Investigation and optimization of high-valent Ta-doped SrFeO_3–δ as air electrode for intermediate-temperature solid oxide fuel cells

1.   Introduction

Solid oxide fuel cells (SOFCs), due to the high energy conversion efficiency and good fuel flexibility, are treated as a promising next-generation energy technology [1–7]. The commercial utilization of SOFCs has been majorly restricted by their typical working temperature (>800°C), the high operating temperature causes many practical matters such as thermal & chemical compatibility between SOFC components, sealing issues, and tardiness start-up. However, the reduced operation temperature is often accompanied by severe polarization resistance loss and sluggish oxygen reduction reaction (ORR) activity. To realise the reduced temperature operation of SOFCs, for improved components durability and materials compatibility and accelerate the start-up process, nowadays there are tremendous research efforts focus on the air electrode materials with desirable ORR activity under lower temperature range [8–10].

Cobalt-based oxides with mixed ionic and electronic conductivity (MIEC) are thoroughly investigated as air electrode materials for IT-SOFCs due to their expectative ORR activity and electrical conductivity. For example, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF), SrCo_0.8Nb_0.1Ta_0.1O_3–δ (SCNT), and Sm_0.5Sr_0.5CoO₃ (SSC) are epidemic MIEC air electrodes for IT-SOFCs. Attractively low values of electrode polarization resistance have been investigated for these cobalt-based cathodes at temperatures as low as 650°C [11–13]. However, cobalt-based perovskites usually exhibited high thermal expansion coefficients (TECs) from the transition of Co³⁺ to Co⁴⁺, and the low spin to high spin. Thermal mismatch between the air electrode and electrolyte layer could lead to electrode stratification and reduce the working life of SOFCs [14].

Recently, many efforts have been focused on overcoming this problem by developing high-performance cobalt-free perovskite oxide cathodes. As well known, SrFeO_3–δ is a typical ABO₃-type mixed-ionic electric conductor, it consists of both brownmillerite and cubic phases, thus restraining ORR kinetics and accompanied by incompatibility between cell components. A-site or B-site doped SrFeO_3–δ based perovskite oxides, La_1–xSr_xFeO_3–δ, Sr_1–xBi_xFeO_3–δ, SrNb_0.1Fe_0.9O_3–δ (SNF), SrTi_0.1Fe_0.9O_3–δ, and Pr_xSr_1–xFeO_3–δ have been investigated as potential cathode materials [15–18]. However, those cobalt-free electrodes typically have lower TECs along with a trade-off electrochemical performance, and the TEC values are still much higher than electrolytes. Therefore, it is crucial to develop air electrode materials with enhanced ORR activity and prolong thermal stability for IT-SOFCs [19–20]. In our previous work, high-valent metal cations Nb⁵⁺ was used as dopants to tailor the electroactivity of SrFeO_3–δ, and demonstrated that adding a certain amount of Nb⁵⁺ to the B-site in SrFeO_3–δ is an effective way to stabilize the cubic perovskite structure, thereby improving the ORR activity and stability. Because Nb⁵⁺ and Ta⁵⁺ are well known VB group transition metals, they have fixed +5 oxidation states and similar ionic radius, which is close to (0.64 Å). Therefore, we propose to doping Ta⁵⁺ into the B-site of SrFeO_3–δ materials as a novel cathode material for IT-SOFC.

Herein, we investigated 10mol% high-valent Ta doped SrFeO_3–δ as potential air electrode material. SrTa_0.1Fe_0.9O_3–δ (STF) shows extremely low TEC values and excellent polarization resistance. The ORR activity was further enhanced by introducing 30wt% SDC powder to form the SrTa_0.1Fe_0.9O_3–δ + Sm_0.2Ce_0.8O_1.9 (STF + SDC, 70:30 mass ratio) composite cathode. The area-specific resistance (ASR) of the STF + SDC composite cathode is 0.012, 0.019, 0.044, 0.115, and 0.233 Ω·cm² at 750, 700, 650, 600, and 550°C, respectively. The ASR at 550°C is even slightly lower than the benchmark reduction electrode material, BSCF, which is 0.24 Ω·cm² at 600°C under similar conditions.

