Electrochemical investigation of LSGMC-composite cathodes on LSGM-substrate

As potential intermediate-temperature SOFC cathodes, perovskite composite cathode materials were evaluated in combination with a La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3- δ (LSGM) electrolyte. resistive 4 3 O 7 in the calcined of composite cathodes LSGM. A bulk Ω ·cm 2 an interfacial resistance 0.16 Ω for LSCF/LSGMC composite cathode LSGM at 800ºC


Introduction
There is a growing interest in reducing the operating temperature of the solid oxide fuel cells (SOFC) from 1000ºC to between 600 and 800ºC (1). A lower operating temperature range can resolve many degradation problems associated with the conventional SOFC operating temperatures, such as nickel catalyst coarsening by the Ostwald ripening mechanism, delamination of the SOFC tri-layer due to thermal stresses, and/or failure of sealant materials. In addition, the lower operating temperature may allow the use of lower cost components such as metal interconnects and balance of plant made from less exotic alloys. There are notable challenges associated with operating a conventional YSZ-based SOFC at intermediate temperature. The most significant is that of insufficient electrolyte conductivity and high cathodic interfacial resistance (i.e. high cathodic activation polarization). The current commercial YSZ electrolyte can achieve ionic conductivity of 0.1 S/cm at 1000ºC but only 0.02 S/cm at 800ºC (2); therefore, it is desirable to find alternative electrolyte and cathode materials for intermediate temperature SOFCs (IT-SOFCs). In recent years, Sr-and Mg-doped LaGaO 3 (LSGM) perovskite materials have gained considerable attention as potential electrolytes for intermediate temperature (IT) SOFCs. Many of the reported LSGM compositions exhibited ionic conductivity at 800ºC (3)(4)(5)(6) higher or comparable to that of YSZ at 1000ºC. The highest electrical conductivity, 0.17 S/cm at 800ºC, was achieved with La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3−δ by Ishihara et al. (7). When LSGM was doped with cobalt (LSGMC) at the Ga site, electrical conductivity as high as 0.43 S/cm at 800ºC was achieved with 15 mol % Co (8). However, electronic conductivity was found by Ishihara et al. to be too high for electrolyte applications when cobalt content was 8.5 mol % or higher (9).
The conventional cathode, Sr-doped LaMnO 3 (LSM), has shown good performance for high-temperature SOFC operation (i.e. 900 to 1000ºC) (2); however, LSM becomes almost exclusively an electronic conductor when there is an absence of oxygen vacancies in its structure. This restricts the locations that oxygen can be reduced to the triple-phase boundary (TPB) regions, where air, cathode, and electrolyte are in contact. Therefore, LSM experiences rapidly decreasing performance at lower temperatures (10). This motivates the search for a mixed ionic and electronic conducting (MIEC) oxide that can be used as a cathode material for IT-SOFCs. Several of the potential cathode materials are based on cobalt-containing perovskites such as Ba 1-x Sr x Co 1-y Fe y O 3-δ (BSCF), La 1x Sr x Co 1-y Fe y O 3-δ (LSCF), Pr 1-x Sr x Co 1-y Fe y O 3-δ (PSCF), and Sr 0.5 Sm 0.5 O 3 (SSC). These cathode materials in general have good electrical conductivities but high thermal expansion coefficient (TEC) mismatch with the LSGM electrolyte (11-11.5×10 -6 K -1 ).
Previous studies on BSCF have shown not only high TEC of 19.7×10 -6 K -1 but also relatively low conductivity of 25.4 S/cm at 800ºC in air for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (11). Pena-Martinez et al. (12) reported the compatibility of this BSCF composition on a La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 electrolyte, but they were only able to achieve a power density of 160 mW/cm 2 at 800ºC. Even though BSCF formed good interfacial contact with the LSGM electrolyte, XRD analysis suggested a surface reaction caused by diffusion of Co above 1000ºC that resulted in high cathode overpotential.
