# Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells

L. Bernadet, F. Buzi, F. Baiutti, J. Segura-Ruiz, J. Dolado, D. Montinaro, M.

Torrell, A. Morata, A. Tarancón

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L. Bernadet, F. Buzi, F. Baiutti, J. Segura-Ruiz, J. Dolado, et al.. Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells. APL Energy, 2023, 1 (3), pp.036101. 10.1063/5.0159765. [hal-05077991](https://hal.science/hal-05077991v1)

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# Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells

L. Bernadet ; F. Buzi ; F. Baiutti ; J. Segura-Ruiz ; J. Dolado ; D. Montinaro ; M. Torrell ;

A. Morata ; A. Tarancón

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APL Energy 1, 036101 (2023)

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# Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells

Cite as: APL Energy 1, 036101 (2023); doi: 10.1063/5.0159765

Submitted: 26 May 2023 • Accepted: 4 October 2023 •

Published Online: 17 October 2023

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L. Bernadet,<fnref n="1" />,ª ID F. Buzi,<fnref n="1" /> ID F. Baiutti,<fnref n="1" /> ID J. Segura-Ruiz,<fnref n="3" /> ID J. Dolado,<fnref n="3" /> ID D. Montinaro,<fnref n="2" /> ID M. Torrell,<fnref n="1" /> ID

A. Morata,<fnref n="1" /> ID and A. Tarancón<fnref n="1" />,<fnref n="4" /> ID

## AFFILIATIONS

## ABSTRACT

Highly efficient solid oxide cells are one of the most promising technologies for a sustainable future based on renewable hydrogen. The diffusion barrier layer employed between zirconia-based electrolytes and state-of-the-art oxygen electrodes aims to limit the formation of electrically insulating secondary phases that dramatically reduce the cells' performance. Conventional barrier layers manufactured by screen-printing technology lead to porous microstructures that enable the formation of insulating $SrZrO_3$, partially blocking the active area of the cells. Opposite, homogeneous and dense barrier layers have proven to be the ultimate solution to limit interdiffusion, substantially improving the cells' performance. Despite the relevance of this solution, the impact of the barrier layer thickness on the final performance of the cells is still unknown. In this work, gadolinia-doped ceria barrier layers with thicknesses between 200 and 800 nm made by pulsed laser deposition were studied in button cells. Excellent electrochemical performance was obtained for all the cells, improving 45% of the power output of the reference counterparts. Moreover, durability tests performed on the cell with the thinnest layer (200 nm) did not show any measurable degradation for 3500 h of continuous operation under high current densities of 0.77 A cm⁻<fnref n="2" /> (~0.87 V) at 750 °C. Post-mortem characterization by synchrotron nano-x-ray fluorescence of a pristine cell and the aged cell allowed us to observe that some spots of $SrZrO_3$ were present at the cathode/electrolyte interface since the cell manufacturing step without increasing during long-term operation. Indeed, the discontinuity of this insulating phase seems not to be critical for cell operation.

© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0159765

## INTRODUCTION

Hydrogen as an energy vector is now considered a solution to reduce the fossil fuel dependency of society. Its production by electrolysis with electricity from renewable sources and its subsequent consumption through fuel cells represent a carbon-free closed loop. Among the different types of fuel cells, Solid Oxide Fuel Cells (SOFCs) present the advantage of being the most efficient ones both in electrolysis and fuel cell mode. Solid Oxide Cells (SOCs) are typically composed of four main layers: a dense electrolyte made of yttria-stabilized zirconia (YSZ), two porous electrodes made of

Ni-YSZ cermet for the hydrogen electrode and $(La,Sr)(Co,Fe)O_{3-δ}$ (LSCF) mixed ionic-electronic conductors for the oxygen electrode, respectively, and finally a gadolinium doped ceria (CGO) barrier layer placed between the electrolyte and the oxygen electrode.<fnref n="1" /> The introduction of this diffusion barrier layer was a key advance in making the use of robust electrolytes such as YSZ in combination with highly performant LSCF electrodes compatible. The use of this layer avoids the formation of insulating phases such as strontium zirconate ($SrZrO_3$) and lanthanum zirconate $(La_2Zr_2O_7)$ and the subsequent decomposition of the cathode material.<fnref n="2" />⁻<fnref n="4" /> However, state-of-the-art barrier layers fabricated by screen printing