2.   Experimental

STF and SrFeO_3–δ were synthesized via solid state method combined with high-energy ball milling. Stoichiometric amounts of SrCO₃ (AR, Aladdin), Ta₂O₅ (AR, Aladdin), and Fe₂O₃ (AR, Aladdin) were ball milled, Pulverisette 6, FRITSCH (400 r/min, 30 min) in alcohol media. The precursor was further calcinated at 1200°C for 20 h twice to achieve a highly pure STF sample.

The STF + SDC (mass ratio, 70:30) powder was premixed by high-energy ball-milling (400 r/min, 30 min). Symmetrical cells of STF |SDC| STF (or STF + SDC |SDC| STF + SDC) were fabricated by spraying STF (or STF + SDC) slurry symmetrically onto the SDC disks. The symmetrical cells with STF air electrodes were calcinated at 800, 900, and 1000°C in an ambient atmosphere for 2 h. Half cells with the configuration of SDC (~20 μm) | SDC–Ni (40:60 mass ratio) were constructed via dry pressing. The STF cathode was sprayed onto the SDC electrolyte layer and subsequently calcinated at 800°C for 2 h. The single fuel cell with STF + SDC composited cathode was fabricated using the same process.

The X-ray diffraction (XRD) of STF, SF, and STF + SDC composite oxides were conducted by Bruker AXS D8 Advance. Field emission scanning electron microscope (FE-SEM, JEOL-S4800) was employed in detecting the microstructure of the electrode and the single cell. FEI Titan G2 F30 (300 kV) was utilized for high-resolution transmission electron microscopy (HR-TEM), high-angle annular dark field scanning TEM (HAADF-STEM), and energy dispersive X-ray spectroscopy (EDS) mapping. Oxygen temperature-programmed desorption (O₂-TPD) was examined via self-building equipment, a programmed temperature furnace combined with a mass spectrometer. The room temperature oxygen nonstoichiometric (δ) in STF oxide was measured by iodometric titration. 100 mg STF powder was dissolved in HCl solution (0.6 mol·L^–1) under nitrogen atmosphere. Starch solution was added to indicate end-point achievement until an abrupt color change. Room temperature oxygen nonstoichiometric (δ) was then calculated according to the consumption of thiosulfate solution. Thermo-gravimetric analysis (TGA) of STF powder was investigated in DTA/TA (NETZSH, STA 449 F3). The oxygen nonstoichiometric between 550 and 750^oC was calculated with the following equation:

where m₀ represents starting weight, m represents actual weight, M represents STF molar mass of stoichiometric form, and δ₀ represents room temperature oxygen nonstoichiometric, calculated from the iodometric titration.

For conductivity testing, a bar-shaped STF was prepared by dry-pressing and then calcinated at 1250°C for 5 h. Four-probe DC configuration was applied for electrical conductivity (Keithley 2420 Source Meter). STF, SrFeO_3–δ, SrNb_0.1Fe_0.9O_3–δ, and SDC powders were pressed into pellets and pre-sintered. Netzsch dilatometer (DIL 402C/3/G) was exploited for TEC values of these samples from 200 to 1000°C.

ASR values of STF/STF + SDC symmetrical cells were evaluated by Solartron electrochemical workstation (1287 + 1260A). The current–voltage–power (I–V–P) performances of SOFCs were measured via Keithley 2420 SourceMeter.

