# In-situ surface activation of polycrystalline LaNiO3 electrocatalyst for the oxygen evolution reaction

Giuditta de Amicis, Anna Testolin, Cristina Cazzaniga, Francesco d'Acapito,
Alessandro Minguzzi, Paolo Ghigna, Alberto Vertova

► To cite this version:

Giuditta de Amicis, Anna Testolin, Cristina Cazzaniga, Francesco d'Acapito, Alessandro Minguzzi, et al.. In-situ surface activation of polycrystalline LaNiO3 electrocatalyst for the oxygen evolution reaction. International Journal of Hydrogen Energy, 2024, 87, pp.890-901. 10.1016/j.ijhydene.2024.09.038. hal-05097669

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# In-situ surface activation of polycrystalline LaNiO₃ electrocatalyst for the oxygen evolution reaction

Giuditta De Amicis<sup>a</sup>, Anna Testolin<sup>b</sup>, Cristina Cazzaniga<sup>b</sup>, Francesco D'Acapito<sup>c</sup>,
Alessandro Minguzzi<sup>d,e,f,*</sup>, Paolo Ghigna<sup>a,d</sup>, Alberto Vertova<sup>d,e</sup>

## ARTICLE INFO

Handling Editor: Prof I Tolj

Keywords:

Alkaline water electrolysis

Oxygen evolution reaction

Surface disorder

Polycrystalline LaNiO₃

Operando X-ray absorption

## ABSTRACT

The efficiency of hydrogen production from water electrolysis is limited by the sluggish kinetics of the Oxygen Evolution Reaction (OER), necessitating catalytic material to avoid the high overpotentials. This study investigates LaNiO₃ as a promising electrocatalyst for the OER in Alkaline Water Electrolysis selected for its cost-effectiveness and easy scale-up. LaNiO₃, prepared by a co-precipitation method, shows good OER activity and electrochemical stability. Operando X-Ray Absorption Spectroscopy (XAS), reveals a crucial role of the Ni(II)/Ni (III) redox couple. Notably, a layer of a Ni(II) compound formed at the catalyst surface is oxidized under anodic potentials and possibly becomes the active sites for the adsorption of OH⁻ and for the OER reaction. High resolution transmission electron microscopy confirms the formation of a defective outer layer during operation, indicating the presence of disordered structure on the surface of the electrocatalyst, which increases the number of active sites for the OER.

## 1. Introduction

Hydrogen, as an energy carrier, could have a key role in a clean, secure, and affordable energy for the future. However, at the moment, hydrogen production is dominated by reforming of natural gas [1], gasification of coal and petroleum coke [2], or gasification and reforming of heavy oil [3]. Recently, different sources of hydrogen production have coded with colors. Black, grey or brown refer to the production of hydrogen from coal, natural gas and lignite respectively [4]. Blue is commonly used for the production of hydrogen by steam methane reforming with Carbon Capture Utilization and Storage (CCUS), using natural gas or biomass [5]. Green is a term applied to production of hydrogen from water by electrolysis using electricity from renewable energy sources. At present, only 4% of the global hydrogen supply is green [6]. Overall, water electrolysis is limited by the sluggish electrode kinetics associated with the Oxygen Evolution Reaction (OER) at the anode [7,8], rather by the cathodic hydrogen evolution reaction (HER). This is due to OER, which involves a four-electron

transfer, compared to two-electron transfer for the HER [9,10]. The overall change in free energy, for the four electrons water splitting reaction, amounts to 4.92 eV [11]. In this situation, the ideal anodic material would require a potential of 1.23 V (vs RHE). For now, IrO₂ and RuO₂ are the most efficient catalysts for OER, but are extremely expensive, as Ir and Ru are two of the least abundant elements on the earth [12–14]. It is also well-known that these oxides are the most active and stable noble metal in acidic electrolytes [15], but are not equally stable in alkaline ones [16,17].

Operating in alkaline (basic) conditions, in principle, permits the use of non-noble and low-cost metal electrocatalysts [18,19], leading to the possibility of cheaper hydrogen production [20]. Numerous studies [21–25] have already shown the high catalytic activity of nickel oxide electrocatalyst in alkaline electrolytes applicable in electrolysers, and the research of this class of materials is still ongoing, as demonstrated by the most recent literature. For example, Ni-doped CoP encapsulated by B- and N- co-doped carbon (Ni–CoP/BNC) electrocatalysts synthesized by hydrothermal synthesis, followed by a thermal treatment [26] and a

<footnote><sup>a</sup> Dipartimento di Chimica, Università di Pavia, Viale Taramelli 16, 27100, Pavia, Italy</footnote>

<footnote><sup>b</sup> Industrie De Nora S.p.A., Via Leonardo Bistolfi 35, 20134 Milan, Italy</footnote>

<footnote><sup>c</sup> CNR-IOM-OGG LISA-CRG c/o ESRF, 71 Avenue des Martyrs, Grenoble, France</footnote>

<footnote><sup>d</sup> Istituto Nazionale di Scienza e Tecnologia dei Materiali, via Giusti 9, Firenze, Italy</footnote>

<footnote><sup>e</sup> Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133, Milan, Italy</footnote>

<footnote><sup>f</sup> Dipartimento di Energia, Politecnico di Milano, Via Lambruschini, 4a - 20156, Milano, Italy</footnote>

<footnote>* Corresponding author. Istituto Nazionale di Scienza e Tecnologia dei Materiali, via Giusti 9, Firenze, Italy.
E-mail address: alessandro.minguzzi@unimi.it (A. Minguzzi).</footnote>
novel core-shell heterostructure that involves 1D NiMoO₄ nanorod core and 2D Co(OH)₂ nanosheet shell, which function as bifunctional electrocatalysts for the oxygen evolution reaction (OER) via a facile two-step method [27].

