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Effect of Gd-doped ceria interlayer on the stability of solid oxide electrolysis cell
Solid State Ionics 295 (2016) 25–31
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Effect of Gd-doped ceria interlayer on the stability of solid oxide electrolysis cell Sun Jae Kim a, Sun Woong Kim a, Young Min Park b, Kun Joong Kim a, Gyeong Man Choi a,⁎ a b
Fuel Cell Research Center, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Fuel Cell Project, Research Institute of Industrial Science and Technology (RIST), Pohang 790-330, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 April 2016 Received in revised form 18 July 2016 Accepted 19 July 2016 Available online xxxx
a b s t r a c t Gd0.2Ce0.8O2-δ (GDC) interlayers, positioned between La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) anode (air electrode) and yttria-stabilized zirconia (Y0.16Zr0.84O2-d, YSZ) electrolyte of solid oxide electrolysis cells (SOECs) to prevent mutual reaction, are compared that used either thin-film deposition method to produce dense layers or screenprinting to produce porous layers. Two cells with 2 μm-thick GDC layers are sputter deposited, one annealed at 1300 °C and the other at 1100 °C. During SOEC operation with an anodic current density of −800 mA cm−2 in 80% H2O + 20% H2 at 800 °C, the two cells show similar Ohmic and polarization resistances at the beginning of measurement, but the cell annealed at 1300 °C is more stable after 100 h than the film annealed at 1100 °C due to the stability of pinhole free nature of GDC layer. In a third cell with ~11 μm-thick screen-printed GDC layer, GDC is porous and therefore the cell degrades more rapidly during SOEC operation for 100 h than does a cell with ~2 μm-thick, pin-hole free GDC film. The good stability of the cell with dense, pin-hole free GDC thin film is attributed to its blocking of Sr. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered as promising fuel for sustainable future [1–3]. Thus, we need solid oxide electrolysis cells (SOECs) which decompose water and generate H2 gas at high temperature (500– 1000 °C) when electricity is applied. SOEC is promising because it shows higher efficiency than low-temperature electrolysis devices (e.g., polymer or alkaline electrolysis cells) [4]. SOECs exploit the same electrochemical reaction that occurs in solid oxide fuel cells (SOFCs), but in reverse. Water steam reduced by electrons produces H2 fuel at the cathode side (fuel electrode), O2– ions are simultaneously oxidized to O2 gas and release electrons at the anode side (air electrode) [5–7]. When yttria-stabilized zirconia (YSZ) is used as the electrolyte, Ni-YSZ cermet is used as a fuel electrode. As an air electrode, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is popular because it is a mixed ionic and electronic conductor (MIEC). Oxygen desorption occurs on the entire surface of LSCF and LSCF reduces oxygen pressure at the interface between LSCF and GDC due to its MIEC nature during electrolysis. Thus LSCF is also more stable than electronic conducting anodes such as (La,Sr)MnO3-δ (LSM) which often delaminates from GDC interlayer due to the increased oxygen pressure at the interface [8–11]. LSCF is a good air electrode, but when it is annealed at high temperature (~1000 °C), an interlayer (or diffusion barrier layer) between LSCF and YSZ must be included to prevent the reaction and the formation of electrically-insulating compounds such as La2Zr2O7 [12–14]. Gd-doped ceria (GDC) [15] is a typical interlayer, but with it LSCF still degrades or the polarization resistance Rp increases during operation of solid
http://dx.doi.org/10.1016/j.ssi.2016.07.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
oxide fuel cell (SOFC). The degradation is ascribed to Sr diffusion from LSCF matrix to the interface between GDC interlayer and YSZ electrolyte [16,17]. Sr diffuses through the porous GDC interlayer either during firing of the LSCF layer or during cell operation. Sr-rich phase forms at the interface between the porous GDC interlayer and dense YSZ electrolyte after LSCF firing at 1040 °C [18] to 1250 °C [19]. Sr also segregates during SOFC operation at 750 °C [20]. Sr can also accumulate at the surface of LSCF at 750–800 °C [21,22]. Thus, when a porous GDC interlayer is used, Sr diffusion to the surface or interface is unavoidable during SOFC operation above 750 °C. Porous screen-printed GDC cannot prevent Sr diffusion during SOEC operation at anodic current −1.0 A cm−2 at ~780 °C [23]. Because porous GDC cannot block Sr diffusion, use of a dense GDC interlayer has been suggested. However, Sr diffusion through dense GDC film coated by pulsed laser deposition (PLD) method and formation of Sr2Zr2O7 phase at the interface between YSZ and GDC have been reported during SOFC operation under cathodic current [24,25]. Pinholes or cracks in the thin film may provide paths along which Sr diffuses during SOFC operation [24]. However, Sr diffusion through dense GDC thinfilm during SOEC operation has not been reported. In this study, we prepared porous GDC interlayer films and dense GDC interlayer films to compare how they affect the degree of Sr diffusion and degradation of SOEC cells. Porous thick-films were prepared by screen printing; dense thin-films were prepared by sputtering. The stability of the cell was tested and compared for 100 h by measuring impedance and observing microstructure.
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2. Experimental Fuel-electrode (cathode) supported cells (Ni-YSZ support/Ni-YSZ fuel electrode/YSZ electrolyte) were fabricated by tape-casting, lamination, and co-firing (TLC method) as reported elsewhere [26]. Laminated green tapes were co-fired at 1400 °C for 5 h in air. The thicknesses of the layers were ~ 0.6 mm (Ni-YSZ support), ~ 10 μm (NiO-YSZ fuel electrode), and ~ 15 μm (YSZ electrolyte). After co-firing, a Gd0.2Ce0.8O2-δ (GDC) interlayer was deposited on the surface of the YSZ electrolyte; thick-films were prepared using screen printing, and thin-films were prepared using sputtering. 2.1. Screen printing of GDC GDC powder was synthesized by a solid state reaction. First, 80 mol% CeO2 and 20 mol% Gd2O3 (both 99.9%, Kojundo Chemical, Japan) were mixed by ball milling with zirconia balls in ethanol for 24 h. The mixed powder was calcined at 1200 °C for 2 h, then planetary milled (Pulverisette 6, Fritsch, Germany) with zirconia balls in ethanol for 4 h. GDC pastes for coating were prepared by mixing GDC powder with an appropriate solution of α-terpineol and ethylene cellulose. GDC paste was coated on top of the fired YSZ half-cell (YSZ/Ni-YSZ) by screen-printing, then fired at 1300 °C for 5 h. The cell with screenprinted GDC was named SP13 (Table 1). 2.2. Sputtering of GDC The GDC target for sputtering was prepared by uni-axial pressing of powder to form a pellet, then sintered. First, 91 mol% CeO2 and 9 mol% Gd2O3 of stoichiometric ratio (Gd0.09Ce0.91O2-δ) were mixed by planetary milling with zirconia balls in ethanol for 4 h. After uni-axial pressing of the powder, a green pellet was obtained and sintered at 1400 °C for 4 h. Power of RF sputtering was 250 W and vacuum pressure in the chamber was 1.5 × 10−2 Torr (Ar:O2 = 4:1). During sputtering, substrate temperature was maintained at 500 °C. Sputtered GDC after annealing at 1100 or 1300 °C showed composition of Gd0.15Ce0.85O2-δ as shown by Energy-dispersive X-ray spectroscopy (EDS, Philips electron optics B.V., Netherlands). The cells with GDC coating were annealed at 1100 °C for 2 h (named TF11) or at 1300 °C for 5 h (TF13) (Table 1). La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) powder (AGC Seimi Chemical, Japan) was mixed with a solution of α-terpineol and ethylene cellulose to form paste, which was screen-printed on top of the GDC interlayer and fired at 1040 °C for 2 h. The area of the LSCF electrode was 0.502 cm2. Pt mesh (52 mesh, Alfa Aesar, USA) as a current collector was attached to the electrode by using Pt paste (Item 6082, Heraeus, Germany) and fired at 850 °C for 2 h. The cells for test were mounted on an alumina tube and sealed with ceramic sealant (Aremco Products, USA). Fuel gas had 80% H2O + 20% H2 and flow rate was ~150 cm3 min−1 . The air electrode (anode) was exposed to open air. Humidity was controlled by flowing H2 gas through a water tank which was heated at ~94 °C to produce 80% H2O + 20% H2 mixture gas and positioned in a SOFC test station (P&P Energytech, Korea). Humidity was confirmed by measuring the open circuit voltage (OCV) of a single cell. The electrochemical performance of the cell was measured at 800 °C by using an impedance analyzer (VSP, Bio Logic Science instruments, Table 1 Cell notation, GDC coating method, firing temperature and time of GDC coated YSZ/Ni-YSZ cell, and thickness of GDC interlayer. YSZ/Ni-YSZ layers were tape cast, laminated, and cofired at 1400 °C for 5 h before GDC coating. GDC thin films (TF11, TF13) were sputter coated on YSZ/Ni-YSZ at 500 °C. Cell notation TF11 TF13 SP13
