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AC impedance characteristics for anode-supported microtubular solid oxide fuel cells
Electrochimica Acta 67 (2012) 159–165
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
AC impedance characteristics for anode-supported microtubular solid oxide fuel cells Hirofumi Sumi a,∗ , Toshiaki Yamaguchi a , Koichi Hamamoto a , Toshio Suzuki a , Yoshinobu Fujishiro a , Toshiaki Matsui b , Koichi Eguchi b a b
Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
a r t i c l e
i n f o
Article history: Received 30 November 2011 Received in revised form 6 February 2012 Accepted 7 February 2012 Available online 15 February 2012 Keywords: Solid oxide fuel cell (SOFC) Anode microstructure AC impedance Distribution of relaxation times
a b s t r a c t The DRT method was applied to analyze the AC impedance spectra for anode-supported microtubular solid oxide fuel cells (SOFCs). The anode microstructure has a more severe effect on the electrochemical properties of the anode-supported microtubular SOFCs because of the small inside diameter and thickness of the anode. For example, the maximum current density and fuel utilization for the cells sintered at 1400 ◦ C were lower than that at 1350 ◦ C due to the low porosity of the anode. However, it is difficult to separate the anode and cathode impedances by the general method of complex non-linear least squares (CNLS) with equivalent circuit models, because several components overlap for the impedance spectra measured between the anode and cathode without a reference electrode. The DRT method can easily separate the impedance spectra into five polarization components for the anode-supported microtubular SOFCs. The activation energy of the anode diffusion resistance was only 24–33 kJ/mol. The walls of the pores in the anode were relatively well-packed with the nickel and YSZ particles for the cells made by the use of acrylic resin as a pore former. The anode diffusion resistance decreased by only a small amount with a rise of operating temperature for the anode-supported microtubular SOFCs. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are expected to serve as power generation systems with high energy conversion efficiency. Recently, many researchers focused on lowering the operating temperature of SOFCs from 800–1000 ◦ C to 600–700 ◦ C due to improvement of durability, start-up time and so on. Two approaches have been particularly popular for use in lowering the operating temperature of SOFCs. One approach is to use electrolyte materials of high ionic conductivity at lower temperatures such as La0.9 Sr0.1 Ga0.8 Mg0.2 O3 (LSGM) [1] and Gd-doped CeO2 (GDC) [2], and the other is to reduce the thickness of the electrolyte made of conventional materials such as Y2 O3 -stabilized zirconia (YSZ) [3–6]. Tubular cells generally reduce the problems associated with brittleness and sealing as compared to planar cells. However, tubular cells have the disadvantage of a small volumetric power density. A cell-stack with large volume and heat capacity cannot start up rapidly. The power density, the start-up time and so on are proportional to the inverse of cell diameter. Kendall et al. [7,8] and Yashiro et al. [9] demonstrated microtubular SOFCs with a diameter on the
∗ Corresponding author. Tel.: +81 52 736 7592; fax: +81 52 736 7405. E-mail address: [email protected] (H. Sumi). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.02.021
millimeter scale. A rapid start-up (i.e. within several seconds) was achieved through the butane fuel for the microtubular SOFCs. Previously, the authors have developed anode-supported microtubular SOFCs with high power density via the “Advanced Ceramic Reactor Project” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The highest power density per cathode area reached 1 W/cm2 at a low temperature of 550 ◦ C for microtubular cells with a diameter of 0.8 mm [10]. Furthermore, the power of 2 W can be obtained at 490 ◦ C for a 3 × 3-cell module with a volume of 1 cm3 [11]. The microtubular SOFC modules are expected to be applied to micro combined heat and power (CHP) systems for residences, and power sources for portable devices and transportation. Anode microstructure affects electrochemical properties more severely for the anode-supported microtubular SOFCs because of the small inside diameter and thickness of the anode. Suzuki et al. reported that cell performance was improved by decreasing the sintering temperature of the anode [12]. The polarization resistance was confirmed to decrease by the increase in porosity of the anode. However, it is difficult to analyze AC impedance spectra by complex non-linear least squares (CNLS) with equivalent circuit models, as several electrode processes generally overlap within the actual impedance spectra. Some mathematical techniques have been developed to identify each electrode process. Schichlein et al.
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suggested the distribution of relaxation times (DRT) method for the analysis of impedance spectra for SOFCs [13,14]. A polarization component is generally analyzed by an equivalent circuit with a parallel resistance–conductance (RC) element. An ideal RC element has a single relaxation time defined by the appropriate R and C values ( = RC). However, the actual polarization resistance for SOFCs cannot be fitted perfectly by one RC element, because relaxation times are generally distributed for actual electrode processes. Therefore, the polarization resistances are generally fitted by the equivalent circuit with RQ element (constant phase elements in parallel with a resistor) for the CNLS method [15]. The DRT method can directly detect the number of electrode processes by mathematical techniques without assuming the equivalent circuits of the polarization components [13,14]. Previously, the effects of the gas concentration in the electrode and the operating temperature were considered by the DRT analysis for planar SOFCs [16–18]. The correlations between the durability and the mass transfer were also investigated [19,20]. Furthermore, the DRT method was applied for several types of SOFCs such as half-cells with patterned electrodes [21,22], metal-supported planar [23], and segmentedin-series tubular cells [24,25]. In this study, the effects of anode microstructure on electrochemical characteristics were investigated by the DRT analysis of AC impedance spectra for the anode-supported microtubular SOFCs. The problems associated with the microtubular cells using a pore former of acrylic resin were investigated from the analysis of the activation energy of each electrode resistance.
