Simulated Solid Oxide Fuel Cell Research Articles

The preparation and properties of Mn–Co–O spinel coating for SOFC metallic interconnect

To meet the performance requirements of solid oxide fuel cell (SOFC) metallic interconnect, the Mn–Co–O spinel coating is prepared on the surface of AISI430 by pack cementation method to reduce the growth kinetics of oxides and inhibit the outward diffusion of Cr. The microstructural characterization shows that a dense, uniform, defect-free spinel coating is successfully fabricated on the surface of AISI430. Under the simulated SOFC cathode environment, the weight gain of coated steel (0.608 mg cm−2) after oxidation at 800 °C for 800 h is significantly lower than that of uncoated (1.586 mg cm−2). In addition, the area specific resistance (ASR) of the coated steel oxidized for 500 h is 17.69 mΩ cm2, much smaller than that of the bare steel, indicating that the oxidation resistance and electrical conductivity of AISI430 are significantly improved by Mn–Co–O spinel coating. Cross-sectional observations of the Mn–Co–O spinel coating are conducted to assess the compatibility of substrate with the adjacent coating and its effectiveness in reducing the growth of the Cr2O3 layer.

11–23% Cr steels for solid oxide fuel cell interconnect applications at 800 °C – How the coating determines oxidation kinetics

The present work investigates the low-cost steels AISI 441, AISI 430, and AISI 444 against the tailor-made high Cr steel Crofer 22 APU (22.9 wt% Cr) at 800 °C in simulated solid oxide fuel cell (SOFC) cathode conditions. Furthermore, a low Cr steel, AISI 409 (11.4 wt% Cr) is included in the study. The oxidation, chromium evaporation, and area-specific resistance (ASR) of the uncoated and Ce/Co-coated steels are studied for up to 3000 h. Ce/Co-coated steels showed significant improvement in behaviour compared to their uncoated counterparts. The oxidation and chromium evaporation behaviour between the uncoated steels varied substantially while the Ce/Co coated steels exhibited highly similar behaviour. The area-specific resistance of the coated low-cost steels was on par with Crofer 22 APU. However, 430 formed a continuous silica layer, resulting in a higher ASR after 3000 h. Cross-sections of the uncoated and Ce/Co-coated steels were analysed using a scanning electron microscope and energy dispersive X-ray spectroscopy.

Modelling of solid oxide fuel cells with internal glycerol steam reforming

The direct application of glycerol in solid oxide fuel cell (SOFC) for power generation has been demonstrated experimentally but the detailed mechanisms are not well understood due to the lack of comprehensive modeling study. In this paper, a numerical model is developed to study the glycerol-fueled SOFC. After model validation, the simulated SOFC demonstrates a performance of 7827 A m−2 at 0.6 V, with a glycerol conversion rate of 49% at 1073 K. Then, parametric analyses are conducted to understand the effects of operation conditions on cell performance. It is found that the SOFC performance increases with decreasing operating voltage or increasing inlet temperature. However, increasing either the fuel flow rate or steam to glycerol ratio could decrease the cell performance. It is also interesting to find out that the contribution of H2 and CO to the total current density is significantly different under various operating conditions, even sometimes CO dominates while H2 plays a negative role. This is different from our conventional understanding that usually H2 contributes more significantly to current generation. In addition, cooling measures are needed to ensure the long-term stability of the cell when operating at a high current density.

Thermal conversion process of CuFe2O4 coating on steel interconnect for solid oxide fuel cell applications

This study reports the thermal conversion process of CuFe2O4 coating as a protective layer on chromia-forming steels. A CuFe1.8 alloy layer is obtained on steel substrate via magnetron sputtering technology, and then sample is exposed to a simulated solid oxide fuel cell (SOFC) cathode atmosphere (in air at 800 °C) for up to 60 min. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal that Fe is preferentially oxidized (Fe2O3 externally and Fe3O4 internally). Then, CuO is formed atop Fe2O3, while the CuFeO2 grows within the internal oxidation zone. The CuFe2O4 spinel layer further grows in different ways with increasing time. The Cr2O3 layer forms at the steel/converted oxide layers interface after the alloy layer disappears.

Noise suppressed and bias field corrected image segmentation method for porous Ni-YSZ anode microstructure

Accurate three-phase identification from Solid Oxide Fuel Cell (SOFC) anode micrograph is challenged by both noise and intensity inhomogeneity. In this paper, a novel framework is proposed for porous Ni-YSZ cermet anode Optical Microscopy (OM) image segmentation. The proposed framework takes advantage of a statistical model in which an observed image is decomposed into two multiplicative components (bias field and true image) and one additive component (noise). A two-stage Principal Component Analysis with Local Pixel Grouping (LPG-PCA) denoising algorithm is firstly performed to suppress additive noise, it can preserve more image local structural features by modeling a pixel and its nearest neighbors as a vector variable and selecting training samples with similar contents to this variable in a local window for PCA transformation. In order to enhance the robustness to noise, uneven illumination and other outliers, a kernel metric is introduced into fuzzy clustering method embedded with bias field correction for image segmentation. The proposed method has been compared to other state-of-the-art segmentation algorithms on both simulated images and real SOFC anode OM images. Extensive experiments results have demonstrated that the proposed framework can successfully eliminate the influence of both uneven illumination and noise on real Ni/YSZ anode OM images to obtain a better three-phase identification accuracy. The high-quality segmentation results lay firm foundation for accurate microstructural parameter extraction.