3.   Results and discussion

3.1   Structure characterization

Fig. 1(a) shows XRD patterns of STF and SrFeO_3–δ oxide. At the Miller Index (1 1 0), the SrFeO_3–δ sample exhibits a small peak beside the main peak, indicating that SrFeO_3–δ perovskite consists of brownmillerite and cubic. It has been reported that the brownmillerite phase could converted to the cubic phase, when the temperature exceeds 900°C [21]. The corresponding Rietveld refinement result (Fig. 1(b)) reflects that the STF sample displayed cubic phase single perovskite (~100wt%), which has a space group of $Pm \bar 3 m$ and lattice parameter a = b = c = 3.88732(4) Å. The fitting parameters of R_wp = 9.48%, R_p = 6.58%, and χ² = 0.6576 (R_wp is weighted profile R-factor, R_p is expected R-factor, and χ is goodness of fitting) confirm the reliability of the Rietveld refinement, indicating a reliable fitting. EDS mapping images and HAADF-STEM demonstrate a homogeneous distribution of all the constituent elements of Sr, Ta, Fe, and O, as shown in Fig. 1(c)–(d). Above results revealed that 10mol% Ta⁵⁺ is successfully doped into the perovskite primitive cell. HR-TEM image and corresponding selected area electron diffraction patterns of STF (Fig. 1(e)–(f)) further confirm the cubic structure, and the lattice fringes of 0.287 nm along with the zone axes [$1\bar1\bar1$].

Fig. 1.  (a) XRD patterns of synthesized SrFeO_3–δ and STF powders, (b) Rietveld refinement profile of the STF powders at room temperature using XRD data, (c) EDS results of the STF powder, (d) HADDF-STEM image and corresponding EDS analysis of STF powder, (e–f) HR-TEM image and corresponding FFT of the STF sample along the [$\mathbf{1\overline{1}\overline{1}}$] direction, respectively (inset of Fig. 1(e) was the magnified image).

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To further investigated the comprehensive characterization of the high-valent doped SF, Fig. 2 depicts X-ray photoelectron spectra (XPS) analysis results of SrFeO_3–δ, SNF, and STF. Fig. 2(a) given the O 1s spectra of SrFeO_3–δ, SNF, and STF. The four fitted peaks listed from high to low binding energy, attributed to adsorbed H₂O, adsorbed oxygen (OH⁻/O₂), highly oxidative oxygen (${\mathrm{O}}_2^{2-}$/O⁻), and the lattice oxygen (O²⁻), respectively. Table 1 shows peak deconvolution results of O 1s spectra, the percentages of lattice oxygen of SrFeO_3–δ, SNF, and STF are 15.6%, 24.7%, and 18.6%, respectively. Fig. 2(b) and Table 2 display peak deconvolution results of Fe 2p_3/2 spectra, the average valence state of Fe elements in SrFeO_3–δ, SNF, and STF are 3.333, 3.394, and 3.280. The XPS results illustrated that there are two different charge compensation methods of the Nb⁵⁺ and Ta⁵⁺ doping in SrFeO_3–δ. It was found that there was a dramatical increase of lattice oxygen ratio of SNF, while there was a decrease of the average valence state of Fe elements of STF. Considering the similar ionic radius, high-valent, and doping level, these two charge compensation methods could be ascribed to the different Pauling electronegativity of Ta⁵⁺ (1.80) and Nb⁵⁺ (1.87).

Fig. 2.  XPS spectra of (a) O 1s and (b) Fe 2p of SrFeO_3–δ, SNF, and STF.

