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Fabrication of ceramic composite anode at low temperature for high performance protonic ceramic fuel cells
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Fabrication of ceramic composite anode at low temperature for high performance protonic ceramic fuel cells Erdienzy Anggia, Eun-Kyung Shin, Jun-Tae Nam, Jong-Sung Park∗ Department of Material Science and Engineering, Myongji University, Yongin, Gyeonggi-do, 449-728, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Protonic ceramics Composite electrodes Hydrocarbon fuels PCFC Fuel cells LSCM
A ceramic composite anode composed of (La0.8Sr0.2) (Cr0.5Mn0.5)O3-δ (LSCM), Ba(Zr0.75Y0.15)O3-δ (BZY), and catalysts was applied in hydrocarbon fuels for protonic ceramics fuel cells. LSCM and BZY served as an electronic conductor and a protonic ceramic, respectively. The single phase of LSCM, a promising electronically conductive ceramic, could be obtained by performing calcination when exposed to air and hydrogen reduction at 973 K, which was much lower than the conventional calcination temperature (approximately 1273 K). The LSCM-BZY composite anode was fabricated successfully at such a low temperature using the infiltration method. By testing the composite electrodes at different temperatures, namely 973, 873, 1223, and 1373 K, the effect of the calcination temperature of LSCM on anode performance in hydrogen and methane fuels was successfully investigated. The composite anode with LSCM calcined at 973 and 1073 K showed three-fold improved performance in H2 fuel and two-fold in CH4 fuel than that of the composite anode with LSCM calcined at 1373 K.
1. Introduction
As aforementioned, almost all of the PCFCs were based on the Nicermet supported cells because nickel typically has good catalytic activity, high electronic conductivity and enhances the sintering of the electrolyte in its capacity as sintering agent [6]. However, there are still some limitations of Ni-cermet, such as poor redox stability, reduction of proton conductivity due to NiO doping into the protonic ceramics, and high steam vapor pressure (generally > 60 vol% in hydrocarbon fuel), that can suppress coking on the nickel surface [4,6,7]. One of the options for overcoming such limitations of Ni-cermet is the use of ceramic composite anodes, which have electrically conductive ceramics instead of Ni, such as (La0.8Sr0.2) (Cr0.5Mn0.5)O3, (La,Sr)TiO3 and (La,Sr)VO3 [8–11]. These ceramic composite anodes have been actively investigated in oxygen ion conducting fuel cells; however, there have been limited reports regarding their application to PCFCs. The (La,Sr)VO3 has been previously applied to a PCFC and galvanic hydrogen pump, but this composite anode could also be operable when the oxygen partial pressure is lower than 10−17 [2,12]. In the case of LSCM with proton ceramics, there are some reports in which LSCM was used for the air side electrode rather than the fuel side electrode whilst LSCM has been widely investigated for the fuel side electrodes in the case of oxygen ion conducting fuel cells [8,13]. It has been suggested that LSCM has a marginal electrical conductivity and good redox stability. But the calcination temperature for synthesizing the pure perovskite structure of LSCM has been reported to
Protonic ceramics are the ion conducting oxide in which the proton can exist inside the crystal lattice in a wet atmosphere and diffuse through the lattice [1]. The ionic conductivity of protonic ceramics is higher than that of conventional oxygen ion conductors, such as yttriastabilized zirconia or Gd-doped ceria, in intermediate temperature ranges from 673 to 873 K [1]. Moreover, a composite electrode with protonic ceramics shows a better performance than that with oxygen ion conductors at intermediate temperatures below 873 K [2]. Because of these characteristics of protonic ceramics, protonic ceramics fuel cells (PCFCs) are more suitable for developing the intermediate temperature fuel cells to increase the durability and decrease the production cost. In the past decade, the performance of PCFCs has been considerably improved by developing cathode materials for PCFCs and optimizing the fabrication method to decrease the thickness of electrolytes in the Ni-cermet support [3–5]. One research group reported that a power density of 1.3 Wcm−2 at 873 K was obtained from a 5 × 5 cm2 PCFC, supported by Ni-cermet, after optimizing the fabrication process [3]. Good performance and high durability at 873 K in various types of hydrocarbon fuels such as methane, ethanol, and butane were reported; the mechanism of coking cleaning using the hydroxyl ion on the protonic ceramics–nickel composite anode was proposed [4].
∗
Corresponding author. E-mail address: [email protected] (J.-S. Park).
https://doi.org/10.1016/j.ceramint.2019.08.256 Received 29 July 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Erdienzy Anggia, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.256
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catalyst, such as 2.0 wt% CeO2 and 0.5 wt% Pd, was added into LSCM by infiltrating the precursor solution of Ce(NO3)3 (Alfa Aesar, 99.5%) and tetraamminepalladium(II) nitrate (Alfa Aesar, 99.9%). The concentrations of precursor solutions for CeO2 and Pd were 1.0 M and 0.5 M respectively. After infiltration of each precursor solution for CeO2 or Pd, the cell was treated at 723 K for 30 min in air. In the case of full-cells, the component with a higher calcination temperature was infiltrated at first. The LSCM for LSCM_1273 and LSCM_1373 was infiltrated before the calcination of LSF at 1123 K while the LSCM for LSCM_973 and LSCM_1073 was infiltrated after the calcination of LSF at 1123 K. After calcination of LSF and LSCM, the catalysts were infiltrated into LSCM and treated at 723 K for 30 min in air. To assist with the measurement of the half-cells and full-cells, silver paste was painted on an electrode and two silver wires were placed on each electrode and attached to a current and voltage probe. The four half-cells with LSCM, calcined at four different temperatures, were placed in the humidified hydrogen atmosphere (3%H2O) and CH4 atmosphere (20%H2O) at 973 or 873 K and the impedance spectra was measured by a potentiostat (Zive, Wonatech corp.) at a frequency range of 1 MHz to 0.01 Hz with a 20 mV AC perturbation at zero bias voltage. The measured impedance spectra was also analyzed based on equivalent circuits model using the software, MEISP (Korea Kumho Petrochemical Co., LTD.) [18]. The full-cells were mounted on the alumina tube using a glass sealant (Mico power, Korea) and H2 (3%H2O) and CH4 (20%H2O) were supplied as fuels to the LSCM-BZY anode side while the air (3%H2O) was supplied to the LSF-BCZY cathode side. I-V curves and impedance spectra in fuel cell mode were also measured at 873 and 973 K using the potentiostat. LSCM powders were also fabricated by drying the precursor solution on a hot plate. The dried LSCM powder was calcined at four different temperatures (973, 1073, 1223, and 1373 K). The four different types of LSCM powders were treated under each fuel cell measurement condition, for 5 h, sequentially: H2 (3%H2O) at 973 K and CH4 (20%H2O) at 873 K. The crystal structures and surface area of the LSCM powders, treated under different conditions, were analyzed using X-ray diffraction (XRD) and the BET method. The microstructure of the LSCM-BZY composite anode, calcined at different temperatures, was observed by the scanning electron microscope (SEM).
