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Electrodeposition and isothermal aging of Co and Mn layers on stainless steel for interconnectors: Initial stages of spinel phase formation
... To increase SOFC lifetime, the application of protective coatings is required. Coatings with perovskite or spinel structures based on transition elements (Mn, Co, Cu, Ni, etc.) are believed to be the most promising [6,[11][12][13][14][15][16][17][18][19][20]. These coatings have low electrical resistance and act as a barrier for chromium migration to the interconnect surface. ...
... These coatings have low electrical resistance and act as a barrier for chromium migration to the interconnect surface. Numerous techniques of protective coating application are known, including wet powder spraying [4], physical vapor deposition [21], sol-gel technology [22], electrophoresis [23,24], and electrochemical deposition [17,[25][26][27]. Electrochemical deposition seems more promising due to the simplicity and the ability to control the thickness of the coating by varying the electric charge passed through an electrolytic cell. ...
... To determine whether co-electrodeposition of cobalt and manganese is possible, cyclic current-voltage curves were obtained for aqueous solutions containing ions of the corresponding metals (Figure 1a). As mentioned in some studies [11,17], the addition of complexing agents causes shifting of the metal reduction potentials at the cathode that promotes the co-electrodeposition of cobalt and manganese. However, the introduction of dimethyl sulfoxide (DMSO) as a complexing agent into the electrolyte only leads to an increase in the difference in the reduction potentials of Co and Mn up to 540 mV ( Figure 1b). ...
This work suggests a method for obtaining heat-resistant protective coatings for 08Kh17T stainless steel that can be used as interconnect material for solid oxide fuel cells. The suggested approach is based on the layer-by-layer precipitation of nickel, cobalt, and manganese, followed by heat treatment in a vacuum and oxidizing atmosphere. XRD results show that the coatings consist of a mixture of metal oxides and compounds with a spinel structure. The obtained coatings demonstrate high resistance to high-temperature oxidation for 100 h. The coating with the ratio of the thicknesses of the cobalt and manganese layers of 1.5/0.5 μm obtained by electrodeposition is the most stable. The specific electrical resistance of this coating is 3.50·10−3 Ω·cm2 after 100 h of exposure at 850 °C, which meets the requirements for SOFC interconnect materials.
... The final product was dissolved in 10 mL of 6 mol/L H 2 [45][46][47][48][49]. After cooling to room temperature, the solutions were filtered and the pH adjusted to 3.0 [27,29,[50][51][52] with the addition of sodium acetate. 49]. ...
... 49]. After cooling to room temperature, the solutions were filtered and the pH adjusted to 3.0 [27,29,[50][51][52] with the addition of sodium acetate. The concentrations of the electrolyte solutions from the recycling process were analyzed by Atomic Absorption Spectrometry (Intralab model AA-1275ª) and the results, S1 and S2, are showed at Table 3. From these results, the concentrations were adjusted with the addition of deionized water to obtain five solutions with different ratios between cobalt and manganese ions (A1-A5) ( Table 3). ...
The recycling of exhausted lithium-ion batteries from mobile phones originate five solutions with different Co and Mn proportions that were used as electrolytic solutions to obtain Mn-Co spinel coatings on the surface of AISI430 stainless steel. The coatings are intended to contain chromium volatility in the working conditions of Solid Oxide Fuel Cells (SOFC) metallic interconnectors. Potentiostatic electrodeposition was the technique used to obtain Mn-Co coatings from low concentration electrolytes at pH = 3.0 and potential applied −1.3 V. Charge efficiency data were used for sample optimization. Three optimized samples were subjected to oxidation heat treatment at 800 °C for 300 h and then characterized by XRD, SEM and EDS. The results showed that the addition of manganese ions instead of cobalt ions in the electrolytic bath produces more stable and well-distributed deposits as the ratio of the two ions becomes equal in the electrolytic bath. Thin, homogeneous and stable spinel coatings (Mn, Co)3O4 2.8 μm and 3.9 μm thick were able to block chromium volatility when exposed to SOFC operating temperature.
... (Mn,Co)3O4 spinel coatings of different thickness have been produced by many different deposition techniques: slurry deposition of ceramic powders [15,[18][19][20], electrodeposition followed by oxidation [21][22][23], physical vapor deposition (PVD), thermal oxidation [24,25] and thermal spray [26][27][28][29]. ...
... Other authors [22,53] reported on Co and Mn spinel based produced by electrodeposition. ...
Electrophoretic deposition, thermal co-evaporation and RF magnetron sputtering methods are used for the preparation of Mn-Co based ceramic coatings for solid oxide fuel cell steel interconnects. Both thin and relatively thick coatings (1–15 μm) are prepared and characterised for their potential protective behaviour. Mn-Co coated Crofer22APU samples are electrically tested for 5000 hours at 800 °C under a 500 mA cm⁻² current load to determine their Area Specific Resistance increase due to a growing chromia scale. After tests, samples are analysed by scanning and transmission electron microscopy. Analysis is focused on the potential chromium diffusion to or through the coating, the oxide scale thickness and possible reactions at the interfaces. The relationships between the coating type, thickness and effectiveness are reviewed and discussed. Out of the three Mn-Co coatings compared in this study, the one deposited by electrophoretic deposition presents the best protection against Cr diffusion and offers long term stability.
... 10 For coating applications, these materials have been prepared by electrodeposition of the metals from aqueous solutions, followed by oxidation. 11,12 As discussed above, both Co and Mn have also been successfully deposited into porous scaffolds as metals. 7,8 Regarding use as SOFC cathodes, MCO-based materials have been reported to have good catalytic activities for oxygen reduction and evolution reactions. ...
