Design of nanofiber-based electrodes for solid oxide electrochemical cells with high performance and stability

Abstract

We demonstrated a La_0.6Sr_0.4CoO_3−d (LSC) nanofiber-based electrode for solid oxide electrochemical cells operating at intermediate temperatures. A thin powder layer deposited at the interface between the nanofiber layer and electrolyte significantly enhanced the adhesion strength, facilitating operation of a porous and hollow nanofiber structure with a high specific surface area and high concentration of oxygen vacancies at a low sintering temperature. The optimized nanofiber-based single cell achieved a significantly improved peak power density of 1 W cm⁻² in fuel-cell mode and current density of 0.79 A cm⁻² at 1.3 V under 50% H₂–50% steam conditions in electrolysis-cell mode at 600 °C with excellent thermal stability under static and reversible cyclic operations. These results demonstrated the feasibility of the nanofiber-based electrode in achieving high performance and stability in solid oxide electrochemical cells operating at intermediate temperatures.

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Introduction

Solid oxide electrochemical cells are efficient and environmentally friendly energy conversion devices^1–3 that can operate as both fuel cells, generating electricity through a direct conversion of various chemical fuels, and steam-electrolysis cells, producing chemical fuels from electrical energy at a high efficiency. However, their high operating temperature (>800 °C) for producing electricity and fuel limits their widespread industrial utilization.^2,4 In particular, such high operational temperature leads to thermal deterioration and performance degradation in cells and systems, impeding further commercial viability of solid oxide electrochemical cells.^4–6 Therefore, extensive efforts have been dedicated to reducing the operating temperature to intermediate values of less than 700 °C.⁷

Decreasing the operating temperature of solid oxide electrochemical cells presents several challenges; the most critical one is the substantial reduction in the electrochemical performance due to the exponential increase in the polarization resistance (R_p) with reduced temperatures.^8–10 In particular, the air electrode, where the oxygen reduction reaction (ORR) in fuel-cell (FC) mode and the oxygen evolution reaction (OER) in electrolysis-cell (EC) mode occur, constitutes the predominant portion of the total resistance. Therefore, the most effective approach to enhance the electrochemical performance at reduced operating temperatures is to mitigate the resistance associated with the air electrode.^9,11 This can be achieved by improving the catalytic activity and increasing the reaction sites by modulating key reaction factors in the ORR and OER, such as developing catalytically active materials,^11,12 controlling the concentration and distribution of charged defects,^9,13 and engineering microstructures at and near the interface between the air electrode and electrolyte for the efficient gas diffusion of reactants and products.^14–17

Nanofiber-based electrodes have been extensively investigated owing to their unique advantages that can effectively reduce the R_p associated with the air electrode. Precise control over the structural characteristics of nanofibers facilitates the fabrication of desired porous structures with a high specific surface area, promoting efficient gas diffusion and extending reaction sites.^18–20 A lower phase formation temperature of nanofibers fabricated from a precursor solution compared to conventional powders fabricated from the solid-state reaction processes facilitates sintering at lower temperatures, resulting in a high density of grain boundaries.^18,21 These grain boundaries have been reported as active reaction sites for the ORR and OER owing to the facile formation of oxygen vacancies.^22–24 Moreover, improvements in the adsorption and desorption reactions of surface oxygen due to the formation of oxygen vacancies have been reported.^25,26