<footnote n="1">¹ IREC, Catalonia Institute for Energy Research, Department of Advanced Materials for Energy Applications, Jardins de les Dones de Negre 1, Sant Adrià del Besòs, 08930 Barcelona, Spain</footnote>

<footnote n="2">² SolydEra SpA, Viale Trento 117, Mezzolombardo 38017, Italy</footnote>

<footnote n="3">³ ESRF, The European Synchrotron, 71 Avenue des Martyrs, Grenoble 38043, France</footnote>

<footnote n="4">⁴ ICREA, Passeig Lluís Companys 23, Barcelona 08010, Spain</footnote>

<footnote>ª Author to whom correspondence should be addressed: lbernadet@irec.cat. Tel.: + 34 933 562 615</footnote>
are always porous (due to a limitation in the maximum sintering temperature fixed by the formation of deleterious YSZ-CGO solid solutions) and do not completely avoid the formation of previously mentioned secondary phases while introducing additional resistance due to their porosity.⁵⁻⁷ Indeed, the porous CGO allows the Sr vapor species to be transported up to the YSZ electrolyte, as well as the CGO-free surfaces seem to be preferential pathways for Zr and Sr diffusion, ending up in the formation of detrimental zirconates.⁶,⁸ Those two phenomena principally happen during the manufacturing process when the oxygen electrode is sintered on top of the barrier layer at a temperature range of 1000–1250 °C.⁴,⁶,⁹,¹⁰

As previously outlined, barrier densification to avoid the formation of insulating phases is a challenge since sintering the barrier layer above 1200 °C results in a strong cation interdiffusion of CGO and YSZ species with an associated resistance increase, as reported by Szàsz *et al.*⁵ In this regard, Matsui *et al.* studied the conduction properties of several CGO-YSZ solutions and showed that they all had lower ionic conductivity.¹¹,¹² Importantly, beyond the impact on the overall performance, it has been pointed out as the cause of severe degradation mechanisms after long-term operation, such as extensive fractures appearing all along the electrolyte.¹³ Longer sintering times at lower temperatures have the same effect, also impacting the performance of the cell.¹⁴ Moreover, even at 1400 °C, the densified barrier layer could not completely block the SrZrO₃ formation.⁵,¹⁴ To maintain the sintering temperature below 1200 °C while densifying the barrier layer, some other techniques were employed, such as infiltration,¹⁵,¹⁶ hydrogel solution deposition by spin-coating,¹⁷ or spray-pyrolysis.¹⁸ Alternatively, thin film deposition techniques were also applied to obtain dense layers at lower temperatures. In particular, good results were obtained by magnetron sputtering,¹⁹⁻²² Electron Beam Physical Vapor Deposition (EB-PVD),⁹,²³⁻²⁵ and Pulsed Laser Deposition (PLD).²²,²⁶,²⁷ In most of the cases, an annealing step was added to reduce the number of grain boundaries of the dense films, along which a strong Zr- and Sr-diffusivity was observed.²³,²⁸,²⁹ Especially relevant results were obtained by Morales *et al.*, showing a performance increase of 70% when using a PLD barrier layer in state-of-the-art SOFC cells from SolydEra.²⁷

Due to the diffusive nature of the phenomenon, the barrier layer thickness and density play fundamental roles in its cation-blocking capability, independent of the employed deposition technique.²⁸ Railsback *et al.* studied the presence of Sr inside a non-fully dense barrier layer by changing its thickness from 1.5 to 6.0 µm. They found out that a minimum layer thickness of 4.5 µm is required to avoid zirconate formation.³⁰ In the same direction, Molin *et al.* managed to produce an efficient barrier layer of 0.7 µm by spray pyrolysis, whereas cells based on 0.3 µm-thick CGO barrier layers do not present any advantage.¹⁸ On their side, Riegraf *et al.* found that 0.5 µm-thick barrier layers fabricated by EB-PVD were enough to avoid the formation of SrZrO₃, while thinner barrier layers of 0.15 µm in thickness were not sufficient.²⁴ In this regard, optimizing the minimum thickness of a barrier layer is of special relevance in the case of time-consuming physical vapor deposition methods.