GDC Coating method Sputtering Screen Printing
Firing condition
GDC Thickness (μm)
1100 °C / 2 h 1300 °C / 5 h
2 11 ± 3
France). Current-voltage (I-V) curves were obtained, then impedance was measured. I-V curves were sequentially measured in SOFC mode and SOEC mode. During a test for 100 h, impedance was measured at open circuit condition after stopping the electrolysis (or anodic) current of −800 mA cm−2. The electrolysis current was applied again after impedance measurement. The microstructure and distributions of atomic elements (Zr, Ce, Sr, Co) across the cell were examined before and after the SOEC test by using a scanning electron microscope (FE-SEM, Philips electron optics B.V., Netherlands) and energy dispersive spectroscopy (EDS). Phases on the GDC surface were identified using X-ray diffraction (XRD, Rigaku, Japan). To clarify the effect of the GDC interlayer, impedance spectra were fitted using a complex nonlinear least square (CNLS) method of an equivalent circuit. An Excel-based program (“Z-Dance”) was used for the fitting which was developed in our laboratory. The program utilizes fitting algorithms similar to many commercial programs and Excel functions. 3. Results and discussion Although GDC films show slight difference in composition (screenprinted: Gd0.2Ce0.8O2-δ; sputtered: Gd0.15Ce0.85O2-δ), the difference in the ionic conductivity can be ignored with Gd content between 0.15 and 0.20 [27]. The surface and cross-section of GDC films were observed and compared (Fig. 1). GDC thin film annealed at 1100 °C (TF11) showed more cracks on the surface along the boundaries (Fig. 1a), but GDC thin film annealed at 1300 °C (TF13) showed nearly crack-free surface (Fig. 1d). Cross sections of both TF 11 (Fig. 1b) and TF13 (Fig. 1e) cells showed ~2 μm-thick GDC layers on YSZ electrolytes with no obvious cavities or pores. No clear boundaries can be found between GDC and YSZ in SEM image, however, boundary lines are drawn for easy of view with the help of EDS profile which was discussed later. In the TF11 cell the GDC film consisted of columnar grains (Fig. 1b), but in the TF13 cell this structure was absent, and GDC film was nearly indistinguishable from YSZ (Fig. 1e). High annealing temperature and long duration (1300 °C for 5 h) apparently eliminated pinholes by additional sintering of GDC film. The GDC layer of the SP13 cell showed large interconnected pores on GDC surface (Fig. 1g), and a porous GDC layer (thickness ~ 11 μm) (Fig. 1h) that was ~5 times thicker than thin-film GDC. In a previous study [18], a cell with GDC interlayer (~6 μm) that had been tape cast, laminated with YSZ layer and support, and cofired at 1300 °C for 5 h showed delamination of GDC from YSZ after a similar electrolysis test. The microstructure (Fig. 1c, f, i) after electrolysis test will be discussed later. “The strongest peak of GDC at ~28.62° (2θ) (111) was observed for all three samples as expected [27,28] and chosen as a reference, and data of three samples were compared over 25° ≤ 2θ ≤ 50°. The reaction of GDC films with underlying YSZ may be examined by observing the change of lattice constant of GDC after annealing at 1100 °C (TF11) or 1300 °C (TF13, SP13). Smaller lattice constant of thick-film SP13 (~0.541 nm) than that of bulk GDC (~0.543 nm) [29,30] may indicate slight Zr+ 4 (0.084 nm) substitution into Ce+ 4 (0.097 nm) site [30]. However, the increased lattice constants of TF11 (~0.545 nm) or TF13 (~0.546 nm) thin-films from the bulk value do not prove the clear reaction since thin film may have additional change due to strain in the thin film.” The grain size of the GDC layer was ~ 50 nm in TF11 (Fig. 2a) and ~ 100 nm in TF13 (Fig. 2b) cells, as estimated using the full width at half maximum (FWHM). The grain sizes of GDC, estimated from SEM surface image, were slightly larger than those from XRD, ~ 90 nm for TF11 (Fig. 1a) and ~140 nm for TF13 (Fig. 1d) cells, respectively. Grain size of GDC in TF13 cell was 1.5–2 times larger than that of TF11 cell due apparently to higher annealing temperature. The grain size of GDC in SP13 cell was much larger than those of TF11 or TF13 cells, ~ 600 nm, since GDC particles in screen-printing paste are ~ 100 nm that increased in size after firing.
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Fig. 1. SEM images of cells with GDC interlayers: (a, b, c) TF11, (d, e, f) TF13 and (g, h, i) SP13. (a, d, g) GDC surface, cross-section of GDC/YSZ; (b, e, f) before test, (c, f, i) after test. LSCF layers are also used during test (c, f, i). Note that the scale used for SP13 is different from that used for TF11 and TF13.
YSZ peaks were detected at 30.06° (2θ) (101) and 34.88° (2θ) (110) on the GDC surface in both TF11 and TF13 cells (YSZ: JPDS 01-0821244). The TF11 cell showed large YSZ peaks, due possibly to the xray penetration through the cracks in the film. Thus, XRD was successfully used to find cracks or pinholes in the GDC (Fig. 1b). On the other hand, the TF13 cell (Fig. 2b) showed weak YSZ peaks since nearly pinhole free GDC layer blocks most of x-ray penetration through the layer. The SP13 cell (Fig. 2c) did not show any YSZ peak, X-rays cannot penetrate ~11 μm-thick GDC layer despite its large pores. I-V curves of three cells were measured and compared at the time (0 h) when cell temperature reached 800 °C (Fig. 3). I-V curves were sequentially measured in SOFC mode and SOEC mode. The curves were measured in SOFC mode, then in SOEC mode. The measured OCV (~0.87 V) was slightly lower than the theoretical value (0.94 V) calculated using the Nernst equation and H2O/H2 ratio [31]. With anodic current of − 800 mA cm−2, the voltages of all three cells were ~ 1.1 V,
Fig. 2. XRD patterns of GDC interlayer before SOEC test obtained from GDC surface. GDC (111) peak was used as a reference peak. (a) TF11, (b) TF13 and (c) SP13.
which is lower than thermal neutral voltage (~1.3 V) [32,33]. At 1.3 V, electrolysis current may reach ~1.5 A cm−2. The three cells show similar slopes at open circuit condition (J = 0 mA cm−2), but the slopes changed when SOFC or SOEC current was high. As SOFC current increased, the voltage decrease was fastest in the TF11 cell and slowest in the TF13 cell among three cells. Because the cathodes (Ni-YSZ) are the same, the difference must be a result of the difference in GDC interlayers. The difference is smaller in SOEC mode than in SOFC mode, but the trend is similar. The I-V behavior of the SP13 cell with screenprinted GDC layer is similar to that of a previous cell with tape-cast GDC layer [18]. From this observation, we conclude that the morphology of GDC layer has more effect on the I-V curve in SOFC mode than in SOEC mode. In other words, the TF13 cell with dense GDC layer shows the best I-V performance.