be higher than 1273 K, and the LSCM composite electrode was typically fabricated at around 1473 K [8,13–15]. Such a high fabrication temperature is not desirable for reducing the chemical reaction with protonic ceramics and increasing the surface area of composite electrode. Therefore, in this work, the possibility of reducing the calcination temperature of LSCM is investigated and supported by evidence that the ceramic composite anode with LSCM and protonic ceramics can be fabricated at 973 K, using the infiltration method, and that improvement of their anode performance in hydrogen fuel and methane fuel by decreasing the fabrication temperature of composite anode is possible. 2. Experimental procedure Protonic ceramics, such as Ba(Zr0.85Y0.15)O3-δ (BZY), Ba (Ce0.55Zr0.30Y0.15)O3-δ (BCZY), Ba(Ce0.85Y0.15)O3-δ (BCY), Ba (Ce0.69Zr0.10Y0.10Yb0.10Cu0.01)O3-δ (BC10ZYYb-Cu), and Ba (Ce0.30Zr0.50Y0.10Yb0.10Cu0.01)O3-δ (BC50ZYYb-Cu), were fabricated by performing a solid oxide reaction. BCZY and BZY were used to form the cathode and anode composite electrodes, respectively; BCY, BC10ZYYbCu, and BC50ZYYb-Cu were used to make a three-layered electrolyte. A total of 1 mol% of CuO in BC50ZYYb-Cu and BC10ZYYb-Cu was considered as the sintering agent for sintering at 1773 K [16]. The appropriate amount of raw powders, such as BaCO3 (99.8%, Alfa Aesar, USA), CeO2 (99.9%, Alfa Aesar, USA), ZrO2 (99.9%, Alfa Aesar, USA), Y2O3 (99.9%, Alfa Aesar, USA), Yb2O3 (99.9%, Alfa Aesar, USA), and CuO (99.9%, Alfa Aesar, USA), were mixed with zirconia ball and ethanol in the jar for 24 h (h). The mixed powders were calcined at 1573 K for 2 h followed by milling for 40 h with the zirconia ball and ethanol. The tape for the porous scaffold of the electrode and the dense electrolyte was fabricated using the tape casting process. The protonic ceramic powders, binder, plasticizer, and dispersant were mixed in the organic solvents, ethanol and xylene, to make the slurry. In the case of the tape for the porous scaffold, the crystalline graphite (Alfa Aesar, USA) was added to the slurry. The detailed process for making the slurry can be found in external reports [17]. Different types of tapes were laminated and sintered at 1773 K for 2 h to make the anode for half-cells and full-cells. In the case of the halfcells, the anode used was the LSCM-BZY composite anode and the electrolyte was BZ50ZYYb-Cu; the configuration of the laminated tapes for the half-cell was the porous BZY/dense electrolyte (BZ50ZYYb-Cu)/ porous BZY. By infiltrating the LSCM solution into the porous BZY scaffold, the composite anode of LSCM-BZY can be fabricated. In the case of the full-cells, BCZY as the cathode scaffold and a three-layered electrolyte were used in order to achieve a small ohmic resistance and the required chemical stability against CO2 and H2O. Hence, the configuration of laminated tapes for the full-cells, including the anode, electrolyte, and the cathode, was porous BZY/dense electrolyte (BC50ZYYb-Cu/BCY/BC10ZYYb-Cu)/porous BCZY. The precursor solution of LSCM and (La0.8Sr0.2)FeO3-δ (LSF) for the anode and cathode was fabricated by dissolving the precursors and citric acid into distilled water. The concentration of the precursor solution was 0.8 M and the raw materials were La(NO3)3·6H2O (99.9%, Alfa Aesar), Sr(NO3)2 (99%, Alfa Aesar), Cr(NO3)3·9H2O (99.3%, Alfa Aesar), Mn(NO3)2·xH2O (99.9%, Alfa Aesar), Fe(NO3)3·9H2O (99.6%, Alfa Aesar) and citric acid (99.5%, Alfa Aesar). The LSCM and the LSF were infiltrated into the anode and cathode sides, respectively, until the target vol% of LSCM (30 vol%) and LSF (20 vol%) were obtained. Between each infiltration, the cells were fired at 973 K to remove the organic species. After reaching the target vol% of LSF or LSCM, the cells were treated at each calcination temperature. The calcination temperature of LSF was 1123 K and LSCM was calcined at four different temperatures, 973, 1073, 1223, and 1373 K, to check the effects of calcination temperature. Henceforward, the LSCM calcined at four different temperatures will be referred to as LSCM_973, LSCM_1073, LSCM_1223, and LSCM_1373. Following the calcination of LSCM, the
3. Results and discussion Fig. 1 shows the XRD pattern of LSCM when calcined in air at 973 K.
Fig. 1. XRD pattern of LSCM when calcined in air at 973 K. 2
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Fig. 2. XRD patterns of LSCM (a) after calcination in air at 973 K, 1073 K, 1223 K, and 1373 K, and (b) after reduction in H2 at 973 K.
The XRD pattern agrees with that of a rhombohedral structured perovskite, La0.9Sr0.1MnO3 (JSPDS 47-0444), which clearly shows split peaks of 2θ at around 40° and 60°. However, there was also a small amount of secondary phases such as SrCO3 (JSPDS 05-0418) and SrCrO4 (JSPDS 35-0743). Fig. 2 shows the XRD patterns of LSCM (a) after calcination in air at i) 973 K, ii) 1073 K, iii) 1223 K, and iv) 1373 K, and Fig. 2 (b) after reduction in H2 at 973 K. As shown in Fig. 2 (a), secondary phases such as SrCO3 and SrCrO4 were still identifiable after calcination at 1073 K and 1223 K, but these had completely disappeared after calcination at 1373 K. This is consistent with the reported results that LSCM single phase can be synthesized at a higher temperature than 1273 K [14,15]. The crystal structure of LSCM, calcined at each temperature, was also the rhombohedral structure. After reducing these powders in H2 at 973 K, the change in crystal structure of LSCM was found and is shown in Fig. 2 (b). Each split peak, at around 40° and 60° of 2θ, merged into single peak, implying that the crystal structure of LSCM had changed from a rhombohedral structure to a cubic structure (JCPDS 51-1516). Such crystal structure change of LSCM in a reduction atmosphere is consistent with previously reported results. It has been reported that the crystal structure of LSCM can be changed with oxygen partial pressure. The crystal structure of LSCM calcined at 1673 K in air was transformed from a hexagonal structure to an orthorhombic or cubic structure after treatment in 5% H2 at 1073 K [19,20]. More interestingly, the secondary phases in LSCM_973 and LSCM_1073 had disappeared. The disappearance of secondary phases after reduction has not been reported on yet. Considering that the composite anode is used in the hydrogen atmosphere during the fuel cell operation in H2 fuel, the LSCM, calcined at a low temperature such
Fig. 3. XRD patterns of i) LSCM_973, ii) LSCM_1073, iii) LSCM_1223 and iv) LSCM_1373, reduced in H2 at 973 K, followed by treatment in (a) CH4 (20% H2O) or (b) Ar balanced 4% H2 with 3%H2O at 873 K for 5 h.
