Highly Conductive Mn-Co Spinel Powder Prepared by Cu-Doping Used for Interconnect Protection of SOFC

: Mn-Co Spinel is considered as one of the most promising materials for the interconnect protection of solid oxide fuel cells; however, its conductivity is too low to maintain a high cell performance as compared with cathode materials. Element doping is an effective method to improve the spinel conductivity. In this work, we proposed doping Mn-Co spinel powder with Cu via a solid phase reaction. Cu δ Mn 1.5 − x Co 1.5 − y O 4 with δ = 0.1, 0.2, 0.3, and x + y = δ was obtained. X-ray diffraction (XRD) and thermogravimetry-differential scanning calorimetry (TG-DSC) were used to evaluate the Cu-doping effect. After sintering at 1000 ◦ C for 12 h, the yield exhibited the best crystallinity, density, and element distribution, with a phase composition of MnCo 2 O 4 /Cu x Mn 3 − x O 4 (x = 1, 1.2, 1.4 or 1.5). X-ray photoelectron spectroscopy (XPS) was used to semi-quantitatively characterize the content changes in element valence states. The areal fraction of Mn 2+ and Co 3+ was found to decrease when the sintering duration increased, which was attributed to the decomposition of the MnCo 2 O 4 phase. Finally, coatings were prepared by atmospheric plasma spraying with doped spinel powders and the raw powder Mn 1.5 Co 1.5 O 4 . It was found that Cu doping can effectively increase the conductivity of Mn-Co spinel coatings from 23 S/cm to 51 S/cm. Although the dopant Cu was found to be enriched on the surface of the coatings after the conductivity measurement, which restrained the doping effect, Cu doping remains a convenient method to signiﬁcantly promote the conductivity of spinel coatings for SOFC applications. structure of the sieved powders was checked by X-ray diffraction (XRD, Bruker D8 Advance, Bruker Cop., Karlsruhe, Germany) in the range of 10–90 ◦ (0.01 ◦ /step, 0.167 s/step). The micromorphology and element distribution of the sieved powders were characterized by a scanning electron microscope (SEM, Nova-Nano-450, FEI, Hillsboro, OR, USA) and energy dispersive spectrometer (EDS, XLT TEM-SDD, AMETEK EDAX, Philadelphia, PA, USA). The solid-phase sintering process was investigated using a thermogravimetry-differential scanning calorimetry (TG-DSC, STA-449-F5, NETZSCH, Selb, Germany) test with a heating rate of 5 ◦ C/min in air. The valence states of the sintered powders were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientiﬁc Nexsa, Thermo Fisher Scientiﬁc Inc., Waltham, MA, USA), and then the content changes in the element valence states were characterized semi-quantitatively. The XPS spectra were obtained with monochromatized Al K α radiation. All spectra were recorded at a take-off angle of 90 ◦ . The carbon C 1s peak with a binding energy of 284.5 eV was used for calibration.