Fig. 3. Current-Voltage (I-V) curves of fuel-electrode (Ni-YSZ) supported cells with different GDC interlayers at 0 h: (a) TF11, (b) TF13 and (c) SP13. Electrolysis current is −800 mA cm−2. SOFC was measured first, then SOEC.
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Fig. 4. Impedance spectra of cells with different GDC interlayers: (a) TF11, (b) TF13, and (c) SP13, for 100 h with 80% H2O + 20% H2. The impedance spectra were obtained at open circuit condition after stopping electrolysis current (J = −800 mA cm−2) at 800 °C.
After the I-V curve test, the impedance spectra of three cells TF11, TF13, and SP13 GDCs were obtained at open circuit condition after stopping the electrolysis current (−800 mA cm−2) after 1, 10, 20, 50, and 100 h operation at 800 °C (Fig. 4). Total ASR (area specific resistance) Rtot is the sum of Ohmic RΩ and polarization Rp resistances. At the beginning of measurement (0 h), TF11 cells had Rtot ~ 0.25 Ω cm2 and TF13 cells had Rtot ~ 0.23 Ω cm2; these values were similar and reasonably close to those calculated (~ 0.27 Ω cm2, ~ 0.22 Ω cm2, respectively) from the slopes of I-V curves near OCV (Fig. 3a, b). TF11 and TF13 both had RΩ ~ 0.07 Ω cm2. At 0 h, TF11 had Rp ~ 0.18 Ω cm2 and TF13 had Rp ~ 0.16 Ω cm2. Although CeO2 reacts with ZrO2 to form solidsolution phase which is more resistive than either CeO2 or ZrO2 at high temperature, the difference in annealing temperature had little effect on initial RΩ as similarly reported [34]. However, during SOEC operation for 100 h, the responses of RΩ and Rp of cells differed widely. In TF11, beginning at 20 h, RΩ increased rapidly to 0.45 Ω cm2 at 100 h, and Rp increased to ~1.4 Ω cm2 (Fig. 4a). This rapid increase or degradation is attributed to delamination of LSCF from GDC layer; similar sudden increases in RΩ and Rp due to delamination in SOEC operation have been reported previously [26]. After SOEC operation for 100 h, microstructure of the TF11 cell (Fig. 1c) showed delamination of LSCF from GDC, and cracks in GDC layer. Sr in LSCF may have diffused through the pinholes of GDC film, and may have thereby contributed to the formation of SrZrO3 or Sr3Zr2O7 and the delamination of LSCF [24]. It is unclear why LSCF in TF11 cell delaminates from GDC. One possibility is that the second phase at GDC/YSZ interface may have affected the oxygen partial pressure (Po2) at LSCF/GDC interface and the increased Po2 during electrolysis may have cracked thin GDC layer and delaminated LSCF layer [18, 26]. In the TF13 cell, over the 100 h, RΩ increased slightly from 0.07 to 0.09 Ω cm2 and Rp increased slightly from 0.163 to 0.165 Ω cm2 (Fig. 4b). The TF13 cell showed little delamination of LSCF, and dense GDC was maintained after electrolysis for 100 h (Fig. 1f). The relative stability of RΩ and Rp was attributed to the crack-free microstructure of GDC film during the SOEC test. SP13 had Rtot ~ 0.26 Ω cm2, which is close to the value (~0.28 Ω cm2) calculated from the I-V curve (Fig. 3c). At 0 h, the SP13 cell had RΩ ~ 0.11 Ω cm2, which is ~ 1.5 times larger than the RΩ ~ 0.07 Ω cm2 of the TF13 cell. Although the SP13 cells are ~11 μm thick whereas the
TF13 cells are 2 μm thick their calculated RΩ assuming dense GDC layers are 0.054 Ω cm2 for SP13 cells and 0.051 Ω cm2 for TF13 cells. The similarity is due to the high conductivity of GDC (~0.30 S cm−1 at 800 °C) [35,36]. RΩ is determined mostly by the thick (~ 15 μm) and resistive (~0.03 S cm−1 at 800 °C) YSZ layer. If we assume that the interface reaction between GDC and YSZ is similar in SP13 and TF13 cells, because they were both fired at 1300 °C, the only factor that affects RΩ is the contact area between GDC and YSZ. Thus the smaller RΩ ~ 0.07 Ω cm2 of TF11 or TF13 cell than the RΩ ~ 0.11 Ω cm2 of SP13 cell is ascribed to the good contact between dense GDC thin-film and LSCF, or between GDC and YSZ layers. A previous cell with tape-cast GDC layer had RΩ ~ 0.27 Ω cm2, which is more than double the RΩ ~ 0.11 Ω cm2 of SP13 cell, possibly due to the poor contact which is the reason for the delamination of GDC from YSZ layer after 20 h [18]. Rp of SP13 was ~ 0.16 Ω cm2, slightly smaller than that of TF 11 (~0.18 Ω cm2) and similar to that of TF13 (~0.16 Ω cm2) at 0 h. A previous cell with tape-cast GDC had Rp ~ 0.17 Ω cm2 [18]. Therefore, at 0 h, the cells with thin GDC film (TF11 and TF13) show relatively small RΩ, whereas the cell with thick and porous GDC (SP13) shows small Rp. After 100 h, RΩ and Rp of the SP13 cell increased substantially to ~0.13 Ω cm2 and ~0.21 Ω cm2, respectively. The layers did not delaminate, and GDC was still porous after the test. Therefore, the cell with thin and dense GDC (TF13) has advantages because it has lower Rtot and slower degradation rate than do cells with thick and porous GDC (SP13). Cell voltage while applying electrolysis current (− 800 mA/cm2) was recorded as function of operation time for 100 h (Fig. 5). Cell voltage of TF11 rapidly increased after 20 h, 1.14 V to 2.53 V after 100 h; this change corresponds to the Rp degradation shown in impedance spectra (Fig. 4a). This abrupt change was also observed in a cell with tape-cast GDC and was similarly ascribed to delamination [18]. However, the initial voltage (1.08 ± 0.01 V) of the three cells was similar (Fig. 3). Cell voltages of TF13 and SP13 increased slowly for 100 h, but were still lower than thermal neutral voltage (~1.3 V). Thus, electrolysis current may be further increased to ~1500 mA cm−2 to reach thermal neutral voltage as shown in I-V curves (Fig. 3). Rates of voltage increase were calculated either using initial and final voltages, 1.09 V to 1.19 V and 1.09 V to 1.13 V for SP13 and TF13, respectively, or by linear fitting of all voltage data of cells for 100 h. Both calculations showed similar values. TF13 cell with degradation rate RD ~ 0.4 × 10− 3 V h−1 is apparently more stable than SP13 cell with RD ~ 1.0 × 10− 3 V h− 1. Delamination of tape-cast GDC from YSZ is prevented by the introduction of contact layer (Ce0.43Zr0.43Gd0.1Y0.04O2-δ) between them, and the degradation rate (1.16 V to 1.21 V for 100 h; i.e., RD ~ 0.5 × 10−3 V h−1) of stabilized cell with contact layer is close to that of the TF13 cell [18]. Thus the TF13 cell shows the smallest voltage drop and slowest degradation rate.
Fig. 5. Voltage of TF11, TF13, and SP13 cells while applying electrolysis current (−800 mA cm2) was recorded for 100 h. Note that two different y-axis scales were used.