... Because of this, the coulombic efficiency for Mn electrodeposition was about 30%, a value similar to that reported in a previous study of deposition in porous scaffolds, 8 but lower than that expected for flat surfaces. 11 In the present study, the electrodepositing current was fixed at 2.86 mA cm −2 by using a voltage drop across the two electrodes of 2.0 ± 0.3 V. It should be noted that Mn(OH) 2 is not formed during electrodeposition due to the low pH of the solution and the addition of (NH 4 ) 2 SO 4 as a buffer. ...
MnCo2O4(MCO)-YSZ composite electrodes for solid oxide fuel cells were prepared by alternating electrodeposition cycles of Mn and Co into a composite scaffold of YSZ and by infiltration of Mn and Co salts. Measurements with 10-wt%, 25-wt%, and 40-wt% MCO-YSZ composites showed that good conductivities, greater than 1 S cm-1, could be achieved for 25-wt% and 40-wt% loadings. Also, the ohmic resistances of symmetric cells with 25-wt% and 40-wt% MCO, produced by either electrodeposition or infiltration, were equal to the value calculated for the YSZ electrolyte. However, the non-ohmic losses in MCO-YSZ cells produced by electrodeposition were significantly lower than those produced by infiltration, probably due to the presence of catalytic amounts of Co3O4 in the electrodeposited composites, observed as an impurity phase in XRD. The addition of 5-wt% La0.8Sr0.2FeO3-δ (LSF) to a 40-wt% MCO-YSZ composite further increased electrode performance.
... Although the MneCo spinel coating suppresses both oxygen inward diffusion and Cr outward diffusion, spallation occurs at the coating/metal interface [78]. The initial coating evolution of the sequentially electrodeposited Co and Mn layer can be discussed from the complex interaction between Mn layer oxidation, formation of Mn oxides, and interdiffusion mechanism [75]. The XRD results confirmed that the metallic Mn phase is almost completely converted to Mnbased oxides (MnO and Mn 2 O 3 ) after 1 h of annealing in inert atmosphere but is not affected by oxidation time in oxidizing atmospheres. ...
... The XRD results confirmed that the metallic Mn phase is almost completely converted to Mnbased oxides (MnO and Mn 2 O 3 ) after 1 h of annealing in inert atmosphere but is not affected by oxidation time in oxidizing atmospheres. The formation of the spinel phase by the interdiffusion of Co (outward) and Mn (inward) is accelerated with increasing annealing time and temperature [75]. The spinel phase, likely (Co, Mn) 3 O 4, is formed with a composition that varies with depth, that is, higher Mn content closer to the surface. ...
With the reduction of solid oxide fuel cells (SOFCs) operating temperature to the range of 600 °C–800 °C, metallic alloy with high oxidation resistance are used to replace traditional ceramic interconnects. Metallic interconnects is advantageous over ceramic interconnects; in terms of manufacturability, cost, mechanical strength, and electrical conductivity. To date, promising candidates for metallic interconnects are all Cr-containing alloys, which are susceptible to volatile Cr migration that causes cell degradation. As such, protective coatings have been developed to effectively inhibit Cr migration; as well as maintain excellent electrical conductivity and good oxidation resistance. This article reviews the progress and technical challenges in developing metallic interconnects; different types of protective coatings and deposition techniques for metallic interconnects for intermediate-temperature SOFC applications.
... However, the weak binding force between coating and substrate leads to it being difficult for long-term operation under an SOEC stack environment. Electrodeposition can improve the adhesion of Co-Mn spinel coating and substrate, and obtained coating is dense [25,26]. However, there is a large disparity between Co 2+ /Co and Mn 2+ /Mn, leading to the co-deposition of Co 2+ and Mn 2+ being difficult to achieve. ...
In this study, CoMnO spinel was applied via atmospheric plasma spray onto 441 SS as SOEC interconnect coating. The performance of oxidation corrosion, electrical resistance, and Cr migration are evaluated. The influence rule was elucidated as the higher the plasma torch power and the thicker coating, the higher the deposition efficiency for the coated specimens. The long-term isothermal oxidation measurement was conducted under a simulated environment for 504 h. The CoMnO35 specimen had a small kp at 6.54 × 10−5 mg2 cm−4 h−1 below the CoMnO30 (7.1 × 10−5) one, and the bare steel sample (1.3 × 10−3). The area-specific resistance (ASR) depends on the temperature and time measured. The CoMnO35 specimen had a smaller Ea (0.61 eV) than the bare steel sample (0.91 eV) and CoMnO38 (0.85 eV). In addition, the CoMnO35 had a lower ASR (27.33 mΩ cm2) than the uncoated one (1.58 Ω cm2 for 670 h).
... Tabela 1: Composição química (%m/m) dos principais ligas metálicas utilizadas como interconectores [22]. Em se tratando dos revestimentos, estes podem ser obtidos pela técnica de eletrodeposição seguida de oxidação em altas temperaturas [6,[30][31][32]. O processo envolve a deposição de um revestimento ou liga metálica sobre uma superfície condutora por meio da eletrólise. ...
