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Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3−δ anode running on humidified methane fuel in solid oxide fuel cells
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Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells Jeong Myeong Lee, Jeong Woo Yun n School of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2015 Received in revised form 12 February 2016 Accepted 18 February 2016
Yttrium and iron co-doped strontium titanium oxide Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). SYTF synthesized by the Pechini method exhibits excellent phase stability during the cell fabrication processes, SOFC operation, and good electrical conductivity (about 1.03 S/cm, porosity 30%) at 900 °C in methane. Because of the slow electrochemical reaction on the SYTF surface, the SYTF anode was modified by a SDC thin film coating on the anode pore wall surface, to increase the number of reaction sites and also accelerate the electrochemical reaction kinetics of the anode. The SDC-modified SYTF anode cell was stable for 300 hour using methane fuel. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Solid oxide fuel cell Strontium titanate Samarium doped ceria Anode modification Electrochemical oxidation Carbon deposition
1. Introduction Solid oxide fuel cells (SOFCs) are a promising alternative energy conversion device because of high energy efficiency and fuel flexibility. Due to high operating temperatures (700–800 °C), commercial hydrocarbons such as natural gas, coal gasified gas, gasoline and diesel can be used as a fuel directly in SOFCs via internal reforming or direct electrochemical oxidation [1–5]. The fuel flexibilities of SOFCs can lower operating costs by minimizing the external reforming process and also allow the direct use of economical hydrocarbon fuels, leading to enhancement of the overall system efficiency. Nickel/yttria-stabilized zirconia (Ni/YSZ) cermet has been widely used as an anode material in SOFCs. Nickel provides good electronic conductivity and catalytic activity, while YSZ provides good oxygen ion conductivity and good thermal expansion match between Ni/YSZ anode and YSZ electrolyte at SOFC operating temperature. The Ni/YSZ anode, however, may not be appropriated for direct use of commercial hydrocarbon fuels because of carbon deposition and sulfur poisoning on the Ni phase. Carbon deposition could deactivate the electrochemical properties of the Ni-based cermet and prevent the electrochemical reaction at the triple phase boundary (TPB) [6–9]. Moreover, carbon can deposit in the anode pore and block the flow of fuel gas to the TPB, n Correspondence to: Chonnam National University, 300 Bukgu, 500-757, Republic of Korea. E-mail address: [email protected] (J.W. Yun).
which results in an increase the gas diffusion resistance [7,10]. The anode structure can be disrupted by deposited carbon diffusing into the nickel phase, leading to permanent damage of the cell. Moreover, nickel in the Ni/YSZ anode is easily agglomerated at SOFCs operating temperature during long-term operation [11]. In addition, sulfur compounds, contained in the commercial hydrocarbon fuel, can be adsorbed physically and/or chemically on the nickel surface or react to form nickel sulfide during long-term operation, which causes deactivation of the electrochemical reaction site [12–14]. Recently, many efforts have been devoted to the development of Ni-free alternative anodes for the direct utilization of practical hydrocarbon fuels. Both sulfur tolerant properties and carbon resistant characteristics are required as alternative anode materials for hydrocarbon fuels. Several Cu-based cermet have been reported due to high resistance to carbon formation [1,15]. However, Cu-based cermet may limit the operating conditions because of the relatively low melting point ( 1080 °C) of Cu. In addition, Cu only provides the electronic conductivity to the anode and does not catalytically affect electrochemical oxidation [16,17]. Dopedceria is a good alternative anode because of excellent ionic conductivity at reducing conditions as well as providing excellent catalytic activity of hydrocarbons due to high oxygen storage [18,19]. Various perovskite structure-based anodes have been also reported as alternative anode materials to overcome carbon deposition and sulfur poisoning for use of hydrocarbon fuels. Doped perovskite structure materials such as doped strontium titanate
http://dx.doi.org/10.1016/j.ceramint.2016.02.104 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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operating condition [31]. Y-doped SrTiO3 (SYT), for example, has been reported as alternative anode candidate but exhibits poor electrochemical properties of oxidation reactions with hydrogen [23]. Hui et al. investigated the electrochemical properties of SrTiO3 containing rare earth dopants including Y, La, Pr, Sm, Nd, Gd or Yb in reducing atmospheres [21]. Tao et al. reported that small amounts ( 5 vol%) of Ni as a catalyst in SYT anodes improve the electro-catalytic activity of the SYT anode [26]. Gorte et al. reported SYT/YSZ composites as anodes to reduce the electrochemical resistance at the interface between the SYT anode and the YSZ electrolyte [23]. Rare doped titanium oxide materials in anodes were also investigated [32–34]. In our previous study, electrochemical properties of SDC (samarium-doped ceria)-coated SYT anodes [35] and Pd catalyzed SDC/SYT anodes [36] were investigated under hydrogen and methane fuel. In the present research, we investigated the electrochemical properties of Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) and the polarization performance of the SYTF anode under humidified hydrogen and humidified methane fuel to assess its feasibility as an alternative anode. In addition, the surface modification effect on the SYTF anode was studied by coating with Sm0.2Ce0.8O2 δ(SDC) sol to form a porous SDC film on the pore wall surface. Using the surface modification, we aimed to improve the electrochemical properties of the SYTF anode.
2. Experimental Fig. 1. Schematic diagram of the cell reactor.
Fig. 2. XRD patterns of SYT after heat treatment at 400, 600, 800, 1000, 1200 °C for 10 h in 5 vol% H2 balanced with N2.
