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Effect of Ce0.43Zr0.43Gd0.1Y0.04O2−δ contact layer on stability of interface between GDC interlayer and YSZ electrolyte in solid oxide electrolysis cell
Journal of Power Sources 284 (2015) 617e622
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Effect of Ce0.43Zr0.43Gd0.1Y0.04O2d contact layer on stability of interface between GDC interlayer and YSZ electrolyte in solid oxide electrolysis cell Sun Jae Kim, Kun Joong Kim, Gyeong Man Choi* Fuel Cell Research Center/Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
h i g h l i g h t s We have fabricated the solid oxide electrolysis cell by tape-casting and co-firing. Delamination of Gd doped-Ceria (GDC) was observed under electrolysis mode. Contact layer (Ce0.43Zr0.43Gd0.1Y0.04O2d) can enhance the interface stability. Low polarization resistance of anode (LSCF) was observed with contact layer.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2014 Received in revised form 6 January 2015 Accepted 27 February 2015 Available online 5 March 2015
In solid oxide electrolysis cells (SOECs), a Gd0.2Ce0.8O2d (GDC) interlayer is often positioned between the (La,Sr) (Co,Fe)O3d (LSCF) anode (air electrode) and the yttria-stabilized zirconia (YSZ) electrolyte to reduce the reaction between the materials. However, delamination of the GDC interlayer from the YSZ electrolyte can occur during SOEC operation. In this study, a Ce0.43Zr0.43Gd0.1Y0.04O2d (CZGY) contact layer is inserted between YSZ and GDC to increase interface stability. SOEC performance is tested and compared with and without contact layer for 100 h. The CZGY contact layer significantly improves the cell's chemical stability and electrochemical performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Solid oxide electrolysis cell (SOEC) Interface stability GDC delamination LSCF air electrode Contact layer Impedance spectroscopy
1. Introduction Steam-electrolysis method using solid oxide electrolysis cells (SOECs) is considered as a highly-effective fuel generation system because hydrogen is produced relatively easily by splitting water vapor at the high temperature. SOECs have advantages due to its high operation temperature. Electrodes of SOECs are relatively inexpensive because they do not require noble metal catalysts (Pt, Pd, etc.) for oxygen molecular desorption reactions. In addition, SOECs generate hydrogen efficiently from water steam at high temperature because steam electrolysis is an endothermic reaction
* Corresponding author. E-mail address: [email protected] (G.M. Choi). http://dx.doi.org/10.1016/j.jpowsour.2015.02.156 0378-7753/© 2015 Elsevier B.V. All rights reserved.
[1e3]. In SOEC electrochemical reactions, oxidation and reduction reactions are taken place reversely with the reactions of solid oxide fuel cells (SOFCs). The water steam reduced by electrons may produce hydrogen fuel at the fuel electrode (1); simultaneously, oxygen ions are oxidized to oxygen gas and release the electrons (2):
H2 O þ 2e /H2 þO2
(1)
O2 /1=2O2 þ2e
(2)
SOECs use similar components with SOFCs, e.g. yttria-stabilized zirconia (YSZ) as an electrolyte, NiOeYSZ cermet as a fuel electrode and (La, Sr) (Co, Fe)O3d (LSCF) or (La, Sr)MnO3d (LSM) as an air electrode. The LSM air electrode has been heavily studied under
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Many mechanisms were suggested to explain delamination. Previous studies suggest that the delamination is due to the oxygen ions trapped at the interface [12e14] or due to a physically weak interface under high oxygen pressure [14]. In this study, a Ce0.43Zr0.43Gd0.1Y0.04O2d (CZGY) contact layer was chosen and introduced between the GDC interlayer and the YSZ electrolyte to reduce the delamination between them. The contact layer has an intermediate composition between those of YSZ and GDC [15]. A fuel electrode (NieYSZ) supported cell was used for the test. The performance of cells with and without a contact layer was compared under electrolysis operation at high current density (800 mA/cm2).
2. Experimental procedure
Fig. 1. A schematic figure of the experimental setup for the electrochemical test for the fuel electrode supported SOEC. All layers (GDC/contact layer/YSZ/NiOeYSZ/NiOeYSZ), except the LSCF air electrode and Pt current collectors, are tape casted and their thicknesses are shown. A cell without the contact layer was also tested.
electrolysis condition. However, cell degradation is reported due to delamination of air electrode and cation segregation problems. Knibbe et al. reported that degradation is mainly resulted from the oxygen gas accumulation in YSZ grain boundaries close to the LSM/ YSZ composite oxygen electrode at an electrolysis potential [4]. Several studies have shown that the degradation is due to the delamination between electrolyte and air electrode [5,6]. The delamination is resulted from a low oxygen ion conductivity of LSM. Other groups report that the delamination is accelerated by La2Zr2O7 during electrolysis operation [7,8]. Thus, LSCF has been suggested as an alternative because it has higher oxygen ion conductivity than does LSM. However, to prevent reactions between LSCF and YSZ electrolyte, Gd0.2Ce0.8O2d (GDC) is used as an interlayer [9]. LSCF also shows performance degradation under SOEC operation. Most degradation is associated with air electrode problems; e.g. delamination and cation segregation. The delamination occurs between the YSZ electrolyte and the GDC interlayer [10,11].
