Mn-Co Spinel Research Articles

Improving the stability of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ/Crofer 22 H glass-sealed joints in humid oxidizing environments with a Mn-Co spinel coating

Sealing the BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) electrolyte to stainless steel interconnect is necessary for proton ceramic fuel cell stack packaging. However, oxidation of the interconnect in the glass-sealed joint presents a significant challenge, especially in humid and oxidizing environments. In this work, a Mn-Co spinel coating was fabricated on Crofer 22 H using microwave heating to extend its service life. The coated Crofer 22 H was successfully sealed to BZCYYb by a SiO2-TiO2-K2O-Na2O-Al2O3-ZnO glass sealant, achieving a sufficient joining. The joint is defect-free and dense, with a shear strength of 20 MPa. Detailed analysis using transmission electron microscopy revealed the underlying sealing mechanism. The Mn-Co spinel coating not only enhances the corrosion resistance of Crofer 22 H but also significantly improves the service life of the joint without compromising strength. The joint remains robust and defect-free after aging in a humid oxidizing atmosphere at 600 °C for 300 h.

A bimetallic MOF-derived MnCo spinel oxide catalyst to enhance toluene catalytic degradation.

MnCo spinel oxide catalysts were successfully synthesized by the calcination of bimetallic Mn/Co-MOFs as sacrificial templates. The derived catalysts exhibited optimal catalytic activity, reusability and thermal stability for toluene oxidation, which was ascribed to their large specific surface area, higher number of octahedral metal ions and the weakest metal-oxygen bonds.

Oxidation behaviour of Mn-Co spinel coating on AISI 430 ferritic stainless steel with and without Cu in Ar-CO2-H2O atmosphere

AISI 430 ferritic stainless steel is a promising candidate for utilising as interconnects of solid oxide fuel cells due to its cost effectiveness and durability. Many methods for applying coating on steel substrates have been developed in order to decrease the degradation of steel due to oxidation rate and chromium volatile problems. Manganese-cobalt spinel exhibits high conductivity, thermal expansion compatible with ferritic stainless steels, and forms a barrier to inhibit chromium migration during oxidation. Copper can be added to manganese-cobalt spinel to improve electrical conductivity of the spinel coating. This work investigated oxide scale formation and oxidation rate of Mn-Co and Mn-Co-Cu coated samples in comparison with uncoated steel. The coated samples were prepared on the AISI 430 ferritic stainless steel using the electrodeposition technique. The oxidation rate was tested at 800℃ in Ar-20% CO2-5% H2O for 96 h. The results showed that both Mn-Co and Mn-Co-Cu coated samples could be formed continuous oxide layers. The SEM image showed a chromium oxide layer under the manganese-cobalt coating layer. The oxidation rate of the samples coated with Mn-Co spinel and Mn-Co-Cu spinel was lower than that of the uncoated steel.

Achieving high-temperature corrosion resistance and conductivity of SUS430 by xCr-MnCo dual-structured coating

The xCr-MnCo dual-structured coating was proposed on SUS430 steel to achieve high corrosion resistance and conductivity. The results show that Cr barrier layer (x) thickness significantly affect coating performance. Slightly increasing the Cr2O3 thickness will not only enhance the coating corrosion resistance (Kp =2.23 × 10−14 g2·cm−4 s−1) but also improve the conductivity by allowing moderate amount of Fe diffuse into the Mn-Co spinel. However, further increasing the Cr2O3 thickness will increase coating spallation risks due to the thermal expansion coefficients mismatch between Cr2O3, SiO2 and substrate. Finally, 0.03Cr-MnCo coating achieved an optimized balance on corrosion resistance and conductivity.

Exploring the effects of Mn content in the Mn-Co spinel coating on its formation and slowing the outward Cr diffusion

Effects of Mn content on the performance of Mn-Co spinel coatings are clarified systematically. Two dense spinel coatings are fabricated on Crofer 22 H by microwave sintering and subsequent oxidation. After oxidation at 800 ℃ in air for 400 h, the thickness of the Cr-containing layer of MnCo2O4-coated sample is significantly lower than that of (Mn, Co)3O4-coated sample. The growth mechanism of Cr-containing layer is elucidated from aspects of stabilizing energy and configurational entropy. Controlling Mn3+ content reduces the outward diffusion channels for Cr and avoids the excessive growth of the Cr-containing layer, achieving a lower resistance as well.