Despite the advantages of nanofibers and their network structures for the electrodes of solid oxide electrochemical cells, their utilization to significantly improve the electrochemical performance remains difficult, with only a few examples.^18,20,21,27 Although a highly porous network structure is a major advantage, it has imposed difficulties in ensuring sufficient contact points between the nanofiber-based electrode and a dense and planar electrolyte due to the limited adhesion.^18,21 Considering the critical contribution of the interfacial resistance between the electrode and electrolyte to the total resistance, insufficient contact points could decrease the electrochemical performance and induce mechanical failures, such as delamination and cracks.^28,29 One of the simplest approaches to overcome this issue is to employ a high sintering temperature to achieve sufficient adhesion between the nanofiber-based electrode and electrolyte. However, this considerably compromised the structural advantages of nanofiber-based electrodes, such as high porosity, high specific surface area, and hollow morphology, owing to the structural instability at elevated temperatures, such as grain growth and agglomeration, resulting in unsatisfactory electrochemical performance.³⁰ Alternative approaches have been explored to utilize nanofiber electrodes through various fabrication techniques, such as flash sintering, the template method, or direct assembly methods that bypass the high temperature sintering process.^31–33 Therefore, a rational design of the electrode structure and fabrication process is essential to fully utilize the structural advantages of nanofibers as an electrode structure.

In this study, we demonstrated a high-performance solid oxide electrochemical cell by utilizing the structural and chemical advantages of electrospun nanofibers. The deposition of a thin adhesion layer between the nanofiber layer and electrolyte can achieve sufficient contact points at the interface and facilitate the use of a lower sintering temperature for a porous and hollow nanofiber structure. The specific surface area of the fabricated nanofiber-based electrode substantially increased with a higher concentration of oxygen vacancies compared to a powder-based electrode, extending reaction sites with high ORR and OER activities. Consequently, the nanofiber-based single cell exhibited significantly reduced R_p compared to the powder-based single cell, resulting in a 50% higher peak power density (1 W cm⁻²) in FC mode and 59% higher current density (0.79 A cm⁻²) at 1.3 V under 50% H₂–50% steam conditions in EC mode, both at 600 °C. Excellent thermal stability was confirmed both under static operation conditions with a constant current of 1 A cm⁻² for the FC mode and −1 A cm⁻² for the EC mode at 700 °C for 300 h, under reversible cyclic operation between FC and EC modes with the dynamic voltage from 0.9 V to 0.5 V and from 1.1 V to 1.5 V, respectively, exhibiting negligible degradation. Our results demonstrate the feasibility of a nanofiber-based electrode to achieve high performance and stability in solid oxide electrochemical cells operating at intermediate temperatures.

Experimental

Preparation of La_0.6Sr_0.4CoO_3−d (LSC) nanofibers

The LSC nanofibers were fabricated using a custom-made electrospinning system. N,N-Dimethylformamide (Alfa Aesar) and ethanol were mixed in a volumetric ratio of 4 : 1, respectively, and polyvinylpyrrolidone (Sigma-Aldrich) was added for appropriate solution viscosity. La(NO₃)₃·6H₂O, Sr(NO₃)₂, and Co(NO₃)₂·6H₂O (Sigma-Aldrich) were dissipated in the solution in a molar ratio of 0.6 : 0.4 : 1 to form the LSC perovskite phase as an air electrode material. The solution was stirred at 50 °C overnight on a heating plate for complete dissolution. The LSC precursor solution was transferred into a syringe pump with a single plastic nozzle (NanoNC; NNC-PN-25 GA). Electrospinning was carried out at an electric field of 1 kV cm⁻¹ and flow rate of 1.3 mL h⁻¹. A grounded Al foil was used to collect the electrospun nanofibers. The as-spun nanofibers were sintered at 650 °C for 3 h for phase formation.

