Improving tubular protonic ceramic fuel cell performance by compensating Ba evaporation via a Ba-excess optimized proton conducting electrolyte synthesis strategy

Protonic ceramic fuel cells (PCFCs) are emerging as a promising technology for reduced temperature ceramic energy conversion devices. The BaCe0.4Zr0.4Y0.1Yb0.1O3−δ (BCZYYb4411) electrolyte is notable for its high proton conductivity. However, the tendency of barium to volatilize in BCZYYb4411 during high-temperature sintering compromises its chemical stability and performance. This study investigates the effects of intentionally incorporating excess barium into BCZYYb4411, formulated as Ba1+x Ce0.4Zr0.4Y0.1Yb0.1O3−δ (where x = 0, 0.1, 0.2, and 0.3), with the aim of compensating barium evaporation and enhancing the physical and chemical properties. We find that excess barium results in a greater shrinkage rate, facilitating a denser electrolyte structure. This barium-enriched electrolyte demonstrates improved electrochemical performance by effectively counteracting the deleterious effects of barium evaporation. Applying this strategy to tubular PCFCs, we achieved a peak power density of 480 mW∙cm−2 at 600 °C. This unique approach provides a simple, tunable, and easy-to-implement processing modification to achieve high-performance tubular PCFC.


Introduction
Protonic ceramic fuel cells (PCFCs) have generated interest for intermediate-temperature electrochemical energy conversion applications due to their potentially superior performance vs. solid oxide fuel cells (SOFCs) [1].Protonic ceramic electrolytes typically exhibit higher ionic conductivity and lower ion-migration activation energy than traditional oxygen ion conductors used in SOFCs due to the inherently higher mobility of protons vs. oxygen ions.Within this class of materials, barium-based perovskite oxides, particularly the acceptor-doped barium zirconate (BaZrO 3 ) and barium cerate (BaCeO 3 ) families, are notable and representative proton-conducting electrolytes [2].To introduce proton conductivity in these materials, acceptor dopants (particularly Y and/or Yb) are incorporated on B-site, e.g., BaZr 1-x (Y,Yb) x O 3−δ (BZ(Y,Yb)) and BaCe 1-x (Y,Yb) x O 3−δ (BC(Y,Yb)) [3].The zirconates are distinguished by their enhanced chemical stability compared to the cerates [4].However, they pose challenges in terms of sinterability, often requiring temperatures exceeding 1700 • C [5].In contrast, the barium cerates, while exhibiting high proton conductivity and improved sinterability, demonstrate lower chemical stability, especially in CO 2 or high H 2 O-containing atmospheres [6].Because BaZr (x) Ce (1-x) O 3 forms a complete solid solution, the B-site Zr and Ce concentrations can be tuned for specific applications.For example, BaCe 0.7 Zr 0.1 Y 0.1 Yb 0.1 O 3−δ (BCZYYb7111) exhibits superior electrical conductivity and a lower sintering temperature compared to more Zr-rich compositions, thereby offering improved performance [7].However, its chemical stability is unsuitable for CO 2 -containing environments [8].On the other hand, the more recently developed BaCe 0.4 Zr 0.4 Y 0.1 Yb 0.1 O 3−δ (BCZYYb4411) composition shows remarkable chemical stability under a wide range of atmospheres, at the cost of slightly reduced electrical conductivity and higher sintering temperatures [9].
Unfortunately, both BCZYYb7111 and BCZYYb4411 face the challenge of barium evaporation due to their high sintering temperature [9,10].This issue is ascribed to the relatively low enthalpy of evaporation of barium when compared to other cations like Zr, Ce, and Y. Ba evaporation adversely impacts the chemical stability, leading to stoichiometric imbalances and the formation of secondary phases [11][12][13].These second phases typically comprise of Y or Yb oxides, which result in reduced ionic conductivity and diminished performance [14].In addition, these secondary phases can hinder grain growth, thereby resulting in smaller grain size, which also adversely reduces the proton conductivity.This