The present work aims to find the minimum thickness of a CGO barrier layer made by PLD, starting from the outstanding performance reported by the authors for a 1.5 µm-thick layer deposited by PLD at button-cell and large-area cell levels.²⁷ Beyond the performance increase, and more interestingly, such an efficient barrier

layer already proved long-term stability in real SOFC operation conditions after 14 000 h of operation. As reported by the authors, the 1.5 µm-thick PLD barrier layer implemented in large-area cells was intact after more than two years of operation as part of a 5-cell stack, ensuring higher cell performance all along the test and avoiding the formation of a crack inside the electrolyte that was observed for reference cells (with screen-printed barrier layers) in the same stack.¹³ In the present study, the authors explore barrier layers with thicknesses in the range between 200 and 800 nm to ensure the scalability of the concept in industrial PLD equipment. Button cells with such barrier layers were fabricated and electrochemically characterized, including a 3500 h long-term test for the thinnest barrier layer. Finally, an advanced post-mortem characterization by nano-x-ray fluorescence (XRF) of pristine and aged cells is presented to determine the efficiency of the diffusion barrier layer.

## EXPERIMENTAL METHODS

### Solid oxide cells fabrication

Anode-supported button half cells made with a porous Ni-YSZ cermet of 260 µm and a dense 8YSZ electrolyte were used in this study. They were manufactured by aqueous tape-casting and co-sintered at 1400 °C. A Ce₀.₈Gd₀.₂O₃⁻δ layer was deposited on top of the electrolyte by means of the large-area PLD technique using PVD5000 (PVD Products) equipment able to host four different targets up to 10 cm and cover the whole surface of a circle of 12 cm in diameter. A sample holder was allowed to perform the deposition onto six Ø20 mm half cells at the same time. Deposition parameters were the following: distance between the sample and the target set to 90 mm, oxygen pressure in the chamber of 20 mTorr, chamber temperature fixed to 600 °C, laser fluence of 1 J cm⁻² per pulse at a frequency of 10 Hz, and target rotation. To control the thickness of the deposited barrier layer, the number of laser pulses was varied. Three sets of samples were prepared using 96 000, 48 000, and 24 000 pulses, corresponding approximately to 800, 400, and 200 nm. To favor grain growth, an annealing of the layer was then performed at 1150 °C for 2 h. This temperature had been optimized in the following work.²⁷ A two-layer cathode composed of LSCF-CGO and LSCF6428 and a LaSrCoO₃⁻δ (LSC) layer as a current collector were screen printed on top of the barrier layer and sintered at 1100 °C.

A reference cell was made with a Ce₀.₉Gd₀.₁O₃⁻δ barrier layer deposited by standard screen-printing techniques and then sintered at ~1300 °C. The rest of the layers (cermet, electrolyte, oxygen electrode, and current collector) were similar to the ones from the cells with the PLD barrier layer.

### Electrochemical characterization

The electrochemical characterization of the cells was performed in a commercial ProboStat™ (NorECs AS) test station placed in a vertical tubular furnace. The cell support and gas pipes were all made of alumina to avoid Cr contamination. Nickel mesh on the fuel side and gold mesh on the oxygen side, together with nickel paste and gold paste, respectively, were used to collect the current. Gas tightness between the fuel chamber and the air chamber was ensured by using a Ceramabond™ (Aremco) sealant. A Maynuo M9812 electronic load was used to regulate the current flow. In addition, a Velleman LABPS3005D power source was connected in series to
compensate for the voltage losses through the cables. Electrochemical impedance spectroscopy (EIS) measurements were performed with a PARSTAT® 2273 potentiostat at Open Circuit Voltage (OCV) and 0.9 V. Spectra were acquired in a frequency range of 10⁵ to 10⁻¹ Hz with an amplitude of 50 mA. Polarization curves were obtained by applying a sweeping rate of 20 mA s⁻¹ in galvanostatic mode. Finally, a long-term durability test was performed for 3500 h at 0.77 A cm⁻². All the tests were performed in SOFC mode. Gas flows are indicated in each related figure.