One of the current challenges is the search for efficient and low-cost materials for the components of Solid Oxide Fuel Cells (SOFCS), since these devices stand out for the clean generation of electric energy and high efficiency. Among these components is the interconnector, ferritic stainless steel can be used in its manufacture. However, protective coatings are necessary to block the volatility of the chrome of the metallic surface of the interconnector under the operating conditions of the PaCOS. In this work, the coatings will be obtained from electrolytic solutions containing Co²⁺ and Mn²⁺ from cathodes of exhausted lithium-ion batteries (LIB’s). Potentiostatic electrodepositions will be performed to obtain cobalt and manganese alloys on the AISI 430 steel samples surface. The charge efficiency data were calculated to optimize the samples, and the more acidic pH condition was shown to be less efficient. However, the film showed a homogeneous microstructure, less porous and composed of an internal (Mn, Cr)2O3 layer and an external MnCo2O4 layer, capable of blocking the loss of Cr, as demonstrated by the analyzes of XRD, SEM and EDS. For the less acidic pH, the presence of Mn in the deposit composition was not identified, and the coating formed by Co3O4 was not able to block the loss of chromium by volatility.
Keywords
Interconnectors; SOFC; electrodeposition. recycling; Ti6Al4V; lithium-ion batteries
... Specifically, they have of good electrical conductivity, an excellent coefficient of thermal expansion that matches that of ferritic stainless steel interconnects and other components such as the anode and cathode, and their high capability for absorbing chromium species migrating from the chromia-rich scale [5,6]. Slurry (including spraying [7], screen printing [8], and plasma spraying [9]) and electrodeposition methods [10][11][12][13] have mainly been used to prepare spinel coatings on stainless steel substrates. Although slurry coating processes are simple for producing thick coatings, the coatings are usually porous, and these techniques have limitations with respect to producing a homogenously thick layer in the case of complex shaped interconnects [5]. ...
After subsequent proper heat treatment, a Co-Mn alloy coating is potentially a protective and conductive coating on ferritic stainless steel interconnects in solid oxide fuel cells. In this paper, we use electrochemical measurements to report the effects of EDTA (disodium ethylenediaminetetraacetate) in a chloride electrolyte on the Co-Mn electrodeposition process. The addition of EDTA to the chloride electrolyte significantly shifts the reduction reaction potential of cobalt in a more negative direction while slightly altering the discharging potential of manganese and most importantly, shortening to a great extent the discharge potential difference between Co2+ and Mn2+. Also, it is crucial to control the [Co2+]/[EDTA] ratio to form completely complexed cobalt ions and a very small amount of complexed manganese ions, and thus to promote the deposition of the Co-Mn alloy. The addition of EDTA to the chloride electrolyte is a promising way to prepare a Co-Mn alloy coating via electrodeposition.
... Furthermore, it has been recently reported [2] that when Mn coating is obtained on stainless steel, and post-annealed with the flow of nitrogen, the manganese nitride coating is formed, which significantly increases the corrosion resistance of stainless steel in NaCl medium. In another example [3], the Mn and Co layers were sequentially deposited over stainless steel and annealed in oxidizing and inert atmospheres. In this manner, Co and Mn spinel phase oxides were prepared for application as coatings for interconnectors in solid oxide fuel cells. ...
Pure manganese coatings were prepared on the steel (AISI 4340) electrode by a non-conventional electrodeposition method, in the presence of 8 mol dm-3 of urea as a plating additive. The influence of urea on the electrodeposition of Mn was investigated by cyclic voltammetry. The morphology of the coat-ings was studied by scanning electron microscopy (SEM), and their elemental composition by energy dispersive X-ray spectrometry (EDS). The results showed that the presence of urea in the solution in-creased the current efficiency for metal reduction for around 20%, and depending on the applied deposi-tion potential, urea may act either as a complexing agent or through the adsorption mechanism. Moreover, urea improves the characteristics of Mn deposits, i.e. their adhesiveness, porosity, compactness, and ap-pearance. Except for oxygen, as part of the Mn corrosion product at the coating surface, no carbon or ni-trogen incorporation was detected in the deposits by EDS.
... However, the replacement of ceramic materials for metallic alloys provides a reduction of manufacturing cost [8], improved mechanical strength and coefficient of thermal expansion compatible with other parts of the cell, higher electrical and thermal conductivity, and ease of manufacturing to complex geometries [9,10]. In this context, Intermediate Temperature Solid Oxide Fuel Cells (ITSOFC), which operate at temperatures ranging from 600 1C to 800 1C allow the use of ferritic stainless steels such as AISI 430 interconnectors [4,11]. However, some issues have to be solved concerning those materials, such as the formation of a layer of chromium (Cr 2 O 3 ), the increase in electrical resistance and volatilization of chromium oxide, and irreversible damage of the system [12,13]. ...
Although oxide coatings of perovskite type deposited on ferritic stainless steel AISI 430 provide an efficient barrier to oxygen diffusion, they have shown degradation at high temperatures by formation of intermediate phases with chrome. Other coatings, such as oxide of spinel type, form an effective barrier to the diffusion of chromium; however, they do not prevent oxygen diffusion into the substrate. In this context, in order to protect against oxidation at high temperatures ferritic stainless steel AISI 430 for application as interconnects in ITSOFC, the present work aimed to develop dual layer coatings obtained by deposition of a perovskite oxide, La0.6Sr0.4CoO3, on the coating of a spinel oxide NiFe2O4. The results showed that the combination of oxide coatings increases oxidation resistance of the ferritic stainless steel and it tends to prevent degradation of the perovskite, which is caused by the diffusion of chromium from substrate to coating creating undesired phases.
... The effect of the applied voltage on the microstructure, Co/Mn ratio and ASR in (Mn,Co) 3 O 4 spinel coatings was investigated. Zhang et al. [34] reported on the microstructure, oxidation kinetics, and electrical behavior of MneCo spinel coating produced by EPD for interconnect applications in solid oxide fuel cells; some authors [35,36] reported Co and Mn spinel based produced by electrodeposition. These papers were mainly focused on the processing and characterization of the MnCo-based coatings but no consideration has been given to the compatibility of the obtained MnCo-based coated metallic interconnect with a glass-ceramic based sealant. ...