as 973 and 1073 K, can also be applied to the anode in fuel cell mode. In order to confirm the feasibility of the four LSCMs in CH4 fuel at intermediate temperatures, such as 873 K, four LSCM powders, reduced in H2 at 973 K, were treated in CH4 (20%H2O) at 873 K, which was equal to the operating atmosphere for the methane fuel. Generally, the steam is added into the methane for the steam reformation of methane. Fig. 3 (a) shows the XRD patterns of LSCM powders, reduced in H2 at 973 K, followed by treatment in CH4 (20%H2O) at 873 K. The pure single phase of LSCM was maintained without impurities in the atmosphere containing CH4 at 873 K, whilst there was some variance in the crystal structure of LSCM dependent on the calcination temperatures. In the case of LSCM_1223 and LSCM_1373, (111) peak and (112) peak in the XRD patterns of LSCM cubic phase, found after reduction in hydrogen at 973 K, were split into multiple peaks after treatment in CH4 at 873 K due to the crystal structure change from a cubic structure to an initial rhombohedral structure. However, the LSCM calcined at 973 and 1073 K still had the cubic crystal structure when the atmosphere was changed from H2 (3%H2O) at 973 K to CH4 (20%H2O) at 873 K. Such change in crystal structure of CH4 (20%H2O) can be correlated with the effect of oxygen partial pressure in the crystal structure of LSCM, rather than the carbon species. Considering that the oxygen partial pressure of CH4 (20%H2O) is higher than H2 (3%H2O), the results in Fig. 3 (a) mean that the cubic phase of LSCM under a reducing
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H2 at 973 K followed by treatment in Ar balanced 4% H2 with 3%H2O for 5 h at 873 K. All XRD patterns of the four LSCM powders in Fig. 3 (b) were nearly equal to that of the LSCM powders after treatment in CH4 (20%H2O). LSCM_973 and LSCM_1073 showed a cubic phase while LSCM_1223 and LSCM_1373 showed a rhombohedral structure. The results confirm that the change in LSCM crystal structure in a methane fuel condition is due to the increase of oxygen partial pressure, rather than the existence of carbon species, and that the crystal structure of LSCM in a methane fuel condition can differ dependent on the initial calcination temperature in air. Fig. 4 shows the microstructure of the LSCM-BZY composite anodes after calcination at four different temperatures in air, of 973, 1073, 1223, and 1373 K. The BZY porous scaffold was completely covered by infiltrated LSCM and the surface morphology of LSCM was different depending on the calcination temperature of the LSCM composite anodes. The particle size of LSCM was decreased by reducing the calcination temperature from 1373 K to 973 K. The particle size of LSCM on the LSCM-BZY composite anode calcined at 1373 K and was about 200 nm, while that of LSCM calcined at 973 K and was about 30 nm. Such a tendency can also be confirmed in the specific surface areas of LSCM powders, calcined at four different temperatures and measured using the BET method. The specific surface areas of the LSCM powders calcined at 973, 1073, 1223, and 1373 K were 17.0, 11.7, 6.2 and 4.8 m2/g, respectively. The surface area of the LSCM powder was increased by decreasing the calcination temperature, due to the smaller particle size of LSCM. Fig. 5 shows the microstructure of the LSCM-BZY composite anode with catalysts CeO2 and Pd after measurement in H2 and CH4 fuel. Each calcination temperature of LSCM-BZY composite electrode is (a) 973 K, (b) 1073 K, (c) 1223 K, and (d) 1373 K. The size of catalysts for LSCMBZY composite anode, calcined at 1373 K, was about 30 nm, but the catalysts for LSCM-BZY composite anode, calcined at 973 and 1073 K, were much smaller. Considering the catalysts were infiltrated into the composite anode after the final calcination of LSCM at each temperature, this can be explained by the fact that a large surface area facilitated the dispersion of the catalysts and suppressed the coarsening during the measurement in H2 at 973 K and CH4 fuel at 873 K. Fig. 6(a) shows the impedance spectra, measured in H2(3%H2O) at 973 K, of half cells with LSCM, calcined at 973, 1073, 1223, and 1373 K. In the impedance spectra, the point of intersection with the xaxis at high and low frequencies represents the ohmic resistance and total resistance, respectively. The difference between the total and ohmic resistance is an electrode polarization resistance, termed nonohmic resistance. In order to compare the electrode polarization more distinctly, the impedance spectra were shifted to origin along the x-axis and the x and y scale were divided by 2.0 in consideration of the symmetric configuration of the electrode of the half-cells. The electrode polarization resistances in H2, at 973 K of the LSCM composite electrode calcined at 973, 1073, 1223, and 1373 K were 0.046, 0.052, 0.061 and 0.126 Ω cm2, respectively. There was no significant difference in the polarization resistance between LSCM composite electrode calcined at 973 and 1073 K, but, above the 1073 K, the anode polarization resistance augmented after increasing the calcination temperature. In particular, an increase in polarization resistance resulted from the low frequency arc, at around 10 Hz. In order to analyze the impedance spectra in detail, the impedance spectra were simulated by composing equivalent circuits of (RHQH) (RLQL), as shown in the inlet of Fig. 6 (b). RH and RL represent electrode polarization resistances at high and low frequencies, while QH and QL represent the constant phase element related to RH and RL [21]. Fig. 6 (b) shows the obtained RH and RL as a function of calcination temperature. Above 1073 K, the increase of RL is more dominant than the increase of RH. This means the improvement of anode performance by decreasing the calcination temperature is due to the enhancement of catalytic activity in a low frequency region. Generally, the impedance
Fig. 4. SEM images of LSCM-BZY composite anodes after calcination at four different temperatures in air, of (a) 973 K, (b) 1073 K, (c) 1223 K, and (d) 1373 K.
condition can be maintained at a higher oxygen partial pressure when the LSCM is calcined at a lower temperature, such as 973 and 1073 K. In order to confirm the fact, the reduced LSCM powders were treated in Ar balanced 4% H2 with 3% H2O for 5 h at 873 K and oxygen partial pressure of about 10−22, this oxygen partial pressure is higher than that of CH4 (20%H2O). Fig. 3 (b) shows the XRD patterns of LSCM_973, LSCM_1073, LSCM_1223 and LSCM_1373 after reduction in 4
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Fig. 6. (a) Impedance spectra, measured in H2(3%H2O) at 973 K, of half cells with LSCM, calcined at i) 973, ii) 1073, iii) 1223 and iv) 1373 K, (b) non-ohmic area specific resistance (ASR) from the impedance arc at high and low frequency.
Fig. 5. Microstructures of LSCM-BZY composite anode with catalysts CeO2 and Pd after measurement in H2 and CH4 fuel; each calcination temperature of LSCM-BZY composite electrode is (a) 973 K, (b) 1073 K, (c) 1223 K and (d) 1373 K.
arc at low frequency is related to the electrochemical reaction for dissociative adsorption or gas diffusion, while the impedance arc at high frequency is related to the charge transfer reaction in the interface between the electrode and the electrolyte [22]. Therefore, the reduction of RL when decreasing the calcination temperature can result from the improvement of catalyst dispersion and the increase of a three-phase boundary due to the increase in surface area of the LSCM composite anode, as shown in Figs. 4 and 5. Fig. 7 shows the impedance spectra of half-cells with LSCM calcined
Fig. 7. Impedance spectra of half-cells with LSCM calcined at i) 973 K, ii) 1073 K, iii) 1223 K, and iv) 1373 K and measured at 873 K in (a) H2 (3%H2O) and (b) CH4 (20%H2O).
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Fig. 8. Microstructure of three-layered electrolyte of the electrolyte-supported PCFC.
at i) 973 K, ii) 1073 K, iii) 1223 K, and iv) 1373 K and measured at 873 K in (a) H2 (3%H2O) and (b) CH4 (20%H2O). In both the H2 and CH4 atmospheres, the half-cell with LSCM, calcined at 1373 K, showed the largest non-ohmic resistance in comparison to the other half-cells. The non-ohmic resistances of the half-cell with LSCM calcined at 973 and 1073 K was similar to the other and smaller than that of the halfcell with LSCM calcined at 1223 and 1373 K. According to the nonohmic resistance of half-cells, by decreasing the calcination temperature of LSCM, from 1373 to 1073 K or 973 K, a three-fold improvement of the anode performance is observed in H2 fuel and almost two-fold in CH4 fuel. Such improvements were obtained because of the reduction in the impedance arc at a low frequency range, which was ascribed to the fine dispersion of catalysts on the large surface area as mentioned previously. The full-cells with LSCM-BZY composite anode calcined at 1073 K were fabricated using the three-layered electrolyte, BC50ZYYb-Cu/ BCY/BC10ZYYb-Cu, in order to acquire a good chemical stability against CO2 and a high ionic conductivity of the electrolyte. Fig. 8 shows the microstructure of the three-layered electrolyte in the electrolyte-supported PCFC. The total thickness of the electrolyte was about 50 μm; the middle layer had a thickness of 30 μm and consisted of BCY, which has high ionic conductivity but poor chemical stability. Considering that methane fuel requires a higher chemical stability, a thin layer of BC50ZYYb-Cu and BC10ZYYb-Cu was placed near the anode and cathode, respectively, because the chemical stability improves when Zr4+ is substituted for Ce4+ [1]. Fig. 9 shows (a) I-V polarization curves and (b) impedance spectra in H2 and CH4 fuels measured at 873 K. The open circuit voltages (OCVs) in H2 and CH4 fuels were 1.10 V and 0.97 V. The OCV in CH4 fuel was lower than that in H2 fuel. The peak power densities in H2 fuel and CH4 fuel were 331 and 156 mW/cm2, respectively. The decrease of peak power density in CH4 fuel was mainly due to the increase in polarization resistance, as opposed to the ohmic resistance, because the non-ohmic resistance increased from 0.38 to 0.81 Ωcm2 when H2 fuel was changed to CH4 fuel, as shown in Fig. 9 (b). Interestingly, the ohmic resistance slightly decreased, from 0.50 Ωcm2 to 0.46 Ωcm2, on changing the fuel from H2 to CH4. This can be explained by the fact that the p-type electrical conductivity of LSCM grows when increasing the oxygen partial pressure and the oxygen partial pressure in CH4 fuel is higher than that in H2 fuel due to the high steam vapor pressure as well as the increase of proton conductivity of electrolyte with increasing the steam vapor pressure [19]. Such characteristics of LSCM can be considered advantageous in comparison to the Ni in Ni-cermet, which can be easily oxidized at a high steam vapor pressure and has a poor redox stability [7].
