Key Roles of Initial Calcination Temperature in Accelerating the Performance in Proton Ceramic Fuel Cells via Regulating 3D Microstructure and Electronic Structure

Developing cathode materials with high performance in oxygen reduction reaction (ORR) is desirable for proton ceramic fuel cells (PCFCs) for energy conversion technology. BaCo0.4Fe0.4Zr0.1Y0.1O3–δ (BCFZY) is widely investigated as a cathode. Herein, BCFZY cathode is used as a paradigmatic example to study the impact of calcination temperature on microstructure, electronic structure, and ORR performance. Ion beam‐scanning electron microscopy indicates BCFZY prepared at 800 °C (BCFZY800) exhibits the largest specific surface area and cathode/electrolyte contact area. BCFZY800 exhibits a peak power density of 1.32 W cm−2 at 650 °C, which is 37% and 193% higher than that of BCFZY prepared at 700 °C (BCFZY700) and 1100 °C (BCFZY1100), respectively. Furthermore, BCFZY800 demonstrates high long‐term stability over 500 h. Soft X‐Ray absorption spectra indicate that the oxidation state of BCFZY800 is reduced, suggesting more catalytically active sites than those of BCFZY700 and BCFZY1100 after the ORR. This work provides a new understanding for enhanced PCFCs performance by proper porosity structure via fine‐tuning the calcination temperature.


Introduction
As the world grapples with an energy crisis, there is an urgent demand for clean and sustainable energy solutions, leading to extensive research on reversible solid oxide cells (ReSOCs). [1]versible proton ceramic cells (RePCCs) that employ protonic conducting electrolytes (H þ ) are garnering significant attention because of the superior performance at lower operating temperatures (300-600 °C) compared to conventional ReSOCs employing oxygen ion (O 2À ) conducting electrolytes. [2]The utilization of protonic conducting electrolytes in RePCCs offers advantages, such as reduced operational and manufacturing costs and extended cell lifespan. [3]These benefits significantly enhanced the applicability and widespread adoption of this technology. [4]dditionally, the promising aspect of RePCCs is their ability to function as both proton ceramic fuel cells (PCFCs) and proton ceramic electrolysis cells (PCECs) within a single electrochemical device.This characteristic is especially significant as it enables the efficient storage of hydrogen energy by converting intermittently generated electricity from renewable energy sources.Nevertheless, as the operating temperature is further reduced, the performance of the cells deteriorates significantly.This deterioration primarily occurs due to the increase in polarization resistance resulting from the sluggish kinetics of the oxygen reduction reaction (ORR) at the air electrode in the fuel cell (FC) mode. [5]ignificant advancements have been achieved in the development of novel materials for RePCCs using ABO 3 perovskite oxides, which are known as mixed ions and electronic conductors.These materials include simple perovskites like La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3Àδ and double perovskites such as PrBaCo 2 O 5þδ .This research developed more efficient and advanced materials for RePCCs. [6]In contrast to solid oxide fuel cells, where the ORR involves both electrons (e À ) and O 2À , the ORR of PCFCs cathode simultaneously involves e À , O 2À , and H þ .Therefore, incorporating specific proton conduction properties into the cathode material can extend the three-phase boundaries (TPBs) throughout the cathode region, leading to higher ORR activity and cell performance toward PCFCs. [7]n this regard, progress had been made in the development of triple conduction oxide for PCFC cathode, such as BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3Àδ (BCFZY) and NaBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5þδ (NBSCF). [8]Particularly, NBSCF was initially chosen for PCFCs applications and was subsequently refined to PrBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5þδ (PBSCF). [9]Furthermore, BCFZY demonstrated excellent performance and stability during chemical and thermal cycling. [10]nderstanding the microstructure of the cathode is crucial, as it encompasses factors such as the phase distribution, particle size, and the abundance of active sites and pores.These elements significantly influence specific electrochemical performance metrics.To further enhance cell performance, a viable approach involves design and optimization of the particle size, structure, and morphology of the electrode materials. [11]Porous nanofiber structures of LaNi 0.6 Fe 0.4 O 3Àδ were fabricated for application in PCFC cathodes.11c] The nanoparticle-scale electrode structure was prepared in a single step using the spray-pyrolytic deposition method, which provided an extended TPB length for the ORR.Additionally, the special morphology of the scaffold provided an efficient conduction path for oxide ions beyond the electrolyte/electrode interface. [12]The effect of different calcination temperatures on ORR activity can alter the phase structure, microstructure, and surface area of materials such as PBC cathodes, resulting in a significant improvement in cell performance. [13]aterials synthesized by calcination at high temperatures tend to have a reduced specific surface area.This reduction in surface area led to a decrease in the number of TPBs, ultimately resulting in higher area-specific resistance (ASR). [14]Currently, there is a lack of comprehensive studies exploring the impact of calcination temperature on the intrinsic properties of materials.Factors such as the particle size, specific surface area, oxygen defects, and changes in the electrode structure have not been systematically investigated in relation to the calcination temperature.Previous studies focused predominantly on planar structures.However, this study adopts a different approach by establishing 3D structures, which offers several advantages for analyzing structure-activity correlations.This 3D framework enables a more straightforward, logical, and accurate analysis of the relationship between the structure of a material and its activity.
In this study, we demonstrate the potential for enhancing electrochemical efficiency by modifying the initial calcination temperature of BCFZY.To achieve this, we investigated the phase structure, microstructure, and electrochemical properties of BCFZY samples prepared at temperatures ranging from 700 to 1100 °C.It was observed that the cathode calcinated at 800 °C has a higher ORR activity and long-term stability than that of other samples.Furthermore, notable results were obtained, including a high-power density of 1.32 W cm À2 at 650 °C and a low polarization impedance of 0.046 Ω cm 2 .The study demonstrates that by simply adjusting the initial calcination temperature to 800 °C, we can attain improved electrochemical properties.This approach holds promise for enhancing the performance of other cathode materials, making it widely applicable.

