The Effect of Adding CeO2 Nanoparticles to Cu–Ni–Al Alloy for High Temperatures Applications

This work presents the effect of CeO2 nanoparticles (CeO2–NPs) on Cu–50Ni–5Al alloys on morphological, microstructural, degradation, and electrochemical behavior at high temperatures. The samples obtained by mechanical alloying and spark plasma sintering were exposed to a molten eutectic mixture of Li2CO3–K2CO3 for 504 h. The degradation of the materials was analyzed using gravimetry measurements and electrochemical impedance spectroscopy. Different characterization techniques, such as X-ray diffraction and scanning electron microscopy, were used to investigate the phase composition, parameter lattice, and microstructure of Cu–Ni–Al alloys reinforced with CeO2–NPs. The hardness of the composite was also examined using the Vickers hardness test. Gravimetry measurements revealed that the sample with 1 wt.% CeO2–NPs presented the best response to degradation with a less drastic mass variation. Impedance analysis also revealed that by adding 1 wt.% CeO2–NPs, the impedance modulus increased, which is related to a lower porosity of the oxide film or a thicker oxide layer. The microhardness also significantly increased, incorporating 1 wt.% CeO2–NPs, which reduced with higher CeO2–NPs content, which is possibly associated with a more uniform distribution using 1 wt.% CeO2–NPs in the Cu–Ni–Al matrix that avoided the aggregation phenomenon.


Introduction
Fuel cells (FC) are considered a promising technology for being an alternative source of electric power [1].These devices convert chemical energy into electricity [2,3] and can be classified according to the work temperature [4].For example, a molten carbonate fuel cell (MCFC), the operational temperature of which is 650 • C, is one of the most efficient FCs, and is contemplated as a carbon capture and storage (CSS) technology because it can capture and convert CO 2 [5].The high-temperature fuel cell (600-1000 • C) uses nickel or other non-precious catalytic materials to decrease the electrode cost [6].The electrode used as the cathode is a porous nickel oxide, where O 2 and CO 2 are injected, and on the anode side, nickel aluminum alloy is employed, generally Ni5Al, and supplies H 2 .According to the report by Lee et al. [7], a charge transfer process controls the electrochemical reactions in a slow reaction system and a mass transfer process in a rapid reaction system.Even though the MCFC has been working for decades, there are still problems to resolve due to the high operational temperature.It is essential to consider that the corrosion of the electrodes and equipment that operate at high temperatures can decrease the FC's service life, as Hacker and Mitsushima proposed [8].More specifically, few works have focused on the corrosion phenomenon on the anode of MCFC.In this context, Accardo et al. [9] studied the addition of copper [10,11] and cerium oxide nanoparticles (CeO 2 -NPs) to improve the mechanical and electrochemical behavior of the Ni5-Al commercial anode.In this sense, as copper has good electrical and thermal conductivity and mechanical resistance [11,12], the Cu-Ni-Al alloy can be an option for the MCFC anode.On the other hand, some authors reported that incorporating CeO 2 -NPs in the Cu-Ni alloy increased the catalyst performance on the anode for the H 2 oxidation reaction [13], and incorporating it in the Ni-5Al alloy reduced the creep strain because it can keep the pore structure stable [9].However, no available data evaluate the micro-macrostructural and corrosion behavior of a Cu-Ni-Al alloy reinforced with CeO 2 nanoparticles used for the anodes in MCFC.
Therefore, the effect of nanoparticle addition on the degradation of Cu-50Ni-5Al in the electrolyte Li 2 CO 3 -K 2 CO 3 (62-38 mol.%) is analyzed in this paper.The microstructural and morphological changes during this process provide valuable insights into the potential use of nanoparticles in the anode.

Sample Obtention
Mechanical alloying was performed using pure powders: Nickel (<10 µm, 99+, Merck, Darmstadt, Germany), Copper (<63 µm, >99%, Sigma-Aldrich, Darmstadt, Germany), and Aluminum (<60 µm, 99.9%, Good Fellow, Hamburg, Germany), which were mechanically alloyed (MA) in a Planetary mill PQ4 Across International, obtaining powder compositions of Cu-50Ni-5Al (wt.%).The milling conditions included a ball-a-powder ratio (BPR) of 10:1 and 2 wt.% stearic acid as a control agent under an inert Ar atmosphere.The milling time used was 100 h effective and there was an on/off cycle of 30/15 min at a speed of 350 r.p.m.Subsequently, 1, 3, and 5 wt.% of the nanoparticles CeO 2 (CeO 2 -NPs) (<25 nm, >99.9%,Sigma-Aldrich, Hamburg, Germany) were added to the alloy using Mixer Y-type Astecma for 1 h.Table 1 shows the chemical composition of Cu-50Ni-5Al + xCeO 2 (wt.%) alloys.Cu-50Ni-5Al without and with the CeO 2 -NPs samples were consolidated by Spark Plasma Sintering (SPS) using a Fuji Electronic Industrial Co model DR.SINTER ® SPS1050.The disks were 10 mm in diameter and 7 mm in thickness, and were produced using a high-density graphite die.The samples were heated from room temperature to 800 • C at a heating rate of 100 • C min −1 , applying a pressure of 50 MPa simultaneously during the heating and holding time of 5 min at the sintering temperature.The entire SPS process was kept under a vacuum of approximately 20 Pa.Finally, the samples were free-cooled to room temperature at a cooling rate of roughly 10 • C s −1 in the SPS chamber.Figure 1 shows the sample preparation (powders) diagram by mechanical alloying and mechanical mixing and a schematic representation of the consolidation process by the SPS system.
rinsed with ethanol for 10 min in a bath cleaning sonicator.Afterward, the samples were cleaned with distilled water and dried at room temperature.

