Characteristics of Ag-doped LaMnO₃ perovskite oxide and its application as a solid oxide fuel cell cathode

Abstract

Ag-doped LaMnO₃ perovskite oxides were synthesized as new cathode materials for solid oxide fuel cells (SOFCs), and their phase stability, reactivity with yttria-stabilized zirconia (YSZ), electronic state of Mn, electrical conductivity, and power generation properties were investigated. La_0.9Ag_0.1MnO_3±δ (LAM01) showed no evidence of Ag metal deposition, whereas La_0.9Ag_0.2MnO_3±δ (LAM02) showed Ag metal deposition, suggesting that Ag dissolved in a perovskite-type structure and its solid solution limit for the La-site was 10%. LAM01 did not react with the YSZ electrolyte at 975 °C for 100 h. The electronic state of Mn was between that of the trivalent and tetravalent states at room temperature, suggesting that the Ag acceptor dopant was charge-compensated by the oxidation of Mn. Electrical conductivity and Seebeck coefficient measurements indicated that the main charge carrier was electron–hole. Stable power generation properties were obtained using the LAM01 cathode and indicated that the Ag acceptor was stable and was compatible with the YSZ electrolyte. Therefore, Ag-doped LaMnO₃ is promising as a novel SOFC cathode.

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1. Introduction

Solid oxide fuel cells (SOFCs) have attracted much attention in recent years due to their high power generation efficiency.^1–5 Yttria-stabilized zirconia (8 mol% Y₂O₃–ZrO₂; YSZ) is widely used as an electrolyte in SOFCs because of its excellent performance, low cost, and material stability.^6–8 In a conventional SOFC cell, a Ni/YSZ cermet is used for the anode^1–3 and a metal oxide, such as (La, Sr)CoO_3−δ (LSC),^9–13 (La, Sr)MnO_3−δ (LSM),^13–15 and La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF),^16–19 is used for the cathodes. However, a challenge in SOFCs with an YSZ electrolyte is the formation of a high-resistance reaction phase between the electrolyte and the cathode; when LSC or LSCF, which has high catalytic activity, is used, the A-site element reacts with Zr in the electrolyte. In particular, the A-site element Sr reacts with ZrO₂ to form SrZrO₃, which has low oxide ion conductivity, leading to a large increase in the ohmic resistance of the SOFC cell.^20–23 An interlayer, such as Ce_0.9Gd_0.1O_2−δ (GDC), can be introduced between these cathode materials and YSZ, but the formation of SrZrO₃ cannot be prevented completely.²⁴ Another problem emerges from the introduction of a GDC interlayer; GDC and YSZ have the same fluorite structures, and thus they easily react, resulting in the formation of a CeO₂–ZrO₂ solid solution with low oxide ion conductivity.²⁵ Mn-type perovskite oxides as cathodes have been proposed as a solution to this problem because of their higher chemical compatibility with the YSZ electrolyte than Co-type perovskite oxides. Yokokawa et al.²¹ reported that chemical thermodynamic considerations showed that the A-site elements are unlikely to react with ZrO₂ in the LaMnO₃ system, and Chen et al.²⁶ also reported that Sr in La_1−xSr_xMnO_3±δ does not react under cathode baking conditions. However, LaMnO₃-based materials also form SrZrO₃ under a low-oxygen atmosphere;^26,27 during SOFC operation, the cathode side reaches a low oxygen partial pressure depending on the electrode overpotential. Therefore, even if SrZrO₃ is not formed during the electrode sintering, it will eventually be formed during the SOFC operation. The formation of a high-resistance reaction phase has also been reported for Ca²⁺-doped materials;²⁸ thus, using alkaline earth metal elements as dopants, which are essential for increasing electrode activity, results in the formation of a ReZrO₃-based high-resistance phase.