Results and Discussion
BCFZY samples were prepared through two sintering temperatures.First, BCFZY powders were synthesized at various temperatures ranging from 700 to 1100 °C.Then, they were calcinated under 950 °C for 4 h for acquiring the BCFZY electrode.These final BCFZY samples are marked as BCFZY700, BCFZY800, BCFZY900, BCFZY1000, and BCFZY1100, respectively.The crystal structures of the BCFZY samples were investigated using X-Ray diffraction (XRD).As shown in Figure 1a, the XRD patterns reveal that all BCFZY samples exhibited a pristine perovskite phase with cubic symmetry (PDF#97-023-8817) and no detectable impurities.Furthermore, an increase in the initial calcination temperature led to a corresponding enhancement in peak intensity and a more pronounced narrowing of the peak shape.This observation indicates that elevated temperatures are conducive to enhancing crystallinity; however, it should be noted that this may also result in larger grain sizes. [15]The XRD patterns of the BCFZY samples at other temperatures are shown in Figure S1a, Supporting Information.Figure S1b, Supporting Information, also presents the XRD pattern of the initially prepared BCFZY powder before being calcinated at 950 °C.It is noticeable that BCFZY powder with synthesis temperatures above 900 °C exhibited a pure perovskite phase.However, when the synthesis temperature falls below 900 °C, a few additional peaks are observed, such as BaCoO 3 with P63/mmc space groups, BaCO 3 with Pmcn space groups, and cobalt iron oxide.Further calcinated at 950 °C for 4 h in air, all samples can be well indexed to a single cubic perovskite structure without any impurity peaks (Figure 1a and S1a, Supporting Information), suggesting that electrode materials in all cells are in the pure BCFZY phase.The XRD Rietveld refinement results shown in Figure S2 and Table S1, Supporting Information, indicate that all samples were well crystalline.
Figure S3, Supporting Information, displays the scanning electron microscopy (SEM) images of BCFZY powders, revealing a significant alteration in particle shape and size as the initial calcination temperature increases from 700 to 1100 °C.The particles underwent a transition from irregular shapes to structures resembling spheres and eventually transformed into larger and more massive particles at elevated temperatures.After further calcination at 950 °C for 4 h in air, the morphologies of the samples exhibit uniform and ultrafine sphere-like particles in the range of 700-900 °C, as shown in Figure 1b.The particle size of BCFZY samples exhibits a consistent increase from 0.13 μm (BCFZY700) to 1.1 μm (BCFZY1100), as depicted in Figure 1c.The tendency for particle growth was particularly pronounced at elevated temperatures, which is consistent with the XRD analysis findings.To validate the particle size of the BCFZY samples at various initial calcination temperatures, a barrett-emmett-teller (BET) specific surface area test was conducted, and the corresponding results are presented in Figure1c and Table S2, Supporting Information.As the initial calcination temperature decreased, the specific surface area of the BCFZY samples gradually increased.13a] Based on their larger specific surface areas, it was initially anticipated that BCFZY700 and BCFZY800 would demonstrate superior electrochemical performance.However, the actual results deviated from this expectation, and this discrepancy is further discussed in the subsequent sections.
Figure 1d illustrates the thermogravimetric (TG) profiles of BCFZY samples, showing the variation in weight loss under air as a function of temperature, spanning from 150 to 800 °C.The weight of the sample steadily decreases from 200 to 800 °C as a result of oxygen depletion.This process primarily results in the liberation of oxygen from the crystal lattice through Co─O bond cleavage. [16]The weight loss of BCFZY samples synthesized at a lower initial calcination temperature exhibits higher values.Specifically, the weight loss percentages for BCFZY800 and BCFZY900 are ≈1.39%,which significantly surpasses the value of 1.08% observed for BCFZY1100.Within the temperature range of 300-800 °C, the corresponding TG weight loss of BCFZY1100 is approximately 0.95%, which is very close to the value reported in the literature. [17]During cell operation, the increased formation of oxygen vacancies enhances the activity of the ORR at the surface and accelerates oxygen kinetics. [18]igure 1e illustrates the results of the oxygen temperatureprogrammed desorption experiment (O 2 -TPD), which qualitatively offers insight into the migration of oxygen species. [19]A lower onset temperature and peak temperature of lattice oxygen desorption in O 2 -TPD curves indicate a faster oxygen exchange and oxygen ion migration within the perovskite. [19,20]20b] Therefore, the BCFZY800 and BCFZY900 exhibit enhanced oxygen desorption capabilities.
To verify the electrochemical cathodic performance of these BCFZY samples in PCFCs, electrochemical impedance spectroscopy (EIS) tests were first carried out at 500-700 °C using a symmetric cell with a configuration of BCFZY| BaZr 0.1 Ce 0.7 Y 0.1 Yb 0.1 O 3-δ (BZCYYb)|BCFZY in a 3 vol% H 2 Oair atmosphere.Due to its nature as a mixed H þ /O 2À ion conducting perovskite, the BZCYYb electrolyte can experience enhanced ORR activity with the introduction of water.This benefit arises from the formation of hydroxyl groups, which promote ORR. [21]Based on the information presented in Figure S4 and Table S3, Supporting Information, it is evident that BCFZY800 demonstrates the lowest ASR values among all the samples, particularly at lower temperatures (≤600 °C).As shown in Figure 2a, the EIS of BCFZY800 is superior to that of other samples at 600 °C.This indicates that the samples prepared at an optimal initial calcination temperature have the potential to serve as promising cathode candidates for PCFCs.Furthermore, the results calculated and depicted in Figure 2b and S4f, Supporting Information, demonstrate that BCFZY samples exhibit an activation energy of ≈0.7 eV when initially calcinated from 700 to 900 °C.These values are relatively lower compared to those of the other samples, especially those that underwent higher calcination temperatures (>1000 °C).The ASR values of BCFZY1100 and BCFZY1000 were significantly higher than those of the other samples, which could be attributed to their smaller specific surface areas, lower oxygen vacancy concentrations, and slower oxygen migration rates.Nevertheless, it is evident that the samples calcined at lower temperatures exhibited outstanding electrochemical performance for the ORR in PCFCs.