In the present work, we studied the perovskite LaNiO₃ as a catalytic material for anodic in Alkaline Water Electrolysis (AWE). Among several perovskite oxides, LaNiO₃ is one of the most promising candidate due to its low charge transfer resistance, high conductivity and good electrocatalytic properties for OER [28,29]. Furthermore, among several perovskite oxides, LaNiO₃ is particularly promising due to its favorable electrocatalytic kinetic and grater performance for OER, ranking among the best-performing in terms of activity between perovskites in the pure state [30].

LaNiO₃ has the intrinsic activity for OER because of its single electron-filled e_g orbital on Ni³⁺ at B-site [31]. In turn, this allows a favorable bond energy for surface (B-site)-O bonding in OER [32]. Perovskite oxides have an ideal cubic structure with stoichiometry ABO₃ in which lanthanides or alkaline earth cations occupy the A-site, and transition metal cations the B-site. Lanthanides and transition metals are highly abundant elements, which allows the purchase of bulk materials for industrial processes [33]. Their flexible crystal and electronic structure provide them ideal candidates to study the correlations between OER activity and structure [34,35]. An indispensable step for studying electrocatalysis is the investigation of the surface composition of the transformed surface that indicated a strong correlation between OER activity and surface composition [36].

One of the most important aspects to study in OER electrocatalysis is the accessibility of active sites, related to the possibility of ion diffusion through the active layer [37], that is related to the hydrous/amorphous vs crystalline nature of the material. The increased performances (and instability) of hydrous materials, compared to “crystalline” was evidenced on several OER electrocatalysts and particularly on IrO₂ [38–43]. In parallel, the formation of amorphous/hydrated layers in the outer portion of the electrocatalyst surface was observed in many cases [34,44,45].

For what concerns LaNiO₃, Baeumer et al. determined the tunability of surface transformation of LaNiO₃ epitaxial thin films showing that the Ni termination is considerably more active than the La termination, and that this difference originates from a thermodynamically stable, Ni oxyhydroxide-type single-layer (disordered) surface phase NiO₂ which forms during the operation of Ni-terminated surfaces and is kinetically inaccessible when starting with a La termination [46].

This interesting finding prompted us to investigate on this phenomenon but on LaNiO₃ electrodes synthesized with a scalable method, which can be applied at the industrial level. In particular, we adopted a synthesis based on a co-precipitation method followed by calcination at 700 °C [47]. The powder was then fully characterized by structural, morphological and electrochemical probes. We also adopted operando X-ray absorption spectroscopy (XAS), aiming at better understanding of the reaction mechanisms in the case of the OER on LaNiO₃ anodes, an approach that we have already adopted with other electrode and photoelectrode materials [48–53]. In fact, XAS has now become an essential tool for the study of charge state, local geometry and electronic structure of specific elements (nickel in this case) during the occurrence of physico-chemical phenomena [54]. This technique allows for a deeper understanding of the real active sites involved in these processes [55].

We observed the formation of a disordered layer on the surface polycrystalline LaNiO₃ electrocatalyst combining XAS, high resolution transmission electron microscopy (HR-TEM), cyclic voltammetry and quasi-steady state polarization curves. Specifically, we demonstrated that, under sufficiently cathodic conditions, a layer of a Ni(II) compound is formed at the catalyst surface, that is then reversibly oxidized under anodic potentials forming Ni(III) sites at the surface, that possibly become the active sites for the adsorption of the reagent and the subsequent OER reaction. The number of active sites increase during operation thus increasing the active area of the electrocatalyst, i.e. the

number of exposed Ni centers.

Similar observations have recently been made for Co-MOF electrocatalysts, showing that the electroactivation of the composite requires the involvement of the OER. This process leads to a remarkable increase in the electroactive population of cobalt centers [56].

It seems therefore that reaching the right crystallinity degree is one of the possible pathways to reach a high activity/high stability material for the OER. In other words, it is one of the possible approaches to apply the so-called “rational design” of a catalyst [57].

Still, we need further information in the field, and this is at the bases of the present work, where we succeeded in the study of the polycrystalline LaNiO₃ system as electrocatalyst for the OER, evidencing the beneficial contribution of its surface disorder for water splitting reaction in alkaline media.

## 2. Materials and methods

### 2.1. Synthesis of lanthanum nickel oxide

LaNiO₃ was synthesized by a co-precipitation method [47], as previously reported [58]. Briefly, 0.01 mol of La(NO₃)₂·6H₂O (puriss. p.a., ≥99.0% titration, Fluka) and 0.01 mol of Ni(NO₃)₂·6H₂O (99.999% trace metals basis, Aldrich) were dissolved in deionized water (Milli-Q) and stirred at room temperature until complete dissolution. Then, a solution of 2 M KOH was dropped-in slowly, up to pH ≥ 13. After the formation of a precipitate, it was washed with deionized water until neutral pH, and finally dried thanks to Buchner filtration. After that, the powder was put in an oven in air at 60 °C for 3 h. Finally, the perovskite powder was obtained after a calcination step in a muffle furnace at 700 °C for 3 h in air.

### 2.2. Structural and morphological characterization

Phase composition and crystal structure of LaNiO₃ was checked by X-ray diffraction (XRD). The XRD pattern was recorder with a Multi-Purpose Diffractometer (Panalytical X'Pert PRO MPD) using a Cu anode (Kα radiation, λ = 1.5405 Å). The crystalline phases have been identified using the ICDD (International Center for Diffraction Data) database. Rietveld refinement of the experimental diffraction was carried out [58].

Moreover, SEM analysis to check surface morphology and the chemical composition of the catalyst were performed using a SEM-FEG Inspect F-50 instrument, with secondary electrons image working with a distance of 10 mm and a high tension from 10 kV to 20 kV.

High resolution transmission electron microscopy (HR-TEM) was carried out using a FEI Tecnai F20 a Field Emission Gun (FEG, probes <3 nm) to confirm the particle size and in particular to study the surface behavior of perovskite and its changes.