To overcome this problem, we are investigating noble dopants other than alkaline earth metal elements. Ag has been selected as a candidate noble dopant, and Ag-doped LaMnO₃ materials have been investigated for potential applications including magnetic memory devices, and magnetic field sensors, and their thermoelectric and magnetic properties have been studied.^29,30 In the SOFC cathode, Ag is not expected to react with ZrO₂ or Y₂O₃. Ag is used as a catalyst and current collector in SOFC systems,^31–35 and exsolved Ag nanocatalysts on perovskite-type oxides have been reported. Kim et al.³⁵ evaluated the catalytic properties of Ag on BaCoO₃³⁵ and Sažinas et al.³¹ evaluated them on LaSrMnO_3−δ. In these studies, the exsolution of Ag nanoparticles was caused by annealing in a hydrogen atmosphere from 300 to 600 °C. Thus, Ag does not interfere with the electrode reaction even if it is precipitated.

In the present study, which also uses Ag, the catalytic properties of Ag metal are not the target. We focused on Ag as the acceptor dopant and evaluated the Ag-doped LaMnO₃ materials as the cathodes for SOFCs. The cathode properties of LaMnO₃ materials were evaluated using Ag as a low-valence element doped into the A-site of perovskite-type oxides to search for an alternative acceptor-dopant to alkaline earth metals. Ag-doped LaMnO₃ was synthesized by using the sol–gel method, and its electrical conductivity, Seebeck coefficient, and electronic structure were investigated. Finally, Ag-doped LaMnO₃ was used as a SOFC cathode, and its performance was measured.

2. Experimental section

2.1 Sample preparation

The sol–gel method was used to synthesize 10 mol% Ag-doped LaMnO₃ (La_0.9Ag_0.1MnO_3±δ, LAM01) and 20 mol% Ag-doped LaMnO₃ (La_0.8Ag_0.2MnO_3±δ, LAM02) from La (NO₃)₃·6H₂O (99.9%, FUJIFILM Wako Pure Chemical Corp.), AgNO₃ (99.8%, FUJIFILM Wako Pure Chemical Corp.), Mn(NO₃)₂·6H₂O (99.9%, FUJIFILM Wako Pure Chemical Corp.), and citric acid monohydrate.³⁶ The starting materials were dissolved in distilled water at a molar ratio of 0.9 : 0.1 : 1 : 2 and 0.8 : 0.2 : 1 : 2, were stirred with a hot stirrer until the water was fully evaporated, and the resulting precursor was calcined at 400 °C for 1 h in air, mixed in a mortar, and sintered at 925 °C for 2 h. La_0.9Sr_0.1MnO_3±δ (LSM01) was prepared in a similar way using Sr(NO₃)₂ (98%–102%, FUJIFILM Wako Pure Chemical Corp.) as the Sr source. The crystal structures of the powders and annealed powders were evaluated using X-ray diffraction (XRD; Miniflex600 or Smart Lab, Rigaku Corp.) with Cu Kα radiation at room temperature.

2.2 Characterization

For in situ X-ray absorption fine structure (XAFS) measurements, the samples were mixed with boron nitride and adjusted to a suitable concentration for the XAFS transmission method. In the experiment, the sample was placed in an electric furnace installed at the beamline, and the temperature was increased at the rate of 10 °C min⁻¹ up to 800 °C in O₂ gas. O₂ gas was continuously supplied at 60 cc min⁻¹ during the measurements. The in situ XAFS measurements of the Mn K-edges were performed at beamline BL01B1 at SPring-8 (Hyogo, Japan) using a transmission ionization chamber and Si (111) double-crystal monochromator.³⁷ The energy scale was calibrated using a Cu foil. Ag K-edge X-ray absorption near edge absorption structure (XANES) was measured using a laboratory-type spectrometer (R-XAS Looper, Rigaku Corp.) operated in transmission mode at room temperature using Ge(840) monochromator crystals.³⁸ The Mn K-edge XANES and extended X-ray absorption fine structure (EXAFS) spectra were analyzed via the IFEFFIT program code using ATHENA.³⁹ For Ag K-edge XANES, background removal and normalization of an X-ray absorption spectrum were performed using a self-made macro programmed using Igor Pro 6 (WaveMatrics, USA) by Yamamoto et al.⁴⁰