As a protective coating of the interconnects in solid oxide fuel cells, spinel-structured Cu1.35Mn1.65O4 powder was coated onto 460FC stainless steel by using the electrophoretic deposition method. A suitable amount of iodine was added to ethanol to charge the spinel powder with a high zeta potential value. Stainless steel substrates were immersed in a slurry, and a DC voltage in the range of 20–60 V was applied for 30–120 s. Because a low-temperature densification of the coated film is crucial for minimizing Cr out-diffusion from the stainless steel substrate, the coated spinel was decomposed into Cu and MnO by applying a heat treatment at 800 °C in a 5% H2/95% N2 atmosphere. Then, it was oxidized at 700 °C in air, leading to appropriate densification. The area-specific resistance of the films was 15–29 mΩ cm² after 1000 h at 700 °C in air.
With the reduction of solid oxide fuel cell (SOFC) operating temperature to the range of 600 − 800℃, Cr-containing ferritic alloys have become the preferred interconnect material, which unfortunately are susceptible to continuous scale growth and Cr volatility at the SOFC operating temperatures. The (Mn,Co)3O4 spinel system is widely regarded as the most effective coating for SOFC interconnect protection, due to its high thermal and electrical conductivity, adequate coefficient of thermal expansion, and excellent Cr blocking capability. This article reviews the physical and chemical properties of the (Mn,Co)3O4-based spinels; different types of coating precursors and deposition techniques; and the effects of spinel composition, quality and thickness on the coating performance. It is concluded that the spinel coating composition, quality, and thickness are more critical than the coating process in affecting the overall coating performance.
ENE-FARM type S equipped with solid oxide fuel cell (SOFC) stack has been commercialized since 2012. To further expand the market of ENE-FARM type S, higher electrical efficiency, higher durability, and lower cost of SOFC stack are required. A coating on a SOFC metallic interconnector is the key technology for the long-term durability of SOFC. In this research, we achieved a significant reduction in resistance and improved durability by improving the highly mass-producible electrodeposition coating method, one of the ceramic coating methods for the SOFC metallic interconnector. Furthermore, the cost of metallic interconnector has been reduced using commodity stainless steel due to its high durability. The SOFC stack of the 2016 model equipped with the electrodeposition coating on the commodity ferritic stainless steel had higher performance and durability than the 2012 model, even if the current density was increased approximately 1.5 times. The electrodeposition coating on commodity ferritic stainless steel has been commercialized and installed in the 2016 and 2020 models of ENE-FARM type S.
A protective/conductive coating can be used to limit the formation of chromia in ferritic steel interconnects in solid oxide fuel cells (SOFCs). One of the most promising candidates for restricting chromia formation is the addition of reactive elements oxides to the coating. Here in, Ni-Co-CeO2 composite coating was applied to Crofer 22 APU alloy via the electroless method. The oxidation and electrical behavior of uncoated and coated samples were evaluated at 800 °C. The coating structure was examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results showed Cr2O3-scale thicknesses, and a reduction in oxidation rates for Ni-Co-CeO2-coated samples compared to uncoated samples. Also, area specific resistance (ASR) results indicated that the formation of NiFe2O4, (Mn,Ni)2O4, Co3O4, and CoFe2O4 compositions and the presence of CeO2 reduced ASR. The ASR values of Ni-Co-CeO2 coated and uncoated Crofer 22 APU alloy after 300 h of isothermal oxidation at 800 °C was 11.8 mΩ cm2 and 35.7 mΩ cm2, respectively.
Solid oxide fuel cell (SOFC) is the modern eco-friendly technology of fuel cell power generation system. It generates electricity from a redox chemical reaction without producing hazardous gases. It consists of anode, cathode and electrolyte. It is operated in the form of stack connected by interconnects to boost-up power output. The recent development of low-temperature (600 °C–800 °C) brings an opportunity to use metallic interconnects over ceramics. Cr-based metallic interconnects are one of the prominent metallic interconnects. They offer chemical inertness, thermal stability, compatible coefficient of thermal expansion and highly dense structure. However, the Cr-migration towards the cathode side is the major problem in them which adversely affect the SOFCs performance. Therefore a good oxidation resistance without sacrificing electrical conductivity is required. To resolve this issue, several alloying elements and spinel coatings have experimented. These spinel coatings are the thin solid films of Mn, Co, Cu and rare earth metals. This review concluded that the Mn–Co based spinal coating showed excellent performance in reducing the Cr-migration in specially designed expensive Crofer 22 APU interconnect. However, the emerging low-cost ferritic interconnects also show their best results with Cu–Fe based spinel coating. Among them, the SUS-430 interconnect shows the equivalent performance of Crofer 22 APU interconnect after surface treatment and appropriate Cu–Fe based spinel coating. Therefore, it can replace the Crofer 22 APU interconnect on a cost basis.