### 2.3. Electrochemical characterizations

The OER performance of the synthesized material was studied in a three-electrode cell set up using a Rotating Disk Electrode (RDE) from Pine instruments and Autolab (PGSTAT30 – EcoChemie, The Netherlands) potentiostat. The thin-film RDE approach reported in Ref. [59] was followed. The Working Electrode (WE) was a perovskite-modified glassy carbon, the Reference Electrode (RE) was a mercury/mercury oxide (Hg/HgO – internal solution 1 M KOH) and the Counter Electrode (CE) was a Platinum wire as these are experiments to study the OER. Prior to the experiments, the WE glassy carbon surface was polished with the #4000 SiC paper and demineralized water. Then, 2 mg mL⁻¹ of catalyst ink was prepared by weighing a selected amount of LaNiO₃, in a mixture of 1:4 Milli-Q water:isopropanol (C₃H₇OH). Nafion® Dupont was used as a binder in a ratio of 0.1w/w Nafion: catalyst. Then the mixture was ultrasonicated for 30 min. After that, 20 µL of the ink were deposited by drop casting on the WE (surface 0.196
cm²) to ensure a catalyst load of 200 µg cm⁻².

All the RDE electrochemical characterizations were performed following the procedure reported in Ref. [58]. The measured potentials were corrected for the ohmic-drop measured by Krastjic and Trasatti method [60].

The LaNiO₃ catalyst was also characterized using Screen-Printed Carbon Electrodes, C-SPE, Metrohm DropSens Electrode 110. The substrate is ceramic-based and is3.38 cm long, 1.02 cm wide, and 0.05 cm thick. The WE, RE and CE are made of carbon, silver and carbon, respectively. The WE diameter is 4 mm corresponding to an area of 0.1256 cm².

52 µL of the LaNiO₃ ink, the same adopted for RDE analysis, was deposited by drop casting on the carbon WE. The SPE was then dipped in 1 M KOH supporting electrolyte solution. In this work, we preferred not to use the CE and RE present on the SPE, particularly for the small area of the CE, compared to the much higher surface area of the deposited powder and for the poorer definition of the reference potential of a pseudo-Ag reference electrode. We therefore opted for using external electrodes, a Pt wire, and a silver/silver chloride (Ag/AgCl) in 3 M aqueous KCl as the counter and reference electrode, respectively. For C-SPE measurements, the pre-conditioning step of the catalyst layer was the same as for the RDE protocol. After that, a series of CV curves in the potential range 0.5–2 V (vs RHE) at 5 mV s⁻¹ in 1 M KOH were recorded. Thus, a total amount of 0.1 mg of LaNiO₃ has been deposited on Carbon surface, obtaining a Ni concentration of 0.2 mg cm⁻².

To evaluate the catalyst stability in the electrochemical cell for XAS experiment, the activity towards the OER was determined as follows. NaOH is used for XAS experiments because sodium absorbs less X-rays than potassium. For this reason, all the robustness experiments were performed using 1 M NaOH. 0.4 mL of the LaNiO₃ ink, used previously for RDE and C-SPE analysis, was deposited by drop casting on a 1 cm² glassy carbon electrode (WE). An Ag wire was used as pseudo-RE and a Pt lamina as CE. The electrochemical cell was the same used for operando XAS experiment described in paragraph 2.4) and showed in Fig. 1 (panel B). Even in this case, the same pre-conditioning step of the catalyst layer, used in the RDE protocol, was performed. Due to the long times required for XAS experiments, the material stability under OER conditions was tested by recording 10,000 CVs between 1.4 and 1.9 V (vs RHE) at 1 V s⁻¹. Before and after, CVs at a lower scan rate, 5 mV s⁻¹, were recorded and compared.

All the measured currents were converted to current densities and all the potentials values were converted versus RHE.

## 2.4. Operando XAS experiment

For the detection of Ni oxidation state and possible variations thereof, operando X-Ray absorption spectroscopy (XAS) experiments were carried out at the nickel K-edge at room temperature in the fluorescence mode at the *Linea Italiana di Spettroscopia di Assorbimento* (LISA) beamline (BM08) [61] at the European Synchrotron Radiation Facility (ESRF). The energy resolution of this beamline is ca. 0.1 eV at 8 keV [61].

A Si(111) fixed exit double crystal monochromator was used; the harmonic rejection and focusing was realized by Si mirrors with a cutoff energy of 15 keV. In particular, the first mirror (cylindrical) is a collimator permitting to obtain a quasi-theoretical energy resolution from the monochromator (~1.5 eV at the Ni–K edge) whereas the second toroidal mirror is used to focus the beam down to a dimension of about 100 µm on the sample. A high-purity germanium fluorescence detector array (12 elements) was used for measuring the fluorescence from the sample. The energy calibration was performed by measuring the absorption spectrum of metallic nickel foil at the Ni K-edge. The energy stability of the monochromator was checked by measuring the absorption spectrum of the Ni foil several times during the experiment. All data were obtained at room temperature. Spectra of NiO and β-NiOOH were acquired in the transmission mode and used as standards for the Ni(II) and Ni(III) oxidation states, respectively [62]. For those measurements, an appropriate amount of sample (to give a unit jump in the absorption coefficient) was mixed with cellulose and pressed into a pellet. A customized electrochemical cell was designed to ensure both electrochemical and XAS experiments [54]. The cell was designed and built using a 3D printer with a hollow-PTFE rectangular shape. The cell was equipped with an upper external part that allowed to hold RE and CE, respectively. A frontal circular hollow window of 0.196 cm² was created to ensure illumination by X-Ray, where the Working Electrode was placed. An Ag wire was used as RE, a Pt lamina as CE and a perovskite-modified Glassy Carbon (GC) as WE. The set-up is shown in Fig. 1.