An electrolyte-supported cell was fabricated for the power generation test. Electrolyte pellets 0.3 mm thick were prepared by sintering 8YSZ (TZ-8Y, Tosoh Corp.) at 1500 °C for 10 h. The anode was prepared by mixing NiO and YSZ in a ratio of 7 : 3 wt%. The anode was screen-printed on the 8YSZ pellet and sintered at 1400 °C for 5 h. LaMnO₃ powder was used for the cathode. The electrode powders were mixed with ethylcellulose (Kishida Chemical Co., Ltd) and a thinner to form a paste. The cathode paste was also screen-printed, 80 mesh Pt current collectors were attached to the anode and cathode, and then the cell was sandwiched with Pyrex glass sealing rings and heated at 925 or 1000 °C for 1 h to sinter the cathode before the power generation tests. For the power generation tests, humidified H₂ with P(H₂O) = 2.3% gas and dry O₂ gas were introduced into the anode and cathode compartments, respectively, at a flow rate of 100 cc min⁻¹. The electrochemical properties were evaluated using an electrochemical impedance analyzer (HZ-Pro, Meiden Hokuto Denko Corp.), and a potentiostat/galvanostat (HA-151B, Meiden Hokuto Denko Corp.). The electric conductivity was measured using a standard AC four-terminal method (IM3590, Hioki Corp.). The thermoelectric power was measured following the electrical conductivity measurement by using the outer two electrodes of the four-probe conductivity bar for temperatures ranging from 300 to 900 °C in air. The temperature gradients were applied by flowing the air to one side of the sample. Electromotive force across the sample bar was measured with a nanovoltmeter (Keithley 2182A/J, Tektronix, Inc.). The Seebeck coefficient of the specimen was determined from the electromotive force slope and temperature difference after compensation for the thermoelectric power of Pt.^41,42

Powder morphology was observed by scanning electron microscopy (SEM; JSM-6510A, JEOL Ltd) and field emission-SEM (S-4700, Hitachi High-Tech Corp.). Selected area electron diffraction patterns and field emission-TEM observations (JEM-2100F, JEOL Ltd) were used to obtain high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images. Energy dispersive X-ray spectrometry (EDX) was used to determine the elemental composition of individual particles. X-ray photoelectron spectroscopy (XPS) was performed on a photoelectron spectrometer (PHI5000 Versa Probe II, ULVAC-PHI Inc.) with monochromatized Al–K radiation (1486.6 eV).

3. Results and discussion

3.1 Crystal structure, stability of the Ag dopant, and compatibility with the electrolyte

Fig. 1(a) shows XRD profiles of the LAM01 and LAM02 samples. For comparison, XRD profile of undoped LaMnO₃ is also shown. The XRD profile of LAM01 revealed peaks that were all attributed to the LaMnO₃ perovskite, which has a trigonal structure (space group: R3̄c, No. 167). The XRD profile of LAM02 was also mainly attributed to the LaMnO₃ perovskite, but a small peak was observed at around 38° that corresponded to the main peak of Ag. Rietveld refinements were conducted for LAM01 and LAM02 XRD profiles with the trigonal (R3̄c) structure (Fig. S1 and Table S1, ESI†). The lattice constants of LAM01 and LAM02 were similar, indicating that the state of the solved Ag in the LaMnO₃ structure of LAM01 and LAM02 was also similar. SEM images showed that LAM01 and LAM02 were micron-sized powders (Fig. 2). However, the LAM02 powders had nanosized particles on the surface of the micron-sized particles in Fig. 2, suggesting the Ag particles were deposited in LAM02. Hence, the solution limit of Ag in the La-site of LaMnO₃ was ∼10%.

Fig. 1 XRD profiles of (a) Ag- and undoped LaMnO₃ and (b) LAM01 powder and mixed powder of LAM01 and 8YSZ before and after annealing at 975 °C.

Fig. 2 SEM images of (a) LAM01 and (b) LAM02 pristine powders.

