# LaPrNiO 4 +δ Nano-Columnar Thin Films as Oxygen Electrodes for Reversible Solid Oxide Cells

Silvère Panisset, Kosova Kreka, David Jauffres, Carmen Jiménez, Albert

Tarancón, Mónica Burriel

► To cite this version:

Silvère Panisset, Kosova Kreka, David Jauffres, Carmen Jiménez, Albert Tarancón, et al. LaPrNiO 4 +δ Nano-Columnar Thin Films as Oxygen Electrodes for Reversible Solid Oxide Cells. ENERGY & ENVIRONMENTAL MATERIALS, 2025, 9 (1), pp.e70080. 10.1002/eem2.70080. hal-05408795

HAL Id: hal-05408795

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Submitted on 10 Dec 2025

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# LaPrNiO$_{4+δ}$ Nano-Columnar Thin Films as Oxygen Electrodes for Reversible Solid Oxide Cells

Silvère Panisset ID, Kosova Kreka ID, David Jauffres ID, Carmen Jiménez ID, Albert Tarancón ID, and Mónica Burriel* ID

This work explores the potential of La₁-xPrxNiO₄+δ thin films fabricated by Pulsed Injection Metal–Organic Chemical Vapor Deposition as oxygen electrodes for low-temperature solid oxide cells. La₁-xPrxNiO₄+δ materials offer promising mixed ionic and electronic conductivity and high oxygen reduction reaction kinetics. In this study, we focus on the microstructural and electrochemical properties of LaPrNiO₄+δ thin films deposited at various temperatures (600–650 °C), revealing that a two-temperature deposition process yields nano-architECTured films with a dense bottom film and a porous nano-columnar top layer of the same material. Electrochemical impedance spectroscopy and electrical conductivity relaxation experiments demonstrate enhanced surface exchange coefficients compared to bulk LaPrNiO₄+δ and La₂NiO₄+δ and high performance, with polarization resistances as low as 0.10 Ω cm² at 600 °C and 1.00 at 500 °C. To better understand the electrochemical behavior of these electrodes, we investigated the limiting mechanisms of oxygen reduction by analyzing the kinetic response to varying oxygen partial pressures and performing detailed impedance analyses. These nano-columnar LaPrNiO₄+δ oxygen electrodes were also deposited on commercial half-cells, enabling the resulting full cells to operate successfully in both reversible solid oxide fuel cell and electrolysis cell modes, reaching a performance of 0.34 W cm⁻² at 600 °C in reversible solid oxide fuel cell mode. This work underscores the promise of LaPrNiO₄+δ thin films for efficient low-temperature-solid oxide cells while addressing challenges in durability and stability.

## 1. Introduction

Solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) are highly efficient energy conversion devices, offering promising sustainable solutions for electricity generation and hydrogen production. However, their commercial widespread implementation has been impeded by the high operating temperatures required for adequate

400–600 °C range. Above this temperature, the conductivity decreases due to the loss of oxygen content, as evidenced by TGA measurements, which reduces the hole carrier density. Pr-rich nickelates show higher conductivity (e.g., 110–120 S cm⁻¹ at 600 °C for x = 2) than La-rich nickelates (e.g. 60–70 S cm⁻¹ at 600 °C for x = 0).[7,8] The structural mismatch between the NiO₂ and Ln₂O₂ layers induces constraints that are relaxed by the intercalation of oxygen, leading to the oxidation of Ni²⁺ to Ni³⁺. The ionic radii of La³⁺ (1.16 Å) and Pr³⁺ (1.126 Å) influence the amount of interstitial oxygen needed to stabilize the structure. La-rich compositions require a lesser amount of interstitial oxygen (δ ≈ 0.16) compared to Pr-rich compositions (δ ≈ 0.22).[9] XRD studies indicate that all La₂ₓPrₓNiO₄+δ compositions possess orthorhombic symmetry at room temperature. Pr-rich nickelates (x = 1, 1.5, 2) belong to the Bmab space group, while La-rich nickelates (x = 0, 0.5) belong to the Fmmm space group.[8,9]

The oxygen diffusion coefficient D* of La₂ₓPrₓNiO₄+δ increases with increasing Pr content from 8.2 10⁻⁹ cm² s⁻¹ (x = 0.5) to 2.2

ionic conductivity of the electrolyte material. These elevated temperatures accelerate the materials degradation and impose strict requirements on component compatibility that increase the overall system costs.[1] Consequently, reducing the operating temperature to an intermediate range (600–750 °C) or even to a lower temperature range (400–600 °C) has become a critical research priority.[2,3] Achieving this goal necessitates the development of advanced electrode[4] and electrolyte[5] materials, alongside the optimization of electrode microstructure,[6] to enable efficient cell operation at reduced temperatures.