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Table  1.  Peak deconvolution results of O 1s spectra for SrFeO_3–δ, SNF, and STF

Sample	O2–			${\rm O}_2^{2-}$/O–			OH–/O2			H2O
	Binding energy / eV	Content / %		Binding energy / eV	Content / %		Binding energy / eV	Content / %		Binding energy / eV	Content / %
SrFeO3–δ	528.1	15.6		529.1	25.7		531.3	51.4		533.0	7.3
SNF	528.3	24.7		529.3	24.4		531.5	42.5		533.1	8.4
STF	528.3	18.6		529.4	24.9		531.6	50.9		533.1	5.6

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Table  2.  Peak deconvolution results of Fe 2p_3/2 spectra for SrFeO_3–δ, SNF, and STF

Sample	Fe4+			Fe3+			Fe2+
	Binding energy / eV	Content / %		Binding energy / eV	Content / %		Binding energy / eV	Content / %
SrFeO3–δ	712.0	46.7		710.5	39.9		709.5	13.4
SNF	712.1	56.4		710.6	26.6		709.5	17.0
STF	712.3	46.9		710.6	34.2		709.4	18.9

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3.2   Thermal-induced characterization

To study the thermal-induced characteristics of STF, electrical conductivity, O₂-TPD, TGA, iodometric titration, and thermal expansion measurements were conducted. Fig. 3(a) shows the electrical conductivity of SrFeO_3–δ, SNF, and STF. There is an inflection around 420°C, which is mainly associated with the thermal reduction of Fe⁴⁺, and is accompanied by the reduction of charge carrier at elevated temperatures. It can be seen that below 420°C, the electrical conductivity is increased with increasing temperature, whereas upon 420°C, the electrical conductivity is decreased with increasing temperature. At this temperature, the maximum conductivity achieves 21 S·cm^–1. The electrical conductivity of STF during the working temperature range from 550 to 750°C is 10–17 S·cm^–1. It is noted that the electrical conductivity of SNF and STF are smaller than SrFeO_3–δ, illustrating the introduction of high valent Nb⁵⁺ and Ta⁵⁺ into the B-site of SrFeO_3–δ restricted the electron transfer occurs through the B–O–B bond. Furthermore, the conductivity of STF is lower than SNF within the same temperature range, that is probably related to Nb⁵⁺ with higher electronegative will draw electron density from neighbouring Fe, and the average valence of Fe in SNF is higher than STF.

Fig. 3.  (a) Electrical conductivity, (b) O₂-TPD curve, (c) TGA curves, (d) oxygen nonstoichiometry (δ), (e) average valence state of iron ions (n), and (f) thermal expansion behavior of SrFeO_3–δ, SNF, STF, and SDC.

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The oxygen desorption onset temperature of STF occurred at 420°C (Fig. 3(b)), which is excellent in consistent with the inflection shown for STF electrical conductivity as a function of temperature. This result indicates that the reduction of transition metal (Fe⁴⁺ to Fe³⁺) in STF occurred above 420°C, which is higher than the onset temperature for SNF (~350°C) [16]. It is obvious that oxygen desorption peaked at 518°C, and the oxygen continued to be released even above the highest examination temperature of 930°C. When compared with Nb⁵⁺, the higher onset temperature results reveal that the doping of Ta⁵⁺ restricted the reduction of iron ions.