## Compositional and microstructural characterization

First of all, the barrier layer thickness was evaluated right after deposition by spectroscopic ellipsometry (UVISEL, Horiba Scientific). The studied spectral range was from 1.5 to 5.0 eV, setting the incident angle to 70°. The layer thickness was then obtained by using a Tauc-Lorentz dispersion model consisting of one oscillator for the CGO and three oscillators for the substrate.³¹ X-ray diffraction (XRD) measurements were performed to verify the absence of potential interdiffusion between YSZ and CGO after the barrier layer deposition. Bruker-D8 Advance equipment using Cu-Kα radiation with a nickel filter and Lynx Eye detector was used at room temperature in the 2q range from 20° to 90°. Raman spectra were recorded on top of the NiO-YSZ/YSZ/CGO half-cell using a high-resolution, confocal XploRA PLUS confocal Raman microscope (OLYMPUS BX43, Horiba Scientific). An excitation line of 532 nm and a 100× objective were employed.

Post-mortem characterization of the cells was then conducted by Scanning Electron Microscopy (SEM, Zeiss Auriga) to observe the cell microstructure and by nano-XRF to determine the chemical composition and the phases around the barrier layer with nanoscale precision. To perform those experiments, thin lamellas were prepared by FIB from the cross-section of embedded cells. This way, the lamellas, 1–2 µm thick and 20 µm long, contained all the layers of interest: electrolyte, barrier layer, and oxygen electrode. Nano-XRF was performed at the ID16B beamline of the European Synchrotron Radiation Facility (ESRF).³² The focus area of the x-ray beam was equal to 51 × 51 nm² (V × H). Two multi-element Si-drift detectors (3 and 7 elements, 50 mm² of active area each) provided high sensitivity for XRF signal detection. To excite the Kα,β-XRF lines of the Zr and obtain a clear signal for this element, the x-ray energy of 29.5 keV was used. Indeed, lower energies used in other studies present in the literature could not detect the small changes in the distribution where the L-lines of this element were detected.³³,³⁴ The scanning area was set to 7.5 × 4.5 µm² using a piezo-motor stage with a step size of 25 × 25 nm² and accumulation times of 1000 ms. PyMca software was used to fit the XRF data on a pixel-by-pixel basis. Intensity maps for each elemental XRF emission line were obtained from fitting.³⁵

## RESULTS AND DISCUSSION

Large-area PLD (LA-PLD) was employed to deposit CGO barrier layers on top of SOC half-cells of NiO-YSZ/YSZ fabricated by SolydEra. The as-deposited barrier layers were characterized by XRD and Raman, as seen in Fig. 1 for the 200 nm-thick CGO layer. The XRD pattern in Fig. 1(a) can be indexed with crystalline

gadolinia-doped ceria, showing the presence of a single phase without remarkable secondary phases. The Raman spectrum in Fig. 1(b) presents the characteristic bands from CGO (F₂g mode at 460 cm⁻¹) and from 8YSZ. It corresponds to a t''-ZrO₂ phase that was previously reported for doped zirconia with 8 to 9 mol.% Y₂O₃.³⁶ The same results were obtained for the two other types of cells. The interdiffusion of YSZ and CGO after the barrier layer annealing process was verified by XRD. Figure S1 in the supplementary material shows the complete pattern (a) and a zoom on the main YSZ and CGO peaks (b). No changes in the peak position could be detected before and after annealing, confirming that the annealing conditions that were optimized in previous work from the group for a thicker barrier layer were still valid for thinner ones.²⁷

The thickness along the cell diameter of the different samples was evaluated by fitting spectroscopic ellipsometry data, shown in Fig. S2, using a Tauc-Lorentz dispersion model. From the experimental light intensities components Iₛ and I_c presented in Fig. S2, one can already see how the higher layer thickness flattens the first portion of the plot around 1.5–3 eV, which is due to the higher absorption of light from thicker CGO that reduces the amount of light crossing the film and getting reflected out. Data fitting was performed by considering a semi-infinite YSZ substrate with a dense CGO layer on top and a thin slightly porous layer of CGO (density set to 93%) to emulate the PLD layer roughness. Figure 2 shows the PLD layer thicknesses along the cell diameter for the three types of cells, which are estimated to be 196 ± 30 nm, 442 ± 28 nm, and 832 ± 38 nm. The thickness variation is equal to 27%, 12%, and 9%, respectively.