[20–25] and doped lanthanum chromite [26–28] as well as double perovskite materials such as Sr2MgMoO6 δ [29,30] have been investigated as alternative anode materials. Such perovskite materials have frequently mixed ionic and electronic conductive (MIEC) properties, leading to extended electrochemical active site in the anode beyond TPB area. The perovskite-base anode, however, does not have sufficient electrochemical properties as an alternative anode materials comparing to the Ni-based cermet in SOFC
2.1. Cell preparation Strontium nitrate (Sr(NO3)3 H2O, Aldrich), yttrium nitrate hexahydrate (Y(NO3)3 6H2O, Aldrich), titanium isopropoxide (Ti[OCH (CH3)2]4, Junsei), and iron nitrate nonahydrate (Fe(NO3)3 9H2O, Aldrich) were synthesized Sr0.92Y0.08Ti0.6Fe0.4O3 δ (SYTF) by the Pechini method. Titanium isopropoxide and citric acid were solubilized in ethanol (99.9%) for stabilization. Nitric acid (HNO3) as a peptizing agent, citric acid (C6H8O7) and ethylene glycol (C2H6O2) as a dispersant agent were added in the solution at 70 °C for 3 h. Strontium nitrate and yttrium nitrate hexahydrate were mixed in the solution, and then iron nitrate noahydrate was added in the solution, respectively. The solution was mixed at 120 °C for 24 h. After drying at room temperature, the solution-gel was burned at 600 °C to form SYTF compounds. The synthesized crystal structures were analyzed by an X-ray diffractometer (XRD, Rigaku, RINT-5200). We fabricated YSZ electrolyte substrates from 8 mol% yttia-stabilized zirconia (YSZ, Tosoh, Japan) powders by uniaxial dry pressing and sintered at 1400 °C for 10 h. The thickness and diameter of the YSZ electrolyte were 1.5 mm and 25.2 mm, respectively. The SYTF slurry was coated on the electrolyte with the size of 0.69 cm2 by tape-casting method as the anode electrode and then sintered at 1400 °C for 10 h in reducing atmospheric condition. For the cathode electrode, Pt paste was coated with the area of 0.69 cm2 by the same tape-casting method and sintered at 1000 °C for 2 h. The anode and cathode layers were approximately 50 μm and 20 μm thick, respectively. Ni/YSZ anode cell was also fabricated in the same processes to compare with the performance of SYTF. The fabrication of the SYTF phase was analyzed in TGA/DTA (STA PT 1600, TA, USA). The microstructure of the SYTF anode was analyzed in a scanning electron microscope (FE-SEM, Hitachi, S-4200, Japan). Samaria-doped ceria ((CeO2)0.8(Sm2O3)0.2; SDC) sol was used to modify the SYTF anode. Ceria sol was prepared by diluting a commercial CeO2 colloidal dispersion (10 nm–20 nm particles in H2O, Alfa Aesar) in de-ionized water. Samaria nitrate ((Sm(NO)3)3 6H2O, 99%, Aldrich) was diluted in deionized water and then added slowly to the diluted ceria sol to attain 20 mol% samaria-doped ceria sol. The SYTF anode was then modified using the SDC sol. The cell was dipped in the prepared SDC sol followed by drying at room temperature and
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Fig. 3. TGA and DTA curves of the precursor resin synthesized by the Pechini method.
Fig. 5. XRD patterns (a) YSZ, SYTF, and SYTF/YSZ mixture, and (b) SYTF and SYTF/ SDC mixture. Fig. 4. SYTF conductivity in H2 and CH4.
calcining at 700 °C. To ensure coating in entire anode pore wall, the coating process was repeated by 5 times. 2.2. Cell characteristics To measure the electrical characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF), the SYTF powder was pressed at 7600 kPa into cuboids and sintered at 1400 °C for 10 h under hydrogen condition. The electrical conductivity was measured by the standard four-probe direct current (DC) method, which was conducted using a multi-meter device (Model 1000 series, Kithessly Co.). The electrode characteristics were also
measured using an impedance analysis device (SP-150, Biologic Science Instrument). The impedance spectra were recorded in the frequency range from 10 2–106 Hz with an exciting voltage of 30 mV to ensure a linear response. The electric loader (Pureun Tech, Korea) was used to investigate the polarization performance and durability of the SYTF anode cell and the SDC-coated anode cell. Fig. 1 shows a schematic diagram of the reactor to measure the cell performance. The cell was sealed by Pyrex glass on the anode side. A perforated Pt plate (1 cm2 in area) and Pt wire (0.5 mm in diameter) were used as a current collector. In addition, the hydrogen, methane and oxygen used in the experiments were humidified using a bubbler at room temperature (25 °C). Gas flow rate was 200 ml/min in both cathode and anode.
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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donor at A-site as well as the size of donor at B-site. Because the ionic radius of Y3 þ (1.19 Å for twelve coordination) is smaller than that of Sr3 þ (1.44 Å for twelve coordination), Sr2 þ in SrTiO3 may be substituted by Y3 þ to form Y2O3. This substitution creates a different charge valence of the cation and subsequently forms lattice defects to maintain the electrical neutrality of crystals and behaves as n-type semiconductor [37]. In addition, the radius of Fe3 þ (0.65 Å for six coordination) is very similar to Ti4 þ (0.61 Å for six coordination). The Ti4 þ ion in SrTiO3 can be likely substituted by Fe3 þ ion to form Fe2O3, which results in the formation of an oxygen vacancy with electron hole to maintain the electrical neutrality of crystals [38,39]. The free electron as n-type defect formation mechanism (1) and the oxygen vacancy formation mechanism (2) are given by SrO
Y2O3 ⎯⎯→ 2Y •Sr + 3OOX + VSr + 2e TiO
2 Fe2 O3 ⎯⎯⎯⎯→ 2Fe′Ti + 3OOX + V •• O
(1) (2)
2þ
Fig. 6. Impedance spectra of the half cells with SYTF anode in H2 and CH4 at 800 °C and 900 °C.
