# Optimizing methane direct utilization: The advanced Sr2CoMoO6-δ anode

Seyedmostafa Nasrollahpourshirvani, Nakhi Amin, Amirmohammad Karimi, Mobli Mostafa

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Seyedmostafa Nasrollahpourshirvani, Nakhi Amin, Amirmohammad Karimi, Mobli Mostafa. Optimizing methane direct utilization: The advanced Sr2CoMoO6-δ anode. International Journal of Science and Research Archive, 2023, 10, pp.607 - 617. 10.30574/ijsra.2023.10.2.0987. hal-05328910

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# Optimizing methane direct utilization: The advanced Sr<sub>2</sub>CoMoO<sub>6-δ</sub> anode

Seyed Mostafa Nasrollahpour Shirvani<sup>1</sup>, Amin Nakhi<sup>2</sup>, Amirmohammad Karimi<sup>3</sup> and Mostafa Mobli<sup>4, *</sup>

International Journal of Science and Research Archive, 2023, 10(02), 607-617

Publication history: Received on 22 October 2023; revised on 01 December 2023; accepted on 03 December 2023

Article DOI: https://doi.org/10.30574/ijsra.2023.10.2.0987

## Abstract

The rising global demand for energy, combined with environmental concerns linked to conventional energy sources, has spurred interest in sustainable and diverse alternatives. This study focuses on enhancing the efficiency of energy conversion devices, specifically solid oxide fuel cells (SOFCs). While conventional Ni/YSZ cermet anodes show promise for the anode of SOFCs, they face challenges, prompting exploration into alternative materials such as Sr<sub>2</sub>MMoO<sub>6-δ</sub> (M=Fe, Co, Ni) perovskites. This work investigates the electro-oxidation of methane on Sr<sub>2</sub>CoMoO<sub>6-δ</sub> (SCM) anodes and explores the impact of Pd infiltration as a metallic element together with LaFe<sub>0.6</sub>Co<sub>0.4</sub>O<sub>3</sub> and as a dopant in LaFe<sub>0.58</sub>Co<sub>0.37</sub>Pd<sub>0.05</sub>O<sub>3-δ</sub> (LFCP). Experimental results reveal improved performance, reduced polarization resistance, and enhanced catalytic activity in SCM anodes impregnated with LFCP, attributed to the homogeneous dispersion of nanoparticles within the anode structure. The study provides insights into optimizing SOFC anode materials for efficient methane electro-oxidation, with potential applications in clean energy technologies.

**Keywords:** Solid Oxide Fuel Cell; Anode Electrode; SCM; Electrochemical Impedance Spectroscopy; Methane Oxidation; Impregnation

## 1. Introduction

The escalating global demand for energy, coupled with the considerable environmental repercussions associated with conventional energy sources, has stimulated the exploration of sustainable and diverse energy alternatives [1-9]. A central focus of this endeavor centers on enhancing the efficiency of energy conversion devices, specifically emphasizing solid oxide fuel cells (SOFCs) [10-18] and batteries [19-21]. SOFCs, recognized as highly efficient energy conversion devices, generate electricity through the electrochemical reaction of gaseous fuels, with hydrogen and its carriers serving as the primary fuel sources [22-29]. Notably, SOFCs distinguish themselves from other fuel cell types by their capability to directly utilize hydrocarbon fuels through internal reforming processes [30-35]. Methane, being the simplest hydrocarbon with a singular carbon atom in its structure, stands out for its absence of a robust C-C bond, facilitating reforming processes for direct integration within SOFCs [32, 36-39]. Furthermore, methane boasts a favorable hydrogen-to-carbon ratio, contributing a substantial number of hydrogen molecules for the anode electrode in SOFCs.

In conventional SOFC configurations, Ni/YSZ (Zr<sub>x</sub>Y<sub>1-x</sub>O<sub>2</sub>) cermet serves as a promising material for the anode electrode, offering advantages such as low impedance in the presence of hydrogen and catalytic activity for methane steam reforming [40-47]. However, this material presents certain drawbacks, notably low redox stability, and heightened catalytic activity leading to coking and sulfur poisoning when utilized with hydrocarbon fuels [48-50]. To address these challenges, alternative ceramic anodes, including double perovskite materials such as Sr<sub>2</sub>Mg<sub>1-x</sub>Mn<sub>x</sub>MoO<sub>6-δ</sub> [51] and

<footnote n="1"><sup>1</sup> Department of Mechanical Engineering, Babol Noshirvani University of Technology, Babol, Iran.</footnote>