Fig. 9. (a) I-V polarization curves and (b) impedance spectra in H2 fuel and CH4 fuels measured at 873 K.
The ohmic resistance from the 20 μm thick electrolyte of BC10ZYYb on Ni-Cermet was reported to be about 0.28 Ωcm2 in CH4 fuel, corresponding to ionic conductivity of 0.007 Scm−1 and in accordance with the following equation in which σ and L are the conductivity and thickness of electrolyte, respectively [4]:
Area specific resistance (Ωcm2) =
L(cm) σ (Scm−1)
The ohmic resistance of 0.46 Ωcm2 in CH4 fuel from the 50 μm thick electrolyte of the three-layered electrolyte can be said to be significant considering its larger thickness, because the ohmic resistance of 0.46 Ωcm2 from 50 μm thickness corresponds to the ionic conductivity of 0.011 Scm−1 and is approximately equal to the ionic conductivity of BCY at 873 K [1]. This highlights the potential of the electrolyte supported cells, using the multi-layered electrolyte, although the absolute value of the ohmic resistance should be further reduced in order to optimally improve the peak power density. 4. Conclusion The effects of calcination temperatures of LSCM on the crystal structure, microstructure, and electrode performance have been successfully investigated. The LSCM calcined at a temperature of below 1273 K with secondary phases, but these secondary phases disappeared following reduction in H2 at 973 K. By decreasing the calcination temperature from 1373 to 1073 K or 973 K, the anode performances in H2 fuel and CH4 fuel were much improved. This can be attributed to the smaller particle size of LSCM and a finer dispersion of catalysts on the surface of LSCM. Full-cells were fabricated with the LSCM composite electrode calcined at 1073 K and the multi-layered electrolyte, which can impart high ionic conductivity and good chemical stability against CO2. The LSCM-BZY composite anode which was calcined at 1073 K functioned correctly in CH4 fuel as well as in H2 fuel. Notably, the ohmic resistance of the full-cell in CH4 fuel with a higher oxygen partial pressure was lower than that in H2 fuel, due to the p-type conductivity 6
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of LSCM.
[10] P.I. Cowin, C.T.G. Petit, R. Lan, J.T.S. Irvine, S. Tao, Recent progress in the development of anode materials for solid oxide fuel cells, Advanced Energy Materials 1 (2011) 314–332. [11] S.-E. Yoon, J.-Y. Ahn, B.-K. Kim, J.-S. Park, Improvements in co-electrolysis performance and long-term stability of solid oxide electrolysis cells based on ceramic composite cathodes, Int. J. Hydrogen Energy 40 (2015) 13558–13565. [12] J. Choi, M. Shin, B. Kim, J.-S. Park, High-performance ceramic composite electrodes for electrochemical hydrogen pump using protonic ceramics, Int. J. Hydrogen Energy 42 (2017) 13092–13098. [13] G. Wu, K. Xie, Y. Wu, W. Yao, J. Zhou, Electrochemical conversion of H2O/CO2 to fuel in a proton-conducting solid oxide electrolyser, J. Power Sources 232 (2013) 187–192. [14] D.S. Jung, H.C. Jang, J.H. Kim, H.Y. Koo, Y.C. Kang, Y.H. Cho, J.-H. Lee, Characteristics of nano-sized La0.75Sr0.25Cr0.5Mn0.5O3 powders prepared by spray pyrolysis, J. Ceram. Process. Res. 11 (2010) 322–327. [15] M.A. Raza, I.Z. Rahman, S. Beloshapkin, Synthesis of nanoparticles of La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) perovskite by solution combustion method for solid oxide fuel cell application, J. Alloy. Comp. 485 (2009) 593–597. [16] J.-S. Park, J.-H. Lee, H.-W. Lee, B.-K. Kim, Low temperature sintering of BaZrO3based proton conductors for intermediate temperature solid oxide fuel cells, Solid State Ion. 181 (2010) 163–167. [17] R. Kngas, J.S. Kim, J.M. Vohs, R.J. Gorte, Restructuring porous YSZ by treatment in hydrofluoric acid for use in SOFC cathodes, J. Am. Ceram. Soc. 94 (2011) 2220–2224. [18] D. Harrington, Multiple electrochemical impedance spectra parameterization (MEISP+), J. Am. Chem. Soc. 124 (2002) 1554–1555. [19] S. Tao, J.T.S. Irvine, Synthesis and characterization of (La0.75Sr0.25)Cr0.5Mn0.5O3−δ, a redox-stable, efficient perovskite anode for SOFCs, J. Electrochem. Soc. 151 (2004) A252–A259. [20] S. Zha, P. Tsang, Z. Cheng, M. Liu, Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1−xMnxO3 under anodic conditions, J. Solid State Chem. 178 (2005) 1844–1850. [21] Q.-A. Huang, R. Hui, B. Wang, J. Zhang, A review of AC impedance modeling and validation in SOFC diagnosis, Electrochim. Acta 52 (2007) 8144–8164. [22] S.B. Adler, Limitations of charge-transfer models for mixed-conducting oxygen electrodes, Solid State Ion. 135 (2000) 603–612.
Acknowledgements This research was supported by a grant from Korea Institute of Energy Technology Evaluation and Planning (KETEP) (20173010032290-G032190911). References [1] K.D. Kreuer, Proton-conducting oxides, Annu. Rev. Mater. Res. 33 (2003) 333–359. [2] S.-H. Song, S.-E. Yoon, J. Choi, B.-K. Kim, J.-S. Park, A high-performance ceramic composite anode for protonic ceramic fuel cells based on lanthanum strontium vanadate, Int. J. Hydrogen Energy 39 (2014) 16534–16540. [3] H. An, H.-W. Lee, B.-K. Kim, J.-W. Son, K.J. Yoon, H. Kim, D. Shin, H.-I. Ji, J.-H. Lee, A 5 × 5 cm2 density of 1.3 W cm–2 protonic ceramic fuel cell with a power at 600 °C, Nature Energy 3 (2018) 870–875. [4] C. Duan, R.J. Kee, H. Zhu, C. Karakaya, Y. Chen, S. Ricote, A. Jarry, E.J. Crumlin, D. Hook, R. Braun, N.P. Sullivan, R. O'Hayre, Highly durable, coking and sulfur tolerant, fuel-flexible protonic ceramic fuel cells, Nature 557 (2018) 217–222. [5] Y.-E. Park, H.-I. Ji, B.-K. Kim, J.-H. Lee, H.-W. Lee, J.-S. Park, Pore structure improvement in cermet for anode-supported protonic ceramic fuel cells, Ceram. Int. 39 (2013) 2581–2587. [6] H. An, D. Shin, H.-I. Ji, Effect of nickel addition on sintering behavior and electrical conductivity of BaCe0.35Zr0.5Y0.15O3-δ, J. Korean Chem. Soc. 56 (2019) 91–97. [7] N. Nasani, Z.-J. Wang, M.G. Willinger, A.A. Yaremchenko, D.P. Fagg, In-situ redox cycling behaviour of Ni–BaZr0.85Y0.15O3−δ cermet anodes for Protonic Ceramic Fuel Cells, Int. J. Hydrogen Energy 39 (2014) 19780–19788. [8] G. Kim, S. Lee, J.Y. Shin, G. Corre, J.T.S. Irvine, J.M. Vohs, R.J. Gorte, Investigation of the structural and catalytic requirements for high-performance SOFC anodes formed by infiltration of LSCM, Electrochem. Solid State Lett. 12 (2009) B48–B52. [9] J.-S. Park, I.D. Hasson, M.D. Gross, C. Chen, J.M. Vohs, R.J. Gorte, A high-performance solid oxide fuel cell anode based on lanthanum strontium vanadate, J. Power Sources 196 (2011) 7488–7494.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication of ceramic composite anode at low temperature for high performance protonic ceramic fuel cells Erdienzy Anggia, Eun-Kyung Shin, Jun-Tae Nam, Jong-Sung Park∗ Department of Material Science and Engineering, Myongji University, Yongin, Gyeonggi-do, 449-728, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Protonic ceramics Composite electrodes Hydrocarbon fuels PCFC Fuel cells LSCM
A ceramic composite anode composed of (La0.8Sr0.2) (Cr0.5Mn0.5)O3-δ (LSCM), Ba(Zr0.75Y0.15)O3-δ (BZY), and catalysts was applied in hydrocarbon fuels for protonic ceramics fuel cells. LSCM and BZY served as an electronic conductor and a protonic ceramic, respectively. The single phase of LSCM, a promising electronically conductive ceramic, could be obtained by performing calcination when exposed to air and hydrogen reduction at 973 K, which was much lower than the conventional calcination temperature (approximately 1273 K). The LSCM-BZY composite anode was fabricated successfully at such a low temperature using the infiltration method. By testing the composite electrodes at different temperatures, namely 973, 873, 1223, and 1373 K, the effect of the calcination temperature of LSCM on anode performance in hydrogen and methane fuels was successfully investigated. The composite anode with LSCM calcined at 973 and 1073 K showed three-fold improved performance in H2 fuel and two-fold in CH4 fuel than that of the composite anode with LSCM calcined at 1373 K.