Gravimetric Measurements
The Cu−50Ni−5Al samples were immersed in molten eutectic Li2CO3-K2CO3 (62-38 mol.%) [14,15] for 504 h (21 days) in an aerated atmosphere or not−controlled medium at 550 ± 5 °C.The gravimetric measurements of samples were carried out as described previously by Arcos et al. [16].The samples were removed and cleaned to eliminate the deposits and corrosion products on the surface.The bulk samples were submerged in hot distilled water (~100 °C) in a sonicator bath (Elma D−78224 Singen/Htw) for 30 min.Subsequently, the samples were dried with hot air and weighed until they reached a constant value, as reported by the ASTM G1−03 [17].The average mass (%) was calculated using Equation (1).
where m and m are the initial and final sample masses at different exposure times.

Morphological and Chemical Characterization
The porosity was determined through Archimedes' method, according to the Standard Test ASTM C373−88 [18].To reveal the microstructure, an etching was employed: 5 g of Fe3Cl, 10 mL of HCl, and 100 mL of distilled water for 8 s.A field−emission scanning electron microscope (FE-SEM), QUANTA FEG 250, was used to obtain the samples' images.
X−ray diffraction (XRD) was implemented using Bruker D2 PHASER with Cu−Kα radiation to characterize the material's structure.The diffraction patterns were recorded from 2θ between 40° and 100° with a 0.02° step and counting time of 1 s/step.

Mechanical Characterization
The microhardness of the samples was calculated by a micro−Vickers durometer, Wilson ® VH1150 Macro Vickers Hardness Tester, under 0.3 kgf of force before the gravimetric measurements.

Electrochemical Measurements
The electrochemical behavior of the Cu−50Ni−5Al samples was studied using open circuit potential and electrochemical impedance spectroscopy (EIS) measurements in the molten Li2CO3−K2CO3 (62:38 mol.%) as an electrolyte at 550 ± 5 °C under an aerated atmosphere.The electric contact for the working electrode, the Cu−50Ni−5Al samples, was The metal samples were polished using sandpaper from #800 to #4000 and then with colloidal silica suspension to reveal their microstructure.The polished samples were rinsed with ethanol for 10 min in a bath cleaning sonicator.Afterward, the samples were cleaned with distilled water and dried at room temperature.

Gravimetric Measurements
The Cu-50Ni-5Al samples were immersed in molten eutectic Li 2 CO 3 -K 2 CO 3 (62-38 mol.%) [14,15] for 504 h (21 days) in an aerated atmosphere or not-controlled medium at 550 ± 5 • C. The gravimetric measurements of samples were carried out as described previously by Arcos et al. [16].The samples were removed and cleaned to eliminate the deposits and corrosion products on the surface.The bulk samples were submerged in hot distilled water (~100 • C) in a sonicator bath (Elma D-78224 Singen/Htw) for 30 min.Subsequently, the samples were dried with hot air and weighed until they reached a constant value, as reported by the ASTM G1-03 [17].The average mass (%) was calculated using Equation (1).
where m i and m f are the initial and final sample masses at different exposure times.

Morphological and Chemical Characterization
The porosity was determined through Archimedes' method, according to the Standard Test ASTM C373-88 [18].To reveal the microstructure, an etching was employed: 5 g of Fe 3 Cl, 10 mL of HCl, and 100 mL of distilled water for 8 s.A field-emission scanning electron microscope (FE-SEM), QUANTA FEG 250, was used to obtain the samples' images.
X-ray diffraction (XRD) was implemented using Bruker D2 PHASER with Cu-Kα radiation to characterize the material's structure.The diffraction patterns were recorded from 2θ between 40 • and 100 • with a 0.02 • step and counting time of 1 s/step.

Mechanical Characterization
The microhardness of the samples was calculated by a micro-Vickers durometer, Wilson ® VH1150 Macro Vickers Hardness Tester, under 0.3 kgf of force before the gravimetric measurements.

Electrochemical Measurements
The electrochemical behavior of the Cu-50Ni-5Al samples was studied using open circuit potential and electrochemical impedance spectroscopy (EIS) measurements in the molten Li 2 CO 3 -K 2 CO 3 (62:38 mol.%) as an electrolyte at 550 ± 5 • C under an aerated atmosphere.The electric contact for the working electrode, the Cu-50Ni-5Al samples, was performed using conductive silver printing ink (resistivity 5-6 µΩ cm) around the sample and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm in length was used as a counter electrode, and an Ag wire that was 25 cm in length and placed inside a quartz glass tube with a porous plug in the tip was used as the reference electrode.The electrochemical measurements were carried out with a Potentiostat Solartron Analytical.

Gravimetric Measurements
Figure 2 shows the effect of CeO 2 -NPs on the weight gain of the Cu-50Ni-5Al samples after exposure to molten carbonates, revealing a reduction in the weight by adding CeO 2 -NPs.In addition, Figure 2 shows that during the initial stage, the weight increased rapidly for all the Cu-50Ni-5Al samples, which was lower for Cu-50Ni-5Al + 1 wt.%CeO 2 -NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a longer exposure time, the weight gain reaches a stationary state associated with a passive oxide layer [16] formed after 80 h for Cu-50Ni-5Al + 3 wt.%CeO 2 -NPs, and after 160 h for Cu-50Ni-5Al + 0 wt.%CeO 2 -NPs.It should be noted that with 5 wt.%CeO 2 -NPs, the weight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%CeO 2 -NPs.Therefore, the sample that suffered the least degradation at high temperatures was 1 wt.%CeO 2 -NPs, which could be attributed to the homogeneous distribution of the CeO 2 -NPs.
performed using conductive silver printing ink (resistivity 5-6 µΩ cm) aroun and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm i used as a counter electrode, and an Ag wire that was 25 cm in length and pl quartz glass tube with a porous plug in the tip was used as the reference el electrochemical measurements were carried out with a Potentiostat Solartron