To improve oxidation resistance, prevent Cr evaporation and maintain appropriate electrical conductivity of AISI 430 stainless steel (430 SS) as the solid oxide fuel cells' (SOFCs) interconnect, a double-layered Co-Mn-O spinel coating is fabricated successfully on 430 SS via a simple double glow plasma alloying process (DGPA) followed by heating in the air (preoxidation treatment). The double-layered Co-Mn-O spinel coating is composed of a thick MnCo2O4 spinel outlayer and a thin mutual-diffused (MnCoFe)3O4 oxide innerlayer. The isothermal and cyclic oxidation measurements are used to investigate the oxidation resistance, and the ASR test is performed to evaluate the conductivity for the coated and uncoated specimens. The coated specimen has a lower oxidation kinetics rate constant (9.0929 × 10⁻⁴ mg² cm⁻⁴ h⁻¹) than the uncoated one (1.900 × 10⁻³ mg² cm⁻⁴ h⁻¹) and the weight gain of the coated specimen (0.84 mg cm⁻²) is less than that of bare steel (1.29 mg cm⁻²) after 750 h oxidation. Meanwhile, the coated specimen holds a lower area specific resistance (0.029 Ω cm²) compared to the uncoated one (2.28 Ω cm²) after 408 h oxidation. Furthermore, the compact Co-Mn-O spinel coating can effectively impede Cr-volatilization. Additionally, the probable mechanism of the Co-Mn alloy conversion into spinel and the electronic conduction behavior in the spinel are discussed. The effects of mutual-diffused oxide innerlayer on oxidation behavior and conductivity are investigated.
In order to decrease oxide growth kinetics, maintain suitable conductivity and prevent Cr-volatilization of AISI 430 stainless steels (430 SS) as the interconnect for intermediate temperature solid oxide fuel cells (SOFCs), a CoNiO spinel oxide protective coating has been successfully fabricated on the 430 SS specimen using a simple and cheap process with two steps: 1) electroplation of CoNi alloy layer and 2) pre-oxidation treatment to convert the CoNi alloy into spinel oxide. The CoNiO spinel layer on the 430 SS (CoNiO 430 SS) is dense and uniform with 8–10 μm thickness. And the CoNiO spinel oxide protective coating consists of a main face-centered-cubic (fcc) NiCo2O4 spinel phase and a minor fcc NiO phase. Compared with bare 430 SS, the oxidation resistance and the conductivity of the CoNiO 430 SS have been improved remarkably under simulated typical SOFC operating cathode conditions (at 800 °C in air). After an isothermal oxidation test at 800 °C, the area specific resistance (ASR) of CoNiO 430 SS is much lower and stable (0.1 Ω cm² for 100 h and 0.9 Ω cm² for 600 h) than that of bare 430 SS (1.2 Ω cm² for 100 h and 2.4 Ω cm² for 600 h). These performances of CoNiO 430 SS imply that it can be a promising candidate interconnect for solid oxide fuel cell.
Because of good high temperature conductivity (Mn,Co)3O4 is a promising oxide coating for solid oxide fuel cell interconnect. High-temperature oxidation of Co-Mn alloys at 750 °C and 850 °C at 105 Pa oxygen reveals that formation of a continuous (Co,Mn)3O4 spinel layer significantly decreases oxidation rate constant but not the same for the discontinuous spinel layer. Co-40Mn fine grain alloy coatings are prepared by high-energy micro-arc alloying process (HEMAA). Microstructures and compositions after preparation and oxidation were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). When pre-oxidized under an oxygen pressure of 105 Pa at 850 °C for 10 h, Co-40Mn coatings are transformed into the external oxide which is mainly composed of (Co,Mn)3O4 and Mn3O4. The results disclose Cr outward transport from the stainless steel is significantly suppressed by the as-prepared (Co,Mn)3O4 oxide coatings, when coatings oxidized at 800 °C for 100 h in wet air. The composited spinel coatings have excellent high temperature oxidation resistance and electrical performance.
Co-40Mn and Co-10Mn alloy coatings were prepared by using high-energy micro-arc alloying process (HEMAA) and then oxidized to form oxide layers. Microstructures and compositions of Co-Mn coatings after preparation and oxidation were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Two sets of coating parameters were used for coating preparation and the effects of individual coating parameters and Co-Mn composition on coating deposition were discussed based on energy consumption and mass-transfer characteristics of different electrode materials. XRD results showed the presence of Co with dissolved Mn and Fe. Oxide scales of both coatings prepared by using set 1 parameters were composed of CoFe2O4 and hematite at 800 °C, and exhibited good adherence with the substrate and high oxidation resistance. While oxide scales of coating samples deposited by set 2 parameters were mainly (Co,Mn)3O4 and Mn3O4 for Co-40Mn coating, and (Co,Mn)3O4 and CoFe2O4 for Co-10Mn coating. Continuous spinel oxide layer was not found at 800 °C in air for all coatings. Oxide scales with high content of Co and Mn experienced breakaway when cooled to room temperature. All the Co-Mn coatings efficiently suppressed Cr outward diffusion and the values of area specific resistance (ASR) were measured to be low.
Electrolytic codeposition was employed as a low-cost alternative process to fabricate composite coatings containing Mn3O4 particles in a Co matrix for potential applications in solid oxide fuel cells. The effects of codeposition parameters on the Mn3O4 particle incorporation, cathode current efficiency, and coating uniformity were investigated using a Design of Experiments (DoE) approach. Concentration of Mn3O4 particles in the plating solution, agitation rate, current density, and solution pH were the four factors considered in the fractional factorial 2(4-1) design. With different combinations of the deposition parameters, the amount of Mn3O4 particles incorporated in the composite coatings ranged from 0 to 12vol.%. The DoE results indicate that the pH of the plating solution exhibited the greatest importance on both particle incorporation and current efficiency, which were decreased significantly below pH2. The Mn3O4 concentration in the plating bath showed the second strongest effect on particle incorporation, followed by the agitation rate. While the applied current density did not appear to affect the Mn3O4 particle incorporation, it had a strong influence on coating thickness uniformity.
