Heterogeneous Composite Fibrous Cathode Undergoing Emphasized Active Oxygen Dissociation for La(Sr)Ga(Mg)O3‐Based High‐Performed Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are of great importance in terms of reducing operational costs and benefiting from system durability. However, SOFCs exhibit sluggish oxygen reduction reactions (ORRs) at cathodes. To overcome this, extensive research is focused on the development of cathodes with superior ORR properties for SOFCs. In this study, a fibrous composite cathode composed of strontium‐doped samarium cobaltate (SSC) and samarium‐doped ceria (SDC) is fabricated via electrospinning. Composite fiber cathodes prepared at specific ratios exhibit ORR properties and oxygen‐ion conductivity, which are attributed to the growth of small SSC particles in the fibrous electrode and nanolattice strain of SSC at the SSC and SDC interface. As such, the enhanced cathode performance contributes to an improvement in the power‐generation characteristics of SOFCs (1.33 W cm−2 at 1073 K using 300 μm thick La(Sr)Ga(Mg)O3 electrolyte). This structural engineering of dual‐phase fiber is a promising design that promotes cathodic ORR performance.


Introduction
Solid oxide fuel cells (SOFCs) are promising energy conversion systems because of their high energy efficiency and fuel flexibility. [1,2]The field of these SOFCs primarily focuses on the catalytic activity of the electrodes.Particularly, the development of high-performance cathodes is essential in SOFCs due to the sluggish electrochemical reaction occurring at the cathode.This aspect constitutes the largest portion of energy loss within the SOFC system. [3]11][12][13][14][15][16][17][18][19][20][21] Conventional composite cathodes are typically manufactured by mixing catalytic electrode materials and oxygen-ion conductors.Various methods such as infiltration, [15][16][17] mechanical mixing, [10,11] chemical vapor deposition, [9] spray pyrolysis, [18,19] and pulsed laser deposition [12][13][14] have been adopted to fabricate composite cathodes.Notably, Ishihara et al. introduced the formation of a nanogradient at the interface of samarium-doped ceria (SDC, Sm 0.2 Ce 0.8 O 2Àδ ) and strontium-doped samarium cobaltate (SSC, Sm 0.5 Sr 0.5 CoO 3Àδ ) via pulsed laser deposition. [13,14]The cobalt and samarium atoms are located at the interfaces in SSC and SDC columns, respectively, and a SmCoO 3Àδ nanogradient is formed at the interface between the SDC and SSC columns.Furthermore, tensile strain in the SDC column and compressive strain in the SSC column were observed at the interface.These atomic locations and lattice changes improved the power-generation properties of the SOFCs by increasing the oxygen-ion conductivity of the composite cathode.However, it is difficult to form a well-developed interface between cathode materials and oxygen-ion-conductor electrolyte materials in bulk composite electrodes.
Electrospinning is a technique in which voltage is applied to overcome the surface tension of a solution and form an electromagnetic field.The nanofiber morphology fabricated by electrospinning is largely dependent on the viscosity of the solution, applied electric field strength, and distance between a syringe and collector.[24][25][26] Many research groups have studied the preparation of metal oxide composite nanofibers via electrospinning to apply a solution containing precursors to SOFCs. Lee et al. prepared a La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3Àδ -Gd 0.1 Ce 0.9 O 1.95 (LSCF-GDC)-nanofiber-based cathode and reported that a cell using the composite cathode exhibited a power density of %1 W cm À2 at 923 K with hydrogen fuel. [25]The cell using this composite cathode could produce two times more power than a cell using the conventional LSCF-GDC cathode because of the higher number of active sites and the lower-mass-transfer resistance in the composite cathode.Kim et al. prepared GDC-embedded Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àδ nanofibers with an enhanced phase boundary and used them as a cathode; the corresponding cell exhibited increased power generation. [26]any researchers have improved the performance of SOFCs by manufacturing fibrous composite anodes via electrospinning.However, studies on the appropriate mixing ratio of the MIEC and oxygen-ion conductor used in the composite electrode and the change in cell performance according to the mixing ratio have not yet been conducted.Therefore, in this study, changes in the shape and electrochemical properties of composite fibers were observed by varying the mixing ratio of SSC (which is an MIEC) to SDC (which is an oxygen-ion conductor).The effects of these changes on the power-generation characteristics of SOFCs were also studied.