Fig. 3(c) given TGA curves for SrFeO_3–δ, SNF, and STF, and weight loss of these samples are 2.01wt%, 1.54wt%, and 0.86wt%, respectively. In the TGA curve for STF, the weight loss was initialized at about 420°C, which agrees with both the onset temperature of oxygen desorption and the electrical conductivity inflection. The release of oxygen from these samples along with the generation of extra oxygen vacancies. The iron ions undergo thermal reduction based on the local electrical neutrality principle. Assuming the interactions among the oxygen vacancies are negligible, the oxygen nonstoichiometry (δ) values can be taken as a measurement of the actual oxygen vacancy concentration. Partial replacement of iron ions in SrFeO_3–δ with high valence ions of Ta⁵⁺ influences the actual Fe³⁺/Fe⁴⁺ ratio and oxygen vacancy concentration. The room temperature oxygen nonstoichiometric (δ) and iron average valence states (n) of STF, SNF, and SrFeO_3–δ are listed in Table 3, as measured by iodometric titration. Compared to the δ and n of SNF, STF demonstrates higher oxygen vacancy concentration and lower average Fe valence. The room temperature δ values of SrFeO_3–δ, STF, and SNF determined by iodometric titration were 0.268, 0.246, and 0.181, respectively. It shows that the Nb⁵⁺ doping of SrFeO_3–δ could trigger a massive decrease in oxygen vacancies. Otherwise, the Ta⁵⁺ with the same high valence state exhibits completely different behaviour. The room temperature n values of SrFeO_3–δ, STF, and SNF determined by iodometric titration were 3.464, 3.486, and 3.343, respectively, which demonstrates similar trends of the XPS fitting results. These results suggested that, compared with Nb⁵⁺ doping, the charge compensation of Ta⁵⁺ doping is mainly achieved by reducing Fe oxidation state rather than sacrificing the oxygen vacancy concentration. Temperature-dependent δ and average valence states of iron ions are shown in Fig. 3(d)–(e). At the SOFC working temperatures of 450–750°C, the δ value of STF increases from 0.262 to 0.331 and, accordingly, the state of iron ions decreases from 3.307 to 3.156, suggesting a dramatic increase in the content of oxygen vacancy concentration and a relatively moderate reduction in Fe⁴⁺ at higher temperatures. Besides, the behavior of TGA and iodometric titration are consistent with the conductivity trends, the lower electronic conductivity could be attributed to the restricted charge carriers.

Table  3.  Average valence of Fe and oxygen nonstoichiometry for STF, SNF, and SrFeO_3–δ calculated from the iodometric titration

Sample	δ	n	Ref.
STF	0.246	3.343	This work
SNF	0.181	3.486	[16]
SrFeO3–δ	0.268	3.464	This work

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The TEC of the cathode material should be matched to that of the electrolyte to avoid delamination during the thermal cycle. The thermal expansion behaviour of STF was evaluated, as shown in Fig. 3(f), the TEC values at the selected temperature ranges are presented in Table 4. Compared with SrFeO_3–δ and SNF, the curve of STF shows an inflection at approximately 400°C, which is associated with the valence state change of iron ions as proved by O₂-TPD results. The average TECs at the temperature range from 200 to 1000ºC are 34.1 × 10^–6, 22.1 × 10^–6, 14.6 × 10^–6, and 13.3 × 10^–6 K^–1 for SrFeO_3–δ, SNF, STF, and SDC, respectively. The TEC of the STF sample is significantly lower and quite nearly the SDC electrolyte [21–22]. This suggests that the substitution of Ta⁵⁺ restricted the thermal reduction of iron in SrFeO_3–δ, thus enhancing thermal compatibility with the electrolyte. In addition, the TECs of STF cathode show a good match with SDC electrolyte, which is optimized for the commercial utilization of solid oxide fuel cells.

Table  4.  Average TEC values of SrFeO_3–δ, SNF, STF, and SDC at selected temperature ranges 10^–6 K^–1

Temperature / °C	SrFeO3–δ	SNF	STF	SDC
200–400	26.7	15.6	13.6	14.1
400–1000	39.6	27.5	14.7	12.3
200–1000	34.1	22.1	14.6	13.3

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3.3   Electrochemical performance

The ORR activity of STF cathode was measured by electrochemical impedance spectra (EIS) examination. To investigate the relationship between electrode sintering temperature and electrochemical performance, the STF cathode was sintered at 800, 900, and 1000°C, respectively. ASR value was calculated from the intercept between high frequency and low frequency, which stands for ORR activity, as shown in Fig. 4(a). Fig. 4(b) displays the Nyquist plots of STF electrodes at 650°C with different cathode sintering temperatures, and fitting results are listed in Table 5. The fitting lines were well matched to the experimental data. It is revealed that higher sintering temperatures resulted in the enlargement of ASR values. The ASRs of the STF cathode sintered at 800°C are 0.015, 0.028, 0.067, 0.152, and 0.376 Ω·cm² from 750 to 550°C at an interval of 50°C, which are only 28%–35% of the values obtained for the SrNb_0.1Fe_0.9O_3–δ cathode. These values achieve the lowest ASRs among cobalt-free cathodes and exceed even those of some high-performance cobalt-containing cathodes. The activation energy (E_a) for STF is 113 kJ·mol^–1, which is superior to the SrNb_0.1Fe_0.9O_3–δ cathode (132 kJ·mol^–1). This superior ORR activity of STF air electrode is mainly ascribed to the extremely high oxygen vacancy concentration.

