electrochemical vapor deposition for metal-supported solid oxide fuel cells

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Densification of an YSZ electrolyte layer prepared by chemical/ electrochemical vapor deposition for metal-supported solid oxide fuel cells Erik Hermawana,b, Gyun Sang Leea, Ghun Sik Kima, Hyung Chul Hama,b, Jonghee Hana, Sung Pil ⁎ Yoona,b, a National Agenda Research Division, Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil-5, Seongbuk-gu, Seoul 02792, Republic of Korea b Department of Clean Energy and Chemical Engineering, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

A R T I C L E I N F O

A BS T RAC T

Keywords: YSZ Metal supports Chemical/electrochemical vapor deposition (CVD/EVD) Metal-supported solid oxide fuel cells (MSSOFCs)

A densification process based on chemical/electrochemical vapor deposition (CVD/EVD) was successfully performed to produce a dense and gas-tight YSZ electrolyte on a metal support for solid oxide fuel cell applications. Micro Ni/YSZ (7:3 wt%) was deposited by screen printing and YSZ was deposited by an atmospheric plasma spray (APS) process on a metal support prior to the CVD/EVD refinement process. The initial nitrogen permeation flux through the YSZ layer prepared by the APS process was in the range of 1.8– 2.7×10−7 mol/s cm2 at 25 °C, which shows that residual pores/pinholes existed in the YSZ layer. After YSZ density refinement by the CVD/EVD process, a dense and gas-tight YSZ layer can be obtained after five hours of deposition. An additional 4–7 µm of YSZ was observed after the refinement process was finished. The average film growth rate during CVD/EVD was approximately 1.14 µm/h. From XRD analysis, the YSZ layer prepared after CVD/EVD showed a dominant cubic structure; nonetheless, a secondary phase was also observed. From the SEM and elemental mapping analyses, the YSZ layers showed a homogeneous distribution on the surface of the metal support. The present results showed that the CVD/EVD process is capable of refining the YSZ electrolyte density/tightness by plugging residual pores/pinholes, along with increasing the YSZ thickness, for application in metal-supported solid oxide fuel cells.

1. Introduction Solid oxide fuel cells (SOFCs) are promising candidates for generating green energy and have several advantages over traditional energy generator systems, such as a relatively high power density, high efficiency, modularity, reliability, fuel flexibility, resistance against thermal cycling, and high robustness with shock resistance and low air pollution [1]. Compared with conventional electrode- and electrolyte-supported cells, metal-supported solid oxide fuel cells (MS-SOFCs) offer various benefits and provide potential advantages. In fact, the production engineering costs are lower than those of devices based on ceramics or cermets (ceramic-metal). At the same time, these systems have large potential to be scaled up, high mechanical strength, high thermal conductivity, short start up-time and improved sealing using ordinary metal joining methods, such as welding or brazing [1–3]. Even with these potential benefits, the optimization of MS-SOFCs is necessary because it address the durability, performance, material cost,

compatibility and potency to be scaled up for mass production. One of the most important processing challenges in the preparation of MSSOFCs is the densification of the electrolyte layer, which typically needs to be dense and gas tight to separate the fuel from the oxidizing atmosphere [4,5]. Yttria-stabilized zirconia (YSZ) is arguably the most important ceramic-based electrolyte material for SOFC systems due to its low cost, high ionic conductivity, high bend strength and chemical inertness. Thus, YSZ can endure mechanical stresses from operational conditions and residual stresses from cell fabrication processes [6–8]. Several processing methods have been developed to fabricate the YSZ electrolyte, such as atomic layer deposition (ALD), atmospheric plasma spray (APS), chemical/electrochemical vapor deposition (CVD/ EVD), electrophoretic deposition (EPD) and other processing methods [4,9–11]. Among these techniques, CVD/EVD has been demonstrated to be a very promising technique/process to fabricate dense and gastight electrolyte material on porous supports for application in SOFCs. However, the CVD/EVD deposition/growth rate is quite slow at 1–

⁎ Corresponding author at: National Agenda Research Division, Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil-5, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail address: [email protected] (S. Pil Yoon).

http://dx.doi.org/10.1016/j.ceramint.2017.05.085 Received 10 March 2017; Received in revised form 11 May 2017; Accepted 11 May 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Hermawan, E., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.085

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2 µm/h; therefore, the time required to achieve sufficient thickness of the electrolyte will be much longer [12–14]. A combination of APS and CVD/EVD may be an opportune way to fabricate dense and gas-tight electrolyte in less time. The APS process possesses a faster deposition rate than the CVD/EVD process; however, the APS process cannot produce a dense and gas-tight electrolyte layer because residual pores/ pinholes still exist after the APS process is performed [6]. Therefore, in combination with the CVD/EVD refinement process, the residual pores/pinholes remaining after the APS process can be plugged, and thus a dense and gas-tight electrolyte can be prepared. CVD/EVD possesses several characteristics that make this technique unique compared to other processing techniques. First, by using the CVD/EVD process, the desired location (in any location of the support) to be coated is easily prepared with changing the experimental parameters. Second, the CVD/EVD process is a self-plugging process, which can close/plug all of the pores as the reaction proceeds [15]. Third, in the CVD/EVD process, it is possible to determine the tightness of the electrolyte by conducting gas permeation tests while the reaction is ongoing (in situ test). The CVD/EVD working process is briefly outlined here for clarity. In the CVD/EVD process for growing oxide materials, a porous support separates the reactor into two parts, the metal chloride source chamber and the oxygen/water source chamber. Fundamentally, the CVD/EVD process consist of two steps, the first step (Phase I) is the pore closure by a CVD reaction between water vapor (or oxygen) and metal chloride. During CVD process, the pore substrates will be plugged with the oxide materials. If the deposited oxide is an oxygen conducting material, after the pores are closed, film growth then proceeds due to different electrochemical potential across the deposited films. This second step (Phase II) is termed as EVD process. In this step, oxygen ions formed from water vapor diffuse to the film and react with the metal chloride source to lead to the growth of oxide on the support surface [16,17]. In previous CVD/EVD studies performed by other investigators, the supports that are often used are either ceramic-based materials or cermet-based materials. No experimental studies have reported the deposition of dense and gas-tight YSZ electrolyte on porous metal supports through the CVD/EVD process. Therefore, the main objective of this paper is to innovatively prepare a dense and gas-tight YSZ layer on the surface of the porous metal support for MS-SOFCs. Prior to YSZ electrolyte deposition, the SOFC anode material (micro Ni/YSZ) with a weight ratio of 7:3 was deposited on the metal support by the screen printing method. Furthermore, the metal support was spray coated via the atmospheric plasma spray (APS) technique to deposit a sufficient thickness of YSZ, and then residual pores/pinholes are plugged with YSZ through the CVD/EVD process to achieve a dense and gas-tight material. Plugging of the residual pores/pinholes by the CVD/EVD process was studied, and the YSZ layer on the metal support was characterized.