<footnote n="2"><sup>2</sup> Nanotechnology and Advanced Materials Department, Materials and Energy Research Center, Karaj, Iran.</footnote>

<footnote n="3"><sup>3</sup> Department of Chemical Engineering, Azad University, Tehran, Iran.</footnote>

<footnote n="4"><sup>4</sup> Department of Mechanical Engineering, University of South Carolina, Columbia, United States.</footnote>
Sr₂Fe₁.₄Ni₀.₁Mo₀.₅O₆₋₈ [16, 52], lanthanum chromate based perovskites (e.g. (La,Sr)(Cr,Mn)O₃₋₈) [26], and strontium titanate based perovskites (e.g. (La,Sr)(Ti)O₃₋₈) [53], have demonstrated promising properties for application in SOFC anodes.

Sr₂MMoO₆₋₈ (M=Co, Ni) perovskites have garnered attention as a promising anode material owing to their notable tolerance to carbon and sulfur, as well as their stability in redox and thermal cycling [54]. Huang et al. [55] examined the behavior of Sr₂CoMoO₆₋₈ (SCM) anode during methane utilization as a fuel, revealing a maximum power density of approximately 0.527 W cm⁻² at 900 °C, comparable to that of the Ni/YSZ-based anode in wet H₂. Dos Santos-Gomez et al. [54] reported a polarization resistance of 6.5 Ω cm² at 750 °C for 5%H₂-Ar oxidation reaction on SCM anode, with no observed carbon deposition following electrochemical impedance spectroscopy. Nevertheless, there remains a need for enhancement in the conductivity and catalytic activity of ceramic anodes such as SCM [56].

A well-established and effective methodology for improving the performance of SOFC electrodes involves the impregnation of nanoparticles composed of oxides or precious metals into the electrode matrix [26, 41, 57-60]. Rath et al. [61] conducted a comparative analysis of the Sr₂FeMoO₆₋₈ (SFM) anode infiltrated with Pd against Co-Ni-Mo (0.1:5:1 molar ratio, CNM) nanoparticles for operation in humidified hydrogen. Their findings revealed that the Pd-impregnated SFM anode exhibited a slightly higher electrode polarization resistance (0.087 Ω cm²) in comparison to the CNM-impregnated SFM (0.060 Ω cm²). Recent investigations have explored LaFe₀.₇Co₀.₃O₃₋₈ and LaFe₀.₆₇Co₀.₃Pd₀.₀₃O₃₋₈ perovskites as symmetric electrode materials for SOFC [62], demonstrating commendable electro-catalytic activity for both hydrogen oxidation and oxygen reduction reactions. Specifically, at 750 °C, the power density of symmetric LaFe₀.₇Co₀.₃O₃₋₈/Sm₀.₂Ce₀.₈O₁.₉ (SDC) and LaFe₀.₆₇Co₀.₃Pd₀.₀₃O₃₋₈/SDC was reported as 291 and 535 mW cm⁻², respectively. Additionally, Pd doping was observed to significantly enhance the catalytic activity of LaFe₀.₇Co₀.₃O₃₋₈ for H₂ oxidation reaction.

In the present study, the electro-oxidation of methane on the SCM anode was systematically investigated. To enhance the performance of the SCM electrode, the impact of Pd infiltration, along with solution of LaFe₀.₆Co₀.₄O₃ (LFC+Pd) and doped-in LaFe₀.₅₈Co₀.₃₇Pd₀.₀₅O₃₋₈ (LFCP) perovskite structures, was thoroughly explored. The mechanism of the methane oxidation reaction on the promoting anode was investigated using comprehensive electrochemical measurements, and both pure and impregnated samples were subjected to microstructural analysis.

## 2. Materials and Methods

In order to manufacture electrolyte pellets, we utilized an 8 mol % yttria-stabilized zirconia (YSZ) material sourced from Tosoh in Japan. This material underwent a cold pressing process and was subsequently sintered at a temperature of 1400 °C for a duration of 6 hours. As a result, the obtained pellets exhibited an approximate diameter of 22 mm and a thickness of 0.9 mm. To function as the counter electrode with a surface area of 0.5 cm², a layer of Pt paste was applied specifically to the central region of the YSZ pellets. In addition, a reference electrode was implemented in the form of a ring. Both the counter and reference electrodes were subjected to an air environment and heated at 850 °C for a period of 1 hour.