Among emerging materials, K₂NiF₄-type layered oxides such as La₂ₓNiO₄+δ (L2NO4) have shown promise due to their mixed ionic and electronic conductivity (MIEC), excellent oxygen reduction reaction kinetics, and overstoichiometry, which leads to enhanced oxygen diffusion and surface reactivity. However, despite their good performance and stability, L2NO4 electrodes suffer from relatively low electronic conduction. Recently, several studies have shown that La₂ₓPrₓNiO₄+δ materials exhibit improved conductivity and electrochemical properties. All mixed lanthanum-praseodymium nickelate compositions exhibit semiconducting behavior below 600 °C, with maximum conductivity observed in the

Dr. S. Panisset, Dr. C. Jiménez, Prof. M. Burriel
Univ. Grenoble Alpes, CNRS, Grenoble INP, LMGP, 38000, Grenoble, France
E-mail: monica.burriel@grenoble-inp.fr

Dr. S. Panisset, Dr. D. Jauffres
Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMaP, 38000, Grenoble, France
Dr. K. Kreka, Prof. A. Tarancón
Catalonia Institute for Energy Research (IREC), 08930, Barcelona, Spain

ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/eem2.70080.

DOI: 10.1002/eem2.70080
$10^{-8} \text{ cm}^2 \text{ s}^{-1}$ ($x=1.5$) at $600^{\circ}\text{C}$.[7] These diffusion coefficients are among the highest reported in the literature for the low-temperature range ($<600^{\circ}\text{C}$), with activation energies between 0.6 to 0.7 eV, which are significantly lower than those of other perovskites such as LSCF or LSC (typically 1.4–1.8 eV).[10] Limited data are available on the surface exchange coefficient for $La_{2-x}Pr_xNiO_{4+\delta}$ compounds. For $x=0.5, 1$, and $1.5$, Vibhu et al. reported values between $La_2NiO_{4+\delta}$ (L2NO4) and $Pr_2NiO_{4+\delta}$ (PNO) with no clear trend on the $k^*$ coefficient relative to the temperature and the Pr content, as the $k^*$ values are highly dependent on the surface state.[7]

Vibhu et al. aimed to find the best compromise between $La_2NiO_{4+\delta}$, exhibiting high stability even at low $p_{O_2}$, and $Pr_2NiO_{4+\delta}$, having excellent electrochemical properties at intermediate temperature but being less stable.[9] They reported that, contrary to Pr-rich nickelates, La-rich nickelates powders are chemically stable under air in the temperature range of 600–800 $^{\circ}\text{C}$ (over 1 month).[11,12] After 1 month, PNO completely dissociated into $PrNiO_{3-\delta}$, $Pr_4Ni_3O_{10+\delta}$, and $Pr_6O_{11}$ at $700^{\circ}\text{C}$ under air. From $x=1$ to $x=2$, new phases ($(La,Pr)NiO_{3-\delta}$, $(La,Pr)_4Ni_3O_{10+\delta}$, and $Pr_6O_{11}$) appeared in the XRD patterns. In addition, Sharma et al. found that the equimolar La-Pr composition, $LaPrNiO_{4+\delta}$, is stable at $700^{\circ}\text{C}$ for 30 days but starts to decompose at $800^{\circ}\text{C}$ after 30 days.[13] They showed that after electrochemical aging under OCV, the polarization resistance of all $La_{2-x}Pr_xNiO_{4+\delta}$ half-cell electrode compositions (from $x=0$ to $x=2$) are stable, while under cathodic polarization the electrodes are significantly damaged by the presence of delamination.[14] On the other hand, remarkable performance and stability were measured under anodic polarization at $700^{\circ}\text{C}$ after 1800 h.

Several studies agreed that the electrode polarization resistance ($R_{pol}$) of $La_{2-x}Pr_xNiO_{4+\delta}$ oxygen electrodes decreases with increasing Pr content.[8,9,11,13] $LaPrNiO_{4+\delta}$ (LPNO, $x=1$) was reported to show a good compromise between electrochemical performance and chemical stability. Vibhu et al. reported a $R_{pol}$ of $0.29 \Omega \text{ cm}^2$ at $600^{\circ}\text{C}$ using LPNO synthesized via the citrate-nitrate route.[7] Sharma et al. documented a polarization resistance of $1.67 \Omega \text{ cm}^2$ at $600^{\circ}\text{C}$ on a symmetrical cell with an LPNO 3D coral type microstructure deposited by electrostatic spray deposition (ESD) on $Ce_{0.9}Gd_{0.1}O_{2-\delta}$ (GDC).[13] Optimized electrodes using a multilayer strategy combining ESD and screen-printing (SP) methods further reduced polarization resistance to $0.12 \Omega \text{ cm}^2$ at $600^{\circ}\text{C}$ as reported by Khamid et al.[15] The developed triple-layer system consists of a bottom layer of GDC deposited by SP to provide better adhesion, topped by a layer of LPNO deposited by ESD, and then coated by a layer of LPNO deposited by SP to improve the current collection.[15]

To the best of our knowledge, the deposition of LPNO thin films, with thicknesses on the order of hundreds of nanometers, has not been attempted to date. Given the growing concerns about material scarcity and environmental impact, thin film electrodes offer significant advantages by reducing the consumption of critical materials required. Furthermore, thin film deposition methods promote the nano-engineering of the electrode, enabling advanced architectures such as infiltration-based designs, vertical aligned nanostructures, or nano-columnar architectures, all of which have demonstrated strong potential for improving electrochemical performance.[16–18] In this context, nanostructured thin films exhibit exceptionally high specific surface areas, which reduce the electrochemical penetration depth and enhance oxygen surface exchange activity. As a result, such films can achieve performance levels comparable to those of much thicker, conventionally processed electrodes, while offering additional benefits in

terms of material efficiency and integration into miniaturized SOC architectures.

