# Design of interfaces in efficient Ln2NiO4+δ (Ln = La, Pr) cathodes for SOFC applications

Rakesh K. Sharma, Mónica Burriel, Laurent Dessemond, Jean-Marc. Bassat,

Elisabeth Djurado

► To cite this version:

Rakesh K. Sharma, Mónica Burriel, Laurent Dessemond, Jean-Marc. Bassat, Elisabeth Djurado. Design of interfaces in efficient Ln2NiO4+δ (Ln = La, Pr) cathodes for SOFC applications. Journal of Materials Chemistry A, 2016, 4 (32), pp.12451-12462. 10.1039/c6ta04845e. hal-01426062

HAL Id: hal-01426062

https://hal.science/hal-01426062v1

Submitted on 28 Jan 2026

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# Design of interfaces in efficient Ln₂NiO₄+δ (Ln = La, Pr) cathodes for SOFC applications†

R. K. Sharma,abc M. Burriel,d L. Dessemond,ab J. M. Bassatc and E. Djurado*ab

In this work, a novel architecture of Ln₂NiO₄+δ (LnNO; Ln = La, Pr) cathodes is prepared on the Ce₀.₉Gd₀.₁O₂−δ (CGO) electrolyte by sequentially using screen-printing (SP) and electrostatic spray deposition (ESD) techniques for the first time. Both LnNO samples crystallize into a single *Fmmm* orthorhombic layered Ruddlesden-Popper structure. The role of the electrode/air and electrode/electrolyte interfaces has been evaluated by impedance spectroscopy. A drastic reduction in polarization resistance (*R*<sub>pol</sub>) from 3.33 to 0.42 Ω cm² and from 0.83 to 0.08 Ω cm² is obtained at 600 °C for LaNO and PrNO, respectively, when the ESD electrode (with a dense thin, ~100 nm, LnNO sub-layer) is topped by a SP current collector. A further *R*<sub>pol</sub> reduction down to 0.16 and 0.04 Ω cm² is successfully obtained for LaNO and PrNO, respectively, when the LnNO sub-layer is replaced by a thicker (~3 µm) porous CGO/LnNO composite sub-layer. This composite sub-layer plays a main role in obtaining the best electrochemical properties of nickelates available in the literature, to the best of our knowledge. Moreover, the values of polarization resistance for both electrodes are constant at 650 °C for 15 days in air, proving their suitability as SOFC cathodes at this intermediate temperature.

## 1. Introduction

Mixed ionic-electronic conductors (MIECs) with a K₂NiF₄-type structure have been intensively studied¹,² as promising IT-SOFC cathode materials due to their attractive electrochemical performance.³⁻⁸ It has been reported that the Ruddlesden-Popper-type nickelate compounds such as Ln₂NiO₄+δ (Ln = La, Pr and Nd) show sufficient electronic conductivity due to the metal mixed valence (~80 to 100 S cm⁻¹) and good ionic transport properties.⁹,¹⁰ Indeed, they can accommodate hyper-stoichiometric oxygen in interstitial lattice sites in the rocksalt layers of the structure.¹¹ In addition, the lanthanide nickelates show a promising electrocatalytic activity for the oxygen reduction reaction (ORR).¹² They exhibit high values of surface exchange and diffusion coefficients (k* and D*) compared to perovskites.¹²,¹³ Moreover, they are characterized by low thermal expansion coefficient (TEC) values,²,³,¹⁴ that are comparable to the most commonly used electrolytes, such as yttria stabilized zirconia (YSZ) and gadolinia doped ceria (CGO).¹⁵⁻¹⁷ However, the polarization resistance (*R*<sub>pol</sub>) is still too high and its mini-mization is required to be used as cathodes for SOFCs. Several strategies to improve the electrochemical performance have

been explored including the addition of a dense base-layer between the electrolyte and porous electrode, the control of the microstructure, the introduction of an ionically conducting phase to form a composite electrode and the design of the electrode’s architecture. For instance, it is possible to fabricate a layer adjacent to the electrolyte with a smaller particle size to maximize the electrocatalytic activity in this region.¹⁸ This sub-layer can be made denser than the bulk porous electrode with a view to improving the electrolyte/electrode contact and to enhance the oxygen transfer to the electrolyte. Thus, the *R*<sub>pol</sub> value of La₂NiO₄+δ was found to decrease from 7.4 to 1.0 Ω cm² at 700 °C, as reported by Sayers *et al*.¹⁹ and it was decreased by ~30% at 600 °C (from 1.2 to 0.85 Ω cm²) with respect to the best porous cell as reported by Hildenbrand.²⁰ Another approach is to increase the surface area of the bulk electrode to enhance the number of active surface sites, yielding better electrochemical performances.²¹ The most promising Pr₂NiO₄+δ electrode has been reported for a mean particle size of ~0.4 µm (*R*<sub>pol</sub> value down to 0.08 Ω cm² at 600 °C).²² Moreover, composite cathodes, consisting of a matrix phase cathode (electronic conducting or MIEC) and a second phase electrolyte, are gaining importance not only due to further reduction in the *R*<sub>pol</sub> value but also due to good cathode-electrolyte adhesion properties.²³⁻²⁵ Composite electrode preparation *via* infiltration is one of the most effective methods used to increase the triple phase boundary (TPB) area and to improve the cathode efficiency at lower temperatures.²⁶,²⁷ Recently, Nicollet *et al*.²⁷,²⁸ have studied two different composite cathodes based on La₂NiO₄+δ and Pr₂NiO₄+δ infiltrated into a Gd-doped ceria backbone. Very low *R*<sub>pol</sub> values of 0.15 Ω cm²