All layers except the LSCF air electrode were fabricated using tape-casting and co-firing. YSZ paste for tape casting was prepared by ball milling 8 mol% yttria-stabilized zirconia (TZ-8YS, Tosoh, Japan) powder with organic solution (polymeric solution þ binder) for 72 h with zirconia balls. To prepare the fuel electrode, NiO (99.97%, Kojundo Chemical, Japan) and YSZ powders in a 3:2 weight ratio were mixed by planetary milling (Pulverisette 6, Netzsch, Germany) with ethanol and zirconia balls for 4 h. To prepare the support layer, NiOeYSZ powder in a 3:2 weight ratio and 10 wt% corn starch (Samchun Chemicals, Korea) as a pore former were mixed by ball milling for 72 h with zirconia balls with ethanol. 20 mol% Gd-doped ceria (Gd0.2Ce0.8O2d, GDC) powder was synthesized by mixing and ball milling 20 mol% Gd2O3 (99.9%, Kojundo Chemical) powder with 80 mol% CeO2 (99.9%, Kojundo Chemical) powder for 72 h with zirconia balls followed by calcining at 1000 C for 2 h. A contact layer (Ce0.43Zr0.43Gd0.1Y0.04O2d, CZGY) was mixed by ball milling 43 mol% CeO2, 43 mol% ZrO2 (Junsei Chemical, Japan), 4 mol% Y2O3 (99.9%, Kojundo Chemical) and 10 mol% Gd2O3 for 24 h with ethanol. The mixed powder was calcined at 1200 C for 2 h, then powders were planetary-milled with zirconia balls and ethanol for 4 h. Tape casting of each layer was performed using a tape caster (Hansung System, Korea). The thickness of a green tape was ~15e65 mm. Green tapes of GDC interlayer, without or with CZGY contact layer, YSZ electrolyte, NiOeYSZ fuel electrode and NiOeYSZ support were laminated with high pressure (650 MPa) at
Fig. 2. Impedance spectra of cells co-fired at 1300 C (a) without and (b) with contact layer for 100 h. Impedance data were recorded at the open circuit mode after interrupting an anodic current of 800 mA/cm2 at 800 C with fuel gas of 80% steam þ 20% H2. Note that the scales are different for two figures.
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Fig. 4. EDS graph of cells co-fired at 1300 C for 5 h, after electrolysis test at 800 C with 80% H2O þ 20% H2 as a fuel, (a) without and (b) with contact layer. Anodic current was applied at 800 mA/cm2 for 100 h. Air electrode components were diffused into GDC interlayer. Fig. 3. EDS graph of cells after co-firing at 1300 C for 5 h, before the electrochemical test, (a) without and (b) with contact layer. Dotted lines in the middle of figures indicate the boundaries after co-firing between GDC and YSZ in (a), among GDC, CZGY, and YSZ in (b).
60 C for ~15 min. The laminated tape was cut into disks (diameter ¼ 24 mm) using a punch machine. Green tape disks were co-fired at 1300 C for 5 h La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) pastes were prepared for screen printing by mixing LSCF powders (AGC Seimi Chemical, Japan) with an organic solution containing alphaterpineol and ethylene cellulose. After co-firing the laminated circular tapes (GDC/CZGY/YSZ/NiO-YSZ) at 1300 C in air, LSCF was coated on top of the GDC interlayer by screen-printing and firing at 1040 C for 2 h. The area of the air electrode was 0.502 cm2. Platinum mesh (52mesh, Alpha 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 Fig. 1 shows the schematic setup for the SOEC cell for electrochemical test. The typical thicknesses of the layers are shown. A single cell was positioned on an alumina tube and sealed using ceramic sealant (Aremco Products, USA). The flow rate of fuel consisting of 80% H2O þ 20% H2 was 150 cm3/min and open air was used. The humidity was controlled
by positioning the entire test set-up inside a solid oxide fuel cell (SOFC) test station (P&P Energytech, Korea). The humidity was confirmed by measuring the open circuit voltage of a single cell. The impedance was measured using an impedance analyzer (VSP, Bio Logic Science instruments, France) at open circuit conditions after stopping the anodic current. The electrochemical performances of the SOECs were tested and compared at 800 C with an anodic current of 800 mA/cm2. The microstructure was examined before and after the SOEC test by using a scanning electron microscope (FE-SEM, Philips electron optics B.V., Netherlands). Energy-dispersive X-ray spectroscopy (EDS, Philips electron optics B.V., Netherlands) analysis was used to determine the linear distributions of materials. 3. Results and discussion To observe the stability of interface between GDC interlayer and YSZ electrolyte, impedance plots of single cells without (Fig. 2a) and with (Fig. 2b) contact layers were compared under test conditions of 800 C and anodic current of 800 mA/cm2 for 100 h.
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Fig. 5. The IeV curves of cell (a) without contact layer and (b) with contact layer, measured at 800 C with 80% H2O þ 20% H2 as a fuel.
The interface layer had little effect on the area-specific Ohmic resistance (RU) of the cells. The cell without the contact layer had RU ¼ ~0.27 U cm2 (Fig. 2a), which is ~4 times higher than the estimated value (~0.059 U cm2) using the dimensions and the known bulk conductivities of GDC (~6 mm, ~0.3 S/cm) and YSZ (~16 mm, ~0.03 S/cm); this difference may be due to the porous nature of GDC layer and the reaction layer formed between YSZ and GDC during co-firing at 1300 C. The cell with the CZGY contact layer had RU ¼ ~0.29 U cm2 (Fig. 2b); Thus the increase in the Ohmic resistance due to the introduction of contact layer was small (~0.02 U cm2). So the sum of Ohmic resistance of ~7 mm-thick CZGY contact layer (~6.03 103 S/cm) [15] and those of two additional interfaces, one between GDC and CZGY and another between CZGY and YSZ, is similar to that of a resistive layer formed between GDC and YSZ. Thus the introduction of CZGY interface is a good strategy if it increases contact strength without much increasing the Ohmic resistance. The introduction of the contact layer also influenced the areaspecific polarization resistance (RP) of the cells. The cell without the contact layer had the polarization resistance (RP ¼ ~0.17 U cm2) at 0e10 h; the cell with contact layer had the polarization resistance (RP ¼ ~0.09 U cm2) over the same interval. The observed value of electrode polarization resistance is the sum of both anodic and cathodic polarization resistances. Because both cells used the same type of cathode (or fuel