3. Results and discussion 3.1. Physical properties Fig. 2 shows the XRD patterns of Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) powder prepared by Pechini method. To investigate the crystalline formation temperature of perovskite phase, the SYTF powder was calcined at different temperature (400 °C, 600 °C, 800 °C, 1000 °C, and 1200 °C) for 10 h under 5% H2/N2 of reducing atmospheric condition. At 400 °C, specific peaks were not detected, indicating the perovskite structure was not formed under at 400 °C. The SYTF structure was exhibited in a single perovskite phase around 600 °C and became slightly crystallized at higher temperature. The results corresponded to the TGA/DTA data as shown in Fig. 3. The first large exothermic peak around 450 °C in the DTA curves may be the point of decomposition of the organic compounds and the additives for synthesizing the SYFT powder. The next small exothermic peak at 600 °C may be attributed to crystallization of the SYTF perovskite phase. By-products such as Y2Ti2O7, which are normally observed in compositions of Y2O3 greater than 8 mol% at a dopant with YSZ [37], were not detected up to 1200 °C except for the SYTF phase. The XRD results indicate that SYTF may be relatively stable at high temperatures in reducing atmosphere conditions. Fig. 4 shows the electrical conductivity of SYTF sample at varying temperature in hydrogen and methane. To measure the electrical conductivity by 4-point probe DC method, we prepared rectangular bar SYTF samples, sintered at 1400 °C in reducing conditions where the porosity of the sample was around 35%, which affected to electrical conductivity. The conductivities were 0.31, 0.40, 0.56, and 0.88 S/cm in the hydrogen and 0.36, 0.51, 0.71, 1.03 S/cm in methane at 600 °C, 700 °C, 800 °C and 900 °C, respectively. The conductivity increased with increasing temperature because the conducting behavior in ceramic materials depends upon varying temperatures. The conductivity was slightly higher in the methane than the hydrogen due to the formation of a thin carbon layer on the SYTF anode surface via thermal decomposition of methane. The electrical conductivity of doped-SrTiO3 with perovskite structure (ABO3) may be affected by the size of
For the A-site, Sr cation lattices in SrTiO3 could be substituted by Y3 þ . To maintain the electrical neutrality of crystals, the strontium vacancies and 2 free electrons would be also formed at the same time, acting as n-type semiconductor. For the B-site, Ti4 þ could be substituted by Fe3 þ to form oxygen vacancy with 2 2 electron holes (V•• ) O ), which could act as an oxygen ion (O conductor. Moreover, TiO2 in SrTiO3 could be reduced to TiO2 δ in reducing atmospheric conditions and form oxygen vacancies leading to an increase in the ionic conductivity. Yoon et al. researched the conductivity of Sr0.92Y0.08Ti1 xFexO3 δ at 800 °C in oxidizing and reducing atmospheric conditions [40]. They reported the conductivity of Sr0.92Y0.08Ti1 xFexO3 δ at x¼ 0.4 was greater than that of the x¼ 0.2 material and the conductivity was 0.2 S/cm at 800 °C in hydrogen atmospheric condition. The reasons for the different electrical conductivity values compared to our values could be the porosity of the sample from the different synthetic processes such as heat treatment or sintering time. 3.2. Chemical compatibility Fig. 5(a) shows the XRD patterns of YSZ, SYTF and SYTF/YSZ mixture to investigate the chemical compatibility of the SYTF anode with the YSZ electrolyte. To verify the by-product formation between SYTF anode and YSZ electrolytes at high temperature, the electrolyte and electrode must be co-sintered to fabricate the fuel cell and the SYTF powder mixed by wet ball milling with YSZ powder in a 50:50 weight ratio for 10 h. The SYTF/YSZ mixed powder was sintered at 1400 °C for 10 h in reducing conditions. The XRD pattern of the SYTF/YSZ mixed sample was compared to those of YSZ and SYTF which were also sintered at 1400 °C. No apparent reaction by-products were detected other than the SYTF phase and the YSZ phase. An XRD analysis between SYTF and SDC was also performed to identify the by-products as shown in Fig. 5 (b). The SDC/SYTF composite powders were prepared via solid mixing. The powders were calcined at 900 °C for 5 h in conditioned air. No additional by-product patterns were detected; however, the SYTF, and SDC patterns were obvious. 3.3. Electrode polarization of SYTF anode Fig. 6 shows impedance spectra of SYTF anode at 800 °C and 900 °C in the humidified hydrogen and in the humidified methane conditions. The experimental was carried out at OCV condition. We were prepared an SYTF anode symmetric cell with a Pt reference electrode. At 800 °C, the polarization resistance in the methane was two-times larger than in hydrogen with 33 Ω cm2 and 23 Ω cm2, respectively. At 900 °C, the polarization resistance
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Fig. 7. Controlling the anode microstructure using a SDC coating on the SYTF anode pore surface.
Fig. 8. Scanning electron microscopy images: the SYTF anode surface(top) and the SDC-modified SYTF anode surface(bottom).
in the methane was similar to hydrogen at 20 Ω cm2 and 16 Ω cm2, respectively. Thin carbon layer formed at the SYTF surface via thermal decomposition of methane may increase the electrical conductivity of the anode. The results agree with the measurement of the electrical conductivity shown in Fig. 4. The electrochemical reaction may occur over the entire surface of SYTF anode because the SYTF has MIEC property. However, the anode polarization resistance was much larger than the Ni/YSZ anode, which has typically much less than 1 Ω cm2 at 800–900 °C. Very poor intrinsic catalytic activity of the SYTF for oxidation of both hydrogen and methane may be one explanation. In addition, the
Fig. 9. Activation plots of the anode polarization resistance(RP) of the SYTF anode and the SDC-modified SYTF anode in (a)H2 and (b)CH4.
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Fig. 10. Impedance polarization resistances of the SYTF anode cell and the SDC modified anode cell in (a)H2 and (b) CH4 at 900 °C.
slow reaction of o0.1 Hz, possibly due to surface diffusion of the SYTF anode, could be another explanation. In our earlier research, we discussed about the impedance spectra of ceramic based anode and compared to Ni-based anode [35]. The low frequency range of o0.1 Hz would be the contribution of slow and non-chargetransfer process including oxygen surface exchange and solid surface diffusion. Therefore, to improve the SYTF anode performance, surface modification is needed to accelerate the surface electrochemical reactions. We reported the polarization resistance of Sr0.92Y0.08TiO3 δ (SYT) and the polarization resistance of SYT to be 30 Ω cm2 and 90 Ω cm2 in the methane at 850 °C and 900 °C, respectively, which are larger values than that of the SYTF. Electronic conductivity would be the major contribution to electrical conductivity in the SYT by A-site substitution via the reaction (1), even though the ionic conductivity could contribute to electrical
Fig. 11. IV-characteristics of the SYTF anode cell and the SDC-modified SYTF anode cell in (a)H2 and (b)CH4 at 900 °C.
conductivity by TiO2 δ formation in reducing atmospheric conditions. For SYTF, however, a B-site cation was also substituted to improve the oxygen ion conductivity via the reaction (2) as well as an A-site substitution to improve electronic conductivity. Improvement of ionic conductivity could result in lower polarization resistance of SYTF compared to SYT. 3.4. Effects of SDC modification Samarium-doped ceria (SDC) has several advantages as a SOFC anode material. The much higher ionic conductivity of SDC, comparing to that of YSZ, can improve the transport of oxygen ions
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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anode surface as well as the number of active electrochemical reaction site on the SYTF anode. Fig. 10 shows the anode polarization resistance of the bare SYFT anode (a) and the SDC-modified SYTF anode (b) in hydrogen and methane at 900 °C. The polarization resistance significantly decreased in hydrogen and methane because of the SDC modification. The polarization resistance of the bare SYTF anode was 15.8 Ω cm2 in hydrogen and 19.9 Ω cm2 in methane, respectively. The polarization resistance of the SDC-modified SYTF anode was decreased to 2.4 Ω cm2 in hydrogen and 3.9 Ω cm2 in methane. The results of the activation energy analysis as shown in Fig. 9 and the polarization resistance analysis as shown in Fig. 10 clearly indicate that SDC modification on the SYTF anode can enhance electrochemical catalytic properties such as adsorption, desorption or surface diffusion including charge-transfer reactions. In addition, the results indicate increasing the electrochemical reaction site. 3.5. Cell performance
Fig. 12. Long-term test of the Ni/YSZ anode cell [45], the SYTF anode cell, the SDCmodified SYT anode cell [35], and SDC-modified SYTF anode cell at 900 °C.