<footnote>ªUniv. Grenoble Alpes, LEPMI, F-38000 Grenoble, France. E-mail: elisabeth.djurado@lepmi.grenoble-inp.fr; Fax: +33-476826777; Tel: +33-476826684</footnote>

<footnote>ᵇCNRS, LEPMI, F-38000 Grenoble, France</footnote>

<footnote>ᶜICMCB, CNRS, Université Bordeaux, 33608 Pessac, France</footnote>

<footnote n="4">⁴Univ. Grenoble Alpes, CNRS, LMGP, F-38000 Grenoble, France</footnote>
for La₂NiO₄+δ and 0.075 Ω cm² for Pr₂NiO₄+δ at 600 °C were found in comparison to those of pure La₂NiO₄+δ (0.93 Ω cm² at 600 °C)²⁹ and Pr₂NiO₄+δ (0.15 Ω cm² at 600 °C)²⁹ cathodes deposited by screen-printing on the YSZ electrolyte. On the other hand, the infiltration procedure demands several repeated impregnation-calcination steps which limits its use for industrial applications.³⁰,³¹ Recently, we reported a drastic reduction of the Rₚol from 3.33 to 0.42 Ω cm² at 600 °C when a 3-D coral nanocrystalline La₂NiO₄+δ film (mean particle size ~150 nm) with a continuous nanometric dense interface was topped by a La₂NiO₄+δ current collector.³²

In this study, we propose for the first time, an alternative fabrication route for Ln₂NiO₄+δ (LnNO; Ln = La, Pr) electrodes on the CGO electrolyte, in three steps, as follows: First, a thin porous CGO film is deposited by SP and sintered. Then, in the second step, it is covered by a 3-D LnNO porous coral-type microstructure by ESD and sintered. Third, the LnNO layer is topped by SP as a current collector and sintered. The innovative triple layer architecture of the full LnNO electrode resides on the presence of a CGO:LnNO composite thin porous sub-layer (3–4 µm thick) as a consequence of the penetration of the ESD LnNO film into the first SP CGO sub-layer. This new architecture is used with the objective of decreasing the polarization resistance by improving the electrode/electrolyte contact and enlarging the TPB region. A comparison of the electrochemical properties of the triple layer LnNO electrodes on CGO with two other architectures is performed: a single ESD LnNO layer and a double ESD + SP LnNO layer.

## 2. Experimental

### 2.1 Materials and solution preparation

Dense CGO (Ce₀.₉Gd₀.₁O₂−δ) pellets, made by pressing discs (20 mm diameter and 1.2 mm thickness) of commercial powder (Solvay) and subsequently fired at 1400 °C for 4 h, were used as the electrolyte.

Nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O, 99.9%, Aldrich], lanthanum nitrate hexahydrate [La(NO₃)₃·6H₂O, 99.9%, Alfa Aesar], praseodymium nitrate hexahydrate [Pr(NO₃)₃·6H₂O, 99.9%, Aldrich], citric acid [C₆H₈O₇, 99.9%, Alfa Aesar], and ethanol (CH₃CH₂OH, >99.9%, Prolabo) were used as precursors. ESD precursor solutions of 0.02 M (total cation concentration) were prepared in a mixture of 30 mL of ethanol and water (1 : 2) by adding a stoichiometric amount of La(NO₃)₃·6H₂O, Pr(NO₃)₃·6H₂O and Ni(NO₃)₂·6H₂O salts and citric acid (20 mol% in excess). For the SP layers, the CGO and the LnNO current collector, inks based on the KD2921 (Zschimmer and Schwarz) solvent were prepared with commercial CGO powder (Solvay) and with LnNO (Ln = La and Pr) powders obtained by auto-combustion. For this process, the same precursor solution was used as in the ESD process, but prepared in pure EtOH. Then, the solutions were heated at 150 °C till the completion of an auto-combustion process in order to obtain amorphous powders. These amorphous powders were sintered at 950 °C for 6 h (LaNO) and 1100 °C for 4 h (PrNO) in air, respectively, in order to obtain crystallized and single-phase powders.

### 2.2 Cathode preparation and characterization

The preparation process of the triple layer cathode can be described in three steps interspersed with heat treatments (Table 1, samples 5 and 6). First a CGO base-layer porous film is deposited by SP on the CGO electrolyte and sintered at 1200 °C for 4 h in air. Then, it is covered by a LnNO (Ln = La, Pr) porous film using the ESD technique, described elsewhere.³³,³⁴ The optimized ESD conditions in order to obtain a 3-D coral type microstructure are those previously reported in ref. 31: a deposition time of 180 minutes, a substrate temperature of 350 °C, a nozzle to substrate distance of 50 mm and a flow rate of 1.5 mL h⁻¹. This LnNO ESD film partially penetrates into the first CGO porous film leading to a denser CGO-LnNO composite base-layer. The crystallization of this LnNO ESD coating is obtained after a heat treatment of only 2 h at 900 °C for LaNO. In