Role of vanadium ions substitution on spinel MnCo2O4 towards enhanced electrocatalytic activity for hydrogen generation

Improving efficient electrocatalysts (ECs) for hydrogen generation through water splitting is of significant interest in tackling the upcoming energy crisis. Sustainable hydrogen generation is the primary prerequisite to realizing the future hydrogen economy. This work examines the electrocatalytic activity of hydrothermally prepared vanadium doped MnCo spinel oxide microspheres (MC), MnVxCo2−xO4 (Vx-MnCo MC, where x ≤ 0.4) in the HER (hydrogen evolution reaction) process. Magnetization measurements demonstrated a paramagnetic (at high temperatures) to a ferrimagnetic (at low temperatures) transition below the Curie temperature (Tc) in all the samples. The magnetization is found to intensify with the rising vanadium content of MCs. The optimized catalyst Vx-MnCo MCs (x = 0.3) outperformed other prepared ECs with a Tafel slope of 84 mV/dec, a low onset potential of 78.9 mV, and a low overpotential of 85.9 mV at a current density of 10 mA/cm2, respectively. The significantly improved HER performance of hydrothermally synthesized Vx-MnCo MCs (x = 0.3) is principally attributable to many exposed active sites, accelerated electron transport at the EC/electrolyte interface, and remarkable electron spectroscopy for chemical analysis (ECSA) value was found ~ 11.4 cm2. Moreover, the Vx-MnCo MCs (x = 0.3) electrode exhibited outstanding electrocatalytic stability after exposure to 1000 cyclic voltametric cycles and 36 h of chronoamperometric testing. Our results suggest a feasible route for developing earth-abundant transition metal oxide-based EC as a superior electrode for future water electrolysis applications.

Oxidation and Cr-evaporation behavior of MnCo based spinel and composite coated AISI 430 steel

This work investigates the importance of the chemical composition and porosity of the coating on reducing the oxidation and Cr-evaporation rate of AISI 430 steel in O2-5%H2O at 800 °C for solid oxide fuel cell (SOFC) interconnect applications. Electrophoretic deposition (EPD) is used to deposit eight different coatings. Mn1.25−0.5xCo1.75−0.5xMxO4 (M = Fe, Cu, and Fe-Cu; and X = 0 and ~0.3) powders are synthesized by the glycine nitrate process (GNP) and applied as spinel (without La2O3) or composite (with La2O3) coatings. The coatings initially show porous structure, and the coating density improvement depends on coating composition. The coating porosity for composite co-doped Fe-Cu MnCo spinel (MCFeCuL) is 28% less than MnCo spinel (MC) after 504 h. All coated samples show lower oxidation and Cr-evaporation rates than uncoated samples. The parabolic oxidation rates are lower for composite coated than the spinel ones, possibly due to the formation of LaCrO3 reducing Cr outward diffusion. Composite MnCo-La2O3 (MCL) shows the lowest oxidation rate. In contrast, coating effectiveness in reducing Cr-evaporation depends on coating density. Cu-doped coatings show less coating porosity with better oxidation and Cr-evaporation behavior than Fe-doped coatings. All coated samples form a densified layer at the thermally grown oxide (TGO) scale/coating interface, playing a crucial role in oxidation resistance and migration of Cr from the TGO scale to the coating. The changes in Cr-evaporation fluxes depending on coating porosity and pore size are calculated and compared with Cr-loss rate measurements. Among the coatings, composite Cu-doped MnCo spinel (MCCuL) and MCFeCuL show better performance than the other ones. This study shows that it is possible to decrease coating porosity and improve the performance of MnCo spinel coatings against corrosion and Cr-evaporation without two-step sintering (at T ≤ 900 °C) procedures.

Synthesis of Anti-poisoning Spinel Mn-Co-C as Cathode Catalysts for Low-Temperature Anion Exchange Membrane Direct Ammonia Fuel Cells.