To gain further insight into the influence of electrochemical processes on the ORR kinetics of the BCFZY cathodes, the electrochemical performance of the BCFZY samples was tested using a symmetrical cell with the configuration BCFZY| Sm 0.8 Gd 0.2 O 2 (SDC)|BCFZY.Distribution of relaxation time (DRT) analysis was used to numerically reconstruct and distinguish electrochemical processes based on discrete EIS data.DRT analysis is a useful approach for understanding the reaction kinetics and transport phenomena of certain electrochemical systems based on the relaxation time differences among all steps. [22]he electrochemical processes from high to low frequency can be divided into three parts: transfer of oxygen ions from the electrolyte to the electrode at the triple-phase boundary (P1), surface charge exchange and ions diffusion (P2), and gas diffusion and adsorption-desorption (P3). [23]The collective EIS and DRT analysis results (Figure 2c) suggest that BCFZY800 exhibits the minimum peak size in all frequency ranges, which can be attributed to its enhanced charge transfer, accelerated surface reaction rates, as well as efficient gas diffusion and adsorption-desorption processes.Consequently, the spectra were fitted using the equivalent circuit (Figure S5a, Supporting Information), where R o represents a serial resistance associated with the total ohmic losses of the symmetrical cells.The equivalent circuit is composed of three (RQ) elements associated with three distinct processes: one at high frequency (HF), R HF , another at medium frequency (MF), R MF , and a third at low frequency (LF), R LF .R HF , R MF , and R LF resistance contributions for SDC-based symmetrical cells with BCFZY samples are shown in Figure S5, Supporting Information.The impedance of BCFZY800 at each frequency is relatively low.The decrease in high-frequency impedance can be attributed to the increased contact surface area, which enhances charge transfer between interfaces; the reduction in medium-frequency impedance is due to the larger electrode surface and enhanced reactivity sites, facilitating migration and reaction of reactive oxygen species; the decline in lowfrequency impedance can be ascribed to the improved electrode structure, promoting mass transfer as well as oxygen adsorption and desorption processes.Furthermore, Table S4, Supporting Information, presents the activation energy values for each frequency, demonstrating that BCFZY800 exhibits superior performance. [12,16]These results confirmed that tuning the initial calcination temperature is an effective way to enhance the cathode activity of PCFCs.Benefiting from these excellent electrochemical properties, BCFZY800 exhibited the lowest ASR in air among all the samples (Figure 2d).
Furthermore, the feasibility of utilizing the BCFZY samples as cathodes for PCFCs was evaluated through single-cell tests with a NiO-BZCYYb|BZCYYb|BCFZY configuration.The corresponding results presented in Figure 3 and S6, Supporting Information, indicate that FCs with an initial calcination temperature ranging from 800 to 900 °C exhibit the most optimal performance.Among them, BCFZY800 exhibits a peak power density (PPD) of up to 1.32 W cm À2 at 650 °C, representing an increase of 37% and 193% compared to BCFZY700 (0.96 W cm À2 ) and BCFZY1100 (0.45 W cm À2 ), respectively.As shown in Figure S7, Supporting Information, nearly identical electrolyte thicknesses suggest that the differences in cell performance primarily arise from variations in the electrodes.Additionally, the power density of the BCFZY-based single cell was superior to that of most other reported high-performance PCFCs (Table 1).The results clearly show that the BCFZY cathode calcined at the appropriate temperature exhibits superior performance compared to that of the BCFZY perovskite prepared at both higher and lower initial calcination temperatures.Figure S6b, Supporting Information, shows the Nyquist AC EIS of all cells at 650 °C under open-circuit voltage conditions.The total area-specific polarization resistance (R p ) and ohmic resistance (R o ) were extracted as shown in Figure 3b.13a] However, by further lowering the initial calcination temperature from 800 °C, R p began to increase.Ro is predominantly influenced by the thickness of electrolyte and electrode-electrolyte interface.As illustrated in Figure S7, Supporting Information, wherein the thickness of the electrolyte remains uniform, no discernible closed pores are present.11c,24] Additionally, cell life is a crucial indicator of the commercial viability of high-performance systems.Figure 3c depicts the durability test results of the cell with the BCFZY800 cathode at 300 mA cm À2 and °C.After continuous operation for over 500 h under a current density of 300 mA cm À2 , negligible degradation of performance is observed, showing excellent electrochemical stability.To check the chemical compatibility between BCFZY samples and electrolyte (BZCYYb), all BCFZY samples calcined at different initial calcination temperatures were mixed with BZCYYb and calcinated at 950 °C for 4 h.The XRD results (Figure S8a, Supporting Information) did not detect any other phases after heat-treating, and the corresponding refined results (Figure S8 and Table S5, Supporting Information) indicated that there were no changes in lattice parameters.This suggests that BCFZY and BZCYYb exhibit excellent chemical compatibility throughout the cell preparation process, further supporting their long-term operational stability.