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50 ples after exposure to molten carbonates, revealing a reduction in the weig CeO2−NPs.In addition, Figure 2 shows that during the initial stage, the weig rapidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5 CeO2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exp longer exposure time, the weight gain reaches a stationary state associated w oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, an for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%Ce weight gain did not reach a plateau of up to 504 h of exposure, like the sample CeO2−NPs.Therefore, the sample that suffered the least degradation at high t was 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distri CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h o molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.sure, porosity can be observed for all the samples, agreeing with the analysi For example, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the CeO2−NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0. performed using conductive silver printing ink (resistivity 5-6 µΩ cm) around the sample and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm in length was used as a counter electrode, and an Ag wire that was 25 cm in length and placed inside a quartz glass tube with a porous plug in the tip was used as the reference electrode.The electrochemical measurements were carried out with a Potentiostat Solartron Analytical.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al samples after exposure to molten carbonates, revealing a reduction in the weight by adding CeO2−NPs.In addition, Figure 2 shows that during the initial stage, the weight increased rapidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%CeO2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a longer exposure time, the weight gain reaches a stationary state associated with a passive oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the weight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%CeO2−NPs.Therefore, the sample that suffered the least degradation at high temperatures was 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distribution of the CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h of exposure to molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.Before exposure, porosity can be observed for all the samples, agreeing with the analysis performed.For example, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the addition of CeO2−NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0.3% porosity, Cu−50Ni−5Al + 3 wt.%CeO2−NPs had 2.0 ± 0.2% porosity, presenting some spots more lightly over the surface, and Cu−50Ni−5Al + 5 wt.%CeO2−NPs had 1.0 ± 0.1% porosity.One reason for this phenomenon is that nanoparticles can easily remain in the pores and performed using conductive silver printing ink (resistivity 5-6 µΩ cm) around the sample and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm in length was used as a counter electrode, and an Ag wire that was 25 cm in length and placed inside a quartz glass tube with a porous plug in the tip was used as the reference electrode.The electrochemical measurements were carried out with a Potentiostat Solartron Analytical.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al samples after exposure to molten carbonates, revealing a reduction in the weight by adding CeO2−NPs.In addition, Figure 2 shows that during the initial stage, the weight increased rapidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%CeO2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a longer exposure time, the weight gain reaches a stationary state associated with a passive oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the weight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%CeO2−NPs.Therefore, the sample that suffered the least degradation at high temperatures was 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distribution of the CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h of exposure to molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.Before exposure, porosity can be observed for all the samples, agreeing with the analysis performed.For example, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the addition of CeO2−NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0.3% porosity, Cu−50Ni−5Al + 3 wt.%CeO2−NPs had 2.0 ± 0.2% porosity, presenting some spots more lightly over the surface, and Cu−50Ni−5Al + 5 wt.%CeO2−NPs had 1.0 ± 0.1% porosity.One reason for this phenomenon is that nanoparticles can easily remain in the pores and performed using conductive silver printing ink (resistivity 5-6 µΩ cm) around the sample and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm in length was used as a counter electrode, and an Ag wire that was 25 cm in length and placed inside a quartz glass tube with a porous plug in the tip was used as the reference electrode.The electrochemical measurements were carried out with a Potentiostat Solartron Analytical.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al samples after exposure to molten carbonates, revealing a reduction in the weight by adding CeO2−NPs.In addition, Figure 2 shows that during the initial stage, the weight increased rapidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%CeO2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a longer exposure time, the weight gain reaches a stationary state associated with a passive oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the weight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%CeO2−NPs.Therefore, the sample that suffered the least degradation at high temperatures was 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distribution of the CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h of exposure to molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.Before exposure, porosity can be observed for all the samples, agreeing with the analysis performed.For example, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the addition of CeO2−NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0.3% porosity, Cu−50Ni−5Al + 3 wt.%CeO2−NPs had 2.0 ± 0.2% porosity, presenting some spots more lightly over the surface, and Cu−50Ni−5Al + 5 wt.%CeO2−NPs had 1.0 ± 0.1% porosity.One reason for this phenomenon is that nanoparticles can easily remain in the pores and performed using conductive silver printing ink (resistivity 5-6 µΩ cm) around the sample and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm in length was used as a counter electrode, and an Ag wire that was 25 cm in length and placed inside a quartz glass tube with a porous plug in the tip was used as the reference electrode.The electrochemical measurements were carried out with a Potentiostat Solartron Analytical.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al samples after exposure to molten carbonates, revealing a reduction in the weight by adding CeO2−NPs.In addition, Figure 2 shows that during the initial stage, the weight increased rapidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%CeO2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a longer exposure time, the weight gain reaches a stationary state associated with a passive oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the weight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%CeO2−NPs.Therefore, the sample that suffered the least degradation at high temperatures was 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distribution of the CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h of exposure to molten carbonate (Li 2 CO 3 -K 2 CO 3 62-38 mol.%) in the aerated atmosphere.Before exposure, porosity can be observed for all the samples, agreeing with the analysis performed.For example, Cu-50Ni-5Al had 16 ± 0.8% porosity, which reduced the addition of CeO 2 -NPs.The Cu-50Ni-5Al + 1 wt.%CeO 2 -NPs samples had 1.0 ± 0.3% porosity, Cu-50Ni-5Al + 3 wt.%CeO 2 -NPs had 2.0 ± 0.2% porosity, presenting some spots more lightly over the surface, and Cu-50Ni-5Al + 5 wt.%CeO 2 -NPs had 1.0 ± 0.1% porosity.One reason for this phenomenon is that nanoparticles can easily remain in the pores and voids of the nanocomposite matrix due to their small size [19].After exposure, a strong surface modification was observed for all the samples, possibly due to the corrosion product formation, which could be a passive film, as suggested by the gravimetric measurements.Ren et al. [20] studied a Cu-35Ni-10Al alloy in molten carbonate (Li 2 CO 3 -K 2 CO 3 62-38 mol.%) for 48 h in an aerated atmosphere, reporting the formation of porous corrosion products composed mainly of Al 2 O 3 and Ni-Al oxides, which is in agreement with the SEM images of Cu-50Ni-5Al (see Figure 3a), which present a porous surface after exposure.anomaterials 2023, 13, x FOR PEER REVIEW 5 of 12 voids of the nanocomposite matrix due to their small size [19].After exposure, a strong surface modification was observed for all the samples, possibly due to the corrosion product formation, which could be a passive film, as suggested by the gravimetric measurements.Ren et al. [20] studied a Cu−35Ni−10Al alloy in molten carbonate (Li2CO3-K2CO 62-38 mol.%) for 48 h in an aerated atmosphere, reporting the formation of porous corrosion products composed mainly of Al2O3 and Ni−Al oxides, which is in agreement with the SEM images of Cu−50Ni−5Al (see Figure 3a), which present a porous surface after exposure.[23].After 21 days of exposure to the Li2CO3−K2CO3, peaks associated with NiO (Fm 3m; JCPDS 010731519), Cu2O (Pn-3m; JCPDS 010751531), and Al2O3 (R-3c; JCPD 010772135) can be seen in all the samples.In addition, the intensities of the reflectio associated with CeO2−NPs can be seen, which can be attributed to the fact that t CeO2−NPs migrate to the surface, as reported by Accardo et al. [9].Note that the increa in the intensity of the reflections associated with CeO2 is much lower for the sample w 5 wt.% of CeO2−NPs than in the samples with 1 wt.% and 3 wt.% of CeO2−NPs, possib because the Ce content decreased on the sample surface, confirming what was previous mentioned in Figure 4.