electrode), we infer that the change of polarization resistance is mostly due to the change in anodic (or air electrode) polarization. Operation for 100 h increased the total area-specific resistance (Ohmic þ polarization) more in the cell without the contact layer than in the cell with it. At 0 h total area-specific resistance (ASR) was slightly higher in the cell without contact layer (~0.44 U cm2) than in the cell with it (~0.37 U cm2), but after operation for 100 h, total ASR of two cells were very different, because operation increased both RU and RP more in the cell without the contact layer than in the cell with it. RU of the cell without the contact layer increased significantly from ~0.27 U cm2 to ~0.57 U cm2 (Fig. 2a), whereas RU of the cell with contact layer increased only slightly from ~0.29 U cm2 to ~0.30 U cm2 (Fig. 2b). RP of the cell without the contact layer rapidly increased from ~0.16 U cm2 to ~1.49 U cm2, but RP of the cell with a contact layer increased only slightly from ~0.09 U cm2 to ~0.14 U cm2. The larger initial Rp value (~0.16 U cm2) for the cell without contact layer than that (~0.09 U cm2) with
contact layer is probably due to the slight initial delamination before 0 h during cell preparation for test. Therefore, overall RU and RP of the cell without the contact layer were approximately 2 and 10 times those of the cell with the contact layer, respectively. The large increase in the polarization resistance over time in the cell without contact layer is mostly attributed to progressive delamination of GDC from YSZ [16] as also will be shown later in S.E.M. image. The results suggest that the contact layer increased the stability of the interface between YSZ and GDC after 100 h of operation. EDS profiles (Fig. 3) were obtained from cells without and with a contact layer before the electrochemical test. In the cell without the contact layer (Fig. 3a), Ce and Zr concentrations ([Ce], [Zr]) in the reaction layer between GDC interlayer and YSZ electrolyte changed gradually because both YSZ and GDC have the same fluorite structure and form a complete solid solution [17]. The dotted vertical lines indicate approximate original position of each layer. The layer that showed an abrupt change of distance between Ce and Zr peak position was ~5 mm thick. Sr and Co in LSCF also diffused into the GDC interlayer. More Sr than Co was found at the interface between YSZ and GDC. In the cell with a contact layer (Fig. 3b) the interface between GDC interlayer and CZGY contact layer (distance between Ce peak position and Zr or Ce plateau position, ~5 mm) shows gradual decrease in [Ce] and gradual increase in [Zr] with distance from GDC layer. [Ce] and [Zr] also changed between CZGY contact layer and YSZ electrolyte (the distance between Zr peak position and the plateau position of Ce or Zr was ~2.5 mm). The thickness of the remaining CZGY contact layer was ~2.5 mm, which was less than the initial thickness of ~7 mm. The total thickness of the reaction zone was ~10 mm, which was much thicker than that (~5 mm) in the cell without contact layer (Fig. 3a) due primarily to the existence of CZGY layer. Sr and Co also diffused into GDC and CZGY layers. However, little Sr or Co segregation was observed at the interface between YSZ and CZGY. The resistivity of intermediate composition between GDC and YSZ is known to show a maximum and thus the reaction layer should show higher resistivity than that of either GDC or YSZ [18]. For example, the composition of CZGY used as a contact layer in this study is an intermediate composition between YSZ and GDC and is much less conductive (more resistive) (~6.03 103 S/cm) than either GDC (~0.3 S/cm) or YSZ layer (~0.03 S/cm) [15]. Thus a large increase of RU is expected due to the formation of a reaction layer, so the observation that RU (0.28 ± 0.01 U cm2) of the cell without or with the CZGY contact layer is much higher than that (~0.055 U cm2) calculated from the thickness and conductivity of GDC and YSZ layers is reasonable. However, the similarity in the RU of the two types of cell (Fig. 2) is difficult to understand. A possible explanation is that the two layers (Fig. 3a and b) have a similar RU although the reaction layer (~5 mm) in Fig. 3a is only half as thick as that (~10 mm) Fig. 3b; this implies that the reaction layer in Fig. 3a has about twice the resistivity as that in Fig. 3b. The observation requires more detailed studies of the contact between the layers. The cell without CZGY contact layer may have bad or weak contact. The quality of contact improves with the increase in the co-firing temperature [16]. EDS profiles of cells were measured after the electrolysis test. [Ce] and [Zr] profiles of two cells without the contact layer (Fig. 4a) changed little from Fig. 3a, but [Sr] and [Co] profiles showed substantial changes. Sr and Co diffused from the LSCF electrode to the GDC interlayer. Sr segregated strongly at the interface between YSZ and GDC, preferentially on the YSZ side. The comparison of Sr segregation before and after electrolysis implies that the applied current is a major source of Sr segregation, and may be a reason for the degradation in electrode performance. In the cell with the
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Fig. 6. Microstructure of cells are shown; (a) before (after co-firing at 1300 C) and (c) after electrolysis test for 100 h of the cell without contact layer; (b) before and (d) after test for 100 h of the cell with contact layer. From top to bottom: LSCF anode, GDC interlayer, with or without CZGY contact layer, YSZ electrolyte.
contact layer (Fig. 4b), Sr and Co still diffused out from LSCF to the GDC interlayer. Reduced concentration of acceptor doping (Sr) in LSCF is expected to decrease the number of charge carriers (oxygen vacancies) and may be responsible for the decrease in the rate of oxygen (O2) oxidation [19]. The phase change due to the loss of Sr in LSCF may also be a reason [16].
Fig. 7. Voltage change of cell (a) without and (b) with contact layer with 800 mA/cm2 for 100 h. Rapid increase after 10 h was shown for the cell without contact layer. The CZGY contact layer increases performance stability with voltage degradation rate of ~0.04%/h. Measurement conditions are 800 C temperature and 20% H2 þ 80% H2O as a fuel.