All the experiments were performed using a 1 M sodium hydroxide solution (NaOH, Sigma Aldrich) serving as supporting electrolyte. An O-ring was used to prevent any solution leak. One glassy carbon electrode (WE) was modified with LaNiO₃ via drop casting (following the same ink preparation procedure reported in *paragraph 2.3*), with a Ni load of 0.2 mg cm⁻². The surface was activated as described in *paragraph 2.3*. The activated surface was then placed facing the electrolyte in the cylinder, with the back side touching the circular hole. The X-Ray beam impinged at the back of the sample, passing through the glassy carbon electrode: this has been maintained thin enough (0.5 mm) to ensure a support

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Customized spectro-electrochemical cell for XAS experiments with three electrode setup: A) synchrotron setup and B) frontal view with circular hollow window where the Working Electrode (WE) was placed.</figcaption></figure>
negligible X-Ray absorption at the Ni K-edge. The oxidation state of nickel was evaluated by recording a series of X-Ray Absorption Near Edge Structure (XANES) spectra at different fixed potential values, selected from previous electrochemical characterization. The selected values were: 0.16 V–0.96 V – 1.3 V–1.4 V – 1.46 V–1.6 V (vs RHE). For the XANES analysis, the raw spectra were first background subtracted with a straight-line fitting to the pre-edge, and then normalized to unit absorption at 800 eV above the edge energy, where the EXAFS oscillations are no longer visible. The EXAFS (extended X-ray absorption fine structure) were first extracted by means of the Athena code [63–65], and then the data analysis was performed by using the EXCURVE code, using a k² weighting scheme and full multiple scattering calculations.

# 3. Results and discussion

## 3.1. Synthesis of lanthanum nickel oxide

LaNiO₃ was synthetized with co-precipitation method, which is a simple and inexpensive method. This technique obeys industrial requirements and criteria in terms of low cost and ease scale-up. The efficiency of this approach was validated by measuring the structural and morphological properties through appropriate characterization analysis.

## 3.2. Structural and morphological analysis

The crystalline structure and phase composition of the LaNiO₃ was determined by XRD (see Fig. 2). The sample is almost monophasic, as it is shown by comparison with a standard pattern of rhombohedral LaNiO₃ (ICDD N° 04-013-6811) shown by the green vertical lines in Fig. 2. The presence of a weak peak at ca. a 44° can be ascribed to a small impurity of rock salt NiO [66].

The refinement of the experimental diffraction pattern of LaNiO₃ was obtained by the Rietveld method using the General Structure and Analysis Software (GSAS-II). The results of the analysis are reported in Table 1.

The determined unit cell parameters and the unit cell volume are in accordance with the standard unit cell parameters: a = b = 5.4596 Å, c = 13.1421 Å and volume = 339.3 Å³.

Fig. 3 represents the SEM images of the LaNiO₃ and highlights the perovskite morphology. At low magnification the sample appears as formed by agglomerates and flakes. Pictures collected at higher magnifications reveal that the agglomerates are formed by particles with nanometric dimensions.

A comprehensive investigation of the surface behavior of the perovskite was carried out using high resolution electron transmission microscopy (HR-TEM). The investigation was done on the as prepared perovskite, on the material kept in 1 M KOH overnight, and on the same material after having worked under OER conditions. This three-step investigation allows to correlate the intrinsic properties of the material with its variations after the OER.

The HR-TEM images reveal that the perovskite LaNiO₃ as prepared exhibits a geminate periodicity, highlighted in red in Fig. 4A. The

<figure><img src="image_3.png" /><figcaption>Fig. 2. Powder XRD pattern of LaNiO₃.</figcaption></figure>

structure is remarkably homogeneous, leading to the observed periodicity that continues up to the edge, as evident in Fig. 4B. These nano-crystals demonstrate consistency, characterizing the whole structure, without any evidence of amorphous or disordered regions. The particles maintain their crystalline nature up to the edge. Fig. 4C shows the HR-TEM image of the LaNiO₃ that was kept in 1 M KOH overnight. Also in this case, the particles appear to be almost single domain, without any significant evidence of disorder near the surface. This confirms that crystal morphology is not affected by the contact with the electrolytic solution.

Conversely, in the LaNiO₃ OER, a significant amount of disorder near the surface is found, as depicted in Fig. 5. In this case, we found the formation of nano-domains (highlighted in the red boxes) near the surface, the dimension of each nano-domain being of the order of 3–4 nm. Here, the surface shows a high level of defectivity, resembling a mosaic structure rather than a single crystal.

After the electrochemical treatment, the gemination has not increased, as it is a characteristic of the bulk material. On the other hand, the disorder confined to the surface is only observed in the LaNiO₃ OER powder. As shown in Fig. 5B, the exterior appears disordered, while going inward (highlighted in the red box), larger particles remain continuous (a feature present in the bulk even in the LaNiO₃ as prepared). It is evident that, after the OER, disoriented nanoparticles formed or grew on the surface. The fact that after treatment overnight in 1 M KOH, the morphology of the perovskite is unchanged confirms that this increasing disorder is caused by the electrochemical reaction.

## 3.3. Electrochemical activity assessment

The OER performance of LaNiO₃ electrode, determined using RDE, is shown in Fig. 6. The perovskite catalyst shows a good OER activity, since the OER starts at ca. 1.63 V (vs RHE). The onset potential is calculated as the intersection of the tangent at the inflection point.

The potential E(V) value at 10 mA cm⁻² is chosen as the metric OER catalyst: 10 mA cm⁻² is the current density that approximately corresponds to the solar spectrum for an efficiency of 10 % under 1 solar illumination [67]. At this current density, the overpotential is 0.43 V, that is close the values reported while benchmarking OER electrocatalysts [16,17]. At this current density, the overpotential is 0.43 V, that is close the values reported while benchmarking OER electrocatalysts [68]. The same references reported overpotentials equal to 0.32 and 0.29 in aqueous 1 M NaOH for IrO₂ and RuO₂, respectively.