Mechanical Properties
Figure 6 shows the hardness of the Cu−50Ni−5Al + x wt.% CeO2−NPs samples before exposure.The results indicate that the sample with 0% CeO2−NPs presents the lowest hardness value, corresponding to 205 ± 21 HV0.3.This can be attributed to the higher porosity of the sample, which reaches 16%, by incorporating different amounts of CeO2−NPs in the sample.Cu−50Ni−5Al +1 wt.% CeO2−NPs perform better due to the CeO2−NPs allowing a decrease in the porosity and a homogenous distribution of CeO2−NPs in the matrix, as mentioned above.However, if the concentration of CeO2−NPs exceeds 1 wt.%, they agglomerate at grain boundaries, reducing the hardness value, as seen in Figure 4. Zawrah et al. [24] concluded that adding Al2O3−NPs to pure Cu improves hardness due to their uniform distribution.The improved hardness can be attributed to the relative contribution of the Orowan strengthening effect, mainly when the reinforcement size is less than 100 nm [25].The CeO2−NPs are very small and hard, impeding the movement of dislocations in the Cu−50Ni−5Al matrix, leading to an improvement in the hardness of the microstructure.

Mechanical Properties
Figure 6 shows the hardness of the Cu-50Ni-5Al + x wt.% CeO 2 -NPs samples before exposure.The results indicate that the sample with 0% CeO 2 -NPs presents the lowest hardness value, corresponding to 205 ± 21 HV 0.3 .This can be attributed to the higher porosity of the sample, which reaches 16%, by incorporating different amounts of CeO 2 -NPs in the sample.Cu-50Ni-5Al +1 wt.% CeO 2 -NPs perform better due to the CeO 2 -NPs allowing a decrease in the porosity and a homogenous distribution of CeO 2 -NPs in the matrix, as mentioned above.However, if the concentration of CeO 2 -NPs exceeds 1 wt.%, they agglomerate at grain boundaries, reducing the hardness value, as seen in Figure 4. Zawrah et al. [24] concluded that adding Al 2 O 3 -NPs to pure Cu improves hardness due to their uniform distribution.The improved hardness can be attributed to the relative contribution of the Orowan strengthening effect, mainly when the reinforcement size is less than 100 nm [25].The CeO 2 -NPs are very small and hard, impeding the movement of dislocations in the Cu-50Ni-5Al matrix, leading to an improvement in the hardness of the microstructure.

Electrochemical Measurements
Figure 7 shows the effect of the addition of 1 wt.%CeO 2 -NPs to the open circuit potential (E OC ) of Cu-50Ni-5Al after exposure to Li 2 CO 3 -K 2 CO 3 at 550 • C and an aerated atmosphere.After a shorter exposure time, the E OC was shifted to more negative values by incorporating CeO 2 -NPs, suggesting an activation of the corrosion phenomena.However, for a longer exposure time, the E OC reached similar values to the sample without CeO 2 -NPs, which can be associated with a stable oxide layer formed on the metal surface.Meléndez-Ceballos et al. [26] studied a Ni porous sample coated by CeO 2 -NPs using the atomic layer deposition in Li 2 CO 3 -K 2 CO 3 at 650 • C in a CO 2 /air 30/70 vol.% atmosphere.
The authors determined an initial potential close to -0.76 V vs. Ag/Ag + ; the reference electrode is a silver wire submerged in Ag 2 SO 4 (10 −1 mol kg −1 ) saturated in Li 2 CO 3 -K 2 CO 3 , which was shifted to more positive values as a function of exposure time, attributed to a delay in the Ni oxidation process due to the presence of CeO 2 -NPs film.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the w ples after exposure to molten carbonates, revealing a CeO2−NPs.In addition, Figure 2 shows that during th rapidly for all the Cu−50Ni−5Al samples, which wa CeO2−NPs, reaching a maximum weight gain of onl longer exposure time, the weight gain reaches a statio oxide layer [16] formed after 80 h for Cu−50Ni−5Al + for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be no weight gain did not reach a plateau of up to 504 h of ex CeO2−NPs.Therefore, the sample that suffered the lea was 1 wt.%CeO2−NPs, which could be attributed to t CeO2−NPs.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the w ples after exposure to molten carbonates, revealing CeO2−NPs.In addition, Figure 2 shows that during rapidly for all the Cu−50Ni−5Al samples, which w CeO2−NPs, reaching a maximum weight gain of o longer exposure time, the weight gain reaches a stat oxide layer [16] formed after 80 h for Cu−50Ni−5A for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be n weight gain did not reach a plateau of up to 504 h of CeO2−NPs.Therefore, the sample that suffered the l was 1 wt.%CeO2−NPs, which could be attributed to CeO2−NPs.