To fabricate the anode electrode, Sr₂CoMoO₆₋₈ (SCM) powder was mixed with an appropriate quantity of Ink Vehicle to create a well-suited SCM slurry. This slurry was applied to the opposite side of the counter electrode and subjected to sintering at a temperature of 1200 °C for a duration of 1 hour under ambient air conditions. Following the sintering process, the anode exhibited a surface area of 0.5 cm² and a thickness of 35 µm.

The preparation of LaFe₀.₆Co₀.₄O₃ (LFC) and LaFe₀.₅₈Co₀.₃₇Pd₀.₀₅O₃ (LFCP) solutions for impregnation involved a sequential process. First, a stoichiometric mixture of La(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O, Co(NO₃)₂·6H₂O, and Pd(NO₃)₂ (all sourced from Merck, Germany) in specific ratios of 1:0.6:0.4:0 for LFC and 1:0.58:0.37:0.05 for LFCP, was prepared using an aqueous solution. The concentration of both LFC and LFCP solutions was maintained at 0.3 M. To serve as a complexing agent, ethylene glycol (EG) was added to the solution in a ratio of 2.4:1 M. Subsequently, a single drop of either LFC or LFCP solution was carefully applied to the surface of a SCM anode. After impregnation, the electrode surface was gently wiped with a tissue and left to dry in ambient air. The impregnated cells were then subjected to heating at 900 °C for a duration of 2 hours under ambient air conditions. In order to compare the catalytic impact of palladium (Pd) as a dopant within the perovskite structure versus its role as an individual metallic particle adjacent to a perovskite, a single drop of Pd(NO₃)₂ solution with a concentration of 0.3 M was trickled onto the surface of the electrode containing LFC, following the same drying procedure.

The drying of LFC and LFCP solutions was carried out in an oven and subsequently subjected to calcination at a temperature of 900 °C for a duration of 2 hours in an ambient air environment, following the method employed by
Rostaghi Chalaki et al. [26] for the calcination of LFC and LFCP. The phase composition of the calcined LFC and LFCP powders was evaluated using X-ray diffraction (XRD) analysis conducted at room temperature. A Philips PW 1730 instrument equipped with Cu-Kα radiation (λ=1.5406 Å) was utilized, covering a range of 20°<2θ<80° with a step size of 0.02°. To assess the microstructure of the impregnated SCM anodes, field emission scanning electron microscopy (FESEM) was employed using a TESCAN MIRA3 instrument.

Electrochemical measurements were conducted using a three-electrode configuration. To serve as a current collector, platinum paste was applied onto the surface of the anode electrode, while platinum wires were selected as the current leads. Methane was employed as the fuel during the experiments. The counter and reference electrodes were exposed to the surrounding atmosphere. Prior to the initiation of electrochemical testing, all anodes underwent activation in methane at a temperature of 850 °C for a duration of one hour. Electrochemical impedance spectroscopy (EIS) was performed using an Autolab data analyzer (PGSTAT302N) at open circuit potential (OCP) across a frequency range spanning from 0.01 Hz to 100 kHz. The Zsimpwin software was utilized to determine the electrode polarization resistance (R<sub>E</sub>) and electrode ohmic resistance (R<sub>Ω</sub>).

**3. Result and Discussion**

Figure 1 illustrates the X-ray diffraction (XRD) outcomes obtained from the LFC and LFCP powders subjected to calcination at 900 °C in an air atmosphere. The XRD pattern of the LFC calcined powder displayed a close resemblance to that of LaFe<sub>0.6</sub>Co<sub>0.4</sub>O<sub>3</sub> (JCPDS card no. 00-044-0361), indicating the absence of any impurities. Notably, the XRD pattern of the LFCP powder exhibited a noticeable shift towards lower angles compared to that of LFC. This shift can be attributed to the reduction in interplanar distances between crystals, which occurs due to the introduction of Pd doping. Consequently, it can be inferred that LFC and LFCP perovskite structures can be formed within the porous framework of the SCM anode electrode subsequent to the impregnation of LFC and LFCP solutions.

<figure><img src="image_3.png" /><figcaption>**Figure 1** XRD patterns of LFC and LFCP solutions calcined at 900 °C for 2 h in air.</figcaption></figure>

Figure 2 depicts the impedance spectra of a SCM anode. The measurements were carried out at OCP and various temperatures while using methane as the fuel. The impedance spectra were evaluated using an equivalent circuit denoted as R<sub>Ω</sub>(R<sub>h</sub>Q<sub>h</sub>)(R<sub>m</sub>Q<sub>m</sub>)(R<sub>I</sub>Q<sub>I</sub>), wherein R<sub>Ω</sub> signifies the ohmic resistance of the cell. Additionally, R<sub>h</sub>, R<sub>m</sub>, and R<sub>I</sub> represent the polarization resistances of the electrodes (R<sub>E</sub> = R<sub>h</sub> + R<sub>m</sub> + R<sub>I</sub>), while Q<sub>h</sub>, Q<sub>m</sub>, and Q<sub>I</sub> correspond to constant phase elements observed at high, medium, and low frequencies, respectively.