Introduction
Solid oxide fuel cells (SOFC) are high-temperature energy conversion devices that can directly convert the chemical energy of fuel into electrical energy. They mainly consist of a porous anode/cathode, a dense electrolyte, and an interconnect. As an important part, the interconnect provides electrical contact between cells while isolating fuel gas from air. Ferritic stainless steel (FSS) has been widely used as interconnect material due to its outstanding oxidation resistance and suitable thermal expansion coefficient [1][2][3]. However, at elevated temperature, the Cr of FSS is oxidized to form Cr 2 O 3 , CrO 3 , or even CrO 2 (OH) 2 . These substances can accumulate at the cathode and react with cathode materials, forming a low active phase and deteriorating cell performance. At present, the above problems are mainly solved by introducing protective coatings [4,5].
(Mn,Co) 3 O 4 spinel coating is often used to diminish the evaporation of Cr and thus reduce the performance degradation and prolong the service life of the stack. However, the theoretical conductivity of (Mn,Co) 3 O 4 spinel at elevated temperature is only 60 S/cm, which is much lower than that of the cathode material, such as LSCF > 200 S/cm at 600-800 • C [6][7][8][9]. Many efforts have been made to improve the conductivity of Mn-Co spinel coating. Fe doping can reduce the growth of Cr 2 O 3 film and improve the oxidation resistance, but this reduces the conductivity of spinel coatings [10][11][12][13]. Ce doping can enhance the adhesion and improve the interfacial stability of oxide film, but the effects depend on the substrate materials [14][15][16]. Cu doping can improve the conductivity and adjust the thermal expansion coefficient of spinel coatings [17][18][19].
For promoting the conductivity of Mn-Co spinel coatings, in this work, we prepared Cu-doped Mn-Co spinel powder via the solid-phase reaction method. The doped powder was denoted as Cu δ Mn 1.5−x Co 1.5−y O 4 (δ = 0.1, 0.2, 0.3, x + y = δ). The first part of this work was to study the optimal doping amount of Cu, and then the doping conditions were studied to improve the doping effect. Finally, the doped powder was used to prepare coatings on FSS substrate to evaluate the doping effect using a plasma spray.

Powder Preparation and Characterization
The raw materials, as shown in Figure 1, were Cu powders (Tijo, Changsha, China) with D 50 = 21.58 µm, and Mn 1.5 Co 1.5 O 4 powder with D 50 = 19.2 µm (Terio, Qingdao, China). Three groups of doping powder were prepared with controlled Cu-doping as shown in Table 1. The raw powders were mechanically mixed in a roller mixer at a speed of 600 r/min for 20 h. After mixing, the powders were transferred into a muff furnace for solid-phase reaction sintering, and then the sintered powders were sieved to maintain the particle size below 50 µm. The phase structure of the sieved powders was checked by X-ray diffraction (XRD, Bruker D8 Advance, Bruker Cop., Karlsruhe, Germany) in the range of 10-90 • (0.01 • /step, 0.167 s/step). The micromorphology and element distribution of the sieved powders were characterized by a scanning electron microscope (SEM, Nova-Nano-450, FEI, Hillsboro, OR, USA) and energy dispersive spectrometer (EDS, XLT TEM-SDD, AMETEK EDAX, Philadelphia, PA, USA). The solid-phase sintering process was investigated using a thermogravimetry-differential scanning calorimetry (TG-DSC, STA-449-F5, NETZSCH, Selb, Germany) test with a heating rate of 5 • C/min in air. The valence states of the sintered powders were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Nexsa, Thermo Fisher Scientific Inc., Waltham, MA, USA), and then the content changes in the element valence states were characterized semi-quantitatively. The XPS spectra were obtained with monochromatized Al Kα radiation. All spectra were recorded at a take-off angle of 90 • . The carbon C 1s peak with a binding energy of 284.5 eV was used for calibration.
For promoting the conductivity of Mn-Co spinel coatings, in this work, we prepared Cu-doped Mn-Co spinel powder via the solid-phase reaction method. The doped powder was denoted as CuδMn1.5−xCo1.5−yO4 (δ = 0.1, 0.2, 0.3, x + y = δ). The first part of this work was to study the optimal doping amount of Cu, and then the doping conditions were studied to improve the doping effect. Finally, the doped powder was used to prepare coatings on FSS substrate to evaluate the doping effect using a plasma spray.