PSCF has also gained considerable attention recently as a possible cathode for IT-SOFCs. Qiu et al. (16) observed that for various doping levels of Fe, the overpotential of Pr 1-x Sr x Co 1-y Fe y O 3-δ (x = 0.2, 0.3) cathodes increased with increasing Fe content from 0 to 0.8. In addition, higher Fe content led to lower TEC, bringing it closer to the TEC of the electrolytes. Alternatively, Kim et al. (17) reported that Pr 0.3 Sr 0.7 Co 0.3 Fe 0.7 O 3-δ has the lowest ASR value, 0.13 Ω cm 2 with 0.84 eV of activation energy but a high TEC value of 18.9×10 -6 K -1 at 700ºC. anode attained a power density of 222 mW/cm 2 at 800ºC (19). Xia et al. (20) reported that the electrochemical performance of an SSC cathode could be further improved by adding samaria-doped ceria (SDC; Sm 0.2 Ce 0.8 O 1.9 ). SDC not only provides more ionic conducting elements but it also maintains porosity in the cathode by preventing SSC particles from densifying during calcinations and sintering. By testing various composite SSC cathodes, Xia et al. (20) determined that the best performing composite was a 70:30 weight percent SSC:SDC mixture at 600ºC. Qin et al. (21) confirmed that composite cathode SSC/SDC of 7:3 weight ratio narrowly edged out the 6:4 composite as having the lowest ASR at 600ºC. The activation energy of SSC/SDC(6:4) was determined by Baek et al. (22) to be lower than that of SSC/SDC(7:3) (i.e. 0.9 eV versus 0.94 eV, respectively). The authors also found that the TEC value of the 6:4 mixture was lowered to 12.3×10 -6 K -1 at 600ºC. Thus, the SSC/SDC(6:4) might perform better for the temperature range of 600-800ºC.
In this work, we study the performance of LSGMC-compounded cathodes on LSGM electrolytes. The addition of LSGMC as a cathode compounding element serves several purposes. The first is to reduce the TEC mismatch between the cobalt-based cathode materials and LSGM and improve interfacial adhesion. This compounding element also adds electronic conductivity to the cathode thereby increasing the number of triple-phase boundaries (TPBs) and promoting percolation of more ionic pathways to the electrolyte. The cathodes are of selected perovskite materials La 0.6 Sr 0.4 Co 0. (SSC-SDC) are each compounded with La 0.8 Sr 0.2 Ga 0.8 Mg 0.1 Co 0.1 O 3-δ (LSGMC) at an 8:2 mixing ratio by weight. It is understood that this mixing ratio is below that required for full percolation of the compounding element. This is intentional as each of the perovskite cathode materials studied herein already possess high ionic conductivity.

Powder preparation
All powders were synthesized by glycine-nitrate combustion synthesis. The precursor metal nitrates were dissolved in de-ionized water at the desired stoichiometry for each material composition. Glycine was added as a combustion fuel and complexing agent to the nitrate solution in n NO3 :n glycine (i.e. molar ratio of nitrate ion to glycine) of 1 to 0.75. The solution was heated to boil off the water. The solution auto-ignited and combusted to yield an ashy product. For the electrolyte, LSGM powder was calcined at 900ºC for 3 h to burn off any residual carbon and resolve any remaining unreacted species. Calcination was followed by a 48 h ball-milling process with plasticizers, binders, and ethanol to break up agglomerates and aid the pellet pressing process. The powder was then dried and uniaxially pressed in a 32 mm cylindrical die under 45.0 MPa then sintered in air at 1450ºC for 4 h. The finished product was a dense pellet with dimensions of 19-21 mm in diameter and ~1 mm in thickness. Cathode powders of BSCF, LSCF, PSCF, SSC-SDC were produced via a similar GNP method and were fired at 1000ºC for 8 h, 1050ºC for 2 h, 1000ºC for 2 h, and 1250ºC for 1 h, respectively. The LSGMC compounding element of the cathode was sintered as a loose powder at 1400ºC for 4 h to achieve pure perovskite phase. The composite cathode was prepared by mixing respective cathode powder with LSGMC powder in an 8:2 weight ratio and ball-milled with ethanol for 24 h. The resulting mixture was dried and reserved for the screenprinting step.