This study investigates the potential of LPNO thin films, deposited using Pulsed Injection Metal–Organic Chemical Vapor Deposition (PIMOCVD), as high-performance oxygen electrodes for low-temperature solid oxide cells (LT-SOCs). Our goal is to assess the viability of this material and deposition method by addressing key challenges such as electrochemical performance, limiting mechanisms, and thermal stability at low operating temperatures. The work begins with the optimization of thin film deposition and microstructural design, followed by a detailed assessment of electrode performance using electrical conductivity relaxation (ECR) and electrochemical impedance spectroscopy (EIS) in a symmetrical cell configuration, complemented by aging tests. Next, the underlying reaction mechanisms are studied to identify rate-limiting processes, and the films are integrated into full cells to measure their performance in both SOFC and SOEC modes, providing a comprehensive understanding of the potential of LPNO thin films for LT-SOC applications.

## 2. Results and Discussion

The LPNO thin films studied in this work were deposited under varying conditions to investigate the influence of deposition temperature and film thickness on their microstructure and electrochemical performance. Two deposition temperatures, 600 and 650 $^{\circ}\text{C}$, were employed, resulting in distinct microstructures. The 200 nm films deposited at 650 $^{\circ}\text{C}$ exhibit a dense microstructure (see **Figure 1a–c**). At 600 $^{\circ}\text{C}$, several porous films were grown from 150 to 600 nm (see Figure S1, Supporting Information for additional SEM images), with a focus here on a representative sample with a thickness of 560 nm. Additionally, a two-temperature deposition process (50 nm at 650 $^{\circ}\text{C}$ followed by several hundreds of nanometers at 600 $^{\circ}\text{C}$) was used to produce nano-architected films with a thin dense bottom layer providing good in-plane conductivity and a porous structure on top (**Table 1**).

### 2.1. LPNO Thin Film Chemical, Structural, and Morphological Characterization

As thin film electrodes are typically limited by the surface exchange reactions, it is crucial to optimize the specific surface area – defined as the surface in contact with the atmosphere divided by the total volume of the film. To do so, a porous nano-columnar microstructure was targeted, as in our previous studies for pure $La_2NiO_{4+\delta}$ films.[19,20] Figure 1 illustrates three different LPNO samples with distinct

<table><caption>**Table 1.** Summary of the studied samples.</caption><thead><tr><td>Electrode thickness</td><td>Configuration</td><td>Sample deposition temperature</td></tr></thead><tbody><tr><td>200 nm</td><td>Asymmetrical cell</td><td>650 °C</td></tr><tr><td>150 nm</td><td>Symmetrical cell</td><td>600 °C</td></tr><tr><td>430 nm</td><td>Symmetrical cell</td><td>600 °C</td></tr><tr><td>560 nm</td><td>Symmetrical cell</td><td>600 °C</td></tr><tr><td>400 nm</td><td>Symmetrical cell</td><td>650/600 °C</td></tr><tr><td>700 nm</td><td>Symmetrical cell</td><td>650/600 °C</td></tr><tr><td>700 nm</td><td>Full cell</td><td>650/600 °C</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** SEM and TEM images of a a–c) 200 nm dense, d–f) 560 nm porous, and g–i), 700 nm porous LPNO thin films deposited at 650, 600, and 650/600 °C, respectively. The cross-sectional SEM images in h) secondary electron and i) backscattered electron modes show the dense first 50 nm (deposited at 650 °C) and the remaining porous nano-columnar film (deposited at 600 °C).</figcaption></figure>

morphologies. The first one is deposited at 650 °C (a–c) and exhibits a relatively dense structure with a high surface roughness, as shown by the cross-sectional TEM view (b). On the other hand, the sample deposited at 600 °C exhibits a more porous structure (e) with the growth of nano-columns formed by small nano-grains (10–20 nm). This porous structure could be nicely maintained over the film growth at least up to 700 nm. The Thornton structure zone model provides a framework for understanding the evolution of film microstructures as a function of deposition temperature.[21,22] According to this model, at low deposition temperatures, the limited surface diffusion of adatoms leads to competitive grain growth and the shadowing effect, which together result in the formation of porous and columnar structures. As the temperature increases, surface diffusion becomes the dominant mechanism, enabling adatoms to travel longer distances and promoting the development of a V-shaped columnar microstructure. At even higher temperatures, bulk diffusion becomes active, leading to grain coarsening and densification of the film. In this study, the complex microstructures observed in films deposited by CVD at relatively low temperatures align with these predictions, highlighting the interplay between deposition temperature and microstructural evolution. In both

