A collaborative study of sintering and composite effects for a PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ IT-SOFC cathode

Abstract

Recently, a novel cathode material PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) has been proposed as a solution to overcome the drawbacks of a conventional cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Here we report systematic procedures to optimize the sintering temperature and the composite for PBSCF as an IT-SOFC cathode. For optimization of the heat treatment conditions for a PBSCF composite cathode, the effects of sintering temperature on the microstructure and electrical transport properties of the material are examined. We also suggest the optimization processes to effectively expand the electrochemical reaction zone based on a combination of a mixed ionic and electronic conductor (MIEC) electrode and an ionically conducting phase (PBSCF-Ce_0.9Gd_0.1O_1.95 (GDC)x, x = 0, 20, 40, 50, and 60 wt%). The optimal intersection point between these two processing systems is revealed to be 50 wt% of GDC containing a composite cathode sintered at 950 °C for 4 h. The area specific resistance (ASR) of PBSCF-GDC50 sintered at 950 °C for 4 h reaches a minimum value of 0.052 Ω cm² at 600 °C, which is consistent with the electrochemical performance results representing peak power density of ∼2.0 W cm⁻² at 600 °C.

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Introduction

Recent interest in a hydrogen-based energy economy has refocused attention on solid oxide fuel cells (SOFCs) as an alternative energy source to fossil fuels. SOFCs are electrochemical energy conversion devices that directly convert the chemical energy in fuel to electricity with high conversion efficiency and low pollution emission.^1–3 Notwithstanding these advantages, the high operating temperature of conventional SOFCs (∼1000 °C) leads to some problems, including material compatibility challenges and high costs. In this respect, recent research has been aimed at developing intermediate temperature SOFCs (IT-SOFCs) operating from 500 to 700 °C. However, there are major obstacles to the practical use of IT-SOFCs, including poor oxide-ion conductivity and inadequate catalytic activity of the conventional cathodes stemming from the reduced operating temperature.^4–6 In the field of IT-SOFC research and development, considerable efforts have therefore been invested in the following two areas crucial to the successful performance of IT-SOFCs: enhancing the ionic conductivity of electrolytes and reducing the polarization resistance of electrodes.⁷

These two approaches are frequently adopted to raise the ionic conductivity using alternative electrolytes, such as Ce_0.8Sm_0.2O_1.9 (SDC) and Ce_0.9Gd_0.1O_1.95 (GDC), as well as the catalytic activity of the cathode.^8,9 One strategy for raising the catalytic activity of the cathode is to utilize mixed ionic and electronic conductivity (MIEC) materials containing Mn, Fe, Co, and/or Ni with perovskite structures, such as Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), Pr_1−xSr_xCoO₃ (PSC), Sm_0.5Sr_0.5CoO₃ (SSC), and La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF).^10–13 These MIEC materials not only possess high oxygen permeability and electro-catalytic activity but also help extend the three-phase boundary (TPB) to the entire cathode–gas interface, which leads to dramatically improved cathode electrochemical performance.^14,15

Many groups have recently studied layered perovskite oxides having the general formula AA′B₂O_5+δ because they offer much higher chemical diffusion and surface exchange coefficients relative to those of ABO₃-type simple perovskite oxides.^16,17 This layered structure reduces the oxygen bonding strength in the [AO] layer and provides crystalline channels for ion motion, which enhance oxygen ion diffusion and surface oxygen exchange. LnBaCo₂O_5+δ (Ln = La, Pr, Nd, Sm, and Gd) (LnBCO) compounds are well known layered perovskite oxides with high mixed ionic and electronic conductivity and faster oxygen transport, even at temperatures below 500 °C, which afford high catalytic activity for the oxygen reduction reaction (ORR).¹⁸ The high concentration of mobile oxygen species may be responsible for the high diffusivity of oxide ions in the bulk and the enhanced surface activity toward the ORR.^16,19–21 Recently, Choi et al. proposed a novel cathode material, PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF).²² The partial co-substitution of Ba and Co by Sr and Fe, respectively, improves not only electrical conductivity and also chemical and thermal compatibility of the material.^22–28 Particularly, Fe substitution of Co enhances the oxygen ion diffusivity, ORR activity, and stability of the cathodes due to increased 3d metal–oxygen bonding energy and also decreases thermal expansion.^29–31 Accordingly, optimized co-substitution of PBSCF, rather than sole substitution, could lead to synergistic effects with respect to the electrochemical properties and long-term thermal stability of the cathode material.²²