2. Experimental Anode microtubes were made from NiO (Sumitomo metal mining), 8 mol% Y2 O3 stabilized ZrO2 (8YSZ; Tosoh), pore former (acrylic resin; Sekisui Plastic) and binder (Cellulose; Yuken Kogyo) powders. The weight ratio of NiO to 8YSZ was 6:4, and the particle size of pore former was ca. 5 m. These powders were mixed by kneading machine with adding the proper amount of water for 2 h. The hardness of the mixture clay was ca. 12 points which measured by a type A durometer (ISO 7619). The anode microtubes were extruded using a piston cylinder with a metal hold of 2.4 mm (outside diameter) and 2.0 mm (inside diameter). After extrusion, the tubes were dried overnight in air at room temperature. A slurry was prepared by mixing 8YSZ, a binder (polyvinyl butyral; Sekisui Chemical) a dispersant (tallow propylene diamine, Kao) and a plasticizer (dioctyl adipate; Wako Pure Chemical Industries) into ethanol and toluene solvents for 48 h. The 8YSZ electrolyte was formed by dip-coating at a pulling rate of 1 mm/s. The 8YSZ electrolyte thin-film and NiO–8YSZ anode microtube were cosintered for 1 h in air. The sintering temperatures were 1350 ◦ C and 1400 ◦ C which modified the anode microstructure. The interlayer of Ce0.9 Gd0.1 O1.95 (Shin-etsu Chemical) and the cathode of La0.6 Sr0.4 Co0.2 Fe0.8 O3 (Dowa Electronics Materials) were coated by a similar manner. The pulling rates of the dip-coater were 1 and 2 mm/s for the interlayer and the cathode, respectively. The interlayer and cathode thin-film were sintered sequentially for 1 h in air at 1200 ◦ C and 1050 ◦ C, respectively. Fig. 1 shows a photograph of the anode-supported microtubular SOFCs used in this study. The outside diameter of microtube was 1.8 mm, and the area of cathode was 0.6 cm2 after sintering. The thicknesses of the anode, electrolyte, interlayer and cathode were ca. 200, 5, 1 and 20 m, respectively. Silver wires were used as current collector. The characteristics of power generation and AC impedance were evaluated with a potentiostat/galvanostat (Solartron Analytical 1287) and an impedance analyzer (Solartron Analytical 1255B). A mixture of x%H2 − 3%H2 O − (100 − x) %N2 (x = 10, 20, 40) was supplied as fuel at a flow rate of 50 mL/min to an anode side, and air
Fig. 1. Photograph of anode-supported microtubular SOFCs with an outside diameter of 1.8 mm and a cathode area of 0.6 cm2 after sintering. Current collectors are silver wires.
was supply as oxidant at 50 mL/min to a cathode side. Operating temperatures were 595 ◦ C, 646 ◦ C and 697 ◦ C. Current–potential (j − E) characteristics were measured from open circuit potential (OCP) to 0.4 V at a sweep rate of 5 mV/s. The AC impedance was measured under OCP in the frequency range from 1 MHz to 0.1 Hz with 20 steps per logarithmic decade. The DRT analysis has been described in detail in Refs. [12,13]. In this study, polarization processes are considered as an equivalent circuit composed of a large number of RC elements in series. However, Liu et al. reported that it was impossible to obtain good results for the DRT analysis for impedance spectra with an inductance at high frequency [24]. The inductance components were removed by the same method as described in Ref. [24] before the DRT analysis for the impedance spectra in this study. The software program of FTIKREG [26] was used to solve an ill-posed inverse problem in the DRT analysis by Tikhonov regularization. 3. Results and discussion The sintering temperatures affect the anode microstructure for the anode-supported microtubular SOFCs. Fig. 2 shows the scanning electron images of (i) oxidized and (ii) reduced anodes sintered at (a) 1350 ◦ C and (b) 1400 ◦ C. The macropores with a diameter of several microns were observed for the all samples, which were made by burning the pore formers of acrylic resin during sintering. Although the walls of the pores were packed with the nickel oxide and YSZ particles in the oxidized state, the small gap between these particles was produced by the shrinkage of metallic nickel after reducing. The formation of similar macropores was reported by the other researches using the pore formers of acrylic resin [27,28]. On the other hand, the micropores were introduced on nanoscale by using the other pore formers such as carbon black [29], rice starch [30], and flour [31]. The size and distribution of the pores can be controlled by a use of each pore former in the anode substrate. The open porosity evaluated by Archimedes’s method were 34% and 31% for the oxidized anode microtubes sintered at 1350 ◦ C and 1400 ◦ C, respectively. The porosity increases by ca. 10 points after reducing the both anode microtubes. In this study, the effects of the difference in the anode porosity on electrochemical properties were investigated precisely for the anode-supported microtubular SOFCs. Fig. 3 shows the j − E characteristics for the anode-supported microtubular SOFCs sintered at 1350 ◦ C and 1400 ◦ C with a fuel flow rate of 50 mL/min. The operating temperature was 595 ◦ C or 697 ◦ C, and the fuel composition was x = 10 or 40 in
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Fig. 2. Scanning electron images of (i) oxidized and (ii) reduced anodes sintered at (a) 1350 ◦ C and (b) 1400 ◦ C.