cases, the high resolution TEM images (c–f) show good adhesion of the LPNO film to the YSZ substrate. Despite the lattice parameter mismatch, which contributes to the polycrystalline growth of the film, the good adhesion suggests compatibility in terms of thermal expansion coefficient. In the case of the sample deposited at lower temperature (600 °C) (e, f), the white regions indicate that porosity almost reaches the bottom of the film, which is beneficial for the oxygen gas to diffuse. However, this porosity could lead to the decrease of in-plane conductivity of the film due to the discontinuity and the columnar nature of the film, which may be detrimental for the uniformity of the current collection and thus to the electrochemical activity of the nano-columns. The beneficial effect of a thin dense layer at the electrode/electrolyte interface on the in-plane conductivity has been shown using Van der Pauw measurements on the LPNO thin films, as shown in Figure S2, Supporting Information. Thus, a two-temperature deposition process was employed to control the formation of a ~50 nm dense bottom layer deposited at 650 °C, and of a porous film microstructure deposited subsequently at 600 °C. Figure 1g–i show surface and cross-sectional SEM images, revealing a well-preserved porous structure along with the presence of a thin and dense bottom layer.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** XRD patterns of 400 nm (gray) and 700 nm (red) LPNO porous nano-columnar thin films deposited at 650/600 °C on YSZ single crystal substrates. LaPrNiO<sub>4.19</sub> (ICDD: 01-086-8668) was used as the reference pattern.</figcaption></figure>

All the LPNO thin films are characterized by EDS to measure the stoichiometry of the material. The film stoichiometry depends on the deposition temperature and the ratio Ln/Ni and Pr/La ratio in the solution due to different volatility, reactivity, and transport dynamics of the different precursors. For films deposited at 600 °C and 650/600 °C, the ratio measured in the film is Ln/Ni = 2.17 ± 0.19 and Pr/La = 0.99 ± 0.09 (calculated over 9 films with thicknesses between 200 and 700 nm). These values are consistent with the targeted LaPrNiO<sub>4+δ</sub> stoichiometry. The difference in reactivity between the Ni and La precursors was already known from the previous L2NO4 deposition optimization.[23] The small difference between the La and Pr precursors can be attributed to the similarities in their organometallic structures.

The diffraction pattern of the 400 nm film deposited at 650/600 °C (Figure 2), all the peaks on the diffractogram match with the LaPrNiO<sub>4.19</sub> tetragonal phase (ICDD: 01-086-8668). The film is clearly polycrystalline with preferential orientations of the (200) and (220) lattice planes at 32.9° and 47.2°, respectively. For the 700 nm film, two additional peaks at 30.5° and 63.6° can be identified as Pr<sub>2</sub>O<sub>3</sub> impurities (ICDD: 04-015-5005). The diffraction pattern of the 200 nm dense LPNO thin film reported in Figure S3, Supporting Information reveals a fully polycrystalline structure with no detectable impurity peaks (see Supporting Information).

## 2.2. LPNO Thin Film Electrical and Electrochemical Properties

The in-plane conductivity of the 200 nm dense thin film was measured using the Van der Pauw method under synthetic air from room temperature to 500 °C. At 300 °C, the in-plane conductivity of the 200 nm dense LPNO is 57 S cm<sup>-1</sup>, which is twice as high as the 27 S cm<sup>-1</sup> measured for 200 nm dense L2NO4 films.[19] This factor is consistent with values reported in the literature, where LPNO conductivity is nearly double that of L2NO4. However, the conductivity values for our

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Chemical surface exchange coefficient for an LPNO dense thin film (t.f.) measured by ECR and compared to literature values for L2NO4 thin films[19,26,28] and bulk samples.[24,29]</figcaption></figure>

films are approximately half of those reported for LPNO bulk pellets,[7,24] which could be related to the existence of a large density of grain boundaries and a high surface roughness.

When oxygen transport is dominated by the surface exchange reaction – occurring when the electrode thickness is significantly lower than the penetration length (estimated to exceed 10 µm at 600 °C[7]), the concentration of oxygen ions within the film remains uniform. Under these conditions, the normalized conductivity, g(t), measured by ECR, is well-fitted using a single exponential, as exemplified in Figure S4, Supporting Information, and is given by:

$$g(t) = 1 - \exp\left(-\frac{t}{\tau}\right) \qquad (1)$$

For surface controlled processes,[25] the chemical surface exchange coefficient k<sub>chem</sub> can be expressed using the surface exchange kinetics characteristic time constant, τ, as follows:

$$k_{\text{chem}} = \frac{1}{a \cdot \tau} \qquad (2)$$

where a is the specific surface area. As it is very complex to accurately measure the exposed active surface, k<sub>chem</sub> is calculated using a dense film and assuming a flat surface, where a is equal to the inverse of the film thickness. Considering that a 200 nm thick film is surface limited, the k<sub>chem</sub> of LPNO is calculated through Equation 2 and is equal to 9.06 10<sup>-7</sup> cm s<sup>-1</sup> at 400 °C with an activation energy of 1.27 eV in the temperature range of 350 to 425 °C. Figure 3 compares the calculated k<sub>chem</sub> coefficients (light blue circles) to literature reports showing that the measured values are in the same range as those reported for L2NO4 bulk and thin films measured by EIS and ECR.[19,24,26,27] Notably, the k<sub>chem</sub> is higher, by a factor 2 to 3, than that of a dense L2NO4 thin film measured by ECR in the same temperature range.[19]
**Figure 4a** shows the surface exchange kinetics characteristic time constant for the 200 nm dense (650 °C) LPNO film and the 400 and 700 nm dense/porous (650/600 °C) LPNO films measured by ECR from 10 to 210 mbar (oxidation). Additional ECR data are given in Figure S5, Supporting Information for the reduction and oxidation at low pO₂ (10 ↔ 210 mbar) and for high pO₂ (210 ↔ 1000 mbar) to provide a more complete picture of the thin film performance under practical reversible SOC operating conditions. As expected, the dense LPNO film exhibits a much higher relaxation time constant compared to the porous LPNO films, due to the much smaller active surface area. In Figure 4b, the τ values measured at 250 °C in oxidation conditions (10–210 mbar) are compared with previously reported values for L2NO4 nanocolumnar thin films with thicknesses ranging from 200 to 1200 nm.[23] The LPNO films demonstrate improved kinetics, with the relaxation time constant of the 400 nm film being three times lower than that of L2NO4 under similar conditions. Such improvement cannot be attributed solely to an improvement in microstructure and is likely to be the result of the higher surface exchange coefficient as observed for the dense films. The decrease in τ by a factor 2.7 is in good agreement with the increase observed in kchem when comparing the dense L2NO4 and LPNO thin films. It was previously reported that in La₂-xPrxNiO₄+δ, the oxygen diffusion coefficient (D*) increases with higher Pr content, typically ranging between the values measured for La₂NiO₄+δ and Pr₂NiO₄+δ. However, the results for the oxygen exchange coefficient (k*) did not show a clear dependence on the praseodymium substitution, likely due to the strong dependence of the oxygen exchange activity with the surface state of the samples.[7] Our results confirm the higher surface exchange activity of LPNO compared to previously reported L2NO4 thin films, with an improvement by a factor of 2–3.

Concerning the decrease in activation energy from dense films to nanocolumnar films, similar results were observed for L2NO4 thin films.[19] This change has been attributed to the exposure of additional crystal orientations associated with changes in surface termination and surface chemistry, and the possible presence of highly active nano-features such as kinks and edges along the lateral surfaces. The lower activation energy of the thicker nano-columnar films could also be interpreted as the limitation of the ionic diffusion contribution in the ECR measurements.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** ECR measurements in oxidation (10 → 210 mbar) on LPNO thin films. a) Arrhenius plot of the time constant for: dense 200 nm (black squares) and dense/porous nano-columnar 400 nm (red circles), and 700 nm (blue triangles) microstructures. b) Comparison of the time constant at 250 °C of the dense/porous 400 nm and 700 nm LPNO with nanocolumnar 200, 400, and 1200 nm La₂NiO₄+δ thin films.[23]</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Arrhenius plots of the polarization resistance of the 400 nm symmetrical dense/porous nanocolumnar LPNO electrode compared with the best performing LPNO electrodes reported in the literature.[11,13,15]</figcaption></figure>

High performance was measured by electrochemical impedance spectroscopy (EIS) for the dense/porous LPNO symmetrical cells (400 nm) in the temperature range of 400–600 °C. As shown in **Figure 5**, the electrode polarization ASRpol reaches values as low as 0.10 and 1.0 Ω cm² at 600 and 500 °C, respectively. Compared to the previously reported LPNO bulk electrodes (Figure 5), the 400 nm thin film exhibits higher performance than the single- and double-layer electrodes from Vibhu's[7,9] and Khamidy's[15] studies, which have ASRs of 0.6 and 1.7 to 1.2 Ω cm² at 600 °C, respectively. The LPNO double layer developed by Sharma et al. demonstrates twice the performance of our thin film electrodes at 500 °C but shows similar performance at 600 °C.[13] These results demonstrate that thin film electrodes can achieve performances comparable to those of bulk electrodes. However, it should be noted that the value of the ASRpol increases over time due

to both film densification and the formation of secondary phases during aging (see **Supporting Information** for full details). Aging tests performed at 500 and 600 °C (see Figure S6, Supporting Information) revealed a progressive increase in polarization resistance, accompanied by microstructural changes such as grain coarsening and reduction in porosity, as observed by SEM (see Figure S7, Supporting Information). This morphological evolution is also correlated with the chemical decomposition of LPNO, as evidenced by the XRD patterns showing the emergence of La₄Ni₃O₁₀ (ICDD: 04-009-1774) and possibly minor phases such as Pr₄Ni₃O₁₀ and Pr₆O₁₁, consistent with previous studies.[12] These changes adversely impact the active surface area and electrochemical properties of the electrode, leading to a progressive decline in performance.
The Nyquist spectra presented in **Figure 6a,b** and analyzed by DRT Figure 6c consist of two semi-circles: one dominant in the medium frequency (MF) range and a second one in the low frequency (LF) range. The DRT spectra reveal two peaks that increase in amplitude and shift to longer characteristic times (lower frequencies) with decreasing temperature. The high-frequency peak is attributed to an artifact caused by distortion of the impedance spectra from the electrolyte. Both semi-circles increase as the temperature decreases exhibiting two thermally activated contributions. The spectra are fitted using an equivalent circuit consisting of $R_1 + R_2//CPE_2 + R_3//CPE_3$, where $R_1$ corresponds to the series