With the aim of effectively expanding the electrochemical reaction zone, composite cathodes that include a mixture of a cathode and an ionic conducting material, such as PBSCO-GDC, SmBa_xSr_1−xCo₂O_5+δ (SBSCO)-GDC, NdBa_0.5Sr_0.5Co₂O_5+δ (NBSCO)-GDC, SSC-SDC, and La_0.5Sr_0.5MnO₃(LSM)-yttria stabilized zirconia (YSZ),^{23,24,28,32,33} have recently gained widespread interest. The combination of an ionically conducting phase and MIEC electrode may boost the electrochemical reaction by providing additional TPB sites for the ORR. Kim et al. have reported that a composite cathode consisting of NBSCO and GDC exhibits lower interfacial polarization resistances, as compared with those of pure NBSCO.²⁴ The authors provide the mechanisms of the ORR for an electronic conducting cathode, MIEC cathode, and MIEC-GDC composite cathode. The electrochemically active areas of composite cathodes can be divided into three pathways: the TPB point where the electrolyte, cathode, and gas are in contact, the 2PB point on the surface of the pure MIEC cathode, and the extended TPB point by the addition of GDC into the cathode.²⁴

Sintering is also an important processing parameter that influences microstructure evolution, grain growth, and densification of a material. Generally, sintering at high temperature increases the grain size of an electrode, which in turn decreases the surface area-gas solid interface (TPB), consequently resulting in a high polarization resistance.³⁴ The electrode particles, however, require adequate sintering temperature (typically at least 800 °C) to strongly adhere to the electrolyte surface.³⁵ Therefore, obtaining fine particles of the cathode and proper adhesion to the electrolyte are in a tradeoff relationship with respect to the sintering temperature.

In this work, the optimization processes for heat treatment of PBSCF-GDC50 and proportional addition of GDC to PBSCF (PBSCF-GDCx, x = 0, 20, 40, 50, and 60 wt%) are systematically investigated to evaluate their potential as cathode materials for IT-SOFCs based on structural characteristics and electrochemical properties of these materials.

Experimental

The PBSCF powders were synthesized by the Pechini process using Pr(NO₃)₃·6H₂O (Aldrich, 99.9%, metal basis), Ba(NO₃)₂ (Aldrich, 99+%),Sr(NO₃)₂ (Aldrich, 99+%), Co(NO₃)₂·6H₂O (Aldrich, 98+%), and Fe(NO₃)₃·6H₂O (Aldrich, 98%) with the addition of ethylene glycol and citric acid as heterogeneous agents in distilled water, followed by a self-combustion process to form submicron powder particles. These powders were calcined at 600 °C for 4 h and then ball-milled in acetone for 24 h. The calcined powders were then dry-pressed into pellets at 5 MPa and sintered at 1150 °C for 12 h in air. For measurement of the cell performances of PBSCF cathodes, slurries consisting of powders, GDC, and an organic binder (Heraeus V006) were used.

The phase identification of PBSCF-GDCx (x = 0, 20, 40, 50, and 60 wt%) was carried out by X-ray powder diffraction (XRD) (Rigaku-diffractometer, Cu Ka radiation) with a scanning rate of 0.5° min⁻¹ in the 2θ range of 20° to 60°. The microstructures of the interface between the GDC electrolyte and PBSCF-GDCx cathodes were investigated using a field emission scanning electron microscope (SEM) (Nova SEM).