Results and Discussion
We utilized electrospinning to fabricate continuous fibrous materials with varying SSC and SDC contents.Figure 1a illustrates the electrospinning process and electrospun composite fibers with different SDC contents.As depicted in Figure 1b, these composite fibers are designed to form a well-integrated dual lattice structure, wherein strain is generated at the interface between SSC and SDC. [27,28]At the interface of this dual-phase composite cathode, the gradient lattice combine is expected to enhance oxygen reduction reaction (ORR) kinetics due to the effect of lattice strain [29,30] (Figure 1c).
Figure 2 shows the scanning electron microscopy (SEM) images of the SSC, SDC, and composite fibers with different SSC:SDC ratios sintered at 1223 K in air.All nanofibers fabricated through the electrospinning process showed a solidtype structure, with diameters in the range of 237-284 nm.
Furthermore, the surface of the composite fibers became rougher with increasing SDC content.The SSC fibers exhibited a smooth surface morphology with large grains, as shown in Figure 2a, whereas the SDC-containing composite fibers exhibited a slightly rough surface morphology with small, well-developed grains (Figure 2b-d).The surface of the SDC fibers (Figure 2e) was composed of smaller particles than that of the SSCSDC composite fibers.The small particle size is attributed to the growth inhibition of the SDC and SSC particles during the formation of the SSCSDC composite fibers.These characteristics emerged because of the different sintering characteristics of the perovskite oxide using cobalt for the B-site and doped ceria.33] In contrast, owing to the high melting temperature of SDC (2673 K), particle agglomeration does not occur at the low sintering temperature used in this experiment. [34]-ray diffraction (XRD) of the composite fibers synthesized at 1223 K displayed the characteristic diffraction peaks of both the SSC and SDC phases, as shown in Figure 3a,b.The hkl planes of each peak and the corresponding 2θ values are summarized in Table S1, Supporting Information.The SSC fibers exhibited diffraction peaks corresponding to perovskite oxide (ICDD card# 00-053-0112), and the diffraction peaks in the XRD pattern of the SDC fibers were assigned to the fluorite structure (ICCD card# 01-080-5538).The XRD patterns of the composite fibers exhibited distinct peaks corresponding to both SSC and SDC phases.There are no serious impurity peaks suggests that no impurity reactions occurred between SSC and SDC during the electrospinning process, confirming the dual-phase cathode composite fiber.Based on these results, the intensity of the peak ascribed to each phase was found to depend on the SSC:SDC ratio in the composite fibers.Furthermore, a slight shift in SSC toward lower angles and SDC toward higher angles with varying SDC content, implying a change in the lattice parameter.Therefore, XRD Rietveld refinement was conducted to clarify these composite structures; the results are summarized in Table S2, Supporting Information, and shown in Figure S2, Supporting Information.In Figure 3c,d, the lattice parameter of cubic SDC increased with increasing SDC content of the composite fiber.In addition, with the introduction of SDC into the composite fibers, the orthorhombic SSC phase showed an increase in lattice parameter b.However, as the SDC content increased, lattice parameter b remained unchanged, whereas a and c increased.These variations in lattice parameters impart tensile and compressive strains (Figure 1c), which cause changes in the electrochemical properties of the composite structures.[14] Furthermore, transmission electron microscopy (TEM) was performed to determine the microstructures and lattice characteristics of the SSC, SDC, and SSCSDC composite fibers.microstructures of the SSC and SDC fibers.Detailed high-resolution TEM (HR-TEM) and fast Fourier transform images were obtained to identify the crystal structures of the fibers.In Figure 4e-h, the selected area electron diffraction patterns correspond to those assigned to the orthorhombic and cubic SDC structures, which is in agreement with the aforementioned XRD results.As shown in Figure 4i, the SSCSDC composite fiber was different from the existing core-cell structure, and SDC nanocrystals with sizes of 40-100 nm were observed on the surface of the SSC fiber.The microstructure was further confirmed by the high-angle annular dark-field (HAADF)-scanning TEM (STEM) and EDS results, as shown in Figure 4l.The nonuniform distribution of cerium and cobalt indicates that SDC and SSC formed two separate phases.This is probably due to the difference in the melting points of the two phases during sintering.Moreover, the lattice spacings were 0.268 nm for SSC and 0.196 nm for SDC, which correspond respectively to the (121) and (220) planes in the SSCSDC composite fiber (Figure 4j-k).Interestingly, the lattice spacing of the (121) plane in a single SSC fiber (d 121 = 0.266 nm) was slightly lower than that in the SSCSDC fiber, whereas there was no change in the SDC phase, which was almost consistent with the Rietveld refinement results of this study.The lattice expansion of the SSC phase may play a role in increasing the diffusivity of oxygen ions in the SSCSDC7030 composite fibers.[14] Therefore, the SSCSDC composite fibers are expected to enhance the ORR kinetics on the cathode in SOFCs.