Fig. 4.  (a) Arrhenius plots of ASR for STF sintered at 800, 900, 1000, and STF + SDC (70:30, mass ratio) sintered at 800°C, (b) EIS for symmetrical cells with the configuration of STF |SDC| STF at 650°C with different cathode sintering temperatures, (c) XRD patterns of the STF with SDC electrolyte at different sintering temperatures, (d) ASR of SrFeO_3–δ, SNF, STF, and STF + SDC at 550, 600, 650, 700, and 750°C, respectively, (e) EIS for symmetrical cells SrFeO_3–δ, SNF, STF, and STF + SDC at 650°C, respectively, and (f) DRT plots of STF and STF + SDC (70:30, mass ratio) electrode at 650°C (γ(t) represents distribution of relaxation time).

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Table  5.  Fitting parameters for different cathodes at 650°C

Sample	R1 / (Ω·cm2)	CPE1 / (F·cm2)	R2 / (Ω·cm2)	CPE2 / (F·cm2)	Rtotal / (Ω·cm2)
STF-800°C	0.023	0.95	0.043	0.87	0.065
STF-900°C	0.026	0.93	0.094	0.92	0.120
STF-1000°C	0.034	0.90	0.156	0.88	0.190
STF + SDC-800°C	0.018	0.92	0.026	0.90	0.044

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The chemical stability between the STF and SDC electrolyte was demonstrated. The STF and SDC powders were mixed with a 1:1 mass ratio via high energy ball milling at 400 r/min for 1 h, then co-fired at 800, 900, and 1000°C for 2 h, respectively. XRD measurement was applied to detect the phase composition of STF, SDC, STF + SDC-800°C, STF + SDC-900°C, and STF + SDC-1000°C. XRD results shown in Fig. 4(c) manifested there is no any detectable new phase formation, suggesting the satisfactory chemical stability between STF and SDC electrolyte.

By mixing primary material with the second phase to form composite electrode is usually used to improve the performance of air electrode. In order to further improve the ORR activity of the STF cathode, 30wt% SDC electrolyte powder was mixed with STF via high-energy ball milling. STF + SDC composite cathodes were painted onto two layers of SDC electrolyte, then calcinated at 800°C for 2 h.

The ASRs of STF + SDC (70:30, mass ratio) composite cathode, between 550 and 750°C are shown in Fig. 4(d). It was found that the introduction of SDC electrolyte material improved the ORR activity, especially at low temperatures. The ASRs of the composite cathode are 0.012, 0.019, 0.044, 0.115, and 0.233 Ω·cm² from 750 to 550°C at an interval of 50°C. The ASR of the composite cathode at 550°C is even slightly lower than for the benchmark cobalt cathode material (BSCF), which is 0.24 Ω·cm² at the same temperature [20]. The Arrhenius plot in Fig. 4(a) demonstrates that the activation energy (E_a) for the STF + SDC (70:30, mass ratio) composite cathode is 108 kJ·mol^–1, which is better than the pure STF cathode (113 kJ·mol^–1) and SrNb_0.1Fe_0.9O_3–δ cathode (132 kJ·mol^–1). Fig. 4(e) displays the Nyquist plots of pure SF, SNF, STF electrode, and STF + SDC composite electrode at 650°C. The high ORR activity of the composite cathode may be due to the SDC electrolyte material extending the active oxygen reduction sites by increasing oxygen ionic conduction, therefore greatly reducing the cathode polarisation at lower working temperatures. The oxygen reduction reaction process of STF and STF + SDC electrodes was analysed by the distribution of relaxation time (DRT) technique. Based on the electrochemical impedance spectroscopy, DRT curves can be categorized as high-frequency (HF) portion (>10⁴ Hz) which corresponds to charge transfer at triple phase boundaries (TPBs), the intermediate frequency (IF) portion (10–10⁴ Hz) related to the surface exchange and ion diffusion in the air electrode, and low-frequency (LF) portion (10^–2–10 Hz), associated with the gas diffusion [23]. Fig. 4(f) shows that HF and IF sections of resistances are significantly reduced through SDC introduction. The above result suggested that the addition of SDC electrolyte material efficiently improves surface exchange and bulk diffusion of oxygen ion, transformation in surface, and extension length of TPBs enhance the charge transfer between STF and SDC electrolyte.