Fabrication of single cells

A Ni–yttria-stabilized zirconia (YSZ) fuel electrode supported single cell was fabricated for the electrochemical analysis. NiO powder (Kojundo Chemical), YSZ powder (Tosoh), and poly(methyl methacrylate) (Alfa Aesar) were uniformly dispersed at a weight ratio of 6 : 4 : 1 in ethanol. HypermerTM KD-6 (Croda) and polyvinyl butyral (Sigma-Aldrich) were added to the fuel electrode ball-milling solution as the dispersant and binder, respectively. The solution was ball milled for 24 h under 250 rpm. The completely dried Ni–YSZ powder after ball milling was pressed with unidirectional force to obtain a circular substrate, followed by sintering at 1000 °C for 3 h. The fuel electrode functional layer and YSZ electrolyte were fabricated through a spin coating process. The functional slurry layer composed of NiO and YSZ powders (6 : 4 weight ratio), which were identically used in the fuel-electrode formation, was dispersed in ethanol with ethyl cellulose (Sam Chun) and KD-6 as the binder and dispersant, respectively. The slurries were coated onto the surface of the fuel electrode substrate and rotated at 3000 rpm for 1 min. A YSZ slurry was prepared through a technique equivalent to that used for the functional layer slurry. The deposited NiO–YSZ functional layer and YSZ electrolyte were co-sintered at 1400 °C for 3 h.

A Gd_0.2Ce_0.8O_2−δ (GDC) diffusion barrier was fabricated through spin coating. GDC powder (Rhodia) was dispersed in 20 mL ethanol with ethyl cellulose at a weight ratio of 10% relative to the powder. After 24 h of ball milling, the solution was spin coated twice onto the electrolyte at 3000 rpm to obtain a thickness of approximately 2 μm. The GDC diffusion barrier was sintered at 1200 °C for 3 h.

The LSC powder layer was fabricated using a screen-printing process with a mixture of La_0.6Sr_0.4CoO_3−δ powder (LSC, K-ceracell, Korea) and the ink vehicle. The LSC powder paste was screen-printed to form a thickness of 6 μm and sintered at 900 °C for 3 h. The LSC nanofiber layer was fabricated using a screen-printing process with a mixture of electrospun LSC nanofibers and the ink vehicle. The LSC powder paste, nanofiber, and powder pastes were screen printed sequentially to form thicknesses of 2, 12, and 4 μm, respectively, and then sintered at 800 °C for 3 h.

Characterization

Scanning electron microscopy (SEM, JSM7000F, JEOL) was used to analyze the structural properties. The crystal structures and grain sizes of the LSC powder and nanofiber were analyzed using X-ray diffraction (XRD, D8 ADVANCE, Bruker Corp.) with Cu K-alpha radiation (λ = 1.5406 Å) at room temperature. The chemical properties were analyzed using X-ray photoelectron spectroscopy (XPS, ESCA Lab 250 XPS spectrometer, VG Scientific Instruments) with a monochromatic Al K-alpha source. The specific surface area was measured using the Brunauer–Emmett–Teller (BET, TRISTAR 3020, Micromeritics) isothermal technique with N₂ adsorption using a surface area analyzer. To compare the relative surface oxygen adsorption and desorption energies, the O₂-temperature programmed desorption (O₂-TPD, AUTOCHEM II 2920, Micromeritics) test was conducted with a thermal conductivity detector. Electrochemical characteristics, including current–voltage (I–V) measurements and electrochemical impedance spectroscopy (EIS) measurements, were conducted using a potentiostat (Interface 1010E, Gamry Instruments) in the frequency range of 0.1–10⁶ Hz. For the single-cell FC mode, dry air and 97% H₂–3% H₂O with a flow rate of 200 sccm were provided to the air and fuel electrodes, respectively. For the single-cell EC mode, dry air and 50% H₂–50% H₂O with a flow rate of 200 sccm were provided to the air and fuel electrodes.

Results and discussion

The structural and chemical characteristics of the electrospun LSC nanofibers, including those affecting the charge-transfer and surface-exchange reactions, such as specific surface area, defect concentration, and adsorption and desorption activation energy, were compared with those of the LSC powders. Fig. 1(a) and (b) show the SEM images of the LSC powders and nanofibers, respectively. The LSC powder has a particle diameter of 300–400 nm. The LSC nanofibers exhibit a porous and hollow morphology with a diameter of 150–250 nm, forming a highly porous network structure.^21,34,35 Based on the BET analysis, the specific surface area of the LSC nanofibers (9.7055 m² g⁻¹) is 4.15 times higher that of the LSC powder (2.3381 m² g⁻¹). The XRD patterns in Fig. 1(d) show identical rhombohedral perovskite phases without secondary phases.³⁴ The grain size of the LSC nanofibers (30.3 ± 1.0 nm) is smaller than that of the LSC powder (36.1 ± 2.0 nm).