1. Introduction
As aforementioned, almost all of the PCFCs were based on the Nicermet supported cells because nickel typically has good catalytic activity, high electronic conductivity and enhances the sintering of the electrolyte in its capacity as sintering agent [6]. However, there are still some limitations of Ni-cermet, such as poor redox stability, reduction of proton conductivity due to NiO doping into the protonic ceramics, and high steam vapor pressure (generally > 60 vol% in hydrocarbon fuel), that can suppress coking on the nickel surface [4,6,7]. One of the options for overcoming such limitations of Ni-cermet is the use of ceramic composite anodes, which have electrically conductive ceramics instead of Ni, such as (La0.8Sr0.2) (Cr0.5Mn0.5)O3, (La,Sr)TiO3 and (La,Sr)VO3 [8–11]. These ceramic composite anodes have been actively investigated in oxygen ion conducting fuel cells; however, there have been limited reports regarding their application to PCFCs. The (La,Sr)VO3 has been previously applied to a PCFC and galvanic hydrogen pump, but this composite anode could also be operable when the oxygen partial pressure is lower than 10−17 [2,12]. In the case of LSCM with proton ceramics, there are some reports in which LSCM was used for the air side electrode rather than the fuel side electrode whilst LSCM has been widely investigated for the fuel side electrodes in the case of oxygen ion conducting fuel cells [8,13]. It has been suggested that LSCM has a marginal electrical conductivity and good redox stability. But the calcination temperature for synthesizing the pure perovskite structure of LSCM has been reported to
Protonic ceramics are the ion conducting oxide in which the proton can exist inside the crystal lattice in a wet atmosphere and diffuse through the lattice [1]. The ionic conductivity of protonic ceramics is higher than that of conventional oxygen ion conductors, such as yttriastabilized zirconia or Gd-doped ceria, in intermediate temperature ranges from 673 to 873 K [1]. Moreover, a composite electrode with protonic ceramics shows a better performance than that with oxygen ion conductors at intermediate temperatures below 873 K [2]. Because of these characteristics of protonic ceramics, protonic ceramics fuel cells (PCFCs) are more suitable for developing the intermediate temperature fuel cells to increase the durability and decrease the production cost. In the past decade, the performance of PCFCs has been considerably improved by developing cathode materials for PCFCs and optimizing the fabrication method to decrease the thickness of electrolytes in the Ni-cermet support [3–5]. One research group reported that a power density of 1.3 Wcm−2 at 873 K was obtained from a 5 × 5 cm2 PCFC, supported by Ni-cermet, after optimizing the fabrication process [3]. Good performance and high durability at 873 K in various types of hydrocarbon fuels such as methane, ethanol, and butane were reported; the mechanism of coking cleaning using the hydroxyl ion on the protonic ceramics–nickel composite anode was proposed [4].
∗
Corresponding author. E-mail address: [email protected] (J.-S. Park).
https://doi.org/10.1016/j.ceramint.2019.08.256 Received 29 July 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Erdienzy Anggia, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.256
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catalyst, such as 2.0 wt% CeO2 and 0.5 wt% Pd, was added into LSCM by infiltrating the precursor solution of Ce(NO3)3 (Alfa Aesar, 99.5%) and tetraamminepalladium(II) nitrate (Alfa Aesar, 99.9%). The concentrations of precursor solutions for CeO2 and Pd were 1.0 M and 0.5 M respectively. After infiltration of each precursor solution for CeO2 or Pd, the cell was treated at 723 K for 30 min in air. In the case of full-cells, the component with a higher calcination temperature was infiltrated at first. The LSCM for LSCM_1273 and LSCM_1373 was infiltrated before the calcination of LSF at 1123 K while the LSCM for LSCM_973 and LSCM_1073 was infiltrated after the calcination of LSF at 1123 K. After calcination of LSF and LSCM, the catalysts were infiltrated into LSCM and treated at 723 K for 30 min in air. To assist with the measurement of the half-cells and full-cells, silver paste was painted on an electrode and two silver wires were placed on each electrode and attached to a current and voltage probe. The four half-cells with LSCM, calcined at four different temperatures, were placed in the humidified hydrogen atmosphere (3%H2O) and CH4 atmosphere (20%H2O) at 973 or 873 K and the impedance spectra was measured by a potentiostat (Zive, Wonatech corp.) at a frequency range of 1 MHz to 0.01 Hz with a 20 mV AC perturbation at zero bias voltage. The measured impedance spectra was also analyzed based on equivalent circuits model using the software, MEISP (Korea Kumho Petrochemical Co., LTD.) [18]. The full-cells were mounted on the alumina tube using a glass sealant (Mico power, Korea) and H2 (3%H2O) and CH4 (20%H2O) were supplied as fuels to the LSCM-BZY anode side while the air (3%H2O) was supplied to the LSF-BCZY cathode side. I-V curves and impedance spectra in fuel cell mode were also measured at 873 and 973 K using the potentiostat. LSCM powders were also fabricated by drying the precursor solution on a hot plate. The dried LSCM powder was calcined at four different temperatures (973, 1073, 1223, and 1373 K). The four different types of LSCM powders were treated under each fuel cell measurement condition, for 5 h, sequentially: H2 (3%H2O) at 973 K and CH4 (20%H2O) at 873 K. The crystal structures and surface area of the LSCM powders, treated under different conditions, were analyzed using X-ray diffraction (XRD) and the BET method. The microstructure of the LSCM-BZY composite anode, calcined at different temperatures, was observed by the scanning electron microscope (SEM).