from electrolyte to anode. Because of mixed ionic and electronic conductive properties in reducing conditions, SDC can provide additional reaction area beyond the triple-phase boundaries (electrolyte, electrode, and gas). In addition, SDC has been known as an oxygen storage and transfer material, which may increase the reaction rates of methane oxidation and deposited carbon oxidation. Due to these benefits as an SOFC anode material, SDC has been known as a candidate alternative anode. Recently, we reported the SDC thin film formation by sol coating at electrode surface to improve the electrode performance by increasing the number of TPB sites [41–44]. The SDC-modified cells were improved by 30–50% in the cell performance due the MIEC property and high oxygen ion conductivity of SDC. In this study, SDC sol was coated on the SYTF anode pore wall surface by sol–gel coating method to accelerate the oxygen ion exchange and solid-state diffusion on the anode surface. It also could provide catalytic active sites for methane fuel. The SDC modification on the anode pore wall surface is illustrated in Fig. 7. The cut view of the anode microstructure for the bare SYTF anode (a) and the SDC-modified SYTF anode are shown in Fig. 8. The SDC thin film was coated continuously on the SYTF anode and the thickness of SDC film was 10–50 nm. The polarization resistance of the SDC-modified SYTF anode was compared to the bare SYTF anode at different temperatures by Arrhenius plots shown in Fig. 9. In hydrogen and methane, the polarization resistance of the SDC-modified SYTF anode was lower than the bare SYTF anode. In addition, the activation energy of the SYTF anode was lowered by SDC surface modification. The activation energy of the bare SYTF anode was 126.3 kJ/mol and 208.4 kJ/mol in hydrogen and methane, respectively, while the SDC-modified SYTF anode was 89.6 kJ/mol and 148.1 kJ/mol in hydrogen and methane, respectively. The results indicate that the SDC modification on the SYTF anode surface can improve electrochemical reaction kinetics such as oxygen diffusion on the
Fig. 11 shows the I–V characteristics of the electrolyte-supported single cell with the bare SYTF anode and the SDC-modified SYTF anode in hydrogen (a) and methane (b) atmospheric conditions at 900 °C. After stabilizing the cell in either hydrogen or methane for approximately 30 min at the operating temperature, the I–V characteristics were measured with 200 ml min 1 of wet hydrogen (3 vol% of H2O) or wet methane gas (3 vol% of H2O) as an anode fuel and excessive air as a cathode gas. The maximum powder density was 37 mW/cm2 for the bare SYTF anode and 152 mW/cm2 for the SDC-modified SYTF anode in hydrogen. For the SDC-modified SYTF anode, the cell performance improved by approximately 4 times compared to the bare SYTF anode, due to high ionic conductivity and electrochemical catalytic activity as well as by extending the TPB area. In methane, the maximum power density was 19 mW/cm2 for the bare SYTF anode and 99 mW/cm2 for the SDC-modified SYTF anode. The maximum cell performance was significantly improved by the SDC modification on the SYTF anode surface. To analyze the long-term stability of the bare SYTF anode and the SDC-modified SYTF anode, the performance of the YSZ electrolyte-supported single cell was measured under a constant current of 20 mA/cm2 and 40 mA/cm2 at 900 °C, as shown in Fig. 12. After 10 h of stabilizing the cell in humidified H2, the humidified CH4 was introduced as an anode fuel. The cell performance was observed for 300 h, and both anodes rapidly decreased performance in the first few hours after introducing methane fuel. The performance was compared to the Ni/YSZ (Y0.08Zr0.82O3 δ) anode and SDC-modified SYT (Sr0.92Y0.08TiO3 δ) anode which we reported previously [35]. For the Ni/YSZ anode, the cell performance rapidly decreased in methane fuel because of the agglomeration of Ni phase and carbon deposition, which we discussed elsewhere [45]. Otherwise, the cell performance of the SYTF, the SDC-modified SYT, and SDC-modified SYTF anode did not decrease for 300 h without any significant degradation due to inhibiting the carbon deposition on the anode surface. The noise in the SDC modified SYTF anode compared to the other materials as shown in Fig. 12 could be related to reduction/re-oxidation of the ceria and also be related to carbon formation/oxidation. The SDC-modified SYTF anode was slightly higher than that of the SDC-modified SYT anode due to additional B-site modification that led to additional oxygen vacancies.
4. Conclusion Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) has been investigated as an alternative anode material to commercial hydrocarbons in SOFCs. To
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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improve the ionic conductivity of the Sr0.92Y0.08TiO3 δ (SYT) anode, Ti4 þ at the B-site was substituted to Fe3 þ , leading to introduction of oxygen vacancies in the SYT. The stability of the SYTF anode at high temperatures and reducing atmospheric conditions was as good as the SYT anode. The SYTF anode was compatible with YSZ at the SOFC operating condition. The electrochemical reactions of the SYT anode in both hydrogen and methane were limited by relatively slow reactions, such as non-charged processes including oxygen surface exchange and solid surface diffusion. To improve such poor electrochemical activities, the surface of the SYTF anode was modified by the SDC coating. The polarization resistance and the cell performance in hydrogen and methane were significantly improved by the SDC modification. For hydrogen fuel, the maximum power density of the cell at 900 °C was 37 mW/cm2 in the bare SYT anode and 152 mW/cm2 in the SDCmodified SYT anode. For methane fuel, the maximum power density of the cell at 900 °C was 19 mW/cm2 in the bare SYTF anode and 99 mW/cm2 in the SDC-modified SYT anode. The SDCcoated SYT anode cell was successfully operated with methane for more than 300 hours without any degradation of the cell performance.
Acknowledgement This research was supported by Nano Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning. (NRF-2015M3A7B4050495).