Comparing the over-potential at 10 mA cm⁻² of the investigated perovskite with that of other pure perovskite oxides-based electrocatalysts on glassy carbon electrode [30] it can be noted that LaNiO₃ has the lowest over-potential value. This implies that, among several perovskite oxides, LaNiO₃ stands out as one of the most promising candidate due to its good electrocatalytic properties for OER and favorable electrocatalytic kinetics.

The Tafel slope for the LaNiO₃ electrode was determined by fitting the linear portion of the Tafel plots (Fig. 7a) using Equation (1).

$$ \eta = \pm \ln 10 \frac{RT}{\alpha F} \log \frac{j}{j_0} = a + b \log j \quad (1) $$

where j is the current density, j₀ is the exchange current density and b is the Tafel slope [69]. The Tafel slope of 47 mV·dec⁻¹ and the j₀ of 1.06·10⁻⁹ A cm⁻² are in excellent agreement with the literature for what concerns LaNiO₃ anodes [70]. The Tafel slope indicates how fast the current increases against overpotential (η), which means that a small Tafel slope, as in our case, suggests good electrocatalytic kinetics [69]. In addition, the Tafel slope of 47 mV·dec⁻¹ (that is that is sufficiently close to 2RT/3F, equal to 40 mV·dec⁻¹), suggests that the rate determining step (rds) is a one-electron transfer electrochemical step, for example, an EErdsC [71].

$$ S + OH^{-} \rightarrow S-OH + e^{-} $$
<table><caption>**Table 1**
Rietveld refinement of LaNiO₃.</caption><thead><tr><th>Space Group</th><th>Structure</th><th>a/Å</th><th>b/Å</th><th>c/Å</th><th>Volume/Å³</th><th>Crystallite Size/nm</th></tr></thead><tbody><tr><td>R - 3c</td><td>Rhombohedral LaNiO₃</td><td>5.4469</td><td>5.4469</td><td>13.1948</td><td>339.030</td><td>23.9</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** SEM images showing the surface morphology and particles distribution of LaNiO₃.</figcaption></figure>

$$ \begin{gathered} \mathrm{S-OH} + \mathrm{OH}^{-} \rightarrow \mathrm{S-O} + \mathrm{H}_{2}\mathrm{O} + \mathrm{e}^{-} \\ 2\mathrm{S-O} \rightarrow 2\mathrm{S} + \mathrm{O}_{2} \end{gathered} $$

or an ECE<sub>rds</sub> [72]:

$$ \begin{gathered} \mathrm{S-O} + \mathrm{OH}^{-} \rightarrow \mathrm{S-OOH} + \mathrm{e}^{-} \\ \mathrm{S-OOH} + \mathrm{OH}^{-} \rightarrow \mathrm{S-OO}^{-} + \mathrm{H}_{2}\mathrm{O} \\ \mathrm{S-OO}^{-} \rightarrow \mathrm{S} + \mathrm{O}_{2} + \mathrm{e}^{-} \end{gathered} $$

Where S is the active site.

We also compared the Tafel slope with that of the IrO₂ electrode at pH 14 (1 M NaOH) from our previous paper [68], as presented in Fig. 7a with line blue. It can be observed that the slope for IrO₂ (48.8 mV·dec⁻¹) is comparable to that of our perovskite LaNiO₃. This confirms that the material is highly active for the OER, considering that IrO₂ is commonly used as benchmark electrocatalyst for this reaction,

In Fig. 7b the turnover frequency (TOF) is also shown with a orange line, as it is used to evaluate the morphological effect on the electro-catalyst’s performance toward OER [73,74]. Additionally, we also compared the TOF with that of the IrO₂ electrode at pH 14 (1 M NaOH) from our previous paper [68] (Fig. 7b line blue). Although a better quantification of the number of active sites should be required [75], we estimated the TOF according to the following equation:

$$ \mathrm{TOF} = \frac{jA}{4nF} \qquad (2) $$

where j represents the current density, A is the surface area of the electrode, F is the Faraday constant and n is the number of moles of Ni (III) reacted [76].

The cyclic voltammogram performed using C-SPE with 0.025 mg of nickel deposited is shown in Fig. 8; cathodic and anodic peaks are visible at about 1.3 V and 1.4 V (vs RHE), respectively. These peaks can be attributed to the Ni(II)/Ni(III) redox and are related to the oxidation/reduction of Ni(OH)₂/NiOOH couple [77]. The percentage of Ni moles reacting can be calculated, using equation (3), via peak integration, equation (2). A value of 1.6·10⁻⁵ A V and 1.1·10⁻⁵ A V were found for anodic and cathodic peak areas, respectively.

$$ \mathrm{mol}(\mathrm{Ni})_{\text{reacted}} = \frac{\text{PeakArea}(\mathrm{A} \cdot \mathrm{V})}{\text{scan rate}(\mathrm{V} \cdot \mathrm{s}^{-1}) \cdot \text{Faraday Constant}(96485.31 \mathrm{C}/\mathrm{mole}^{-1})} \qquad (3) $$

$$ \% \text{ mol (Ni) active in the reaction} = \frac{\text{mol (Ni)}_{\text{reacted}}}{\text{mol (Ni)}_{\text{tot}}} \qquad (4) $$

The amount of nickel on electrode surface was calculated from the stoichiometry of the deposited perovskite.

The fraction of active Ni is about 8.3%, using the anodic peak area, and about 6%, using the cathodic peak area.