x%H2 − 3%H2 O − (100 − x) %N2 mixtures. The performances for the cells sintered at 1350 ◦ C and 1400 ◦ C were almost the same at an operating temperature of 697 ◦ C in 40%H2 . The current density reached to 1.6 A/cm2 at a cell potential of 0.5 V. However, the fuel utilization was only 35% at 1.6 A/cm2 in 40%H2 . The OCP for the cell sintered at 1350 ◦ C was slightly lower than that at 1400 ◦ C. This means that the small gas leakage occurred due to the low relative density of the electrolyte thin-film sintered at the lower temperature. The area specific resistances were derived from the slopes of j − E curves near OCP to be 0.41 and 0.51 cm2 in 40%H2 at
697 ◦ C for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. This difference of resistances was caused by the change of the anode overpotential and the gas partial pressure near OCP. On the other hand, the cell potential dropped at 0.9 A/cm2 in 10%H2 at 697 ◦ C for the cell sintered at 1400 ◦ C, while the current density reached 1.1 A/cm2 for the cell sintered at 1350 ◦ C. The maximum fuel utilizations were 93% and 72% in 10%H2 at 697 ◦ C for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. This means that the low porosity in the anode mainly affects the decrease in performance at high fuel utilizations. The difference in the performances for the
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Fig. 3. Current–potential characteristics for anode-supported microtubular SOFCs sintered at 1350 ◦ C and 1400 ◦ C with a fuel flow rate of 50 mL/min.
cells sintered at 1350 ◦ C and 1400 ◦ C was small in 10%H2 at 595 ◦ C because of the low fuel utilization at 0.6 A/cm2 and 50 mL/min. The difference in the performances, of course, seems to appear at lower hydrogen concentrations or flow rates even at a temperature of 595 ◦ C. Fig. 4 shows the AC impedance spectra for the anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C measured between the anode and the cathode under OCP state. Two arcs seem to exist in the Nyquist plots of Fig. 4(a)–(d). Total resistances agreed with the area specific resistances derived from the slope of j − E curves near OCPs in Fig. 3. The diameters of the arcs produced at both high and low frequencies increased with a decrease in hydrogen concentration in Fig. 4(a) and (c), which suggests that the activation and diffusion resistances in the anode were large at a low hydrogen concentration. In comparison with the sintering temperatures, the ohmic resistances were almost the
same for the cells sintered at 1350 ◦ C and 1400 ◦ C. However, the arcs at both high and low frequencies for the cells sintered at 1400 ◦ C were larger than those at 1350 ◦ C. This result suggests that the low porosity in the anode causes the increase in not only diffusion but also activation resistances. Fig. 4(b) and (d) shows the impedance spectra measured in 10%H2 at 595 ◦ C, 646 ◦ C and 697 ◦ C. The arcs produced at high frequencies especially increased with a decrease in the operating temperature for the cells sintered at 1350 ◦ C and 1400 ◦ C. The AC impedances in Fig. 4 were measured between the anode and the cathode. Therefore, the spectra contained at least four components of activation and diffusion resistances in the anode and the cathode. However, it is too difficult to exactly separate the over-lapped impedance spectra into more than four components using the CNLS method with equivalent circuit models. Precise impedance analyses were tried by the DRT method in reference to Refs. [12,13] using FTIKREG program [26]. Liu et al. suggests that the inductance component should be removed before the DRT analysis to obtain good results [24]. In this study, the equivalent circuit as shown in Fig. 5(a) was assumed to estimate the inductance. Fig. 5(b) shows the impedance spectra at 697 ◦ C in 10%H2 before and after the inductance removal for the cell sintered at 1350 ◦ C. In the inset of Fig. 5, a large inductance component was observed at high frequency by noise from lead wires for the raw data. The inductance is assumed to be 0.323 H cm2 from the equivalent circuit of Fig. 5(a). After removing the inductance component, the arc was clarified at high frequency from 1 MHz to 1 kHz. The impedance spectra after removing the inductance component were used for the DRT analyses after this. Fig. 6 shows the DRT spectra for the anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C. Five DRT peaks were detected for each spectrum. The effects of the hydrogen concentration were evaluated at 697 ◦ C in Fig. 6(a) and (c). The peak of R5 increased with a decrease in the hydrogen concentration, which corresponds to an anode diffusion resistance. The area ratios of R5 in 10%:20%:40%H2 were 1:0.93:0.88 and 1:0.85:0.73 at 697 ◦ C for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. The peaks of R5 for the cell sintered at 1400 ◦ C were larger than that at 1350 ◦ C. This means that the low porosity causes the increase in the anode diffusion resistance. The peaks of R2 and R3 slightly increased with a decrease in the hydrogen concentration. Liu et al. reported that the change in the activation
Fig. 4. AC impedance spectra for anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C measured between anode and cathode under OCP state.
