Reagents included lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), ammonium hydroxide (NH₄OH), absolute ethanol, potassium hydroxide (KOH), methanol, potassium ferrocyanide trihydrate (K₄[Fe(CN)₆]·3H₂O), Nafion^®, and freshly prepared deionized water. All chemicals were purchased from Sigma Aldrich (99.9%) and used without any purification. For impedance studies, nickel oxide (NiO) was used as an anode material and commercially synthesized gadolinium-doped ceria (Ce_0.9Gd_0.1O_2–δ, 10GDC) was used as an electrolyte; both were purchased from Fiaxell Technologies, Switzerland, and used without additional treatment.

Powders of pure and nickel-doped lanthanum cerate were prepared using a precipitation method (Butt et al., 2020). Nitrate precursors of salts were taken in a proper molar ratio and mixed separately in deionized water with constant stirring until a clear solution was obtained. Ammonia was added, and precipitates were collected at a pH range of 9–11. The precipitates were heated at 80°C to evaporate the solvent. The obtained thick slurry of precipitates was dried in an oven at 110°C for 9–10 h and calcined at 500°C. The calcined product was ball-milled (ground) using acetone for 5 h for homogeneous mixing (Singh et al., 2016). A flowsheet diagram for the synthesis of nickel-doped lanthanum cerate is presented in Figure 1. The Ni-doped lanthanum cerate (LaCe_1-xNi_xO_3±δ; LCNiO), was prepared in three compositions with nickel contents (x) of 0.01, 0.05, and 0.09; details of compositions with sample codes and molecular formulae are provided in Supplementary Table S1.

To explore the capability of LCNiO nanomaterials as cathodes for SOFCs, high-temperature impedance (HT-EIS) measurements were carried out. A complete cell consisting of anode, cathode, and electrolyte material was fabricated. Nickel oxide, which was prepared by a simple co-precipitation method (Deshpande, 2016), was used as anode material. Ammonia was added dropwise to nickel nitrate solution to precipitate out a greenish nickel hydroxide gel. The resultant gel was oven-dried at 110°C and heated at 350°C for 3 h to obtain the corresponding nickel oxide powder. The resultant product was ball-milled to grind the NiO powder to homogeneity. Hence, a greyish-black nickel oxide was obtained, which was used as anode material during the cell’s fabrication for high-temperature impedance studies.

A tri-layered cell (20 mm diameter and 1 mm thickness) was prepared by loose pressing under uniaxial pressure in a 20 mm die, with 0.35 g of each anode and cathode material, while 0.25 g electrolyte was sandwiched between the anode and cathode layers. Nickel-doped lanthanum cerate was used as a cathode material, with gadolinium-doped ceria as an electrolyte and nickel oxide as an anode material. The asymmetric cell was placed directly in an open-flange fuel cell tester with gold mesh as the cathode-current collector and Ni mesh as the anode-current collector. EIS testing was conducted at 973, 1023, 1073, and 1123 K using a Gamry potentiostat (Raza et al., 2010).

Nickel-doped lanthanum cerate materials were synthesized and characterized by various techniques to ensure their successful synthesis and purity. The crystal structure of LCNiO was analyzed by XRD by PANalytical Xpert highscore, using CuKα₁ with a wavelength of 1.54 nm and a scan range of 20°–80°. The ceramics’ microstructures were observed by SEM, and their elemental compositions were confirmed by EDX. SEM and EDX were carried out using a TESCAN scanning electron microscope. Thermogravimetric (TGA) curves were acquired from a Shimadzu TGA/DTA thermal analyzer in air.

Electrochemical studies were performed to investigate the redox potential of nickel-doped lanthanum cerate nanoceramics for the OER. For this purpose, a glassy carbon (GC) electrode with geometric area of 0.07 cm² was modified with the synthesized nanocatalysts and employed as a working electrode. The electrocatalyst-modified electrode was prepared using a drop cast method. The surface of GC was wetted with 2 μL ethanol, and 0.1 mg of catalyst was dropped on the working surface followed by drop-casting of 2 μL of 0.5% V/V Nafion solution (Mujtaba and Janjua, 2016; Butt et al., 2020). The dried electrode (LCNiO/GC) was used to acquire the electrochemical data. Each time, the GC electrode was subjected to the preconditioning measures of polishing and ultrasonication for improved performance.