A series of 60 cyclic voltammetry measurements were carried out to
<figure><img src="image_5.png" /><figcaption>Fig. 4. HR-TEM images of LaNiO₃ as prepared (A and B) and after treatment in 1 M KOH (C).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. HR-TEM images of LaNiO₃ after OER.</figcaption></figure>

determine the progressive activation of the perovskite LaNiO₃ (Fig. 9a). It is observed that there is a discernible and gradual enhancement of both oxidation Ni(III) and reduction Ni(II) peaks with increasing number of cycles. As can be seen from Fig. 9b, the value of peak current density increases with higher cycles. In particular, it doubles after 60 cycles, indicating that the material electrochemical activity significantly increases during operation. The increase of peak current occurs for both

the anodic and the cathodic scans, demonstrating that the increase of redox transition kinetics (and so, of the number of sites involved in this process) are reversible.

This experimental evidence confirms that the electrochemical treatment is responsible for activating the material, making it more capable for electrons and ion transfers. This analysis supports the HR-TEM investigation, pointing to the operando increase of the active
<figure><img src="image_7.png" /><figcaption>Fig. 6. Anodic scan of LaNiO₃ at 5 mV s⁻¹ in 1 M KOH.</figcaption></figure>

area, due to the fact that the material surface becomes disordered after the powder is subjected to electrochemical phenomena, either the redox transition between 1.2 and 1.4 V or the OER.

### 3.4. Operando X-ray absorption experiments

To verify the mechanical and chemical stability of the deposits, which requires to be stable for several hours during XAS experiments, we performed stress tests, cycling the deposited electrocatalytic powder in 1 M NaOH for 10,000 cycles at 1 V s⁻¹ between 1.4 V and 1.9 V vs RHE. Fig. 10A shows the first and subsequent intermediate cycles during the stress test. These results illustrate the initial activation of the material, particularly observed at the 4,500th cycle, confirming that the electrochemical treatment induces the activation of the electrocatalyst. As the test progresses, the material loses part of its activity while converging to a stable situation.

Fig. 10B shows a comparison of the anodic semi-cycles before (orange curve) and after (green curve) the stress test lowering the scan rate to 5 mV s⁻¹. A weak electrochemical deactivation can be observed after 10,000 cycles, due to a small visible powder loss from the glassy carbon surface and found in the electrolyte after the experiment.

In conclusion, the system is sufficiently stable for the timing of the XAS experiment; the OER activity is still good after 10,000 cycles and there are no significant problems due to the bubbles formation. Taken together, these results confirm the effectiveness of the electrochemical treatment in activating the material and underline the durability and reliability of the system over extended cycles.

The XAS experiment results at the Ni K-edge on the LaNiO₃ material

are shown in Fig. 11. In Fig. 11A, we first show the XANES spectrum of the pristine material, along with the corresponding derivative (Fig. 11B). The spectra of NiO and β-NiOOH are shown as references for the Ni(II) and Ni(III) oxidation states, respectively.

The exact edge energy position in XAS is a measure of the effective nuclear charge. This is due to the fact that the edge energy is the binding energy of a core electron (the Ni 1s electron in the present case). This binding energy is in turn due to the electrostatic interaction between the electron and the nucleus. This interaction is screened by the outer (valence) electrons. This screening means that the effective nuclear charge sensed by the electron is less than the formal nuclear charge. Now, an increase (say) of the oxidation state means that the number of outer electrons is decreased, the screening is lesser and the effective nuclear charge is increased. As a consequence, the edge energy position shifts towards higher energy with increasing oxidation state. Of course, the opposite effect is observed when decreasing the oxidation state. Thus, the edge energy position is used for an exact determination of the oxidation states of elements in materials. This effect is known as the chemical shift in XAS. It is apparent by comparison with the XANES spectra of the reference materials, that for the pristine material the edge energy position is coincident with that of β-NiOOH, which is a standard for Ni(III), and this allows to assign the mean oxidation state of Ni to Ni (III).

The XANES spectra of the materials under operando conditions are reported in Fig. 11C where a comparison is made of the spectrum of the pristine material with that of the material at 0.16 V, and then in all the

<figure><img src="image_8.png" /><figcaption>Fig. 8. Cyclic voltammogram of LaNiO₃ done with Screen-Printed Carbon Electrodes at 5 mV s⁻¹ in 1 M KOH.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. In a) Tafel plot of LaNiO₃ (η vs log j) compared with that of IrO₂ from our previous paper [68]. In b) the calculation of the TOF (η vs log TOF) for LaNiO₃ is presented, again compared with that of IrO₂ from our previous paper [68]. Data for IrO₂ are reproduced from Ref. 60 with permission from the Royal Society of Chemistry.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** In a) 60 cyclic voltammograms of LaNiO₃ at 50 mV s⁻¹ in 1 M KOH: clear evidence of the progressive growth of oxidation (Ni³⁺) and reduction (Ni²⁺) peaks with increasing the number of cycles. In b) the trend of the anodic and cathodic peak current for all the cycles reported in a).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** A) Anodic curves (first and subsequent intermediate cycles) of LaNiO₃ in 1 M NaOH at 1 V s⁻¹ during the stress test. B) Anodic scan, before and after the stress test, of LaNiO₃ in 1 M NaOH at 5 mV s⁻¹ to evaluate the catalyst robustness.</figcaption></figure>

other conditions in Fig. 11D (enlargement in Fig. 11F) and along with the corresponding derivative (Fig. 11E); also in this case, the derivative spectra are shown for the sake of better evidence. At 0.16 V, LaNiO₃ shows an edge shifting towards lower energies compared to the pristine material, which is indicative of a partial reduction of Ni at this potential: the spectrum of metallic Ni is extremely different [78], and therefore this reduction can only ascribed to the formation of Ni(II) on the surface. The spectra at the different potentials look very similar, with the remarkable exception of a small shift of the edge energy position towards higher energies, which is increasing in intensity, going to more positive (anodic) potentials. We want here to stress that the shift of the edge energy position is small, but still detectable. This is because we preferred to study a sample whose structural and morphological features are close to those required for future industrial application, rather than being optimized for XAS experiments.