Cobalt depositions from supercritical carbon dioxide (scCO2) were conducted on various surfaces including the native oxide surface of silicon wafers, tantalum nitride (TaN), carbon, and copper in a cold-wall reactor using bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II) as the precursor. Deposition onto TaN barrier layers at temperature above 300°C yielded high purity cobalt films as determined by X-ray photoelectron spectroscopy with grain sizes of 200 nm or less. The volume resistivities, of about 200 nm thick films estimated from the surface resistivities, were about 2.5 times higher than that of the literature value of pure cobalt. Cobalt films could also be deposited on both carbon and copper surfaces with morphologies that varied depending on the amount of precursor loaded. Moreover, the cobalt film protected copper surface from oxidation in solution and also improved its resistance to oxidation in air as demonstrated respectively by cyclic voltammetry and X-ray photoelectron spectroscopy depth profiles.
Ultrathin Co-Co(OH)2 composite nanoflakes have been fabricated through electrodeposition on 3D nickel foam. As electrochemical capacitor electrodes, they exhibit a high specific capacitance of 1000 F g(-1) at the scan rate of 5 mV s(-1) and 980 F g(-1) at the current density of 1 A g(-1), respectively, and the retention of capacitance is 91% after 5000 cycles.
Cathodes for solid oxide fuel cells (SOFC) should possess many properties, including high electrical conductivity, high catalytic activity for oxygen reduction, and compatibility with other cell components. The most important properties of cathodes are their catalytic activity for oxygen reduction and their compatibility with the electrolyte (including thermal expansion match and chemical non-reactivity). In addition to compatibility with the electrolyte, compatibility of the cathode with interconnect is also important. Both oxide ceramic and metallic materials are used as interconnects in SOFCs. As expected, these two types of interconnects present quite different issues in their compatibility with the cathode. In addition to compatibility with the electrolyte, compatibility of the cathode with interconnect is also important. Both oxide ceramic and metallic materials are used as interconnects in SOFCs. As expected, these two types of interconnects present quite different issues in their compatibility with the cathode. Generally, cathodes are made by powder processing routes. Cathode material powders are either made by solid state reaction of constituent oxides, or high surface area powders are precipitated from nitrate and other solutions as a gel product, which is dried, calcined and comminuted to give crystalline particles in the 1-10 μm size range.
Abstract The performance of the oxide-ion electrolyte of a solid oxide fuel cell (SOFC) is critical to the development of an intermediate-temperature system. Although yttria-stabilized zirconia is the electrolyte used in SOFCs under commercial development, other candidate materials are now available, and there remains a strong motivation to search for new, improved oxide-ion electrolytes. The leading contenders are discussed not only with respect to their oxide-ion conductivity, but also with respect to mechanical and chemical compatibility with the electrodes and the working environment at each electrode.
The positions of the oxygen ions of tetragonal Mn3O4 are determined through the Debey-Scherrer X-ray analysis, by the extension of the Bertaut's method and with least square method. The assigned correction for the anomalous scattering on Mn to Fe-Ka radiation is somewhat larger than the calculated value. The two oxygen parameters ε and δ for the space-group D194n are 0.032-0.036 and 0.008-0.010. These values correspond to the case that the tetragonal deformation comes from the octahedral sites but the tetrahedral sites resist to the deformation.
Manganese cobalt oxides are promising coating materials for reducing chromium volatilization, and thus the associated cathode poisoning, from interconnect alloys in solid oxide fuel cells (SOFCs). Interaction between this coating and the oxide scale formed on the alloy during fuel cell operation can lead to changes in the coating composition and thus its performance. In this study, the properties of manganese cobalt spinel oxides and the reaction between manganese cobalt spinel oxides and chromia were investigated. The reaction product consists of two layers: a layer in contact with chromia that grows by the diffusion of cobalt and manganese from (Mn,Co)3O4 toward the chromia and an intermediate layer that grows by the diffusion of chromium through the reaction layer. The effect of dopants on the coating performance was also investigated. With the addition of iron or titanium, the rate of reaction between the spinel coating and the chromia scale can be decreased significantly, which would reduce the risk of scale spallation and provide an increase in the lifetime of the interconnect and thus the fuel cell.
The oxidation behaviour of the uncoated ferritic Fe-22Cr steel Sanergy HT is compared with an 640 nm Co coated version of the same material. The materials have been subject to corrosion and Cr volatilization measurements in air for up to 3000 h at 850 °C. Oxidation tests have been carried out both isothermal and discontinuously. The volatilization measurements were carried out using a recently developed denuder technique, which allows to quantify Cr evaporation in a time resolved manner. The oxidation process is studied from very initial phases (>15 s) to long term behaviour (3000 h). The formed oxide scales are analysed by XRD, SEM/EDX as well as TEM/EDX.The results show that both materials form an oxide scale with an inner layer of Cr2O3 and a spinel layer on top. In the case of the uncoated material, the spinel layer is of (Cr,Mn)3O4 type while in the presence of a Co coating a (Co,Mn,Fe)3O4 is formed. The Cr evaporation measurements show that despite the fact that the Co coating is very thin (640 nm) it effectively blocks Cr evaporation for at least 3000 h. This is in line with TEM analysis showing that after 3000 h there is only a low Cr content in the outer oxide scale. This long term stability indicates the suitability of the coated material as solid oxide fuel cell (SOFC) interconnect.