Table 1 Composition of the commercial porous metal support media. Metal Support

Blue Tech

Composition (%) Cr

Fe

C

Mn

S

Si

Ni

Cu

14–17

6–10

0.15

1

0.015

0.5

72

0.5

Table 2 Composition of the paste for the screen printing process. Composition

Weight (%)

Powder α-Terpineol Ethyl Cellulose Fish Oil PEG DBP

40 85 7 2 3 3

60

used as the main composition in the screen printing process. To prepare the slurry paste, the powder was mixed with α-terpineol, ethyl cellulose, fish oil, PEG and DBP with a mortar and pestle in the appropriate ratio shown in Table 2 until well mixed [18]. The paste was screen printed on the surface of the metal support with mesh screen number 325 and sintered at the temperature that gives the appropriate porosity (40–50%) for 2 h under reduced atmosphere (10% H2/Ar). To control the anode microstructure, the preferred sintering temperature was investigated by compacting 1 g micro nickel (for reference) and micro Ni/YSZ (7:3) powder with a uniaxial pelleting machine at a pressure of 590 kg with a dwell time of 30 s. Then, the pellets were sintered over a temperature range from 500 to 1100 °C for 2 h under reduced atmosphere (10% H2/Ar). The experimental porosity of each pellet was calculated by the formula shown in Eqs. (A.1) and (A.2) [19], where P is the porosity, ρt is the experimental density, ρi is the theoretical density (obtained from literature), m is the mass of the pellet, r is the radius of pellet and h is the thickness of pellet. Furthermore, the mercury porosimetry analysis (Micromeritics, Autopore IV) was conducted to characterize the pore of the micro Ni/YSZ (7:3) in the temperature which give the appropriate porosity of 40–50%.

⎛ ρ⎞ P = ⎜⎜1− t ⎟⎟ x 100% ⎝ ρi ⎠

(A.1)

m ρt = 2 πr h

(A.2)

2.2. YSZ deposition with APS

2. Experimental methodology

There are three kinds of YSZ materials that are used for APS coating: (1) YSZ powder (Sea Won, Korea) with a particle size of 11 µm, (2) YSZ powder (Sea Won, Korea) with particle size of 3 µm and (3) 18% YSZ sol in an H2O colloidal dispersion (Alfa Aesar, Korea). To form the powder precursor, the YSZ suspension was prepared by mixing YSZ powder with ethyl alcohol (anhydrous 99.9%, Samchun Pure Chemical Co., Ltd., Korea) and the dispersant BYK-110 (BYKChemie GMBH) in a mass ratio of 54:6:1, respectively. Meanwhile, the YSZ sol was used as received without any treatment. The suspension was directly used to coat the surface of the porous metal support to get a sufficient YSZ layer thickness of 20 µm. APS was executed with a commercial plasma spraying system (Axial-III) using Ar-N2-H2 (15:2:3) plasma gas at a total flow rate of 245 L/min. The spraying was performed at a distance of 200 mm, while the electric current was maintained at 220 A. The atomizing gas and slurry feed flow were controlled at 15 L/min and 45 mL/min, respectively. After the APS process, the CVD/EVD process was performed next to plug residual

2.1. Modification of the metal support's microstructure The commercial porous metal supports used during the experimental study were Blue Tech (Puren Technology, Korea). The Blue Tech sample has a thickness of approximately 0.162 cm, diameter of approximately 2.54 cm, active area of 2.84 cm2 and average pore size of 20 µm. The composition of the porous metal support is given in Table 1. To modify the porosity and pore size of the porous metal support, micro nickel powder (Inco Special Product, Canada), YSZ (8 mol% Y2O3) powder (TOSOH, Japan), ethyl alcohol (anhydrous 99.9%, Samchun Pure Chemical Co., Ltd., Korea), α-terpineol (Kanto Chemical), ethyl cellulose (Junsei Chemical), fish oil (Sigma Aldrich), polyethylene glycol (PEG) (Junsei Chemical) and dibutyl phthalate (DBP) (Junsei Chemical) were used as the starting materials. The anode material (micro Ni/YSZ) with a weight ratio of 7:3 was 2

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chambers. He (99.99%) and N2 (99.99%) were used to flush the reactor before and after the CVD/EVD experiment, while H2 (99.99%) and 5% H2/Ar were used as the carrier gas in the water/oxygen chamber and metal chloride chamber, respectively. The water/oxygen chamber precursor was made by bubbling 5% H2/Ar through the water sparger to produce a mixture of hydrogen and water vapor. The hydrogencontaining water mixture could be directly supplied to the back of the porous media support in the water/oxygen chamber.