<table><caption>**Table 1** Sample identification, composition and deposition technique, surface area and sintering conditions of the prepared cathodes on the CGO electrolyte</caption><thead><tr><th>Samples (architecture)</th><th>Composition (Technique)</th><th>Surface area [cm²]</th><th>Sintering in air (temperature/time)</th></tr></thead><tbody><tr><td>Sample 1 (single layer)</td><td>LaNO (ESD)</td><td>2.00</td><td>950 °C/6 h</td></tr><tr><td>Sample 2 (single layer)</td><td>PrNO (ESD)</td><td>2.00</td><td>1100 °C/4 h</td></tr><tr><td>Sample 3 (double layer)</td><td>LaNO (ESD) +<br>LaNO (SP)</td><td>1.54</td><td>950 °C/6 h + 1050 °C/2 h + 1100 °C/0.5 h</td></tr><tr><td>Sample 4 (double layer)</td><td>PrNO (ESD) +<br>PrNO (SP)</td><td>1.54</td><td>1100 °C/4 h<br>1050 °C/2 h + 1100 °C/0.5 h</td></tr><tr><td>Sample 5 (triple layer)</td><td>CGO (SP) +<br>LaNO (ESD) +<br>LaNO (SP)</td><td>1.54</td><td>1200 °C/4 h<br>900 °C/2 h<br>1050 °C/2 h</td></tr><tr><td>Sample 6 (triple layer)</td><td>CGO (SP) +<br>PrNO (ESD) +<br>PrNO (SP)</td><td>1.54</td><td>1200 °C/4 h<br>1000 °C/2 h + 1050 °C/2 h<br>1050 °C/2 h</td></tr></tbody></table>
the case of PrNO, a heat treatment at 1000 °C for 2 h followed by another one at 1050 °C for 2 h in air is required. No reactivity was observed between LnNO and CGO at these temperatures. The third and final step is the deposition of the LnNO current collecting layer (CCL) by SP, calcined at 1050 °C for 2 h in air. Both sintered electrodes, 30 µm thick, present a good adhesion onto the CGO electrolyte as demonstrated by the tape test. The cathode area is 1.54 cm².

Two other LnNO electrodes have been prepared on the CGO electrolyte with a single ESD layer (Table 1, samples 1 and 2) and a double layer (ESD + SP) configuration (Table 1, samples 3 and 4).

Six symmetrical cells were prepared with a single, double and triple layer electrode on both sides of the CGO electrolyte in order to be analyzed by impedance spectroscopy. Electrochemical impedance diagrams were recorded between 500 and 700 °C in air in the frequency range from 0.05 Hz to 100 kHz with an ac signal amplitude of 0.02 V at an open circuit potential (OCP) by using a PGSTAT 302N (Eco Chemie). A gold grid (Heraeus, 1024 meshes cm⁻² woven from 0.06 mm dia. wire) was used as the current collector. The data were fitted using equivalent circuits with the Zview® software (Scribner Associates) and were normalized to the electrode area. The crystal structure was determined using powder X-ray diffraction (XRD, Philips X'Pert-MPD system, Cu Kα radiation, λ = 1.54056 Å) in the Bragg-Brentano configuration. The XRD patterns were refined using Fullprof software in order to determine the lattice parameters.³⁵ Surface and cross-sectional morphology of the

cathodes was characterized using a scanning electron microscope (ZEISS Ultra 55 instrument with a field emission gun, FEG). The particle size of the electrodes was estimated from the SEM images using an image analysis tool (Image J software). The composition analysis was performed by energy-dispersive X-ray spectroscopy (EDX) using an acceleration voltage equal to 20 keV. Oxygen overstoichiometry (δ) of La₂NiO₄+δ and Pr₂NiO₄+δ samples (powder scratched from ESD films) was determined by thermogravimetry measurements (TGA-Q50 instrument) under flowing 5% H₂-Ar gas. Data were collected from 25 to 800 °C at a heating rate of 0.5 °C min⁻¹. Long-term stability measurements were also carried out on the LnNO triple layer cathodes at 650 °C for 15 days in air.

# 3. Results and discussion

## 3.1 Structural characterization of the LnNO films

Fig. 1 shows XRD patterns of the LaNO and PrNO films deposited by ESD and SP on the CGO electrolyte previously recovered by a screen-printed CGO porous layer (samples 5 and 6, Table 1). For the LaNO film, no impurities or secondary phases have been observed neither for the ESD film after sintering at 900 °C for 2 h, nor for sample 5, after sintering at 900 °C for 2 h followed by 1050 °C for 2 h in air. All diffraction peaks matched well with those of the La₂NiO₄+δ phase (ICDD # 01-074-9394) and those of the Ce₀.₉Gd₀.₁O₂−δ phase (ICDD # 04-013-6577) originating from both the substrate and the SP sub-layer. In the case of Pr₂NiO₄+δ