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Fig. 5. (a) Equivalent circuit to estimate the inductance, and (b) impedance spectra measured at 697 ◦ C in 10%H2 before and after inductance removal for the cell sintered at 1350 ◦ C.
resistance was smaller than that in the diffusion resistance against oxygen partial pressure [24]. The peaks of R2 and R3 for the cell sintered at 1400 ◦ C were also larger than those at 1350 ◦ C. The increase in the anode activation resistances was thought to cause by the decrease in triple phase boundaries due to the growth of nickel particles sintered at the higher temperature. The peaks of R1 and R4 did not change for the cell sintered at 1350 ◦ C and 1400 ◦ C, which supposed that R1 and R4 are the cathode activation and diffusion resistances, respectively. Fig. 6(b) and (d) shows the effects of the operating temperature on the DRT spectra in 10%H2 . The peaks from R1 to R4 decreased significantly with a rise of the operating temperatures. The peaks of R2 and R3 were
overlapped at low temperatures. The other researches also reported that the characteristic frequencies were varied against the operating temperatures [16,24]. For the anode-supported microtubular SOFCs, the characteristic frequencies of the anode activation components more changed than that of the other components. The peaks of R5 changed slightly against the operating temperature for the both cells sintered at 1350 ◦ C and 1400 ◦ C. Each resistance from R1 to R5 was derived by the CNLS method using an equivalent circuit of Fig. 5(a) fixing the characteristic frequencies derived by the DRT analyses. For examples, the resistances of R5 were 0.19, 0.16 and 0.13 cm2 at 697 ◦ C in 10%, 20% and 40%H2 , respectively. This resistance ratio of R5 was almost the same as the area
Fig. 6. DRT spectra for anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C.
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between the anodes sintered at 1350 ◦ C and 1400 ◦ C. Actually, the size of macropores was almost the same for the both anodes as shown in Fig. 2. However, the size and distribution of micropores between the nickel and YSZ in the walls of the macropores are different for each anode, which strongly affected the anode diffusion resistance. The anode microstructure should be modified to more decrease the anode diffusion resistance for the microtubular SOFCs made by the use of acrylic resin as pore former.
4. Conclusions
Fig. 7. Arrhenius plot of each resistance in 10%H2 for the cells sintered at (a) 1350 ◦ C and (b) 1400 ◦ C. Table 1 Activation energy derived from the Arrhenius plot and characteristic frequency for each resistance. Activation energy (kJ/mol) ◦
R0 R1 R2 R3 R4 R5
Characteristic frequency ◦
Sintered at 1350 C
Sintered at 1400 C
6.2 × 101 1.1 × 102 2.2 × 102 1.3 × 102 1.4 × 102 3.3 × 101
5.7 × 101 1.0 × 102 1.7 × 102 1.6 × 102 1.1 × 102 2.4 × 101
– 15–30 kHz 2.5–5.0 kHz 370–850 Hz 55–120 Hz 4.8–7.8 Hz
ratio of the DRT peaks. Fig. 7 shows the Arrhenius plot of each resistance in 10%H2 for the cells sintered at (a) 1350 ◦ C and (b) 1400 ◦ C. All resistances decreased with a rise in the operating temperature for the both cells sintered at 1350 ◦ C and 1400 ◦ C. However, the temperature dependence of the anode diffusion resistance (R5) was smaller than that of the other resistances. Table 1 shows the activation energy (Eact ) derived from the following equation and the characteristic frequency for each resistance: log = log 0 +
Eact RT
(1)
where , R and T are resistance, gas constant and temperature, respectively. The activation energy of ohmic resistance (R0) was ca. 60 kJ/mol, which is smaller than the values of ionic conductivity for YSZ in the previous reports [32,33]. Badwal reported that the activation energy of resistivity for YSZ at 850–1000 ◦ C was lower than that at 400–500 ◦ C [32]. The energy of volume resistance was also reported to be smaller than that of the grain boundary resistance [33]. The thickness of the YSZ electrolyte made by dip-coating was only 5 m in this study. The small activation energy of ohmic resistance was realized by reducing the grain boundaries in the YSZ thin-film electrolyte. On the other hand, the activation energies for the electrode activation and the cathode diffusion resistances were 100–200 kJ/mol, which were almost the same as the result in previous reports [14,21]. However, the energy of the anode diffusion resistance was only 24–33 kJ/mol. The percentage of the anode diffusion resistance became large in the all polarization resistances at the high temperature of 697 ◦ C. Furthermore, the anode diffusion resistance (R5) for the cell sintered at 1400 ◦ C was almost twice as large as that at 1350 ◦ C. The difference in porosity was only 3%
In this study, the effects of anode microstructure on electrochemical characteristics were investigated by the DRT analysis of AC impedance spectra for the anode-supported microtubular SOFCs. The anode porosity for the cells sintered at 1400 ◦ C was smaller than that at 1350 ◦ C. The difference in the porosity affected the maximum fuel utilization and anode resistances. The maximum fuel utilizations were 93% and 72% at an operating temperature of 697 ◦ C in 10%H2 for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. Two arcs seemed to exist in the Nyquist plots of AC impedance spectra for the anode-supported microtubular SOFCs. Both of the arcs at high and low frequencies changed against the hydrogen concentration. The DRT analyses can separate five polarization resistances for the impedance spectra after removing an inductance component. The DRT peaks at 0.37–5.0 kHz and 4.8–7.8 Hz changed against the anode sintering temperature and/or the hydrogen concentration for the anode-supported microtubular SOFCs. The former peaks corresponded to anode activation resistances, and the latter ones were the anode diffusion resistances. The anode diffusion resistances decreased only a small amount with respect to rising the operating temperatures, because the wall of the pores were well-packed with nickel and YSZ particles. The anode microstructure should be modified to decrease the anode diffusion resistance for the microtubular SOFCs made by the use of acrylic resin as pore former.