restricted grain growth, combined with the significantly lower (10-100x lower) ionic conductivity across grain boundaries compared to the grain bulk, necessitates higher sintering temperatures [15].Jin et al reported that both the electrical conductivity and grain sizes of Ba x Ce 0.7 Zr 0.1 Y 0.1 Yb 0.1 O 3−δ (x = 0.9-1.1)increase with increasing Ba concentration [16].For example, the conductivity of the stoichiometric electrolyte is about 2.5 times higher than Ba-deficient Ba 0.9 Ce 0.7 Zr 0.1 Y 0.1 Yb 0.1 O 3−δ .While mechanical strength is known to decrease with increasing grain size [17,18], this effect is generally modest compared to the significant impact of grain size on ionic conductivity.Therefore, achieving stoichiometric electrolytes with large grain sizes and excellent electrochemical performance is highly desirable, and this hinges, in part, on mitigating Ba evaporation and maintaining stoichiometry throughout the sintering process.
Several additional challenges must be overcome to enable the practical application of PCFCs beyond electrolyte optimization.Tubular PCFCs offer an attractive candidate design due to their rapid startup/shutdown, thermal cycling stability, superior mechanical strength, and suitability for large-scale fabrication [19,20].However, their performance is typically lower than planar PCFCs.This discrepancy may, in part, be attributed to increased challenges associated with preventing barium evaporation.For planar PCFCs, a commonly adopted method to mitigate barium evaporation involves placing an identical electrolyte pellet on top of the primary cell during the high temperature sintering process.Choi et al reported that placing a 'sacrificial' BCZYYb4411 pellet on top of the primary cell can effectively prevent barium evaporation [21].However, this method is not readily applicable to tubular geometries, where establishing mechanical contact between a sacrificial barium source and the electrolyte presents a significant design challenge.
The use of over-stoichiometric barium zirconate or cerate compositions has emerged as a potential solution, offering the dual benefit of suppressing barium evaporation and enhancing sinterability.The increased sinterability may result from an increase in diffusion coefficients, perhaps facilitated by the larger unit cell volume as well as potentially higher point defect concentrations, and the possible presence of a Ba-rich intergranular liquid oxide phase that can promote the densification process [22,23].Jin et al and Guo et al have demonstrated improved sinterability and conductivity with increasing Ba content in Ba x Ce 0.7 Zr 0.1 Y 0.1 Yb 0.1 O 3-δ (0.9 ⩽ x ⩽ 1.1) and Ba 1±x Ce 0.4 Zr 0.4 Y 0.2 O 3-δ (0 ⩽ x ⩽ 0.2) proton conductors, respectively [16,24].This is attributed to the positive effect of excess Ba on shrinkage rate and sinterability.The potential of super-stoichiometric electrolytes may therefore overcome the geometric challenges posed by tubular PCFCs, thereby enabling an easy way to compensate barium evaporation as well as lower the sintering temperature.
Here, we incorporate excess barium into BCZYYb4411 to compensate for barium evaporation effects during sintering.We characterize the structure of Ba 1+x Ce 0.4 Zr 0.4 Y 0.1 Yb 0.1 O 3-δ (x = 0, 0.1, 0.2, and 0.3) to identify the effect of excess barium incorporation on the unit cell.We then analyze the shrinkage behavior vs. temperature as a function of the barium concentration in the electrolyte.As the barium excess concentration increases, the shrinkage rate of the electrolyte also increases.This enables the fabrication of denser electrolytes at reduced sintering temperatures and compensates for barium evaporation during the sintering process.SEM and EPMA results demonstrate that the Ba excess 20 mol% electrolyte exhibits a single-phase surface without secondary phases and excellent density.As a result, PCFCs fabricated with Ba-excess 20 mol% electrolyte demonstrate enhanced sinterability and improved electrochemical performance.