Potassium ferrocyanide is a standard analyte for examining the conductive behavior of electroactive materials, including electrocatalysts. The conductive nature of LCNiO materials was observed using cyclic voltammetry and electrochemical impedance spectra in the presence of 5 mM potassium ferrocyanide in 1 M KCl, using the potentiostat/galvanostat interface 1000 by Gamry. A three-electrode cell assembly was used for electroanalytic measurements. The catalyst modified GC as a working electrode and calibrated Ag/AgCl (3 M KCl) as a reference electrode (RE), and platinum wire was used as a counter electrode (CE). The response of the modified electrode toward water oxidation was studied in 1 M KOH and methanol as a facilitator (Guzman et al., 2013). Voltammetric investigations were performed in the potential range of 0.2–1.8 V (versus RHE) in argon- and oxygen-saturated environments separately, while EIS and chronoamperometry were performed at the potential at which current density approaches 10 mA cm⁻².

HT-EIS measurements in the solid state were carried out by coupling a potentiostat with an open-flange fuel cell tester, and readings were taken at 973, 1023, 1073, and 1123 K. All measurements were carried out with an initial frequency of 1 MHz and final frequency of 0.1 Hz.

XRD diffractograms of lanthanum cerate (LCO) and nickel-doped lanthanum cerate (LCNiO) series are presented, showing that single-phase morphology was maintained in nickel-doped lanthanum cerate materials. The result obtained for pure LCO is presented in the inset of Figure 2A and matches well with the reference card of lanthanum cerate perovskite materials with cubic symmetry (JCPDS 01-080-5546). The XRD patterns of synthesized nickel-doped lanthanum cerate ceramics with various amounts of the dopant, from x = 0.01 (LCNiO-1) to 0.09 (LCNiO-9), are shown together in Figure 2. These patterns demonstrate no extra peaks for impurity and a slight peak shift with increased intensities, verifying the successful incorporation of nickel dopant into the lanthanum cerate crystal matrix. An additional peak at a 2θ value of 40° was observed for LCNiO, in comparison to the LCO pattern, which corresponds to the assimilation of dopant into the perovskite structure.

All the compositions showed consistency in diffractograms, with the only difference lying in the intensity, which increased as the dopant concentration increased. Peak intensities in the XRD pattern of LCNiO-9 were increased in comparison to LCNiO-1 and LCNiO-5; therefore, LaCe_0.91Ni_0.09O_3±δ is the optimum composition for the tolerance of the doping concentration among those considered. The crystal structure of nickel-doped lanthanum cerates was found to be cubic, with a tolerance factor range of 0.813–0.825 (ideally, it is unity). The crystallite size (D) was calculated using the Debye–Scherrer equation (Monshi et al., 2012): D = k λ / β ⁡ cos ⁡ θ (1) where k is the constant, λ is the wavelength of X-rays used, and ß is the full width at the half-maximum of the peak. The crystallite size for pure lanthanum cerate was 11 nm, which is in the range of 18–28 nm for doped materials (Supplementary Table S2). The increased crystallite sizes of doped compositions compared to their parent material is further evidence of the successful incorporation of nickel ions into lanthanum cerates. This upsurge in particle size upon dopant addition can be attributed to the oxygen vacancies produced to compensate for the charge imbalance; this provides a path toward greater oxygen ion movement and increased grain growth (Mondal et al., 2015; Sankannavar et al., 2018). The crystallite size of LCNiO-9 showed the lowest value (18 nm) among all compositions, which revealed a better electrocatalytic property of this composition for OER (Yang et al., 2019).

FT-IR is used to determine the phase purity and any impurity present in the sample. For doped lanthanum cerates, FTIR spectra in the selected range (4000 cm⁻¹ to 400 cm⁻¹) were recorded in powder form, and spectra of LCNiO-9 are presented in Figure 2B. For metal oxide vibrations, vibrational bands were observed in the range of 400–700 cm⁻¹ (Cheng et al., 2004; Singh et al., 2016), which is due to the internal stretching motion of the Ni ions against oxygen. The vibrational band observed at about 3609 cm⁻¹ is for O-H stretching, which could be due to hydroxyl groups in the surface-adsorbed water. The vibrational peaks correspond to C-O vibration bands for tridentate carbonates observed at about 1407 and 1371 cm⁻¹ (Liao et al., 2002), which are due to adsorbed CO₂. The entire spectrum shows a sharp peak for the metal oxide band and a weak vibrational peak of impurity, confirming the successful formation of perovskite.