<figure><img src="image_2.png" /><figcaption>Fig. 1 XRD patterns of the LnNO films coated by ESD and SP on the CGO electrolyte previously recovered by a SP CGO sub-layer: (a) La₂NiO₄+δ and (b) Pr₂NiO₄+δ.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2 SEM micrographs of sample 1: (a, b) surface, (c, d) cross-section and sample 2: (e, f) surface, (g, h) cross-section.</figcaption></figure>

prepared by ESD, some traces of Pr₆O₁₁ and NiO appear as
intermediate phases after sintering at 1000 °C for 2 h along with
the CGO phase (ICDD # 04-013-6577) coming from both the

substrate and the SP sub-layer. Then, after a successive sinter-
ing at 1050 °C for 2 h in air (sample 6), they recombine, leading
to the pure Pr₂NiO₄+δ phase (ICDD #04-014-1778)). No structural
<figure><img src="image_4.png" /><figcaption>Fig. 3 SEM micrographs of sample 3: (a, b) surface, (c) cross-section and sample 4: (d, e) surface, (f) cross-section.</figcaption></figure>

modification was detected when the Pr₂NiO₄+δ CCL film was successively screen-printed onto the ESD layer and sintered again at 1050 °C for 2 h. Both LnNO films crystallize in an orthorhombic unit cell with the *Fmmm* space group (no. 69). The cell parameters, determined from Fullprof software refinement (Fig. S1 and S2†), are *a* = 5.457(3) Å, *b* = 5.462(8) Å and *c* = 12.690(6) Å for La₂NiO₄+δ and *a* = 5.391(4) Å, *b* = 5.453(8) Å and *c* = 12.446(3) Å for Pr₂NiO₄+δ, in good agreement with previously reported values.²⁹

Moreover, the oxygen overstoichiometry (δ) determined by TGA (Fig. S3†) was found to be 0.16 and 0.24 for La₂NiO₄+δ and Pr₂NiO₄+δ, respectively, which is in good agreement with the literature.²⁹

## 3.2 Microstructural characterization of the LnNO films with different architectures

As described in Table 1, one can consider six LnNO electrodes deposited on the CGO electrolyte with different architectural designs: *the single layer*: a coral-type ESD LnNO layer (sample 1: LaNO and sample 2: PrNO), *the double layer*: an ESD + SP LnNO

layer (sample 3: LaNO and sample 4: PrNO) and *the triple layer*: characterized by the CGO-LnNO composite base-layer, the ESD LnNO coral-type layer and the SP LnNO current collecting layer (sample 5: LaNO and sample 6: PrNO). In the following, a microstructural study of the LnNO electrode is focused on the construction of the electrode starting with the single, then the double and ending with the triple layer architecture. The electrical performance of the triple layer architecture is then investigated and compared to that of both the single and the double layer electrodes.

Fig. 2 shows the SEM surface and cross-sectional images of the samples 1 (LaNO) and 2 (PrNO). Whatever the rare-earth composition (La or Pr), both samples 1 and 2 are approximately 20 µm thick and show a porous 3-D coral type microstructure, as previously reported by Sharma *et al*. for LaNO.³² The ESD process obeys the electrohydrodynamic atomization laws of liquids to produce an aerosol from a liquid precursor which is then directed to a heated substrate. Thus, the microstructure of the resulting film is strongly dependent on the size of the droplets impacting the heated substrate. The droplet size can be controlled through the ESD deposition parameters such as the
<figure><img src="image_5.png" /><figcaption>Fig. 4 SEM micrographs of sample 6: (a), (b) cross-section, (c) CGO-PrNO sub-layer on the CGO electrolyte, (d) schematic of the triple layer design.</figcaption></figure>

solution flow rate, distance between the substrate and the nozzle and substrate temperature.³⁶ ESD is a promising and low-cost process which allows a good reproducibility of the coatings. Using similar ESD conditions to the ones reported in our previous article³² a porous 3-D coral-type microstructure is obtained (a substrate temperature of 350 °C, a nozzle to substrate distance of 50 mm and a flow rate of 1.5 mL h⁻¹). This porous microstructure is the consequence of preferential landing of the aerosol droplets on the CGO substrate since nearly dry aerosol droplets impact the substrate.³⁷ The charges on the substrate surface induced by the strong applied electrostatic field concentrate more where the curvature is greater.³⁸ So, the charged droplets arriving at the surface will be more attracted towards the more curved areas. This is referred to as "preferential landing". This action is the main route for the particle agglomeration, especially when the droplets are small and light. Hwang *et al*.³⁸ have shown that when the substrate roughness increases, preferential landing is promoted, leading to particle agglomeration. The average particle size of samples 1 (LaNO) and 2 (PrNO) is estimated to be ~150 nm (Fig. 2b) and ~200 nm (Fig. 2f), respectively. The crystal growth is expected to be larger for PrNO in comparison to LaNO since a higher sintering temperature is required to obtain the pure Pr₂NiO₄+δ crystalized phase (1100 °C for 4 h for PrNO compared to 950 °C for 6 h for LaNO). The presence of a thin dense base-layer at the electrolyte-electrode interface, ~100 nm thick, can be observed in Fig. 2d and h, for both sample 1 and sample 2, respectively. As reported previously,³² this first thin dense interlayer is deposited at the beginning of the ESD process. As a consequence, the