The microstructural development of Mn1.5Co1.5O4-coated Crofer22 APU has been studied using cross-sectional transmission electron microscopy. Alloy samples were coated via a slurry process involving consolidation by reduction and re-oxidation, and these samples were then oxidized at 800 °C for times of up to 1000 h. All samples exhibited a thin chromia scale at the alloy/coating interface plus spinel phases as a reaction layer between the chromia and the manganese cobaltite coating. The oxidized samples also exhibited pockets of stoichiometric MnCr2O4 spinel at the chromia/alloy interface and internal Ti-rich oxides in the alloy below the chromia. The reaction layer spinels exhibit remarkable changes in thickness, morphology and composition, and these effects are explained on the basis of changes in the diffusive fluxes during the different stages of coating application and subsequent exposure. The possible consequences of these observations for the degradation mechanisms that could affect SOFC interconnects produced from MCO-coated Crofer22 APU are discussed.
A new method was developed to deposit high quality manganese coatings from MnSO4 aqueous solution without any additives (sulphur or selenium compounds). The method was by pre-electrolysis of the plating bath. Thereafter, the plating of manganese proceeded. High cathode current efficiency (71%) was obtained. Studies showed that deposition time and current density influence the current efficiency of manganese deposition.
Recent thermodynamic and electrical conductivity data are evaluated to select the most appropriate electrolyte composition for IT-SOFC operation at 500°C. Ce0.9Gd0.1O1.95 has an ionic lattice conductivity of 10−2 Scm−1 at 500°C, and the Gd3+ ion is the preferred dopant, compared to Sm3+ and Y3+, at this temperature. Thermodynamic investigations indicate that for CeO2–Re2O3 solid solutions at intermediate temperatures it becomes easier to reduce Ce4+ as the concentration of Re2O3 is increased. As the associated electron mobilities do not appear to be a strong function of composition it follows that Ce0.9Gd0.1O1.95 has a wider ionic domain than Ce0.8Gd0.2O1.9 at intermediate temperatures. Particular attention is drawn to the deleterious effects of impurities (principally SiO2) which are responsible for large dopant concentration dependent grain boundary resistivities. These grain boundary resistivities can obscure the intrinsic lattice ionic conductivities and cause investigators to select non-optimal dopant compositions. It follows that the use of clean (SiO2
Samarium-doped Ceria powders for solid electrolyte ceramics were synthesized by a combustion process. Cerium nitrate and samarium
nitrate were used as the starting materials, and glycine was used as fuel. Decomposition of unburned nitrogen and carbon residues
was investigated by simultaneous thermogravimetry analysis and differential thermal analysis experiments. The X-ray diffraction
results showed that the single-phase fluorite structure forms at a relatively low calcination temperature of 800 �C. X-rays
patterns of the SDC powders revealed that the crystallite size of the powders increases with increasing calcination temperature.
The sintering behavior results showed that more than 96% of the relative density is obtained for powders sintered at 1,100 �C
for 8 h. The alternating current impedance spectroscopy results showed that the SDC15 sample sintered at 1,100 �C has ionic
conductivity of 0.015 Scm−1at 650 �C in air. The present work results have indicated that glycine–nitrate route is a relatively low-temperature preparation
technique to synthesize SDC powders with a high sinterability and a good ionic conductivity.
Experimental data on the thermodynamics and the phase diagram of the Mn-O system were reviewed, and by application of the CALPHAD method, a consistent set of thermodynamic model parameters was optimized. The phases pyrolusite (MnO2), bixbyite (Mn2O3), and hausmannite (Mn3O4) were described as stoichiometric compounds. Manganosite (Mn1-xO) was described using the compound-energy model and the liquid described using the two-sublattice model for ionic liquids.
Binary oxide spinels composed of Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, and Zn were synthesized. Electrical conductivity was measured in air at 500°–800°C. Thermal expansion was measured from room temperature to 1000°C. Ferrite spinels have thermal expansion coefficients of 11–12 ppm/K, compared with 7–9 ppm/K for other spinels except Cu–Mn and Co–Mn which show anomalous behavior. The highest electrical conductivity among transition metal spinels was found for MnCo2O4 (60 S/cm at 800°C) and Cu1.3Mn1.7O4 (225 S/cm at 750°C).
High conductivity coatings that resist oxide scale growth and reduce chromium evaporation are needed to make stainless steel interconnect materials viable for long-term stable operation of solid oxide fuel cells (SOFC). Mn1.5Co1.5O4 spinel is one of the most promising coatings for interconnect application because of its high conductivity, good chromium retention capability, as well as good CTE match to ferritic stainless steels. Mn–Co electrodeposition followed by oxidization is potentially a low cost method for fabrication of (Mn,Co)3O4 spinel coatings. This work looks at the co-deposition of Mn–Co alloys for this application. As a guide to optimize the deposition process, characterizations of the cathodic reactions and reaction potentials are done using polarization curves. It was found that as cobalt concentration was varied that the alloy composition became richer in cobalt, indicating that the deposition is regular co-deposition process. It was also found that at 0.05 M Co concentration in excess gluconate the Mn–Co alloys composition could be tuned by varying the current density. Coatings with Mn–Co around 1:1 could be obtained at a current density of 250 mA/cm2. However, the higher potential increased hydrogen production making the films more porous. Oxidation of the alloy coatings showed that much of the porosities could be eliminated during oxidation. It was found in a number of samples that fully dense coatings where obtained. The composition of the oxidized coating was found to become enriched in Mn, possibly due to the Mn fast diffusion from the substrate.