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Impedance spectra of TF13 and SP13 cells may be compared to analyze the difference in anodic resistance and degradation. Anodic and cathodic Rp of electrolyte-supported cell may be separated using a 3-electrode configuration with a reference electrode [37–40]. Separation of cathodic Rp from anodic Rp may also be possible using complex nonlinear least square (CNLS) fitting and an equivalent circuit in a fuelelectrode supported cell [41–43]. Because both cells used the same type of fuel electrode, we assumed that the difference in impedance is mostly due to the air electrode (LSCF), which also normally shows ~ 3 times larger Rp than does the fuel electrode (Ni-YSZ) at 800 °C [37]. The equivalent circuit (Fig. 6a) for fitting of impedance spectra consists of an inductor L and a resistor R0 with resistance R0 connected with four RQ parallel circuits in series. During the impedance measurement, inductance was unavoidable which was originated from test conditions (test jig, lead wire, furnace heater etc.). In this study, inductance value was estimated as ~0.1 μH cm2 (measured value ~0.2 μH). R0 represents RΩ from the electrolyte, and resistors R1, R2, R3 and R4 with resistances R1, R2, R3 and R4 in the RQ circuits represent polarization resistances Rp associated with electrochemical reactions from electrode. Q is a constant-phase element (CPE) for a non-ideal capacitor. Many equivalent circuits have been suggested for SOFCs [42–45]. Anode-supported cell with four R-Q parallel elements has been shown [44]. Although all impedance spectra show two large overlapping depressed semicircles, the curve-fitting results show four possible semicircles. We have not tried to separate the anodic from the cathodic contribution
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due to its complexity. However, previous study shows that the air electrode contributes more dominantly than the fuel electrode [37]. We have further simplified the spectra to two depressed semicircles by combining semicircles: RH = R1 + R2 represents resistance at high frequency, and is mostly due to charge transfer reaction; RL = R3 + R4 represents resistance at low frequency, and is mostly due to surfaceexchange kinetics [45,46]; Rp = RH + RL. In the TF13 cell, RH was N RL at both 0 and 100 h (Fig. 6b); in contrast, in the SP13 cell RH was slightly bRL at 0 h, and both RH and RL had increased by 100 h (Fig. 6c). Capacitance values of two samples were estimated from the fitted CPE Q1–Q4 values and similar at 0 h (Fig. 6a); ~ 1.1 × 10−3, ~ 4.5 × 10−3, ~4.2 × 10−2 and ~8.5 × 10−1 F cm−2, respectively. RΩ, RH, RL, and Rp were plotted versus operation time (Fig. 7). The TF13 cell showed similar RΩ (0.056 Ω cm2) and RL (0.058 Ω cm2) values that are ~1/2 of RH value (0.102 Ω cm2) at 0 h (Fig. 7a). RΩ value fluctuated slightly for 50 h and increased slightly after 50 h. Slight increase of RΩ for 280 h has also been reported for the similar GDC film coated between LSCF and YSZ [47]. Both RH and RL, thus Rp, were stable or even slightly decreased after 20 h. Thus the TF13 cell showed good stability at least for 100 h although slight decrease and increase were shown before 20 h. The SP13 cell had RL = 0.082 Ω cm2 ≈ RH = 0.079 Ω cm2 at 0 h (Fig. 7b). RΩ value increased rapidly for 100 h. Rp increase was in similar trend with RΩ increase after 20 h. Both RH and RL increased with time for initial 5 h, decreased slightly from 5 to 50 h, then increased after
Fig. 6. (a) Equivalent circuit used for fitting of measured impedance spectra. Fittings for (b) TF13 and (c) SP13 at 0 and 100 h.
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We have not shown EDS line profile of TF11 cell since the degradation is clearly shown due to the delamination of LSCF from GDC. In the TF13 cell, dense (initially 2 μm-thick) GDC was located between LSCF and YSZ (Fig. 8a). Although a small amount of Co and Sr diffused into the GDC layer in contact with LSCF, Sr little accumulate at the interface between GDC and YSZ. Thus we expect a limited change of Sr content in LSCF. We further expect RL value of TF13 is stable with time and smaller than that of SP13 due to the stable LSCF composition. In the SP13 cell, a porous and thick (~ 11 μm thick) GDC was clearly seen between LSCF and YSZ (Fig. 8b). Although some Co diffused from LSCF to GDC in contact with LSCF, Sr clearly accumulated at the interface between GDC and YSZ. A similar result was observed in a cell with a porous tape-cast GDC layer and GDC was unable to stop the Sr accumulation at the interface between YSZ and GDC during cell firing, and electrolysis current accelerated the Sr diffusion [18]. From this comparison, we conclude that Sr can only diffuse through a GDC layer if it is porous. Sr diffusion is possible mostly on the surface of GDC particles. Because the GDC layer in TF13 cell shows few cracks or pinholes, little Sr diffusion occurred. Dense GDC film in the TF13 cell is not likely to be epitaxial, so we believe that Sr diffusion through grain boundaries may also not be significant. 4. Conclusions
Fig. 7. Ohmic and polarization ASR (area-specific resistance) parameters determined by fitting of impedance spectra using the equivalent circuit (Fig. 6) were shown as a function of the operation time for (a) TF13 and (b) SP13. RH = R1 + R2; RL = R3 + R4.
50 h to reach 0.11 Ω cm2 after 100 h. We have observed the similar trend of increasing Ohmic (RΩ) and polarization (RΩ) resistance values after 20 h. The increase of RΩ may be attributed to the loss of contact or to the formation of insulating phase at the interface between LSCF and GDC or between GDC and YSZ. Sr accumulation at the interface between GDC and YSZ is more likely a reason for both RΩ and Rp increase. The Rp increase is also attributed to the change of LSCF composition. As the acceptor content (Sr) of LSCF decreases due to the out-diffusion of Sr, the oxygen vacancy concentration may decrease, thereby slowing the surface-exchange kinetics and increasing RL [48]. The TF13 cells had RL that was ~0.5 times that of the SP13 cell. Thus the difference in GDC interlayer may have affected the surface-exchange reaction more than the charge transfer. To confirm Sr diffusion, EDS line profiles of TF13 (Fig. 8a) and SP13 (Fig. 8b) cells were compared after running an SOEC test for 100 h.
GDC interlayers were deposited between LSCF electrode and YSZ electrolyte to form either porous or dense films, and their ability to block Sr diffusion and their effect on the Ohmic resistance (RΩ) and polarization resistance (Rp) of the cell were compared during operation in SOEC mode. Screen printing deposited a ~11 μm-thick GDC layer on YSZ, but the layer was porous, so it could not prevent Sr accumulation at the interface between GDC and YSZ. Sputtering deposited a dense GDC layer on the surface of sintered YSZ; annealing of this layer at 1300 °C prevented Sr accumulation at the interface between GDC and YSZ; this layer was thin (~2 μm) and achieved good contact with the YSZ electrolyte, and therefore minimized RΩ. However, when GDC thin film was annealed at 1100 °C, it had cracks or pinholes, and cells that used it degraded rapidly due to Sr diffusion and the resultant delamination of the LSCF electrode. With applied electrolysis current of −800 mA cm−2, the cell with dense and pinhole free sputtered thin-film (~ 2 μm) GDC showed lower degradation rate of voltage (~8%) than did the cell with porous and thick (~11 μm) screen-printed GDC film (~28%) during operation for 100 h. Therefore, to achieve stable voltage in SOECs, that use GDC film, it should be free of cracks and pinholes. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the
Fig. 8. EDS line profile of cells after SOEC test (J = −800 mA cm−2) for 100 h with GDC interlayers: (a) TF13 and (b) SP13.