first charged liquid droplets containing the precursor solution are spread on the polished flat surface of the grounded substrate. The spreading is larger if the precursor solution is characterized by a low evaporation rate. This happens with an EtOH + water (1 : 2) based precursor solution due to a large amount of liquid in the charged droplets impacting the substrate. This leads to the formation of a continuous thin dense layer which also facilitates the adhesion of the electrode-electrolyte interface without the need for high sintering temperatures. In order to further improve the flatness and contact between the current collecting grid and the electrode, double layer architecture samples 3 (LaNO) and 4 (PrNO) were prepared by depositing a screen-printed CCL of a similar composition on top of samples 1 and 2, respectively (Fig. 3). The current collecting layer, 20 µm thick, partially penetrates (~50%) the 3-D coral type electrode which further improves the lateral percolation. The average particle size of the screen-printed layer sintered under the same conditions is approximately 450 nm for both compositions. Finally, we have designed other cathodes, samples 5 (LaNO) and 6 (PrNO) by modifying the thin dense sub-layer. It consists of a triple layer electrode obtained by inserting a CGO-LnNO composite base-layer onto the electrolyte (Table 1). Fig. 4 shows the SEM micrographs of sample 6, the Å-A cross section, the composite base-layer and a schematic of the triple layer design on the CGO substrate. Similar morphologies can be observed in sample 5 (Fig. S4†). A compact base-layer, ~3–4 µm thick, is observed. It is the result of the penetration of the electrosprayed LnNO into an initial porous SP LnNO layer forming a base-layer of the CGO-LnNO
<figure><img src="image_6.png" /><figcaption>Fig. 5 EDX analysis of the sub-layer region in the LnNO triple layer electrode: (a) sample 5, (b) sample 6.</figcaption></figure>

composite, similar to the infiltration process.³¹ The formation of the CGO-LnNO composite of the base-layer was confirmed by EDX in the base-layer region. As an example, EDX signals, collected from an area estimated to 1 µm × 1 µm located in the centre of the sub-layer, are shown in Fig. 5.

The LnNO electrodes with different designs are characterized by a heterogeneous porosity, larger in the ESD part than in the SP one. An estimation of the porosity of the ESD single layer has been performed by comparing the apparent film density with the LnNO theoretical density. The apparent density was obtained by weighing the CGO substrate before and after the LnNO film deposition and measuring the film thickness and diameter. A porosity of approximately 84 vol% was estimated. This large porosity indicates that plenty of space is available for the transport of oxygen molecules especially in the ESD film. These values are in good agreement with the porosity reported by Sar *et al*.³⁹ using FIB-SEM on the CGO/LSCF film also deposited in our group by ESD.

### 3.3 Electrochemical properties and stability

The electrochemical properties of the electrodes (samples 1–6, Table 1) deposited symmetrically on CGO were investigated by impedance spectroscopy in air at OCV over the temperature range 500–700 °C. In the following, the electrical properties of

the single and the double layer electrode architecture are first presented and then followed by the ones of the triple layer electrode. The impedance diagrams were fitted with an equivalent circuit, as shown in Fig. 6, consisting of a combination of several resistances (R)-constant phase element (CPE) parallel circuits, connected in series with an inductance (L) using ZView® software. This electrical element (L), appearing as a high frequency tail below the real axis, represents the contribution of the connecting leads, measuring device and connecting wires to the measured total impedance.⁴⁰ The high frequency intercept of the diagrams on the real axis corresponds to the overall ohmic resistance (R_s), including the resistive contributions of the electrolyte, electrode, leads and current collectors. The electrode performance can be quantified by the polarization resistance R-pol (2R-pol = 2R₁ + 2R₂ + 2R₃), which is the difference between the real axis intercepts of the impedance arc. According to the selected analysis, the electrode response is

<figure><img src="image_7.png" /><figcaption>Fig. 6 Equivalent electrical circuit model used for impedance spectra fitting.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 7 Nyquist plots recorded at 600 °C in air at OCV: (a) sample 1 and (b) sample 3. The numbers indicate the logarithm of the measuring frequency.</figcaption></figure>

composed of two or three main elementary contributions located at high frequency (1), medium frequency (2) and low frequency (3) depending upon the temperature and composition of the cathodes. The medium frequency arc resistance can be associated with various electrode processes such as the adsorption of gaseous oxygen O₂, dissociation of O₂ and charge transfer-diffusion (O²⁻) in the electrode whereas the high frequency arc is usually attributed to the charge transfer at the electrode/electrolyte interface.⁴⁰⁻⁴² The low frequency contribution can be related to gas impedance due to either porous electrodes or the experimental set-up.⁴³,⁴⁴

Typical experimental and fitted impedance diagrams, plotted in the Nyquist plane are shown in Fig. 7a and b for samples 1 and 3, respectively. Similar data were obtained for the PrNO electrodes (samples 2 and 4). The ohmic resistance (Rₛ) has been subtracted in order to facilitate comparison between the resistance polarization values (Rₚol) of the different samples. It is worth mentioning the important effect of the screen-printed CCL layer on the Rₛ. Adding a SP layer results in a significant decrease of the series resistance regardless of the composition of the electrode. For instance, at 600 °C, Rₛ varies from 14.58 Ω cm²

(sample 1) down to 5.43 Ω cm² (sample 2, not shown in Fig. 7). Indeed, Rₛ depends upon the intimate contact at the electrode/electrolyte and current collector/electrode interfaces. Increasing the contact area at both interfaces yields a homogeneous current distribution within the volume of the electrode, i.e. a lower constriction effect of the electrical current lines. As can be seen in Fig. 2d and h for single layer electrodes, a thin dense layer can be also evidenced at the electrode/electrolyte interface for sample 3 and sample 4. Nevertheless, by using only a gold grid as a current collector (samples 1 and 2), the current lines are not well distributed through the electrode volume. Once the screen-printed layer is added (Fig. 3), a homogeneous distribution of current lines within the electrode can be thus anticipated as well as a decrease of the constriction effect of the current lines in the vicinity of the electrode/electrolyte interface. An argument that further supports this assumption is that the series resistance recorded for both triple layer electrodes is not modified (Table 2). Accordingly, any variation of the polarization resistance between the double (samples 3 and 4) and triple layer (samples 5 and 6) electrodes is expected to be related mainly to the electrode morphology.

