# Elucidating nickel oxide reduction in a Ni-YSZ solid oxide cell via in-situ X-ray nano holo-tomography

Salvatore de Angelis, Julie Villanova, Peter Stanley Jørgensen, Vincenzo Esposito, Jacob Ross Bowen

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Salvatore de Angelis, Julie Villanova, Peter Stanley Jørgensen, Vincenzo Esposito, Jacob Ross Bowen. Elucidating nickel oxide reduction in a Ni-YSZ solid oxide cell via in-situ X-ray nano holo-tomography. Acta Materialia, 2024, 273, pp.119965. 10.1016/j.actamat.2024.119965. hal-05097670

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# Elucidating nickel oxide reduction in a Ni-YSZ solid oxide cell via in-situ X-ray nano holo-tomography

Salvatore De Angelis<sup>a, *</sup>, Julie Villanova<sup>b</sup>, Peter Stanley Jørgensen<sup>a</sup>, Vincenzo Esposito<sup>a</sup>, Jacob Ross Bowen<sup>c</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- X-ray holo tomography
- Solid oxide cells
- Nickel oxide reduction
- In-situ tomography
- Fuel electrode

## ABSTRACT

Reducing nickel oxide in solid oxide cell (SOC) electrodes is a critical step during the early stages of cell operation. It is considered crucial in shaping the final microstructure of the fuel electrode. In this work, we investigate the NiO reduction process in a state-of-the-art Solid Oxide Cell (SOC) using *in-situ* X-ray nano-holo-tomography. Results show a fast reaction kinetics and a reaction front from the outer to the inner regions of the sample. NiO reduction is complete in the first few seconds and the metallic Ni particles present a sponge-like structure, with many nanocrystallites and internal nano porosity. After reduction, the Ni network undergoes coarsening, increasing its particle size. Based on the acquired data, we propose a new chain of reaction mechanisms for forming the Ni nanocrystallites observed after reduction.

## 1. Introduction

Solid Oxide Cells (SOC) are a promising technology for facilitating the penetration of renewable sources into the future energy scenario. SOCs can reversibly convert electrical energy to hydrogen and vice-versa, using high-efficiency, abundant and non-critical materials. Therefore, they can economically store excess electricity from fluctuating energy sources like solar or wind.

A state-of-the-art SOC cell comprises multiple ceramic layers consisting of two porous electrodes and a dense electrolyte, often made of yttria-stabilized zirconia (YSZ). The oxygen electrode is usually an LSM: YSZ (lanthanum strontium manganite and yttria-stabilized zirconia) composite or a mixed conductor like LSCF (La0.6Sr0.4Co0.8Fe0.2O3-δ). The fuel electrode is usually a porous Ni/YSZ cermet produced by sintering a powder mixture of NiO and YSZ in air between 1200 and 1300 °C. The NiO is reduced to Ni during the first instants of operation by the fuel stream [1]. During this important step, the porosity of the cermet increases, and the final electrode morphology is achieved. Therefore, it is important to understand the NiO reduction mechanism, elucidate the fundamental reduction steps, and clarify the overall processes leading to the final Ni-YSZ microstructure.

NiO reduction has been extensively investigated for powders to

understand how the H₂ partial pressure, temperature, and densification influence the reduction mechanisms [2–4]. However, the results of these works are not readily transferable to SOC technology since the YSZ microstructure imposes specific constraints on the reduction process. In previous works, the NiO reduction kinetics in SOCs were studied using thermogravimetric analysis (TGA), which provides information on the reduction rate according to mass change in the sample [5,6]. Other techniques, such as *in-situ* X-ray diffraction (XRD) [7,8] or Raman spectroscopy [9,10], were employed to track the evolution of the Ni phase fraction (compared to NiO) during reduction. In both cases, the kinetics of the reaction was determined as a function of temperature. Also, it was shown that the reaction rate is high at the beginning of the reduction and decreases towards the end [11,12]. These kinds of studies were often coupled with *in-situ* electrochemical impedance spectroscopy (EIS) or conductivity measurements, where indirect information on the evolution of Ni network connectivity could be obtained [13–16].

While these methods provide important insights into NiO reduction in SOCs, they offer information only at the macroscopic level. For a deeper understanding of the processes involved at the micro to nano-scale, imaging techniques are required [17].

In the last decade, *in-situ* SEM and TEM experiments have shown the nucleation and growth of nano porosity at the particle level [18,19].