Table 3 Metal support preparation process. No

Metal Support Preparation Process

Metal Support #1

#2

#3

✓

✓

1

Micro Ni/YSZ (7:3) Screen Printing

✓

2

APS with YSZ Powder (11 µm) APS with YSZ Powder (3 µm) APS with YSZ sol

✓ ✓ ✓

2.4. CVD/EVD experimental conditions and procedures After the metal support was mounted to the end of the Inconel tube using a laser welding process, the sublimation boats were filled with the YCl3 and ZrCl4 powder and placed in a certain position to maintain each sublimation boat at the desired temperature. The reactor was set to the desired temperature at a heating rate (3–4 °C/min), and both chambers were flushed with helium. The pressure of both reactors was maintained at 500 Torr. When the temperature stabilized, the reactor pressure was changed to 10 Torr with an evacuation time (from 500 Torr to 10 Torr) of approximately 10 min. The carrier gas in the metal chloride chamber was changed to 5% H2/Ar and directed to the sublimation boat to carry the metal chloride vapor, and the carrier gas in the water/oxygen chamber was also changed to H2 and bubbled to the water sparger using the mass flow controller. The experimental conditions for the CVD/EVD process are given in Table 4.

pinholes and to refine the density/tightness of the electrolyte. Table 3 shows the preparation of the metal support prior to electrolyte refinement with the CVD/EVD process. 2.3. CVD/EVD apparatus The CVD/EVD experiments were performed in a homemade apparatus. The main parts of the apparatus are the metal chloride source chamber, water/oxygen source chamber, metal chloride vapor delivery system, three-zone furnace, water sparger and vacuum integrity system. Fig. 1 shows a schematic diagram of the CVD/EVD reactor system. The metal chloride chamber is made from alumina, while the water/oxygen chamber is made from Inconel with the metal support mounted in it with laser welding process. A sublimation boat which were made from alumina tubes was placed in the metal chloride chamber. Each sublimation boat contained ZrCl4 (anhydrous powder 99.99%, Sigma Aldrich) and YCl3 (anhydrous powder 99.99%, Sigma Aldrich), which are the metal chloride chamber precursors. The reactor was placed in a three-zone tubular furnace controlled by a temperature controller to achieve the desired temperature profile along the reactor. The water sparger system consisted of a three-neck round-bottom flask filled with distilled water and a heating element to ensure a constant temperature. Vacuum pumps (Jungwoo Motor, Korea) and a pressure controller (Atovac, Korea) were used to control the pressure in the metal chloride and water/oxygen chambers. Three mass flow controllers (Bronkhorst, Korea) were used to control the amount of carrier gas supplied to the metal chloride and water/oxygen

2.5. In situ N2 gas permeation In the in situ gas permeation test, the temperature of the metal support was maintained at a temperature similar to that in the deposition process (1000 °C). The permeation test was started by flushing both chambers with helium for approximately 15 min after deposition. Both chambers were filled with high-purity nitrogen until the pressure in the metal chloride and water/oxygen chambers were maintained at 10 Torr and 40 Torr, respectively. After the pressure stabilized, both chambers were isolated by closing all the inlet and outlet valves. The pressure vs time data in both chambers were automatically harvested by a personal computer. These data were used

Fig. 1. Schematic diagram of the homemade CVD/EVD reactor system.

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Table 4 Experimental conditions for CVD/EVD. CVD/EVD

Deposition temperature Reactor pressure ZrCl4 sublimation bed temperature YCl3 sublimation bed temperature YCl3/ZrCl4 ratio in vapor Gas flow in metal chloride chamber Gas flow in water/oxygen chamber Water sparger pressure Water sparger temperature H2O/H2 ratio in vapor

1000 °C 10 Torr 190 °C 700 °C 3/10 15 mL/min 7 mL/min 15 kPa 28 °C 0.23

to calculate the gas permeation flux using the slope of the pressure vs time. The equation for calculating the permeation flux is shown below in Eq. (A.3) [15].

ji =

⎛ ∆P ⎞ Vr ⎟ ⎜− R. Tr . Sm ⎝ ∆t ⎠

Fig. 3. The porosities of micro nickel and the combination of micro Ni/YSZ (7:3) at each given temperature.

(A.3)

where ji is the permeation flux (mol/cm2∙sec), Vr is the volume of the water/oxygen source chamber (0.16 L), Tr is the average temperature in the water/oxygen source chamber (K), Sm is the permeating area (2.84 cm2), R is the gas constant (0.082 L∙atm/mol∙K), and ∆P /∆t is the slope of the pressure vs time (atm/sec).

Eqs. (A.6) and (A.7) [21,22], where a is the lattice parameter (Å), λ is the x-ray wavelength (1.5418 Å) and θ is the Bragg angle of the cubic (111) reflection.

% − molY2O3 =

2.6. Characterization of the structure and morphology of the deposited layers

λ=2

The phase structure and layer composition were examined by an Xray diffractometer (XRD, Rigaku, MiniFlex II) with Cu Kα radiation with λ=1.5418 Å. During analysis, the samples were measured over an angle (2θ) range of 10° to 90° at a scan rate of 1°/min for qualitative and quantitative analyses. The morphology of the deposited layer and the elemental mapping were analyzed by a scanning electron microscope (SEM) (FE-SEM, Hitachi, S-4200, Japan) connected with an energy-dispersive X-ray spectroscopy analyzer (EDAX, AMETEK, USA). The volume ratio of the monoclinic phase (Vm) was estimated from the XRD peaks using the methods shown in Eqs. (A.4) and (A.5) [20], where Xm is the monoclinic peak intensity ratio, Im and Ic represent the integrated intensity of monoclinic [(111) and (111)] and cubic (111), respectively.