<table><caption>Table 2 Comparison of Rₛ, Rₚol, R₁, R₂, and R₃ values (Ω cm²) of the three architectures for LaNO and PrNO electrodes (estimated error is < 5%)</caption><thead><tr><th rowspan="3">Temperature [°C]</th><th colspan="12">LaNO cathodes</th></tr><tr><th colspan="5">Single layer (sample 1)</th><th colspan="5">Double layer (sample 3)</th><th colspan="5">Triple layer (sample 5)</th></tr><tr><th>R<sub>s</sub></th><th>R<sub>pol</sub></th><th>R<sub>1</sub></th><th>R<sub>2</sub></th><th>R<sub>3</sub></th><th>R<sub>s</sub></th><th>R<sub>pol</sub></th><th>R<sub>1</sub></th><th>R<sub>2</sub></th><th>R<sub>3</sub></th><th>R<sub>s</sub></th><th>R<sub>pol</sub></th><th>R<sub>1</sub></th><th>R<sub>2</sub></th><th>R<sub>3</sub></th></tr></thead><tbody><tr><td>500</td><td>22.78</td><td>13.38</td><td>3.86</td><td>9.52</td><td>—</td><td>8.25</td><td>2.85</td><td>0.82</td><td>2.03</td><td>—</td><td>7.61</td><td>0.97</td><td>0.29</td><td>0.68</td><td>—</td></tr><tr><td>550</td><td>18.96</td><td>7.00</td><td>2.24</td><td>4.76</td><td>—</td><td>6.40</td><td>1.01</td><td>0.38</td><td>0.63</td><td>—</td><td>6.35</td><td>0.35</td><td>0.11</td><td>0.23</td><td>0.01</td></tr><tr><td>600</td><td>14.56</td><td>3.32</td><td>1.63</td><td>1.69</td><td>—</td><td>5.43</td><td>0.42</td><td>0.11</td><td>0.30</td><td>0.01</td><td>5.28</td><td>0.16</td><td>0.04</td><td>0.11</td><td>0.01</td></tr></tbody><thead><tr><th rowspan="3">Temperature [°C]</th><th colspan="12">PrNO cathodes</th></tr><tr><th colspan="5">Single layer (sample 2)</th><th colspan="5">Double layer (sample 4)</th><th colspan="5">Triple layer (sample 6)</th></tr><tr><th>R<sub>s</sub></th><th>R<sub>pol</sub></th><th>R<sub>1</sub></th><th>R<sub>2</sub></th><th>R<sub>3</sub></th><th>R<sub>s</sub></th><th>R<sub>pol</sub></th><th>R<sub>1</sub></th><th>R<sub>2</sub></th><th>R<sub>3</sub></th><th>R<sub>s</sub></th><th>R<sub>pol</sub></th><th>R<sub>1</sub></th><th>R<sub>2</sub></th><th>R<sub>3</sub></th></tr></thead><tbody><tr><td>500</td><td>14.28</td><td>2.38</td><td>0.91</td><td>1.47</td><td>—</td><td>6.30</td><td>0.32</td><td>0.12</td><td>0.20</td><td>—</td><td>7.26</td><td>0.23</td><td>0.10</td><td>0.13</td><td>—</td></tr><tr><td>550</td><td>10.74</td><td>1.30</td><td>0.56</td><td>0.74</td><td>—</td><td>4.76</td><td>0.14</td><td>0.06</td><td>0.08</td><td>—</td><td>5.50</td><td>0.08</td><td>0.03</td><td>0.04</td><td>0.01</td></tr><tr><td>600</td><td>8.32</td><td>0.83</td><td>0.18</td><td>0.65</td><td>—</td><td>4.35</td><td>0.08</td><td>—</td><td>0.07</td><td>0.01</td><td>4.59</td><td>0.04</td><td>—</td><td>0.03</td><td>0.01</td></tr></tbody></table>
<table><caption>**Table 3** Comparison of minimum $R_{pol}$ values obtained in this study with values reported in the literature for LnNO (Ln = La, Pr) cathodes</caption><thead><tr><th>Cathode</th><th>Electrolyte</th><th><i>R</i><sub>pol</sub> [Ω cm<sup>2</sup>]</th><th><i>T</i> [°C]</th><th>Reference</th></tr></thead><tbody><tr><td>LaNO infiltrated CGO</td><td>CGO</td><td>0.15</td><td>600</td><td>Nicollet et al.<sup>27</sup></td></tr><tr><td>PrNO infiltrated CGO</td><td>CGO</td><td>0.07</td><td>600</td><td>Nicollet et al.<sup>28</sup></td></tr><tr><td>LaNO</td><td>CGO</td><td>2.20</td><td>600</td><td>Philippeau et al.<sup>51</sup></td></tr><tr><td>PrNO</td><td>CGO</td><td>0.28</td><td>600</td><td></td></tr><tr><td>LaNO</td><td>YSZ</td><td>0.93</td><td>600</td><td>Vibhu et al.<sup>29</sup></td></tr><tr><td>PrNO</td><td>YSZ</td><td>0.15</td><td>600</td><td></td></tr><tr><td>Triple layer</td><td>CGO</td><td>0.16</td><td>600</td><td></td></tr><tr><td>LaNO</td><td>CGO</td><td>0.04</td><td>600</td><td>Present work</td></tr><tr><td>PrNO</td><td>CGO</td><td>0.04</td><td>600</td><td></td></tr></tbody></table>