To examine the stability of doped Ag, LAM01 powder was annealed at 975 °C for 100 h in air (Fig. 1(b)). The XRD peaks shifted slightly to a lower angle, suggesting that lattice expansion occurred in the annealed samples, and thus that reductive expansion occurred due to the formation of oxygen vacancies. However, the XRD profiles before and after annealing treatment were the same, and the Ag main peak at 38° was not observed after annealing. In addition, the SEM image showed no Ag metal deposition and the Ag atomic ratio of the LAM01 powder showed no major change after annealing treatment (Fig. 3(a) and Fig. S2, and Table 1 and S2, ESI†). Therefore, decomposition of LAM01 or deposition of Ag was not observed after annealing at 975 °C. In addition, the compatibility of LAM01 and 8YSZ was evaluated by mixing LAM01 and 8YSZ powders at a weight ratio of 1 : 1, forming them into φ10 mm pellets, and annealing at 975 °C for 100 h in air. The XRD profiles after annealing (Fig. 1(b)) were identical to those after annealing and no additional peaks due to the reaction of LAM01 and 8YSZ were identified. In addition, Fig. 3(b) shows no Ag metal deposition after annealing. This indicates that LAM01 is compatible with 8YSZ below 975 °C.

Fig. 3 SEM images of (a) LAM01 pristine powders annealed at 975 °C for 100 h in air and (b) LAM01 and 8YSZ powders mixed at a weight ratio of 1 : 1 and annealed at 975 °C for 100 h in air.

Table 1 EDX atomic concentrations of LAM01 measured by SEM-EDX

	La (at%)	Ag (at%)	Mn (at%)
Pristine powder	48.5	3.3	48.2
Powder annealed at 975 °C for 100 h	48.6	3.5	47.9

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3.2 Electronic and local structures

The Mn K-edge XANES spectrum of LAM01 measured at room temperature, the spectra of MnO, LaMnO₃ annealed in an N₂ atmosphere, and CaMnO₃ as references for the Mn²⁺, Mn³⁺, and Mn⁴⁺ states, respectively, are shown in Fig. 4(a). The CaMnO₃ (Mn⁴⁺) reference sample has the orthorhombic perovskite structure (Pnma), and the LaMnO₃ (Mn³⁺) reference sample has the orthorhombic perovskite structure (Pbnm), in which the MnO₆ octahedron is distorted by the Jahn–Teller effect. LaMnO₃ shows a stoichiometric composition at an oxygen partial pressure of 10⁻⁴ atm, whereas it shows an oxygen-rich composition in air.⁴³ Hence, LaMnO₃ annealed in an N₂ atmosphere at 1200 °C was used as the reference spectrum for Mn³⁺. For the divalent ion reference, MnO (Mn²⁺) with a cubic structure (Fm3̄m) was used because a material with a similar local structure could not be obtained. Although the symmetry around Mn is different, the absorption edge shifts to a lower photon energy as the Mn is reduced. The absorption edge energy of LAM01 was higher than that of LaMnO₃, which suggested that the valence state of Mn was higher than the trivalent state but lower than the tetravalent state. Assuming La³⁺, Ag¹⁺, and O²⁻ states, the valence state of Mn for the stoichiometric LAM01 was Mn^3.2+. This state was consistent with the results of Mn K-edge XANES spectra. Therefore, the acceptor Ag¹⁺ dopant was charge-compensated by the oxidation of Mn, and oxygen vacancies were not formed or were negligible at room temperature. Fig. 4(b) shows the Ag K-edge XANES spectra of LAM01 together with the reference samples of Ag foil and Ag₂O. The Ag K-edge energy shifts to lower energy corresponding to the Ag⁰ → Ag⁺ oxidation state change (Fig. 4(b) inset).^44,45 The XANES spectrum of LAM01 was different from that of Ag foil, which implies that Ag ions were doped in LAM01. The different XANES spectrum compared to Ag₂O suggests that the local structure around Ag ion in LAM01 is different from that around the Ag₂O reference sample.⁴⁶

Fig. 4 (a) Ex situ Mn K-edge XANES spectra, (b) ex situ Ag K-edge XANES spectra of LAM01 at room temperature. (c) in situ Mn K-edge XANES spectra and (d) amplitude of Fourier transformed obtained from EXAFS oscillation of LAM01 from room temperature to 800 °C in air. The weighted spectra were Fourier transformed in a k space of 3 to 12 Å⁻¹.