Electrochemical impedance spectroscopy of PBSCF-GDCx was carried out using a symmetrical cell. The GDC electrolyte powders were pressed into pellets, and then sintered at 1350 °C for 4 h in air to obtain a dense electrolyte substrate. Slurries composed of PBSCF and GDC powders were screen-printed onto both sides of the GDC electrolytes to form symmetrical half-cells, followed by calcination at various temperatures (900–1050 °C). A silver paste was used as a current collector for the electrodes. Impedance spectra were recorded under OCV in a frequency range of 1 mHz to 500 kHz with AC perturbation of 14 mV from 500 °C to 650 °C.

Ni–GDC anode-supported cells were fabricated to measure the electrochemical performances of PBSCF-GDCx. The PBSCF-GDCx electrodes were blended with an organic binder (Heraeus V006) for slurries and a Ni–GDC cermet anode was prepared by a mixture of nickel oxide, GDC, and starch prepared via ball-milling in ethanol for 24 h. After drying, the NiO–GDC mixture was pressed into pellets (∼0.6 mm thick and 15 mm diameter). Thin GDC electrolyte membranes were prepared by a refined particle suspension coating technique. A GDC suspension was prepared by dispersing GDC powder (Aldrich) in ethanol with a small amount of binder (polyvinyl Butyral, B-98) and dispersant (Triethanolamine, Alfa Aesar) at a ratio of 1 : 10. The GDC suspension was applied to a NiO–GDC anode support by drop-coating, followed by drying in air and subsequent co-sintering at 1400 °C for 5 h, The PBSCF-GDCx slurries were then screen-printed onto the GDC electrolyte layer. The cells consisting of 3 layers (Ni–GDC as an anode, GDC as an electrolyte, and a cathode) were finally sintered at 950 °C for 4 h under an air atmosphere with an active electrode area of 0.36 cm². Both the electrolyte and the cathode thickness of a single cell are about 15 μm. Ag wires were attached at both electrodes of single cells using an Ag paste as a current collector. An alumina tube and a ceramic adhesive (Aremco, Ceramabond 553) were employed to fix the single cell. Humidified hydrogen (3% H₂O) was applied as fuel through a water bubbler with a flow rate of 20 mL min⁻¹ and static air was used as an oxidant during single cell tests. I–V curves were examined using a BioLogicPotentiostat at operating temperature from 500 °C to 650 °C.

Results and discussions

The effects of sintering temperature variations on microstructure and electrical transport

Fig. 1 shows the XRD patterns of PBSCF-GDC50 obtained after heat treatment at various temperatures from 900 to 1050 °C with 50 °C intervals for 4 h in air. Generally, a phase reaction between the electrode and the electrolyte can result in the formation of an undesired insulating layer at the interface, which hinders the oxide-ionic and electronic transport.³⁶ Therefore, the chemical compatibility between PBSCF and GDC should be investigated as a preliminary study. Fig. 1(a) confirms that a pure layered perovskite phase is obtained with the diffraction peaks indexed in a tetragonal structure (space group P4/mmm) after heat treatment at 1150 °C for 12 h.²² For a comparison, the XRD pattern of GDC powder is also presented in Fig. 1(b). All the peaks presented in Fig. 1(c)–(f) can be attributed to either PBSCF and GDC, indicating that PBSCF is chemically compatible with GDC under the given circumstances, even at temperatures much higher than IT-SOFC operating temperature (∼700 °C). Moreover, the peak intensities of PBSCF-GDC50 become more intense and sharp with increasing sintering temperature due to well crystallized phases.³⁷

Fig. 1 X-ray diffraction patterns of (a) PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ sintered at 1150 °C for 12 h, (b) Ce_0.9Gd_0.1O_1.95 (GDC) sintered at 1350 °C for 4 h, and PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDC50 sintered at (c) 900 °C, (d) 950 °C, (e) 1000 °C, and (f) 1050 °C for 4 h.

