostructured MEAs also increased the cell performance due to the increased active surface area. Chao et al. reported a corrugated MEA by nanosphere lithography which active membrane area was enhanced to 1.6-twofold [40,41]. Su et al. also reported a cup-shape MEA at micrometer scale which roughness factor of active surface area was increased to ~ 5 [42]. In order to achieve extremely large roughness factors, a natural occurrence is to employ the nanotubular geometry with high aspect ratio. However, the resulting power density was rather disappointed down to only 1 μW cm −2 [43], calling for the emergent design using zirconia-based nanotubes with high aspect ratio as solid electrolytes.
To fabricate the 3D nanostructured MEAs, ALD is one of the most ideal deposition technique of choice. ALD is a gas phase thin film deposition based on alternate, self-limiting surface reaction of precursors [44]. ALD allows for fabricating high aspect-ratio and complex surface structures employing templates such as AAO [45][46][47][48], Opals [49], and aerogel structures [50]. And also, nanoscale laminated films can be grown by alternative depositions at each atomic layer with desired ratio of the number of deposition cycles [51]. In this paper, we studied on the formation of YSZ NTs by templatedirected ALD. By controlling the atomic layer depositing ratio of ZrO 2 and Y 2 O 3 , we were able to achieve the cubic phase YSZ-NTs with high aspect-ratio up to ~ 110. To expose the large active surface area of YSZ-NTs, partial etching procedures were developed. The resulting free-standing YSZ-NT-based MEA was prepared having porous Pt electrodes at the both sides with an active area of 7 mm in diameter. The preliminary results exhibit promising resistance values under H 2 ambient at 400 °C.

Experimental
AAO templates were fabricated by a two-step anodization method base on aluminum (99.999%, Goodfellow, UK) [52][53][54] 3 Y] (Strem Chem., USA) were used as metal containing reactants preheated at 80 and 130 °C, respectively. Deionized water was used as oxygen source and delivered at room temperature. Dried N 2 was used as purge/carrier gas. Both ZrO 2 and YSZ thin films depositions were used in expose-mode protocol, where a full ALD cycle is consisting of 10 s pulse, 50 s waiting, and 60 s purging. For the deposition of cubic phase YSZ films, 7 ALD cycles of ZrO 2 layer and 1 ALD cycle of Y 2 O 3 layer were repeatedly deposited for desired thickness [16]. Porous Pt thin films were deposited on both cathode and anode by DC sputtering (Cressington 308R, Cressington, UK) at room temperature in 10 Pa Ar ambient [15].
Physical dimensions of the resulting tubes and MEAs were inspected by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7000F, Japan & Carl Zeiss AG SIGMA, Germany). The structures of asgrown and post-annealed nanotubes were investigated by X-ray diffraction (XRD, Rigaku Ultima IV, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-4010, Japan). And also, specific crystal structure of nanotubes was obtained using a synchrotron radiation source at beam line 5A with the wavelength 0.7653 Å at Pohang Light Source using Mar 345-image plate. For converting image to the 2D diffraction pattern, FIT2D program was used [55]. The 2θ of XRD pattern was recalculated to the corresponding angles of λ = 1.54 Å (Cu-Kα radiation). The electrochemical impedance spectroscopy was measured to determine the polarization resistance of the cells using potentiostat and frequency analyser (1252, Solartron, UK) in the frequency range of 100 kHz-0.1 Hz with AC amplitude of 20 mV.