The resistance of electrode polarization during the oxidation of methane was found to be 15.81 Ω cm² at a temperature of 750 °C. As the temperature was increased to 800 and 850 °C, the resistance decreased to 11.91 and 7.30 Ω cm², respectively. The findings from the impedance spectra analysis are summarized in Table 1. To understand the different stages of the methane oxidation reaction, the activation energy for the high-, medium-, and low-frequency arcs was determined and found to be 1.04, -0.042, and 0.87 eV, respectively. These results suggest that the high- and low-frequency arcs are thermally activated, while the medium-frequency arc shows slight thermal deactivation.
<figure><img src="image_4.png" /><figcaption>**Figure 2** SCM anode electrode polarization resistance at (a) 750 (b) 800 and (c) 850 °C in methane. Points are experimental data and solid lines are fitted data with the equivalent circuit $R_Ω(RQ)_1(RQ)_2(RQ)_3$. Dashed lines show each impedance arc. Closed symbols mark the frequency points from 0.01 Hz to 100 kHz at each frequency decade.</figcaption></figure>

<table><caption>**Table 1** Fitted impedance results for the methane oxidation reaction on the SCM anode at different temperatures</caption><thead><tr><th rowspan="3">Temp (°C)</th><th rowspan="3">Ohmic resistance (Ω cm²)</th><th colspan="9">Electrode Polarization resistance</th></tr><tr><th colspan="3">High frequency arc</th><th colspan="3">Medium frequency arc</th><th colspan="3">Low frequency arc</th></tr><tr><th>R₁ (Ωcm²)</th><th>Q₁ (Ωcm²sⁿ)</th><th>n₁</th><th>R₂ (Ωcm²)</th><th>Q₂ (Ωcm²sⁿ)</th><th>n₂</th><th>R₃ (Ωcm²)</th><th>Q₃ (Ωcm²sⁿ)</th><th>n₃</th></tr></thead><tbody><tr><td>750</td><td>4.54</td><td>0.93</td><td>5.5×10⁻⁴</td><td>0.8</td><td>1.71</td><td>0.14</td><td>0.7</td><td>13.17</td><td>0.36</td><td>0.9</td></tr><tr><td>800</td><td>3.83</td><td>0.58</td><td>3.7×10⁻⁴</td><td>0.8</td><td>1.95</td><td>0.19</td><td>0.7</td><td>9.16</td><td>0.35</td><td>0.9</td></tr><tr><td>850</td><td>3.15</td><td>0.29</td><td>2.9×10⁻⁴</td><td>0.8</td><td>2.06</td><td>0.21</td><td>0.7</td><td>4.95</td><td>0.37</td><td>0.9</td></tr></tbody></table>

At 850 °C, the high-frequency arc exhibited a peak frequency of 2511 Hz and an activation energy of 1.04 eV. In the context of the methane oxidation reaction on the LSCM/YSZ composite anode in wet methane, Jiang et al. [63] observed three distinct impedance arcs. They attributed the high-frequency arc to the transport of charged species across the interface of LSCM/YSZ. Another study compared the $La_{0.4}Sr_{0.6}Ti_{1-x}Mn_xO_{3-δ}$ (LSTM) anode with the LSTM/YSZ composite anode, where the latter showed a remarkably smaller high-frequency arc than the LSTM anode. The researchers concluded that the composite anode's LSTM/YSZ interface area was significantly larger, resulting in enhanced charge transfer [64]. Given the similarities between the characteristics of the high-frequency arc in this study and the findings
in the references [63, 64], it can be inferred that the high-frequency arc is associated with the transfer of charged species in the SCM anode.

Primdahl and Mogensen [65] studied the electrochemical oxidation of wet H₂ on Ni/YSZ electrode at 1000 °C in a three-electrode setup. They observed an arc with summit frequency around 1 Hz in the impedance spectra that has small negative activation energy (-0.09 eV). They attributed this arc to the gas conversion on the surface of Ni/YSZ anode electrode. In this study, the composition of fuel on the anode electrode is completely different from the counter and reference electrode side. The summit frequency of medium-frequency arc, at 850 °C, is 0.9 Hz and the activation energy of this arc is -0.042 eV. Comparison of this arc with references [64, 65], attributes the medium-frequency arc to the gas conversion on the surface of SCM anode electrode.