Fig. 1 Structural and chemical properties of the LSC powder and nanofiber. SEM images of (a) LSC powder and (b) LSC nanofibers. (c) BET curves. (d) XRD patterns. (e) XPS O 1s spectra. (f) O₂-TPD profiles.

Fig. 1(e) shows the XPS O 1s spectra deconvoluted into four components: lattice oxygen (O_lattice), oxygen vacancy (O_vacancy), surface-adsorbed oxygen (O_ads), and adsorbed water molecule (H₂O). We calculated the relative intensity ratios of O_vacancy/O_lattice as a quantitative index to compare the concentration of the oxygen vacancies.⁵ The LSC nanofibers exhibit an intensity ratio of 0.896, which is approximately 34% higher than that of the LSC powders (0.670), confirming the higher concentration of oxygen vacancies in the LSC nanofibers. The higher concentration of oxygen vacancies in the LSC nanofibers can be attributed to their smaller grain size and consequently, higher grain boundary density where the formation enthalpy for the oxygen vacancy is lower than that of the bulk grain.^22–24 Furthermore, the relative intensity ratio of O_ads/O_lattice is higher in the LSC nanofibers (2.380) than that in the LSC powders (1.666), indicating more favorable oxygen adsorption at and near oxygen vacancies.²⁵

The O₂-TPD analysis results in Fig. 1(f) show that oxygen adsorption and desorption are facilitated by the higher concentration of oxygen vacancies in the LSC nanofibers. The desorption profile below 400 °C can be attributed to α-oxygen, representing the desorption of weakly adsorbed molecular oxygen on oxygen vacancies.^36,37 The peaks appeared at a substantially lower temperature of 266 °C for the LSC nanofibers compared to 377 °C for the LSC powders, verifying the lower adsorption and desorption energy at the surface of the LSC nanofibers.

We designed the nanofiber-based electrode structures in the Ni–YSZ fuel electrode-supported single cell configuration. Fig. 2(a) presents the cross-sectional image of the LSC powder cell after sintering the LSC electrode at a typical sintering temperature of 900 °C, indicating sufficient adhesion at the interface.^17,38 However, the structural advantages of the nanofibers, such as high porosity and specific surface area, significantly decreased at sintering temperatures above 900 °C, as shown in Fig. S1.† In contrast, at a lower sintering temperature of 800 °C, the adhesion at the interface is insufficient, increasing R_p, as shown in Fig. S2.† Therefore, the design strategy to utilize the advantageous structural characteristics, while securing sufficient adhesion at the interface is necessary to fully utilize the advantageous structural characteristics of nanofiber-based electrodes. To address this issue, we inserted an additional powder-based adhesion layer between the LSC nanofiber and GDC buffer layers, as shown in Fig. 2(b). The effects of the adhesion layer were compared in the LSC nanofiber cells after sintering at 800 °C, as shown in Fig. S3.† The ohmic resistance (R_ohm) decreases by 48.7% from 0.150 Ω cm² for the LSC nanofiber cells without the adhesion layer to 0.077 Ω cm² for the LSC nanofiber cells with the adhesion layer.

Fig. 2 Cross-sectional SEM images of the fuel-electrode-supported single cells with the (a) LSC powder-based air electrode and (b) LSC nanofiber-based air electrode.