3) electrolyte pellets and electrochemical characterization
BCZYYb powder was synthesized via solid-state reactive sintering (SSRS) [25].Stoichiometric amounts of BaCO 3 (Alfa Aesar, 99%), CeO 2 (Alfa Aesar, 99%), ZrO 2 (Alfa Aesar, 99%), Y 2 O 3 (Alfa Aesar, 99%), and Yb 2 O 3 (Alfa Aesar, 99%) were combined with 1 wt% NiO (Alfa Aesar, 99%) added as a sintering aid.Precursors are ball-milled in isopropanol for 24 h and dried in a heating oven.To fabricate the electrolyte pellets, 1 g of electrolyte was thoroughly mixed with 0.02 g of binder (10% polyvinyl alcohol 20 000 MW dissolved in water) and then dry pressed in a 12.7 mm diameter carbon-aided stainless-steel die.The pellets were pressed at 20 MPa for 1 min to form a white-colored electrolyte pellet.Pellets were individually fired at 1475 • C for 5 h.After that, the bottom sides of the electrolyte pellets were polished with sandpaper, and the pellets were ground down to a thickness of 0.5-0.55mm.

Dilatometer measurements
A pellet for dilatometry measurements was fabricated using the same method described above.The diameter of the pellet was reduced to 6.35 mm to accommodate instrument specifications.Linear shrinkage was measured with a Netzsch Dilatometer 402 A with 50 sccm synthetic air (21% O 2 + 79% N 2 ) from 25 to 1500

Electrical conductivity characterization of electrolyte pellets
For current collection, Pt mesh and wire were used.Those were attached with Pt paste and cured on a hot plate at 150 • C for 30 min.The electrical conductivities of the electrolyte pellets were measured by electrochemical impedance spectroscopy (EIS).A Gamry Reference 3000 was used for EIS measurement from 0.1 to 1 MHz with an AC excitation amplitude of 10 mV.The electrical conductivity was measured from 500 to 700

Anode-supported tubular PCFC fabrication
The anode substrate cermet precursor was prepared for extrusion with stoichiometric ratios of BaCO 3 , CeO 2 , ZrO 2 , Y 2 O 3 , Yb 2 O 3 , NiO, corn starch (Sigma Aldrich), and methylcellulose 4000 cP (Sigma Aldrich).Mass ratios were 53 wt% NiO, 28 wt% BCZYYb4411, 16 wt% corn starch, and 3 wt% methyl cellulose.The mixture was ball-milled in isopropanol for 24 h, and then dried on a hot plate.After that, 0.4 wt% of PVP 40 000 (Alfa Aesar) was added and the mixture was ball-milled a further 24 h without any solvent.The powder was then mixed with 28 wt% water in a mortar and pestle to make a ceramic clay.
The clay was extruded with a small-scale laboratory extruder; a homemade electronic oil pump maintained a constant extrusion pressure to the extrusion machine.The setup consisted of a hydraulic press connected to the pump to maintain a constant pressure on the spider die.The clay was extruded through the die to produce green tubes.The extruded tubes were dried at room temperature for 24 h is a tube-shaped wooden die.The wooden die consisted of two halves with four semicircular rows down the length of the die.The two die halves were weighted down to ensure the tubes remained straight during the drying process.The tubes were then dried at 50 • C for 1 h to facilitate further handling.The green tubes were hang-fired at 1100 • C for 2 h inside aluminum oxide guide tubes.