Correspondingly, a shoulder in the derivative spectra at ca. 8348.5 eV increases in amplitude, reaching the larger value at 1.6 V, where the OER is ongoing. It is worth noting here that XAS is bulk sensitive technique, and that the probed depth at the Ni K-edge in the LaNiO₃ catalyst can be estimated to be around few microns. In addition, in the CV, cathodic and anodic peaks are observed at about 1.3 V and 1.4 V (vs RHE), which are ascribed to the Ni(II)/Ni(III) couple. Therefore, we can attribute all our experimental finding to the formation of a surface layer of a Ni(II) compound, possibly Ni(OH)₂, at 0.16 V. This layer, going to

anodic potentials, is oxidized to NiOOH, possibly forming active Ni(III) sites at the surface, and then the OER can proceed via these sites. It should be noted that the fraction of Ni involved in this reaction can be estimated by relating the shift observed in Fig. 11F with the shift between the edge energy position of NiO and β-NiOOH, which in turn amounts to ca. 2 eV. A fraction of ca. 10% of Ni is obtained, which is in excellent agreement with the values reported above and obtained via peak integration of the CV plots and that confirms the existence of a defective shell around the particles, notwithstanding the partial deactivation observed in Fig. 10. According to these last results, the partial deactivation can be thus associated to a partial loss of the less adherent particles from the rotating electrode, due to the mechanical action of oxygen bubbles.

The formation of a Ni(III) containing layer is confirmed by the Δμ spectra reported in Fig. 11G. Here the Δμ signal is obtained by subtracting to the spectra at each of the potential, the spectrum at –0.16 V used as a reference. The negative peak, increasing in amplitude going to more positive potential, confirms the shift of the edge to higher energies.

### 3.5. Surface disorder

Recalling that XANES is also a probe for high-order correlation functions between the photoabsorber and the neighboring, the XANES spectra reported above show that the catalyst is stable in all the
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Ni K-edge XANES spectra and relevant derivative of pristine LaNiO₃ (A - B), compared with a spectrum of the material at 0.16 V (C), and of the material under operando conditions at different potentials D (enlargement in panel F). In A the spectra of NiO and NiOOH are shown for reference of the edge position of Ni(II) and Ni(III), and the spectra are shifted along the y axis for better clarity. The derivative spectra reported in A and D are shown in the panels B and F, respectively. The panel G shows the Δμ spectra obtained by subtracting at the spectra at each of the potential the spectrum at –0.16 V used as a reference.</figcaption></figure>

investigated conditions.

A confirmation of this conclusion can be obtained by the EXAFS spectra shown in Fig. 12A and Fourier Transform (FT) shown in Fig. 12B. The different spectra refer to cathodic (0.16 V) and anodic conditions (1.46 V). Oxygen bubble formation at potentials larger than 1.46 V did not allow the collection of EXAFS spectra without disturbing spikes at more anodic conditions. The EXAFS data can be fitted against a theoretical model, with R<sub>j</sub> (radial distance of the j-th coordination shell from the photo-absorber) and σ<sub>j</sub><sup>2</sup> (variance in R<sub>j</sub>) as fitting parameters. We remind here that the EXAFS FT is similar to a radial distribution function around the photo-absorber. The fitting parameters are therefore shown in Table 2, where the crystallographic distances, as calculated from the crystal structure [58], are also reported for reference. It can be noted that: i) the agreement between the EXAFS distances and the crystallographic distances is of good quality, and ii) there are no significant variations in the parameters in all the conditions, apart from a small increase in the EXAFS Debye-Waller factors (σ<sup>2</sup>), which may be attributed to a small increase in the disorder in the local chemical environment of Ni. It is then concluded that the crystal structure of LaNiO<sub>3</sub> is

stable during the water splitting reaction, both in anodic and cathodic investigated potentials [0.16 V–0.96 V – 1.3 V–1.4 V – 1.46 V–1.6 V (vs RHE)].

It can be observed that the OER activity of LaNiO₃ has a strong correlation with the corresponding crystalline structure and surface transformation. In LaNiO₃, Ni(III) cations are stabilized in the bulk of the perovskite structure, although this is maybe no longer valid at the surface, particularly at the oxide aqueous interface [79]. The surface composition of perovskite oxides, in contact with alkaline solutions, reasonably forms a thin hydrate layer that can be attributed to the formation of a Ni(II) compound, likely Ni(OH)₂, under cathodic potential. The layer is subsequently re-oxidized, under anodic potentials, forming fresh Ni(III) sites which are probably the active sites for adsorption and for the OER reaction [34].

Baeumer et al. derived a strong relationship between surface transformation, surface composition and OER activity in LaNiO₃ films. They demonstrated the tunability of surface transformation of the LaNiO₃ epitaxial thin films by modifying a single atomic layer at the surface showing that the Ni termination is significantly more active than the La
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** A) EXAFS spectra in different operando conditions and B) the relevant Fourier Transform.</figcaption></figure>

termination. This difference originates from a thermodynamically stable, Ni oxyhydroxide-type one atomic layer (disordered) surface phase NiO₂ evolved on Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination [46].

According to the results of the present work, we witness a similar behavior but on a polycrystalline material, where the La- and Ni-terminated are randomly distributed over the particles surface. The structure of the layer, formed on the surface, is likely very disordered, which may explain the increase of the EXAFS Debye-Waller factors with increasing potential. We assume that the change in the oxidation state of Ni, leading to the formation of Ni(II), is confined to the surface, causing a reconstruction of the structure by forming extremely small domains, in agreement with the HR-TEM inspections.