Fig. 5(a) shows I–V–P plots of the single fuel cell STF |SDC (~20 μm)| SDC–Ni (40:60, mass ratio), and pure hydrogen was taken as fuel at a flow rate of 60 mL·min^–1. The peak power densities (PPDs) of fuel cells with STF cathode are 891, 749, 511, 285, and 136 mW·cm⁻² from 650 to 450°C at an interval of 50°C. Open circuit voltages (OCVs) at recorded temperatures all exceed 0.8 V, represents that the tested SOFCs were intact and well-sealed. I–V–P properties of composite electrode were investigated under the cell structure STF + SDC (70:30, mass ratio) |SDC (~20 μm)| SDC–Ni (40:60, mass ratio). Fig. 5(b) shows the PPDs of SOFC with the STF + SDC composite cathode are 1117, 907, 635, 400, and 212 mW·cm⁻² from 650 to 450°C at an interval of 50°C. The higher cell performance achieved by the 30wt% SDC composite cathode might be due to the favorable electrochemical characteristics, resulting from the optimized TPBs. Fig. 5(c)–(d) shows cross-section SEM images of the tested cell with pure STF and STF + SDC composite electrode, respectively. The SDC electrolytes have similar thickness, around 20 μm. The interfacial contacts between air electrode and electrolyte are satisfactory. It can be seen that STF electrode and STF + SDC composite electrode layers are tightly attached to SDC electrolyte, with no any detectable delimitations, verified the thermal compatibility.

Fig. 5.  I–V–P plots and SEM images of SOFCs with configurations: (a, c) Ni + SDC |SDC| STF; (b, d) Ni + SDC |SDC| STF + SDC.

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4.   Conclusion

In summary, STF cubic phase single perovskite oxide was synthesized via a solid-state method assisted with high-energy ball milling. The charge compensation of Ta⁵⁺ doping is mainly achieved by increasing the Fe³⁺/Fe⁴⁺ ratio rather than sacrificing the oxygen vacancy concentration. The average TEC of STF was 14.6 × 10^–6 K^–1 between 200 and 1000°C, which is quite near that of SDC electrolyte. STF cathode reached relatively low ASR of 0.015, 0.028, 0.067, 0.152, and 0.376 Ω·cm² at 750, 700, 650, 600, and 550°C, respectively. With SDC as electrolyte and STF as cathode, the fuel cell provided PPDs of 891, 745, 510, 283, and 134 mW·cm^–2 at 650, 600, 550, 500, and 450°C, respectively. To further improve the ORR activity of the STF cathode, 30wt% SDC powder was introduced, and the ASRs of STF + SDC composite cathode were 0.012, 0.019, 0.044, 0.115, and 0.233 Ω·cm² at 750, 700, 650, 600, and 550°C, respectively. By introducing 30wt% SDC, the PPD of fuel cells with STF and STF + SDC composite cathode increased from 891 to 1117 mW·cm^–2 at 650°C.