We compared the effects of the sintering temperature of the LSC nanofiber cells with the adhesion layer, as shown in Fig. S4.† Although R_ohm is similar for both cells owing to the adhesion layer, R_p significantly increased with the sintering temperature, that is, 0.137 and 0.208 Ω cm² for the LSC nanofiber cell sintered at 800 and 900 °C, respectively. The thickness of the adhesion layer was optimized to 2 μm, as shown in Fig. S5.† The thickness of the nanofiber layer was optimized to 12 μm, as shown in Fig. S6.† The current-collecting layer was deposited with LSC powder in the final step. Therefore, the fabrication process for the nanofiber-based cell was optimized with a sintering temperature of 800 °C for the nanofibers and a powder-based adhesion layer with a thickness of 2 μm, achieving structural advantages of nanofiber-based electrodes.

The electrochemical performance of the LSC powder and nanofiber cells was evaluated in a fuel electrode-supported single-cell configuration in the operating temperature range of 550–700 °C, as shown in Fig. 3. Humidified H₂ with 3% steam in FC mode and 50% steam in EC mode was supplied to the fuel electrode, whereas a dry air was supplied to the air electrode for both modes. Detailed fabrication and evaluation processes are provided in the characterization section. The LSC nanofiber cell exhibits considerably improved electrochemical performance compared to the LSC powder cell in both modes. Specifically, the LSC nanofiber cell has substantially higher peak power densities in FC mode with more pronounced improvements at lower temperatures. For example, the peak power densities of the LSC nanofiber cell are 2.19, 1.57, 1.00, and 0.51 W cm⁻² at 700, 650, 600, and 550 °C, respectively, which are 1.25, 1.31, 1.50, and 1.50 times higher than those of the LSC powder cell. Similarly, the LSC nanofiber cell exhibits higher current densities in EC mode at the thermoneutral voltage (1.3 V) with more pronounced improvements at lower temperatures. For example, the current densities of the LSC nanofiber cell are 2.60, 1.50, 0.79 and 0.35 A cm⁻² at 700, 650, 600, and 550 °C, respectively, which are 1.48, 1.48, 1.52, and 1.59 times higher than those of the LSC powder cells.

Fig. 3 Electrochemical performance of the LSC powder and nanofiber cells in FC and EC modes. I–V–P curves in FC mode of the (a) LSC powder cell and (b) LSC nanofiber cell. (c) Comparison of the peak power densities as a function of the operating temperature. I–V curves in EC mode of the (d) LSC powder cell and (e) LSC nanofiber cell. (f) Comparison of current densities at 1.3 V as a function of the operating temperature. All single cells were evaluated in the temperature range of 550–700 °C with a fuel supply of 97% H₂–3% steam in FC mode and 50% H₂–50% steam in EC mode.

The EIS measurements indicate that the improved electrochemical performance of the LSC nanofiber cell can be primarily attributed to a substantial reduction in the R_p, as shown in Fig. 4. In FC mode with a supply of 97% H₂–3% steam to a fuel electrode, the LSC nanofiber cell exhibits R_p values of 0.137 and 0.271 Ω cm² at 700 and 600 °C, respectively, which are 0.92 and 0.65 times smaller than those of the LSC powder cell, as shown in Fig. 4(a). The reduction in R_p values of the LSC nanofiber cell is more pronounced at lower temperatures, as shown in Fig. 4(b), consistent with its smaller activation energy of 0.65 ± 0.01 eV compared to 0.83 ± 0.01 eV for the LSC powder cell, as shown in Fig. 4(c). A similar trend is observed in EC mode with a supply of 50% H₂–50% steam to a fuel electrode. The LSC nanofiber cell exhibits R_p values of 0.053 and 0.194 Ω cm² at 700 and 600 °C, respectively, which are 0.74 and 0.61 times smaller than those of the LSC powder cell, as shown in Fig. 4(d). The reduction in the R_p values of the LSC nanofiber cell is more pronounced at lower temperatures, as shown in Fig. 4(e), with its smaller activation energy of 1.00 ± 0.01 eV compared to 1.14 ± 0.01 eV for the LSC powder cell, as shown in Fig. 4(f). In contrast, no noticeable differences in R_ohm are observed between the LSC nanofiber and powder cells in both modes.