ults and Discussion
igure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al samfter exposure to molten carbonates, revealing a reduction in the weight by adding NPs.In addition, Figure 2 shows that during the initial stage, the weight increased ly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a r exposure time, the weight gain reaches a stationary state associated with a passive layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h −50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the t gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%NPs.Therefore, the sample that suffered the least degradation at high temperatures wt.% CeO2−NPs, which could be attributed to the homogeneous distribution of the NPs.icrostructural and Chemical Characterization igure 3 shows the morphology of the samples before and after 504 h of exposure to n carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.Before expoporosity can be observed for all the samples, agreeing with the analysis performed.xample, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the addition of NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0.3% porosity, Ni−5Al + 3 wt.%CeO2−NPs had 2.0 ± 0.2% porosity, presenting some spots more y over the surface, and Cu−50Ni−5Al + 5 wt.%CeO2−NPs had 1.0 ± 0.1% porosity.eason for this phenomenon is that nanoparticles can easily remain in the pores and ) 3% wt.% CeO 2 -NPs, and (

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al sames after exposure to molten carbonates, revealing a reduction in the weight by adding O2−NPs.In addition, Figure 2 shows that during the initial stage, the weight increased pidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%O2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a nger exposure time, the weight gain reaches a stationary state associated with a passive ide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h r Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the eight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%O2−NPs.Therefore, the sample that suffered the least degradation at high temperatures as 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distribution of the O2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h of exposure to olten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.Before expore, porosity can be observed for all the samples, agreeing with the analysis performed.r example, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the addition of O2−NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0.3% porosity, −50Ni−5Al + 3 wt.%CeO2−NPs had 2.0 ± 0.2% porosity, presenting some spots more htly over the surface, and Cu−50Ni−5Al + 5 wt.%CeO2−NPs had 1.0 ± 0.1% porosity.ne reason for this phenomenon is that nanoparticles can easily remain in the pores and ) 5% wt.% CeO 2 -NPs before exposure.performed using conductive silver printing ink (resistivity 5-6 µ and copper wire of 25 cm in length.In addition, a Pt wire that used as a counter electrode, and an Ag wire that was 25 cm in le quartz glass tube with a porous plug in the tip was used as the electrochemical measurements were carried out with a Potentio

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain ples after exposure to molten carbonates, revealing a reduction CeO2−NPs.
In addition, Figure 2 shows that during the initial st rapidly for all the Cu−50Ni−5Al samples, which was lower fo CeO2−NPs, reaching a maximum weight gain of only 1.2% at longer exposure time, the weight gain reaches a stationary state oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%C for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that w weight gain did not reach a plateau of up to 504 h of exposure, lik CeO2−NPs.
Therefore, the sample that suffered the least degrada was 1 wt.%CeO2−NPs, which could be attributed to the homoge CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated a sure, porosity can be observed for all the samples, agreeing wit ) Cu-50Ni-5Al and ( Nanomaterials 2023, 13, x FOR PEER REVIEW performed using conductive silver printing ink (resistivity 5-6 µΩ and copper wire of 25 cm in length.In addition, a Pt wire that w used as a counter electrode, and an Ag wire that was 25 cm in len quartz glass tube with a porous plug in the tip was used as the electrochemical measurements were carried out with a Potentios 3. Results and Discussion