As previously reported,²⁰,⁴⁵ the presence of a thin dense layer at the cathode/electrolyte interface (samples 1 and 2) favors the oxygen transfer from the cathode towards the electrolyte and hence leads to better electrode performance. For instance, the polarization resistance was reported to decrease from 0.67 to 0.21 Ω cm² and from 1.2 to 0.85 Ω cm² at 600 °C for LSCF and LaNO, respectively, when a thin dense layer was deposited by PLD on the TZ3Y electrolyte prior to the screen-printed porous electrode.²⁰,⁴⁵ It is not the case for samples 1 and 2; indeed, the $R_{pol}$ values are found to be 3.33 and 0.83 Ω cm² at 600 °C (Table 2), respectively. These values are found to be higher than the lowest values reported in the literature,⁵¹, (2.20 Ω cm², LaNO and 0.28 Ω cm², PrNO) (see Table 3). The coral microstructure of samples 1 and 2 is highly porous (Fig. 2a and e) and the contact area with Au grids is too low and thus the whole electrode volume is not totally active for the ORR. Regardless of the electrode microstructure and the related behavior, the oxygen reduction reaction (ORR) from molecular oxygen O₂ into oxygen ions, O²⁻ requires the O₂ supply to the active reaction sites. Consequently, sufficient porosity is needed to facilitate gas permeation and the electronic conductivity must be also

maximized at the same time to improve the current collection. This could be the reason for the relatively poor performance of samples 1 and 2. In order to improve the contact between cathode and the grids and hence the effective electronic conductivity of the cathode sheet, the double layer cathodes (samples 3 and 4) were fabricated by depositing a SP layer of the respective composition on the top of the 3-D coral microstructure as a current collector and studied. As for the series resistance, a drastic improvement in $R_{pol}$ is obtained when adding a La₂NiO₄+δ (sample 3) and Pr₂NiO₄+δ SP layer (sample 4), respectively (Fig. 7). Double layer cathodes show much better performance: 0.42 Ω cm² at 600 °C (sample 3) and 0.08 Ω cm² at 600 °C (sample 4), respectively. At 600 °C, the $R_{pol}$ decreases by a factor of 8 for sample 3 and by a factor of 10 for sample 4. This is also in good agreement with the improvement in $R_s$ values.

This drastic improvement of the electrode performance (Table 2) of sample 4 over sample 3 (or sample 2 over sample 1) is also due to better mixed ionic-electronic conductivity as well as diffusion properties of PrNO in comparison to LaNO for a similar architecture. Indeed, these properties are dependent upon oxygen nonstoichiometry (δ); the higher the δ, larger the ionic conductivity and diffusion. From TGA (Fig. S3†), PrNO shows larger oxygen nonstoichiometry (δ = 0.24) in comparison to LaNO (δ = 0.16) in good agreement with the literature.²⁹ As shown in Fig. 7, the frequency distribution of the electrode impedance is not modified by adding the CCL, indicating that the ORR remains the same. In order to further reduce the $R_{pol}$ values, a composite sub-layer was added for samples 5 and 6 (Fig. 4). For a given electrode composition, the frequency distribution of the electrode impedance is not strongly modified (Fig. 8). The ORR is likely to remain similar. On the other hand, the shape of the Nyquist plot (electrode characteristic) is modified since a third elementary contribution (labeled as 3) is clearly observed for temperatures higher than 550 °C (Table 2). The fitted and experimental spectra at 600 °C from the inset of Fig. 8b, is shown in Fig. S5.† In order to assess the robustness of the fittings, capacitances and exponents of CPE for all 6 architectures are further given in Table S1.† It must be emphasized that the $R_{pol}$ value decreases further down to 0.04 Ω cm² for sample 5 (LaNO) and to 0.02 Ω cm² for sample 6 (PrNO) at

<figure><img src="image_9.png" /><figcaption>**Fig. 8** Nyquist Plots recorded as a function of the temperature (from 500 to 700 °C) at OCV; (a) sample 5 and (b) sample 6. The inset picture is a zoom-in of the high temperature plot. The numbers indicate the logarithm of the measuring frequency.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 9 Arrhenius plot of the R<sub>pol</sub> in air for six electrodes (OCV conditions) on the CGO electrolyte.</figcaption></figure>