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Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells Jeong Myeong Lee, Jeong Woo Yun n School of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2015 Received in revised form 12 February 2016 Accepted 18 February 2016
Yttrium and iron co-doped strontium titanium oxide Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). SYTF synthesized by the Pechini method exhibits excellent phase stability during the cell fabrication processes, SOFC operation, and good electrical conductivity (about 1.03 S/cm, porosity 30%) at 900 °C in methane. Because of the slow electrochemical reaction on the SYTF surface, the SYTF anode was modified by a SDC thin film coating on the anode pore wall surface, to increase the number of reaction sites and also accelerate the electrochemical reaction kinetics of the anode. The SDC-modified SYTF anode cell was stable for 300 hour using methane fuel. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Solid oxide fuel cell Strontium titanate Samarium doped ceria Anode modification Electrochemical oxidation Carbon deposition
1. Introduction Solid oxide fuel cells (SOFCs) are a promising alternative energy conversion device because of high energy efficiency and fuel flexibility. Due to high operating temperatures (700–800 °C), commercial hydrocarbons such as natural gas, coal gasified gas, gasoline and diesel can be used as a fuel directly in SOFCs via internal reforming or direct electrochemical oxidation [1–5]. The fuel flexibilities of SOFCs can lower operating costs by minimizing the external reforming process and also allow the direct use of economical hydrocarbon fuels, leading to enhancement of the overall system efficiency. Nickel/yttria-stabilized zirconia (Ni/YSZ) cermet has been widely used as an anode material in SOFCs. Nickel provides good electronic conductivity and catalytic activity, while YSZ provides good oxygen ion conductivity and good thermal expansion match between Ni/YSZ anode and YSZ electrolyte at SOFC operating temperature. The Ni/YSZ anode, however, may not be appropriated for direct use of commercial hydrocarbon fuels because of carbon deposition and sulfur poisoning on the Ni phase. Carbon deposition could deactivate the electrochemical properties of the Ni-based cermet and prevent the electrochemical reaction at the triple phase boundary (TPB) [6–9]. Moreover, carbon can deposit in the anode pore and block the flow of fuel gas to the TPB, n Correspondence to: Chonnam National University, 300 Bukgu, 500-757, Republic of Korea. E-mail address: [email protected] (J.W. Yun).
which results in an increase the gas diffusion resistance [7,10]. The anode structure can be disrupted by deposited carbon diffusing into the nickel phase, leading to permanent damage of the cell. Moreover, nickel in the Ni/YSZ anode is easily agglomerated at SOFCs operating temperature during long-term operation [11]. In addition, sulfur compounds, contained in the commercial hydrocarbon fuel, can be adsorbed physically and/or chemically on the nickel surface or react to form nickel sulfide during long-term operation, which causes deactivation of the electrochemical reaction site [12–14]. Recently, many efforts have been devoted to the development of Ni-free alternative anodes for the direct utilization of practical hydrocarbon fuels. Both sulfur tolerant properties and carbon resistant characteristics are required as alternative anode materials for hydrocarbon fuels. Several Cu-based cermet have been reported due to high resistance to carbon formation [1,15]. However, Cu-based cermet may limit the operating conditions because of the relatively low melting point ( 1080 °C) of Cu. In addition, Cu only provides the electronic conductivity to the anode and does not catalytically affect electrochemical oxidation [16,17]. Dopedceria is a good alternative anode because of excellent ionic conductivity at reducing conditions as well as providing excellent catalytic activity of hydrocarbons due to high oxygen storage [18,19]. Various perovskite structure-based anodes have been also reported as alternative anode materials to overcome carbon deposition and sulfur poisoning for use of hydrocarbon fuels. Doped perovskite structure materials such as doped strontium titanate
http://dx.doi.org/10.1016/j.ceramint.2016.02.104 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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operating condition [31]. Y-doped SrTiO3 (SYT), for example, has been reported as alternative anode candidate but exhibits poor electrochemical properties of oxidation reactions with hydrogen [23]. Hui et al. investigated the electrochemical properties of SrTiO3 containing rare earth dopants including Y, La, Pr, Sm, Nd, Gd or Yb in reducing atmospheres [21]. Tao et al. reported that small amounts ( 5 vol%) of Ni as a catalyst in SYT anodes improve the electro-catalytic activity of the SYT anode [26]. Gorte et al. reported SYT/YSZ composites as anodes to reduce the electrochemical resistance at the interface between the SYT anode and the YSZ electrolyte [23]. Rare doped titanium oxide materials in anodes were also investigated [32–34]. In our previous study, electrochemical properties of SDC (samarium-doped ceria)-coated SYT anodes [35] and Pd catalyzed SDC/SYT anodes [36] were investigated under hydrogen and methane fuel. In the present research, we investigated the electrochemical properties of Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) and the polarization performance of the SYTF anode under humidified hydrogen and humidified methane fuel to assess its feasibility as an alternative anode. In addition, the surface modification effect on the SYTF anode was studied by coating with Sm0.2Ce0.8O2 δ(SDC) sol to form a porous SDC film on the pore wall surface. Using the surface modification, we aimed to improve the electrochemical properties of the SYTF anode.
2. Experimental Fig. 1. Schematic diagram of the cell reactor.
Fig. 2. XRD patterns of SYT after heat treatment at 400, 600, 800, 1000, 1200 °C for 10 h in 5 vol% H2 balanced with N2.