Current densityeVoltage (IeV) curves of the cells in both SOFC and SOEC regions were extracted at the beginning of the test (0 h) just after the impedance measurement. SOFC curves were obtained first, followed by SOEC curves. The IeV curve of the cell without contact layer (Fig. 5a) was non-linear in the high-current SOFC region but that of the cell with contact layer (Fig. 5b) was nearly linear in both regions; the result shows current-limiting behavior, and therefore that the direction of oxygen ion transport is important. The current-limiting behavior only occurred when oxygen ions diffused from air electrode to fuel electrode. The air electrode may be responsible for this behavior because it was observed only for the cell without contact layer. This is reasonable since the electrolyte layer with no resistive CZGY interlayer is more conductive than that with interlayer and allows faster diffusion of oxygen from air electrode to fuel electrode. Thus the oxygen content at the interface between air electrode and conductive electrolyte is decreased resulting in the current limiting behavior. The CZGY contact layer improves the contact between GDC and YSZ. The slope of linear part of curve was higher for the cell without the contact layer than in the cell with it. ASR calculated from the slope of the SOEC without the contact layer was ~0.42 U cm2, which is similar to the impedance data (~0.44 U cm2). Similarly, the ASR with the contact layer calculated in the SOEC region was ~0.37 U cm2, similar to the total resistance value calculated from impedance data (~0.38 U cm2). Thus the cell without contact layer, with larger ASR, shows the current-limiting behavior. The values of open-circuit voltage (OCV) were ~0.88 V for both cells in the presence of 80% H2O þ 20% H2 fuel, slightly lower than the theoretical (~0.94 V) electromotive force. Microstructures of cells were obtained for cells immediately after sintering without (Fig. 6a) and with (Fig. 6b) the contact layer,
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and after the SOEC test for 100 h without (Fig. 6c) and with (Fig. 6d) the contact layer. After the electrolysis test, in the cell without the contact layer the GDC interlayer delaminated from the YSZ electrolyte (Fig. 6c). This delamination may be the main cause of the rapid increase of impedance with time (Fig. 2a). In contrast, in the cell with the contact layer, no delamination was observed after the SOEC test for 100 h (Fig. 6d), and no cavities were observed between YSZ and CZGY or between CZGY and GDC layers. Without contact layer, cell firing temperature (1300 C) is not high enough for the reaction (or diffusion) between GDC and YSZ, and thus weak bonding and delamination are resulted. However, with CZGY of intermediate composition, the bonding is stronger between GDC and CZGY or YSZ and CZGY than between GDC and YSZ. Cell voltage was measured at 800 mA/cm2 for 100 h (Fig. 7) in cells without and with a contact layer. In both cases, the initial voltage (V0) was less than the thermal neutral voltage (<1.287 V), at which the endothermic energy of electrolysis equals the energy from Ohmic heating (Joule heating) energy in the cell [20]. The cell without the contact layer had V0 ¼ ~1.23 V, as also shown in the IeV curves (Fig. 5). The voltage was relatively stable for 0e10 h, then increased rapidly to ~2.44 V over time. This tendency was also shown in the impedance plot (Fig. 2a) and is a result of delamination of GDC from YSZ (Fig. 6c). The cell with the contact layer had V0 ¼ ~1.16 V; the voltage increased slightly to ~1.21 V after 100 h (increase rate ~0.04%/h). The stable performance and slight degradation may be due to the enhanced interface stability (GDC/ YSZ) and the small amount of Sr segregation, respectively. 4. Conclusions Electrolysis performance and interface stability were compared with and without CZGY contact layers under 80% H2O humidity. The CZGY contact layer improved the stability of contact with the neighboring layers and thus prevented delamination during electrolysis tests as shown in the microstructure. Thus the contact layer significantly improved the performance of electrolysis cell as
shown by the impedance and currentevoltage relations. The increase in the Ohmic resistance due to contact layer was small (~0.02 U cm2 at 800 C). But, the LSCF air electrode still degraded slowly over time due to Sr segregation. Acknowledgments This research (paper) was performed for the Hydrogen Energy R&D Center, part of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology of Korea. The authors are thankful to Korea Institute of the Energy Research (KIER), Korea. References [1] A. Brisse, J. Schefold, M. Zahid, Int. J. Hydrogen Energy 33 (2008) 5375. [2] M. Ni, M.K.H. Leung, D.Y.C. Leung, Int. J. Hydrogen energy 33 (2008) 2337. [3] A. Hauch, S.D. Ebbesen, S.H. Jensen, M. Mogensen, J. Mater. Chem. 18 (2008) 2331. [4] R. Knibbe, M.L. Traulsen, A. Hauch, S.D. Ebbesen, M. Mogensen, J. Electrochem. Soc. 157 (2010) B1209. [5] M.A. Laguna-Berceroa, R. Campanaa, A. Larrea, J.A. Kilner, V.M. Orera, J. Power Sources 196 (2011) 8942. [6] J.H. Kim, H.I. Ji, H.P. Dasari, D.W. Shin, H.S. Song, J.H. Lee, B.K. Kim, H.J. Je, H. W Lee, K.J. Yoon, Inter. J. Hydrogen Energy 38 (2013) 1225. [7] K. Chen, S.P. Jiang, Inter. J. Hydrogen Energy 36 (2011) 10541. [8] M. Keane, M.K. Mahapatra, A. Verma, P. Singh, Inter. J. Hydrogen Energy 37 (2012) 16776. [9] J.M. Ralph, C. Rossignol, R. Kumar, J. Electrochem. Soc. 150 (2003) A1518. [10] P. Hjalmarsson, X. Sun, Y. Liu, M. Chen, J. Power Sources 223 (2013) 349. [11] F. Tietz, D. Sebold, A. Brisse, J. Schefold, J. Power Sources 223 (2013) 129. [12] J.A. Kilner, Solid State Ionics 129 (2000) 13. [13] G. Balducci, J. Kaspar, P. Fornasiero, M. Graziani, M.S. Islam, J.D. Gale, J. Phys. Chem. B101 (1997) 1750. [14] X. Zhou, B. Scarfino, H.U. Anderson, Solid State Ionics 175 (2004) 19. [15] A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709. [16] S.J. Kim, G.M. Choi, Solid State Ionics 262 (2014) 303. [17] C.H. Lee, G.M. Choi, Solid State Ionics 135 (2000) 653. [18] A. Tsoga, A. Naoumidis, D. Stover, Solid State Ionics 135 (2000) 403. [19] L.W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 273. [20] J.S. Herring, J.E. O'Brien, C.M. Stoots, G.L. Hawkes, J.J. Hartvigsen, M. Shahnam, Inter. J. Hydrogen Energy 32 (2007) 440.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Effect of Ce0.43Zr0.43Gd0.1Y0.04O2d contact layer on stability of interface between GDC interlayer and YSZ electrolyte in solid oxide electrolysis cell Sun Jae Kim, Kun Joong Kim, Gyeong Man Choi* Fuel Cell Research Center/Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