Powder Preparation and Characterization
The raw materials, as shown in Figure 1, were Cu powders (Tijo, Changsha, China) with D50 = 21.58 μm, and Mn1.5Co1.5O4 powder with D50 = 19.2 μm (Terio, Qingdao, China). Three groups of doping powder were prepared with controlled Cu-doping as shown in Table 1. The raw powders were mechanically mixed in a roller mixer at a speed of 600 r/min for 20 h. After mixing, the powders were transferred into a muff furnace for solidphase reaction sintering, and then the sintered powders were sieved to maintain the particle size below 50 μm. The phase structure of the sieved powders was checked by X-ray diffraction (XRD, Bruker D8 Advance, Bruker Cop., Karlsruhe, Germany) in the range of 10-90° (0.01°/step, 0.167 s/step). The micromorphology and element distribution of the sieved powders were characterized by a scanning electron microscope (SEM, Nova-Nano-450, FEI, Hillsboro, OR, USA) and energy dispersive spectrometer (EDS, XLT TEM-SDD, AMETEK EDAX, Philadelphia, PA, USA). The solid-phase sintering process was investigated using a thermogravimetry-differential scanning calorimetry (TG-DSC, STA-449-F5, NETZSCH, Selb, Germany) test with a heating rate of 5 °C/min in air. The valence states of the sintered powders were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Nexsa, Thermo Fisher Scientific Inc., Waltham, MA, USA), and then the content changes in the element valence states were characterized semi-quantitatively. The XPS spectra were obtained with monochromatized Al Kα radiation. All spectra were recorded at a take-off angle of 90°. The carbon C 1s peak with a binding energy of 284.5 eV was used for calibration.    Table 2. Before spraying, substrates were ultrasonically cleaned with alcohol to remove any oil and impurities on the surface.

Coating Characterization
The sectional morphology, density, and element distribution of the spinel coating were analyzed by SEM and EDS. The phase composition of the coating was characterized by XRD. The conductivity of coatings was determined by the four-probe method. The conductivity test setup is shown in Figure 2. Testing samples were placed in a tubular furnace, which was heated up to 800 • C at a rate of 5 • C/min. A constant current of 0.2 A was provided between I + and I − through silver lines, and the corresponding voltage between U + and U − was recorded. The coating conductivity (ε, S/cm) was obtained as ε = 1 ÷ (C × R × W), where C denotes the wiring coefficient, R denotes the calculated resistance (Ω), and W denotes the thickness of the coating (cm).

Coating Characterization
The sectional morphology, density, and element distribution of th were analyzed by SEM and EDS. The phase composition of the coating w by XRD. The conductivity of coatings was determined by the four-pro conductivity test setup is shown in Figure 2. Testing samples were plac furnace, which was heated up to 800 °C at a rate of 5 °C/min. A constant was provided between I + and I − through silver lines, and the correspond tween U + and U − was recorded. The coating conductivity (ε, S/cm) was ob (C × R × W), where C denotes the wiring coefficient, R denotes the calcu (Ω), and W denotes the thickness of the coating (cm).    Cu-doping can promote the formation of the cubic spinel phase. After increasing the amount of Cu-doping, the diffraction peaks were found to shift slightly toward the right because of the replacement of Co 2+ (0.074 nm) with Cu 2+ (0.073 nm), and the diffraction intensity of (Co,Mn)(Co,Mn) 2 O 4 and CuO was found to decrease, which indicates the doping reactions had taken place.

Phases and Structures
When both CuO and CoMn 2 O 4 were present in the coating, the uniformity and conductivity of the powder were reduced [18]. It was found that the (Co,Mn)(Co,Mn) 2 O 4 and CuO of Cu 0.3 Mn 1.5−x Co 1.5−y O 4 powder exhibited the lowest diffraction intensity and the best crystallinity of the spinel phase.
atings 2021, 11, x FOR PEER REVIEW When both CuO and CoMn2O4 were present in the coating, the uniform tivity of the powder were reduced [18]. It was found that the (Co,Mn)(Co, of Cu0.3Mn1.5−xCo1.5−yO4 powder exhibited the lowest diffraction intensity a tallinity of the spinel phase.