<footnote><sup>a</sup> Department of Energy Conversion and Storage, Technical University of Denmark, Kgs. Lyngby 2800, Denmark</footnote>

<footnote><sup>b</sup> ESRF - the European Synchrotron Radiation Facility, 71 Avenue des Martyrs, Grenoble 38000, France</footnote>

<footnote><sup>c</sup> Xnovo Technology ApS, Galoche Alle 15, 1st Floor, Køge 4600, Denmark</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: sdea@dtu.dk (S. De Angelis).</footnote>
However, both SEM and TEM provide two-dimensional information. Furthermore, the thin samples (< 100 nm) required for TEM analysis amplify surface-related effects and are not fully representative of describing the mechanism in the material bulk.

Post-mortem X-ray nanotomography measurements have been

performed to study SOCs properties [20–24] and to acquire the input structure to simulate their electrochemical behavior [25–27]. Further, *in-situ* tomography was increasingly developed, trying to improve spatial and temporal resolution (that are often exclusive [21]) as well as the ratio of the field of view (FOV) versus resolution. However, previous

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Volume rendering of the region of interest acquired using X-ray nano holo-tomography showing the overall sample shape obtained by FIB milling; (b) Volume rendering acquired at low resolution (micro-CT) showing the sample mounted on the alumina rod; (c) Schematic representation of the sample environment; (d) Schematic layout of the in-situ X-ray nano-imaging setup at ID16B.</figcaption></figure>
in-situ works on SOCs showed a large field of view but low resolution (FOV: 350 µm; pixel size: 395 nm) [28] or high resolution but a small field of view (FOV: ~10–20 µm, pixel size: 15–100 nm [29,30]).

Recently, ptychographic X-ray computed tomography (PXCT) was employed for studying NiO reduction in a representative SOC fuel electrode [31]. High-resolution tomograms of NiO-YSZ cermet were acquired before and after 1 h of reduction in a 5 % H₂-reducing atmosphere.

However, due to the long scanning time, only the initial and final stage of the reduction process was examined.

In this work, we capitalize on the combined high temporal and spatial resolution of X-ray nano-holo-tomography to acquire more time steps during the reduction process, elucidating the reduction kinetic in a representative SOC electrode, under representative conditions, and in three dimensions.

## 2. Experimental

### 2.1. Sample preparation

The sample was prepared by precision mechanical polishing from a typical ~300 µm thick NiO-3YSZ (yttria-stabilized zirconia, 3 mol.% Y₂O₃) anode-supported SOFC half-cell with a ~10 µm thick NiO-8YSZ functional layer and a ~15 µm thick 8YSZ electrolyte. Details of the cell production can be found elsewhere [31].

A pillar of approximately 60×60×3000 µm³ is obtained by precision polishing. In the region of interest for tomography, the pillar is refined to 54×40 µm² by focused ion beam (FIB) milling using a Zeiss CrossBeam 1540XB and a final probe current of 1 nA (Fig. 1a). The sample is fixed on an alumina rod of 1 mm diameter using platinum paste (Fig. 1b). The sample and alumina rod are encapsulated in a glass capillary filled with a mix of H₂ 5 % and N₂ 95 %, to ensure a controlled atmosphere. The capillary is fixed on a Huber pin and sealed with UV glue inside a glove bag.

### 2.2. Tomographic procedure

The nano-tomographic experiment was performed at the nano-analysis beamline ID16B of the European Synchrotron Radiation Facility (ESRF) [32]. Phase contrast nano holo-tomography [33] was performed using a conical and quasi-monochromatic beam (ΔE/E = 10⁻²) with an energy of 29.6 keV and a high photon flux of 10¹¹ pH/s. For each holo-tomographic dataset, four tomograms were acquired at four different sample-to-detector distances along the beam propagation direction (Fig. 1d). For each tomogram, 2998 projections with a pixel size of 27×27nm² were acquired using a fast PCO edge 5.5 detector (with CMOS sensors) over a 360° rotation, with an exposure time of 0.5 s per projection. The total acquisition time for a full holo-tomography scan was approximately 2 h.

For the heat treatments, a resistive furnace developed in-house at ID16B [34] was employed. The furnace can achieve high temperatures (> 800 °C) and is integrated with the sample stage, allowing in-situ imaging capabilities.