h i g h l i g h t s We have fabricated the solid oxide electrolysis cell by tape-casting and co-firing. Delamination of Gd doped-Ceria (GDC) was observed under electrolysis mode. Contact layer (Ce0.43Zr0.43Gd0.1Y0.04O2d) can enhance the interface stability. Low polarization resistance of anode (LSCF) was observed with contact layer.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2014 Received in revised form 6 January 2015 Accepted 27 February 2015 Available online 5 March 2015
In solid oxide electrolysis cells (SOECs), a Gd0.2Ce0.8O2d (GDC) interlayer is often positioned between the (La,Sr) (Co,Fe)O3d (LSCF) anode (air electrode) and the yttria-stabilized zirconia (YSZ) electrolyte to reduce the reaction between the materials. However, delamination of the GDC interlayer from the YSZ electrolyte can occur during SOEC operation. In this study, a Ce0.43Zr0.43Gd0.1Y0.04O2d (CZGY) contact layer is inserted between YSZ and GDC to increase interface stability. SOEC performance is tested and compared with and without contact layer for 100 h. The CZGY contact layer significantly improves the cell's chemical stability and electrochemical performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Solid oxide electrolysis cell (SOEC) Interface stability GDC delamination LSCF air electrode Contact layer Impedance spectroscopy
1. Introduction Steam-electrolysis method using solid oxide electrolysis cells (SOECs) is considered as a highly-effective fuel generation system because hydrogen is produced relatively easily by splitting water vapor at the high temperature. SOECs have advantages due to its high operation temperature. Electrodes of SOECs are relatively inexpensive because they do not require noble metal catalysts (Pt, Pd, etc.) for oxygen molecular desorption reactions. In addition, SOECs generate hydrogen efficiently from water steam at high temperature because steam electrolysis is an endothermic reaction
* Corresponding author. E-mail address: [email protected] (G.M. Choi). http://dx.doi.org/10.1016/j.jpowsour.2015.02.156 0378-7753/© 2015 Elsevier B.V. All rights reserved.
[1e3]. In SOEC electrochemical reactions, oxidation and reduction reactions are taken place reversely with the reactions of solid oxide fuel cells (SOFCs). The water steam reduced by electrons may produce hydrogen fuel at the fuel electrode (1); simultaneously, oxygen ions are oxidized to oxygen gas and release the electrons (2):
H2 O þ 2e /H2 þO2
(1)
O2 /1=2O2 þ2e
(2)
SOECs use similar components with SOFCs, e.g. yttria-stabilized zirconia (YSZ) as an electrolyte, NiOeYSZ cermet as a fuel electrode and (La, Sr) (Co, Fe)O3d (LSCF) or (La, Sr)MnO3d (LSM) as an air electrode. The LSM air electrode has been heavily studied under
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S.J. Kim et al. / Journal of Power Sources 284 (2015) 617e622
Many mechanisms were suggested to explain delamination. Previous studies suggest that the delamination is due to the oxygen ions trapped at the interface [12e14] or due to a physically weak interface under high oxygen pressure [14]. In this study, a Ce0.43Zr0.43Gd0.1Y0.04O2d (CZGY) contact layer was chosen and introduced between the GDC interlayer and the YSZ electrolyte to reduce the delamination between them. The contact layer has an intermediate composition between those of YSZ and GDC [15]. A fuel electrode (NieYSZ) supported cell was used for the test. The performance of cells with and without a contact layer was compared under electrolysis operation at high current density (800 mA/cm2).
2. Experimental procedure
Fig. 1. A schematic figure of the experimental setup for the electrochemical test for the fuel electrode supported SOEC. All layers (GDC/contact layer/YSZ/NiOeYSZ/NiOeYSZ), except the LSCF air electrode and Pt current collectors, are tape casted and their thicknesses are shown. A cell without the contact layer was also tested.
electrolysis condition. However, cell degradation is reported due to delamination of air electrode and cation segregation problems. Knibbe et al. reported that degradation is mainly resulted from the oxygen gas accumulation in YSZ grain boundaries close to the LSM/ YSZ composite oxygen electrode at an electrolysis potential [4]. Several studies have shown that the degradation is due to the delamination between electrolyte and air electrode [5,6]. The delamination is resulted from a low oxygen ion conductivity of LSM. Other groups report that the delamination is accelerated by La2Zr2O7 during electrolysis operation [7,8]. Thus, LSCF has been suggested as an alternative because it has higher oxygen ion conductivity than does LSM. However, to prevent reactions between LSCF and YSZ electrolyte, Gd0.2Ce0.8O2d (GDC) is used as an interlayer [9]. LSCF also shows performance degradation under SOEC operation. Most degradation is associated with air electrode problems; e.g. delamination and cation segregation. The delamination occurs between the YSZ electrolyte and the GDC interlayer [10,11].