Fig. 2 presents SEM images of PBSCF-GDC50 sintered at different temperatures. Fig. 2(a) indicates that there is an uneven distribution of grains as well as poorer porosity and insufficient adhesion to the GDC electrolyte, likely due to insufficient heat treatment. The powders can be regarded as raw materials that are not fully reacted until 900 °C. From Figs. 2(b)–(d), the progress of grain growth with increasing sintering temperature is observed. The particles are faceted and the size distribution is wider when they are produced at higher temperatures. Fig. 2(b) reveals homogeneously distributed particles and relatively small grain size with good connectivity between the electrode and electrolyte as well as reasonable porosity, which ensures effective gas diffusion. Fig. 2(c) and (d) present much larger grain size and agglomerations of particles, possibly due to excessive heat treatment, which may decrease the electrochemically active surface area. These results demonstrate that the microstructure of the PBSCF-GDC50 composite cathode sintered at 950 °C for 4 h in air can considered optimal.

Fig. 2 Cross-sectional SEM images of PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDC50 cathode/GDC electrolyte interface sintered at (a) 900 °C, (b) 950 °C, (c) 1000 °C, and (d) 1050 °C.

To assess the electro-catalytic activity of PBSCF-GDC50 sintered at various temperatures, the impedance spectroscopy of the symmetrical cell was investigated under open circuit voltage (OCV) conditions. For comparison, Nyquist plots of PBSCF-GDC50/GDC/PBSCF-GDC50 sintered at various temperatures are shown in Fig. 3(a) with the specific area specific resistance (ASR) values at 600 °C, defined as ASR = (R_p × 0.36 cm²)/2, provided in the inset. The intercept on the real axis at high frequency corresponds to the ohmic-resistance of the cell (R_ohm) whereas the low frequency intercept gives the total resistance (R_ohm + R_p). Therefore, the ASR values are determined by the impedance intercept between high frequency and low frequency with the real axis of the Nyquist plot. The minimum ASR value reaches 0.053 Ω cm² at 600 °C, obtained from the PBSCF-GDC50 sample sintered at 950 °C, which is much lower value than ASR values of the other samples, as listed in the inset of Fig. 3(a). These data indicate that PBSCF-GDC50 sintered at 950 °C may have the smallest polarization resistance as well as the best electrochemical performance. One possible origin of the lower ASR value of the material sintered at 950 °C could be micro-structural enhancement, as mentioned above. Over the investigated temperature range, 950 °C can be considered the optimal sintering temperature for the PBSCF-GDC50 composite cathode in terms of microstructure and electro-catalytic activity. The temperature dependence of the polarization resistance for the PBSCF-GDC50 electrode on the GDC electrolyte with various heat treatments is illustrated by an Arrhenius plot in Fig. 3(b). The apparent Ea values of PBSCF-GDC50 cathodes sintered at different temperatures are also shown in the inset of Fig. 3(b). The relatively lower Ea value of PBSCF-GDC50 sintered at 950 °C may suggest a lower chemical barrier for oxygen reduction compared to that of the other samples, subsequently resulting in high electro-catalytic activity.^38–40

Fig. 3 (a) Impedance spectra of PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDC50 composite cathodes on GDC symmetrical cells sintered at various temperatures and measured at 600 °C under OCV. (b) Temperature dependence of polarization resistance for PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDC50 composite cathodes on GDC electrolyte sintered at various temperatures by Arrhenius plots.