Low-temperature anion exchange membrane direct ammonia fuel cells (AEM-DAFCs) have emerged as a potential power source for transportation applications with the recognition that liquid ammonia is a carbon-free hydrogen carrier and facilitates storage, refill, and distribution. However, ammonia crossover from the cell anode to cathode can decrease the fuel efficiency, drop the voltage, and poison the cathode catalysts. In this work, the Mn-Co spinel on three different carbon supports [BP2000, Vulcan XC-72R, and multiwalled carbon nanotubes (MWCNTs)] has been successfully synthesized and demonstrated a high oxygen reduction reaction (ORR) activity with good ammonia tolerance. The structure and composition of the obtained Mn-Co-C catalysts were characterized by high-angle annular dark-field scanning transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All three catalysts exhibit superb ammonia tolerance, and Mn-Co-BP2000 demonstrates the highest ORR activity, even better than the commercial Pt-C in the presence of ammonia. When paired with the commercial PtIr-C anode, the Mn-Co-BP2000 cathode improved the peak power density of single cells from 100.1 mW cm-2 for the Pt-C cathode to 128.2 mW cm-2 under a 2 bar backpressure in both electrodes at 80 °C. All the results have manifested that Mn-Co-BP2000 is a good cathode catalyst for low-temperature AEM-DAFCs.

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. CuδMn1.5−xCo1.5−yO4 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 MnCo2O4/CuxMn3−xO4 (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 Mn2+ and Co3+ was found to decrease when the sintering duration increased, which was attributed to the decomposition of the MnCo2O4 phase. Finally, coatings were prepared by atmospheric plasma spraying with doped spinel powders and the raw powder Mn1.5Co1.5O4. 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.

TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

Steel interconnect robustness under SOFC conditions for extended operation remains a challenge. The observation and analysis of the interconnect microstructure aged in real conditions for long periods provides analysis data to help to understand degradation and potentially to design accelerated testing.In this study, TEM observations of the cross section of interconnects aged in real conditions (9000h and 20000h stacks) are presented. The interconnects are Crofer22APU with the air side coated with manganese cobalt oxide (MCO) to prevent chromium evaporation and increase oxidation resistance. FIB-prepared lamella from the inlet and outlet air side are analysed by TEM imaging and EDX. A major difference between inlet and outlet is the position of Mn-rich grains in the chromia scale: at the inlet, Mn-rich grains are located at the metal/chromia interface (figure 1(a)) while at the outlet, Mn-rich crystals are located close but not directly adjacent to the metallic bulk (figure 1(b)). The inlet microstructure has been observed in the literature in an oxidation study of MCO coated steel [1]. However, the outlet case which was observed for both the 9000h and 20000h aged stack samples was not yet reported in the literature, to the best of our knowledge. The difference between the inlet and outlet position was observed as well on a 15000h aged sample by SEM, and may be explained by differences between the inlet and outlet conditions: higher temperature at the outlet, as well as lower oxygen content. In addition to this particular Mn position, Fe particles are also observed in the chromia scale, confirming results previously described in the literature[2]. The analysis of the MCO coating also shows an inner denser layer, as expected after ageing at high temperature[3]. Analysis of the segregation of cobalt oxide at the MCO grain boundaries is also presented.More investigations are necessary to explain the complete oxidation mechanisms of the MCO-coated Crofer interconnect. However, the important role of the coating in interconnect oxidation is shown. The understanding of all reactions and the analysis of interconnects aged in real conditions, on different locations, should help designing more reliable combinations, better predictive models and possibly accelerated tests.[1] S. Molin et al., « Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects », J. Eur. Ceram. Soc., vol. 37, no 15, p. 4781-4791, déc. 2017, doi: 10.1016/j.jeurceramsoc.2017.07.011.[2] L. V. Gambino, N. J. Magdefrau, et M. Aindow, « Microstructural effects of the reduction step in reactive consolidation of manganese cobaltite coatings on Crofer 22 APU », Mater. High Temp., vol. 32, no 1-2, p. 142-147, janv. 2015, doi: 10.1179/0960340914Z.00000000090.[3] B. Talic, V. Venkatachalam, P. V. Hendriksen, et R. Kiebach, « Comparison of MnCo2O4 coated Crofer 22 H, 441, 430 as interconnects for intermediate-temperature solid oxide fuel cell stacks », J. Alloys Compd., vol. 821, p. 153229, avr. 2020, doi: 10.1016/j.jallcom.2019.153229. Figure 1

Preparation, Evaluation, and Application of SUS430/441 Interconnect with Mn-Co Coating in Solid Oxide Fuel Cells

Mn-Co spinel is one of the most promising coatings for ferrite stainless steel interconnects. This study combines ex-situ and in-situ experiments, evaluating corrosion behavior and electrical performance of SUS430/441 interconnects with Mn-Co spinel coatings prepared by physical vapor deposition (PVD) in air. The ex-situ area-specific resistance (ASR) test indicated that the ASR growth rate of the coated sample at 750℃ was 0.37%/kh, suggesting good corrosion resistance. Under actual operating conditions, the stack with a Mn-Co spinel coating and heat treatment had the lowest ohmic resistance growth rate and performance degradation rate. Thus, a Mn-Co spinel coating applied to the cathode side of the interconnect by PVD can efficiently improve stack stability. Meanwhile, heat treatment at the initial operation of stacks is beneficial to the formation of the Mn-Co coating and the elimination of Cr diffusion.