All layers except the LSCF air electrode were fabricated using tape-casting and co-firing. YSZ paste for tape casting was prepared by ball milling 8 mol% yttria-stabilized zirconia (TZ-8YS, Tosoh, Japan) powder with organic solution (polymeric solution þ binder) for 72 h with zirconia balls. To prepare the fuel electrode, NiO (99.97%, Kojundo Chemical, Japan) and YSZ powders in a 3:2 weight ratio were mixed by planetary milling (Pulverisette 6, Netzsch, Germany) with ethanol and zirconia balls for 4 h. To prepare the support layer, NiOeYSZ powder in a 3:2 weight ratio and 10 wt% corn starch (Samchun Chemicals, Korea) as a pore former were mixed by ball milling for 72 h with zirconia balls with ethanol. 20 mol% Gd-doped ceria (Gd0.2Ce0.8O2d, GDC) powder was synthesized by mixing and ball milling 20 mol% Gd2O3 (99.9%, Kojundo Chemical) powder with 80 mol% CeO2 (99.9%, Kojundo Chemical) powder for 72 h with zirconia balls followed by calcining at 1000 C for 2 h. A contact layer (Ce0.43Zr0.43Gd0.1Y0.04O2d, CZGY) was mixed by ball milling 43 mol% CeO2, 43 mol% ZrO2 (Junsei Chemical, Japan), 4 mol% Y2O3 (99.9%, Kojundo Chemical) and 10 mol% Gd2O3 for 24 h with ethanol. The mixed powder was calcined at 1200 C for 2 h, then powders were planetary-milled with zirconia balls and ethanol for 4 h. Tape casting of each layer was performed using a tape caster (Hansung System, Korea). The thickness of a green tape was ~15e65 mm. Green tapes of GDC interlayer, without or with CZGY contact layer, YSZ electrolyte, NiOeYSZ fuel electrode and NiOeYSZ support were laminated with high pressure (650 MPa) at
Fig. 2. Impedance spectra of cells co-fired at 1300 C (a) without and (b) with contact layer for 100 h. Impedance data were recorded at the open circuit mode after interrupting an anodic current of 800 mA/cm2 at 800 C with fuel gas of 80% steam þ 20% H2. Note that the scales are different for two figures.
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Fig. 4. EDS graph of cells co-fired at 1300 C for 5 h, after electrolysis test at 800 C with 80% H2O þ 20% H2 as a fuel, (a) without and (b) with contact layer. Anodic current was applied at 800 mA/cm2 for 100 h. Air electrode components were diffused into GDC interlayer. Fig. 3. EDS graph of cells after co-firing at 1300 C for 5 h, before the electrochemical test, (a) without and (b) with contact layer. Dotted lines in the middle of figures indicate the boundaries after co-firing between GDC and YSZ in (a), among GDC, CZGY, and YSZ in (b).
60 C for ~15 min. The laminated tape was cut into disks (diameter ¼ 24 mm) using a punch machine. Green tape disks were co-fired at 1300 C for 5 h La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) pastes were prepared for screen printing by mixing LSCF powders (AGC Seimi Chemical, Japan) with an organic solution containing alphaterpineol and ethylene cellulose. After co-firing the laminated circular tapes (GDC/CZGY/YSZ/NiO-YSZ) at 1300 C in air, LSCF was coated on top of the GDC interlayer by screen-printing and firing at 1040 C for 2 h. The area of the air electrode was 0.502 cm2. Platinum mesh (52mesh, Alpha 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 Fig. 1 shows the schematic setup for the SOEC cell for electrochemical test. The typical thicknesses of the layers are shown. A single cell was positioned on an alumina tube and sealed using ceramic sealant (Aremco Products, USA). The flow rate of fuel consisting of 80% H2O þ 20% H2 was 150 cm3/min and open air was used. The humidity was controlled
by positioning the entire test set-up inside a solid oxide fuel cell (SOFC) test station (P&P Energytech, Korea). The humidity was confirmed by measuring the open circuit voltage of a single cell. The impedance was measured using an impedance analyzer (VSP, Bio Logic Science instruments, France) at open circuit conditions after stopping the anodic current. The electrochemical performances of the SOECs were tested and compared at 800 C with an anodic current of 800 mA/cm2. The microstructure was examined before and after the SOEC test by using a scanning electron microscope (FE-SEM, Philips electron optics B.V., Netherlands). Energy-dispersive X-ray spectroscopy (EDS, Philips electron optics B.V., Netherlands) analysis was used to determine the linear distributions of materials. 3. Results and discussion To observe the stability of interface between GDC interlayer and YSZ electrolyte, impedance plots of single cells without (Fig. 2a) and with (Fig. 2b) contact layers were compared under test conditions of 800 C and anodic current of 800 mA/cm2 for 100 h.
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Fig. 5. The IeV curves of cell (a) without contact layer and (b) with contact layer, measured at 800 C with 80% H2O þ 20% H2 as a fuel.
The interface layer had little effect on the area-specific Ohmic resistance (RU) of the cells. The cell without the contact layer had RU ¼ ~0.27 U cm2 (Fig. 2a), which is ~4 times higher than the estimated value (~0.059 U cm2) using the dimensions and the known bulk conductivities of GDC (~6 mm, ~0.3 S/cm) and YSZ (~16 mm, ~0.03 S/cm); this difference may be due to the porous nature of GDC layer and the reaction layer formed between YSZ and GDC during co-firing at 1300 C. The cell with the CZGY contact layer had RU ¼ ~0.29 U cm2 (Fig. 2b); Thus the increase in the Ohmic resistance due to the introduction of contact layer was small (~0.02 U cm2). So the sum of Ohmic resistance of ~7 mm-thick CZGY contact layer (~6.03 103 S/cm) [15] and those of two additional interfaces, one between GDC and CZGY and another between CZGY and YSZ, is similar to that of a resistive layer formed between GDC and YSZ. Thus the introduction of CZGY interface is a good strategy if it increases contact strength without much increasing the Ohmic resistance. The introduction of the contact layer also influenced the areaspecific polarization resistance (RP) of the cells. The cell without the contact layer had the polarization resistance (RP ¼ ~0.17 U cm2) at 0e10 h; the cell with contact layer had the polarization resistance (RP ¼ ~0.09 U cm2) over the same interval. The observed value of electrode polarization resistance is the sum of both anodic and cathodic polarization resistances. Because both cells used the same type of cathode (or fuel