To investigate the correlation between the electrode structure and cell performance, 3D structures of the monitored BCFZY cathodes before and after electrochemical testing were reconstructed and presented in Figure S9 and S10, Supporting Information, and Figure 4a1-e1.The yellow, purple, and green components represent the pores, BCFZY, and interfaces between the pores and BCFZY, respectively.Figure S9, Supporting Information, shows each phase of the reconstructed BCFZY cathode after FC tests.To quantify the evolution of 3D microstructure, interfacial area density (specific surface area per unit volume in μm À1 ), pores volume fraction, and tortuosity factor were calculated based on the reconstructed result and listed in Table 2.The BCFZY800 cathode exhibits the highest pore volume fraction (58.5%), which is higher than those of BCFZY700 (49.7%),BCFZY900 (55.1%),BCFZY1000 (43.8%), and BCFZY1100 (37.9%).This trend is consistent with the decrease in low-frequency arcs observed in the DRT (Figure 2c).Furthermore, the volume fraction of BCFZY700 was lower than that of BCFZY800, which may be caused by the shrinkage of the material during the second phasing.The high pore volume fraction in BCFZY800 facilitated gas diffusion and reduced the polarization impedance.Additionally, the BCFZY800 cathode exhibited a relatively low tortuosity factor, thereby facilitating fuel gas dispersion and ion transfer while also improving cell performance to some extent. [25]Furthermore, the interfacial area densities were calculated, as shown in Table 1.BCFZY is a triple-conducting oxide (H þ , O 2À , and e À ) with a contact interface between the perovskite and pores that serves as the ORR reaction site.The density of this interface was positively correlated with the reaction rate.The interfacial area density of BCFZY800 (7.781 μm À1 ) is higher than that of other materials, which is advantageous for oxygen reduction reactions. [24,26]For comparison, 3D structural reconstructions of the electrode structure before the FC testing were conducted, and the results are presented in Figure S10 and Table S6, Supporting Information.It was found that the cells with BCFZY700, BCFZY800, and BCFZY900 cathode powders prepared at lower temperatures indicated a relatively stable pore volume fraction before and after PCFC operation compared to that of the cell with the cathode prepared at higher temperatures.
We conducted a comprehensive analysis of the microstructure at the interface between BZCYYb and BCFZY in the reconstructed cells, as illustrated in Figure 4a2-e2 and a3-e3.To provide crucial insights into the electrode-electrolyte connection, 2D images of the directly obtained SEM slices perpendicular to the electrode/electrolyte interface were carefully examined.We then quantified the gradients of the pore fraction at the electrode/ electrolyte interface and plotted them against the electrode depth, as shown in Figure 4a3-e3.As anticipated, both BCFZY1000 and BCFZY1100 exhibited the highest porosity and the smallest electrolyte contact area within a depth of 1.5 μm.In contrast, BCFZY800 displays the lowest porosity and the largest contact surface, thereby facilitating charge transfer at the interface. [9]he porosities at the electrode/electrolyte interfaces of BCFZY800 and BCFZY900 were nearly identical to those of the electrode bulk region.However, for the other samples, porosity gradually stabilized as the depth within the electrode increased.Uniform porosity across the entire region can potentially achieve sustained long-term operation.
Soft X-Ray absorption spectroscopy (XAS) measurements were conducted to identify the active sites and their electronic  ) are included as references. [28]he Co L 3 -edge spectra in Figure 5a indicate the presence of Co 2þ , Co 3þ , and Co 4þ ions, suggesting their coexistence in all BCFZY samples before cell testing, with minimal changes observed in both energy position and spectral shape.This observation was also confirmed by Co K-edge X-Ray absorption near edge structure (XANES) (Figure S11a, Supporting Information).However, noticeable differences in multiplet spectral features were observed after the cell tests.The spectral weight of the lower energy shoulders (denoted as A1, A2, and A3) below the main  -OH have moderate bond strengths for the ORR. [29]We further examined the electronic structure of Fe using Fe L 3 -edge XAS (Figure S13, Supporting Information) and Fe K-edge XANES (Figure S11b, Supporting Information).These spectra exhibit no discernible differences for any of the BCFZY samples before and after the electrochemical test, indicating a predominantly stable HS Fe 3þ state.27c,30] As shown in the Co oxide references, the pre-edge peaks below 532 eV originate from the unoccupied O 2p orbitals hybridized with Co 3d states. [28]hese pre-edge peaks shift toward lower photon energies with increasing valence states due to the increase in the hybridization strength.27a,c,f] After the ORR, the spectrum of BCFZY800 in Figure 5d was positioned at higher photon energies than those of BCFZY700 and BCFZY1100, consistent with its lower average valence state, as concluded from the Co-L 3 spectra in Figure 5b.In summary, both the Co-L 3 and O-K spectra provide evidence of a significant decrease in the Co valence state and an increase in the number of oxygen vacancies after ORR in the BCFZY800 sample, despite the similar initial valence states of the Co ions in all the BCFZY samples.
Additionally, the electrocatalytic activities of the BCFZY samples as PCEC anodes were explored.As shown in Figure 6a, the single cell with the BCFZY800 anode demonstrates superior performance with current densities of 1.31 A cm À2 at 1.3 V in 10 vol% H 2 O-air at 600 °C.This can be attributed to the high oxygen vacancy concentration favoring H 2 O adsorption and the high porosity and low tortuosity benefiting the diffusion transport of H 2 O and O 2 . [31]As shown in Figure 6b, the current densities of a single cell with the BCFZY800 anode at 600, 550, 500, and 450 °C are 1.31, 0.79, 0.41, and 0.19 A cm À2 at 1.3 V in 10 vol% H 2 O-air, respectively.Additionally, Figure S15, Supporting Information, illustrates the I-V curves for cells with other samples.The electrolysis current exhibited by the BCFZY800 cell was significantly higher compared to those reported in most previous studies on PCECs, as summarized in Table S7, Supporting Information.The Faradaic efficiency was calculated at electrolysis current densities of À500 and À1500 mA, and the results are presented in Figure S16 and Table S8, Supporting Information.It is evident that BCFZY800 exhibits superior Faradaic efficiency.In both the EC and FC modes, the BCFZY800 electrode exhibited excellent electrochemical performance compared to those of the BCFZY electrodes initially calcined at temperatures too low or too high, indicating that the electrode materials prepared at an appropriate initial calcination temperature demonstrate superior performance.