[20–25] and doped lanthanum chromite [26–28] as well as double perovskite materials such as Sr2MgMoO6 δ [29,30] have been investigated as alternative anode materials. Such perovskite materials have frequently mixed ionic and electronic conductive (MIEC) properties, leading to extended electrochemical active site in the anode beyond TPB area. The perovskite-base anode, however, does not have sufficient electrochemical properties as an alternative anode materials comparing to the Ni-based cermet in SOFC
2.1. Cell preparation Strontium nitrate (Sr(NO3)3 H2O, Aldrich), yttrium nitrate hexahydrate (Y(NO3)3 6H2O, Aldrich), titanium isopropoxide (Ti[OCH (CH3)2]4, Junsei), and iron nitrate nonahydrate (Fe(NO3)3 9H2O, Aldrich) were synthesized Sr0.92Y0.08Ti0.6Fe0.4O3 δ (SYTF) by the Pechini method. Titanium isopropoxide and citric acid were solubilized in ethanol (99.9%) for stabilization. Nitric acid (HNO3) as a peptizing agent, citric acid (C6H8O7) and ethylene glycol (C2H6O2) as a dispersant agent were added in the solution at 70 °C for 3 h. Strontium nitrate and yttrium nitrate hexahydrate were mixed in the solution, and then iron nitrate noahydrate was added in the solution, respectively. The solution was mixed at 120 °C for 24 h. After drying at room temperature, the solution-gel was burned at 600 °C to form SYTF compounds. The synthesized crystal structures were analyzed by an X-ray diffractometer (XRD, Rigaku, RINT-5200). We fabricated YSZ electrolyte substrates from 8 mol% yttia-stabilized zirconia (YSZ, Tosoh, Japan) powders by uniaxial dry pressing and sintered at 1400 °C for 10 h. The thickness and diameter of the YSZ electrolyte were 1.5 mm and 25.2 mm, respectively. The SYTF slurry was coated on the electrolyte with the size of 0.69 cm2 by tape-casting method as the anode electrode and then sintered at 1400 °C for 10 h in reducing atmospheric condition. For the cathode electrode, Pt paste was coated with the area of 0.69 cm2 by the same tape-casting method and sintered at 1000 °C for 2 h. The anode and cathode layers were approximately 50 μm and 20 μm thick, respectively. Ni/YSZ anode cell was also fabricated in the same processes to compare with the performance of SYTF. The fabrication of the SYTF phase was analyzed in TGA/DTA (STA PT 1600, TA, USA). The microstructure of the SYTF anode was analyzed in a scanning electron microscope (FE-SEM, Hitachi, S-4200, Japan). Samaria-doped ceria ((CeO2)0.8(Sm2O3)0.2; SDC) sol was used to modify the SYTF anode. Ceria sol was prepared by diluting a commercial CeO2 colloidal dispersion (10 nm–20 nm particles in H2O, Alfa Aesar) in de-ionized water. Samaria nitrate ((Sm(NO)3)3 6H2O, 99%, Aldrich) was diluted in deionized water and then added slowly to the diluted ceria sol to attain 20 mol% samaria-doped ceria sol. The SYTF anode was then modified using the SDC sol. The cell was dipped in the prepared SDC sol followed by drying at room temperature and
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Fig. 3. TGA and DTA curves of the precursor resin synthesized by the Pechini method.
Fig. 5. XRD patterns (a) YSZ, SYTF, and SYTF/YSZ mixture, and (b) SYTF and SYTF/ SDC mixture. Fig. 4. SYTF conductivity in H2 and CH4.
calcining at 700 °C. To ensure coating in entire anode pore wall, the coating process was repeated by 5 times. 2.2. Cell characteristics To measure the electrical characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF), the SYTF powder was pressed at 7600 kPa into cuboids and sintered at 1400 °C for 10 h under hydrogen condition. The electrical conductivity was measured by the standard four-probe direct current (DC) method, which was conducted using a multi-meter device (Model 1000 series, Kithessly Co.). The electrode characteristics were also
measured using an impedance analysis device (SP-150, Biologic Science Instrument). The impedance spectra were recorded in the frequency range from 10 2–106 Hz with an exciting voltage of 30 mV to ensure a linear response. The electric loader (Pureun Tech, Korea) was used to investigate the polarization performance and durability of the SYTF anode cell and the SDC-coated anode cell. Fig. 1 shows a schematic diagram of the reactor to measure the cell performance. The cell was sealed by Pyrex glass on the anode side. A perforated Pt plate (1 cm2 in area) and Pt wire (0.5 mm in diameter) were used as a current collector. In addition, the hydrogen, methane and oxygen used in the experiments were humidified using a bubbler at room temperature (25 °C). Gas flow rate was 200 ml/min in both cathode and anode.
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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donor at A-site as well as the size of donor at B-site. Because the ionic radius of Y3 þ (1.19 Å for twelve coordination) is smaller than that of Sr3 þ (1.44 Å for twelve coordination), Sr2 þ in SrTiO3 may be substituted by Y3 þ to form Y2O3. This substitution creates a different charge valence of the cation and subsequently forms lattice defects to maintain the electrical neutrality of crystals and behaves as n-type semiconductor [37]. In addition, the radius of Fe3 þ (0.65 Å for six coordination) is very similar to Ti4 þ (0.61 Å for six coordination). The Ti4 þ ion in SrTiO3 can be likely substituted by Fe3 þ ion to form Fe2O3, which results in the formation of an oxygen vacancy with electron hole to maintain the electrical neutrality of crystals [38,39]. The free electron as n-type defect formation mechanism (1) and the oxygen vacancy formation mechanism (2) are given by SrO
Y2O3 ⎯⎯→ 2Y •Sr + 3OOX + VSr + 2e TiO
2 Fe2 O3 ⎯⎯⎯⎯→ 2Fe′Ti + 3OOX + V •• O
(1) (2)
2þ
Fig. 6. Impedance spectra of the half cells with SYTF anode in H2 and CH4 at 800 °C and 900 °C.