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AC impedance characteristics for anode-supported microtubular solid oxide fuel cells Hirofumi Sumi a,∗ , Toshiaki Yamaguchi a , Koichi Hamamoto a , Toshio Suzuki a , Yoshinobu Fujishiro a , Toshiaki Matsui b , Koichi Eguchi b a b
Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
a r t i c l e
i n f o
Article history: Received 30 November 2011 Received in revised form 6 February 2012 Accepted 7 February 2012 Available online 15 February 2012 Keywords: Solid oxide fuel cell (SOFC) Anode microstructure AC impedance Distribution of relaxation times
a b s t r a c t The DRT method was applied to analyze the AC impedance spectra for anode-supported microtubular solid oxide fuel cells (SOFCs). The anode microstructure has a more severe effect on the electrochemical properties of the anode-supported microtubular SOFCs because of the small inside diameter and thickness of the anode. For example, the maximum current density and fuel utilization for the cells sintered at 1400 ◦ C were lower than that at 1350 ◦ C due to the low porosity of the anode. However, it is difficult to separate the anode and cathode impedances by the general method of complex non-linear least squares (CNLS) with equivalent circuit models, because several components overlap for the impedance spectra measured between the anode and cathode without a reference electrode. The DRT method can easily separate the impedance spectra into five polarization components for the anode-supported microtubular SOFCs. The activation energy of the anode diffusion resistance was only 24–33 kJ/mol. The walls of the pores in the anode were relatively well-packed with the nickel and YSZ particles for the cells made by the use of acrylic resin as a pore former. The anode diffusion resistance decreased by only a small amount with a rise of operating temperature for the anode-supported microtubular SOFCs. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are expected to serve as power generation systems with high energy conversion efficiency. Recently, many researchers focused on lowering the operating temperature of SOFCs from 800–1000 ◦ C to 600–700 ◦ C due to improvement of durability, start-up time and so on. Two approaches have been particularly popular for use in lowering the operating temperature of SOFCs. One approach is to use electrolyte materials of high ionic conductivity at lower temperatures such as La0.9 Sr0.1 Ga0.8 Mg0.2 O3 (LSGM) [1] and Gd-doped CeO2 (GDC) [2], and the other is to reduce the thickness of the electrolyte made of conventional materials such as Y2 O3 -stabilized zirconia (YSZ) [3–6]. Tubular cells generally reduce the problems associated with brittleness and sealing as compared to planar cells. However, tubular cells have the disadvantage of a small volumetric power density. A cell-stack with large volume and heat capacity cannot start up rapidly. The power density, the start-up time and so on are proportional to the inverse of cell diameter. Kendall et al. [7,8] and Yashiro et al. [9] demonstrated microtubular SOFCs with a diameter on the
∗ Corresponding author. Tel.: +81 52 736 7592; fax: +81 52 736 7405. E-mail address: [email protected] (H. Sumi). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.02.021
millimeter scale. A rapid start-up (i.e. within several seconds) was achieved through the butane fuel for the microtubular SOFCs. Previously, the authors have developed anode-supported microtubular SOFCs with high power density via the “Advanced Ceramic Reactor Project” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The highest power density per cathode area reached 1 W/cm2 at a low temperature of 550 ◦ C for microtubular cells with a diameter of 0.8 mm [10]. Furthermore, the power of 2 W can be obtained at 490 ◦ C for a 3 × 3-cell module with a volume of 1 cm3 [11]. The microtubular SOFC modules are expected to be applied to micro combined heat and power (CHP) systems for residences, and power sources for portable devices and transportation. Anode microstructure affects electrochemical properties more severely for the anode-supported microtubular SOFCs because of the small inside diameter and thickness of the anode. Suzuki et al. reported that cell performance was improved by decreasing the sintering temperature of the anode [12]. The polarization resistance was confirmed to decrease by the increase in porosity of the anode. However, it is difficult to analyze AC impedance spectra by complex non-linear least squares (CNLS) with equivalent circuit models, as several electrode processes generally overlap within the actual impedance spectra. Some mathematical techniques have been developed to identify each electrode process. Schichlein et al.
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suggested the distribution of relaxation times (DRT) method for the analysis of impedance spectra for SOFCs [13,14]. A polarization component is generally analyzed by an equivalent circuit with a parallel resistance–conductance (RC) element. An ideal RC element has a single relaxation time defined by the appropriate R and C values ( = RC). However, the actual polarization resistance for SOFCs cannot be fitted perfectly by one RC element, because relaxation times are generally distributed for actual electrode processes. Therefore, the polarization resistances are generally fitted by the equivalent circuit with RQ element (constant phase elements in parallel with a resistor) for the CNLS method [15]. The DRT method can directly detect the number of electrode processes by mathematical techniques without assuming the equivalent circuits of the polarization components [13,14]. Previously, the effects of the gas concentration in the electrode and the operating temperature were considered by the DRT analysis for planar SOFCs [16–18]. The correlations between the durability and the mass transfer were also investigated [19,20]. Furthermore, the DRT method was applied for several types of SOFCs such as half-cells with patterned electrodes [21,22], metal-supported planar [23], and segmentedin-series tubular cells [24,25]. In this study, the effects of anode microstructure on electrochemical characteristics were investigated by the DRT analysis of AC impedance spectra for the anode-supported microtubular SOFCs. The problems associated with the microtubular cells using a pore former of acrylic resin were investigated from the analysis of the activation energy of each electrode resistance.