Figure 3 shows cross-section SEM images of the barrier layers from pristine cells after sintering and LSCF oxygen electrode attachment. Independently on the film thickness, the PLD barrier layers are perfectly attached and are highly covering the surface of the electrolyte, ensuring a sharp transition between the YSZ and CGO (as observed in Figs. 3(a)–3(c) due to the notable brightness contrast between the two materials). Figure S3 shows the top views before and after the PLD barrier layer deposition and after the annealing treatment for both the cells with a 400 and a 200 nm CGO layer. One can see that the CGO is growing with the same orientation as the underneath YSZ grains, conforming to the common epitaxial growth of PLD CGO thin film on YSZ. Indeed, the columnar grain microstructure typically observed for PLD-grown polycrystalline layers is not present, but rather only large grains similar to the ones from the YSZ substrate. Some roughness could be observed from the top images [Figs. S3(c) and S3(f)]. Nevertheless, the cross-section images showed homogeneous layers without, as expected, any visible grain boundary. Barrier layer thicknesses measured by SEM, with a variation of 10% (750 ± 75 nm, 390 ± 39 nm, and 180 ± 18 nm), are in good agreement with the ones obtained by ellipsometry. Finally, the cross-section images of the cells also show oxygen electrodes with open porosity, good percolation, and excellent oxygen electrode attachment without the presence of delamination or significant formation of secondary phases. The thickness of the electrolyte (8 µm) can be appreciated in Fig. S4.

The electrochemical characterization of full cells made with PLD barrier layers with different thicknesses is presented in Fig. 4. Figure 4(a) shows voltage-current density and power density curves measured at 750 °C with an atmosphere of pure H₂ at the fuel electrode and air at the oxygen electrode. First, an Open Circuit
<figure><img src="image_10.png" /><figcaption>FIG. 1. (a) X-ray diffraction pattern and (b) Raman spectra of the top surface of the as-deposited 200 nm CGO barrier layer on a NiO-YSZ/YSZ half-cell. Peaks from the XRD pattern related to each lattice plane are identified with Miller indices (hkl).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>FIG. 2. Thicknesses of the 200, 400, and 800 nm PLD-deposited CGO barrier layers along the cell diameter. A picture of one half-cell with the distribution of the points where the measurements were performed is present as an inset.</figcaption></figure>

Voltage (OCV) as high as 1.22 V (close to the theoretical value) was obtained for the three cells, indicating the absence of gas leakages, i.e., adequate gas tightness. Additionally, the complete set of cells showed identical performance, reaching very high current densities of 1.50 A cm⁻² and power densities of 1.05 W cm⁻² at the typical SOFC operating voltage of 0.7 V. Those values represent a 45% increase in reference cell counterparts made with screen-printed CGO barrier layers (tested with the same operating conditions).²⁷ More specifically, reference cells provide a current density of 1.11 A cm⁻², equivalent to a power density of 0.79 W cm⁻², at 0.7 V and 750 °C [see Fig. 4(a)]. As an additional electrochemical characterization, V-I curves at 650 and 700 °C are presented in Fig. S5. At 0.7 V, a power density around 0.6 and 0.9 W cm⁻² is obtained for the three types of cells.

EIS measurements of all the cells were performed at the same temperature for OCV and 0.9 V operation conditions [see Nyquist plots in Figs. S6 and 4(b)]. EIS spectra were all fitted using an electrical equivalent circuit consisting of an inductive contribution related to the setup, in series with the serial resistance attributed to the