Xm =

Vm =

Im (111) + Im (111) Im(111) + Im(111) + Ic(111)

1.115Xm 1 + 0.115Xm

(a−5.1159) x 100% 0.001547

a sinθ 3

(A.6) (A.7)

3. Results and discussion 3.1. Metal support microstructure control Fig. 2 shows SEM images of the metal support before and after micro Ni/YSZ (7:3) was screen printed on the porous metal support, in which micro Ni/YSZ was deposited on the surface of the metal support. To control the anode microstructure/porosity on the metal support to be approximately 40–50%, the appropriate sintering temperature of micro Ni/YSZ (7:3) was investigated. Fig. 3 shows the porosity at each given temperature of micro nickel (for reference) and combination of micro Ni/YSZ (7:3). From this porosity determination, the sintering temperature that gives the best microstructure properties/porosity can be found. For micro nickel only, the sintering temperature that gives 40–50% porosity was 600 °C (porosity: 42.4%); meanwhile, that for micro Ni/YSZ (7:3) was 1000 °C (porosity: 44.97%). From the mercury porosimetry analysis, the porosity of micro Ni/YSZ (7:3) which was sintered at 1000 °C, showed the value porosity of 43.78%. These two observations give almost the same value with low deviations (~1%),

(A.4)

(A.5)

While the yttria composition was determined using the shown in

Fig. 2. SEM photograph of the porous metal support before and after screen printing.

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therefore, after the micro Ni/YSZ (7:3) screen printing process, the sample was treated at 1000 °C. As we can see, the addition of YSZ to the micro nickel helps to maintain the appropriate porosity at high temperature. At 1000 °C, the micro nickel only had 5% porosity remaining; however, after the addition of YSZ, the porosity can be increased to approximately 45%. Furthermore, YSZ supports the nickel particles to inhibit coarsening and provide an acceptable thermal expansion coefficient close to that of other fuel cell components [23]. Maintaining the porosity of micro Ni/ YSZ on the metal support provides easy gas-phase diffusion to the active site in the SOFC system and facilitates the plugging of the YSZ surface through the diffusion of gas reactants in the CVD/EVD process for YSZ electrolyte refinement. In the CVD/EVD process, one of the rate-limiting steps during pore closure and layer growth is the diffusion of gas reactants through the porous support via Knudsen regime diffusion (occurring at low pressure and in small-sized pores) through both sides of the porous metal exposed to metal chloride and water/oxygen sides. Knudsen diffusion occurs when the mean free path of a gas molecule is on the same order as the pore dimensions. Generally, Knudsen diffusion processes mainly occur at low pressure and small pore diameter. Three mechanisms can occur when gas molecules travel through the porous support, e.g., molecular diffusion, viscous diffusion and Knudsen diffusion. The Knudsen diffusion regime can be distinguish by the Knudsen number (Kn), which is the ratio between the mean free path (λ) and the pore size of the porous support (dp). The Knudsen number calculation is described in Eqs. (A.8) and (A.9) [24]. Where KB is the Boltzmann constant (1.3807×10–23 J/K), T is the temperature of gas (K) (CVD/EVD process conducted in 1273 K), p is the gas pressure (atm) (CVD/EVD process conducted in a pressure of 10 Torr=0.01316 atm) and dg is the effective diameter of a gas molecule (m) (dg of water = 2.75×10−10 m)

Kn =

λ=

λ dp

Fig. 4. The pore size analysis of micro Ni/YSZ (7:3) sintered at 1000 °C derived from mercury porosimetry.

the pore size are the pore closure time and the minimum thickness of the layer that is gas tight. The pore closure time is crucial in determining the ultimate properties of the deposited layer. If the pore size is quite big, the time needed to plug the pores is longer than the time to plug small-sized pores. The difference in the pore size of the sample will result in a different relative resistance for transport of the gas reactants during the CVD/EVD process [26]. From the investigation by Lin et al. [27], the minimum thickness of the YSZ layer on the metal support to achieve gas-tight conditions greatly relies on the pore size of the supports on which the film grows. As a rule of thumb, the minimum thickness to achieve a gas-tight YSZ layer on the porous metal support is approximately 10 times the pore diameter size of the support. Therefore, if the porous support has a smaller pore size, the minimum thickness to achieve gas-tight conditions can be reduced, and thus the deposition time needed to obtain the appropriate thickness can also be reduced.

(A.8)

KBT 2 pπdg 2

(A.9)

3.2. YSZ coating with APS

If the Knudsen number (Kn) number is greater than 10, collisions between gas molecules and the porous support are more dominant than the collision of gas molecules with other gas molecules, which cause negligible molecular diffusion and viscous diffusion. If Kn is lower than 0.1, collisions between the gas molecules are dominant, and Knudsen diffusion becomes negligible, and if Kn ranges between 0.1 and 10, three mechanisms govern the gas transport process [24]. The original pore size of bare metal support is 20 µm. Therefore, the bare Blue Tech metal support has a Knudsen number of 1.96 at 10 Torr and 1000 °C (at the CVD/EVD experimental conditions). Thus, when CVD/EVD is performed, the gas transport mechanism will follow three mechanism regimes (molecular diffusion, viscous diffusion and Knudsen diffusion) instead of Knudsen diffusion only. However, from the mercury porosimetry analysis which is shown in Fig. 4, the pore size of micro Ni/YSZ (7:3) after the screen printing process and heat treatment at 1000 °C on the surface of the metal support was appeared to be 0.031 µm. The Knudsen number of the metal support after the screen printing process is improved to above 10, compared to that of the bare metal support. From the previous explanation above, if Kn is more than 10, the Knudsen diffusion regime will occur during CVD/ EVD, in which deposition is dominant. This is because, during the screen printing processes, the original pores in the metal support are partially plugged with micro Ni/YSZ. Therefore, the reduction of the pore size in the support by the micro Ni/YSZ (7:3) screen printing process helps to optimize the CVD/EVD process by improving the Knudsen regime area, in which deposition can be facilitated [25]. The pore size of the support also affects the performance of the CVD/EVD deposition process. The parameters that depend strongly on