Fig. 4 Electrochemical impedance analysis of the LSC powder and nanofiber cells under an open-circuit voltage condition. In FC mode with a supply of 97% H₂–3% steam to a fuel electrode: (a) Nyquist plot measured at 700 and 600 °C, (b) comparison of the polarization resistance (R_p), and (c) Arrhenius plots and activation energies for the ohmic resistance (R_ohm) and R_p. In EC mode with a supply of 50% H₂–50% steam to a fuel electrode: (d) Nyquist plot measured at 700 and 600 °C, (e) comparison of R_p, and (f) Arrhenius plots and activation energies for R_ohm and R_p.

The systematic investigation of R_p with control of operating conditions further supports that the origin of the reduced R_p values of the LSC nanofiber cell is owing to the structural and chemical characteristics of nanofibers. As the electrochemical reactions and corresponding R_p vary depending on the fuel conditions and operating modes, the electrochemical impedance was analyzed separately for the FC and EC modes using the distribution of the relaxation time (DRT) analysis with control of pO₂ and Arrhenius plots to investigate the reasons for the performance improvement.^14,39 First, we identified five distinct peaks in the Bode plot of the LSC powder cell with DRT analysis, as shown in Fig. 5(a). The resistances of P1–P5 were plotted as a function of pO₂ to identify individual peaks, and each reaction order was calculated, as shown in Fig. 5(b).

Fig. 5 Electrochemical impedance analysis of LSC powder and nanofiber cells with pO₂ control. (a) DRT analysis of the LSC powder cell as a function of the frequency at different pO₂ values in the range of 0.063–0.21 atm at 700 °C. (b) R_p values of the LSC powder cell of P1–P5 as a function of pO₂ and reaction order of P1–P5. (c) DRT analysis in FC mode with a supply of 97% H₂–3% steam to a fuel electrode at 550 °C. (d) Arrhenius plots and activation energies for P3 and P4 of the LSC powder and nanofiber cells. (e) DRT analysis in EC mode with a supply of 50% H₂–50% steam to a fuel electrode at 550 °C. (f) Arrhenius plots and activation energies for P3 and P4 of the LSC powder and nanofiber cells.

The electrochemical reactions corresponding to the reaction order values are provided in Table S1.† DRT analysis in FC mode (dry air to an air electrode and 97% H₂–3% steam to a fuel electrode), as shown in Fig. 5(c) and Tables S2–S4,† revealed that two reactions, P3 and P4, are substantially reduced in the LSC nanofiber cell compared to the LSC powder cell, whereas other reactions remained similar in both cells. The P3 and P4 reactions are associated with the oxygen charge transfer, dissociation, and association, respectively.^2,39 The Arrhenius plot reveals a concurrent decrease in the resistances of the P3 and P4 reactions in the LSC nanofiber cell with activation energies that are 0.40 and 0.56 times smaller than that of the LSC nanofiber cell (0.34 and 0.29 eV) and LSC powder cell (0.84 and 0.51 eV), as shown in Fig. 5(d). Similarly, the DRT analysis in EC mode (dry air to an air electrode and 50% H₂–50% steam to a fuel electrode), as shown in Fig. 5(e) and Tables S5–S7,† reveals the substantial reduction in P3 and P4 in the LSC nanofiber cell compared to the LSC powder cell. The LSC nanofiber cell exhibits a concurrent decrease in the resistances of the P3 and P4 reactions to 0.79 and 0.90 times smaller activation energy of 1.74 and 1.36 eV than 2.19 and 1.51 eV for the LSC powder cell, respectively, as shown in Fig. 5(f).