Results and discussion
Doping of aliovalent oxides stabilizes the cubic phase of ZrO 2 [56], and the substitutional cations (Ca 2+ , Mg 2+ , Y 3+ ) will generate oxygen vacancies for charge compensation which makes a fast ionic conductor. The amount of Y 2 O 3 in the ZrO 2 matrix could be controlled by varying the ratio of number of cyclic ALD process of ZrO 2 and Y 2 O 3 . Shim et al. reported that ZrO 2 :Y 2 O 3 = 7:1 ratio is estimated to be 7-8 mol% of Y 2 O 3 in the ZrO 2 matrix [16], as an optimum for high ionic conductivity [57]. Our results also exhibited similar doping behaviors (see Fig. 1d). Figure 1a shows a mosaic of TEM images of YSZ-NTs by the template-directed ALD method and subsequent wet-chemical etching of the templates. The resulting YSZ-NTs have no-cracks and pinhole-free with a high aspect ratio over 120, ~ 110 nm in diameter, and ~ 14 μm in length. Since YSZ-NTs function as electrolyte for the oxygen transport as well as anti-fuel-crossover layers, even the small pinholes and cracks could result in severe damage to the performance and failure of cells. HR-TEM image and selected electron area diffraction (SAED) patterns confirmed that YSZ-NTs were polycrystalline with the grain size of sub 10 nm in the cubic polymorphism (Fig. 1b, c). Figure 1d shows the energy-dispersive X-ray spectroscopy (EDS) elemental line profile taken from the YSZ-NT. The green line indicated for yttrium which confirmed the presence of Y 2 O 3 as dopant in the ZrO 2 matrix (to be 6-8 mol%).
With and without applying the Y 2 O 3 sub-cycles, the resulting structures were analyzed by XRD upon thermal annealing at different temperatures up to 600 °C. The observed XRD patterns showed the noticeable difference at around 35 each other. The ZrO 2 NTs without doping stabilized in the tetragonal phase, which is in good agreement with our previous works, due to the developments of nanosized grains [58,59]. An XRD pattern for the tetragonal phase of ZrO 2 NTs is shown in Fig. 2a. The XRD pattern clearly observed the peak splitting at  (002) (110) and (013) (121), respectively. Remarkably, the structures of the resulting YSZ-NTs differ from those of ZrO 2 NTs although the tetragonal and cubic zirconia results in very similar XRD patterns. The cubic phase of YSZ NTs exhibited the only single peaks that assigned to (200) and (311) at 34.9° and 59.6 of 2θ, respectively. Note that the measurement conditions were identical for all the samples in Fig. 2. To confirm the exact structure whether or not our YSZ-NTs have the cubic phase as-grown, a synchrotron source was used to obtain diffraction patterns further. The high intensity and collimation of synchrotron radiation allowed for high resolution studies and also results very close to Bragg reflections. As shown in Fig. 3, the peaks at 30.1°, 34.9°, 50.2°, and 59.6 of 2θ unambiguously assigned the cubic phase, corresponding to the (111), (200), (220) and (311) planes, respectively.
Once the target materials and structures were achieved, we developed a fabrication strategy for preparing YSZ-NT-based MEA structures as illustrated in Fig. 4. First, YSZ thin films were deposited into the AAO template by ALD as described above. To expose the close ends of YSZ-NTs surface, the Al and AAO layers were partially removed by wet-chemical etching in the mixed solution of CuCl 2 + HCl and 10 wt.% H 3 PO 4 . Finally, porous Pt electrodes were deposited on both the top and bottom sides of free-standing YSZ NTs with AAO templates. Figure 4 show the photographs of free-standing MEAs with Al foil and AAO template as mechanical supports. On the top side, the Pt electrode was formed inside the alumina membranes to prevent electrical contact with the aluminum ring. From the bottom side, we clearly verified that the dimension of the free-standing MEA area is 7 mm in diameter. The SEM images show that porous Pt electrode continuously covered the both sides of YSZ-NTs. On the bottom side, the end-closed YSZ-NTs were exposed from the template by partial etching, and the height of exposed YSZ-NTs approximately 300 nm. This nanotubular array structures offered a geometrical surface area ~ 2 times larger than the projected area of the planar structure.
The charge transfer resistance at moderate temperatures pertains to application in solid oxide fuel cells. We monitored the resulting resistance of our free-standing YSZ-NT-based MEA structures at 400 °C using H 2 as fuel. Figure 5 shows the corresponding impedance spectra. The high-frequency intercepts, corresponding to the ohmic resistance due to ionic transport through the electrolyte [60], were approximately 1.84 Ω cm 2 . With the porous electrodes, the ohmic resistance was higher than similar roughness factor of nanostructued MEAs   [25], because of the distance between top and bottom electrode over 10 μm and hollow structure of the NTs. Toward the higher power density, the pores inside the YSZ-NTs should be completely filled with porous electrodes to shorten the electrode-electrode distance which in turn reduces the resistance of the cell further (Fig. 6).

Conclusion
In summary, we studied on the formation of YSZ-NTs with high aspect ratio up to ~ 110 by template-directed ALD method. The crystal structures of tetragonal phase ZrO-2 NTs and cubic Phase YSZ NTs were demonstrated using both conventional XRD and high-resolution synchrotron radiation diffraction, complementarily. And also, the YSZ NTs based MEAs with AAO template as mechanical support structures were introduced by wet chemical etching technique. Using this fabrication process, the total area of the free-standing MEA and the roughness factor have achieved up to 7 mm in diameter and approximately ~ 2 times, respectively. The present study will open a new venue for realizing the micro-SOFCs with ultra-high efficiency and low-temperature operation capability. Fig. 6 a, b Photographs of free-standing YSZ-NT-based MEA with an active area of 7 mm in diameter. a Golden color shows porous Pt electrode on the YSZ NTs; gray color, YSZ NTs on AAO template; metal color, Al disk. b The corresponding back-side image, exposing the free-standing YSZ-NTs (dark green). c Cross-sectional SEM image of the top-side MEA, displaying porous Pt electrodes with ~ 80 nm thickness on the YSZ-NT membranes by sputtering. d SEM micrographs of porous Pt electrode coated on the exposed tubes in the partially etched alumina template. Inset, the plane view