current collection and the ionic conductivity from the electrolyte and the barrier layer, and the connection in series of RpnQn elements that represent the polarization impedance of the electrodes. Following the rule of using as few RpnQn elements as possible, two were needed to fit the cells made with the PLD barrier layer, while three were used to fit the spectra from the reference cell. One can see that the three types of cells give similar results. Under polarization, at 0.9 V, the values of serial resistance are close for the three cells (0.09 Ω cm² with 400 nm and 0.10 Ω cm² with 200 nm-thick and 800 nm-thick barrier layer; please note that those values are lower than the ones visible in Fig. 4(b) because they were obtained through the spectra fittings to compensate for the high-frequency arc deformation by the inductive contribution). This difference can hardly be attributed to the change in barrier layer thickness, as it is within the range of the cell and test setup variability. One can see that the Nyquist plot for the reference cell mainly differs from the serial resistance, with a value twice higher than the one from the PLD barrier layer. This difference is consistent with previous results and was attributed to the presence of SrZrO₃ at the interface between the electrolyte and the barrier layer, to the YSZ/CGO interdiffusion during the manufacturing process, and to the porosity of the CGO layer.¹,³,²⁷ Moreover, the Bode plot present in Fig. 4(c) allows us to see that the middle range frequency arc (10¹⁻¹⁰² Hz) is strongly impacted by the change of barrier layer. This contribution is known to be possibly related to the O²⁻ transfer at the interlayer/electrolyte interface, which is aligned with this result.²⁴,²⁶ The use of a dense barrier layer leads to a decrease in this middle-range frequency arc by most likely avoiding the oxygen transfer limitation caused by the presence of SrZrO₃ in screen-printed interlayers, as well as maximizing the transfer area by having a dense layer.

At OCV, see Fig. S6, the cell made with the 400 nm barrier layer shows a bigger polarization resistance than the two other cells, but the values of serial resistance are the same as under polarization. Indeed, the low-frequency contribution is related to the gas conversion process, which is highly dependent on the steam content of the inlet gas. Even though the inlet gas is supposed to be pure H₂, some traces of H₂O are inevitably present. Therefore, a slight variation in steam concentration can strongly impact the low-frequency anode resistance.³⁷,³⁸
<figure><img src="image_12.png" /><figcaption>FIG. 3. SEM images of the cells made with a PLD barrier layer using (a) 96 000, (b) 48 000, and (c) 24 000 laser pulses.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>FIG. 4. (a) V-I curves for full cells of Ni-YSZ/YSZ/PLD_CGO/CGO-LSCF/LSCF/LSC with different thicknesses of PLD-deposited barrier layers and one reference cell made with a screen-printed CGO barrier layer, measured in SOFC mode at 750 °C. (b) Nyquist and (c) Bode plots of EIS measurements carried out for the same fuel cells at 0.9 V. Gas flows were equal to 16 NmL·min⁻¹·cm⁻² of H₂ at the fuel electrode and 40 NmL·min⁻¹·cm⁻² of air at the cathode.</figcaption></figure>

Overall, the obtained results indicate that all the cells perform similarly within the experimental accuracy.

Due to presenting similar performance, long-term durability and stability tests under operation were carried out in cells with

the thinnest barrier layers, i.e., cells with 200 nm-thick CGO barrier layers. Figure 5(a) shows the voltage evolution with time under current densities as high as 0.77 A cm⁻². Vertical spikes correspond to measurement stops carried out to perform intermediate cell characterization (V-i curves and EIS). Despite some issues with nitrogen supply at the fuel electrode, the cell showed extremely stable performance over the whole duration of the test, i.e., 3500 h. Regardless, an initial small voltage decrease occurred during the first 150 h, compensated by a slight improvement in performance. A positive voltage difference of +3 mV was recorded at the end of the test, indicating a virtual non-degradation of the cell. This result is confirmed by the overlapping of the initial and final polarization curves shown in Fig. 5(b). Complementary, the evolution of the EIS spectra at OCV and 0.9 V is presented in Figs. 5(c) and 5(d), respectively. At OCV, one can see the second arc of the EIS spectra decreasing between 0 and 1540 h and then increasing until the end of the test without recovering the initial value. The spectra obtained at 0.9 V, however, do not significantly change during the durability test. The variation observed at OCV, mainly coming from the low frequency arc (10° Hz maximum range), which corresponds to the gas conversion into the anode chamber,³⁷⁻³⁹ can most likely be attributed to the variation of steam content in the fuel atmosphere. Even though the measurement was performed after the OCV stabilization after stopping the current (around 30 min), different OCV values were obtained along the 3500 h durability test. Figure S7 reports the evolution with time of the OCV and the total resistance obtained from the EIS spectra, which follow the same trend, with the highest resistance corresponding to the highest OCV and so the lowest steam proportion.