Conclusion
In this work, we studied the influence of porosity distribution and electronic structure on the electrocatalytic activity of the BCFZY system.We found that the size and density of porosity can be well tuned by the initial calcination temperature of BCFZY from 700 to 1100 °C.Among these samples, the BCFZY cathode initially calcined at 800 °C exhibits the highest pores volume fraction and interfacial area density, while having the smallest tortuosity factor, giving the best PCFC performance and a longterm stability.X-Ray absorption spectroscopic studies indicated that the oxidation state of Co ions in BCFZY800 was strongly reduced after ORR compared with those of other BCFZY samples, although the initial valence states of Co ions for all BCFZY samples were nearly the same.The electrochemical performance test reveals that the BCFZY800 has a power density of 1.32 W cm À2 at 650 °C, which is 37% and 193% higher than that initially calcined at 700 and 1100 °C, respectively.Additionally, BCFZY800 exhibited high stability in the FC mode for more than 500 h and high electrolytic performance in the EC mode.We can conclude that the BCFZY800 sample has the most optimized microstructure and the largest number of active sites on the electrode, which are responsible for its high catalytic activity and long-term stability.

Figure 1 .
Figure 1.a) XRD patterns; b) SEM images; c) size distribution extracted from SEM images and BET-specific surface areas; d) TG profiles; and e) O 2 -TPD curves of different BCFZY samples after being calcinated at 950 °C for 4 h.