S.J. Kim et al. / Solid State Ionics 295 (2016) 25–31
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Effect of Gd-doped ceria interlayer on the stability of solid oxide electrolysis cell Sun Jae Kim a, Sun Woong Kim a, Young Min Park b, Kun Joong Kim a, Gyeong Man Choi a,⁎ a b
Fuel Cell Research Center, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Fuel Cell Project, Research Institute of Industrial Science and Technology (RIST), Pohang 790-330, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 April 2016 Received in revised form 18 July 2016 Accepted 19 July 2016 Available online xxxx
a b s t r a c t Gd0.2Ce0.8O2-δ (GDC) interlayers, positioned between La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) anode (air electrode) and yttria-stabilized zirconia (Y0.16Zr0.84O2-d, YSZ) electrolyte of solid oxide electrolysis cells (SOECs) to prevent mutual reaction, are compared that used either thin-film deposition method to produce dense layers or screenprinting to produce porous layers. Two cells with 2 μm-thick GDC layers are sputter deposited, one annealed at 1300 °C and the other at 1100 °C. During SOEC operation with an anodic current density of −800 mA cm−2 in 80% H2O + 20% H2 at 800 °C, the two cells show similar Ohmic and polarization resistances at the beginning of measurement, but the cell annealed at 1300 °C is more stable after 100 h than the film annealed at 1100 °C due to the stability of pinhole free nature of GDC layer. In a third cell with ~11 μm-thick screen-printed GDC layer, GDC is porous and therefore the cell degrades more rapidly during SOEC operation for 100 h than does a cell with ~2 μm-thick, pin-hole free GDC film. The good stability of the cell with dense, pin-hole free GDC thin film is attributed to its blocking of Sr. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered as promising fuel for sustainable future [1–3]. Thus, we need solid oxide electrolysis cells (SOECs) which decompose water and generate H2 gas at high temperature (500– 1000 °C) when electricity is applied. SOEC is promising because it shows higher efficiency than low-temperature electrolysis devices (e.g., polymer or alkaline electrolysis cells) [4]. SOECs exploit the same electrochemical reaction that occurs in solid oxide fuel cells (SOFCs), but in reverse. Water steam reduced by electrons produces H2 fuel at the cathode side (fuel electrode), O2– ions are simultaneously oxidized to O2 gas and release electrons at the anode side (air electrode) [5–7]. When yttria-stabilized zirconia (YSZ) is used as the electrolyte, Ni-YSZ cermet is used as a fuel electrode. As an air electrode, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is popular because it is a mixed ionic and electronic conductor (MIEC). Oxygen desorption occurs on the entire surface of LSCF and LSCF reduces oxygen pressure at the interface between LSCF and GDC due to its MIEC nature during electrolysis. Thus LSCF is also more stable than electronic conducting anodes such as (La,Sr)MnO3-δ (LSM) which often delaminates from GDC interlayer due to the increased oxygen pressure at the interface [8–11]. LSCF is a good air electrode, but when it is annealed at high temperature (~1000 °C), an interlayer (or diffusion barrier layer) between LSCF and YSZ must be included to prevent the reaction and the formation of electrically-insulating compounds such as La2Zr2O7 [12–14]. Gd-doped ceria (GDC) [15] is a typical interlayer, but with it LSCF still degrades or the polarization resistance Rp increases during operation of solid
http://dx.doi.org/10.1016/j.ssi.2016.07.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
oxide fuel cell (SOFC). The degradation is ascribed to Sr diffusion from LSCF matrix to the interface between GDC interlayer and YSZ electrolyte [16,17]. Sr diffuses through the porous GDC interlayer either during firing of the LSCF layer or during cell operation. Sr-rich phase forms at the interface between the porous GDC interlayer and dense YSZ electrolyte after LSCF firing at 1040 °C [18] to 1250 °C [19]. Sr also segregates during SOFC operation at 750 °C [20]. Sr can also accumulate at the surface of LSCF at 750–800 °C [21,22]. Thus, when a porous GDC interlayer is used, Sr diffusion to the surface or interface is unavoidable during SOFC operation above 750 °C. Porous screen-printed GDC cannot prevent Sr diffusion during SOEC operation at anodic current −1.0 A cm−2 at ~780 °C [23]. Because porous GDC cannot block Sr diffusion, use of a dense GDC interlayer has been suggested. However, Sr diffusion through dense GDC film coated by pulsed laser deposition (PLD) method and formation of Sr2Zr2O7 phase at the interface between YSZ and GDC have been reported during SOFC operation under cathodic current [24,25]. Pinholes or cracks in the thin film may provide paths along which Sr diffuses during SOFC operation [24]. However, Sr diffusion through dense GDC thinfilm during SOEC operation has not been reported. In this study, we prepared porous GDC interlayer films and dense GDC interlayer films to compare how they affect the degree of Sr diffusion and degradation of SOEC cells. Porous thick-films were prepared by screen printing; dense thin-films were prepared by sputtering. The stability of the cell was tested and compared for 100 h by measuring impedance and observing microstructure.
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2. Experimental Fuel-electrode (cathode) supported cells (Ni-YSZ support/Ni-YSZ fuel electrode/YSZ electrolyte) were fabricated by tape-casting, lamination, and co-firing (TLC method) as reported elsewhere [26]. Laminated green tapes were co-fired at 1400 °C for 5 h in air. The thicknesses of the layers were ~ 0.6 mm (Ni-YSZ support), ~ 10 μm (NiO-YSZ fuel electrode), and ~ 15 μm (YSZ electrolyte). After co-firing, a Gd0.2Ce0.8O2-δ (GDC) interlayer was deposited on the surface of the YSZ electrolyte; thick-films were prepared using screen printing, and thin-films were prepared using sputtering. 2.1. Screen printing of GDC GDC powder was synthesized by a solid state reaction. First, 80 mol% CeO2 and 20 mol% Gd2O3 (both 99.9%, Kojundo Chemical, Japan) were mixed by ball milling with zirconia balls in ethanol for 24 h. The mixed powder was calcined at 1200 °C for 2 h, then planetary milled (Pulverisette 6, Fritsch, Germany) with zirconia balls in ethanol for 4 h. GDC pastes for coating were prepared by mixing GDC powder with an appropriate solution of α-terpineol and ethylene cellulose. GDC paste was coated on top of the fired YSZ half-cell (YSZ/Ni-YSZ) by screen-printing, then fired at 1300 °C for 5 h. The cell with screenprinted GDC was named SP13 (Table 1). 2.2. Sputtering of GDC The GDC target for sputtering was prepared by uni-axial pressing of powder to form a pellet, then sintered. First, 91 mol% CeO2 and 9 mol% Gd2O3 of stoichiometric ratio (Gd0.09Ce0.91O2-δ) were mixed by planetary milling with zirconia balls in ethanol for 4 h. After uni-axial pressing of the powder, a green pellet was obtained and sintered at 1400 °C for 4 h. Power of RF sputtering was 250 W and vacuum pressure in the chamber was 1.5 × 10−2 Torr (Ar:O2 = 4:1). During sputtering, substrate temperature was maintained at 500 °C. Sputtered GDC after annealing at 1100 or 1300 °C showed composition of Gd0.15Ce0.85O2-δ as shown by Energy-dispersive X-ray spectroscopy (EDS, Philips electron optics B.V., Netherlands). The cells with GDC coating were annealed at 1100 °C for 2 h (named TF11) or at 1300 °C for 5 h (TF13) (Table 1). La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) powder (AGC Seimi Chemical, Japan) was mixed with a solution of α-terpineol and ethylene cellulose to form paste, which was screen-printed on top of the GDC interlayer and fired at 1040 °C for 2 h. The area of the LSCF electrode was 0.502 cm2. Pt mesh (52 mesh, Alfa Aesar, USA) as a current collector was attached to the electrode by using Pt paste (Item 6082, Heraeus, Germany) and fired at 850 °C for 2 h. The cells for test were mounted on an alumina tube and sealed with ceramic sealant (Aremco Products, USA). Fuel gas had 80% H2O + 20% H2 and flow rate was ~150 cm3 min−1 . The air electrode (anode) was exposed to open air. Humidity was controlled by flowing H2 gas through a water tank which was heated at ~94 °C to produce 80% H2O + 20% H2 mixture gas and positioned in a SOFC test station (P&P Energytech, Korea). Humidity was confirmed by measuring the open circuit voltage (OCV) of a single cell. The electrochemical performance of the cell was measured at 800 °C by using an impedance analyzer (VSP, Bio Logic Science instruments, Table 1 Cell notation, GDC coating method, firing temperature and time of GDC coated YSZ/Ni-YSZ cell, and thickness of GDC interlayer. YSZ/Ni-YSZ layers were tape cast, laminated, and cofired at 1400 °C for 5 h before GDC coating. GDC thin films (TF11, TF13) were sputter coated on YSZ/Ni-YSZ at 500 °C. Cell notation TF11 TF13 SP13