<figure><img src="image_5.png" /><figcaption>**Figure 3** Impedance spectra of SCM anodes impregnated with (a, c, and e) LFC+Pd and (b, d, and f) LFCP at (a, b) 750, (c, d) 800, and (e, f) 850 °C in methane. Points are experimental data and solid lines are fitted data with the equivalent circuit $R_Ω(R_hQ_h)(R_mQ_M)(R_lQ_l)$. Dashed lines show each impedance arc. Closed symbols mark the frequency points from 0.01 Hz to 100 kHz at each frequency decade.</figcaption></figure>

The impedance spectra of a Ni/YSZ anode in a moist H₂ environment at 850 °C were investigated by Jiang et al. [66]. They discovered an arc with a peak frequency of 0.02 Hz and an activation energy of 0.42 eV, which they attributed to the process of hydrogen dissociative adsorption on Ni particles. Since the low-frequency arc is thermally activated and reaches a peak frequency of 0.87 Hz at 850 °C, it can be inferred that this arc corresponds to the hydrogen dissociative adsorption on the SCM anode electrode [66].

Figure 3 presents the impact of treating the SCM electrode with LFC+Pd and LFCP solutions on its performance under varying methane temperatures. At 750 °C, the polarization resistance of the electrode decreased from 15.81 to 5.05 and 4.98 Ω cm² after impregnation with LFC+Pd and LFCP solutions, respectively. When the temperature was raised to 800 °C, the resistance decreased from 11.91 to 2.29 and 1.96 Ω cm² for LFC+Pd and LFCP solutions, respectively. This trend continued at 850 °C, with the resistance dropping from 7.30 to 1.07 and 0.86 Ω cm² for LFC+Pd and LFCP solutions,
respectively. Across all three temperatures, LFCP demonstrated greater enhancement compared to LFC+Pd, possibly due to the heightened activity of Pd ions within the perovskite structure compared to Pd single atoms. Notably, the reduction in resistance was significantly more pronounced at 850 °C, which can be attributed to the increased catalytic activity of LFC and LFCP perovskites under high-temperature conditions.

Examining Tables 2 and 3, which present the obtained impedance outcomes concerning the methane oxidation process on both LFC+Pd and LFCP-infiltrated SCM anodes, offers additional insights for delving into the electrochemical impact of LFC+Pd and LFCP on the SCM electrode. Figure 6 portrays the influence of each solution on parameters such as electrode polarization resistance as well as the characteristic arcs at high, medium, and low frequencies.

<table><caption>**Table 2** Fitted impedance results for the methane oxidation reaction on the SCM anode impregnated with LFC+Pd solution at different temperatures.</caption><thead><tr><th rowspan="3">Temp (°C)</th><th rowspan="3">Ohmic resistance (Ω cm²)</th><th colspan="9">Electrode polarization resistance</th></tr><tr><th colspan="3">High frequency arc</th><th colspan="3">Medium frequency arc</th><th colspan="3">Low frequency arc</th></tr><tr><th>R₁ (Ωcm²)</th><th>Q₁ (Ωcm²sⁿ)</th><th>n₁</th><th>R₂ (Ωcm²)</th><th>Q₂ (Ωcm²sⁿ)</th><th>n₂</th><th>R₃ (Ωcm²)</th><th>Q₃ (Ωcm²sⁿ)</th><th>n₃</th></tr></thead><tbody><tr><td>750</td><td>4.48</td><td>0.23</td><td>4.6×10⁻⁴</td><td>1</td><td>1.02</td><td>0.17</td><td>0.8</td><td>3.80</td><td>0.45</td><td>0.7</td></tr><tr><td>800</td><td>3.79</td><td>0.18</td><td>2.9×10⁻⁴</td><td>1</td><td>0.36</td><td>0.18</td><td>0.8</td><td>1.75</td><td>0.44</td><td>0.7</td></tr><tr><td>850</td><td>2.94</td><td>-</td><td>-</td><td>-</td><td>0.23</td><td>0.19</td><td>0.8</td><td>0.84</td><td>0.41</td><td>0.7</td></tr></tbody></table>