Overall, the thinnest tested barrier layer allowed stable power density for 3500 h without any degradation. Therefore, one can state that a decrease in the barrier layer thickness deposited by PLD does not impact the cell performance, showing an improvement of +45% concerning a reference cell made with a screen-printed barrier layer.

Compositional analysis of pristine and aged cells was performed by nano-XRF using x-ray synchrotron radiation with a focused beam to reach nanometric lateral resolution. The elemental maps of the cross-sections of the cells obtained with nano-XRF are presented in Fig. 6; supplementary material is in Figs. S4, S5, and S6. SEM pictures of the lamellae prepared by Focused Ion Beam (FIB) are gathered in Figs. 6(a) and 6(c). As observed in these pictures, the analyzed area was centered on the barrier layer, with a small portion of the oxygen electrode present on the top and the electrolyte on the bottom. Elemental maps of Ce, Zr, and Sr were
<figure><img src="image_14.png" /><figcaption>FIG. 5. (a) Durability test performed at 750 °C for the Ni-YSZ/YSZ/CGO-200 nm/LSCF cell. (b) Initial and final polarization curves. (c) and (d) EIS spectra along the OCV and 0.9 V tests, respectively.</figcaption></figure>

acquired and are presented in Figs. 6(b) and 6(d) for the pristine and aged samples, respectively, to identify potential interdiffusion of cations between the different layers and to observe the eventual presence of $SrZrO_3$ or alternative secondary phases. First of all, high concentrations of Zr unambiguously delimited the region of the zirconia-based electrolyte. Moreover, it is important to mention that, despite their nm-scale thickness, CGO barrier layers can be straightforwardly correlated with the homogeneous and continuous region of Ce immediately above the electrolyte. A sharp transition between Ce and Zr signals can be observed on the line profiles presented in Figs. S5 and S6, which correspond to the vertical white lines (labeled I and II) on the elemental maps. Based on this sharp interface, dashed lines were added to elemental maps to indicate the YSZ-CGO interface. Using this line as a reference, the logarithmic scale of the elemental maps allows us to observe the presence of Zr above the bottom limit of the barrier layer for both cells, meaning that a small portion of Zr is migrating through the barrier layer. Regarding Sr distribution, one can observe Sr-rich regions located within the electrode and others matching Zr-rich spots out of the 8YSZ electrolyte. This would indicate the presence of small regions of $SrZrO_3$ at the interface between the electrolyte and the oxygen electrode, probably at 8YSZ grain boundaries. Nevertheless, comparing the pristine and aged cells, there is no evidence of increasing the amount of $SrZrO_3$ present after the long-term test. Regarding other cathode elements (La, Co, and Fe), they follow a similar distribution pattern and do not reach the electrolyte even after the degradation period (see elemental maps in Fig. S8 and line profiles in Figs. S9 and S10).

As already mentioned, the secondary-phase formation appears to be discontinuous. In this regard, previous reports indicate that

small amounts of discontinuous strontium zirconate do not have a significant impact on cell performance.⁵,²⁴ Riegraf *et al.* found that an electrolyte-supported cell with a 0.3 or 0.5 µm-thick dense barrier layer was giving similar performance even though the 0.3 µm-thick CGO layer allowed the formation of small amounts of $SrZrO_3$ and the 0.5 µm-thick barrier layer was completely blocking it.²⁴ Accordingly, the durability test performed for 3500 h on cells with 200 nm-thick CGO barrier layers confirmed here that the discontinuous layer of $SrZrO_3$ present at the initial stage does not trigger any further cell degradation. In the same direction, EIS resistance and capacity at high frequencies, which tend to increase with the presence of such a secondary phase formation at the electrolyte/oxygen electrode interface, do not present a significant change in our particular case [Figs. 4(b) and 4(c)]. This confirms that the level of $SrZrO_3$ inside the cell with the 200 nm-thick CGO layer is still below the content, strongly impacting the performance.⁴⁰ In addition, the small amount of Sr migrating to the electrolyte-electrode interface promotes better cathode stability, as previously demonstrated with thicker CGO PLD-deposited layers above 1.5 µm.¹³ Indeed, the post-mortem analysis of aged samples does not evidence any extra phase formation at the electrode level.