SEM is widely used for analyzing the surface morphology of materials. It provides information about the particle shape of the material under analysis, as well as the homogeneity and non-homogeneity of a phase formed on the surface (Echlin, 2013). SEM micrographs of the synthesized samples are presented in Supplementary Figure S1. Small irregular nanoflakes of LCNiO are visible, with an anticipated average particle size of 20–40 nm. The soft, spongy, and sticky nature of ceria particles may result in the agglomeration of nanoparticles (Masui et al., 2002). The stoichiometry of elements in LCNiO was confirmed by EDX analysis. The EDX spectrum of LCNiO-9 is presented in Figure 2C. EDX peaks corresponding to La, Ce, O, and Ni were observed in all spectra, and elemental analysis inferred the stoichiometry of the nanomaterials (Supplementary Table S3).

Thermogravimetric analysis was performed to check the thermal stability of compositions over a temperature range. Thermogravimetric data obtained for LCNiO-1 is presented in the form of a plot (Figure 2D). Initial mass loss was observed due to the removal of adsorbed water, represented by the red region below 300°C. The slope at 330°C (blue region) can be ascribed to the decomposition and burnout of most of the nitrates of lanthanum, cerium, and nickel into precursor powders. The second weight loss between 400 and 600°C (green region) is mostly due to the dehydration of lanthanum, cerium, and nickel hydroxides. A small weight loss at 600–700°C (orange region) is due to residual nitrate conversion and intermediate decomposition, while the straight line (pink region) represents the formation and stability of the requisite compound (Djoudi et al., 2012).

The electrocatalytic potency of a cathode is a function of its oxygen diffusion rate, and the LCNiOs were thus further investigated via solid-state high-temperature impedance spectroscopy (HT-EIS). Ni-doped lanthanum cerate (LaCe_1-xNi_xO_3±δ; x = 0.01, 0.05, and 0.09) nanoceramics were tested as SOFC cathodes, with 10GDC (10 mol% gadolinium-doped ceria) as an electrolyte and nickel oxide (NiO) as anode material in a three-layered co-pressed device. This three-layered cell was placed in the open-flange fuel cell tester, and high-temperature EIS testing was carried out at 973, 1023, 1073, and 1123 K using a highly precise thermocomputer device for pre-setting and controlling temperature profiles. EIS spectra observed at different temperatures for the three-layered 3L-LCNiO-3 device are presented in Figure 3A.

An oxidant is reduced at the cathode, the fuel is oxidized at the anode in SOFCs, and different processes may occur at the electrode/electrolyte interface. These processes may include diffusion, adsorption, charge transfer reactions, and many others (Ding et al., 2019), which can be studied using different resistance values obtained from EIS data at higher temperatures.

Usually, the electrochemical performance of a fuel cell is evaluated by the ohmic, activation, and concentration polarizations/drops. In such an EIS spectrum, the intercept with the Z_real axis at high frequency provides ohmic resistance (R_o), which is mainly contributed by electrolyte, wires, and current collectors. The intercept with the Z_real axis at low frequency provides total resistance (R_t). Polarization resistance (R_p) is the difference between low- and high-frequency intercepts at the Z_real axis, which are mainly due to charge transfer and diffusion processes (Ding et al., 2011; Mroziński et al., 2019; Farsani et al., 2020).

The EIS spectrum for the three-layered cell is presented in Figure 3B, with an equivalent circuit model adopted for the simulation of data in the inset. The EIS spectrum exhibits two overlapped semicircular arcs, as presented in Figure 3A. A small semicircle at the low-frequency region corresponds to the charge transfer process at the electrode/electrolyte interface; however, the other semicircle at the high-frequency region corresponds to diffusion processes at the electrode for the oxygen-exchange reactions at the surface. The low-frequency arc corresponds with the same pattern upon temperature variation; however, a very slight change in the high-frequency arc was observed, owing to its temperature-dependent nature (Mohammadi et al., 2014). The HT-EIS spectra show an explicit alleviation of ohmic and polarization resistances with a temperature rise from 973 to 1123 K. This behavior can be attributed to increased charge transfer conductivity, corresponding to interfacial capacitance and/or ionic conductivity. Figures 4A,B depict the similar EIS experimentation for 3L-LCNiO-1 and 3L-LCNiO-9 respectively, at various temperatures (973, 1023, 1073, and 1123 K) in an oxygen–methane environment.