<table><caption>**Table 3** Fitted impedance results for the methane oxidation reaction on the SCM anode impregnated with LFCP solution at different temperatures.</caption><thead><tr><th rowspan="3">Temp(°C)</th><th rowspan="3">Ohmic resistance (Ω cm²)</th><th colspan="9">Electrode polarization resistance</th></tr><tr><th colspan="3">High frequency arc</th><th colspan="3">Medium frequency arc</th><th colspan="3">Low frequency arc</th></tr><tr><th>R₁ (Ωcm²)</th><th>Q₁ (Ωcm²sⁿ)</th><th>n₁</th><th>R₂ (Ωcm²)</th><th>Q₂ (Ωcm²sⁿ)</th><th>n₂</th><th>R₃ (Ωcm²)</th><th>Q₃ (Ωcm²sⁿ)</th><th>n₃</th></tr></thead><tbody><tr><td>750</td><td>4.39</td><td>0.24</td><td>3.7×10⁻⁴</td><td>0.9</td><td>0.43</td><td>0.06</td><td>1</td><td>4.31</td><td>0.37</td><td>0.8</td></tr><tr><td>800</td><td>3.67</td><td>0.20</td><td>3.8×10⁻⁴</td><td>0.9</td><td>0.16</td><td>0.05</td><td>1</td><td>1.60</td><td>0.31</td><td>0.8</td></tr><tr><td>850</td><td>2.51</td><td>-</td><td>-</td><td>-</td><td>0.11</td><td>0.03</td><td>1</td><td>0.75</td><td>0.29</td><td>0.8</td></tr></tbody></table>

The electrode polarization resistance of the SCM anode in the context of methane electro-oxidation exhibited a marked decrease following the introduction of both impregnation solutions. This reduction in polarization resistance can be primarily attributed to the discernible diminution observed at medium and low frequencies. The decrease in polarization resistance at low frequencies may be attributed to the augmentation of the anode's catalytic activity towards hydrogen adsorption facilitated by the impregnation of both solutions. Moreover, in the realm of medium frequencies, the incorporation of LFCP solution led to a notable reduction in electrode polarization resistance compared to LFC+Pd solution, possibly stemming from the remarkable catalytic prowess of LFCP in facilitating methane decomposition. In contrast, when the LFC+Pd solution was employed for impregnation, a substantial reduction in electrode polarization resistance at high frequencies ensued, which is likely linked to the promotion of ionic conduction within the anode as a result of LFC+Pd solution impregnation. It can be reasonably inferred that the impregnation of LFCP exerts a significant influence across all three frequency arcs.

Figure 7 illustrates the SEM micrographs depicting the SCM anode electrode's morphological changes prior to and subsequent to impregnation with LFC+Pd and LFCP solutions. The SEM imagery distinctly reveals the emergence of nanoparticles on the surface of the SCM anode electrode upon the introduction of both impregnation solutions. Notably, the formation of LFCP nanoparticles appears more pronounced when contrasted with the outcomes observed for LFC and Pd. As previously discussed, the SCM anodes subjected to LFCP solution impregnation exhibited diminished electrode polarization resistance in comparison to those treated with LFC+Pd solution. This enhanced performance
associated with LFCP solution can be attributed to the homogeneous dispersion of LFCP nanoparticles, which are found to be well-formed within the porous framework of the SCM anode.

<figure><img src="image_6.png" /><figcaption>**Figure 6** Effect of (a) LFC+Pd and (b) LFCP solutions impregnation on R₁, R₂, R₃, and R₆ of SCM anode at 750, 800, and 850 °C in methane.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 7** SEM micrograph of SCM anode electrode, (a) before, and after (b) LFC+Pd (c) and (d) LFCP solution impregnation.</figcaption></figure>

**4. Conclusion**

SCM anode had large electrode polarization resistance for methane oxidation at 850 °C. Deconvolution of the impedance spectra by using an equivalent circuit revealed that electro-oxidation of methane on the SCM anode electrode is limited by at least three electrode processes. These processes are related to the charge transfer, at high-frequencies, gas conversion, at medium-frequencies and hydrogen dissociative adsorption on the SCM anode electrode at low-frequencies. The SCM electrode polarization resistance reduced strongly with LFC+Pd solution impregnation. LFC nanoparticles promoted the electro-catalytic property of SCM anode by facilitating the processes of methane decomposition and hydrogen adsorption. However, results revealed that doping Pd in the LFC perovskite structure promotes the electrode performance compared to metallic Pd. The electrode polarization resistance at low frequencies decreased with of LFCP solution more than LFC+Pd solution.
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Disclosure of conflict of interest

No conflict of interest to be disclosed.

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