High-resolution TEM images in
Figure 5 shows the thermogravimetric analysis (TGA) results of SSC, SDC, and SSCSDC composite fibers in air and N 2 atmospheres to evaluate the oxygen release capacity (ORC) and number of generated oxygen vacancies. [35]Additionally, the mass change in the TGA results could be estimated by the oxygen nonstoichiometry (δ) of materials.The estimation of the δ value by temperature can be expressed as ORC as a function of time (t).
In the TGA curves in Figure 5a,b, the SSC fiber and SSCSDC7030 exhibited considerably higher weight loss than the other SSCSDC composite and SDC fibers in the final stage, indicating that more oxygen was released from the lattice.Furthermore, as the SDC content of the SSCSDC composite fiber increased, the weight loss decreased because of the lower ORC of SDC.In contrast, in Figure 5c,d, the SSCSDC7030 fiber exhibited superior ORC, and even the SSCSDC5050 fiber showed a higher ORC than the SSC fibers in a N 2 atmosphere with a low oxygen partial pressure.This trend can be explained by the variation in the unit cell volume owing to the lattice parameters of the SSC or SDC phase in the composite fibers.Furthermore, excluding SDC, the specific surface areas of the SSC and SSCSDC composite fibers were similar, allowing for a comprehensive comparison of the ORCs and electrocatalytic activities on their corresponding surfaces (Supporting Information is found in Figure S4).Therefore, the optimal SSCSDC composite fiber is expected to have improved ORR performance owing to its disordered oxygen vacancies.
Electrochemical impedance spectroscopy (EIS) was performed on La(Sr)Ga(Mg)O 3 (LSGM)-supported symmetrical cells using the SSC, SDC, and SSCSDC composite fiber electrodes to estimate their ORR performances (Figure S5 and S7, Supporting Information).Figure 6a and S7a, Supporting information, present the Arrhenius plots and calculated activation energy (E a ) of R p for these fibrous composite electrodes.Among the four electrodes, the SSCSDC7030 fiber exhibited the lowest R p at all temperatures (1073-923 K).Moreover, E a of the SSC, SSCSDC7030, SSCSDC5050, SSCSDC3070, and SDC fibers were calculated to be 124, 113, 114, 113, and 129 kJ mol À1 , respectively.The E a of the SSCSDC composite fibers was lower than that of the SSC fiber, demonstrating their high potential as fibrous composite cathodes.To obtain a deeper understanding of the ORR, distribution of relaxation time (DRT) analysis of the EIS results at 1023 K was conducted (Figure 6b and S7c, Supporting information). [36,37]Each plot is composed of three distinct peaks, indicating three rate-determining steps in the electrode reaction: high (P1), intermediate (P2), and low frequency (P3).According to the literature, the P1 process is likely associated with transportation processes of O 2À at the cathode/LSGM electrolyte interface. [38]he P2 is related to the surface exchange kinetics, especially the charge-transfer process on the electrode surface. [39]In addition, P3 is likely associated with the oxygen adsorption and dissociation processes of the gaseous reaction. [40]The introduction of the SDC phase optimized the resistance of the charge transfer at the cathode/LSGM electrolyte interface.Since the SSC electrode characterized as an MIEC, the resistance value related to the P2 in Figure 6b was anticipated to demonstrate the lowest resistance owing to its enhanced surface reactivity on the electrode.The resulting value of intensity related to the P2 in Figure 6b is the lowest.Conversely, the P3 process emerges as the predominant factor shaping the overall ORR reaction.Within this context, the SSCSDC7030 fiber stands out with the lowest resistance, primarily due to its robust ORC, which expedites the processes of oxygen absorption and dissociation.To verify whether the enhanced performance of SSCSDC7030 is associated with the surface oxygen exchange kinetics within the lattice, the dependence of R p on the partial pressure of oxygen (P O 2 ) was investigated (Figure 6c).The specific electrochemical reaction of the ORR was described as R p at different P O 2 expressed by the following formula.