Electrolyte and air electrode preparation for tubular PCFC
The electrolyte layer was uniformly deposited onto the anode substrate using ultrasonic spray coating [26].The preparation of the electrolyte suspension involved a stoichiometric mixture of BaCO 3 , CeO 2 , ZrO 2 , Y 2 O 3 , Yb 2 O 3 , and 1 wt% of NiO with a homogeneous mixture of binder, plasticizer and dispersant (Heraeus V-006, Alfa Aesar PEG 400, and Alfa Aesar PVP 40 000, respectively).The slurry solution was comprised of 13 wt% electrolyte precursor powder, 1 wt% PEG 400, 1 wt% PVP 40 000, 2.5 wt% V-006 A, 2.5 wt% -terpineol (Alfa Aesar), and 80 wt% isopropanol.The solution was ball-milled with zirconia grinding media for 48 h to form a homogenous solution.Following mixing, the electrolyte slurry was deposited on the extruded anode substrate with an ultrasonic spray atomizer (Sonotek ALIGN).The tubes were continuously rotated at a constant speed while the electrolyte solution was spray deposited.After that, the electrolyte was dried for 24 h at room temperature to evaporate the IPA.Then the green tubes were fired at 450 • C for 1 h to burn out the organics.The half-cell tubes (anode-electrolyte) were hang-fired inside aluminum oxide guide tubes at 1475 • C for 5 h.BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3 (BCFZY) air electrode material was synthesized by a sol-gel method [1].This powder was mixed with 20 wt% single-phase BCZYYb7111 electrolyte powder to promote adherence with the electrolyte.The BCZYYb7111 powder was purchased from Terra Fuel Cell (South Korea).The composite electrode was mixed with a binder and dispersant (5 wt% Heraeus V006-A and 20 wt% Solsperse 28 000 diluted in alpha-terpineol, respectively) to form an ink that was brush-painted onto the electrolyte.The complete assembly was then sintered at 900 • C in air for 5 h using a heating ramp rate of 2 • C min −1 .The resulting final cells had a diameter of ∼0.58 cm and a length of ∼1 cm, with an electrode active area of 0.45 cm 2 .

Cell testing
After cathode application and firing, Au paste was applied on top of the cathode to improve the current collection.Subsequently, Ag wire was then wrapped on top of the Au layer, and short Ag gridlines were manually painted.An Au paste coating was also applied to the anode side and four Ag grid-lines were painted at about 90 • interval using a silver paste (DAD-87), and then baked at 200 • C for an hour on a hotplate to ensure adhesion.The full cell assembly was mounted on an alumina oxide tube and sealed using Cerambond (552-VFG), and it followed the testing direction of the previous study [27].Briefly, the cell was operated under high-purity hydrogen as the fuel and humidified (3 vol% water vapor) synthetic air (21% O 2 + 79% N 2 ) as the oxidant.The cell was reduced at 600 • C under 100 vol% H 2 to the anode.The humidified air was fed to the cathode during the reducing process of the cell.The electrochemical testing was performed using a Gamry Reference 3000.The EIS measurements were performed at open circuit voltage (OCV) using an AC signal amplitude of 10 mV in the frequency range of 10 6 -1 Hz.Durability testing was performed using the same gas flows at 600 • C under a 200 mA cm −2 constant current load.

Crystal structure and microstructure characterization
Structural information for the synthesized Ba excess electrolyte pellets was acquired by a PANalytical PW3040 x-ray diffractometer (XRD) with Cu-Kα radiation tube voltage 45 kV, and tube current 40 mA.The microstructure was investigated using field emission scanning electron microscopy (FE-SEM, JEOL JSM-7000F).Elemental mapping was conducted by electron probe microanalysis (EPMA, JXA-8100, JEOL) with an attached wavelength-dispersive spectrometer (WDS).The composition and morphology of the electrolyte surface were investigated with EPMA-WDS and FE-SEM, respectively.