Fig. 4(c) shows the in situ Mn K-edge XANES spectra for LAM01 in a 100% O₂ atmosphere from room temperature to 800 °C. The Mn K-edge spectra changed continuously with isosbestic points, indicating the variations of the electronic structure at elevated temperatures. Fig. 4(d) shows the amplitudes of the Fourier transforms χ(R) obtained from k³-weighted EXAFS oscillations of LAM01. The FT peak of the first coordination shell changed with temperature, suggesting a change in the MnO₆ octahedron symmetry. The decrease in the peak intensity at ∼1.5 Å suggests a decrease in the CN around the Mn ions, a distortion in the MnO₆ octahedron symmetry, or both due to O desorption at the elevated temperature. In addition, the peak shifted to a longer distance above 500 °C. It implies the reductive expansion of LAM01 at elevated temperatures. Therefore, the symmetry around Mn in LAM01 changed with increasing temperature, and the concentration of oxygen defects increased at SOFC operating temperatures of up to 800 °C.

3.3 Electrical conductivity and Seebeck coefficient

Fig. 5(a) shows the electrical conductivity, σ, measured in air. The conductivity increased with increasing temperature. The activation energy was 0.170 eV (Fig. S3, ESI†). The relative density of LAM01 was 51%; therefore, the conductivities were calibrated by density correction factors.⁴⁷ Ag doped LaMnO₃ could be a mixed conductor of ions and electrons, but the high conductivity value suggested that it was mainly an electron conductor. Fig. 5(b) shows the Seebeck coefficients measured in air. The Seebeck coefficient of Sr-doped LaMnO₃ measured in 100% O₂ by Mizusaki et al. is also plotted as the reference.⁴³ The Seebeck coefficients were positive, suggesting that the charge carrier was an electron hole. The electronic conductivity measurements suggested electric conduction and the Seebeck coefficient measurements implied that the main charge carrier was the electron–hole.

Fig. 5 (a) Electrical conductivity of Ag doped LaMnO₃ measured in air. The bulk conductivity of Ag doped LaMnO₃ was calibrated by density correction factors.⁴⁷ (b) Absolute Seebeck coefficients of Ag doped LaMnO₃ measured in air. For comparison, those of LSM measured in 100% O₂ are also shown.⁴³

3.4 Power generation characteristics

The I–V/I–P curves and impedance spectra of the SOFC cell using the LAM01 cathode, which was baked at 925 °C, are shown in Fig. 6(a). Stable power generation curves were obtained using the LAM01 cathode, indicating that LAM01 is stable and applicable as an SOFC cathode. The impedance of the cathode showed larger resistance than the anode for the measured temperatures, and cathode resistance increased as temperature decreased (Fig. 6(b)). The EDX analysis data after the power generation test showed a slight decrease in the Ag ratio (Table 2). The EDX analysis suggested that the Ag ratio did not change much after annealing at 975 °C (Table 1). Thus, this slight ratio change was attributed to the effect of the Pt current collector because Pt easily reacts with Ag and forms a Pt–Ag alloy and the cathode was baked after the Pt mesh was attached. Therefore, even though LAM01 is stable under annealing in air, when LAM01 is used as a SOFC cathode, the choice of the current collector is important because Ag is removed from the perovskite structure by alloying. In addition, the Ag-doped LaMnO₃ cathode baked at 925 °C showed delamination during the power generation test (Fig. 7). There are two possible reasons for the delamination; one is insufficient baking temperature, and the other is the large mismatch between the thermal expansion coefficients (TECs) of LAM01 and YSZ. For example, the TECs of La_0.9Sr_0.1MnO₃ and La_0.8Sr_0.2MnO₃ is 12–13 × 10⁻⁶ K⁻¹.⁴⁸ In addition, it has been reported that TECs do not largely differ depending on the dopant species at lower dopant levels.⁴⁹ Thus, we can simply estimate the TEC of LAM01 as 12 to 13 × 10⁻⁶ K⁻¹ from those of La_0.9Sr_0.1MnO₃ and La_0.8Sr_0.2MnO₃, which is very close to that of YSZ (10 × 10⁻⁶ K⁻¹).⁵⁰ Thus, it can be concluded that the main reason for the delamination is the former. To decrease the delamination, we also performed the power generation test using not only the LAM01 cathode baked at 1000 °C but also the LAM01 + YSZ composite cathode baked at 1000 °C (Fig. S4–S6, ESI†). The highest performance of about 220 mW cm⁻² was obtained at the LAM01 : YSZ ratio of 10 : 0 at 800 °C. Assuming the LAM01 cathode is used with anode support cells (ASCs) and also assuming that the electrolyte resistance of 0.95 Ω cm² is subtracted from the total cell resistance since the conductivity of YSZ is 10^−1.5 S cm⁻¹ at 800 °C,⁷ the power generation density when the ASCs used is estimated to reach about 0.6 W cm⁻². Even though the SOFC performance was improved by increasing the baking temperature, the Ag ratio decreased substantially due to alloying (Table S2 and Fig. S2, ESI†). Thus, this result suggests that the choice of cathode baking temperature and an appropriate current collector are important when LAM01 is used as a SOFC cathode. However, the SEM, HAADF-STEM, XPS, SEM-EDX, and XRD results (Fig. S7–S14, ESI†) did not show the deposition of Ag metal and decomposition of the perovskite structure despite the harsh treatment conditions. That is, the high cathodic performance was unlikely to have been achieved by Ag deposition. In addition, the LAM01 cathode cell baked at 1000 °C showed similar performance to that of the LSM01 cathode, also suggesting that LAM01 may be suitable as an SOFC cathode (Fig. S15, ESI†).