The effects of GDC variations on micro-structural and electrochemical properties

Based on the preliminary study to determine the optimal sintering temperature, we present the systematic variation of the GDC ratio in PBSCF to identify optimized two-phase composite cathodes. To further investigate the effects of GDC on the micro-structural properties of PBSCF-GDCx (x = 0, 20, 40, 50, and 60 wt%) composite cathodes, typical SEM images of PBSCF-GDCx are shown in Fig. 4. For a primary powder, with smaller particle size, accordingly larger surface area is obtained, increasing the amount of reactant exposed for the reaction.³⁴ Larger particles and severe coalescence of powder bonded with grains are observed in Fig. 4(a). An increase in the amount of GDC, however, gradually mitigates these agglomerations and homogeneously disperses the particles of the PBSCF-GDCx electrode. Meanwhile, the composite cathodes containing 40, 50, and 60 wt% of GDC display similar microstructure, showing well-bonded porous networks, which facilitate oxygen transport to the activated the TPB sites. Fig. 4(f) presents a cross-sectional view of a single cell consisting of Ni–GDC as an anode, GDC as an electrolyte, and a PBSCF-GDCx composite cathode sintered at 950 °C. The thickness of both the dense GDC electrolyte and the porous cathode of a single cell is approximately 15 μm.

Fig. 4 SEM images of PrBa_0.5Sr_0.5Co_2−xFe_xO_5+δ–GDCx/GDC/Ni-GDC: (a) x = 0 wt%, (b) x = 20 wt%, (c) x = 40 wt%, (d) x = 50 wt%, (e) x = 60 wt%, and (f) the cross-section of a single cell with an approximately 15 μm thick GDC membrane.

The impedance responses on the PBSCF-GDCx/GDC/PBSCF-GDCx symmetrical cells are characterized by two arcs at high-frequency and low-frequency, indicating that there might be at least two electrode processes which limit the ORR on PBSCF-GDCx composite cathodes. Fig. 5(a) shows the experimental electrochemical impedance spectroscopy patterns at 600 °C, simulated with the equivalent circuit model illustrated in the inset of Fig. 5(a) from 0 to 60 wt% of GDC. The impedance analysis results using the model circuit are also listed in Table 1. The real axis values at the high frequency intercepts, R₁, mainly correspond to the electrolyte resistance and wires. In the Nyquist plots, high-frequency arcs are equivalent to R₂, which is caused by charge transfer during the migration and diffusion of oxygen ions from the TPB into electrolyte lattice. Meanwhile, low-frequency arcs correspond to R₃, which is associated with adsorption/desorption of the molecular oxygen and bulk or surface oxygen diffusion.^15,41 For comparison, the specific values of R₂ and R₃ are plotted in Fig. 5(b). The polarization resistance R_p is defined as the sum of two resistances (R₂ and R₃). It is obvious that the addition of the ionically conducting phase GDC to the PBSCF cathode results in a significant reduction of non-charge transfer resistance (R₃) while R₂ is nearly unchanged up to 50 wt% of GDC, which consequently decreases the total polarization resistance (R_p = R₂ + R₃). These results might be attributed to the predominant non-charge transfer contribution to the electrode polarization resistance, likely due to the extended electrochemical reaction zone with the addition of GDC into PBSCF, where the reaction rate of the composite electrodes would be the sum of those on the 2PB sites (the surface of MIEC) and those on the TPB sites (all interface boundaries between PBSCF and GDC that are exposed to oxygen gas).²⁴ This characteristic of the composite cathode could lead to many more ORR sites than those of a single electronic conductor or MIEC. At 60 wt% of GDC, however, a sudden increase in both R₂ and R₃ is observed due to a reduction of effective electron-conducting paths by the excessive amount of GDC particles.²⁴ The constant phase element (CPE) represents a non-ideal capacitor and indicates the similarity of the CPE to a true capacitor. The capacitance (C) values of the electrodes are also provided in Table 1 obtained from the following eqn (1), where n is an empirical constant:^42–44

Fig. 5 (a) Experimental and simulated impedance plots of PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ–GDCx/GDC/Ni–GDC by the equivalent circuit shown as an inset. (b) Comparison of R₂ and R₃ for PrBa_0.5Sr_0.55Co_1.5Fe_0.5O_5+δ–GDCx/GDC/Ni-GDC. (c) Temperature dependence of polarization resistance for PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDCx (x = 0, 20, 40, 50, and 60 wt%) composite cathodes on GDC electrolyte sintered at 950 °C for 4 h by Arrhenius plots.