Solid Phase Reactions
CuδMn1.5−xCo1.5−yO4 (δ = 0, 0.1, 0.2, 0.3) powders were tested by TGthe physical and chemical changes during sintering. According to the TG 4a and the subsequent XRD analysis ( Figure 5), it was found that the some weight at 500-800 °C, which was mainly attributed to the oxidizati However, at 800-1000 °C, the powder weights were found to decrease CuO had disappeared. At 1034 °C, the powder weight was slightly decrea might be related to the decomposition of the spinel phase. Then, the po to lose weight violently between 1200-1300 °C, which indicates that t were decomposed. As shown in Figure 4b, the commercial Mn1.5Co1.5O endothermic peak at 1034 °C, while CuδMn1.5−xCo1.5−yO4 (δ = 0.1, 0.2, 0.3 endothermic peak. According to public reports [9,13,20,21], the formatio and MnCo2O4 is an exothermic process. Since the MnCo2O4 phase of Mn1.5Co1.5O4 powder showed no significant decomposition at 1034 °C, it the endothermic peak at 1034 °C was the result of the decomposition of thermore, in the range 1100-1300 °C, after increasing the amount of C shifted toward the left, and the stable temperature of the spinel phase DSC curves were found to gradually flatten and continuously exhausted in Figure 4c,d, the Cu0.3Mn1.5−xCo1.5−yO4 powder exhibited the best crysta clusion, CuxMn3−xO4 spinel was successfully synthesized between 900 °C powders with δ = 0.3 had the best crystallinity of spinel phases and co

Solid Phase Reactions
Cu δ Mn 1.5−x Co 1.5−y O 4 (δ = 0, 0.1, 0.2, 0.3) powders were tested by TG-DSC to evaluate the physical and chemical changes during sintering. According to the TG curve in Figure 4a and the subsequent XRD analysis ( Figure 5), it was found that the powders gained some weight at 500-800 • C, which was mainly attributed to the oxidization of Cu to CuO. However, at 800-1000 • C, the powder weights were found to decrease slightly and the CuO had disappeared. At 1034 • C, the powder weight was slightly decreased again, which might be related to the decomposition of the spinel phase. Then, the powder was found to lose weight violently between 1200-1300 • C, which indicates that the spinel phases were decomposed. As shown in Figure 4b, the commercial Mn 1.5 Co 1.5 O 4 powder had no endothermic peak at 1034 • C, while Cu δ Mn 1.5−x Co 1.5−y O 4 (δ = 0.1, 0.2, 0.3) powder had an endothermic peak. According to public reports [9,13,20,21], the formation of Cu x Mn 3-x O 4 and MnCo 2 O 4 is an exothermic process. Since the MnCo 2 O 4 phase of the commercial Mn 1.5 Co 1.5 O 4 powder showed no significant decomposition at 1034 • C, it was inferred that the endothermic peak at 1034 • C was the result of the decomposition of Cu x Mn 3-x O 4 . Furthermore, in the range 1100-1300 • C, after increasing the amount of Cu, the TG curve shifted toward the left, and the stable temperature of the spinel phases decreased. The DSC curves were found to gradually flatten and continuously exhausted heat. As shown in Figure 4c

Sintering Conditions
Cu0.3Mn1.5−xCo1.5−yO4 powders were prepared at controlled sintering temperatures and durations. Cu was introduced into the spinel lattice by sintering. The more Cu that entered the spinel lattice, the better the conductivity of the coating. The phase composition, crystallization, and surface morphology of the prepared Cu0.3Mn1.5−xCo1.5−yO4 powders were specifically investigated. According to the XRD patterns of the powders sintered at 500-1300 °C for 4 h, it was found that, with the increase in sintering temperature, the abundance of CuO and (Co,Mn)(Co,Mn)2O4 phases reduced, while the abundance of MnCo2O4/CuxMn3−xO4 spinel phases increased. The crystallinity of the spinel phases increased firstly and then decreased as the temperature increased; thus, the best crystallization temperature was between 1000-1100 °C, as shown in Figure 5. According to the morphology of the sintered powders, the powder density was found to increase when the temperature increased from 900 °C to 1300 °C. After sintering between 1100-1300 °C, the powder was found to be severely agglomerated, as shown in Figure 6, while at 1000 °C, the powder porosity was reduced. After sintering at 1000 °C for 2 h, 4 h, 8 h, 12 h, 16 h, and 24 h, the intensity of the diffraction peak and crystallinity of spinel were found to firstly increase and then decrease after 12 h. The change in diffraction peaks was related to the amount of spinel phases. The diffraction intensity of powder sintered for 12 h was the highest and the crystallinity was