2. Experimental Anode microtubes were made from NiO (Sumitomo metal mining), 8 mol% Y2 O3 stabilized ZrO2 (8YSZ; Tosoh), pore former (acrylic resin; Sekisui Plastic) and binder (Cellulose; Yuken Kogyo) powders. The weight ratio of NiO to 8YSZ was 6:4, and the particle size of pore former was ca. 5 m. These powders were mixed by kneading machine with adding the proper amount of water for 2 h. The hardness of the mixture clay was ca. 12 points which measured by a type A durometer (ISO 7619). The anode microtubes were extruded using a piston cylinder with a metal hold of 2.4 mm (outside diameter) and 2.0 mm (inside diameter). After extrusion, the tubes were dried overnight in air at room temperature. A slurry was prepared by mixing 8YSZ, a binder (polyvinyl butyral; Sekisui Chemical) a dispersant (tallow propylene diamine, Kao) and a plasticizer (dioctyl adipate; Wako Pure Chemical Industries) into ethanol and toluene solvents for 48 h. The 8YSZ electrolyte was formed by dip-coating at a pulling rate of 1 mm/s. The 8YSZ electrolyte thin-film and NiO–8YSZ anode microtube were cosintered for 1 h in air. The sintering temperatures were 1350 ◦ C and 1400 ◦ C which modified the anode microstructure. The interlayer of Ce0.9 Gd0.1 O1.95 (Shin-etsu Chemical) and the cathode of La0.6 Sr0.4 Co0.2 Fe0.8 O3 (Dowa Electronics Materials) were coated by a similar manner. The pulling rates of the dip-coater were 1 and 2 mm/s for the interlayer and the cathode, respectively. The interlayer and cathode thin-film were sintered sequentially for 1 h in air at 1200 ◦ C and 1050 ◦ C, respectively. Fig. 1 shows a photograph of the anode-supported microtubular SOFCs used in this study. The outside diameter of microtube was 1.8 mm, and the area of cathode was 0.6 cm2 after sintering. The thicknesses of the anode, electrolyte, interlayer and cathode were ca. 200, 5, 1 and 20 m, respectively. Silver wires were used as current collector. The characteristics of power generation and AC impedance were evaluated with a potentiostat/galvanostat (Solartron Analytical 1287) and an impedance analyzer (Solartron Analytical 1255B). A mixture of x%H2 − 3%H2 O − (100 − x) %N2 (x = 10, 20, 40) was supplied as fuel at a flow rate of 50 mL/min to an anode side, and air
Fig. 1. Photograph of anode-supported microtubular SOFCs with an outside diameter of 1.8 mm and a cathode area of 0.6 cm2 after sintering. Current collectors are silver wires.
was supply as oxidant at 50 mL/min to a cathode side. Operating temperatures were 595 ◦ C, 646 ◦ C and 697 ◦ C. Current–potential (j − E) characteristics were measured from open circuit potential (OCP) to 0.4 V at a sweep rate of 5 mV/s. The AC impedance was measured under OCP in the frequency range from 1 MHz to 0.1 Hz with 20 steps per logarithmic decade. The DRT analysis has been described in detail in Refs. [12,13]. In this study, polarization processes are considered as an equivalent circuit composed of a large number of RC elements in series. However, Liu et al. reported that it was impossible to obtain good results for the DRT analysis for impedance spectra with an inductance at high frequency [24]. The inductance components were removed by the same method as described in Ref. [24] before the DRT analysis for the impedance spectra in this study. The software program of FTIKREG [26] was used to solve an ill-posed inverse problem in the DRT analysis by Tikhonov regularization. 3. Results and discussion The sintering temperatures affect the anode microstructure for the anode-supported microtubular SOFCs. Fig. 2 shows the scanning electron images of (i) oxidized and (ii) reduced anodes sintered at (a) 1350 ◦ C and (b) 1400 ◦ C. The macropores with a diameter of several microns were observed for the all samples, which were made by burning the pore formers of acrylic resin during sintering. Although the walls of the pores were packed with the nickel oxide and YSZ particles in the oxidized state, the small gap between these particles was produced by the shrinkage of metallic nickel after reducing. The formation of similar macropores was reported by the other researches using the pore formers of acrylic resin [27,28]. On the other hand, the micropores were introduced on nanoscale by using the other pore formers such as carbon black [29], rice starch [30], and flour [31]. The size and distribution of the pores can be controlled by a use of each pore former in the anode substrate. The open porosity evaluated by Archimedes’s method were 34% and 31% for the oxidized anode microtubes sintered at 1350 ◦ C and 1400 ◦ C, respectively. The porosity increases by ca. 10 points after reducing the both anode microtubes. In this study, the effects of the difference in the anode porosity on electrochemical properties were investigated precisely for the anode-supported microtubular SOFCs. Fig. 3 shows the j − E characteristics for the anode-supported microtubular SOFCs sintered at 1350 ◦ C and 1400 ◦ C with a fuel flow rate of 50 mL/min. The operating temperature was 595 ◦ C or 697 ◦ C, and the fuel composition was x = 10 or 40 in
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Fig. 2. Scanning electron images of (i) oxidized and (ii) reduced anodes sintered at (a) 1350 ◦ C and (b) 1400 ◦ C.