where k is a constant.The n value provides information concerning the specific process of the ORR step. [41,42]2,ðgÞ ↔ O 2,ads ; n ¼ 1 O 2;ads ↔ 2O ads ; n ¼ 0.5 where O X O ðsÞ and O o : ðsÞ the lattice oxygen atoms adjacent to the oxygen vacancies on the surface and V öðsÞ is the lattice oxygen.The n value of the SSC fiber electrode was 0.385, and the n value decreased with an increase in the SDC content of the SSCSDC composite fiber, as shown in Figure 6d and S7b, Supporting Information.The n value of the SSCSDC3070 fiber decreased the most to 0.307, indicating lower ORR performance than that of the SSC fiber.For the SSCSDC7030 and SSCSDC5050 fibers, the estimated n values were similar (SSCSDC7030; n = 0.333, SSCSDC5050; n = 0.332, SDC; n = 0.517).This modification resulted in a higher ORR activity than that of the SSC fiber.These results suggest that the ORR kinetics were accelerated through the optimized strains and ratio of each phase in the SSCSDC composite fibers.
To confirm the effects of the composite fiber cathode on the power-generation properties of the SOFC, single cells using the 300 μm thick LSGM electrolyte were prepared with cathodes of different SSC:SDC ratios.Figure 7a shows the power-generation properties of these cells at intermediate temperatures.At all operating temperatures, the cell using the SSCSDC7030 composite nanofiber cathode exhibited the highest maximum power density (MPD).In Figure 7b and S8, Supporting Information, the fibrous cathodes achieved improved cell performance in relation to the SSCSDC7030 powder using the 300 μm thick LSGM electrolyte.The MPD of the cell using the SSCSDC7030 composite fiber electrode at 1073 K was 1.33 W cm À2 -approximately 177% higher than that using the SSCSDC7030 powder electrode (0.48 W cm À2 ).In addition, at 923 K, the cell using the SSCSDC7030 electrode exhibited an MPD (0.34 W cm À2 ) that was approximately 240% higher than that using the SSCSDC 7030 powder electrode (0.10 W cm À2 ).These results suggest that the structural properties of our fiber electrodes improve ORR efficiency, thereby improving the cell performance.Figure 7e shows the EIS results at 923 K for a single cell using the SSCSDC7030 and SSC fiber electrodes.The Ohmic resistance showed values similar to those for the 300 μm thick LSGM electrolyte in the full cell.However, the R p value of the SSCSDC7030 fiber was lower than that of the SSC fiber.As shown in the Bode plots, the SSCSDC7030 fiber exhibited a drastic decrease in the middle frequency (10 3 -10 1 This may be due to the enhancement of ORR kinetics and the change in the lattice structure at the hetero-interface between the SSC and SDC phases in the SSCSDC7030 composite fiber, as confirmed by TEM and XRD. [43]However, from the XRD patterns and Rietveld refinement results shown in Figure 3, it can be predicted that lattice parameter changes also occur in the SSCSDC5050 and SSCSDC3070 composite electrode fibers.Therefore, the lower performance of the cells using the SSCSDC5050 or SSCSDC3070 composite fiber than those using the SSCSDC7030 composite fiber and even using the SSC electrode cannot be explained only by a change in the lattice parameter at the interface between SSC and SDC.This relatively low SOFC performance can be attributed to two factors.Considering the reaction mechanism of the MIEC, oxygen molecules are adsorbed on the surface of the electrode and the three-phase boundary and are reduced to oxygen ions by reacting with electrons.Therefore, for the ORR to occur smoothly at the cathode, an adequately large reaction area and sufficient electron-transport channels are required.The TEM images in Figure 4 show that small SDC particles were generated around SSC in the composite fibers.The distribution of the SDC particles can help improve the cell performance by improving the oxygen-ion conductivity if the SDC particles