resistance, mostly due to the YSZ ionic resistance. Two resistances in parallel to constant phase elements are used to fit the two semi-circles. Figure 6d shows that the activation energy of ASR$_1$ is close to 1.0 eV as expected for the YSZ electrolyte confirming the good current collection and absence of short-circuits.[30] Both ASR$_2$ and ASR$_3$ have similar activation energy (1.32 and 1.26 eV, respectively). At 600 °C, the capacities $C_2$ and $C_3$, calculated from the pseudo-capacities ($C_{eq} = (R^{1-\alpha}CPE)^{-\alpha}$), are equal to $1.44 \times 10^{-2}$ and $1.21 \text{ F cm}^{-2}$, respectively. Considering the ASR$_2$ activation energy, the $C_2$ value, and the characteristic time ($\tau_2 = 10$ ms), the first semi-circle is attributed to surface exchange reaction

process. When compared with the 700 nm symmetrical cell at 600 °C in Figure 6b, the increase in film thickness leads to the reduction of the MF arc by a factor 2.8, likely due to the increase of active surface. At this point, it is difficult to attribute a process to the LF arc as it could not correspond to a gas diffusion-related process due to its related high activation energy. Further ECR and EIS analysis, detailed in the next section, have been carried out to investigate the different contributions and to clearly identify the related mechanisms.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** a) Nyquist plots of the 400 nm symmetrical LPNO cell in the temperature range 450–525°C and b) comparison of the 400 and 700 nm symmetrical LPNO cells at 600 °C, c) DRT spectra and d) associated Arhenius plots of the fitted electrode polarization ASR$_{pol}$ (= ASR$_2$ + ASR$_3$) and the series ASR$_1$ of the 400 nm symmetrical LPNO cell.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 7.** a) Representative example of the determination of the initial reaction rate, $\vec{R}|_{t=0}$, consisting in the tangent at $t=0$ of the curve of the relaxation of the conductance driven by an abrupt oxygen partial pressure change; b) variation of the initial reaction rate, $\vec{R}|_{t=0}$, against various final oxygen partial pressures for two LPNO thin films (porous and dense).</figcaption></figure>

## 2.3. Limiting Mechanisms of the LPNO Thin Film Oxygen Electrodes

The initial reaction rates, $\vec{R}|_{t=0}$, were determined during ECR measurement at various oxygen partial pressures, $p_{O_2}$. In these measurements, the temperature is fixed to 250 °C and the $p_{O_2}$ is varied from 210 mbar down to different $p_{O_2}^{final}$. It has been shown in the literature that the initial reaction rate is linked to $p_{O_2}^{final}$ by:

$$ \vec{R}|_{t=0} \sim (p_{O_2}^{final})^{-n} \quad (3) $$

where the exponent n depends on the limiting process.[31] When molecular oxygen is involved in the process, n = 1, whereas n = 0.5 for processes involving only atomic oxygen. As shown in **Figure 7a**, $\vec{R}|_{t=0}$ corresponds geometrically to the tangent at $t=0$ of the relaxation curves of the conductance driven by an abrupt change of $p_{O_2}$. Figure 7b shows that for both the porous 700 nm LPNO thin film (red) and for the dense 200 nm LPNO thin film (black), the limiting process at 250 °C is either the oxygen ionization or incorporation as their n exponent is close to 0.5.

Additionally, the $p_{O_2}$ dependence was studied by EIS on the LPNO 700 nm thin film electrode, in order to have a better understanding of the limiting processes at
gas conversion phenomenon.[37] However, these gas diffusion-related processes are characterized by a very low activation energy, which is in contradiction with the activation energy observed in the previous section. Thus, a possible explanation could be that the O₂ adsorption (physisorption or chemisorption) becomes limiting in the LF range.

## 2.4. Anode Supported LPNO Cells

The performance of an LPNO oxygen electrode is evaluated for the first time in reversible operation. Both fuel cell and electrolysis modes are measured using an LPNO thin film as an oxygen electrode on a full cell fabricated using a commercial fuel electrode-supported button cell from SolydEra. On top of the YSZ electrolyte, a 350 nm barrier layer was deposited by PLD (600 °C, 20 mT) and a 700 nm porous LPNO thin film was deposited by PI-MOCVD using the same conditions as for the symmetrical cell studied previously. From the surface SEM images (see Supporting Information) it is observed that the porous microstructure was nicely preserved despite the roughness of the electrolyte.