x%H2 − 3%H2 O − (100 − x) %N2 mixtures. The performances for the cells sintered at 1350 ◦ C and 1400 ◦ C were almost the same at an operating temperature of 697 ◦ C in 40%H2 . The current density reached to 1.6 A/cm2 at a cell potential of 0.5 V. However, the fuel utilization was only 35% at 1.6 A/cm2 in 40%H2 . The OCP for the cell sintered at 1350 ◦ C was slightly lower than that at 1400 ◦ C. This means that the small gas leakage occurred due to the low relative density of the electrolyte thin-film sintered at the lower temperature. The area specific resistances were derived from the slopes of j − E curves near OCP to be 0.41 and 0.51 cm2 in 40%H2 at
697 ◦ C for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. This difference of resistances was caused by the change of the anode overpotential and the gas partial pressure near OCP. On the other hand, the cell potential dropped at 0.9 A/cm2 in 10%H2 at 697 ◦ C for the cell sintered at 1400 ◦ C, while the current density reached 1.1 A/cm2 for the cell sintered at 1350 ◦ C. The maximum fuel utilizations were 93% and 72% in 10%H2 at 697 ◦ C for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. This means that the low porosity in the anode mainly affects the decrease in performance at high fuel utilizations. The difference in the performances for the
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Fig. 3. Current–potential characteristics for anode-supported microtubular SOFCs sintered at 1350 ◦ C and 1400 ◦ C with a fuel flow rate of 50 mL/min.
cells sintered at 1350 ◦ C and 1400 ◦ C was small in 10%H2 at 595 ◦ C because of the low fuel utilization at 0.6 A/cm2 and 50 mL/min. The difference in the performances, of course, seems to appear at lower hydrogen concentrations or flow rates even at a temperature of 595 ◦ C. Fig. 4 shows the AC impedance spectra for the anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C measured between the anode and the cathode under OCP state. Two arcs seem to exist in the Nyquist plots of Fig. 4(a)–(d). Total resistances agreed with the area specific resistances derived from the slope of j − E curves near OCPs in Fig. 3. The diameters of the arcs produced at both high and low frequencies increased with a decrease in hydrogen concentration in Fig. 4(a) and (c), which suggests that the activation and diffusion resistances in the anode were large at a low hydrogen concentration. In comparison with the sintering temperatures, the ohmic resistances were almost the
same for the cells sintered at 1350 ◦ C and 1400 ◦ C. However, the arcs at both high and low frequencies for the cells sintered at 1400 ◦ C were larger than those at 1350 ◦ C. This result suggests that the low porosity in the anode causes the increase in not only diffusion but also activation resistances. Fig. 4(b) and (d) shows the impedance spectra measured in 10%H2 at 595 ◦ C, 646 ◦ C and 697 ◦ C. The arcs produced at high frequencies especially increased with a decrease in the operating temperature for the cells sintered at 1350 ◦ C and 1400 ◦ C. The AC impedances in Fig. 4 were measured between the anode and the cathode. Therefore, the spectra contained at least four components of activation and diffusion resistances in the anode and the cathode. However, it is too difficult to exactly separate the over-lapped impedance spectra into more than four components using the CNLS method with equivalent circuit models. Precise impedance analyses were tried by the DRT method in reference to Refs. [12,13] using FTIKREG program [26]. Liu et al. suggests that the inductance component should be removed before the DRT analysis to obtain good results [24]. In this study, the equivalent circuit as shown in Fig. 5(a) was assumed to estimate the inductance. Fig. 5(b) shows the impedance spectra at 697 ◦ C in 10%H2 before and after the inductance removal for the cell sintered at 1350 ◦ C. In the inset of Fig. 5, a large inductance component was observed at high frequency by noise from lead wires for the raw data. The inductance is assumed to be 0.323 H cm2 from the equivalent circuit of Fig. 5(a). After removing the inductance component, the arc was clarified at high frequency from 1 MHz to 1 kHz. The impedance spectra after removing the inductance component were used for the DRT analyses after this. Fig. 6 shows the DRT spectra for the anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C. Five DRT peaks were detected for each spectrum. The effects of the hydrogen concentration were evaluated at 697 ◦ C in Fig. 6(a) and (c). The peak of R5 increased with a decrease in the hydrogen concentration, which corresponds to an anode diffusion resistance. The area ratios of R5 in 10%:20%:40%H2 were 1:0.93:0.88 and 1:0.85:0.73 at 697 ◦ C for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. The peaks of R5 for the cell sintered at 1400 ◦ C were larger than that at 1350 ◦ C. This means that the low porosity causes the increase in the anode diffusion resistance. The peaks of R2 and R3 slightly increased with a decrease in the hydrogen concentration. Liu et al. reported that the change in the activation
Fig. 4. AC impedance spectra for anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C measured between anode and cathode under OCP state.