Considering the fast reaction kinetics of NiO reduction and a scan time of ~ 2 h, this work selected a stop-and-go procedure for the experiment. The furnace was preheated and kept at 850 °C for the entire experiment span. A first holo-tomographic tomogram was acquired for the sample in the pristine state. Subsequently, the furnace was lowered over the sample and raised again after 5 s. After thermal equilibration, a new holo-tomographic tomogram was acquired at room temperature. The same procedure was repeated for successive heat treatments of 5, 9, 17, and 32 s, giving cumulative treatment times of 10, 19, 36, and 68 s.

### 2.3. Tomographic reconstruction and data analysis

The three-dimensional reconstructions were obtained in two steps: i)

phase retrieval calculations were performed on the single projections using an in-house octave script based on a Paganin-like [35] approach using a delta/beta ~35 and ii) filtered back projection was performed with the ESRF software PyHST2 [36] for the final 3D reconstructions. In the reconstructed volumes, ring artifacts were removed using a post-processing algorithm based on image filtering [37] implemented in MATLAB.

The heat-treated tomograms were first registered to the pristine tomogram (0 s) using the iterative closest point algorithm [38] with points sampled from the external edges of the scanned structure. Only data from a rectangular box fully contained in the fuel electrode was used to analyze the changes in the intensity distribution. The pore connectivity analysis was calculated from a coarse segmentation, obtained by setting an intensity threshold halfway between the air and the YSZ histogram peaks intensity grey level in the 68 s tomogram. The segmentation of the Ni network for the particle size distribution was performed by a combination of a non-local means filter and manual thresholding, following a similar procedure as in [39]. The segmentation of the reduction front (Fig. 4) was performed manually slice by slice.

## 3. Results and discussions

### 3.1. Global microstructure evolution and reaction front

Fig. 2 shows a two-dimensional slice obtained from the full tomograms after each heat treatment. The slice is obtained from the location indicated in Fig. 2a while additional locations at a different sample height are shown in Figure S-1 of the supplementary materials. The different phases (NiO and YSZ) can be distinguished for each image, where image intensity is quantitatively proportional to the material electron density. Fig. 2b shows the initial NiO-YSZ state microstructure with the locations of the dense electrolyte layer, porous functional, and support layer marked. The microstructure of all three layers is representative of typical pre-pilot scale production SOC half cells [40]. Fig. 2c-g presents the same cross-section after the subsequent heat treatments. From the evolution of the reduction process, the following key qualitative observations can be made:

(i) After 5 s a large fraction of the NiO in the sample has been reduced to Ni. The newly reduced particles present an internal nano-porous structure.

(ii) A reduction front is visible at 5 and 10 s, and the reduction initiates from the sample exterior, proceeding inwards in the sample (see Figure S-3 for orthogonal views).

(iii) The nano-porous Ni structure coarsens (green circles in Fig. 2) simultaneously with the migrating reaction front.

(iv) For the conditions of this experiment, Ni reduction appears complete after only 19 s of treatment.

(v) Ni detaches (Ni dewetting) from the YSZ backbone after 68 s (red circles in Fig. 2).

The qualitative observations can be confirmed by Fig. 3, which shows the distribution of the raw intensity values in the six tomograms. The 0 s intensity distribution shows two distinct peaks (YSZ and NiO) with a long tail towards lower intensities representing the pore and pore/solid interfaces.

The progressive reduction of NiO into metallic Ni is testified by the increase of the peak related to Ni, corresponding to a normalized intensity of 1.5. Furthermore, the peak related to the NiO, present in the 0 s tomogram, becomes convoluted with the one related to the YSZ. The convolution is due to the formation of the internal nano porosity, which results in many voxels placed at the interface between pores and Ni, with intermediate grayscale levels comparable to the YSZ. The new convoluted peak decreases its intensity, indicating a reduction of NiO content in the volume. Finally, the formation of additional porosity is testified by the growth of the histogram tail for lower normalized intensities,
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** (a) 3D rendering of the sample in its pristine state; (b-g) Cross-section images extracted from the slice shown in (a) after 0 s (b), 5 s (c), 10 s (d), 19 s (e), 36 (f) and 68 s (g) of reduction in 5 % H₂ / 95% N₂ at 850 C. In all images, the darker phase is the pore phase, the light gray is the YSZ, and the brightest phase is either Ni or NiO.</figcaption></figure>

indicating an increase in pore phase fraction in the electrode.

Fig. 4 shows 3D renderings (a-c) for the pristine, 19 s, and 68 s reduction states (all reduction times are shown in Figures S-2, and S-3 of the supplementary materials).

In the 3D renderings, the sample surface is shown in transparent grey, and the percolating porosity, connected to the surrounding reducing atmosphere, is illustrated in green.