Fig. 5 shows SEM images of the metal supports before and after APS coating with YSZ. The YSZ layer deposited on the metal supports has different characteristics depending on the precursor characteristics. As we can see, the YSZ layer derived from larger sized precursor particles is rougher and not uniform. However, using nanosized particles, which exist in YSZ sol, for APS coating provides an YSZ layer that is more uniform and denser than the others. This may occur because, when applying smaller sized particles in the spray coating method, the melting point and fusing rate are better than when larger sized particles are used [28]. Moreover, the layer was dense and had a better microstructure, which can afford a significant reduction in both the amount and the size of the voids (dead spots) [29,30]. Fig. 6 shows the pressure vs time data in the water/oxygen chamber, during in-situ permeation with N2 performed on bare metal support and on samples after APS process at room temperature. Table 5 shows the permeation rate of the bare metal support along with the metal supports after YSZ deposition by the APS technique. From our investigation, the bare metal support (as a reference) has a permeation flux of approximately 1.6×10−6 mol/cm2 s. The bare porous metal support shows a high gas permeation rate; after 1 min of permeation, an equilibrium pressure between the metal chloride and water/oxygen chambers was achieved. However, after YSZ deposition by APS, the gas permeation rate was reduced to 1.8–2.7 (×10−7 mol/ cm2 s) at room temperature, which means the rate was reduced by a factor of 6–8. Sample #3 shows the lowest gas permeation rate among the samples. As explained above, this may occur due to the use of finer particle in APS, which improves the density of the layer. From this 5

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Fig. 5. SEM images of the metal supports before and after APS coating with YSZ: (a) before (b) sample #1, (c) sample #2, and (d) sample #3.

observation, we can see that the APS technique cannot afford a dense and gas-tight YSZ electrolyte, as was also reported by Li et al. [6]. Therefore, refinement of the electrolyte is needed to plug residual pores/pinholes in the YSZ electrolyte layer.

3.3. YSZ electrolyte refinement process by CVD/EVD Fig. 7 shows an SEM photograph of the YSZ electrolyte surface after refinement with the CVD/EVD process, along with elemental mapping. The YSZ layers, which consist of zirconium and yttrium, are spread uniformly on the metal support, where the residual pores of the YSZ electrolyte layer on the top of the metal support are plugged during CVD/EVD, as we can see by comparison with the SEM images prior to CVD/EVD shown in Fig. 5. Under low pressure conditions in the CVD/EVD process, YSZ was deposited on the outer surface of the support because the rate of diffusion of H2O was faster than that of the metal chlorides. In Knudsen regime diffusion (CVD/EVD operation), the gas diffusivity is inversely proportional to the square root of the diffusive gas molar weight. The gas with higher molar weight (metal chloride) has a smaller diffusivity than the gas with lower molar weight (such as oxygen and water vapor). Therefore, the deposited compound is commonly found in the entrance/the support side facing the metal chloride chamber [31,32]. In the CVD/EVD process, the pressure difference in both the metal chloride source chamber and the water/oxygen source chamber plays a critical role in deposition. If the inner pressure of the porous support is higher, the deposition process cannot be facilitated. Therefore, in all CVD/EVD experiments, the total pressure in both chambers should be the same (no absolute pressure difference between them) [33]. The CVD/EVD process can be divided into two major steps. The

Fig. 6. Initial gas permeation tests on the bare metal support and on the metal support after the APS process at room temperature.

Table 5 Initial gas permeation test of the bare metal support and after YSZ deposition by APS at room temperature. Metal Support

Nitrogen Permeation (×10−7mol/cm2 s)

Bare #1 #2 #3

16 2.64 2.02 1.81

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Fig. 8. In situ nitrogen permeation test during CVD/EVD at 1000 °C.

Section 3.1, in the CVD/EVD process, collisions between gas molecules and the porous support are more dominant than collisions between gas molecules. Therefore, it is most likely that in this regime (low pressure, high temperature and small pores) a heterogeneous surface reaction will be formed. The pore closure time and permeability profile of the YSZ layers were determined by an in situ nitrogen permeation test before, during and after the CVD/EVD experiment at the deposition temperature (1000 °C). The in situ nitrogen permeability profile is shown in Fig. 8. The permeability flux right before CVD/EVD process began ranged from 5.5 to 8.5 (×10−8 mol/cm2 s) at the deposition temperature (1000 °C). This value is much smaller than the initial permeation rate observed at room temperature (25 °C), which is mentioned in Table 5. The gas permeation rate is inversely proportional to the temperature; hence, the permeation rate at higher temperatures will be smaller than at room temperature. The gas permeation flux was calculated from the slope of pressure over time using Eq. (A.3). The permeation flux was monitored for each 1 h deposition by halting the deposition. Continuation of the deposition results in a gradual decrease in the permeability, which means that YSZ is deposited on the surface of the metal support. From our experimental observations after 1 h of deposition, the permeability of the YSZ layer on the metal support samples sharply decreased to around zero. This was considered to be the pore closure time. A pore closure time of 1 h was observed in all cases. According to de Haart et al. [26], the pore closure time is proportional to the pore radius because the reaction between metal chloride and water is a first-order reaction against to the metal chloride concentration and a zero-order reaction against to the water vapor concentration. Directly after the pores are plugged, the second step of EVD will be occurred if the deposited layer is an oxygen-conducting material. The oxygen source reactant, i.e., the water vapor can diffuse through the film, be reduced to oxygen ions, bulk transported through the film and react with the metal source, leading to continuous film growth. A deposition temperature of 1000 °C, was also help to facilitate the diffusion of oxygen ions through the YSZ layer and has a significant effect on the growth of the YSZ layer during EVD process. In the experiment, right after pore closure phase is achieved, the deposition was continued to achieve an appropriate thickness of YSZ for gas-tight conditions. All the tested metal supports achieved fully gas-tight conditions after 5 h of deposition by CVD/EVD, which means that the thickness of YSZ is sufficient to ensure gas-tight conditions. The reactions that occur during the growth of YSZ are shown in Eqs. (B.3), (B.4) and (B.5).