3. Results and discussion 3.1. Physical properties Fig. 2 shows the XRD patterns of Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) powder prepared by Pechini method. To investigate the crystalline formation temperature of perovskite phase, the SYTF powder was calcined at different temperature (400 °C, 600 °C, 800 °C, 1000 °C, and 1200 °C) for 10 h under 5% H2/N2 of reducing atmospheric condition. At 400 °C, specific peaks were not detected, indicating the perovskite structure was not formed under at 400 °C. The SYTF structure was exhibited in a single perovskite phase around 600 °C and became slightly crystallized at higher temperature. The results corresponded to the TGA/DTA data as shown in Fig. 3. The first large exothermic peak around 450 °C in the DTA curves may be the point of decomposition of the organic compounds and the additives for synthesizing the SYFT powder. The next small exothermic peak at 600 °C may be attributed to crystallization of the SYTF perovskite phase. By-products such as Y2Ti2O7, which are normally observed in compositions of Y2O3 greater than 8 mol% at a dopant with YSZ [37], were not detected up to 1200 °C except for the SYTF phase. The XRD results indicate that SYTF may be relatively stable at high temperatures in reducing atmosphere conditions. Fig. 4 shows the electrical conductivity of SYTF sample at varying temperature in hydrogen and methane. To measure the electrical conductivity by 4-point probe DC method, we prepared rectangular bar SYTF samples, sintered at 1400 °C in reducing conditions where the porosity of the sample was around 35%, which affected to electrical conductivity. The conductivities were 0.31, 0.40, 0.56, and 0.88 S/cm in the hydrogen and 0.36, 0.51, 0.71, 1.03 S/cm in methane at 600 °C, 700 °C, 800 °C and 900 °C, respectively. The conductivity increased with increasing temperature because the conducting behavior in ceramic materials depends upon varying temperatures. The conductivity was slightly higher in the methane than the hydrogen due to the formation of a thin carbon layer on the SYTF anode surface via thermal decomposition of methane. The electrical conductivity of doped-SrTiO3 with perovskite structure (ABO3) may be affected by the size of
For the A-site, Sr cation lattices in SrTiO3 could be substituted by Y3 þ . To maintain the electrical neutrality of crystals, the strontium vacancies and 2 free electrons would be also formed at the same time, acting as n-type semiconductor. For the B-site, Ti4 þ could be substituted by Fe3 þ to form oxygen vacancy with 2 2 electron holes (V•• ) O ), which could act as an oxygen ion (O conductor. Moreover, TiO2 in SrTiO3 could be reduced to TiO2 δ in reducing atmospheric conditions and form oxygen vacancies leading to an increase in the ionic conductivity. Yoon et al. researched the conductivity of Sr0.92Y0.08Ti1 xFexO3 δ at 800 °C in oxidizing and reducing atmospheric conditions [40]. They reported the conductivity of Sr0.92Y0.08Ti1 xFexO3 δ at x¼ 0.4 was greater than that of the x¼ 0.2 material and the conductivity was 0.2 S/cm at 800 °C in hydrogen atmospheric condition. The reasons for the different electrical conductivity values compared to our values could be the porosity of the sample from the different synthetic processes such as heat treatment or sintering time. 3.2. Chemical compatibility Fig. 5(a) shows the XRD patterns of YSZ, SYTF and SYTF/YSZ mixture to investigate the chemical compatibility of the SYTF anode with the YSZ electrolyte. To verify the by-product formation between SYTF anode and YSZ electrolytes at high temperature, the electrolyte and electrode must be co-sintered to fabricate the fuel cell and the SYTF powder mixed by wet ball milling with YSZ powder in a 50:50 weight ratio for 10 h. The SYTF/YSZ mixed powder was sintered at 1400 °C for 10 h in reducing conditions. The XRD pattern of the SYTF/YSZ mixed sample was compared to those of YSZ and SYTF which were also sintered at 1400 °C. No apparent reaction by-products were detected other than the SYTF phase and the YSZ phase. An XRD analysis between SYTF and SDC was also performed to identify the by-products as shown in Fig. 5 (b). The SDC/SYTF composite powders were prepared via solid mixing. The powders were calcined at 900 °C for 5 h in conditioned air. No additional by-product patterns were detected; however, the SYTF, and SDC patterns were obvious. 3.3. Electrode polarization of SYTF anode Fig. 6 shows impedance spectra of SYTF anode at 800 °C and 900 °C in the humidified hydrogen and in the humidified methane conditions. The experimental was carried out at OCV condition. We were prepared an SYTF anode symmetric cell with a Pt reference electrode. At 800 °C, the polarization resistance in the methane was two-times larger than in hydrogen with 33 Ω cm2 and 23 Ω cm2, respectively. At 900 °C, the polarization resistance
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Fig. 7. Controlling the anode microstructure using a SDC coating on the SYTF anode pore surface.
Fig. 8. Scanning electron microscopy images: the SYTF anode surface(top) and the SDC-modified SYTF anode surface(bottom).
in the methane was similar to hydrogen at 20 Ω cm2 and 16 Ω cm2, respectively. Thin carbon layer formed at the SYTF surface via thermal decomposition of methane may increase the electrical conductivity of the anode. The results agree with the measurement of the electrical conductivity shown in Fig. 4. The electrochemical reaction may occur over the entire surface of SYTF anode because the SYTF has MIEC property. However, the anode polarization resistance was much larger than the Ni/YSZ anode, which has typically much less than 1 Ω cm2 at 800–900 °C. Very poor intrinsic catalytic activity of the SYTF for oxidation of both hydrogen and methane may be one explanation. In addition, the
Fig. 9. Activation plots of the anode polarization resistance(RP) of the SYTF anode and the SDC-modified SYTF anode in (a)H2 and (b)CH4.
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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Fig. 10. Impedance polarization resistances of the SYTF anode cell and the SDC modified anode cell in (a)H2 and (b) CH4 at 900 °C.
slow reaction of o0.1 Hz, possibly due to surface diffusion of the SYTF anode, could be another explanation. In our earlier research, we discussed about the impedance spectra of ceramic based anode and compared to Ni-based anode [35]. The low frequency range of o0.1 Hz would be the contribution of slow and non-chargetransfer process including oxygen surface exchange and solid surface diffusion. Therefore, to improve the SYTF anode performance, surface modification is needed to accelerate the surface electrochemical reactions. We reported the polarization resistance of Sr0.92Y0.08TiO3 δ (SYT) and the polarization resistance of SYT to be 30 Ω cm2 and 90 Ω cm2 in the methane at 850 °C and 900 °C, respectively, which are larger values than that of the SYTF. Electronic conductivity would be the major contribution to electrical conductivity in the SYT by A-site substitution via the reaction (1), even though the ionic conductivity could contribute to electrical
Fig. 11. IV-characteristics of the SYTF anode cell and the SDC-modified SYTF anode cell in (a)H2 and (b)CH4 at 900 °C.
conductivity by TiO2 δ formation in reducing atmospheric conditions. For SYTF, however, a B-site cation was also substituted to improve the oxygen ion conductivity via the reaction (2) as well as an A-site substitution to improve electronic conductivity. Improvement of ionic conductivity could result in lower polarization resistance of SYTF compared to SYT. 3.4. Effects of SDC modification Samarium-doped ceria (SDC) has several advantages as a SOFC anode material. The much higher ionic conductivity of SDC, comparing to that of YSZ, can improve the transport of oxygen ions
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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anode surface as well as the number of active electrochemical reaction site on the SYTF anode. Fig. 10 shows the anode polarization resistance of the bare SYFT anode (a) and the SDC-modified SYTF anode (b) in hydrogen and methane at 900 °C. The polarization resistance significantly decreased in hydrogen and methane because of the SDC modification. The polarization resistance of the bare SYTF anode was 15.8 Ω cm2 in hydrogen and 19.9 Ω cm2 in methane, respectively. The polarization resistance of the SDC-modified SYTF anode was decreased to 2.4 Ω cm2 in hydrogen and 3.9 Ω cm2 in methane. The results of the activation energy analysis as shown in Fig. 9 and the polarization resistance analysis as shown in Fig. 10 clearly indicate that SDC modification on the SYTF anode can enhance electrochemical catalytic properties such as adsorption, desorption or surface diffusion including charge-transfer reactions. In addition, the results indicate increasing the electrochemical reaction site. 3.5. Cell performance
Fig. 12. Long-term test of the Ni/YSZ anode cell [45], the SYTF anode cell, the SDCmodified SYT anode cell [35], and SDC-modified SYTF anode cell at 900 °C.