electrode), we infer that the change of polarization resistance is mostly due to the change in anodic (or air electrode) polarization. Operation for 100 h increased the total area-specific resistance (Ohmic þ polarization) more in the cell without the contact layer than in the cell with it. At 0 h total area-specific resistance (ASR) was slightly higher in the cell without contact layer (~0.44 U cm2) than in the cell with it (~0.37 U cm2), but after operation for 100 h, total ASR of two cells were very different, because operation increased both RU and RP more in the cell without the contact layer than in the cell with it. RU of the cell without the contact layer increased significantly from ~0.27 U cm2 to ~0.57 U cm2 (Fig. 2a), whereas RU of the cell with contact layer increased only slightly from ~0.29 U cm2 to ~0.30 U cm2 (Fig. 2b). RP of the cell without the contact layer rapidly increased from ~0.16 U cm2 to ~1.49 U cm2, but RP of the cell with a contact layer increased only slightly from ~0.09 U cm2 to ~0.14 U cm2. The larger initial Rp value (~0.16 U cm2) for the cell without contact layer than that (~0.09 U cm2) with
contact layer is probably due to the slight initial delamination before 0 h during cell preparation for test. Therefore, overall RU and RP of the cell without the contact layer were approximately 2 and 10 times those of the cell with the contact layer, respectively. The large increase in the polarization resistance over time in the cell without contact layer is mostly attributed to progressive delamination of GDC from YSZ [16] as also will be shown later in S.E.M. image. The results suggest that the contact layer increased the stability of the interface between YSZ and GDC after 100 h of operation. EDS profiles (Fig. 3) were obtained from cells without and with a contact layer before the electrochemical test. In the cell without the contact layer (Fig. 3a), Ce and Zr concentrations ([Ce], [Zr]) in the reaction layer between GDC interlayer and YSZ electrolyte changed gradually because both YSZ and GDC have the same fluorite structure and form a complete solid solution [17]. The dotted vertical lines indicate approximate original position of each layer. The layer that showed an abrupt change of distance between Ce and Zr peak position was ~5 mm thick. Sr and Co in LSCF also diffused into the GDC interlayer. More Sr than Co was found at the interface between YSZ and GDC. In the cell with a contact layer (Fig. 3b) the interface between GDC interlayer and CZGY contact layer (distance between Ce peak position and Zr or Ce plateau position, ~5 mm) shows gradual decrease in [Ce] and gradual increase in [Zr] with distance from GDC layer. [Ce] and [Zr] also changed between CZGY contact layer and YSZ electrolyte (the distance between Zr peak position and the plateau position of Ce or Zr was ~2.5 mm). The thickness of the remaining CZGY contact layer was ~2.5 mm, which was less than the initial thickness of ~7 mm. The total thickness of the reaction zone was ~10 mm, which was much thicker than that (~5 mm) in the cell without contact layer (Fig. 3a) due primarily to the existence of CZGY layer. Sr and Co also diffused into GDC and CZGY layers. However, little Sr or Co segregation was observed at the interface between YSZ and CZGY. The resistivity of intermediate composition between GDC and YSZ is known to show a maximum and thus the reaction layer should show higher resistivity than that of either GDC or YSZ [18]. For example, the composition of CZGY used as a contact layer in this study is an intermediate composition between YSZ and GDC and is much less conductive (more resistive) (~6.03 103 S/cm) than either GDC (~0.3 S/cm) or YSZ layer (~0.03 S/cm) [15]. Thus a large increase of RU is expected due to the formation of a reaction layer, so the observation that RU (0.28 ± 0.01 U cm2) of the cell without or with the CZGY contact layer is much higher than that (~0.055 U cm2) calculated from the thickness and conductivity of GDC and YSZ layers is reasonable. However, the similarity in the RU of the two types of cell (Fig. 2) is difficult to understand. A possible explanation is that the two layers (Fig. 3a and b) have a similar RU although the reaction layer (~5 mm) in Fig. 3a is only half as thick as that (~10 mm) Fig. 3b; this implies that the reaction layer in Fig. 3a has about twice the resistivity as that in Fig. 3b. The observation requires more detailed studies of the contact between the layers. The cell without CZGY contact layer may have bad or weak contact. The quality of contact improves with the increase in the co-firing temperature [16]. EDS profiles of cells were measured after the electrolysis test. [Ce] and [Zr] profiles of two cells without the contact layer (Fig. 4a) changed little from Fig. 3a, but [Sr] and [Co] profiles showed substantial changes. Sr and Co diffused from the LSCF electrode to the GDC interlayer. Sr segregated strongly at the interface between YSZ and GDC, preferentially on the YSZ side. The comparison of Sr segregation before and after electrolysis implies that the applied current is a major source of Sr segregation, and may be a reason for the degradation in electrode performance. In the cell with the
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Fig. 6. Microstructure of cells are shown; (a) before (after co-firing at 1300 C) and (c) after electrolysis test for 100 h of the cell without contact layer; (b) before and (d) after test for 100 h of the cell with contact layer. From top to bottom: LSCF anode, GDC interlayer, with or without CZGY contact layer, YSZ electrolyte.
contact layer (Fig. 4b), Sr and Co still diffused out from LSCF to the GDC interlayer. Reduced concentration of acceptor doping (Sr) in LSCF is expected to decrease the number of charge carriers (oxygen vacancies) and may be responsible for the decrease in the rate of oxygen (O2) oxidation [19]. The phase change due to the loss of Sr in LSCF may also be a reason [16].
Fig. 7. Voltage change of cell (a) without and (b) with contact layer with 800 mA/cm2 for 100 h. Rapid increase after 10 h was shown for the cell without contact layer. The CZGY contact layer increases performance stability with voltage degradation rate of ~0.04%/h. Measurement conditions are 800 C temperature and 20% H2 þ 80% H2O as a fuel.