Results and discussion
We introduced the excess barium BCZYYb4411 electrolyte strategy to compensate for Ba evaporation.The chemical formula of BCZYYb4411 was modified to increase the barium concentration on the A-site with the ratio increasing from 1 to 1.3.The excess barium BCZYYb4411 compositions were denoted by Ba excess 0% to 30 mol%, corresponding to Ba 1+x Ce 0.4 Zr 0.4 Y 0.1 Yb 0.1 O 3-δ (x = 0, 0.1, 0.2, and 0.3).To fabricate the Ba excess pellets, SSRS with 1 wt% of NiO was used.The pellets were individually sintered at 1475 • C for 5 h.To investigate the structure of the Ba excess 0, 10, 20, and 30 mol% pellets, XRD analysis was performed as shown in figure 1.
The XRD patterns reveal a single-phase perovskite oxide with cubic lattice symmetry (Pm3m) for all pellets up to 20 mol% barium excess.This observation indicates the successful incorporation of excess Ba into the lattice.Furthermore, lattice expansion is suggested by a monotonic shift in peak positions to lower angles with increasing Ba excess, which corresponds to the incorporation of Ba 2+ into the lattice as shown in figure 1(b).
This lattice expansion was analyzed through XRD refinement (supplementary figure S1), and the results are summarized in table 1.The lattice volume of Ba excess BCZYYb4411 expands from 79.87 to 80.78 Å 3 with the increase in the excess barium from 0 to 30 mol%.Interestingly, the XRD pattern for the Ba excess 30 mol% show partial phase transformation from cubic to orthorhombic structure [28,29].A secondary peak around 29 • , attributed to the orthorhombic phase, is identified based on refinement by Rietveld analysis (supplementary figure S1(d)).Although this orthorhombic phase constitutes a relatively minor proportion (7.4%) compared to the predominant cubic phase (92.6%), the result suggests that the Ba 30 mol% excess electrolyte may surpass the solubility limit in the A-site of the lattice.
It is known that shrinkage behavior differences between the electrolyte and anode substrate can lead to the formation of pinholes or porous electrolytes during PCFC fabrication [30].The shrinkage rate of the anode substrate typically exceeds that of the protonic conducting electrolyte [31].Consequently, enhancing the sinterability of the electrolyte is critical to minimize the shrinkage mismatch during cell fabrication.Intriguingly, the excess Ba electrolyte compositions lead to enhanced sinterability.Dilatometry results, presented in figure 2, show the correlation between temperature and electrolyte shrinkage rate as a function  of the Ba excess.Notably, the electrolyte shrinkage rate increases significantly with the rise in excess barium concentration.For example, the Ba excess 20 and 30% compositions reach linear shrinkage values of 37 and 54% at 1500 • C vs. 16% for the stoichiometric composition.In addition, the shrinkage rate of Ba excess 20 mol% matches most closely with the shrinkage rate of the anode substrate.This improved match in anode and electrolyte shrinkage rates may therefore further explain the enhanced densification and reduced defect density for the Ba excess 20 mol% electrolyte composition.
We studied the impact of excess barium on the electrolyte conductivity via AC impedance spectroscopy of bulk electrolyte pellets under wet N 2 atmosphere with Pt wire and paste-based electrodes.While barium excess positively enhances the shrinkage rate, it negatively impacts the ionic conductivity, as shown in figure 3.This phenomenon aligns with findings reported by Ma et al, who found that over-stoichiometry on the A-site of the electrolyte leads to reduced conductivity [32].Excess barium is hypothesized to facilitate the formation of an amorphous BaO phase at grain boundaries which hinders proton transport, even though it may contribute to increased sinterability.
To further study the practical application of the barium excess electrolyte strategy, electrolyte layers containing from 0 to 30 mol% Ba excess were deposited on anode-supported tubular PCFCs.These anode/electrolyte "half-cell" tubes were fired at 1475 • C under direct exposure to ambient air without a sacrificial powder bed or cell covering.To evaluate the enhanced sintering effects due to barium excess, FE-SEM was used to examine the as-fired electrolyte surface morphology as shown in figure 4.This analysis revealed notable differences in electrolyte surface quality depending on the level of barium excess.Specifically, electrolytes with lower barium concentrations (0 and 10 mol%) displayed small pinholes and  secondary phases.We attribute the remnant porosity to the lower shrinkage rates associated with these lower levels of barium excess.In contrast, the electrolytes with higher excess barium concentration (20 and 30 mol%) demonstrated improved densification, with a dramatic reduction in pinholes and a slight increase in grain size.The improved surface morphology of the Ba excess 20 and 30 mol% cells indicates that higher barium concentrations effectively enhance electrolyte sintering and densification, resulting in the development of a more uniform grain structure with reduced surface defect density.
To investigate the potential presence of secondary phases on the surface of the Ba excess electrolytes, FE-EPMA with WDX was pursued as shown in figure 5.The surface of Ba excess 0% displayed a distinct secondary phase identified as Y or Yb oxide, in addition to NiO precipitates.The highly heterogeneous, multi-phase nature of the stoichiometric electrolyte cell was attributed to phase instabilities triggered by barium loss.Specifically, we hypothesize that the loss of A-site Ba leads to an enriched concentration of B-site cations on the electrolyte surface.The excess B-site cations, once they surpass the solubility limit within the lattice, lead to the formation of second phase B z O y oxides.