GDC Coating method Sputtering Screen Printing
Firing condition
GDC Thickness (μm)
1100 °C / 2 h 1300 °C / 5 h
2 11 ± 3
France). Current-voltage (I-V) curves were obtained, then impedance was measured. I-V curves were sequentially measured in SOFC mode and SOEC mode. During a test for 100 h, impedance was measured at open circuit condition after stopping the electrolysis (or anodic) current of −800 mA cm−2. The electrolysis current was applied again after impedance measurement. The microstructure and distributions of atomic elements (Zr, Ce, Sr, Co) across the cell were examined before and after the SOEC test by using a scanning electron microscope (FE-SEM, Philips electron optics B.V., Netherlands) and energy dispersive spectroscopy (EDS). Phases on the GDC surface were identified using X-ray diffraction (XRD, Rigaku, Japan). To clarify the effect of the GDC interlayer, impedance spectra were fitted using a complex nonlinear least square (CNLS) method of an equivalent circuit. An Excel-based program (“Z-Dance”) was used for the fitting which was developed in our laboratory. The program utilizes fitting algorithms similar to many commercial programs and Excel functions. 3. Results and discussion Although GDC films show slight difference in composition (screenprinted: Gd0.2Ce0.8O2-δ; sputtered: Gd0.15Ce0.85O2-δ), the difference in the ionic conductivity can be ignored with Gd content between 0.15 and 0.20 [27]. The surface and cross-section of GDC films were observed and compared (Fig. 1). GDC thin film annealed at 1100 °C (TF11) showed more cracks on the surface along the boundaries (Fig. 1a), but GDC thin film annealed at 1300 °C (TF13) showed nearly crack-free surface (Fig. 1d). Cross sections of both TF 11 (Fig. 1b) and TF13 (Fig. 1e) cells showed ~2 μm-thick GDC layers on YSZ electrolytes with no obvious cavities or pores. No clear boundaries can be found between GDC and YSZ in SEM image, however, boundary lines are drawn for easy of view with the help of EDS profile which was discussed later. In the TF11 cell the GDC film consisted of columnar grains (Fig. 1b), but in the TF13 cell this structure was absent, and GDC film was nearly indistinguishable from YSZ (Fig. 1e). High annealing temperature and long duration (1300 °C for 5 h) apparently eliminated pinholes by additional sintering of GDC film. The GDC layer of the SP13 cell showed large interconnected pores on GDC surface (Fig. 1g), and a porous GDC layer (thickness ~ 11 μm) (Fig. 1h) that was ~5 times thicker than thin-film GDC. In a previous study [18], a cell with GDC interlayer (~6 μm) that had been tape cast, laminated with YSZ layer and support, and cofired at 1300 °C for 5 h showed delamination of GDC from YSZ after a similar electrolysis test. The microstructure (Fig. 1c, f, i) after electrolysis test will be discussed later. “The strongest peak of GDC at ~28.62° (2θ) (111) was observed for all three samples as expected [27,28] and chosen as a reference, and data of three samples were compared over 25° ≤ 2θ ≤ 50°. The reaction of GDC films with underlying YSZ may be examined by observing the change of lattice constant of GDC after annealing at 1100 °C (TF11) or 1300 °C (TF13, SP13). Smaller lattice constant of thick-film SP13 (~0.541 nm) than that of bulk GDC (~0.543 nm) [29,30] may indicate slight Zr+ 4 (0.084 nm) substitution into Ce+ 4 (0.097 nm) site [30]. However, the increased lattice constants of TF11 (~0.545 nm) or TF13 (~0.546 nm) thin-films from the bulk value do not prove the clear reaction since thin film may have additional change due to strain in the thin film.” The grain size of the GDC layer was ~ 50 nm in TF11 (Fig. 2a) and ~ 100 nm in TF13 (Fig. 2b) cells, as estimated using the full width at half maximum (FWHM). The grain sizes of GDC, estimated from SEM surface image, were slightly larger than those from XRD, ~ 90 nm for TF11 (Fig. 1a) and ~140 nm for TF13 (Fig. 1d) cells, respectively. Grain size of GDC in TF13 cell was 1.5–2 times larger than that of TF11 cell due apparently to higher annealing temperature. The grain size of GDC in SP13 cell was much larger than those of TF11 or TF13 cells, ~ 600 nm, since GDC particles in screen-printing paste are ~ 100 nm that increased in size after firing.
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Fig. 1. SEM images of cells with GDC interlayers: (a, b, c) TF11, (d, e, f) TF13 and (g, h, i) SP13. (a, d, g) GDC surface, cross-section of GDC/YSZ; (b, e, f) before test, (c, f, i) after test. LSCF layers are also used during test (c, f, i). Note that the scale used for SP13 is different from that used for TF11 and TF13.
YSZ peaks were detected at 30.06° (2θ) (101) and 34.88° (2θ) (110) on the GDC surface in both TF11 and TF13 cells (YSZ: JPDS 01-0821244). The TF11 cell showed large YSZ peaks, due possibly to the xray penetration through the cracks in the film. Thus, XRD was successfully used to find cracks or pinholes in the GDC (Fig. 1b). On the other hand, the TF13 cell (Fig. 2b) showed weak YSZ peaks since nearly pinhole free GDC layer blocks most of x-ray penetration through the layer. The SP13 cell (Fig. 2c) did not show any YSZ peak, X-rays cannot penetrate ~11 μm-thick GDC layer despite its large pores. I-V curves of three cells were measured and compared at the time (0 h) when cell temperature reached 800 °C (Fig. 3). I-V curves were sequentially measured in SOFC mode and SOEC mode. The curves were measured in SOFC mode, then in SOEC mode. The measured OCV (~0.87 V) was slightly lower than the theoretical value (0.94 V) calculated using the Nernst equation and H2O/H2 ratio [31]. With anodic current of − 800 mA cm−2, the voltages of all three cells were ~ 1.1 V,
Fig. 2. XRD patterns of GDC interlayer before SOEC test obtained from GDC surface. GDC (111) peak was used as a reference peak. (a) TF11, (b) TF13 and (c) SP13.
which is lower than thermal neutral voltage (~1.3 V) [32,33]. At 1.3 V, electrolysis current may reach ~1.5 A cm−2. The three cells show similar slopes at open circuit condition (J = 0 mA cm−2), but the slopes changed when SOFC or SOEC current was high. As SOFC current increased, the voltage decrease was fastest in the TF11 cell and slowest in the TF13 cell among three cells. Because the cathodes (Ni-YSZ) are the same, the difference must be a result of the difference in GDC interlayers. The difference is smaller in SOEC mode than in SOFC mode, but the trend is similar. The I-V behavior of the SP13 cell with screenprinted GDC layer is similar to that of a previous cell with tape-cast GDC layer [18]. From this observation, we conclude that the morphology of GDC layer has more effect on the I-V curve in SOFC mode than in SOEC mode. In other words, the TF13 cell with dense GDC layer shows the best I-V performance.