X-Ray Diffraction (XRD) patterns were collected using a Siemens D5000 x-ray diffractometer with a Ni-filtered CuKα radiation source at 40kV and 30mA at room temperature for material phase characterization. The scans were taken at a 2θ rate of 0.02º/sec over the range of 20-90º. The XRD samples were loose powders of LSGMC and other cathode materials calcined at various temperatures and an LSGM electrolyte substrate pellet sintered at 1400ºC for 4 h. The sintered LSGM pellet was pressed with a 13 mm cylindrical die using powders ground with an agate mortar and pestle. The morphology of the surface of the cross-section of a half-cell of the SSC-SDC/LSGMC interface was examined using scanning electron microscopy (SEM, Zeiss Ultra 55).

Half-cell fabrication
A half-cell arrangement of composite cathode on LSGM electrolyte was used for the electrochemical tests. The electrodes were deposited using screen printing. The cathode ink slurries were prepared by mixing the composite cathode powders (i.e. cathode + LSGMC) in a 1:1 ratio by weight with a pre-mixed solution of 98% alpha-terpineol and 2% Butvar. The ink was painted on the electrolyte using a 1 cm 2 square mask to form the active area. All screen-printed cathodes were fired at 1100ºC in air for 2 h. Platinum paste (Engelhard, A3788A) and gauze (Alfa Aesar, 52 mesh woven from 0.1 mm diameter wired, 99.9%) were used as current collectors in a three-probe configuration as shown in Figure 1. The half cell was then fired at 950ºC for 1 h to cure the paste. Scanning electron microscopy (SEM) was used to analyze the cathode morphology and the quality of the screen-printed composite cathode and also the integrity of the bonding between the screen-printed composite cathode and the electrolyte.  Electrochemical impedance test A spring-loaded test rig in combination with the Solartron 1260A frequency analyzer and the 1480A multiplexer potentiostat were used to measure electrochemical characteristics of the half cells.
The electrochemical measurements using the electrochemical impedance spectroscopy (EIS) method were taken in air for temperatures of 700, 750, and 800ºC. The impedance was determined at open circuit voltage (OCV) with a frequency range from 0.1 Hz to 100 kHz and 10 mV of ac-amplitude. Cathodic polarization was determined using a dc-polarization test with CellTest software to acquire the current-overpotential responses of the half cell.

Results and Discussion
The room temperature XRD patterns of the LSGM pellet and LSGMC powder calcined at various temperatures in air for 4 h are shown in Figure 2. XRD patterns have been identified as the desired perovskite phase with the cubic structure (space group Pm-3m) for LSGM (23,24) and LSGMC (25). The secondary phases of LaSrGa 3 O 7 (JCPDS # 45-0637) and LaSrGaO 4 (# 80-1806) were present in both LSGM and LSGMC samples at the low calcination temperature. Both LaSrGa 3 O 7 and LaSrGaO 4 have been reported as minor phases during the synthesis of LSGM (24,26) and of LSGMC (27). Neither of the secondary phases is desirable. LaSrGaO 4 has been shown to be electrically insulating while the LaSrGa 3 O 7 phase was reported to have very low electrical conductivity (23). Since Co has been known to improve the sinterability of perovskites, LSGMC specimens demonstrated relatively less secondary phases at 900ºC and 1400ºC as compared to the LSGM. At 1400ºC and 4 h, pure phase perovskite was obtained for LSGMC powders, while small traces of the LaSrGa 3 O 7 phase still lingered in the LSGM pellets. The LSGM pellets became highly phase pure only after heat treatment up to 1450ºC for 4 h. The Arrhenius plot of the measured bulk electrical conductivity of LSGM electrolyte as compared to that of 10 mol % YSZ in the literature (1) is shown in Figure 3. The measurements were conducted in air using the ac-impedance method with four-probe configuration. The electrolyte pellet used in this test was produced from powders processed by ball-milling with chemical additives to yield an electrical conductivity of 0.10 S/cm at 800ºC.  The XRD patterns of calcined BSCF, LSCF, PSCF, and SSC-SDC powders along with the respective patterns of the screen-printed composite cathode on LSGM pellet and of the bare LSGM pellet are displayed in Figure 4 to 7. As can be seen, all cathode powders attained a relatively pure perovskite structure with minimal secondary or impurity phases. The XRD pattern for the BSCF powder agrees well with that reported for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ powder synthesized by the EDTA-Pechini method (i.e. sintered at 1000ºC for 5 h) with the perovskite structure indexed as cubic (space group Pm-3m) (11). The scan taken from the LSGM pellet with screen-printed composite BSCF/LSGMC showed a dominant perovskite structure identical to that of the LSGM and also small traces of secondary phases of LaSrGaO 4 and LaSrGa 3 O 7 . This suggests possible reaction of LSGM with the cathode due to interdiffusion of cations between the cathode and the electrolyte. Huang et al. (28) have observed precipitation of LaSrGaO 4 and LaSrGa 3 O 7 in LSGM.