Overall, the present results show that CGO barrier layers made by PLD comprised between 200 and 800 nm lead to similar enhanced performance compared to conventionally fabricated barrier layers. Although small amounts of secondary phases are observed at the interface between the electrolyte and the cathode, good cathode stability and excellent long-term performance are preserved even for the cells with the thinnest barrier layer. This is in good agreement with results previously obtained by the authors for much thicker PLD-deposited CGO barrier layers (of 1.5 µm in thickness) on
<figure><img src="image_15.png" /><figcaption>FIG. 6. SEM images of FIB-prepared lamellae analyzed by nano-XRF of (a) a pristine cell and (c) the cell operated for 3500 h with the 200 nm-thick PLD barrier layer. (b) and (d) are the corresponding 2D element mappings on the logarithmic scale obtained from the XRF signal. Vertical white lines correspond to line profiles presented in Figs. S3 and S4. The x and y scales are in µm.</figcaption></figure>

large-area cells operated for two years,¹³ representing a big advan-
tage in terms of fabrication time and cost. Compared with the results
focused on the barrier layer thickness published in the literature, this
200 nm PLD barrier layer is the thinnest ever reported, allowing it
to effectively act as a blocking layer.¹⁸,²⁴,³⁰ To keep decreasing the
layer thickness, further studies need to be performed, paying special
attention during the fabrication step and the upscale to large-area
cells that can present lower thickness homogeneity affecting their
coverage.

CONCLUSION

PLD-grown CGO barrier layers of different thicknesses
between 200 and 800 nm were implemented into button SOCs and
characterized in SOFC mode using H₂ as fuel. The three types of cells
presented similar electrochemical performance with no significant
change in the polarization curves or EIS spectra despite the change in
barrier layer thickness. All of them reached a power density increase
of 44%-46% at 0.7 V and 750 °C compared to a reference-optimized

cell made with a screen-printed CGO barrier layer. Remarkably, the
cell with the thinnest PLD barrier layer (of 200 nm in thickness)
did not show any measurable degradation after being operated for
3500 h at a current density as high as 0.77 A cm⁻² (~0.88 V). Com-
positional analysis using nano-XRF allowed us to prove that small
amounts of SrZrO₃ were forming during the manufacturing process
of the cell with the 200 nm-thick barrier layer but were not detri-
mental either to the cell's initial performance or its durability. It is,
therefore, possible to implement dense and extremely thin barrier
layers as a good protector against secondary phase formation and
electrolyte and cathode destabilization. Consequently, the use of thin
barrier layers will represent a big advantage in decreasing the man-
ufacturing time, energy, and cost of the next generation of highly
performing and stable SOCs.

SUPPLEMENTARY MATERIAL

The supplementary material shows detailed information on the
barrier layer characterization using XRD, ellipsometry, and SEM;
additional electrochemical characterization of the obtained solid oxide cells; and, finally, nano-XRF maps and line scans from the cell made with a 200 nm-thick barrier layer aged for 3500 h.

## ACKNOWLEDGMENTS

The authors acknowledge the ESRF for the beamtime granted for this work and the staff of ID16B for their support. This project has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 101017709 (EPISTORE) and Grant Agreement No. 101101418 (24/_ZEN) and was partially funded by the "Generalitat de Catalunya" Grant No. 2021 SGR 00750.

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author Contributions

**L. Bernadet:** Conceptualization (equal); Data curation (equal); Investigation (equal); Visualization (lead); Writing – original draft (lead); Writing – review & editing (equal). **F. Buzi:** Data curation (equal); Investigation (equal); Visualization (supporting); Writing – original draft (supporting). **F. Baiutti:** Funding acquisition (supporting); Investigation (supporting). **J. Segura-Ruiz:** Formal analysis (supporting). **J. Dolado:** Formal analysis (supporting). **D. Montinaro:** Resources (supporting); Writing – review & editing (supporting). **M. Torrell:** Formal analysis (supporting); Writing – review & editing (supporting). **A. Morata:** Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Supervision (lead); Writing – review & editing (equal). **A. Tarancón:** Conceptualization (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal).

## DATA AVAILABILITY

The data that support the findings of this study are available within the article and its supplementary material.

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