Fig. 3. Current-Voltage (I-V) curves of fuel-electrode (Ni-YSZ) supported cells with different GDC interlayers at 0 h: (a) TF11, (b) TF13 and (c) SP13. Electrolysis current is −800 mA cm−2. SOFC was measured first, then SOEC.
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Fig. 4. Impedance spectra of cells with different GDC interlayers: (a) TF11, (b) TF13, and (c) SP13, for 100 h with 80% H2O + 20% H2. The impedance spectra were obtained at open circuit condition after stopping electrolysis current (J = −800 mA cm−2) at 800 °C.
After the I-V curve test, the impedance spectra of three cells TF11, TF13, and SP13 GDCs were obtained at open circuit condition after stopping the electrolysis current (−800 mA cm−2) after 1, 10, 20, 50, and 100 h operation at 800 °C (Fig. 4). Total ASR (area specific resistance) Rtot is the sum of Ohmic RΩ and polarization Rp resistances. At the beginning of measurement (0 h), TF11 cells had Rtot ~ 0.25 Ω cm2 and TF13 cells had Rtot ~ 0.23 Ω cm2; these values were similar and reasonably close to those calculated (~ 0.27 Ω cm2, ~ 0.22 Ω cm2, respectively) from the slopes of I-V curves near OCV (Fig. 3a, b). TF11 and TF13 both had RΩ ~ 0.07 Ω cm2. At 0 h, TF11 had Rp ~ 0.18 Ω cm2 and TF13 had Rp ~ 0.16 Ω cm2. Although CeO2 reacts with ZrO2 to form solidsolution phase which is more resistive than either CeO2 or ZrO2 at high temperature, the difference in annealing temperature had little effect on initial RΩ as similarly reported [34]. However, during SOEC operation for 100 h, the responses of RΩ and Rp of cells differed widely. In TF11, beginning at 20 h, RΩ increased rapidly to 0.45 Ω cm2 at 100 h, and Rp increased to ~1.4 Ω cm2 (Fig. 4a). This rapid increase or degradation is attributed to delamination of LSCF from GDC layer; similar sudden increases in RΩ and Rp due to delamination in SOEC operation have been reported previously [26]. After SOEC operation for 100 h, microstructure of the TF11 cell (Fig. 1c) showed delamination of LSCF from GDC, and cracks in GDC layer. Sr in LSCF may have diffused through the pinholes of GDC film, and may have thereby contributed to the formation of SrZrO3 or Sr3Zr2O7 and the delamination of LSCF [24]. It is unclear why LSCF in TF11 cell delaminates from GDC. One possibility is that the second phase at GDC/YSZ interface may have affected the oxygen partial pressure (Po2) at LSCF/GDC interface and the increased Po2 during electrolysis may have cracked thin GDC layer and delaminated LSCF layer [18, 26]. In the TF13 cell, over the 100 h, RΩ increased slightly from 0.07 to 0.09 Ω cm2 and Rp increased slightly from 0.163 to 0.165 Ω cm2 (Fig. 4b). The TF13 cell showed little delamination of LSCF, and dense GDC was maintained after electrolysis for 100 h (Fig. 1f). The relative stability of RΩ and Rp was attributed to the crack-free microstructure of GDC film during the SOEC test. SP13 had Rtot ~ 0.26 Ω cm2, which is close to the value (~0.28 Ω cm2) calculated from the I-V curve (Fig. 3c). At 0 h, the SP13 cell had RΩ ~ 0.11 Ω cm2, which is ~ 1.5 times larger than the RΩ ~ 0.07 Ω cm2 of the TF13 cell. Although the SP13 cells are ~11 μm thick whereas the
TF13 cells are 2 μm thick their calculated RΩ assuming dense GDC layers are 0.054 Ω cm2 for SP13 cells and 0.051 Ω cm2 for TF13 cells. The similarity is due to the high conductivity of GDC (~0.30 S cm−1 at 800 °C) [35,36]. RΩ is determined mostly by the thick (~ 15 μm) and resistive (~0.03 S cm−1 at 800 °C) YSZ layer. If we assume that the interface reaction between GDC and YSZ is similar in SP13 and TF13 cells, because they were both fired at 1300 °C, the only factor that affects RΩ is the contact area between GDC and YSZ. Thus the smaller RΩ ~ 0.07 Ω cm2 of TF11 or TF13 cell than the RΩ ~ 0.11 Ω cm2 of SP13 cell is ascribed to the good contact between dense GDC thin-film and LSCF, or between GDC and YSZ layers. A previous cell with tape-cast GDC layer had RΩ ~ 0.27 Ω cm2, which is more than double the RΩ ~ 0.11 Ω cm2 of SP13 cell, possibly due to the poor contact which is the reason for the delamination of GDC from YSZ layer after 20 h [18]. Rp of SP13 was ~ 0.16 Ω cm2, slightly smaller than that of TF 11 (~0.18 Ω cm2) and similar to that of TF13 (~0.16 Ω cm2) at 0 h. A previous cell with tape-cast GDC had Rp ~ 0.17 Ω cm2 [18]. Therefore, at 0 h, the cells with thin GDC film (TF11 and TF13) show relatively small RΩ, whereas the cell with thick and porous GDC (SP13) shows small Rp. After 100 h, RΩ and Rp of the SP13 cell increased substantially to ~0.13 Ω cm2 and ~0.21 Ω cm2, respectively. The layers did not delaminate, and GDC was still porous after the test. Therefore, the cell with thin and dense GDC (TF13) has advantages because it has lower Rtot and slower degradation rate than do cells with thick and porous GDC (SP13). Cell voltage while applying electrolysis current (− 800 mA/cm2) was recorded as function of operation time for 100 h (Fig. 5). Cell voltage of TF11 rapidly increased after 20 h, 1.14 V to 2.53 V after 100 h; this change corresponds to the Rp degradation shown in impedance spectra (Fig. 4a). This abrupt change was also observed in a cell with tape-cast GDC and was similarly ascribed to delamination [18]. However, the initial voltage (1.08 ± 0.01 V) of the three cells was similar (Fig. 3). Cell voltages of TF13 and SP13 increased slowly for 100 h, but were still lower than thermal neutral voltage (~1.3 V). Thus, electrolysis current may be further increased to ~1500 mA cm−2 to reach thermal neutral voltage as shown in I-V curves (Fig. 3). Rates of voltage increase were calculated either using initial and final voltages, 1.09 V to 1.19 V and 1.09 V to 1.13 V for SP13 and TF13, respectively, or by linear fitting of all voltage data of cells for 100 h. Both calculations showed similar values. TF13 cell with degradation rate RD ~ 0.4 × 10− 3 V h−1 is apparently more stable than SP13 cell with RD ~ 1.0 × 10− 3 V h− 1. Delamination of tape-cast GDC from YSZ is prevented by the introduction of contact layer (Ce0.43Zr0.43Gd0.1Y0.04O2-δ) between them, and the degradation rate (1.16 V to 1.21 V for 100 h; i.e., RD ~ 0.5 × 10−3 V h−1) of stabilized cell with contact layer is close to that of the TF13 cell [18]. Thus the TF13 cell shows the smallest voltage drop and slowest degradation rate.
Fig. 5. Voltage of TF11, TF13, and SP13 cells while applying electrolysis current (−800 mA cm2) was recorded for 100 h. Note that two different y-axis scales were used.