They attributed this to La diffusion across the electrode/electrolyte interface resulting in La enrichment or depletion in LSGM. Another possible chemical reactivity is Co diffusion into LSGM as reported by Nauomidis et al. using EDS analysis of various Co-based cathode materials on La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-δ . Extensive Co diffusion is not expected between the cathode materials and the compounding element LSGMC because the Co is already present in the LSGMC. The XRD pattern of LSCF matches that of a cubic structure (space group Pm-3m) as reported by Swierczek and Gozu (14). Figure 5 indicates that the composite LSCF cathode on a LSGM pellet has identical peaks to both LSCF and LSGM due to similar lattice constants of these two perovskites. Peaks of LaSrGaO 4 were detected in the composite cathode XRD pattern but not for LaSrGa 3 O 7 . This agrees with observations of Lin and Barnet, (15) which also had trace presence of LaSrGaO 4 in a fired LSCF/LSGM powder mixture, but the authors were able to prove by XRD analysis that La diffusion did not take place. This suggests that Co diffusion may be the main reason for LaSrGaO 4 precipitation. Single perovskite phase with orthorhombic structure was identified for PSCF powder (Figure 6), which is in agreement with the work of Qiu et al. (16). PSCF and LSGM peaks showed up as distinct peaks in the XRD pattern of composite PSCF/LSGMC on a LSGM pellet. LaSrGaO 4 and LaSrGa 3 O 7 phases again were present in small amount. The XRD scan of the composite SSC-SDC powder gave the expected peaks associated with SSC and SDC (20). There were LaSrGaO 4 and LaSrGa 3 O 7 precipitates detected when the composite cathode SSC-SDC/LSGMC was calcined on LSGM. La diffusion into CeO 2 -based materials is strongly suspected in this case due to previous research (28). Possible La diffusion into SSC-SDC powder could stem from LSGMC powder or LSGM pellet and could lead to a La deficiency in the LSGM phase. La deficiency gives rise to the resistive phase LaSrGa 3     SEM images of the cross-section of a half cell of composite cathode SSC-SDC/LSGMC on a LSGM electrolyte after the electrochemical tests are displayed in Figure 8. The images show a dense and homogeneous phase LSGM electrolyte was produced and that good interfacial contact with porous cathode layer was achieved. The screen-printed cathode layer was about 5-6 µm thick with a fine particle size (<1 µm). The cathode does, however, appear to have low porosity and significant particle agglomeration. This is in agreement with a recent study by Bansal and Zhong (29), which reported that combustion-synthesized SSC powders become increasingly dense when fired as loose powders at temperatures greater than 1000ºC. Cathode porosity in this study was introduced not only by the deposition of the slurry through the porous screen but also by the pyrolysis of organic binder and solvents from the cathode slurry at sintering temperatures. Random integration of already sintered-LSGMC powder amongst calcined-SSC powder in the composite cathode should help hinder extensive diffusion and agglomeration. On the other hand, the low porosity level could also be due to a poor quality binder used in suspending cathode particles in the slurry used for screen-printing. An ink slurry mixture with too low viscosity and binding strength will result in powder particles with more fluid characteristics and higher diffusion at sintering temperatures. Future studies will focus some attention on optimizing the cathode performance by improving the porosity level of the cathodes.   from 5.82 Ω (with 1 cm 2 active cathode area) at 700ºC to 3.81 Ω at 800ºC. The interfacial resistance also decreases with higher operating temperature as evidenced by the smaller magnitudes arcs in the high temperature Nyquist plots of Figure 9b. The polarization resistance at 800ºC (0.16 Ω) is more than an order of magnitude higher than that reported for composite cathode La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ /La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3−δ of a 7:3 weight ratio on LSGM measured in air under OCV conditions at 800ºC by Lin and Barnett (15).