Oxidation and volatilisation behaviour of a type 430 stainless steel coated by Mn-Co oxide by slurry method with pre-oxidation for SOFC interconnect application

An AISI 430 stainless steel was coated by Mn-Co spinel using a slurry method. Pre-oxidation before the coating helped reduce the oxidation rate of the coated steel at 800 °C in O2-5%H2O, relating to the formation of the continuous chromia layer which inhibited the outward diffusion of iron to the coating layer. It also helped reduce the mass flux of Cr loss due to the volatilisation. The combined molecular and Knudsen diffusion of the volatile species through the coating layer was suggested to explain the reduced volatilisation rate of the coated sample with pre-oxidation.

Synthesis and characterization of Ag-Y co-doped Mn-Co spinel for solid oxide fuel cell application

Mn-Co spinel coating for solid oxide fuel cell interconnects shows a suitable thermal expansion match, acceptable electrical conductivity, and good structural stability. In this paper, the addition of different amounts of Ag and Y as dopants on the physical and electrical properties of the Mn-Co spinel is investigated to improve the coating performance. First, the doped and co-doped powders were successfully synthesized using the Pechini sol-gel method. The synthesized powders were then characterized using X-ray diffraction and field emission scanning electron microscopy. The results confirmed that Ag was not completely doped into the spinel structure and instead acted. as an additive, whereas Y caused Mn3O4 impurity phase formation at higher mol%. Next, the powders were pressed and sintered at different temperatures (950 and 1050 °C) to evaluate the effect of dopants on the sintering and electrical behavior of the samples. Finally, the electrical conductivity of the samples was evaluated using a 2-probe direct current technique. Although results showed that room temperature electrical conductivity increased upon doping, adding Y had a better effect on conductivity than Ag.

Mn-Co spinel coatings on Crofer 22 APU by electrophoretic deposition: Up scaling, performance in SOFC stack at 850 °C and compositional modifications

Ceramic coatings for metallic interconnects play a key role in limiting corrosion and chromium evaporation in solid oxide cells. This study presents the upscaling of the electrophoretic deposition (EPD) technique to process Mn-Co spinels on real-dimension Crofer 22 APU interconnects and the test in a SOFC stack. Area specific resistance of long-term test conducted for 5000 h at 850 °C demonstrated that two-steps sintering has a significant influence on the coating performance; an area specific resistance degradation rate of 0.5 mΩ cm2 kh−1 is recorded. Stack test, operated in fuel cell mode at 850 °C for 3000 h under application of 227 mA/cm², including 5 thermal cycles, demonstrated the effectiveness of the electrophoretically deposited Mn-Co spinel in limiting the oxide scale growth on the Crofer 22 APU. An advanced post mortem investigation showed the effectiveness of the EPD ceramic coating, even when considering different and complex surfaces of the Crofer 22 APU.

Pressure assisted flash sintering of Mn-Co based spinel coatings for solid oxide electrolysis cells (SOECs)

Pressure assisted flash sintering was used to process Mn-Co-Cu based spinel coatings, electrophoretically deposited on a Crofer22APU interconnect. This method resulted in highly dense coatings, heat-treated for only a short duration (<10 min) at temperatures lower than conventional sintering temperatures. The electrophoretically deposited coatings showed no delamination or cracks after flash sintering at high heating rates (>200 °C/min). The high heating rate promoted Cu modified Mn-Co spinel and limited the formation of a Cr-oxide scale on the Crofer22APU substrate. Flash sintering was found to be a promising and time efficient sintering technique to overcome some of the issues related to low coating density and oxide scale formation in solid oxide electrolysis cell conditions.