700 °C, respectively. Once again, such an improvement in sample 6 over sample 5 is due to the better mixed ionic-electronic conductivity and diffusion properties of PrNO since the architecture is similar (Fig. 4 and S4†). Remarkably, the obtained R<sub>pol</sub> values are lower than those reported in the literature for LnNO based cathodes (Table 3). When a compact composite layer of CGO and LnNO is inserted between the electrolyte and the double layer cathode, a significant decrease in both the high and medium frequency resistances (R<sub>1</sub> and R<sub>2</sub>) is observed. Since the series resistances do not vary between double and triple layer electrodes for temperatures higher than 500 °C (Table 2), it can be assumed that the constriction effect at the electrode/electrolyte interface is minimized in the chosen temperature range. By considering that the SP layer acts as an optimized current collector for both types of electrodes, and that the microstructure of the ESD and SP layers remains

<figure><img src="image_11.png" /><figcaption>Fig. 10 Variation in the polarization resistance of samples 5 and 6 at 650 °C under OCV conditions.</figcaption></figure>

unchanged (Fig. 3 and 4), one could infer that the polarization resistance remains the same. Nevertheless, adding CGO enhances the ionic conductivity adjacent to the electrolyte and a better contact between CGO and LnNO can be anticipated.<sup>20,23</sup> The improvement of the electrode performance is likely to be mainly related to the increase of the active electrode thickness by inserting a composite layer. These results are in good agreement with the work previously reported by Nicollet *et al.*<sup>27,28</sup> for the composite electrodes, LaNO and PrNO infiltrated into a Gd-doped ceria backbone. Low R<sub>pol</sub> values of 0.15 Ω cm<sup>2</sup> for LaNO and 0.08 Ω cm<sup>2</sup> for PrNO at 600 °C were reported. Thus the proposed cathodes in this work could be considered as a potential cathode material for IT-SOFCs.

As could be expected, R₁ and R₂ decrease by increasing the temperature (Table 2). The R₂ values are always larger than those of R₁ (at least by a factor of 2 between 500 and 700 °C) suggesting that the oxygen desorption or adsorption process is the rate-limiting step for the ORR.<sup>46-48</sup> As shown in Table 2, the magnitude of the (R₃) low frequency contribution (~1 Hz) almost does not vary with temperature. Moreover this contribution is characterized by high capacitance values, C ≈ 2.6 F cm⁻² for the LaNO and ≈3.3 F cm⁻² for the PrNO cathode, respectively. Such a contribution has been attributed to the O₂ gas diffusion process through the porous electrode.<sup>49,50</sup>

The activation energies deduced from the Arrhenius plots of the polarization resistance R<sub>pol</sub> (Fig. 9) are in good agreement with the values reported in the literature.<sup>19,20,27</sup> To the best of our knowledge, the R<sub>pol</sub> values obtained (as low as 0.04 Ω cm<sup>2</sup> at 700 °C for sample 5 and as low as 0.02 Ω cm<sup>2</sup> at 700 °C for sample 6) are far better than the minimum R<sub>pol</sub> values previously reported for pure La<sub>2</sub>NiO<sub>4+δ</sub> and Pr<sub>2</sub>NiO<sub>4+δ</sub> based cathodes, as shown in Table 3. Moreover, all the results have been found to be reproducible. Such excellent performance of triple layer cathodes indicates that an architectural design of LnNO (Ln = La, Pr) cathodes plays a major role in improving their performance by enhancing the electrode to electrolyte charge transfer as well as the ORR.

Finally, the polarization resistance stability was evaluated for two electrodes (samples 5 and 6) for 360 h at 650 °C and neither of the cathodes shows any significant variation in the polarization resistance at OCV, as shown in Fig. 10. Further studies are in progress to evaluate the stability of these nanostructured electrodes after long-term operation under polarization and to test the performance in fuel cells.

In summary, electrostatic spray deposition could be an effective method for the commercial production of large-area electrodes for SOFCs with a series of advantages compared to the wet infiltration process, including easy industrial implementation, less preparation steps and low cost. Moreover, this method may be applicable to other electrochemical systems, such as solid electrolyzers, batteries, and supercapacitors.

## 4. Conclusions

This study demonstrates that the presence of a thin composite layer of CGO and LnNO in between the electrolyte and the double layer LnNO cathodes deposited *via* SP and ESD
procedures, respectively, is a very innovative route to improve
significantly the electrochemical performance. A single phase of
the prepared electrode crystallized in orthorhombic (Fmmm)
has been confirmed by XRD. During the growth of the 3-D coral
microstructure of LnNO on the top of a thin compact CGO film,
the precursor solution penetrates the compact CGO layer and
leads to the formation of the CGO and LnNO composite. In this
work, cathodes containing a CGO-LnNO composite sub-layer
displayed polarization resistances as low as 0.16 Ω cm² at 600 °C
(0.04 Ω cm² at 700 °C) for LaNO and 0.04 Ω cm² at 600 °C (0.02 Ω
cm² at 700 °C) for PrNO, which are among the best values re-
ported so far for SOFC cathodes. The lowest values, found for
these composite cathodes are due to the higher ionic conduc-
tivity of the composites, larger number of active points close to
electrolyte, and larger contact area between CGO and LnNO
which facilitate the charge transfer from the cathode to the
electrolyte. In addition, the Rpol values for both electrodes were
found to be constant at 650 °C for 15 days, further proving their
suitability to be used as SOFC cathodes.

Acknowledgements

This work was performed within the framework of the Centre of
Excellence of Multifunctional Archituctured Materials
“CEMAM” no. AN-10-LABX-44-01 funded by the “Investments
for the Future” Program.

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