Current densityeVoltage (IeV) curves of the cells in both SOFC and SOEC regions were extracted at the beginning of the test (0 h) just after the impedance measurement. SOFC curves were obtained first, followed by SOEC curves. The IeV curve of the cell without contact layer (Fig. 5a) was non-linear in the high-current SOFC region but that of the cell with contact layer (Fig. 5b) was nearly linear in both regions; the result shows current-limiting behavior, and therefore that the direction of oxygen ion transport is important. The current-limiting behavior only occurred when oxygen ions diffused from air electrode to fuel electrode. The air electrode may be responsible for this behavior because it was observed only for the cell without contact layer. This is reasonable since the electrolyte layer with no resistive CZGY interlayer is more conductive than that with interlayer and allows faster diffusion of oxygen from air electrode to fuel electrode. Thus the oxygen content at the interface between air electrode and conductive electrolyte is decreased resulting in the current limiting behavior. The CZGY contact layer improves the contact between GDC and YSZ. The slope of linear part of curve was higher for the cell without the contact layer than in the cell with it. ASR calculated from the slope of the SOEC without the contact layer was ~0.42 U cm2, which is similar to the impedance data (~0.44 U cm2). Similarly, the ASR with the contact layer calculated in the SOEC region was ~0.37 U cm2, similar to the total resistance value calculated from impedance data (~0.38 U cm2). Thus the cell without contact layer, with larger ASR, shows the current-limiting behavior. The values of open-circuit voltage (OCV) were ~0.88 V for both cells in the presence of 80% H2O þ 20% H2 fuel, slightly lower than the theoretical (~0.94 V) electromotive force. Microstructures of cells were obtained for cells immediately after sintering without (Fig. 6a) and with (Fig. 6b) the contact layer,
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and after the SOEC test for 100 h without (Fig. 6c) and with (Fig. 6d) the contact layer. After the electrolysis test, in the cell without the contact layer the GDC interlayer delaminated from the YSZ electrolyte (Fig. 6c). This delamination may be the main cause of the rapid increase of impedance with time (Fig. 2a). In contrast, in the cell with the contact layer, no delamination was observed after the SOEC test for 100 h (Fig. 6d), and no cavities were observed between YSZ and CZGY or between CZGY and GDC layers. Without contact layer, cell firing temperature (1300 C) is not high enough for the reaction (or diffusion) between GDC and YSZ, and thus weak bonding and delamination are resulted. However, with CZGY of intermediate composition, the bonding is stronger between GDC and CZGY or YSZ and CZGY than between GDC and YSZ. Cell voltage was measured at 800 mA/cm2 for 100 h (Fig. 7) in cells without and with a contact layer. In both cases, the initial voltage (V0) was less than the thermal neutral voltage (<1.287 V), at which the endothermic energy of electrolysis equals the energy from Ohmic heating (Joule heating) energy in the cell [20]. The cell without the contact layer had V0 ¼ ~1.23 V, as also shown in the IeV curves (Fig. 5). The voltage was relatively stable for 0e10 h, then increased rapidly to ~2.44 V over time. This tendency was also shown in the impedance plot (Fig. 2a) and is a result of delamination of GDC from YSZ (Fig. 6c). The cell with the contact layer had V0 ¼ ~1.16 V; the voltage increased slightly to ~1.21 V after 100 h (increase rate ~0.04%/h). The stable performance and slight degradation may be due to the enhanced interface stability (GDC/ YSZ) and the small amount of Sr segregation, respectively. 4. Conclusions Electrolysis performance and interface stability were compared with and without CZGY contact layers under 80% H2O humidity. The CZGY contact layer improved the stability of contact with the neighboring layers and thus prevented delamination during electrolysis tests as shown in the microstructure. Thus the contact layer significantly improved the performance of electrolysis cell as
shown by the impedance and currentevoltage relations. The increase in the Ohmic resistance due to contact layer was small (~0.02 U cm2 at 800 C). But, the LSCF air electrode still degraded slowly over time due to Sr segregation. Acknowledgments This research (paper) was performed for the Hydrogen Energy R&D Center, part of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology of Korea. The authors are thankful to Korea Institute of the Energy Research (KIER), Korea. References [1] A. Brisse, J. Schefold, M. Zahid, Int. J. Hydrogen Energy 33 (2008) 5375. [2] M. Ni, M.K.H. Leung, D.Y.C. Leung, Int. J. Hydrogen energy 33 (2008) 2337. [3] A. Hauch, S.D. Ebbesen, S.H. Jensen, M. Mogensen, J. Mater. Chem. 18 (2008) 2331. [4] R. Knibbe, M.L. Traulsen, A. Hauch, S.D. Ebbesen, M. Mogensen, J. Electrochem. Soc. 157 (2010) B1209. [5] M.A. Laguna-Berceroa, R. Campanaa, A. Larrea, J.A. Kilner, V.M. Orera, J. Power Sources 196 (2011) 8942. [6] J.H. Kim, H.I. Ji, H.P. Dasari, D.W. Shin, H.S. Song, J.H. Lee, B.K. Kim, H.J. Je, H. W Lee, K.J. Yoon, Inter. J. Hydrogen Energy 38 (2013) 1225. [7] K. Chen, S.P. Jiang, Inter. J. Hydrogen Energy 36 (2011) 10541. [8] M. Keane, M.K. Mahapatra, A. Verma, P. Singh, Inter. J. Hydrogen Energy 37 (2012) 16776. [9] J.M. Ralph, C. Rossignol, R. Kumar, J. Electrochem. Soc. 150 (2003) A1518. [10] P. Hjalmarsson, X. Sun, Y. Liu, M. Chen, J. Power Sources 223 (2013) 349. [11] F. Tietz, D. Sebold, A. Brisse, J. Schefold, J. Power Sources 223 (2013) 129. [12] J.A. Kilner, Solid State Ionics 129 (2000) 13. [13] G. Balducci, J. Kaspar, P. Fornasiero, M. Graziani, M.S. Islam, J.D. Gale, J. Phys. Chem. B101 (1997) 1750. [14] X. Zhou, B. Scarfino, H.U. Anderson, Solid State Ionics 175 (2004) 19. [15] A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709. [16] S.J. Kim, G.M. Choi, Solid State Ionics 262 (2014) 303. [17] C.H. Lee, G.M. Choi, Solid State Ionics 135 (2000) 653. [18] A. Tsoga, A. Naoumidis, D. Stover, Solid State Ionics 135 (2000) 403. [19] L.W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 273. [20] J.S. Herring, J.E. O'Brien, C.M. Stoots, G.L. Hawkes, J.J. Hartvigsen, M. Shahnam, Inter. J. Hydrogen Energy 32 (2007) 440.