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain o ples after exposure to molten carbonates, revealing a reduction CeO2−NPs.
In addition, Figure 2 shows that during the initial sta rapidly for all the Cu−50Ni−5Al samples, which was lower for CeO2−NPs, reaching a maximum weight gain of only 1.2% at 5 longer exposure time, the weight gain reaches a stationary state a oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%Ce for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that wit weight gain did not reach a plateau of up to 504 h of exposure, like CeO2−NPs.
Therefore, the sample that suffered the least degradat was 1 wt.%CeO2−NPs, which could be attributed to the homogen CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and a molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated at sure, porosity can be observed for all the samples, agreeing with ) Cu-50Ni-5Al + 1 wt.%CeO 2 -NPs exposure to Li 2 CO 3 -K 2 CO 3 at 550 • C and aerated atmosphere over time.
Figure 8 shows the Nyquist diagrams of Cu-50Ni-5Al after 1 h of exposure to Li 2 CO 3 -K 2 CO 3 in an aerated atmosphere at E = E OC and 550 • C, revealing a significant increase in the impedance modulus due to the incorporation of 1 wt.%CeO 2 -NPs, which could be related to the formation of a passive oxide layer on the alloy, as previously mentioned.As can be seen at E = E OC , the impedance responses reveal two time constants at high and low frequency ranges (HF and LF), which can be associated with the cathodic current, not only involving the capacitance of the electric double layer (C dl ) and the oxygen reduction reaction, but also the formation of an oxide layer due to the alloy dissolution.Different equivalent circuits have been proposed to represent the physical model, which can be composed of capacitors, resistances, and constant phase elements (CPE) related to the heterogeneity of the surface [27].Figure 8 shows the Nyquist diagrams of Cu−50Ni−5Al after 1 h of exposure to Li2CO3−K2CO3 in an aerated atmosphere at E = EOC and 550 °C, revealing a significant increase in the impedance modulus due to the incorporation of 1 wt.%CeO2−NPs, which could be related to the formation of a passive oxide layer on the alloy, as previously mentioned.As can be seen at E = EOC, the impedance responses reveal two time constants at high and low frequency ranges (HF and LF), which can be associated with the cathodic current, not only involving the capacitance of the electric double layer (Cdl) and the oxygen reduction reaction, but also the formation of an oxide layer due to the alloy dissolution.Different equivalent circuits have been proposed to represent the physical model, which can be composed of capacitors, resistances, and constant phase elements (CPE) related to the heterogeneity of the surface [27].Figure 9 shows the Bode plots of Cu−50Ni−5Al + 1 wt.%CeO2−NPs after 1 h of exposure to Li2CO3−K2CO3 in an aerated atmosphere at E = EOC and 550 °C, as a representative example of the study system.Figure 9 reveals a capacitive response with two or three time constants at all frequency ranges, possibly associated with the formation of an oxide film and oxygen reduction reaction.Additionally, the Bode plots revealed a higher impedance modulus at the LF range when CeO2−NPs were added, which can be related to the polarization resistance of the system, suggesting an enhancement of the corrosion resistance due to the incorporation of the CeO2−NPs to the Cu−50Ni−5Al matrix [27-29].Moreover, Figure 9 shows the non−corrected ( ) and corrected Bode plots by electrolyte resistance ( ), revealing that this effect is mainly in the high frequency range.Figure 9 shows the Bode plots of Cu-50Ni-5Al + 1 wt.%CeO 2 -NPs after 1 h of exposure to Li 2 CO 3 -K 2 CO 3 in an aerated atmosphere at E = E OC and 550 • C, as a representative example of the study system.Figure 9 reveals a capacitive response with two or three time constants at all frequency ranges, possibly associated with the formation of an oxide film and oxygen reduction reaction.Additionally, the Bode plots revealed a higher impedance modulus at the LF range when CeO 2 -NPs were added, which can be related to the polarization resistance of the system, suggesting an enhancement of the corrosion resistance due to the incorporation of the CeO 2 -NPs to the Cu-50Ni-5Al matrix [27-29].Moreover, Figure 9 shows the non-corrected ( performed using conductive silver printing ink (resistivity 5-6 µΩ cm) around the sample and copper wire of 25 cm in length.In addition, a Pt wire that was 25 cm in length was used as a counter electrode, and an Ag wire that was 25 cm in length and placed inside a quartz glass tube with a porous plug in the tip was used as the reference electrode.The electrochemical measurements were carried out with a Potentiostat Solartron Analytical.

Gravimetric Measurements
Figure 2 shows the effect of CeO2−NPs on the weight gain of the Cu−50Ni−5Al samples after exposure to molten carbonates, revealing a reduction in the weight by adding CeO2−NPs.In addition, Figure 2 shows that during the initial stage, the weight increased rapidly for all the Cu−50Ni−5Al samples, which was lower for Cu−50Ni−5Al + 1 wt.%CeO2−NPs, reaching a maximum weight gain of only 1.2% at 504 h of exposure.For a longer exposure time, the weight gain reaches a stationary state associated with a passive oxide layer [16] formed after 80 h for Cu−50Ni−5Al + 3 wt.%CeO2−NPs, and after 160 h for Cu−50Ni−5Al + 0 wt.%CeO2−NPs.It should be noted that with 5 wt.%CeO2−NPs, the weight gain did not reach a plateau of up to 504 h of exposure, like the sample with 3 wt.%CeO2−NPs.Therefore, the sample that suffered the least degradation at high temperatures was 1 wt.%CeO2−NPs, which could be attributed to the homogeneous distribution of the CeO2−NPs.