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Fig. 5. (a) Equivalent circuit to estimate the inductance, and (b) impedance spectra measured at 697 ◦ C in 10%H2 before and after inductance removal for the cell sintered at 1350 ◦ C.
resistance was smaller than that in the diffusion resistance against oxygen partial pressure [24]. The peaks of R2 and R3 for the cell sintered at 1400 ◦ C were also larger than those at 1350 ◦ C. The increase in the anode activation resistances was thought to cause by the decrease in triple phase boundaries due to the growth of nickel particles sintered at the higher temperature. The peaks of R1 and R4 did not change for the cell sintered at 1350 ◦ C and 1400 ◦ C, which supposed that R1 and R4 are the cathode activation and diffusion resistances, respectively. Fig. 6(b) and (d) shows the effects of the operating temperature on the DRT spectra in 10%H2 . The peaks from R1 to R4 decreased significantly with a rise of the operating temperatures. The peaks of R2 and R3 were
overlapped at low temperatures. The other researches also reported that the characteristic frequencies were varied against the operating temperatures [16,24]. For the anode-supported microtubular SOFCs, the characteristic frequencies of the anode activation components more changed than that of the other components. The peaks of R5 changed slightly against the operating temperature for the both cells sintered at 1350 ◦ C and 1400 ◦ C. Each resistance from R1 to R5 was derived by the CNLS method using an equivalent circuit of Fig. 5(a) fixing the characteristic frequencies derived by the DRT analyses. For examples, the resistances of R5 were 0.19, 0.16 and 0.13 cm2 at 697 ◦ C in 10%, 20% and 40%H2 , respectively. This resistance ratio of R5 was almost the same as the area
Fig. 6. DRT spectra for anode-supported microtubular SOFCs sintered at (a), (b) 1350 ◦ C and (c), (d) 1400 ◦ C.
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between the anodes sintered at 1350 ◦ C and 1400 ◦ C. Actually, the size of macropores was almost the same for the both anodes as shown in Fig. 2. However, the size and distribution of micropores between the nickel and YSZ in the walls of the macropores are different for each anode, which strongly affected the anode diffusion resistance. The anode microstructure should be modified to more decrease the anode diffusion resistance for the microtubular SOFCs made by the use of acrylic resin as pore former.
4. Conclusions
Fig. 7. Arrhenius plot of each resistance in 10%H2 for the cells sintered at (a) 1350 ◦ C and (b) 1400 ◦ C. Table 1 Activation energy derived from the Arrhenius plot and characteristic frequency for each resistance. Activation energy (kJ/mol) ◦
R0 R1 R2 R3 R4 R5
Characteristic frequency ◦
Sintered at 1350 C
Sintered at 1400 C
6.2 × 101 1.1 × 102 2.2 × 102 1.3 × 102 1.4 × 102 3.3 × 101
5.7 × 101 1.0 × 102 1.7 × 102 1.6 × 102 1.1 × 102 2.4 × 101
– 15–30 kHz 2.5–5.0 kHz 370–850 Hz 55–120 Hz 4.8–7.8 Hz
ratio of the DRT peaks. Fig. 7 shows the Arrhenius plot of each resistance in 10%H2 for the cells sintered at (a) 1350 ◦ C and (b) 1400 ◦ C. All resistances decreased with a rise in the operating temperature for the both cells sintered at 1350 ◦ C and 1400 ◦ C. However, the temperature dependence of the anode diffusion resistance (R5) was smaller than that of the other resistances. Table 1 shows the activation energy (Eact ) derived from the following equation and the characteristic frequency for each resistance: log = log 0 +
Eact RT
(1)
where , R and T are resistance, gas constant and temperature, respectively. The activation energy of ohmic resistance (R0) was ca. 60 kJ/mol, which is smaller than the values of ionic conductivity for YSZ in the previous reports [32,33]. Badwal reported that the activation energy of resistivity for YSZ at 850–1000 ◦ C was lower than that at 400–500 ◦ C [32]. The energy of volume resistance was also reported to be smaller than that of the grain boundary resistance [33]. The thickness of the YSZ electrolyte made by dip-coating was only 5 m in this study. The small activation energy of ohmic resistance was realized by reducing the grain boundaries in the YSZ thin-film electrolyte. On the other hand, the activation energies for the electrode activation and the cathode diffusion resistances were 100–200 kJ/mol, which were almost the same as the result in previous reports [14,21]. However, the energy of the anode diffusion resistance was only 24–33 kJ/mol. The percentage of the anode diffusion resistance became large in the all polarization resistances at the high temperature of 697 ◦ C. Furthermore, the anode diffusion resistance (R5) for the cell sintered at 1400 ◦ C was almost twice as large as that at 1350 ◦ C. The difference in porosity was only 3%
In this study, the effects of anode microstructure on electrochemical characteristics were investigated by the DRT analysis of AC impedance spectra for the anode-supported microtubular SOFCs. The anode porosity for the cells sintered at 1400 ◦ C was smaller than that at 1350 ◦ C. The difference in the porosity affected the maximum fuel utilization and anode resistances. The maximum fuel utilizations were 93% and 72% at an operating temperature of 697 ◦ C in 10%H2 for the cells sintered at 1350 ◦ C and 1400 ◦ C, respectively. Two arcs seemed to exist in the Nyquist plots of AC impedance spectra for the anode-supported microtubular SOFCs. Both of the arcs at high and low frequencies changed against the hydrogen concentration. The DRT analyses can separate five polarization resistances for the impedance spectra after removing an inductance component. The DRT peaks at 0.37–5.0 kHz and 4.8–7.8 Hz changed against the anode sintering temperature and/or the hydrogen concentration for the anode-supported microtubular SOFCs. The former peaks corresponded to anode activation resistances, and the latter ones were the anode diffusion resistances. The anode diffusion resistances decreased only a small amount with respect to rising the operating temperatures, because the wall of the pores were well-packed with nickel and YSZ particles. The anode microstructure should be modified to decrease the anode diffusion resistance for the microtubular SOFCs made by the use of acrylic resin as pore former.
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