In addition, a significant number of secondary-phase NiO particles were also observed to segregate on the electrolyte surface.Due to its smaller ionic size (0.69 Å, 6-fold coordination) compared to host ions in the electrolyte A and B sites (such as Ce, Zr, and Ba), Ni is favored to segregate on the surface rather than incorporate into the electrolyte lattice [33][34][35].Phase-segregation is suppressed in the higher Ba excess electrolyte cells.As a result, the Ba excess 20 and 30 mol% samples exhibit a relatively uniform elemental distribution on the surface without a significant number of secondary phases (although a few NiO precipitates are still observed in the 30 mol% Ba excess cell).
Based on the EPMA-WDS results presented above, we hypothesize that the higher Ba-excess electrolytes cells, despite their reduced total conductivity, might lead to better electrochemical performance than the low  Ba-excess electrolyte cells due to the suppression of surface defects and second phase heterogeneities.To examine this hypothesis, anode-supported tubular cells were fabricated using Ba excess electrolyte (0%, 10%, 20%, and 30 mol%).Cell morphology was examined via postmortem SEM as shown in figures 6(a)-(c).The porous anode substrate supports a thin, dense electrolyte layer.The electrolyte thickness was controlled using an ultrasonic sprayer to a consistent value of 7-8 µm, though the 0% Ba excess cell had a slightly thicker electrolyte (∼10 µm).All cells also featured a porous BCFZY4411 + BCZYYb7111 air electrode layer approximately 8-12 µm in thickness.
Cell polarization curves (figure 6(d)) show that Ba excess 10, 20, and 30 mol% electrolyte cells produced stable OCV values of 1.04, 1.06, and 1.016 V, respectively.In contrast, the Ba excess 0% cell exhibited a lower OCV of 0.95 V and showed unstable behavior, despite having a slightly thicker electrolyte (10 µm) compared to the other cells (as shown in figure S2).This instability was attributed to the presence of substantial electrolyte defects (e.g.pinholes) as previously observed from the electrolyte surface morphology investigations.Unfortunately, the current-voltage response of the 0% Ba excess cell could not be measured due to its instability.
The fuel cell based on the Ba excess 20 mol% electrolyte achieved the highest performance, delivering a power density of 480 mW•cm −2 at 600 • C.This power density was notably higher than that of the Ba excess 10% cell (417 mW•cm −2 ) and the Ba excess 30% cell (365 mW•cm −2 ).The Ba excess 20 mol% cell appears to optimally balance improved electrochemical performance thanks to the formation of a homogenous, dense electrolyte free of second phase segregation offset by the reduction in bulk ionic conductivity caused by the excess Ba.These results were further verified by EIS measurements.Nyquist plots in figure 6(e) reveal that the Ba excess 20 mol% cell exhibited the lowest values for both ohmic and polarization resistance with values of 0.29 and 0.153 Ω•cm 2 , respectively.In comparison, the ohmic resistance for the 10 and 30% Ba excess cells were higher at 0.395, and 0.553 Ω•cm 2 and the polarization resistance values reached 0.199 and 0.319 Ω•cm 2 respectively.Since we previously found that Ba excess decreases the bulk ionic conductivity of the electrolyte, the lower ohmic resistance of the Ba excess 20 mol% cell can likely be attributed to a lower-resistance electrolyte/electrode interface due to the improved electrolyte surface quality and the suppression of second-phase segregation.Indeed, XRD measurements demonstrate no secondary phases forming in a mixture of Ba excess 20 mol% electrolyte and NiO powders subjected to heat treatment at the cell sintering temperature of 1475 • C for 5 h (figure S3).The performance of the 20 mol% Ba excess tubular PCFC is compared with other literature reports as shown in supplementary table S1.Remarkably, the Ba excess 20 mol% cell attains record high performance at 600 • C vs. prior work measured under the same operating temperature, despite employing a BCZYYb4411 electrolyte as opposed to the BCZYYb7111 electrolyte used in most previous studies [20 , 36].BCZYYb4411-based cells tend to produce lower performance than BCZYYb7111-based cells and also generally require a higher electrolyte-sintering temperature.
The I-V performance and OCV EIS response of the Ba excess 20 mol% cell was evaluated at temperatures ranging from 600 to 700 • C (figures 7(a) and (b)).This cell reached maximum power densities of 394, 473, 538, 613, and 701 mW•cm 2 at 600, 625, 650, 675, and 700 • C, respectively.Several duplicate cells were also tested, demonstrating similar performance under identical operating conditions, as shown in figure S4.Both the ohmic and polarization resistances show temperature activated behavior; the Arrhenius plots are presented in figures 7(c) and (d).An activation energy of 0.268 eV was obtained for the ohmic resistance component, aligning reasonably well with existing reference data in the range of 0.3-0.5 eV, which is typical for PCFC material [37].Meanwhile, an activation energy of 0.674 eV was obtained for the polarization resistance, again comparable (although on the lower end) to values obtained from other PCFC studies [38].