Moreover, structural disorder facilitates the ion diffusion and the charge transfer through amorphous materials [80]: due to the very small crystal size of the nano-domains that are grown in the present case, we can assume that this explanation holds also here. This fact, combined to

the low amount of this disordered nano-domains, makes the contribution of the surface hydroxide layer to the EXAFS spectra negligible. Similar considerations can be applied also to the XANES part of the X-ray absorption spectra (reported in Fig. 11): the shift of the edge energy position due to the formation of a surface layer of a Ni(II) compound, possibly Ni(OH)₂, accounts for only 10% ca. of the total number of Ni atoms, which is in line with the statement that the bulk of the catalyst is stable in all the investigated conditions.

We can therefore conclude that the combined effect of the formation of the thin disordered nano-domains and the robustness of the bulk LaNiO₃ catalyst, provide it as excellent electrocatalyst for the OER under alkaline conditions.

## 4. Conclusions

In this work, LaNiO₃ prepared by the simple and inexpensive co-precipitation method, has been explored as a highly promising catalyst material for OER in alkaline condition. In addition, morphological and structural analysis, with a strict electrochemical characterization, allow a proper assessment of LaNiO₃ for future industrial electrode fabrication. Remarkably, the material shows excellent OER activity and stands out as the most favorable candidate within the perovskite oxides-based family.

Furthermore, the study assesses the stability of the spectro-electrochemical set-up, ensuring the reliability of the experimental results.

The operando XAS experiments revealed that, under cathodic conditions, a layer of a Ni(II) compound, likely Ni(OH)₂, forms at the catalyst surface, and it is subsequently re-oxidized under anodic potentials. The fresh Ni(III) sites at the surface are possibly the active sites for adsorption and for the OER reaction. We were able to observe a surface disorder on the polycrystalline LaNiO₃ electrocatalyst and combining experimental results.

HR-TEM analysis effectively confirms a remarkable change in the perovskite surface structure. Specifically, it shows that the surface becomes disorder after the powder is subjected to the OER. Thus, this investigation reveals that the formation of Ni(II) is confined to the surface.

We can conclude that the robustness of the bulk LaNiO₃ catalyst combined with the surface disorder afford it as excellent electrode for alkaline water electrolysis, particularly under the specific conditions investigated in this study. LaNiO₃'s dual strengths of stability and reactivity make it a promising candidate for advancing the field of sustainable green hydrogen production.

### CRediT authorship contribution statement

**Giuditta De Amicis:** Writing – original draft, Investigation, Data curation. **Anna Testolin:** Writing – review & editing, Supervision, Methodology, Data curation, Conceptualization. **Cristina Cazzaniga:** Methodology, Investigation, Data curation. **Francesco D'Acapito:** Writing – review & editing, Validation, Resources, Methodology, Investigation, Data curation. **Alessandro Minguzzi:** Writing – review & editing, Supervision, Resources, Project administration, Investigation, Data curation, Conceptualization. **Paolo Ghigna:** Writing – review &

<table><caption>**Table 2**
EXAFS fitting parameters.</caption><thead><tr><th>E (V vs RHE)</th><th>R<sub>1</sub><br>Ni–O/Å</th><th>R<sub>2</sub><br>Ni–La/Å</th><th>R<sub>3</sub><br>Ni–Ni/Å</th><th>σ<sup>2</sup> (Å<sup>2</sup>)</th><th>σ<sup>2</sup> (Å<sup>2</sup>)</th><th>σ<sup>2</sup> (Å<sup>2</sup>)</th><th>R (Å) crystallographic</th></tr></thead><tbody><tr><td>0.16</td><td>1.99(1)</td><td>3.34(1)</td><td>3.82(2)</td><td>0.018(1)</td><td>0.009(1)</td><td>0.013(2)</td><td></td></tr><tr><td>0.96</td><td>1.99(1)</td><td>3.34(1)</td><td>3.82(2)</td><td>0.019(2)</td><td>0.009(1)</td><td>0.014(2)</td><td>1.9138 (Ni–O)</td></tr><tr><td>1.3</td><td>1.99(1)</td><td>3.34(1)</td><td>3.82(1)</td><td>0.017(1)</td><td>0.009(1)</td><td>0.013(2)</td><td>3.2861 (Ni–La)</td></tr><tr><td>1.46</td><td>1.98(1)</td><td>3.34(1)</td><td>3.82(1)</td><td>0.018(1)</td><td>0.008(1)</td><td>0.013(2)</td><td>3.8276 (Ni–Ni)</td></tr><tr><td>1.6</td><td>2.00(2)</td><td>3.36(3)</td><td>3.84(3)</td><td>0.021(3)</td><td>0.011(3)</td><td>0.013(3)</td><td></td></tr></tbody></table>
editing, Writing – original draft, Supervision, Project administration, Funding acquisition, Data curation, Conceptualization. **Alberto Vertova:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

## 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.

## Acknowledgements

We are thankful to PON "Ricerca e Innovazione" 2014–2020, Azione IV.5 "Dottorati su tematiche green" DM1061–10.08.2021.

G.D.A. gratefully acknowledge the Dipartimento di Chimica of the Università degli Studi di Milano for co-financing the project.

Thanks are given to LISA, a project funded by CNR (project DFM. AD006.072). The experiment was carried out under the IHR project IH-EV-36 with doi of the data: https://doi.esrf.fr/10.15151/ESRF-ES-846305705.

A.M. is grateful to a project funded under the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.3 - Call for tender No. 341 of March 15, 2022 of Ministero dell'Università e della Ricerca (MUR); funded by the European Union – NextGenerationEU Project code PE0000021, Concession Decree No. 1561 of October 11, 2022 adopted by Ministero dell'Università e della Ricerca (MUR), CUP D43C22003090001, Project title "Network 4 Energy Sustainable Transition – NEST".

AV is grateful to Piano di Sostegno alla Ricerca 2022 (Linea 2A) – Università degli Studi di Milano.

TEM analyses were performed at the TEM facility of the Unitech COSPECT at the University of Milan (Italy).

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