Microstructural and Chemical Characterization
Figure 3 shows the morphology of the samples before and after 504 h of exposure to molten carbonate (Li2CO3-K2CO3 62-38 mol.%) in the aerated atmosphere.Before exposure, porosity can be observed for all the samples, agreeing with the analysis performed.For example, Cu−50Ni−5Al had 16 ± 0.8% porosity, which reduced the addition of CeO2−NPs.The Cu−50Ni−5Al + 1 wt.%CeO2−NPs samples had 1.0 ± 0.3% porosity, Cu−50Ni−5Al + 3 wt.%CeO2−NPs had 2.0 ± 0.2% porosity, presenting some spots more lightly over the surface, and Cu−50Ni−5Al + 5 wt.%CeO2−NPs had 1.0 ± 0.1% porosity.One reason for this phenomenon is that nanoparticles can easily remain in the pores and Figure 8 shows the Nyquist diagrams of Cu−50Ni−5Al after 1 h of exposure to Li2CO3−K2CO3 in an aerated atmosphere at E = EOC and 550 °C, revealing a significant increase in the impedance modulus due to the incorporation of 1 wt.%CeO2−NPs, which could be related to the formation of a passive oxide layer on the alloy, as previously mentioned.As can be seen at E = EOC, the impedance responses reveal two time constants at high and low frequency ranges (HF and LF), which can be associated with the cathodic current, not only involving the capacitance of the electric double layer (Cdl) and the oxygen reduction reaction, but also the formation of an oxide layer due to the alloy dissolution.Different equivalent circuits have been proposed to represent the physical model, which can be composed of capacitors, resistances, and constant phase elements (CPE) related to the heterogeneity of the surface [27].Figure 9 shows the Bode plots of Cu−50Ni−5Al + 1 wt.%CeO2−NPs after 1 h of exposure to Li2CO3−K2CO3 in an aerated atmosphere at E = EOC and 550 °C, as a representative example of the study system.Figure 9 reveals a capacitive response with two or three time constants at all frequency ranges, possibly associated with the formation of an oxide film and oxygen reduction reaction.Additionally, the Bode plots revealed a higher impedance modulus at the LF range when CeO2−NPs were added, which can be related to the polarization resistance of the system, suggesting an enhancement of the corrosion resistance due to the incorporation of the CeO2−NPs to the Cu−50Ni−5Al matrix [27][28][29].Moreover, Figure 9 shows the non−corrected ( ) and corrected Bode plots by electrolyte resistance ( ), revealing that this effect is mainly in the high frequency range.
), revealing that this effect is mainly in the high frequency range.
Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 12 Figure 8 shows the Nyquist diagrams of Cu−50Ni−5Al after 1 h of exposure to Li2CO3−K2CO3 in an aerated atmosphere at E = EOC and 550 °C, revealing a significant increase in the impedance modulus due to the incorporation of 1 wt.%CeO2−NPs, which could be related to the formation of a passive oxide layer on the alloy, as previously mentioned.As can be seen at E = EOC, the impedance responses reveal two time constants at high and low frequency ranges (HF and LF), which can be associated with the cathodic current, not only involving the capacitance of the electric double layer (Cdl) and the oxygen reduction reaction, but also the formation of an oxide layer due to the alloy dissolution.Different equivalent circuits have been proposed to represent the physical model, which can be composed of capacitors, resistances, and constant phase elements (CPE) related to the heterogeneity of the surface [27].Figure 9 shows the Bode plots of Cu−50Ni−5Al + 1 wt.%CeO2−NPs after 1 h of exposure to Li2CO3−K2CO3 in an aerated atmosphere at E = EOC and 550 °C, as a representative example of the study system.Figure 9 reveals a capacitive response with two or three time constants at all frequency ranges, possibly associated with the formation of an oxide film and oxygen reduction reaction.Additionally, the Bode plots revealed a higher impedance modulus at the LF range when CeO2−NPs were added, which can be related to the polarization resistance of the system, suggesting an enhancement of the corrosion resistance due to the incorporation of the CeO2−NPs to the Cu−50Ni−5Al matrix [27][28][29].Moreover, Figure 9 shows the non−corrected ( ) and corrected Bode plots by electrolyte resistance ( ), revealing that this effect is mainly in the high frequency range.

Figure 1 .
Figure 1.Sample preparation (powder mixture) and schematic representation of the SPS system (consolidation).

Figure 1 .
Figure 1.Sample preparation (powder mixture) and schematic representation of the SPS system (consolidation).

Figure 2 .
Figure 2. The variation of weight of Cu-50Ni-5Al samples after 504 h of exposure to molten carbonates.(

Figure 4
Figure 4 presents the EDS results before and after exposure to analyze the chemica composition of the sample's surface.Before exposure, the alloy's surface is very similar for all the samples, revealing a homogeneous distribution of all the elements (Cu, Ni, Al and O).However, Cu−50Ni−5Al has some Al spots, and Cu−50Ni−5Al + 3 wt.%CeO2−NPs have some zones not identified by the mapping, which could be Li because it has deficient energy and is difficult to detect.Cu−50Ni−5Al+ 5 wt.%CeO2−NPs present some nanoparticles agglomeration (CeO2) corresponding to the element Ce.Frattini et al. [21] observed the same effect when adding small amounts of ZrO2−NPs in the Ni−Al alloy, although with the increase in the amount to 10% ZrO2−NPs, the distribution of the NPs becomes homogeneous.After exposure, the quantity of oxygen (O) increased significantly for al the samples, which can be attributed to the oxide formation on the surface, as proposed above.Nevertheless, potassium (K) was also found on the surface, a component of the molten carbonates (Li2CO3−K2CO3), revealing the possible formation of a deposit.According to Gonzalez−Rodriguez et al. [22], the Ni−50Al alloy was immersed in Li2CO3−K2CO (62:38 mol%) for 100 h in static air at 650 °C.They reported that the main corrosion products were Ni, Al, and K, such as NiO, Al2O3, LiAlO2, and LiKCO2.Also, Ren et al. [20] described mainly Al2O3 and Cu2O as the corrosion products of Cu−35Ni−10Al exposure to Li2CO3−K2CO3 (62:38 mol%) during 1 h in air at 650 °C.Accardo et al. [9] also proposed

Figure 4 Figure 4 .
Figure4presents the EDS results before and after exposure to analyze the chemical composition of the sample's surface.Before exposure, the alloy's surface is very similar for all the samples, revealing a homogeneous distribution of all the elements (Cu, Ni, Al, and O).However, Cu-50Ni-5Al has some Al spots, and Cu-50Ni-5Al + 3 wt.%CeO 2 -NPs have some zones not identified by the mapping, which could be Li because it has deficient energy and is difficult to detect.Cu-50Ni-5Al+ 5 wt.%CeO 2 -NPs present some nanoparticles agglomeration (CeO 2 ) corresponding to the element Ce.Frattini et al.[21] observed the same effect when adding small amounts of ZrO 2 -NPs in the Ni-Al alloy, although, with the increase in the amount to 10% ZrO 2 -NPs, the distribution of the NPs becomes homogeneous.After exposure, the quantity of oxygen (O) increased significantly for all the samples, which can be attributed to the oxide formation on the surface, as proposed above.Nevertheless, potassium (K) was also found on the surface, a component of the molten carbonates (Li 2 CO 3 -K 2 CO 3 ), revealing the possible formation of a deposit.According to Gonzalez-Rodriguez et al.[22], the Ni-50Al alloy was immersed in Li 2 CO 3 -K 2 CO 3 (62:38 mol%) for 100 h in static air at 650 • C.They reported that the main corrosion products were Ni, Al, and K, such as NiO, Al 2 O 3 , LiAlO 2 , and LiKCO 2 .Also, Ren et al.[20] described mainly Al 2 O 3 and Cu 2 O as the corrosion products of Cu-35Ni-10Al exposure to