be higher than 1273 K, and the LSCM composite electrode was typically fabricated at around 1473 K [8,13–15]. Such a high fabrication temperature is not desirable for reducing the chemical reaction with protonic ceramics and increasing the surface area of composite electrode. Therefore, in this work, the possibility of reducing the calcination temperature of LSCM is investigated and supported by evidence that the ceramic composite anode with LSCM and protonic ceramics can be fabricated at 973 K, using the infiltration method, and that improvement of their anode performance in hydrogen fuel and methane fuel by decreasing the fabrication temperature of composite anode is possible. 2. Experimental procedure Protonic ceramics, such as Ba(Zr0.85Y0.15)O3-δ (BZY), Ba (Ce0.55Zr0.30Y0.15)O3-δ (BCZY), Ba(Ce0.85Y0.15)O3-δ (BCY), Ba (Ce0.69Zr0.10Y0.10Yb0.10Cu0.01)O3-δ (BC10ZYYb-Cu), and Ba (Ce0.30Zr0.50Y0.10Yb0.10Cu0.01)O3-δ (BC50ZYYb-Cu), were fabricated by performing a solid oxide reaction. BCZY and BZY were used to form the cathode and anode composite electrodes, respectively; BCY, BC10ZYYbCu, and BC50ZYYb-Cu were used to make a three-layered electrolyte. A total of 1 mol% of CuO in BC50ZYYb-Cu and BC10ZYYb-Cu was considered as the sintering agent for sintering at 1773 K [16]. The appropriate amount of raw powders, such as BaCO3 (99.8%, Alfa Aesar, USA), CeO2 (99.9%, Alfa Aesar, USA), ZrO2 (99.9%, Alfa Aesar, USA), Y2O3 (99.9%, Alfa Aesar, USA), Yb2O3 (99.9%, Alfa Aesar, USA), and CuO (99.9%, Alfa Aesar, USA), were mixed with zirconia ball and ethanol in the jar for 24 h (h). The mixed powders were calcined at 1573 K for 2 h followed by milling for 40 h with the zirconia ball and ethanol. The tape for the porous scaffold of the electrode and the dense electrolyte was fabricated using the tape casting process. The protonic ceramic powders, binder, plasticizer, and dispersant were mixed in the organic solvents, ethanol and xylene, to make the slurry. In the case of the tape for the porous scaffold, the crystalline graphite (Alfa Aesar, USA) was added to the slurry. The detailed process for making the slurry can be found in external reports [17]. Different types of tapes were laminated and sintered at 1773 K for 2 h to make the anode for half-cells and full-cells. In the case of the halfcells, the anode used was the LSCM-BZY composite anode and the electrolyte was BZ50ZYYb-Cu; the configuration of the laminated tapes for the half-cell was the porous BZY/dense electrolyte (BZ50ZYYb-Cu)/ porous BZY. By infiltrating the LSCM solution into the porous BZY scaffold, the composite anode of LSCM-BZY can be fabricated. In the case of the full-cells, BCZY as the cathode scaffold and a three-layered electrolyte were used in order to achieve a small ohmic resistance and the required chemical stability against CO2 and H2O. Hence, the configuration of laminated tapes for the full-cells, including the anode, electrolyte, and the cathode, was porous BZY/dense electrolyte (BC50ZYYb-Cu/BCY/BC10ZYYb-Cu)/porous BCZY. The precursor solution of LSCM and (La0.8Sr0.2)FeO3-δ (LSF) for the anode and cathode was fabricated by dissolving the precursors and citric acid into distilled water. The concentration of the precursor solution was 0.8 M and the raw materials were La(NO3)3·6H2O (99.9%, Alfa Aesar), Sr(NO3)2 (99%, Alfa Aesar), Cr(NO3)3·9H2O (99.3%, Alfa Aesar), Mn(NO3)2·xH2O (99.9%, Alfa Aesar), Fe(NO3)3·9H2O (99.6%, Alfa Aesar) and citric acid (99.5%, Alfa Aesar). The LSCM and the LSF were infiltrated into the anode and cathode sides, respectively, until the target vol% of LSCM (30 vol%) and LSF (20 vol%) were obtained. Between each infiltration, the cells were fired at 973 K to remove the organic species. After reaching the target vol% of LSF or LSCM, the cells were treated at each calcination temperature. The calcination temperature of LSF was 1123 K and LSCM was calcined at four different temperatures, 973, 1073, 1223, and 1373 K, to check the effects of calcination temperature. Henceforward, the LSCM calcined at four different temperatures will be referred to as LSCM_973, LSCM_1073, LSCM_1223, and LSCM_1373. Following the calcination of LSCM, the
3. Results and discussion Fig. 1 shows the XRD pattern of LSCM when calcined in air at 973 K.
Fig. 1. XRD pattern of LSCM when calcined in air at 973 K. 2
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Fig. 2. XRD patterns of LSCM (a) after calcination in air at 973 K, 1073 K, 1223 K, and 1373 K, and (b) after reduction in H2 at 973 K.
The XRD pattern agrees with that of a rhombohedral structured perovskite, La0.9Sr0.1MnO3 (JSPDS 47-0444), which clearly shows split peaks of 2θ at around 40° and 60°. However, there was also a small amount of secondary phases such as SrCO3 (JSPDS 05-0418) and SrCrO4 (JSPDS 35-0743). Fig. 2 shows the XRD patterns of LSCM (a) after calcination in air at i) 973 K, ii) 1073 K, iii) 1223 K, and iv) 1373 K, and Fig. 2 (b) after reduction in H2 at 973 K. As shown in Fig. 2 (a), secondary phases such as SrCO3 and SrCrO4 were still identifiable after calcination at 1073 K and 1223 K, but these had completely disappeared after calcination at 1373 K. This is consistent with the reported results that LSCM single phase can be synthesized at a higher temperature than 1273 K [14,15]. The crystal structure of LSCM, calcined at each temperature, was also the rhombohedral structure. After reducing these powders in H2 at 973 K, the change in crystal structure of LSCM was found and is shown in Fig. 2 (b). Each split peak, at around 40° and 60° of 2θ, merged into single peak, implying that the crystal structure of LSCM had changed from a rhombohedral structure to a cubic structure (JCPDS 51-1516). Such crystal structure change of LSCM in a reduction atmosphere is consistent with previously reported results. It has been reported that the crystal structure of LSCM can be changed with oxygen partial pressure. The crystal structure of LSCM calcined at 1673 K in air was transformed from a hexagonal structure to an orthorhombic or cubic structure after treatment in 5% H2 at 1073 K [19,20]. More interestingly, the secondary phases in LSCM_973 and LSCM_1073 had disappeared. The disappearance of secondary phases after reduction has not been reported on yet. Considering that the composite anode is used in the hydrogen atmosphere during the fuel cell operation in H2 fuel, the LSCM, calcined at a low temperature such
Fig. 3. XRD patterns of i) LSCM_973, ii) LSCM_1073, iii) LSCM_1223 and iv) LSCM_1373, reduced in H2 at 973 K, followed by treatment in (a) CH4 (20% H2O) or (b) Ar balanced 4% H2 with 3%H2O at 873 K for 5 h.