Conclusions
In this work, we investigated a series of barium excess BCZYYb4411 electrolyte formulations to compensate for barium evaporation during the high temperature sintering of PCFCs.We found that excess barium is incorporated in lattice of BCZYYb4411 with a consequent increase in lattice volume.When the barium concentration exceeds the solubility limit (as seen in the 30% Ba excess composition), it induces phase separation.We find that electrolytes formed from Ba excess 20 and 30% compositions exhibit a significantly higher shrinkage rate than those formed from the stoichiometric or 10% Ba excess compositions, but also lead to lower electrolyte conductivity.The higher Ba-excess (20 mol% and 30%) compositions enable improved electrolyte densification, a reduction of pinholes/surface defects and compositional uniformity of the electrolyte surface.Tubular PCFCs were fabricated with the 0, 10, 20, and 30% Ba excess electrolytes to assess the impact on overall electrochemical performance.The Ba excess 20 mol% electrolyte-based cell achieved the highest performance, with a peak power density of 480 mW•cm −2 at 600 • C, exceeding cells with either higher or lower Ba concentrations.We hypothesize that the 20 mol% Ba-excess composition optimally balances the benefits of enhanced electrolyte densification and improved chemical homogeneity against the lower bulk ionic conductivity caused by excess Ba.

Figure 3 .
Figure 3. Temperature dependence of the total conductivity of Ba excess electrolytes in wet N2 from 500 to 700 • C.

Figure 5 .
Figure 5. FE-EPMA WDS results of the electrolyte surface of tubular PCFCs with Ba excess electrolytes.The elemental distribution was investigated for Ba, Ni, Y, and Yb.

Figure 7 .
Figure 7. Temperature dependence electrochemical characterization of the Ba excess 20 mol% tubular PCFC, (a) Polarization curves of tubular PCFC with Ba excess 20 mol% cell from 600 to 700 • C, (b) EIS Nyquist plot of Ba excess 20 mol% cell from 600 to 700 • C, (c) ohmic resistance and, (d) polarization resistance from 600 to 700 • C.