Sintering Conditions
Cu 0.3 Mn 1.5−x Co 1.5−y O 4 powders were prepared at controlled sintering temperatures and durations. Cu was introduced into the spinel lattice by sintering. The more Cu that entered the spinel lattice, the better the conductivity of the coating. The phase composition, crystallization, and surface morphology of the prepared Cu 0.3 Mn 1.5−x Co 1.5−y O 4 powders were specifically investigated. According to the XRD patterns of the powders sintered at 500-1300 • C for 4 h, it was found that, with the increase in sintering temperature, the abundance of CuO and (Co,Mn)(Co,Mn) 2 O 4 phases reduced, while the abundance of MnCo 2 O 4 /Cu x Mn 3−x O 4 spinel phases increased. The crystallinity of the spinel phases increased firstly and then decreased as the temperature increased; thus, the best crystallization temperature was between 1000-1100 • C, as shown in Figure 5. According to the morphology of the sintered powders, the powder density was found to increase when the temperature increased from 900 • C to 1300 • C. After sintering between 1100-1300 • C, the powder was found to be severely agglomerated, as shown in Figure 6, while at 1000 • C, the powder porosity was reduced. After sintering at 1000 °C for 2 h, 4 h, 8 h, 12 h, 16 h, and 24 h, the intensity of the diffraction peak and crystallinity of spinel were found to firstly increase and then decrease after 12 h. The change in diffraction peaks was related to the amount of spinel phases. The diffraction intensity of powder sintered for 12 h was the highest and the crystallinity was the best, as shown in Figure 5c, d. In Figure 7, EDS analysis showed a uniform distribution of Cu, which indicates that the Cu has been sucessfully doped into Mn1.5Co1.5O4 spinel powders. In conclusion, 1000 °C/12 h was the best condition for Cu doping.  After sintering at 1000 • C for 2 h, 4 h, 8 h, 12 h, 16 h, and 24 h, the intensity of the diffraction peak and crystallinity of spinel were found to firstly increase and then decrease after 12 h. The change in diffraction peaks was related to the amount of spinel phases. The diffraction intensity of powder sintered for 12 h was the highest and the crystallinity was the best, as shown in Figure 5c,d. In Figure 7, EDS analysis showed a uniform distribution of Cu, which indicates that the Cu has been sucessfully doped into Mn 1.5 Co 1.5 O 4 spinel powders. In conclusion, 1000 • C/12 h was the best condition for Cu doping.

Stability of Doped Powders
As Cu-Mn spinel is less stable than Mn-Co spinel at elevated temperature, when doping Mn-Co spinel with Cu to improve the conductivity, the stability of the obtained spinel

Stability of Doped Powders
As Cu-Mn spinel is less stable than Mn-Co spinel at elevated temperature, when doping Mn-Co spinel with Cu to improve the conductivity, the stability of the obtained spinel is reduced. After being sintered at 1000 • C for 4 h, 12 h, and 24 h to allow the doping, the Cu 0.3 Mn 1.5−x Co 1.5−y O 4 powder was heated to 800 • C and maintained in that state for 12 h to evaluate the stability. In Figure 8a, the XRD patterns before and after the heat treatment are presented. The MnCo 2 O 4 /Cu x Mn 3−x O 4 phase was detected after the heat treatment, but the diffraction intensity was significantly diminished. TG and DSC tests were performed from room temperature to 800 • C with a ramping rate of 5 • C/min, and a preservation stage of 12 h, in order to study the stability of the doped powders. It was found that during ramping, the DSC plots showed evident exothermic behavior. However, during the following preservation stage, both the TG and DSC curves exhibited a linear behavior in which no endothermic and exothermic peak was visible, as shown in Figure 8b