The electrochemical performance in fuel cell mode was assessed at 500, 550, and 600 °C. All measurements were carried out using a dry synthetic air flow on the oxygen electrode side and a dry hydrogen flow on the fuel electrode side. Figure 9a shows the current density-voltage (j-V) and current density-power density (j-P) curves. The open circuit voltage (OCV) in the range of 1.20–1.22 V indicates adequate sealing and gas separation between the fuel electrode and oxygen electrode chambers. At 600 °C, the cell reached a current density and maximal power density of 0.48 A cm⁻² and 0.34 W cm⁻² at 0.7 V. In the literature, to our best knowledge, only one study reported the i-V curve in SOFC mode using an LPNO oxygen electrode, where a maximal power density of 0.21 W cm⁻² at 600 °C was reported using an LPNO deposited by electrostatic spray deposition on a commercial half-cell (Ni-3YSZ/Ni-8YSZ/8YSZ/CGO).[13] However, it should be noted that after leaving the cell at 600 °C for 2 h, the performance decreased by about 20% down to 0.38 A cm⁻² and 0.23 W cm⁻², likely due to a lack of thermal stability of the electrode and/or the current collector.

In our study, to further evaluate the stability of the LPNO thin film electrode under operating conditions, the full cell was operated at 500 °C during 200 h under galvanostatic conditions (at 57 mA cm⁻²). The recorded evolution of the cell voltage over time is presented in Figure 10. Monitoring the voltage at a fixed current allows direct assessment of performance degradation. The results show a rapid drop in the cell voltage within the first 10 h, followed by a constant decay of

<figure><img src="image_9.png" /><figcaption>**Figure 10.** LPNO/YSZ/Ni-YSZ full cell voltage evolution during thermal aging (200 h) at 500 °C at 57 mA cm⁻².</figcaption></figure>

25 mV per 100 h. This confirms the gradual degradation due to chemical and microstructural changes, as also observed in the aging tests of the LPNO symmetrical cell.

In electrolysis mode, the measurements were carried out under dry synthetic air flow on the oxygen electrode side and a mixture of 10/90% H₂/H₂O and Ar on the fuel electrode side. An OCV in the range of 0.91–0.95 V was obtained. Figure 9b shows that the cell reached a current density of 0.24 A cm⁻² at 600 °C and at 1.4 V. No data were found in the literature on LPNO cells in SOEC mode, and thus this work proves the first successful operation of LPNO oxygen electrodes in electrolysis and reversible modes.

## 3. Conclusions

This study investigates the potential of LaPrNiO₄+δ (LPNO) thin films as oxygen electrodes for low-temperature solid oxide cells (LT-SOCs). Using Pulsed Injection Metal-Organic Chemical Vapor Deposition (PI-MOCVD), LPNO thin films were fabricated with tailored microstructures. A two-temperature deposition process - combining a dense bottom layer for in-plane conductivity with a porous top layer for enhanced surface exchange - resulted in films

with optimized microstructure. The dense/porous LPNO films achieved polarization resistances as low as 0.10 Ω cm² at 600 °C, a performance comparable to or superior to bulk LPNO electrodes. Additionally, the films exhibit high surface exchange coefficients (9.06 × 10⁻⁷ cm s⁻¹ at 400 °C with Eₐ = 1.27 eV), which are critical for efficient oxygen reduction reactions at reduced operating temperatures. Further investigation revealed that at 600 °C, the limiting mechanisms of the LPNO electrode are charge transfer and O₂ adsorption at the electrode surface.

The performance of the LPNO electrodes in fuel cell and electrolysis modes underscores

<figure><img src="image_10.png" /><figcaption>**Figure 9.** Polarization curves of the LPNO/YSZ/Ni-YSZ cell in a) SOFC mode and b) SOEC mode from 500 to 600 °C.</figcaption></figure>
their versatility and potential in both energy generation and storage sys-
tems. In fuel cell mode, a power density of 0.34 W cm⁻² was achieved
at 600 °C, while preliminary tests in electrolysis mode indicate promis-
ing current densities, although with noticeable degradation over time.
The study also addresses the challenges of chemical and thermal stabil-
ity posed by praseodymium lanthanum nickelate materials. Strategies to
mitigate these effects, such as the incorporation of stabilizing elements,
remain critical for advancing LPNO thin films toward practical
applications.

Overall, this work provides a comprehensive evaluation of LPNO thin films as oxygen electrodes, establishing their viability for LT-SOCs while identifying key areas for improvement. Future studies should focus on enhancing the chemical stability of LPNO to fully realize its potential in sustainable energy technologies.

4. Experimental Methods

**Thin film fabrication:** LPNO thin films were deposited using Pulsed Injection Metal–Organic Chemical Vapor Deposition (PI-MOCVD) on yttria-stabilized zirconia (YSZ) (Crystec) substrates with (100) orientation. The precursor solution was prepared by dissolving commercial La(tmhd)₃, Pr(tmhd)₃ and Ni(tmhd)₂ powders (Strem Chemicals) in m-xylene (Alfa Aesar) with a ratio of Ln/Ni = 4 and Pr/La = 0.9, and a solution concentration of 0.02 M. The solution was injected into the reactor using an electrovalve at a frequency of 4 Hz and with an opening time of 2 ms. The solution composition was intentionally chosen to compensate for the known difference in precursor reactivity. For the deposition conditions and reactor used, the Ni(tmhd)₂ precursor has shown to be significantly less reactive – approximately by a factor of two – than the La(tmhd)₃ and Pr(tmhd)₃ precursors, which have similar organometallic structures. The precursors and solvent were vaporized in the evaporation line (220–280 °C) before reaching the substrate, which was heated to either 600 or 650 °C, depending on the desired microstructure. No barrier layer is required as the film synthesis occurs at low temperatures, preventing possible secondary phase reactions with YSZ. At the substrate, nucleation and film growth occurred via gas–solid reactions. The carrier gas composition was fixed at 34% Ar/66% O₂, with a total pressure of 5 Torr. The film thickness was controlled by the number of pulses injected.