are appropriately arranged.The pathway for electrons to participate in the reaction is blocked, resulting in a decrease in ORR activity.In composite fibers containing high proportions of SDC, the decrease in ORR activity significantly increased with temperature change. [44]ased on these results, the ORR performance of the composite electrode is enhanced with the large active surface area owing to relatively small SSC particles as well as the change in the lattice structure formed at the interface between the SSC and SDC particles constituting the composite electrode.However, as the SDC content of the composite electrode increased, the cell performance degraded owing to a decrease in electrical conductivity and active surface area.[47][48][49][50][51] The outstanding cell performance achieved in this study, surpassing that of previously reported 300 μm thick LSGM-based cells, can be attributed to the unique fiberstructured electrodes and enhanced ORR resulting from appropriate lattice changes.

Conclusion
In this study, a composite fiber electrode composed of SSC and SDC was fabricated via electrospinning.The prepared composite fibers contained different ratios of SSC and SDC, depending on the composition of the mixed solution used for spinning.The TEM, XRD, and Rietveld refinement results confirmed that these SSCSDC composite fibers formed extremely uniformed fibrous structures with well blended between SDC and SSC.It might have an optimal heterointerface according to their material ratios, which may cause lattice strains to enhance ORR activity in the cathode.The composite fiber cathode with the optimal ratio exhibited higher ORR activity than the SSC fiber electrode.However, as the SDC content of the composite electrode increased, the active reaction area and electrical conductivity decreased.Enhanced oxygen-ion conductivity in the composite electrode resulted from the well-formed three-phase boundary and the lattice strain at the interface between the SSC and SDC phases.Overall, the results suggest that optimizing the composition and structure of the SSCSDC composite fibers can effectively enhance their electrochemical performance for the ORR.This dual-phase composite fiber is a promising design that promotes cathodic ORR.

Experimental Section
Preparation of Fibrous Composite Cathode: The SSCSDC composite fibers were prepared through the simple electrospinning technique by using a polyvinylpyrrolidone (PVP; MW = %1 300 000 g mol À1 , Sigma-Aldrich) solution containing metal precursors.For the solution, a cosolvent was first prepared by mixing ethanol, water, and acetic acid in a volumetric ratio of 1:1:3.Sm(NO 3 ) 3 •6H 2 O (Alfa-Aesar, 99.9%), Sr(NO 3 ) 2 (Samchun Chemical, 98%), and Co(NO 3 ) 2 •6H 2 O (Samchun Chemical, 97%) were used as precursors and dissolved in the cosolvent in a molar ratio of 0.5:0.5:1 to prepare a 0.1 M SSC stock solution.Sm(NO 3 ) 3 •6H 2 O (Alfa-Aesar, 99.9%) and Ce(NO 3 ) 3 •6H 2 O (Sigma-Aldrich, 99.9%) were dissolved in a molar ratio of 0.2:0.8 to prepare a 0.1 M stock solution of SDC.The SSC and SDC stock solutions were mixed at mass ratios of 0:100, 30:70, 50:50, 70:30, and 100:0.Next, 10 wt% PVP was dissolved in an oil bath at 333 K.The prepared solution was placed in a 10 mL syringe with a capillary tip (d = 0.5 mm).A variable high-voltage power supply (Korea Switching Co.) was used for electrospinning.The anode of the high-voltage power supply was clamped to a syringe needle tip, and the cathode was connected to an aluminum foil collector.The applied voltage was 20 kV, distance between the nozzle and collector was 15 cm, and solution supply rate was 1 mL h À1 .After electrospinning, the composite nanofibers were calcined at 1223 K in air for 3 h.The samples were denoted as SSCSDC7030, SSCSDC5050, and SSCSDC3070 according to the mixing ratio of the SSC and SDC precursors.