Figure 5
Figure 5 compares the XRD patterns recorded before and after exposure Li2CO3−K2CO3 (62:38 mol%).Before exposure, the reflections were identified as corr sponding to typical fcc structures (Fm-3m).No peaks were associated with Ni or Al, ind cating that the solid solution Cu−Ni−Al obtained by mechanical alloying is maintain post−sintering by SPS.The samples reinforced with CeO2−NPs show low-intensity refle tions associated with CeO2 (Fm-3m; JCPDS 010750076).The lattice parameter of the C Ni−Al alloys without CeO2−NPs is 0.358 nm, which remains constant when incorporati the different CeO2−NPs.This indicates that the CeO2−NPs do not react with the Cu−Ni− matrix in the consolidation process because the SPS technique is a fast method for sinte ing[23].After 21 days of exposure to the Li2CO3−K2CO3, peaks associated with NiO (Fm 3m; JCPDS 010731519), Cu2O (Pn-3m; JCPDS 010751531), and Al2O3 (R-3c; JCPD 010772135) can be seen in all the samples.In addition, the intensities of the reflectio associated with CeO2−NPs can be seen, which can be attributed to the fact that t CeO2−NPs migrate to the surface, as reported by Accardo et al.[9].Note that the increa in the intensity of the reflections associated with CeO2 is much lower for the sample w 5 wt.% of CeO2−NPs than in the samples with 1 wt.% and 3 wt.% of CeO2−NPs, possib because the Ce content decreased on the sample surface, confirming what was previous mentioned in Figure4.

Figure 5
Figure 5 compares the XRD patterns recorded before and after exposure to Li 2 CO 3 -K 2 CO 3 (62:38 mol%).Before exposure, the reflections were identified as corresponding to typical fcc structures (Fm-3m).No peaks were associated with Ni or Al, indicating that the solid solution Cu-Ni-Al obtained by mechanical alloying is maintained post-sintering by SPS.The samples reinforced with CeO 2 -NPs show low-intensity reflections associated with CeO 2 (Fm-3m; JCPDS 010750076).The lattice parameter of the Cu-Ni-Al alloys without CeO 2 -NPs is 0.358 nm, which remains constant when incorporating the different CeO 2 -NPs.This indicates that the CeO 2 -NPs do not react with the Cu-Ni-Al matrix in the consolidation process because the SPS technique is a fast method for sintering [23].After 21 days of exposure to the Li 2 CO 3 -K 2 CO 3 , peaks associated with NiO (Fm-3m; JCPDS 010731519), Cu 2 O (Pn-3m; JCPDS 010751531), and Al 2 O 3 (R-3c; JCPDS 010772135) canbe seen in all the samples.In addition, the intensities of the reflections associated with CeO 2 -NPs can be seen, which can be attributed to the fact that the CeO 2 -NPs migrate to the surface, as reported by Accardo et al.[9].Note that the increase in the intensity of the reflections associated with CeO 2 is much lower for the sample with 5 wt.% of CeO 2 -NPs than in the samples with 1 wt.% and 3 wt.% of CeO 2 -NPs, possibly because the Ce content decreased on the sample surface, confirming what was previously mentioned in Figure4.

Figure 7
Figure 7 shows the effect of the addition of 1 wt.%CeO2−NPs to the op tential (EOC) of Cu−50Ni−5Al after exposure to Li2CO3−K2CO3 at 550 °C a atmosphere.After a shorter exposure time, the EOC was shifted to more nega incorporating CeO2−NPs, suggesting an activation of the corrosion phenome for a longer exposure time, the EOC reached similar values to the sa CeO2−NPs, which can be associated with a stable oxide layer formed on the Meléndez-Ceballos et al. [26] studied a Ni porous sample coated by CeO2− atomic layer deposition in Li2CO3−K2CO3 at 650 °C in a CO2/air 30/70 vol.%The authors determined an initial potential close to -0.76 V vs. Ag/Ag + ; the r trode is a silver wire submerged in Ag2SO4 (10 −1 mol kg −1 ) saturated in L which was shifted to more positive values as a function of exposure time, delay in the Ni oxidation process due to the presence of CeO2−NPs film.

Figure 7
Figure 7 shows the effect of the addition of 1 wt.%CeO2−NPs to the open cir tential (EOC) of Cu−50Ni−5Al after exposure to Li2CO3−K2CO3 at 550 °C and an atmosphere.After a shorter exposure time, the EOC was shifted to more negative v incorporating CeO2−NPs, suggesting an activation of the corrosion phenomena.H for a longer exposure time, the EOC reached similar values to the sample CeO2−NPs, which can be associated with a stable oxide layer formed on the metal Meléndez-Ceballos et al. [26] studied a Ni porous sample coated by CeO2−NPs u atomic layer deposition in Li2CO3−K2CO3 at 650 °C in a CO2/air 30/70 vol.% atmo The authors determined an initial potential close to -0.76 V vs. Ag/Ag + ; the referen trode is a silver wire submerged in Ag2SO4 (10 −1 mol kg −1 ) saturated in Li2CO3 which was shifted to more positive values as a function of exposure time, attribu delay in the Ni oxidation process due to the presence of CeO2−NPs film.

Figure 7 .
Figure 7. Open circuit potential variation of (

)
and corrected Bode plots by electrolyte resistance ( Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 12

Figure 9 .
Figure 9. (a,b) Effect of correction of electrolyte resistance on Bode plots and (c) variation of the imaginary part of the impedance of Cu-50Ni-5Al + 1% wt.% CeO 2 -NPs exposure 1 h to Li 2 CO 3 -K 2 CO 3 in aerated atmosphere at 550 • C and E = E OC .