Elemental Composition
To determine the elemental composition and valence states of Cu0.3Mn1.5−xCo1.5−yO4 powders, XPS tests were carried out. As shown in Figure 9a, Co, Mn, Cu, O, and C were identified in the coating. According to the Co 2p spectrum in Figure 9b, the low-energy peaks (780.6 and 796.1 eV) and high-energy peaks (781.6 and 797.4 eV) in the Co 2P3/2 and 2P1/2 regions were ascribed to Co 3+ and Co 2+ , respectively. In addition, the peaks with binding energies of 786.6 eV and 803.6 eV were identified as the satellite peaks of Co 2+ and Co 3+ [22][23][24]. As illustrated in Figure 9c, the peaks at 641.5, 642.3, and 642.8 eV corresponded to Mn 2+ , Mn 3+ , and Mn 4+ , respectively [25][26][27]. In Figure 9d, the peaks at 945 eV were attributed to Cu 2+ satellites, while those at 930.8 and 933.4 eV were evidence of the presence of Cu + and Cu 2+ in the Cu 2P3/2 orbital peaks. Peaks at 953.5 and 950.6 eV can be assigned to the existence of Cu 2+ and Cu + , which were observed in the Cu 2P1/2 orbital [28][29][30]. The peaks at 530.4 eV and 531.9 eV, as shown in Figure 9e, corresponded to the oxygen-metal bond (metal = Mn, Co) and O-H bond in the spinel phase, respectively [3].

Elemental Composition
To determine the elemental composition and valence states of Cu 0.3 Mn 1.5−x Co 1.5−y O 4 powders, XPS tests were carried out. As shown in Figure 9a, Co, Mn, Cu, O, and C were identified in the coating. According to the Co 2p spectrum in Figure 9b, the low-energy peaks (780.6 and 796.1 eV) and high-energy peaks (781.6 and 797.4 eV) in the Co 2P 3/2 and 2P 1/2 regions were ascribed to Co 3+ and Co 2+ , respectively. In addition, the peaks with binding energies of 786.6 eV and 803.6 eV were identified as the satellite peaks of Co 2+ and Co 3+ [22][23][24]. As illustrated in Figure 9c, the peaks at 641.5, 642.3, and 642.8 eV corresponded to Mn 2+ , Mn 3+ , and Mn 4+ , respectively [25][26][27]. In Figure 9d, the peaks at 945 eV were attributed to Cu 2+ satellites, while those at 930.8 and 933.4 eV were evidence of the presence of Cu + and Cu 2+ in the Cu 2P 3/2 orbital peaks. Peaks at 953.5 and 950.6 eV can be assigned to the existence of Cu 2+ and Cu + , which were observed in the Cu 2P 1/2 orbital [28][29][30]. The peaks at 530.4 eV and 531.9 eV, as shown in Figure 9e, corresponded to the oxygen-metal bond (metal = Mn, Co) and O-H bond in the spinel phase, respectively [3].