Due to the limited resolution and other image artefacts, the percolation status of each pore region is semi-quantitative as in some regions, single voxel segmentation errors can alter the status of connected pore volumes (an example of such error is indicated in Figure S-2).

Nevertheless, in the pristine state, it is evident that the functional layer presents very low pore connectivity, testified by the absence of green pores close to the dense electrolyte. The lack of pore connectivity

in the functional layer agrees with the results reported in [41]. Contrarily, Fig. 4a shows that the support layer has a large fraction of pores connected to the sample boundaries, already in the pristine state.

Upon reduction, the overall pore connectivity improves, and more pores in the sample interior connect to the boundaries. After 68 s of cumulative treatment time, most pores appear fully connected.

## 3.2. Local microstructure evolution and reduction mechanism

Fig. 5 shows the mechanism of NiO reduction and Ni coarsening inside a representative SOC electrode at the microscale. Figure S-4 of the supplementary materials presents more locations within the electrode.

In the pristine state, the NiO network (white phase) appears dense and does not present any internal detectable porosity (Fig. 5a). After 5 s
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Distribution of raw gray-value intensities in each tomogram. All intensities are normalized based on the intensity of the YSZ peak position in the 0 s tomogram.</figcaption></figure>

(Fig. 5b), the reduction of the NiO network leads to the formation of metallic Ni which presents a sponge-like structure characterized by nanometer-scale porosity.

Subsequently, the sponge-like Ni undergoes a coarsening process, where the small Ni crystallites grow to form bigger agglomerates (Fig. 5c, 5d). Furthermore, the internal pores in the Ni particles grow in size due to the Ni particles coarsening. Interestingly, Fig. 5f (white dashed circle) shows that in some locations, the Ni coarsening leads to the formation of pores probably located at the original NiO grain boundaries. Further investigations using diffraction-based techniques (e.g. 3D X-ray diffraction tomography) are needed to confirm this hypothesis.

Fig. 5 shows that, for the conditions of this experiment, the NiO reduction is a fast process, occurring within 5 s from the start of the heat treatment. Therefore, due to the fast kinetics of the NiO reduction, the formation of the final Ni network can be considered a two-step process. First, the NiO network undergoes chemical reduction to form the sponge-like metallic Ni. Subsequently, the Ni crystallites coarsen to form the network shown in Fig. 5f.

The mechanism of NiO reduction observed in Fig. 5 is in line with several experimental reports where the formation of nanosized Ni crystallites was already observed *in-situ* TEM studies [19,42–44].

Based on previous literature studies and the results presented here,

we can summarize the mechanism of NiO reduction in SOCs as in Fig. 6.

The NiO reduction starts at the NiO/pore interface, where the NiO network is in contact with the relatively H₂-rich reducing gas (Fig. 6a). The first few nanometers of the NiO surface react with H₂ to form metallic Ni. However, instead of a coherent and dense Ni film, small Ni crystallites are formed on the surface of the NiO network (6 b). This phenomenon can be explained by considering the volume contraction occurring upon reduction, which creates stress in the metallic layer, leading to its fragmentation. The formation of small Ni crystallites (a few nanometers) in the first instants of the reduction process is well visible in Fig. 5b and was previously reported in [45]. The fragmented metallic Ni opens new pathways for the H₂ molecules, which can penetrate and reduce deeper layers of NiO, leading to the sponge-like structure observed in Fig. 5b and schematically represented in Fig. 6b. Fig. 5b shows that, locally, this process occurs in less than 5 s. The fast kinetic of the NiO reduction at 850 °C makes it challenging to acquire enough data for a quantitative kinetic model. Therefore, in order to increase the number of measurements during reduction, we will perform a future study at a lower temperature.

For the conditions of this experiment, Fig. 5c-f shows how the coarsening process occurs in the first 68 s of the heat treatment. The driving force for the coarsening process is minimising the system interface energy, which is governed by the Ni atomic surface mobility [46]. In the first instants of coarsening (Fig. 5b), the system presents high interface energy, and the coarsening proceeds at a faster rate (Fig. 5b,c) to then slow down with the increase of Ni and pore sizes (Fig. 5d-f).

To provide a more quantitative analysis of the observed changes in the Ni network, we extracted a subvolume of 300×300×300 (~8 × 8 × 8 µm³) voxels from the volumes treated for 19, 36, and 68 s. Given the limited resolution, we excluded the first three datasets from this analysis. Fig. 7 (top row) shows the three-dimensional rendering of the reduced nickel network. Note that, for clarity, the 3D renderings only show a subvolume of 200×200×200 voxels.