**Structural, chemical and morphological characterization:** The crystallinity and the purity of the LPNO thin films were carried out by X-ray diffraction using the standard Bragg-Brentano (BB) geometry (0-2θ) on a Bruker D8 Advance series II diffractometer, employing monochromatic Cu-Kα1 radiation with a wavelength (λ) of 1.541 Å. Grazing-Incidence XRD (GI-XRD) was employed on the 200 nm LPNO thin film due to the inherent challenge of obtaining high intensity peaks from polycrystalline thin films (< 200 nm) using standard measurements. GI-XRD measurements were performed on a 5-circle RIGAKU Smartlab diffractometer, employing an incidence angle of 0.4°, a step size of 0.06°, and a scan rate of 0.25° per minute.

Scanning electron microscopy (SEM) images were recorded using a Zeiss FEG
Gemini SEM 500. The typical working conditions for surface and cross-section
SEM were an accelerating voltage of 3 kV, a working distance of 4 mm, and a
30 µm aperture size (using both SE and BSE detectors). Coupled with SEM char-
acterization, energy-dispersive X-ray spectroscopy (EDS) was used to analyze ele-
mental composition. The SEM is equipped with a Brucker EDS detector. EDS was
used to estimate the ratio Ln/Ni (Ln = La + Pr) and Pr/La in the LPNO thin films.
A copper strip was used for calibration before each measurement.

The TEM lamella were prepared in cross-section by the semi-automated polishing tripod technique with the MultiPrepTM system (Allied High Tech Products, Inc.). The PIPS II from GATAN was used for the final polishing. TEM and High Resolution-TEM images were recorded with a JEOL JEM 2010 LaB6 microscope operating at 200 kV with a 0.19 nm point-to-point resolution.

**Electrical and electrochemical characterization:** Both in-plane resistivity measurements and electrical conductivity relaxation (ECR) experiments were performed using the same experimental setup in a high temperature cell (Nextron). The system operates under controlled temperature and gas conditions, with

temperatures ranging from room temperature to 600 °C. The samples are placed onto a 1/2" ceramic heating stage and the surface temperature is calibrated beforehand using a Pt100 thermocouple, which temperature is considered to be equivalent to the thin film one. The chamber atmosphere was precisely controlled using gas mass flow controllers (N₂ and O₂), allowing for the adjustment of the oxygen partial pressure pO₂ within the 0.01–300 mbar range. The pO₂ was monitored using a Rapidox 2100 analyzer. Electrical connections were established using tungsten probes mechanically pinned onto the thin film electrode and the measurements were carried out using a Keithley 2400. The Van der Pauw method was employed to evaluate the in-plane resistivity of the thin film under synthetic dry air or controlled pO₂. ECR was used to investigate the oxygen surface exchange activity of the electrode. During ECR experiments, gas switching was facilitated by a 4-way valve and a mixing chamber, obtaining a flush time of approximately 7 s at a flow rate of 1000 ml min⁻¹.

Electrochemical impedance spectroscopy (EIS) was used to characterize the oxygen electrode performance using a symmetrical cell configuration. The cells consisted of a double-side-polished YSZ substrate with the thin film oxygen electrode deposited on both sides to enable across-plane measurements. Gold paste was used as a current collector to provide good contact between the electrode and the gold grid. The measurements were carried out in dry air, in the frequency range from 1 MHz to 100 mHz with an AC voltage of 50 mV and an active surface area of 8 × 8 mm². The measurements were performed in a home-built setup in the temperature range of 400–600 °C. The setup allows the simultaneous measurement of two cells under the same conditions. EIS measurements were performed using a two-channel Biologic SP-150e potentiostat, and the different contributions to the polarization resistance of the thin film electrode were subsequently assessed.

**Acknowledgements**

This work was funded by the European Union's Horizon 2020 research and inno-
vation program under grant agreements no. 824072 (Harvestore project) and no.
101017709 (EPISTORE) and by the Centre of Excellence of Multifunctional Archi-
tECTured Materials "CEMAM" no. ANR-10-LABX-44-01 as part of the "Investments
for the Future" Program. The authors further acknowledge the scientific and tech-
nical assistance of Laetitia Rapenne for TEM measurements using the CMTC char-
acterization platform of Grenoble INP.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

All sample datasets and materials related to this work are made available
under CC BY 4.0 license in the zenodo repository: 10.5281/zenodo.14809707.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from
the author.

**Keywords**

lanthanum praseodymium nickelate, low temperature Solid Oxide Cells,
SOEC, SOFC, thin films

Received: March 2, 2025

Revised: June 3, 2025

Published online: June 21, 2025
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