Fig. 7. SEM photograph of the surface of the YSZ electrolyte after refinement with CVD/ EVD, along with the elemental mappings: (a) sample #1, (b) sample #2, and (c) sample #3.

first step is CVD process (plugging process) and the second step is EVD process (film growth process). During the first step of deposition, the precursors ZrCl4 and YCl3 in the metal chloride chamber diffuse through the pores of the support and directly contact the water vapor, which also diffused from the water/oxygen chamber to form YSZ on the surface of the porous metal support. The reaction of the metal chloride reactant with water vapor is shown in Eqs. (B.1) and (B.2).

ZrCl4 (g) + 2H2O (g) → ZrO2 (s ) + 2H2 (g) + 2Cl2 (g)

(B.1)

2YCl3 (g) + 3H2O (g) → Y2O3 (s ) + 3H2 (g) + 3Cl2 (g)

(B.2)

The reaction between YCl3/ZrCl4 and H2O inside a porous material can be facilitated in two ways: (1) a homogeneous reaction of the reactants in the gas phase after which the product is deposited on the pore surface or (2) a heterogeneous reaction of the reactants of which at least one is absorbed on the pore surface [34]. As mentioned in

H2O (g) + 2e− → H2 (g) + O 2−

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(B.3)

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ZrCl4 (g) + 2O 2− → ZrO2 (s ) + 2Cl2 (g) + 4e−

(B.4)

2YCl3 (g) + 3O 2− → Y2O3 (s ) + 3Cl2 (g) + 6e−

(B.5)

Table 6 Summary of the CVD/EVD experimental results.

Normally, the reaction between metal chloride and water is thermodynamically unfavorable for metal oxide formation. As an example, the standard Gibbs free energy (ΔG°) for the formation of ZrO2 is about +220 kJ/mol at 1000 °C. In other words, it means, at standard conditions, the oxygen partial pressure in the metal chloride chamber is larger than the oxygen partial pressure in the water/oxygen chamber. That condition imparts a negative driving force against the EVD film. Meanwhile, to increase the driving force for the film growth, EVD experimental process should provide a very low partial pressure of chlorine in the metal chloride chamber in order to lower oxygen partial pressure in the metal chloride chamber. Nonetheless, in the real experimental study, ZrO2 was observed to result from this reaction. This anomaly can be explained by the extreme external conditions, which most likely result from the very low partial pressure of chlorine in the metal chloride chamber, which shifts the thermodynamic equilibrium. Nevertheless, in the real CVD/EVD experiment, the amount of metal chloride that was consumed due to film growth is much lower than the rate of metal chloride sublimation. In the CVD/ EVD experiment, a relatively large carrier gas flow, which was used as the vehicle for metal chloride distribution, can also work to sweep away chlorine (generated from the reaction), resulting in a low partial pressure of chlorine. Therefore, this condition can impart a positive driving force for the EVD process [35]. The theoretical growth rate of the YSZ layer deposited at 1000 °C (where pore diffusion is the rate-limiting step) can be calculated with Eq. (A.10) [36], where dl/dt is the growth rate (μm/h) and T is the temperature (K).

dl 82 = dt T

Sample No.

#1

#2

#3

Pore closure time (h) Deposition time (h) YSZ film thickness after APS (μm) YSZ film thickness after CVD/EVD (μm) Growth rate dl/dt (μm/h) Gas tight

1 5 20 24.7 1.18 ✓

1 5 20 25.56 1.39 ✓

1 5 20 27 1.75 ✓

(A.10)

The theoretical growth rate of YSZ layer at 1000 °C is approximately 2.3 µm/h. However, the average experimental growth rate is somewhat lower than the theoretical value. The experimental growth rates ranged from 1.1 to 1.8 µm/h, with an average of 1.44 µm/h. Brinkman et al. [37], also reported the same condition, where the theoretical growth rate are bigger than the experimental growth rate. The possibility to explain the discrepancies between theoretical and experimental growth rate is that the formed YSZ layer were not totally cubic phase but also consisted of second phases (e.g. monoclinic). The effective values of the electron and also electron hole conductivities of cubic YSZ are lower, because of the relative amount of cubic YSZ is smaller than unity [36,38]. In terms rate limiting steps, the predicted/ theoretical growth rates which were larger than the experimental observations indicates that, the bulk electrochemical transport may not be the rate limiting step (at deposition temperature 1000 °C). At temperature below 1000 °C, the oxygen ion bulk transport should be the rate limiting step because the bulk diffusion resistance in the YSZ larger than that at 1000 °C due to the large activation energy for electron conduction in YSZ [27]. At 1000 °C, it is very likely that the pore diffusion of water vapor through substrate pores is the rate limiting step for the film growth, because of the lower oxygen partial pressure in both reactor chambers. This is confirmed by the order of magnitude for the layer growth between theoretical and experimental are still well comparable [35,37]. On top of the metal supports, an additional YSZ layer with a thickness ranging from 4 to 7 µm was deposited over a total deposition time of 5 h to provide gas-tight conditions after the APS process. A summary of the experimental CVD/EVD results is given in Table 6. Fig. 9 shows the cross-sectional view of the YSZ layer after CVD/ EVD refinement, along with the elemental mappings. The elemental mappings of zirconium, yttrium and oxygen clearly show that the YSZ layer was distributed on the surface of the metal support where