All EIS spectra were model-fitted to obtain the value of resistances (ohmic and polarization) and various electrical parameters like resistivity/conductivity, and activation energies were also estimated (Georgieva et al., 2017). The magnitude of resistances against ion movement was calculated using the simulations, and data were extracted by fitting the EIS spectra at each temperature (Table 2).

The decreasing trend of polarization resistance of the 3L-LCNiO-5-based cell with temperature rise is presented in Figure 5A and shows an increase in the ionic conduction of the composited device at higher temperatures (Nasir et al., 2020). From the observed values of polarization resistances, the conductivity was observed to also increase rapidly from the value at 973 K through 1023 K, while after 1073 K, there was no significant change in conductivity value (Prakash et al., 2017). It can be inferred that 1073 K is the optimum temperature for these materials to work best, with low resistivity and good conductivity, under ambient conditions. The Arrhenius equation gives a clear relationship between the conductivity of the solid, or other EC devices, and temperature. Activation energy (E_a) is a major barrier in solid-state devices, and electrocatalysts are required to lower its value by adopting an alternative energy conversion path. In the cathodic performance of the Ni-doped lanthanum cerates in the 3-L configuration, the activation energy was estimated from the temperature-dependent EIS data using the Arrhenius equation: ln ⁡ σ T = ln ⁡   σ o − E a / R T (4)

The activation energy for ionic conduction was calculated for the composited devices from the slopes of the Arrhenius plots (-E_a/R) (Nasir et al., 2020), as presented in Figure 5B, to be 2.99 eV for LCNiO-1, 1.76 eV for LCNiO-5, and 1.16 eV for LCNiO-9. Electrical bandgaps were calculated using conductivity measurements (electrical bandgap = 2E_a). The electrical bandgap values of LCNiO-1, LCNiO-5, and LCNiO-9 are 5.98, 3.52, and 2.32 eV, respectively, which conform to the better tuning of the LCNiO-9 composition. The low activation energy and bandgap values illustrate the superior conductive nature of LCNiO-9 and the facile oxygen reduction process at the electrode surface, which also corresponds well to the results obtained by electrochemical analysis in the OER.

Voltammetric experiments were carried out to analyze the performance of the synthesized nanomaterials in the OER in basic medium (electrolyte; 1 M KOH), in the optimal potential scan range of 0.2–1.8 V. In this potential range, a distinct anodic peak was observed, which could be ascribed to water electrolytic oxidation or the OER peak.

Before analyzing the OER activity of electrodes, some pre-experimental verifications were performed to examine the intrinsic activity of the electrode in the KOH medium. Firstly, the response of the bare GC electrode was recorded in KOH electrolyte to confirm that there was no response of water-splitting peak by the GC itself in KOH. A comparison of CV profiles for bare GC and LCNiO-1-modified GC electrodes in KOH is given in Figure 6A. This comparison shows that the neat GC electrode scarcely responded (Kapałka et al., 2009), and a ∼0.7 A cm⁻² peak current density for OER on the LCNiO-1-modified electrode confirmed the electroactive nature of these catalyst materials (Figure 6C). Hence, these voltammograms made it easier to decide whether to proceed with this material for water splitting.

Figure 6B represents the cyclic voltammograms of LCNiO-1-modified electrode in aqueous basic medium (i.e., in KOH) and in aprotic solvent (i.e., dimethyl sulphoxide; DMSO), which was recorded as a second verification step to counter-check whether the OER peak is solely due to the presence of water in the aqueous medium.

Peak current (density) is an essential criterion for checking the electroactivity of EC systems, and its absence from the voltammetric profiles may reflect the overall conditional EC process being infeasible. The conditions for such activity may also involve the solvent nature, in addition to many other factors like electrode surface roughness, surface area, and double-layer capacitance. A one-time CV test was performed in DMSO as an aprotic solvent for EC measurements to segregate any effects due to solvent and catalyst nature (Figure 6B). Voltammograms presented in Figure 6C showed no peak with LCNiO-1 modified GC electrode, while a small hump as a Faradaic activity at 1.59 V (vs. RHE) in KOH was observed, suggesting the electro-efficiency of the LCNiO material for the water oxidation reaction in the basic aqueous medium only.