from electrolyte to anode. Because of mixed ionic and electronic conductive properties in reducing conditions, SDC can provide additional reaction area beyond the triple-phase boundaries (electrolyte, electrode, and gas). In addition, SDC has been known as an oxygen storage and transfer material, which may increase the reaction rates of methane oxidation and deposited carbon oxidation. Due to these benefits as an SOFC anode material, SDC has been known as a candidate alternative anode. Recently, we reported the SDC thin film formation by sol coating at electrode surface to improve the electrode performance by increasing the number of TPB sites [41–44]. The SDC-modified cells were improved by 30–50% in the cell performance due the MIEC property and high oxygen ion conductivity of SDC. In this study, SDC sol was coated on the SYTF anode pore wall surface by sol–gel coating method to accelerate the oxygen ion exchange and solid-state diffusion on the anode surface. It also could provide catalytic active sites for methane fuel. The SDC modification on the anode pore wall surface is illustrated in Fig. 7. The cut view of the anode microstructure for the bare SYTF anode (a) and the SDC-modified SYTF anode are shown in Fig. 8. The SDC thin film was coated continuously on the SYTF anode and the thickness of SDC film was 10–50 nm. The polarization resistance of the SDC-modified SYTF anode was compared to the bare SYTF anode at different temperatures by Arrhenius plots shown in Fig. 9. In hydrogen and methane, the polarization resistance of the SDC-modified SYTF anode was lower than the bare SYTF anode. In addition, the activation energy of the SYTF anode was lowered by SDC surface modification. The activation energy of the bare SYTF anode was 126.3 kJ/mol and 208.4 kJ/mol in hydrogen and methane, respectively, while the SDC-modified SYTF anode was 89.6 kJ/mol and 148.1 kJ/mol in hydrogen and methane, respectively. The results indicate that the SDC modification on the SYTF anode surface can improve electrochemical reaction kinetics such as oxygen diffusion on the
Fig. 11 shows the I–V characteristics of the electrolyte-supported single cell with the bare SYTF anode and the SDC-modified SYTF anode in hydrogen (a) and methane (b) atmospheric conditions at 900 °C. After stabilizing the cell in either hydrogen or methane for approximately 30 min at the operating temperature, the I–V characteristics were measured with 200 ml min 1 of wet hydrogen (3 vol% of H2O) or wet methane gas (3 vol% of H2O) as an anode fuel and excessive air as a cathode gas. The maximum powder density was 37 mW/cm2 for the bare SYTF anode and 152 mW/cm2 for the SDC-modified SYTF anode in hydrogen. For the SDC-modified SYTF anode, the cell performance improved by approximately 4 times compared to the bare SYTF anode, due to high ionic conductivity and electrochemical catalytic activity as well as by extending the TPB area. In methane, the maximum power density was 19 mW/cm2 for the bare SYTF anode and 99 mW/cm2 for the SDC-modified SYTF anode. The maximum cell performance was significantly improved by the SDC modification on the SYTF anode surface. To analyze the long-term stability of the bare SYTF anode and the SDC-modified SYTF anode, the performance of the YSZ electrolyte-supported single cell was measured under a constant current of 20 mA/cm2 and 40 mA/cm2 at 900 °C, as shown in Fig. 12. After 10 h of stabilizing the cell in humidified H2, the humidified CH4 was introduced as an anode fuel. The cell performance was observed for 300 h, and both anodes rapidly decreased performance in the first few hours after introducing methane fuel. The performance was compared to the Ni/YSZ (Y0.08Zr0.82O3 δ) anode and SDC-modified SYT (Sr0.92Y0.08TiO3 δ) anode which we reported previously [35]. For the Ni/YSZ anode, the cell performance rapidly decreased in methane fuel because of the agglomeration of Ni phase and carbon deposition, which we discussed elsewhere [45]. Otherwise, the cell performance of the SYTF, the SDC-modified SYT, and SDC-modified SYTF anode did not decrease for 300 h without any significant degradation due to inhibiting the carbon deposition on the anode surface. The noise in the SDC modified SYTF anode compared to the other materials as shown in Fig. 12 could be related to reduction/re-oxidation of the ceria and also be related to carbon formation/oxidation. The SDC-modified SYTF anode was slightly higher than that of the SDC-modified SYT anode due to additional B-site modification that led to additional oxygen vacancies.
4. Conclusion Sr0.92Y0.08Ti0.7Fe0.3O3 δ (SYTF) has been investigated as an alternative anode material to commercial hydrocarbons in SOFCs. To
Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i
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improve the ionic conductivity of the Sr0.92Y0.08TiO3 δ (SYT) anode, Ti4 þ at the B-site was substituted to Fe3 þ , leading to introduction of oxygen vacancies in the SYT. The stability of the SYTF anode at high temperatures and reducing atmospheric conditions was as good as the SYT anode. The SYTF anode was compatible with YSZ at the SOFC operating condition. The electrochemical reactions of the SYT anode in both hydrogen and methane were limited by relatively slow reactions, such as non-charged processes including oxygen surface exchange and solid surface diffusion. To improve such poor electrochemical activities, the surface of the SYTF anode was modified by the SDC coating. The polarization resistance and the cell performance in hydrogen and methane were significantly improved by the SDC modification. For hydrogen fuel, the maximum power density of the cell at 900 °C was 37 mW/cm2 in the bare SYT anode and 152 mW/cm2 in the SDCmodified SYT anode. For methane fuel, the maximum power density of the cell at 900 °C was 19 mW/cm2 in the bare SYTF anode and 99 mW/cm2 in the SDC-modified SYT anode. The SDCcoated SYT anode cell was successfully operated with methane for more than 300 hours without any degradation of the cell performance.
Acknowledgement This research was supported by Nano Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning. (NRF-2015M3A7B4050495).
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Please cite this article as: J.M. Lee, J.W. Yun, Characteristics of Sr0.92Y0.08Ti0.7Fe0.3O3 δ anode running on humidified methane fuel in solid oxide fuel cells, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.104i