Fabrication of LSGM-Based Cells: An anode material, nickel-iron powder was synthesized using the traditional wet impregnation method.Fe(NO 3 ) 3 •9H 2 O (Wako Co., 99%) was dissolved in distilled water after which nickel(II) oxide (Wako Co., 99%) was added to the solution (the molar ratio of nickel to iron in this solution was controlled at 9:1).The aqueous solution was evaporated at 353 K under continuous stirring until gelation.The powder obtained after gelation was subjected to combustion at 673 K for 2 h in air to decompose organic compounds.The resulting powder was sintered at 1223 K in air for 6 h.
The LSGM powder was prepared by the conventional solid-state reaction.Stoichiometric amounts of La 2 O 3 (Sigma-Aldrich, 99.99%), SrCO 3 (Sigma-Aldrich, 99.99%), Ga 2 O 3 (Sigma-Aldrich, 99.99%), and MgO (Sigma-Aldrich, 99.9%) powders were ball milled in ethanol for 24 h.After drying, the mixture was calcined at 1273 K for 6 h in air (heating and cooling rates of 3.3 K min À1 ).Subsequently, the LSGM powder was pressed at 25 MPa and sintered at 1773 K in air for 5 h.Finally, the thickness of the LSGM electrolyte was adjusted to approximately 300 μm by polishing.Symmetric cells were fabricated using LSGM electrolyte pellets with a thickness of 300 μm.The SSC and SSCSDC composite nanofiber cathode materials were screen-printed to achieve a 5 mm diameter on the LSGM electrolyte and calcined at 1273 K for 1 h.The single SOFC cells were prepared via screen printing using a nickel-iron anode and composite cathode.The nickel-iron anode materials were screen-printed to achieve a 5 mm diameter on the LSGM electrolyte and calcined at 1373 K.The SSC and SSCSDC composite nanofiber cathode materials were screen-printed to achieve a 5 mm diameter on the LSGM electrolyte and calcined at 1273 K for 1 h.
Characterization of Fibrous Composite Cathodes and Their Electrochemical Properties: The crystalline structures of the samples and SSC and SDC were analyzed through XRD (MiniFlex600, Rigaku) conducted with Cu K α radiation.In addition, the lattice parameter from XRD spectra was calculated by Rietveld refinement by GSAS software.Field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi) was performed to examine the sample morphology and cross sections of single SOFC cells.The morphologies and lattice structures of the electrodes were analyzed through TEM (Titan G2 60-300, FEI).TGA (STA8122, Rigaku) was employed to evaluate the ORC and the number of generated oxygen vacancies in SSC, SDC, and the SSCSDC composite fibers.The estimated generation of oxygen vacancies (Δδ) was calculated as follows. [52] ¼ M i 15.999 1 À m m i (7)   where M i is the molecular weight of each fibrous material; m_i and m are the initial and final weights of the sample upon heating, respectively; and 15.999 g mol À1 is the atomic weight of oxygen.The specific surface areas were measured using the Brunauer-Emmett-Teller nitrogen adsorptiondesorption method (BELSORP-mini II, Microtrac BEL) at 77 K. EIS of the symmetric cells was performed in ambient air over an operating temperature range of 923-1073 K at intervals of 50 K.EIS was performed using an impedance analyzer (SP-300, Bio-Logic) in the frequency range of 0.1-7 MHz at an AC amplitude of 30 mV and 6 points per decade.The DRT (MATLAB) was analyzed through EIS to effective determine the performance of the electrode.The single SOFCs were tested with 3% humidified hydrogen (100 sccm) and oxygen (100 sccm) as the fuel and oxidant, respectively.The current-voltage (I-V ) characteristics of the SOFCs were recorded at 923, 973, 1023, and 1073 K using a potentiostat (Bio-Logic SP-300).After the I-V performance test, EIS was conducted under open-circuit voltage conditions, with an AC voltage amplitude of 30 mV in the frequency range of 0.01-7 MHz and 6 points per decade.