Coatings Conductivity
To evaluate the doping effect, commercial Mn1.5Co1.5O4 and Cu0.3Mn1.5−xCo1.5−yO4 powder (sintered at 1000 °C and maintained for 8 h, 12 h, and 24 h, respectively) were sprayed on Al2O3 sheets with an atmospheric plasma spray; the parameters are provided in Table  1. During spraying, the spinel powder was injected into the plasma plume, the temperature of which was as high as 15,000 °C, then the injected powder was heated, melted, accelerated, and impacted on the substrate to form coatings. Deoxidization and decomposition of the spinel powder occurred during spraying; therefore, low active phases such as MnO2 and CoO were generated and left in the coating. As a result of the very short spraying time, only a small part of the powder was decomposed, and most of the powder retained the spinel structure. Before the conductivity measurement, the sample was heated to 800 °C and maintained for 12 h to allow the densification of the coating and cure the decomposed phases. As shown in Figure 10, after the heat treatment, significant signals of MnCo2O4/CuMn2O4 were detected, which indicates a transference of low conductive phases to the spinel phase. For this reason, the measured coating conductivity increased with the temperature during preservation with a duration up to 50 h, as shown in Figure  11. The conductivity of the coating prepared with powder sintered for 12 h was about 51 S/cm after 47 h of preservation. For the coating prepared with powder sintered at 1000 °C for 8 h and 24 h, the conductivity was 45 S/cm and 49.2 S/cm, respectively, which are more According to public reports [10,31,32], various valences exist in Mn-Co-Cu spinel. By performing a deconvolution of the XPS spectra, the relative proportion of different valence states of elements can be obtained. The relative proportion is listed in Table 3. It was found that the relative proportion of Mn 3+ increased to 41.3% when sintered for 12 h and then decreased 33.6% after 24 h of preservation. The areal fraction of Mn 2+ and Co 3+ increased first and then decreased when the sintering duration increased. The increase in Mn 3+ indicated an increase in the CuMn 2 O 4 phase, and the decrease in Mn 2+ and Co 3+ was related to the decomposition of the MnCo 2 O 4 phase. Moreover, the increase in Mn 4+ indicated an increase in MnO 2 , which diminished the conductivity of the prepared powders or coatings. According to Table 3 the powder prepared at 1000 • C and sintered for 12 h gave the most MnCo 2 O 4 and CuMn 2 O 4 . sprayed on Al 2 O 3 sheets with an atmospheric plasma spray; the parameters are provided in Table 1. During spraying, the spinel powder was injected into the plasma plume, the temperature of which was as high as 15,000 • C, then the injected powder was heated, melted, accelerated, and impacted on the substrate to form coatings. Deoxidization and decomposition of the spinel powder occurred during spraying; therefore, low active phases such as MnO 2 and CoO were generated and left in the coating. As a result of the very short spraying time, only a small part of the powder was decomposed, and most of the powder retained the spinel structure. Before the conductivity measurement, the sample was heated to 800 • C and maintained for 12 h to allow the densification of the coating and cure the decomposed phases. As shown in Figure 10, after the heat treatment, significant signals of MnCo 2 O 4 /CuMn 2 O 4 were detected, which indicates a transference of low conductive phases to the spinel phase. For this reason, the measured coating conductivity increased with the temperature during preservation with a duration up to 50 h, as shown in Figure 11. The conductivity of the coating prepared with powder sintered for 12 h was about 51 S/cm after 47 h of preservation. For the coating prepared with powder sintered at 1000 • C for 8 h and 24 h, the conductivity was 45 S/cm and 49.2 S/cm, respectively, which are more than double that of the Mn 1.5 Co 1.5 O 4 coating, 23 S/cm after 50 h of preservation. Therefore, by applying Cu doping, the conductivity of Mn-Co spinel coatings can be significantly improved. Moreover, according to Figures 10 and 11, the coating prepared with powder sintered at 1000 • C for 8 h exhibited lower crystallinity and less spinel content, resulting in a coating conductivity lower than that of the 1000 • C/12 h and 24 h samples.   The sectional morphology and element distribution before and after the conductivity by applying Cu doping, the conductivity of Mn-Co spinel coatings can be significantly improved. Moreover, according to Figures 10 and 11, the coating prepared with powder sintered at 1000 °C for 8 h exhibited lower crystallinity and less spinel content, resulting in a coating conductivity lower than that of the 1000 °C/12 h and 24 h samples.  The sectional morphology and element distribution before and after the conductivity measurement of the best coating (1000 °C for 12 h) are provided in Figure 12. After the conductivity measurement, the coating was significantly densified, and no large pores or The sectional morphology and element distribution before and after the conductivity measurement of the best coating (1000 • C for 12 h) are provided in Figure 12. After the conductivity measurement, the coating was significantly densified, and no large pores or cracks were visible on the cross-section. The distribution of Mn and Co exhibited no evident variation, while Cu was found to be enriched on the coating surface. The enriched Cu on the surface was then oxidized to CuO at 800 • C, which limited the conductivity of the coatings. Despite the existence of CuO on the surface restraining the doping effect, Cu doping remains a convenient method to significantly promote the conductivity of spinel coatings for SOFC applications.