Fig. 6 (a) Power generation characteristic curves and (b) impedance profiles of the LAM01 cell baked at 925 °C.

Table 2 EDX atomic concentrations measured by SEM observations of the LAM01 cathode (baked at 925 °C) after the power generation tests

	La (at %)	Ag (at %)	Mn (at %)
LAM01 cathode (baked at 925 °C)	52.0	2.7	45.3

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Fig. 7 SEM images after the power generation tests of the LAM01 cell baked at 925 °C (cell cross section).

The present results showed that the LAM01 cathode is compatible with a YSZ-based cell. Therefore, Ag is an appropriate acceptor dopant for LaMnO₃ cathodes that showed superior compatibility with the YSZ electrolyte and stable cathode performance. From the viewpoint of practical application of SOFC, LAM01 is a promising cathode material. Further improvements via introducing a higher concentration of Ag dopant and choosing appropriate cathode fabrication conditions and an appropriate current collector would improve the cathode performance.

4. Conclusions

Ag-doped LaMnO₃ was synthesized using a sol–gel method, and its various material properties, including thermodynamic stability against YSZ, which are essential for the SOFC cathode, were investigated and discussed. For 10 mol% Ag doping, Ag was dissolved in LaMnO₃ perovskite oxide, whereas 20 mol% Ag doping resulted in the deposition of Ag particles. LAM01 and LAM01 mixed 8YSZ powders were stable at 975 °C for 100 h, showing no change in the crystal structure and confirming material stability at elevated temperatures. LAM01 is an electric conductor in which the main charge carrier is a hole. Power generation tests on 8YSZ electrolyte-supported cells using an LAM01 cathode showed stable cathode properties. Even though problems including determining the cathode baking conditions and choice of the current collector remain, the Ag acceptor is a stable and compatible element with the YSZ electrolyte and Ag-doped LaMnO₃ is a promising material for SOFC cathodes. It is expected that high cathode performance can be achieved by resolving these problems.

Data availability

The data supporting the findings of this work are available within the article and supporting information. Raw data that support the findings of this work are available from the corresponding authors (M. O. and T. S.), upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by Toyota Mobility Foundation (T. M. F.). We thank Mr Tomoyuki Ueki, Tokushima University, for the XPS measurements. We also thank Prof. Ken-ichiro Murai, Tokushima University, for the XRD measurements. X-ray absorption spectroscopy measurements were conducted at SPring-8 under proposal no. 2020A1359 and 2022B1247. This work was also partly supported by Tokushima International Science Office (TISI).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00190g

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