Figure 1 .
Figure 1.a) Schematic of fabrication of strontium-doped samarium cobaltate and samarium-doped ceria (SSCSDC) composite fiber using electrospinning.b) The schematic illustration of oxygen reduction reaction (ORR) of these composite fibers and c) dual phase of the unit cell using its structure.
Figure 4a-d show the

Figure 2 .
Figure 2. a) Scanning electron microscopy image of the SSC fiber and SSCSDC composite fibers with varying SDC contents: b) SSCSDC7030, c) SSCSDC5050, d) SSCSDC3070, and e) SDC fibers.f ) Schematic of the SSCSDC composite fiber fabricated by electrospinning.

Figure 3 .
Figure 3. a) X-ray diffraction (XRD) patterns of the SSCSDC composite fiber with varying content of SDC.b) The enlarged XRD patterns with diffraction angles of 31°-36°and 57°-62°.The Rietveld refinement from XRD data of b) SSC and c) SSCSDC7030 fiber.Refined lattice parameters of the d) cubic SDC and e) orthorhombic SSC phase in the composite fiber.f ) Cell volume of SSC and SDC phase in the composite fiber with varying content of SDC.

Figure 4 .
Figure 4. High-resolution transmission electron microscopy (HR-TEM) images of the nanofibers and enlarged view of the edge of the fibers: a,b) SSC and c,d) SDC fibers.The corresponding e,g) fast Fourier transform plots and f,h) lattice spacing are shown (red box: SSC parts, blue box: SDC parts).i) HR-TEM images of the SSCSDC composite fiber and j,k) corresponding lattice spacing of the SSC and SDC phases.l) HAADF-STEM images and elemental concentration profiles of strontium, samarium, cerium, and cobalt of the SSCSDC composite fiber.

Figure 5 .
Figure 5. Thermogravimetric analysis (TGA) curves of the SSC, SDC, and SSCSDC composite fibers in a) air and c) N 2 .Estimated oxygen nonstoichiometry (δ) of the SSC, SDC, and SSCSDC composite fibers by calculation based on the TGA results at 473-1073 K in b) air and d) N 2 .

Figure 6 .
Figure 6.a) Arrhenius plot of polarization resistance (R p ) from the impedance spectra of La(Sr)Ga(Mg)O 3 (LSGM)-supported symmetrical cells with SSC and SSCSDC composite fiber electrodes in air; b) Distribution of relaxation time plots of the SSC and SSCSDC composite fiber electrodes at 1023 K; c) Electrochemical impedance spectroscopy (EIS) curves at various oxygen partial pressure for the SSCSDC7030 fiber at 1023 K; d) Dependence of R p of the SSC and SSCSDC composite fibers on oxygen partial pressure (P O 2 ) at 1023 K.

Figure 7 .
Figure 7. a) Maximum power densities (MPDs) of the cell using the SSCSDC composite fibers with varying SDC content.b) Current-voltage (I-V ) curves of the cell with the SSC fiber, SSCSDC7030 fiber, SSCSDC7030 powder with 300 μm thickness at 1073 K. c) EIS results of the SSC fiber and SSCSDC7030 electrode at 923 K (insert: Bode plots of the SSC and SSCSDC7030 fiber electrode), and d) comparison of MPDs with various SOFC cells based on the 300 μm thick LSGM electrolyte.