Smaller secondary arcs at low frequency are observed for the impedance spectra measured at 700 and 800ºC. These arcs could be due to the mass transfer limitations in the composite cathode. Mass transfer limitations might be present in the current work due to several possible factors. First, the cathode porosity might not be adequate for the relatively fast surface exchange of O 2 . Low porosity or non-percolating pores could lead to insufficient O 2 transport to the reactive sites. Second, reactions between or amongst phases present in the interface may have occurred (e.g. by interdiffusion of La or Co) as implied by the XRD results. Even though the SEM images ( Figure 8) indicate no noticeable phase separation for both the cathodes, different phases could have similar Z numbers that do not show up on back-scattered mode. Thus, more accurate chemical phase analysis of the cathode/electrolyte interface is needed to verify this possibility. The cathodic polarization curves measured at three temperatures 700, 750, and 800ºC for the composite cathode LSCF/LSGMC half-cell is displayed in Figure 10. The composite cathode, calcined at 1100ºC for 2 h, shows the expected temperature dependent behavior of increasing current density for a given overpotential value with increasing temperature. The polarization curves of the other composite cathode half cells also show the same temperature dependent behavior. Higher overpotential values registered measuring errors for all four composite cathode specimens signifying measurement interference for the cathode/electrolyte interfaces. This is possibly due to the mass transfer limitations or the presence of the resistive LaSrGaO 4 and LaSrGa 3 O 7 phases. The LSCF/LSGMC composite with the lowest observed amount of LaSrGaO 4 in its XRD pattern demonstrated the highest current density improvement with temperature for a given overpotential value.  Figure 11 provides a performance comparison for the four composite cathodes at each temperature of 700, 750, and 800ºC. At 700ºC, BSCF composite cathode has the best performance with the highest current density at each overpotential, achieving 0.1 A·cm -2 at 0.04 V overpotential. BSCF and LSCF composite cathodes showed good performance at 750ºC with both achieving 0.105 A·cm -2 at 0.04 V overpotential. The LSCF half cell was found to have a steeper slope in the Tafel region of the polarization curve, indicating that better performance maybe have been possible if higher overpotentials and current densities could have been achieved. The LSCF composite cathode showed the highest performance at 800ºC achieving 0.11 A·cm -2 at 0.045 V overpotential. The measured performance for PSCF was better than that reported in the literature. CGO-based cells using Pr 0.8 Sr 0.2 Co 0.8 Fe 0.2 O 3-δ as a cathode by Qiu et al. (16) found a cathodic overpotential as great as 0.16 V for a current density of 0.016 A·cm -2 at 700ºC, which is much higher than the measured 0.025 V for this study for Pr 0.3 Sr 0.7 Co 0.3 Fe 0.7 O 3-δ on LSGM electrolyte.

Conclusion
Glycine-nitrate combustion synthesis has been applied to obtain pure phase perovskite powders of various types for use in composite cathodes for SOFCs. Dense LSGM electrolyte substrates were manufactured with electrical conductivity measured at 0.102 S/cm at 800ºC. Composite cathode materials were obtained by mixing sintered-LSGMC powders with each cathode material in an 8 to 2 weight ratio for both introducing MIEC properties and lowering the TEC. It was found that undesirable phase of LaSrGaO 4 and LaSrGa 3 O 7 were present in the calcined layer of the composite cathodes on LSGM for all four of the tested cathode materials. EIS and dc-polarization tests were applied to characterize the cathode/electrolyte interface. Bulk ohmic resistance was measured for LSCF/LSGMC composite cathode giving values of 5.82 Ω·cm 2 and 3.81 Ω·cm 2 at 700 and 800ºC, respectively. An interfacial resistance value of 0.16 Ω at 800ºC was measured for LSCF/LSGMC composite on a LSGM pellet under OCV conditions. DC polarization measurements demonstrated that BSCF/LSGMC composite performed best at 700ºC, while LSCF/LSGMC composite has the best performance at 800ºC. The secondary arcs in the impedance spectra measured for LSCF/LSGMC along with the relatively low current density measured indicate some mass transfer limitations in the composite cathodes. Future work should focus on optimizing the porosity level in the cathode and the firing conditions of screen-printed composite cathodes on LSGM to minimize chemical reactivity of the cathodes with LSGM, and to eliminate resistive phase impurities.