Figure 2 .
Figure 2. a) Nyquist plots and b) Arrhenius plots of the ASR values of BZCYYb-based symmetrical cells with BCFZY samples; c) DRT spectra; and d) Arrhenius plots of the ASR values of SDC-based symmetrical cells with BCFZY samples.The inset of (c) is the corresponding EIS.

Figure 3 .
Figure 3. a) I-V-P curves for cells with BCFZY samples cathode at 650 °C; b) comparison of R p and R o of single cells with BCFZY samples with different initial synthesis temperatures; and c) durability testing of cells in the FC mode at 600 °C.

Figure 5 .
Figure 5. Co L 3 -edge spectra of BCFZY700, BCFZY800, and BCFZY1100 a) before and b) after FC tests, respectively, together with the reference spectra of CoO, LaCoO 3 -20 K, and BaCoO 3 .O K-edge spectra of BCFZY700, BCFZY800, and BCFZY1100 c) before and d) after FC tests, respectively.The spectra of CoO, LaCoO 3 -20 K, and BaCoO 3 references are presented as comparison.

Figure 6 .
Figure 6.a) Comparison of the IÀV curves of cells with H 2 (10 vol% H 2 O) as a fuel source at 600 °C.b) I-V curves for cells with BCFZY800 anode measured in the electrolysis cell mode at 450-600 °C with H 2 (10 vol% H 2 O).

Table 1 .
PCFCs performances reported in literatures and this study with different cathode materials.

Table 2 .
Parameters of BCFZY samples obtained through 3D reconstruction.