Fig. 7 also shows the Ni network's computed continuous particle size distribution (cPSD). Results show an increase in the mean value for the Ni particle radius from ~50 nm (19 s) to ~100 nm for the 36 and 68s-treated volumes. We can also observe that the cPSD for the 68 s volume presents a more pronounced tail towards bigger radii than the 19 s volume.

These results shed light on the very first instants of Ni coarsening in a SOC microstructure. However, comparing the cPSD of the 68 s volume with a state-of-the-art SOC microstructure [45], the mean radius for the

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Three-dimensional renderings of the investigated volume for a) pristine, b) 19 s, and c) 68 s of cumulative reduction time. In a), b), and c), the pores connected to the sample boundaries are shown in green. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Magnified images of the edge of the sample (in the Ni-3YSZ support layer) extracted from a cross-section of the full volume after (a) 0 s, (b) 5 s, (c) 10 s, (d) 19 s, (e) 36, and (f) 68 s of reduction in 5 % H₂ / 95% N₂ at 850 °C.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Schematic diagrams of NiO reduction. a) beginning of reduction where only the NiO surface reacts. b) formation of the Ni crystallites and subsequent new pathways for H₂ molecules. c) formation of the sponge-like Ni structure at the end of reduction.</figcaption></figure>

Ni particles is much smaller (~100 nm vs. ~400 nm), hinting that the coarsening is incomplete. Further, Fig. 5f shows that, after 68 s, the Ni network presents high internal porosity. This situation is not observed in SOC fuel electrodes reduced under technological conditions, where the Ni network is dense and well-attached to the YSZ structure. Considering that the high-temperature step of a standard reduction procedure can last up to 2 h [47], we assume that the coarsening process is not complete. In-situ tomographic experiments with longer heat treatments would be necessary to investigate the full formation of the Ni network in SOCs.

## 4. Conclusions

The NiO reduction in state-of-the-art SOC microstructure was studied using in-situ X-ray nano holo-tomography. The sample was analyzed in its pristine state and after different reduction times (5 s, 10 s, 19 s, 36 s, and 68 s) in a static reducing atmosphere (5 % H₂ in N₂) at 850 °C.

The NiO reduction to metallic Ni was observed in the tomograms and confirmed by analysing the normalized intensities. We report that, for the condition of this experiment, the reduction reaction is rapid and occurs after only 5 s of treatment.

During the first few seconds of treatment, results also show a reaction
<figure><img src="image_10.png" /><figcaption>Fig. 7. (Top) three-dimensional renderings of the Ni network after 19, 36 and 68 s of cumulative treatment. (Bottom) Continuous particle size distribution (cPSD) calculated on the Ni network, showing an increase in mean radius for the Ni particles.</figcaption></figure>

front from the exterior to the sample's interior, while the entire micro-
structure is reduced after 19 s.

From the microstructure analysis, we report the formation of Ni
crystallites as the product of the reduction reaction. Further, we propose
a possible reduction mechanism based on our data and previous litera-
ture. Finally, we studied the first instants of the Ni coarsening, showing
the growth of the Ni particles from a mean radius of ~50 nm (19 s) to
~100 nm after 68 s of cumulative treatment.

These findings contribute to a better understanding of the reduction
kinetics and coarsening behaviour in SOCs, with implications for
designing and optimising SOC materials and production processes.

CRediT authorship contribution statement

**Salvatore De Angelis:** Visualization, Writing – original draft, Conceptualization, Data curation, Investigation, Methodology, Software. **Julie Villanova:** Conceptualization, Investigation, Visualization, Data curation, Methodology, Writing – original draft. **Peter Stanley Jørgensen:** Investigation, Conceptualization, Data curation, Software, Supervision, Writing – review & editing. **Vincenzo Esposito:** Conceptualization, Methodology, Supervision, Validation, Writing – review & editing. **Jacob Ross Bowen:** Conceptualization, Methodology, Project administration, Software, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgment

We acknowledge the European Synchrotron Radiation Facility for
providing synchrotron radiation facilities (proposal MA3500). We want
to thank Rémi Tucoulou, Sylvain Labouré, and Cyril Guilloud for their
technical support. We also thank Vincent Fernandez for providing the
3D rendering of the top of the sample mounting (Fig. 1b) using X-ray
micro-tomography at ID19.

**Supplementary materials**

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.actamat.2024.119965.

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