Fig. 9. Cross-sectional SEM images of the YSZ layer after CVD/EVD refinement along with the elemental mappings: (a) sample #1, (b) sample #2, and (c) sample #3.

deposition and refinement occurred. From the elemental analysis, Zr and Y were observed to penetrate deeper into the metal support. This may originally result from ZrCl4 or YCl3 gas molecules diffusing into the pores as the reaction is heated to the deposition temperature (1000 °C). Therefore, when H2O is supplied to the reactor, it will directly react with ZrCl4/YCl3 to form ZrO2 and Y2O3 [34]. Compared 8

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Table 8 XRD intensity data of the YSZ powder and YSZ prepared after the CVD/EVD process. Orientation (hkl)

I (%) Powder

Cubic zirconia/yttria layer (YSZ) 111 100 200 23 220 43 311 26 222 7 400 5 331 8 420 5

to Zr, Y penetrates deeper and is much more scattered in the elemental mapping image. This may occur because the diffusivity of YCl3 is larger than that of ZrCl4 [26].

3.4. XRD and film composition analysis To examine the phases before and after CVD/EVD, the XRD pattern of YSZ deposited on the metal support are given in Fig. 10. As indicated in the XRD pattern, the YSZ layer before CVD/EVD (after the APS process) showed only the cubic phase. The phase resulting after the CVD/EVD process was also mostly cubic, as shown by the enhancement in the YSZ cubic intensity compared to before the CVD/EVD process. However, a certain amount of the monoclinic phase was observed. YSZ with monoclinic and cubic phases can easily be distinguished by its peaks. For the monoclinic phase, the 2θ of the (111) and (111) peaks are 28.2° and 31.5°; meanwhile, for the cubic phase, the 2θ peak is around 30° [36]. Table 7 shows the composition of yttrium oxide and the fraction of the monoclinic phase in the YSZ layer. The composition of yttrium oxide in the YSZ layer was in the range of 7.3–7.9%. The Y/Zr ratio in the layer is controlled by the vapor pressure of each metal chloride reactant at the given temperature during deposition. Compared to the yttrium fraction in the vapor phase calculated from the equilibrium vapor pressure at the desired sublimation bed temperature, the composition of yttrium oxide in the layer is nearly the same as the desired value (8 mol%). From the investigations of Carolan-Michaels [39] and Xomeritakis-Lin [40], the composition of yttrium oxide in the film will be the same as that in the vapor phase. The discrepancy between the calculated value and the real value may result from several major reasons, such as deactivation of the YCl3 powder reactant, resulting in a lower vapor pressure during the CVD/EVD experiment; the carrier gas flow rate; the flow pattern; and factors associated with the design of the reactor. In all cases, a secondary phase was spotted in the YSZ layer, and a decrease in the yttria content in the layer may

Vm (%)

#1 #2 #3

7.547 7.332 7.815

7.02% 2.48% 2.05%

#3

100 21 45 28 6 7 8 6

100 22 38 23 5 5 6 5

100 21 40 27 6 4 8 5

4. Conclusions The fabrication of an YSZ layer on top of a metal support via the APS process and the refinement of the electrolyte density/tightness through a CVD/EVD process were reported. Deposition of the anode material by screen printing gives properties that are beneficial for pore size reduction by the CVD/EVD process. The pore size of the metal support is reduced by a factor of 20; meanwhile, the Knudsen number increases steeply to over 10, which means that the Knudsen diffusion regime (favored condition) will be much more dominant in the CVD/ EVD process. After YSZ deposition via the APS process, an YSZ layer with a thickness of 20 µm was deposited on the metal support. However, residual pores/pinholes were still observed, which allows gas diffusion, as shown by the room-temperature gas permeation value of 1.8–2.7 (×10−7 mol/cm2 s). The CVD/EVD refinement process to plug the residual pores and provide additional thickness to the YSZ layer was successfully performed. An additional YSZ thickness of 4–7 µm can be prepared after CVD/EVD to ensure the tightness of the YSZ electrolyte. The average film growth rate during CVD/EVD was approximately 1.14 µm/h at 1000 °C and 10 Torr. Gas permeation tests conducted after the CVD/ EVD process showed a zero permeation rate after 5 h of deposition, and a dense and gas-tight YSZ layer was obtained. The XRD, SEM and

Table 7 Yttrium oxide composition and volume fraction of monoclinic (Vm) in the YSZ layer deposited after the CVD/EVD process. Y2O3 (mol-%)