(Invited) Oxygen Electrocatalysis By Transition Metal Spinel Oxides

Exploring efficient and low-cost oxygen electrocatalysts for ORR and OER is critical for developing renewable energy technologies like fuel cells, metal-air batteries, and water electrolyzers. This presentation will present a systematic study on oxygen electrocatalysis (ORR and OER) of transition metal spinel oxides. Starting with a model system of Mn-Co spinel, the presentation will introduce the correlation of oxygen catalytic activities of these oxides and their intrinsic chemical properties. The catalytic activity was measured by rotating disk technique and the intrinsic chemical properties were probed by synchrotron X-ray absorption techniques. It was found that molecular orbital theory is able to well-explain their activities. The attention was further extended from cubic Mn-Co spinels to tetragonal Mn-Co spinels and it was found that the molecular theory is again dominant in determining the catalytic activities. This mechanistic principle is further applied to explain the ORR/OER activities of other spinels containing other transition metals (Fe, Ni, Zn, Li, etc.). The talk further gives insight on surface reconstruction on spinel oxides and how the bulk properties affect such reconstruction during OER. In addition, the talk will also discuss the possibility of the spin-selected charge transfer during oxygen electrocatalysis through transition metal oxides

Obtaining Mn-Co Alloys in AISI 430 Steel from Lithium-Ion Battery Recycling: Application in SOFC Interconnectors

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.

Effect of Off-Stoichiometry on Electrical Conductivity in Ni-Fe and Mn-Co Spinel Systems

This study seeks to clarify the effects of off-stoichiometry on the electrical conductivity of spinels in the (Ni,Fe)3O4 and (Mn,Co)3O4 systems. Spinels with the compositions NixFe3–xO4 (where x = 0.85, 0.875, 0.9, 1.0, 1.05, and 1.15) and MnxCo3–xO4 (where x = 1.0, 1.2, 1.35, 1.5, 1.75, and 2.0) were prepared using conventional solid-state process. Electrical conductivity measurements were taken between 500 °C and 800 °C using the 4-point DC method. The conductivity in Ni-Fe spinels increased with increasing Fe content. Similarly, the conductivity of Mn-Co spinels increased drastically with increasing Co content. The coefficients of thermal expansion (CTEs) of these samples were also evaluated. All the (Ni,Fe)3O4 spinels were found to exhibit excellent CTE matching with prospective stack components. However, thermal expansion characteristics were highly dependent on the composition in the (Mn,Co)3O4 system, and only a narrow range of compositions was well-matched in CTE with prospective stack components. The significant effect of cation off-stoichiometry and sample preparation on the electrical behavior of spinels was discussed as a possible explanation for the large discrepancies in reported conductivity values for both systems.

Promoting toluene oxidation by engineering octahedral units via oriented insertion of Cu ions in the tetrahedral sites of MnCo spinel oxide catalysts.

Cu ions were deliberately introduced into the tetrahedral sites of MnCo spinel oxides by pyrolysis of Prussian blue analogues pre-doped with Cu. The as-synthesized catalysts deliver superior catalytic activity for toluene oxidation and excellent stability owing to the increase of octahedral metal ions and elongation of metal-oxygen bonds induced by the specific substitution of Cu.

Performance and In-Situ Densification of Mn-Co Spinel Coating on SUS430/441 Interconnect by Spraying Method

The oxidation of interconnect in the SOFC stacks results in the degradation of cell performance, and thus the protective coating is needed. In this work, manganese oxide and cobalt oxide with molar ratio of 1:1 were deposited on the interconnect surface by spraying method to form the protective coating, whilst the interconnect materials were SUS430 and SUS441. First, the coated interconnects were heated at 800 oC in air for 1000 h. The results suggest that the two oxides were converted to not dense Mn-Cr spinel, yet the weight-gain and surface chromium content of the coated interconnects were not efficiently inhibited. Next, the coated interconnects were pretreated in 5 % H2/95 % N2 atmosphere for 10 h at 800 oC and the coatings were reduced into MnO and metallic Co. The diffusion sintering of metallic Co during the subsequent oxidation treatment facilitated the formation of dense Mn-Co-Cr spinel, resulting in excellent oxidation resistance, where the oxidation rate constant was an order of magnitude lower and surface chromium content was remarkably reduced. In addition, the phenomena described above for SUS430 and SUS441 were similar. As a result, the low-cost coating by spraying method with H2 pretreated was efficient, which significantly enhanced the oxidation resistance of SUS430 and SUS441 due to the formation of dense spinel.