Table 1 Electrochemical impedance spectroscopy fitting results of PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDCx measured at 600 °C in air

x	R 1 (Ω cm^2)	R 2 (Ω cm^2)	R 3 (Ω cm^2)	R p (Ω cm^2)	CPE2-Q (F cm^−2)	CPE3-Q (F cm^−2)	C 2 (F cm^−2)	C 3 (F cm^−2)
0	2.1052	0.0403	0.0747	0.1150	9.88 × 10^−2	3.70 × 10^−1	4.38 × 10^−4	1.37 × 10^−2
20	2.2403	0.0385	0.0613	0.0998	6.56 × 10^−2	3.47 × 10^−1	4.09 × 10^−4	2.42 × 10^−2
40	2.1658	0.0379	0.0374	0.0753	8.25 × 10^−2	5.65 × 10^−1	2.45 × 10^−4	8.78 × 10^−3
50	2.0605	0.0376	0.0309	0.0685	5.21 × 10^−2	9.40 × 10^−1	3.94 × 10^−4	1.89 × 10^−2
60	2.3051	0.0580	0.1280	0.1860	9.49 × 10^−2	9.41 × 10^−1	3.11 × 10^−4	2.35 × 10^−1

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Fig. 5(c) shows an Arrhenius plot of R_p at 600 °C for PBSCF-GDCx, and the calculated values of activation energy are also shown in the inset of Fig. 6(c). The specific ASR values of PBSCF-GDCx are 0.106, 0.083, 0.057, 0.052, and 0.174 Ω cm² at 600 °C for x = 0, 20, 40, 50, and 60 wt% of GDC, respectively.

Fig. 6 I–V curves and corresponding power density curves of a single cell (PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ-GDCx/GDC/Ni-GDC) under various temperatures: (a) x = 0, (b) x = 20, (c) x = 40, (d) x = 50, and (e) x = 60 wt%.

Fig. 6 presents the power density and voltage as a function of the current density for Ni-GDC/GDC/PBSCF-GDCx cells using humidified H₂ (3% H₂O) as a fuel and static ambient air as an oxidant in a temperature range from 500 to 650 °C. The fuel cell performance increases with increasing amount of GDC in the PBSCF-GDCx composite cathode up to x = 50 wt%. The maximum power density at 600 °C reaches about 2.0 W cm⁻² at x = 50 wt% whereas a sudden drop in performance (∼1.3 W cm⁻²) occurs at x = 60 wt%. These results can be inferred from the symmetrical cell impedance. As mentioned above, the extended TPB sites obtained by introducing the composite cathode might enhance the ORR activities, which would be a leading factor for the optimal electrochemical performances of PBSCF-GDC50.

Conclusion

The sintering process is an important parameter that influences microstructure evolution, grain growth, and densification of a material. For PBSCF-GDC50 sample, the optimal sintering temperature with the smallest polarization resistance was revealed to be 950 °C, and is attributed to enhanced micro-structural evolution under the given processing conditions. Different wt% GDC-containing composite cathodes (PBSCF-GDCx) are investigated in order to induce the desired properties of a high ORR rate. The performances of PBSCF-GDCx composite electrodes in terms of ASR and power density are gradually improved with an increasing amount of GDC up to x = 50 wt%. This can be explained by the optimized ORR mechanisms by the addition of the ionically-conducting GDC phase that extends the TPB (all interface boundaries between PBSCF and GDC). At x = 60 wt%, however, the performance of the composite cathode begins to deteriorate due to a reduction of effective electron-conducting paths by the immoderate amount of GDC.

Acknowledgements

This research was supported by Basic Science Research Program (2012-R1A1A1013380) and Mid-career Researcher Program (2013R1A2A2A04015706 and 2011-0010773) through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology, and the grant from the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (201103020030060) funded by the Korea government Ministry of Knowledge Economy.

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