Conclusions
The powder CuδMn1.5−xCo1.5−yO4 (x + y = δ) with δ = 0.3 showed a better performan than the powder with δ = 0.1 or 0.2. The results of XRD and TG-DSC indicated that creasing the dopant amount resulted in an enhanced doping effect. With δ = 0.3, the po der had a better crystallinity and a more conductive spinel phase.
The best doping condition for Cu0.3Mn1.5−xCo1.5−yO4 powders was 1000 °C for 12 h. U der this condition, the crystallization of the spinel phase (MnCo2O4 and CuxMn3−xO4) w the best and the powder was moderately dense. The dopant Cu was found to be even distributed in the powders. The CuxMn3−xO4 (x = 1, 1.2, 1.4 or 1.5) was found to be main composed of CuMn2O4 and was sufficiently stable at elevated temperatures. According the XPS results, the powder prepared at 1000 °C for 12 h consisted of the most MnCo and CuMn2O4. When the preservation stage was conducted at 1000 °C with a duration less than 12 h or more than 12 h, less MnCo2O4 and CuxMn3−xO4 phases were detected.
Coatings were prepared with doped powders using an atmospheric plasma spr After preservation at 800 °C for a duration of up to 50 h, the measured conductivity of prepared coatings reached 45 S/cm, 51 S/cm, and 49.2 S/cm, respectively, i.e., two tim more than that of the Mn1.5Co1.5O4 coating (23 S/cm). Although Cu was enriched on coating surface after the conductivity measurement, Cu-doping is still an effective meth to improve the conductivity of spinel coatings used for interconnect protection in SOF

Conclusions
The powder Cu δ Mn 1.5−x Co 1.5−y O 4 (x + y = δ) with δ = 0.3 showed a better performance than the powder with δ = 0.1 or 0.2. The results of XRD and TG-DSC indicated that increasing the dopant amount resulted in an enhanced doping effect. With δ = 0.3, the powder had a better crystallinity and a more conductive spinel phase.
The best doping condition for Cu 0.3 Mn 1.5−x Co 1.5−y O 4 powders was 1000 • C for 12 h. Under this condition, the crystallization of the spinel phase (MnCo 2 O 4 and Cu x Mn 3−x O 4 ) was the best and the powder was moderately dense. The dopant Cu was found to be evenly distributed in the powders. The Cu x Mn 3−x O 4 (x = 1, 1.2, 1.4 or 1.5) was found to be mainly composed of CuMn 2 O 4 and was sufficiently stable at elevated temperatures. According to the XPS results, the powder prepared at 1000 • C for 12 h consisted of the most MnCo 2 O 4 and CuMn 2 O 4 . When the preservation stage was conducted at 1000 • C with a duration of less than 12 h or more than 12 h, less MnCo 2 O 4 and Cu x Mn 3−x O 4 phases were detected.
Coatings were prepared with doped powders using an atmospheric plasma spray. After preservation at 800 • C for a duration of up to 50 h, the measured conductivity of the prepared coatings reached 45 S/cm, 51 S/cm, and 49.2 S/cm, respectively, i.e., two times more than that of the Mn 1.5 Co 1.5 O 4 coating (23 S/cm). Although Cu was enriched on the coating surface after the conductivity measurement, Cu-doping is still an effective method to improve the conductivity of spinel coatings used for interconnect protection in SOFC.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.