S.J. Kim et al. / Solid State Ionics 295 (2016) 25–31
Impedance spectra of TF13 and SP13 cells may be compared to analyze the difference in anodic resistance and degradation. Anodic and cathodic Rp of electrolyte-supported cell may be separated using a 3-electrode configuration with a reference electrode [37–40]. Separation of cathodic Rp from anodic Rp may also be possible using complex nonlinear least square (CNLS) fitting and an equivalent circuit in a fuelelectrode supported cell [41–43]. Because both cells used the same type of fuel electrode, we assumed that the difference in impedance is mostly due to the air electrode (LSCF), which also normally shows ~ 3 times larger Rp than does the fuel electrode (Ni-YSZ) at 800 °C [37]. The equivalent circuit (Fig. 6a) for fitting of impedance spectra consists of an inductor L and a resistor R0 with resistance R0 connected with four RQ parallel circuits in series. During the impedance measurement, inductance was unavoidable which was originated from test conditions (test jig, lead wire, furnace heater etc.). In this study, inductance value was estimated as ~0.1 μH cm2 (measured value ~0.2 μH). R0 represents RΩ from the electrolyte, and resistors R1, R2, R3 and R4 with resistances R1, R2, R3 and R4 in the RQ circuits represent polarization resistances Rp associated with electrochemical reactions from electrode. Q is a constant-phase element (CPE) for a non-ideal capacitor. Many equivalent circuits have been suggested for SOFCs [42–45]. Anode-supported cell with four R-Q parallel elements has been shown [44]. Although all impedance spectra show two large overlapping depressed semicircles, the curve-fitting results show four possible semicircles. We have not tried to separate the anodic from the cathodic contribution
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due to its complexity. However, previous study shows that the air electrode contributes more dominantly than the fuel electrode [37]. We have further simplified the spectra to two depressed semicircles by combining semicircles: RH = R1 + R2 represents resistance at high frequency, and is mostly due to charge transfer reaction; RL = R3 + R4 represents resistance at low frequency, and is mostly due to surfaceexchange kinetics [45,46]; Rp = RH + RL. In the TF13 cell, RH was N RL at both 0 and 100 h (Fig. 6b); in contrast, in the SP13 cell RH was slightly bRL at 0 h, and both RH and RL had increased by 100 h (Fig. 6c). Capacitance values of two samples were estimated from the fitted CPE Q1–Q4 values and similar at 0 h (Fig. 6a); ~ 1.1 × 10−3, ~ 4.5 × 10−3, ~4.2 × 10−2 and ~8.5 × 10−1 F cm−2, respectively. RΩ, RH, RL, and Rp were plotted versus operation time (Fig. 7). The TF13 cell showed similar RΩ (0.056 Ω cm2) and RL (0.058 Ω cm2) values that are ~1/2 of RH value (0.102 Ω cm2) at 0 h (Fig. 7a). RΩ value fluctuated slightly for 50 h and increased slightly after 50 h. Slight increase of RΩ for 280 h has also been reported for the similar GDC film coated between LSCF and YSZ [47]. Both RH and RL, thus Rp, were stable or even slightly decreased after 20 h. Thus the TF13 cell showed good stability at least for 100 h although slight decrease and increase were shown before 20 h. The SP13 cell had RL = 0.082 Ω cm2 ≈ RH = 0.079 Ω cm2 at 0 h (Fig. 7b). RΩ value increased rapidly for 100 h. Rp increase was in similar trend with RΩ increase after 20 h. Both RH and RL increased with time for initial 5 h, decreased slightly from 5 to 50 h, then increased after
Fig. 6. (a) Equivalent circuit used for fitting of measured impedance spectra. Fittings for (b) TF13 and (c) SP13 at 0 and 100 h.
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We have not shown EDS line profile of TF11 cell since the degradation is clearly shown due to the delamination of LSCF from GDC. In the TF13 cell, dense (initially 2 μm-thick) GDC was located between LSCF and YSZ (Fig. 8a). Although a small amount of Co and Sr diffused into the GDC layer in contact with LSCF, Sr little accumulate at the interface between GDC and YSZ. Thus we expect a limited change of Sr content in LSCF. We further expect RL value of TF13 is stable with time and smaller than that of SP13 due to the stable LSCF composition. In the SP13 cell, a porous and thick (~ 11 μm thick) GDC was clearly seen between LSCF and YSZ (Fig. 8b). Although some Co diffused from LSCF to GDC in contact with LSCF, Sr clearly accumulated at the interface between GDC and YSZ. A similar result was observed in a cell with a porous tape-cast GDC layer and GDC was unable to stop the Sr accumulation at the interface between YSZ and GDC during cell firing, and electrolysis current accelerated the Sr diffusion [18]. From this comparison, we conclude that Sr can only diffuse through a GDC layer if it is porous. Sr diffusion is possible mostly on the surface of GDC particles. Because the GDC layer in TF13 cell shows few cracks or pinholes, little Sr diffusion occurred. Dense GDC film in the TF13 cell is not likely to be epitaxial, so we believe that Sr diffusion through grain boundaries may also not be significant. 4. Conclusions
Fig. 7. Ohmic and polarization ASR (area-specific resistance) parameters determined by fitting of impedance spectra using the equivalent circuit (Fig. 6) were shown as a function of the operation time for (a) TF13 and (b) SP13. RH = R1 + R2; RL = R3 + R4.
50 h to reach 0.11 Ω cm2 after 100 h. We have observed the similar trend of increasing Ohmic (RΩ) and polarization (RΩ) resistance values after 20 h. The increase of RΩ may be attributed to the loss of contact or to the formation of insulating phase at the interface between LSCF and GDC or between GDC and YSZ. Sr accumulation at the interface between GDC and YSZ is more likely a reason for both RΩ and Rp increase. The Rp increase is also attributed to the change of LSCF composition. As the acceptor content (Sr) of LSCF decreases due to the out-diffusion of Sr, the oxygen vacancy concentration may decrease, thereby slowing the surface-exchange kinetics and increasing RL [48]. The TF13 cells had RL that was ~0.5 times that of the SP13 cell. Thus the difference in GDC interlayer may have affected the surface-exchange reaction more than the charge transfer. To confirm Sr diffusion, EDS line profiles of TF13 (Fig. 8a) and SP13 (Fig. 8b) cells were compared after running an SOEC test for 100 h.
GDC interlayers were deposited between LSCF electrode and YSZ electrolyte to form either porous or dense films, and their ability to block Sr diffusion and their effect on the Ohmic resistance (RΩ) and polarization resistance (Rp) of the cell were compared during operation in SOEC mode. Screen printing deposited a ~11 μm-thick GDC layer on YSZ, but the layer was porous, so it could not prevent Sr accumulation at the interface between GDC and YSZ. Sputtering deposited a dense GDC layer on the surface of sintered YSZ; annealing of this layer at 1300 °C prevented Sr accumulation at the interface between GDC and YSZ; this layer was thin (~2 μm) and achieved good contact with the YSZ electrolyte, and therefore minimized RΩ. However, when GDC thin film was annealed at 1100 °C, it had cracks or pinholes, and cells that used it degraded rapidly due to Sr diffusion and the resultant delamination of the LSCF electrode. With applied electrolysis current of −800 mA cm−2, the cell with dense and pinhole free sputtered thin-film (~ 2 μm) GDC showed lower degradation rate of voltage (~8%) than did the cell with porous and thick (~11 μm) screen-printed GDC film (~28%) during operation for 100 h. Therefore, to achieve stable voltage in SOECs, that use GDC film, it should be free of cracks and pinholes. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the
Fig. 8. EDS line profile of cells after SOEC test (J = −800 mA cm−2) for 100 h with GDC interlayers: (a) TF13 and (b) SP13.
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