as 973 and 1073 K, can also be applied to the anode in fuel cell mode. In order to confirm the feasibility of the four LSCMs in CH4 fuel at intermediate temperatures, such as 873 K, four LSCM powders, reduced in H2 at 973 K, were treated in CH4 (20%H2O) at 873 K, which was equal to the operating atmosphere for the methane fuel. Generally, the steam is added into the methane for the steam reformation of methane. Fig. 3 (a) shows the XRD patterns of LSCM powders, reduced in H2 at 973 K, followed by treatment in CH4 (20%H2O) at 873 K. The pure single phase of LSCM was maintained without impurities in the atmosphere containing CH4 at 873 K, whilst there was some variance in the crystal structure of LSCM dependent on the calcination temperatures. In the case of LSCM_1223 and LSCM_1373, (111) peak and (112) peak in the XRD patterns of LSCM cubic phase, found after reduction in hydrogen at 973 K, were split into multiple peaks after treatment in CH4 at 873 K due to the crystal structure change from a cubic structure to an initial rhombohedral structure. However, the LSCM calcined at 973 and 1073 K still had the cubic crystal structure when the atmosphere was changed from H2 (3%H2O) at 973 K to CH4 (20%H2O) at 873 K. Such change in crystal structure of CH4 (20%H2O) can be correlated with the effect of oxygen partial pressure in the crystal structure of LSCM, rather than the carbon species. Considering that the oxygen partial pressure of CH4 (20%H2O) is higher than H2 (3%H2O), the results in Fig. 3 (a) mean that the cubic phase of LSCM under a reducing
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H2 at 973 K followed by treatment in Ar balanced 4% H2 with 3%H2O for 5 h at 873 K. All XRD patterns of the four LSCM powders in Fig. 3 (b) were nearly equal to that of the LSCM powders after treatment in CH4 (20%H2O). LSCM_973 and LSCM_1073 showed a cubic phase while LSCM_1223 and LSCM_1373 showed a rhombohedral structure. The results confirm that the change in LSCM crystal structure in a methane fuel condition is due to the increase of oxygen partial pressure, rather than the existence of carbon species, and that the crystal structure of LSCM in a methane fuel condition can differ dependent on the initial calcination temperature in air. Fig. 4 shows the microstructure of the LSCM-BZY composite anodes after calcination at four different temperatures in air, of 973, 1073, 1223, and 1373 K. The BZY porous scaffold was completely covered by infiltrated LSCM and the surface morphology of LSCM was different depending on the calcination temperature of the LSCM composite anodes. The particle size of LSCM was decreased by reducing the calcination temperature from 1373 K to 973 K. The particle size of LSCM on the LSCM-BZY composite anode calcined at 1373 K and was about 200 nm, while that of LSCM calcined at 973 K and was about 30 nm. Such a tendency can also be confirmed in the specific surface areas of LSCM powders, calcined at four different temperatures and measured using the BET method. The specific surface areas of the LSCM powders calcined at 973, 1073, 1223, and 1373 K were 17.0, 11.7, 6.2 and 4.8 m2/g, respectively. The surface area of the LSCM powder was increased by decreasing the calcination temperature, due to the smaller particle size of LSCM. Fig. 5 shows the microstructure of the LSCM-BZY composite anode with catalysts CeO2 and Pd after measurement in H2 and CH4 fuel. Each calcination temperature of LSCM-BZY composite electrode is (a) 973 K, (b) 1073 K, (c) 1223 K, and (d) 1373 K. The size of catalysts for LSCMBZY composite anode, calcined at 1373 K, was about 30 nm, but the catalysts for LSCM-BZY composite anode, calcined at 973 and 1073 K, were much smaller. Considering the catalysts were infiltrated into the composite anode after the final calcination of LSCM at each temperature, this can be explained by the fact that a large surface area facilitated the dispersion of the catalysts and suppressed the coarsening during the measurement in H2 at 973 K and CH4 fuel at 873 K. Fig. 6(a) shows the impedance spectra, measured in H2(3%H2O) at 973 K, of half cells with LSCM, calcined at 973, 1073, 1223, and 1373 K. In the impedance spectra, the point of intersection with the xaxis at high and low frequencies represents the ohmic resistance and total resistance, respectively. The difference between the total and ohmic resistance is an electrode polarization resistance, termed nonohmic resistance. In order to compare the electrode polarization more distinctly, the impedance spectra were shifted to origin along the x-axis and the x and y scale were divided by 2.0 in consideration of the symmetric configuration of the electrode of the half-cells. The electrode polarization resistances in H2, at 973 K of the LSCM composite electrode calcined at 973, 1073, 1223, and 1373 K were 0.046, 0.052, 0.061 and 0.126 Ω cm2, respectively. There was no significant difference in the polarization resistance between LSCM composite electrode calcined at 973 and 1073 K, but, above the 1073 K, the anode polarization resistance augmented after increasing the calcination temperature. In particular, an increase in polarization resistance resulted from the low frequency arc, at around 10 Hz. In order to analyze the impedance spectra in detail, the impedance spectra were simulated by composing equivalent circuits of (RHQH) (RLQL), as shown in the inlet of Fig. 6 (b). RH and RL represent electrode polarization resistances at high and low frequencies, while QH and QL represent the constant phase element related to RH and RL [21]. Fig. 6 (b) shows the obtained RH and RL as a function of calcination temperature. Above 1073 K, the increase of RL is more dominant than the increase of RH. This means the improvement of anode performance by decreasing the calcination temperature is due to the enhancement of catalytic activity in a low frequency region. Generally, the impedance
Fig. 4. SEM images of LSCM-BZY composite anodes after calcination at four different temperatures in air, of (a) 973 K, (b) 1073 K, (c) 1223 K, and (d) 1373 K.
condition can be maintained at a higher oxygen partial pressure when the LSCM is calcined at a lower temperature, such as 973 and 1073 K. In order to confirm the fact, the reduced LSCM powders were treated in Ar balanced 4% H2 with 3% H2O for 5 h at 873 K and oxygen partial pressure of about 10−22, this oxygen partial pressure is higher than that of CH4 (20%H2O). Fig. 3 (b) shows the XRD patterns of LSCM_973, LSCM_1073, LSCM_1223 and LSCM_1373 after reduction in 4
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Fig. 6. (a) Impedance spectra, measured in H2(3%H2O) at 973 K, of half cells with LSCM, calcined at i) 973, ii) 1073, iii) 1223 and iv) 1373 K, (b) non-ohmic area specific resistance (ASR) from the impedance arc at high and low frequency.
Fig. 5. Microstructures of LSCM-BZY composite anode with catalysts CeO2 and Pd after measurement in H2 and CH4 fuel; each calcination temperature of LSCM-BZY composite electrode is (a) 973 K, (b) 1073 K, (c) 1223 K and (d) 1373 K.
arc at low frequency is related to the electrochemical reaction for dissociative adsorption or gas diffusion, while the impedance arc at high frequency is related to the charge transfer reaction in the interface between the electrode and the electrolyte [22]. Therefore, the reduction of RL when decreasing the calcination temperature can result from the improvement of catalyst dispersion and the increase of a three-phase boundary due to the increase in surface area of the LSCM composite anode, as shown in Figs. 4 and 5. Fig. 7 shows the impedance spectra of half-cells with LSCM calcined
Fig. 7. Impedance spectra of half-cells with LSCM calcined at i) 973 K, ii) 1073 K, iii) 1223 K, and iv) 1373 K and measured at 873 K in (a) H2 (3%H2O) and (b) CH4 (20%H2O).
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Fig. 8. Microstructure of three-layered electrolyte of the electrolyte-supported PCFC.
at i) 973 K, ii) 1073 K, iii) 1223 K, and iv) 1373 K and measured at 873 K in (a) H2 (3%H2O) and (b) CH4 (20%H2O). In both the H2 and CH4 atmospheres, the half-cell with LSCM, calcined at 1373 K, showed the largest non-ohmic resistance in comparison to the other half-cells. The non-ohmic resistances of the half-cell with LSCM calcined at 973 and 1073 K was similar to the other and smaller than that of the halfcell with LSCM calcined at 1223 and 1373 K. According to the nonohmic resistance of half-cells, by decreasing the calcination temperature of LSCM, from 1373 to 1073 K or 973 K, a three-fold improvement of the anode performance is observed in H2 fuel and almost two-fold in CH4 fuel. Such improvements were obtained because of the reduction in the impedance arc at a low frequency range, which was ascribed to the fine dispersion of catalysts on the large surface area as mentioned previously. The full-cells with LSCM-BZY composite anode calcined at 1073 K were fabricated using the three-layered electrolyte, BC50ZYYb-Cu/ BCY/BC10ZYYb-Cu, in order to acquire a good chemical stability against CO2 and a high ionic conductivity of the electrolyte. Fig. 8 shows the microstructure of the three-layered electrolyte in the electrolyte-supported PCFC. The total thickness of the electrolyte was about 50 μm; the middle layer had a thickness of 30 μm and consisted of BCY, which has high ionic conductivity but poor chemical stability. Considering that methane fuel requires a higher chemical stability, a thin layer of BC50ZYYb-Cu and BC10ZYYb-Cu was placed near the anode and cathode, respectively, because the chemical stability improves when Zr4+ is substituted for Ce4+ [1]. Fig. 9 shows (a) I-V polarization curves and (b) impedance spectra in H2 and CH4 fuels measured at 873 K. The open circuit voltages (OCVs) in H2 and CH4 fuels were 1.10 V and 0.97 V. The OCV in CH4 fuel was lower than that in H2 fuel. The peak power densities in H2 fuel and CH4 fuel were 331 and 156 mW/cm2, respectively. The decrease of peak power density in CH4 fuel was mainly due to the increase in polarization resistance, as opposed to the ohmic resistance, because the non-ohmic resistance increased from 0.38 to 0.81 Ωcm2 when H2 fuel was changed to CH4 fuel, as shown in Fig. 9 (b). Interestingly, the ohmic resistance slightly decreased, from 0.50 Ωcm2 to 0.46 Ωcm2, on changing the fuel from H2 to CH4. This can be explained by the fact that the p-type electrical conductivity of LSCM grows when increasing the oxygen partial pressure and the oxygen partial pressure in CH4 fuel is higher than that in H2 fuel due to the high steam vapor pressure as well as the increase of proton conductivity of electrolyte with increasing the steam vapor pressure [19]. Such characteristics of LSCM can be considered advantageous in comparison to the Ni in Ni-cermet, which can be easily oxidized at a high steam vapor pressure and has a poor redox stability [7].