The improved performance of the LSC nanofiber cell in P3 and P4, which have been reported as the most sluggish reaction steps in both modes, can be attributed to its structural advantages owing to the low sintering temperature, including significantly the high specific surface area for extended reactions sites and the high density of grain boundaries for active surface oxygen exchange owing to the high concentration of oxygen vacancies.^24,40–42 Accordingly, the smaller activation energies for P3 and P4 in the LSC nanofiber cell can result in the more pronounced improvement in the electrochemical performances with lower operating temperatures. However, R_ohm remained similar in the operating temperature range of 550–700 °C, with similar activation energies of 0.66 eV in FC mode and 0.69 eV in EC mode in both the LSC nanofiber and LSC powder cells, indicating no notable impact on the ion conduction due to the high density of grain boundaries, which might otherwise increase R_ohm.

Fig. 6 illustrates the excellent stability of the LSC nanofiber cell during static operation in both FC and EC modes and load-varying reversible cyclic operations between FC and EC modes. Under a constant current density of 1 A cm⁻² for the FC mode and −1 A cm⁻² for the EC mode, the LSC nanofiber cell maintained its initial performance at 700 °C for 300 h, with negligible degradation rates of 0.00006% h⁻¹ and 0.0086% h⁻¹ for FC and EC modes, respectively. This was further validated by the negligible changes in the Nyquist plots, as shown in Fig. 6(c) and (d). The dynamic operation, involving reversible cyclic operation between FC and EC modes (from 0.9 to 0.5 V for the FC mode and 1.1 to 1.5 V for the EC mode with an interval of 1 h), also demonstrated high stability, as shown in Fig. 6(b). In addition, the structural and chemical properties in the nanofiber-based electrode remained unchanged during the stability tests, as shown in Fig. S7.† Therefore, the high performance and stability demonstrated by the nanofiber-based electrodes under both static and dynamic operation conditions promote their feasibility as an appealing strategy for solid oxide cells operating at intermediate temperatures.

Fig. 6 Stability evaluation of the LSC nanofiber cell under various operation conditions at 700 °C. (a) Galvanostatic measurements with a constant current density of 1 A cm⁻² for 300 h in FC mode (97% H₂–3% steam to a fuel electrode) and EC mode (50% H₂–50% steam to a fuel electrode). (b) Reversible cycling measurement between FC and EC modes with the dynamic voltage from 0.9 to 0.5 V for the FC mode (97% H₂–3% steam to a fuel electrode) and from 1.1 to 1.5 V for the EC mode (50% H₂–50% steam to a fuel electrode). Nyquist plot during the stability test for 300 h in (c) FC mode and (d) EC mode.

Conclusions

In this study, we demonstrated the high performance and stability of a nanofiber-based electrode for solid oxide electrochemical cells. A thin powder-based adhesion layer at the interface between the nanofiber layer and electrolyte facilitates operation at lower sintering temperatures compared to the typical sintering process. This enabled the fabrication of porous and hollow nanofiber structures with high specific surface and a high concentration of oxygen vacancies, resulting in extended reaction sites and high ORR and OER activity. Consequently, the electrochemical analysis revealed significant reductions in R_p and activation energies. The optimized nanofiber-based single cell demonstrated a peak power density of 1 W cm⁻² in FC mode and a current density of 0.79 A cm⁻² at 1.3 V under 50% H₂–50% steam conditions in EC mode, both at 600 °C, with excellent thermal stability both under static and reversible cycle operations. Our results confirmed that the design strategy leveraging the structural and chemical advantages of nanofiber-based electrodes exhibited exceptional performance and stability in solid oxide electrochemical cells operating at intermediate temperatures.

Data availability

The data supporting this article have been included as part of the ESI.† No software or code has been included.

Author contributions

Seungwoo Han: conceptualization, methodology, writing – original draft, writing – review & editing. Hyun Sik Yoo: data curation, investigation. Wonyoung Lee: conceptualization, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science and ICT, South Korea) (no. 2022R1A4A1031182 and 2022R1A2C3012372).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta05916f

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