#2

produce this second phase. In this study, the volume fraction of the monoclinic phase (Vm) observed in the yttria/zirconia layers was in the range of 2–7%. Metal chloride aging due to air/oxygen leakage into the reactor (which is practically unavoidable), especially for yttrium chloride, may result in a low Y/Zr content in the vapor phase, leading the YSZ layer to transform into a secondary phase. An inaccurate location of the sublimation bed inside the reactor is another possible cause of this observation. Table 8 summarizes the comparison of the relative peak intensities in each of the YSZ cubic phase orientations for the YSZ powder (for reference) and for the YSZ layer prepared after the CVD/EVD process. From the data, we note that the intensities over the CVD/EVD layers at several orientation were increased, however, there is also another orientation in which the intensities are lower than those of the powder, which may occur because the film grew preferentially in another orientation. The orientation and faceting of the YSZ films reveal that a second transport process occurs in conjunction with oxygen diffusion to the film. If bulk charge transport is the limiting step, then the film will not be oriented and will instead have a smooth surface. On the other hand, if surface diffusion of the reactive species is the limiting step, faceting and orientation will occur. During the surface diffusion process, adsorbed species diffuse more through faster growing crystals than they do through slower growing crystals; therefore, faceting and oriented film growth can be achieved [26,39]. The observed data, which shows a variety of faceted phases, indicate that, in several cases, bulk transport is the limiting factor for film growth; meanwhile, for the rest of the cases, surface diffusion is the limiting factor for growth.

Fig. 10. XRD pattern of YSZ deposited on the porous metal support after the CVD/EVD refinement process. C: cubic, M: monoclinic.

Sample No.

#1

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elemental mapping analyses showed that the YSZ layer was homogeneously deposited on the surface of the metal support, and most YSZ layers exhibited a dominant cubic orientation; however, a secondary phase was also observed. The volume fraction of the monoclinic phase (Vm) observed in the layers was in the range of 2–7%, while the composition of yttrium oxide in the YSZ layer was in the range of 7.3– 7.9%. The present result showed that the refinement of the YSZ electrolyte by plugging the residual pores/pinholes, along with the addition of YSZ thickness, by the CVD/EVD process was successfully performed for use in MS-SOFC applications. Acknowledgement This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153030040930). References [1] R. Fernández-González, E. Hernández, S. Savvin, P. Núñez, A. Makradi, N. Sabaté, et al., A novel microstructured metal-supported solid oxide fuel cell, J. Power Sources 272 (2014) 233–238. [2] V.A. Rojek-Wöckner, A.K. Opitz, M. Brandner, J. Mathé, M. Bram, A novel Ni/ ceria-based anode for metal-supported solid oxide fuel cells, J. Power Sources 328 (2016) 65–74. [3] Y.-M. Kim, P. Kim-Lohsoontorn, S.-W. Baek, J. Bae, Electrochemical performance of unsintered Ba0.5Sr0.5Co0.8Fe0.2O3−δ, La0.6Sr0.4Co0.8Fe0.2O3−δ, and La0.8Sr0.2MnO3−δ cathodes for metal-supported solid oxide fuel cells, Int. J. Hydrog. Energy 36 (2011) 3138–3146. [4] P. Blennow, J. Hjelm, T. Klemensø, Å. Persson, K. Brodersen, A.K. Srivastava, et al., Development of planar metal supported SOFC with novel cermet anode, ECS Trans. 25 (2) (2009) 701–710. [5] G. Accardo, L. Spiridigliozzi, R. Cioffi, C. Ferone, E. Di Bartolomeo, S.P. Yoon, et al., Gadolinium-doped ceria nanopowders synthesized by urea-based homogeneous coprecipitation (UBHP), Mater. Chem. Phys. 187 (2017) 149–155. [6] C.-J. Li, C.-X. Li, X.-J. Ning, Performance of YSZ electrolyte layer deposited by atmospheric plasma spraying for cermet-supported tubular SOFC, Vacuum 73 (2004) 699–703. [7] G. Accardo, C. Ferone, R. Cioffi, D. Frattini, L. Spiridigliozzi, G. Dell'Agli, Electrical and microstructural characterization of ceramic gadolinium-doped ceria electrolytes for ITSOFCs by sol-gel route, J. Appl. Biomater. Funct. Mater. 14 (2016) e35–e41. [8] G. Dell'Agli, L. Spiridigliozzi, A. Marocco, G. Accardo, C. Ferone, R. Cioffi, Effect of the mineralizer solution in the hydrothermalsynthesis of gadolinium-doped (10% mol Gd) ceria nanopowders, J. Appl. Biomater. Funct. Mater. 14 (2016) e189–e196. [9] J.W. Fergus, R. Hui, X. Li, D.P. Wilkinson, J. Zhang, Solid Oxide Fuel Cells: Materials Properties and Performance, CRC Press, USA, 2009. [10] M. Irshad, K. Siraj, R. Raza, A. Ali, P. Tiwari, B. Zhu, et al., A brief description of high temperature solid oxide fuel cell's operation, materials, design, fabrication technology and performance, Appl. Sci. 6 (2016) 75. [11] D. Marcano, G. Mauer, R. Vaßen, A. Weber, "Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS) and plasma spray-physical vapor deposition (PS-PVD), Surf. Coat. Technol. 〈http://doi.org/ http://dx.doi.org/10.1016/j.surfcoat.2016.10.088〉. [12] K. Kikuchi, K. Okada, A. Mineshige, Growth mechanism of thin films of yttriastabilized zirconia by chemical vapor infiltration using NiO–ceria substrate as oxygen source, J. Power Sources 162 (2006) 1060–1066. [13] L.C.D. Jonghe, C.P. Jacobson, S.J. Visco, Supported electrolyte thin film synthesis of solid oxide fuel cells, Annu. Rev. Mater. Res. 33 (2003) 169–182. [14] Gil-Sung Kim, Sang-Chul Hwang, Yoen-Su Kim, H.-S. Shin, Growth of Yttriastabilized zirconia thin films on textured silver substrates by chemical vapor

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