Fig. 9. (a) I-V polarization curves and (b) impedance spectra in H2 fuel and CH4 fuels measured at 873 K.
The ohmic resistance from the 20 μm thick electrolyte of BC10ZYYb on Ni-Cermet was reported to be about 0.28 Ωcm2 in CH4 fuel, corresponding to ionic conductivity of 0.007 Scm−1 and in accordance with the following equation in which σ and L are the conductivity and thickness of electrolyte, respectively [4]:
Area specific resistance (Ωcm2) =
L(cm) σ (Scm−1)
The ohmic resistance of 0.46 Ωcm2 in CH4 fuel from the 50 μm thick electrolyte of the three-layered electrolyte can be said to be significant considering its larger thickness, because the ohmic resistance of 0.46 Ωcm2 from 50 μm thickness corresponds to the ionic conductivity of 0.011 Scm−1 and is approximately equal to the ionic conductivity of BCY at 873 K [1]. This highlights the potential of the electrolyte supported cells, using the multi-layered electrolyte, although the absolute value of the ohmic resistance should be further reduced in order to optimally improve the peak power density. 4. Conclusion The effects of calcination temperatures of LSCM on the crystal structure, microstructure, and electrode performance have been successfully investigated. The LSCM calcined at a temperature of below 1273 K with secondary phases, but these secondary phases disappeared following reduction in H2 at 973 K. By decreasing the calcination temperature from 1373 to 1073 K or 973 K, the anode performances in H2 fuel and CH4 fuel were much improved. This can be attributed to the smaller particle size of LSCM and a finer dispersion of catalysts on the surface of LSCM. Full-cells were fabricated with the LSCM composite electrode calcined at 1073 K and the multi-layered electrolyte, which can impart high ionic conductivity and good chemical stability against CO2. The LSCM-BZY composite anode which was calcined at 1073 K functioned correctly in CH4 fuel as well as in H2 fuel. Notably, the ohmic resistance of the full-cell in CH4 fuel with a higher oxygen partial pressure was lower than that in H2 fuel, due to the p-type conductivity 6
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of LSCM.
[10] P.I. Cowin, C.T.G. Petit, R. Lan, J.T.S. Irvine, S. Tao, Recent progress in the development of anode materials for solid oxide fuel cells, Advanced Energy Materials 1 (2011) 314–332. [11] S.-E. Yoon, J.-Y. Ahn, B.-K. Kim, J.-S. Park, Improvements in co-electrolysis performance and long-term stability of solid oxide electrolysis cells based on ceramic composite cathodes, Int. J. Hydrogen Energy 40 (2015) 13558–13565. [12] J. Choi, M. Shin, B. Kim, J.-S. Park, High-performance ceramic composite electrodes for electrochemical hydrogen pump using protonic ceramics, Int. J. Hydrogen Energy 42 (2017) 13092–13098. [13] G. Wu, K. Xie, Y. Wu, W. Yao, J. Zhou, Electrochemical conversion of H2O/CO2 to fuel in a proton-conducting solid oxide electrolyser, J. Power Sources 232 (2013) 187–192. [14] D.S. Jung, H.C. Jang, J.H. Kim, H.Y. Koo, Y.C. Kang, Y.H. Cho, J.-H. Lee, Characteristics of nano-sized La0.75Sr0.25Cr0.5Mn0.5O3 powders prepared by spray pyrolysis, J. Ceram. Process. Res. 11 (2010) 322–327. [15] M.A. Raza, I.Z. Rahman, S. Beloshapkin, Synthesis of nanoparticles of La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) perovskite by solution combustion method for solid oxide fuel cell application, J. Alloy. Comp. 485 (2009) 593–597. [16] J.-S. Park, J.-H. Lee, H.-W. Lee, B.-K. Kim, Low temperature sintering of BaZrO3based proton conductors for intermediate temperature solid oxide fuel cells, Solid State Ion. 181 (2010) 163–167. [17] R. Kngas, J.S. Kim, J.M. Vohs, R.J. Gorte, Restructuring porous YSZ by treatment in hydrofluoric acid for use in SOFC cathodes, J. Am. Ceram. Soc. 94 (2011) 2220–2224. [18] D. Harrington, Multiple electrochemical impedance spectra parameterization (MEISP+), J. Am. Chem. Soc. 124 (2002) 1554–1555. [19] S. Tao, J.T.S. Irvine, Synthesis and characterization of (La0.75Sr0.25)Cr0.5Mn0.5O3−δ, a redox-stable, efficient perovskite anode for SOFCs, J. Electrochem. Soc. 151 (2004) A252–A259. [20] S. Zha, P. Tsang, Z. Cheng, M. Liu, Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1−xMnxO3 under anodic conditions, J. Solid State Chem. 178 (2005) 1844–1850. [21] Q.-A. Huang, R. Hui, B. Wang, J. Zhang, A review of AC impedance modeling and validation in SOFC diagnosis, Electrochim. Acta 52 (2007) 8144–8164. [22] S.B. Adler, Limitations of charge-transfer models for mixed-conducting oxygen electrodes, Solid State Ion. 135 (2000) 603–612.
Acknowledgements This research was supported by a grant from Korea Institute of Energy Technology Evaluation and Planning (KETEP) (20173010032290-G032190911). References [1] K.D. Kreuer, Proton-conducting oxides, Annu. Rev. Mater. Res. 33 (2003) 333–359. [2] S.-H. Song, S.-E. Yoon, J. Choi, B.-K. Kim, J.-S. Park, A high-performance ceramic composite anode for protonic ceramic fuel cells based on lanthanum strontium vanadate, Int. J. Hydrogen Energy 39 (2014) 16534–16540. [3] H. An, H.-W. Lee, B.-K. Kim, J.-W. Son, K.J. Yoon, H. Kim, D. Shin, H.-I. Ji, J.-H. Lee, A 5 × 5 cm2 density of 1.3 W cm–2 protonic ceramic fuel cell with a power at 600 °C, Nature Energy 3 (2018) 870–875. [4] C. Duan, R.J. Kee, H. Zhu, C. Karakaya, Y. Chen, S. Ricote, A. Jarry, E.J. Crumlin, D. Hook, R. Braun, N.P. Sullivan, R. O'Hayre, Highly durable, coking and sulfur tolerant, fuel-flexible protonic ceramic fuel cells, Nature 557 (2018) 217–222. [5] Y.-E. Park, H.-I. Ji, B.-K. Kim, J.-H. Lee, H.-W. Lee, J.-S. Park, Pore structure improvement in cermet for anode-supported protonic ceramic fuel cells, Ceram. Int. 39 (2013) 2581–2587. [6] H. An, D. Shin, H.-I. Ji, Effect of nickel addition on sintering behavior and electrical conductivity of BaCe0.35Zr0.5Y0.15O3-δ, J. Korean Chem. Soc. 56 (2019) 91–97. [7] N. Nasani, Z.-J. Wang, M.G. Willinger, A.A. Yaremchenko, D.P. Fagg, In-situ redox cycling behaviour of Ni–BaZr0.85Y0.15O3−δ cermet anodes for Protonic Ceramic Fuel Cells, Int. J. Hydrogen Energy 39 (2014) 19780–19788. [8] G. Kim, S. Lee, J.Y. Shin, G. Corre, J.T.S. Irvine, J.M. Vohs, R.J. Gorte, Investigation of the structural and catalytic requirements for high-performance SOFC anodes formed by infiltration of LSCM, Electrochem. Solid State Lett. 12 (2009) B48–B52. [9] J.-S. Park, I.D. Hasson, M.D. Gross, C. Chen, J.M. Vohs, R.J. Gorte, A high-performance solid oxide fuel cell anode based on lanthanum strontium vanadate, J. Power Sources 196 (2011) 7488–7494.
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