The mechanism of manganese electrodeposition from a sulphate bath on to a stainless-steel substrate has been studied by using current efficiency data to resolve the totali-E curves. A simple, two-step electron transfer mechanism:
\textMn\text + + + \texte\xrightarrow\textr\text.d\text.s\textMn\text + {\text{Mn}}^{{\text{ + + }}} + {\text{e}}\xrightarrow{{{\text{r}}{\text{.d}}{\text{.s}}}}{\text{Mn}}^{\text{ + }}
\textMn\text + + \texte ® \textMn{\text{Mn}}^{\text{ + }} + {\text{e}} \to {\text{Mn}}
is proposed to explain the following experimentally obtained parameters: cathodic and anodic transfer coefficients, reaction order and stoichiometric number. The mechanism also explains the effect of pH oni
o,Mn and on the corrosion currents.
One of the challenges in improving the performance and cost-effectiveness of solid oxide fuel cells (SOFCs) is the development of suitable interconnect materials. The interconnect material is in contact with both the anode and the cathode, and thus must be stable with both electrode materials and in oxidizing and reducing environments. The interconnect material must also maintain a low electrical resistance during cell operation to avoid decreased efficiency due to ohmic losses. The common feature of the two approaches (metallic and ceramic) to the development of interconnect materials is the presence of chromium. The most promising ceramic materials are chromites, while the most promising metallic materials are chromia-forming alloys. The focus of this paper is comparison of metallic alloys for use as interconnects in solid oxide fuel cells, in terms of properties including oxidation resistance, volatility, electrical resistance and thermal expansion.
At the usual temperature of solid oxide fuel cell (SOFC) operation, ferritic stainless steels form electrically insulating or poorly conducting oxide scales, which can cause high internal resistance losses and chromium poisoning. In an effort to avoid this problem, we applied conductive copper manganite and cobalt manganite spinel coatings, with nominal composition MnCo2O4 and Cu1.4Mn1.6O4, which were deposited on the surface of UNS 430 stainless steel by electroplating and subsequent air annealing. Microstructural evaluation indicated that the spinel layers inhibited outward diffusion of chromium. Moreover, excellent structural and thermal stability were observed after several thermal cycles at 750 °C and for up to 28 days, and the coating layers showed good adhesion to the substrate.
To add a coating on a metallic interconnect is one option to prevent Cr poisoning of the cathode and to retain high conductivity during solid oxide fuel cells (SOFC) operation. Electroplating of metals or alloys followed by oxidation offers a cost-effective method. In this study, pure Co and Mn/Co alloys formed by electrodeposition are used to protect the substrate, SUS 430. On-cell tests, using uncoated, cobalt-coated and MnCo-coated interconnects were conducted at 375 mA cm−2 for 323, 500 and 820 h, respectively. The results show that cell power degrades at a rate of 33% in 320 h using an uncoated interconnect. Significant improvements are obtained for cell tests utilizing unoptimized coated interconnects with the degradation rate of 5% and 9% per 1000 h for cobalt and MnCo coatings, respectively. Based on the results from SEM and XRD studies, the advantages of both coatings are to successfully inhibit Cr diffusion to the scale surface. However, thin (∼2 μm) cobalt coating allows fast scale growth, while thicker cobalt coatings have the potential to fail due to mismatch in the coefficient of temperature expansion (CTE) between Co3O4 and the SUS 430 substrate. In spite of higher degradation rate for the MnCo coatings evaluated here, the addition of Mn into the cobalt coating not only aids in suppression of scale growth, but also reduces the CTE mismatch. Furthermore, no performance decay after two thermal cycles was observed. Finally, the cell degradation was observed to have a correlation with the cell cathode interlayer microstructure.
Rapidly decreasing electronic conductivity, chromium volatility and poisoning of the cathode material are the major problems associated with inevitable growth of chromia on ferritic stainless steel interconnects of solid oxide fuel cells (SOFC). This work evaluates the performance of a novel, electrodeposited composite Co/LaCrO3 coating for AISI 430 stainless steel. The oxidation behaviour of the Co/LaCrO3-coated AISI 430 substrates is studied in terms of scale microstructure and growth kinetics. Area-specific resistance (ASR) of the coated substrates has also been tested. The results showed that the Co/LaCrO3 coating forms a triple-layer scale consisting of a chromia-rich subscale, a Co–Fe spinel mid-layer and a Co3O4 spinel top layer at 800 °C in air. This scale is protective, acts as an effective barrier against chromium migration into the outer oxide layer and exhibits a low, stable ASR of ∼0.02 Ω cm2 after 900 h at 800 °C in air.
Ferritic stainless steels have become the standard material for solid oxide fuel cell (SOFC) interconnect applications. The use of commercially available ferritic stainless steels, not specifically designed for interconnect application, however, presents serious issues leading to premature degradation of the fuel cell stack, particularly on the cathode side. These problems include rapidly increasing contact resistance and volatilization of Cr from the oxide scales, resulting in cathode chromium poisoning and cell malfunction. To overcome these issues, a variety of conductive/protective coatings, surface treatments and modifications as well as alloy development have been suggested and studied over the past several years. This paper critically reviews the attempts performed thus far to mitigate the issues associated with the use of ferritic stainless steels on the cathode side. Different approaches are categorized and summarized and examples for each case are provided. Finally, directions and recommendations for the future studies are presented.
min (left) and 1 h (right) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article
- Fig
Fig. 6. X-ray diffraction results and phase identification of samples annealed in N 2 : 10 min (left) and 1 h (right). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
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X-ray diffraction results and phase identification of samples annealed in N 2 : 10 min (left) and 1 h (right)
- Fig
Fig. 6. X-ray diffraction results and phase identification of samples annealed in N 2 : 10 min (left) and 1 h (right). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
- A Petric
- H Ling
A. Petric, H. Ling, J. Am. Ceram. Soc. 90 (2007) 1515e1520.