Three phases segmentation from Ni/YSZ anode optical microscopy images using quantum-inspired mixture clustering model

Three-phase segmentation of solid oxide fuel cell (SOFC) anode image is essential for its microstructure quantitative analysis. A quantum-inspired mixture clustering model is developed for Ni/YSZ anode optical microscopy (OM) images. The proposed mixture clustering model focuses on combining Markov random field (MRF)-based fuzzy logic model with Gaussian mixture model (GMM). In the premise part of fuzzy if-then rules, the clique potential MRF function is defined by multiplication of both prior distributions and fuzzy membership functions with a quantum-inspired adaptive fuzzy degree, which takes prior statistics properties and spatial contextual information of SOFC anode OM image into consideration. In addition, GMM is introduced into the consequent part to design a negative log-prior function as the pixel distance metric. The proposed method has been compared to other state-of-the-art segmentation algorithms on both simulated images and real SOFC anode OM images. The experimental results demonstrate that the proposed method is able to achieve a higher segmentation accuracy with a faster convergence speed, which lays firm foundation for microstructural parameters extraction from SOFC electrodes image datasets.

Influence of Alloying Elements on the Behavior of Different Ferritic Steels as Candidate Materials for SOFC Interconnect

The efficiency of a solid oxide fuel cell (SOFC) can be improved by using a Ni-mesh between the ferritic steel interconnect and the Ni/YSZ anode. However, interdiffusion processes can lead to internal oxidation within the Ni-mesh and some microstructural changes, i.e., formation of an austenite zone and accelerated formation of σ-phase at the ferrite/austenite interface. These changes may adversely affect the performance of the cell during long-term operation. The present work focused on the influence of certain alloying elements on the overall behavior of the ferritic steel under simulated SOFC operating conditions to define conditions to minimize σ-phase formation without degrading the oxidation resistance and mechanical properties of the steel. The experimental results indicate that decreasing the amount of Cr and adjusting the amount of Nb, Si and W of the steel is a possible way to achieve that goal.

Performance of Co[sbnd]Ni[sbnd]O spinel oxide coating on AISI 430 stainless steel as interconnect for intermediate temperature solid oxide fuel cell

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.

The surface evolution of La0.4Sr0.6TiO3+δ anode in solid oxide fuel cells: Understanding the sulfur-promotion effect

The ideal solid oxide fuel cells (SOFCs) can be powered by readily available hydrocarbon fuels containing impurities. While this is commonly recognized as a key advantage of SOFC, it also, together with the elevated operating temperature, becomes the main barrier impeding the in-situ or operando investigations of the anode surface chemistry. Here, using a well-designed quenching experiment, we managed to characterize the near-surface structure of La0.4Sr0.6TiO3+δ (LST) anode in SOFCs fuelled by H2S-containing methane. This new method enabled us to clearly observe the surface amorphization and sulfidation of LST under simulated SOFC operating conditions. The ∼1 nm-thick two dimensional sulfur-adsorbed layer was on top of the disordered LST, containing –S, –SH and elemental sulfur species. In SOFC test, such “poisoned” anode showed increased performances: a ten-fold enhanced power density enhancement (up to 30 mW cm−2) and an improved open circuit voltage (from 0.69 V to 1.17 V). Moreover, its anodic polarization resistance in methane decreased to 21.53 Ω cm2, a difference of 95% compared with the sulfur-free anode. Control experiments confirmed that once the adsorbed sulfur species were removed electrochemically, methane conversion slowed down simultaneously till full stop.

LaFeO3 perovskite conversion coatings grown on a 13Cr ferritic stainless steel: a corrosion degradation study in simulated solid oxide fuel cell (SOFC) interconnect conditions at 700 °C

In this work, the corrosion stability of a 13Cr ferritic stainless steel protected by a novel LaFeO3 perovskite conversion coating is studied in demanding solid oxide fuel cell (SOFC) environments. Long‐term degradation behavior of coated 13Cr steel is evaluated and compared with the uncoated steel in air and in dual H2/air atmospheres at 700 °C. The perovskite coating was found to promote high oxidation resistance during single atmosphere air exposure at 700 °C up to 1800 h, with no signs of coating degradation. Oxidation experiments in dual‐atmosphere environments at 700 °C, carried out with humid hydrogen and air flowing, respectively, on the simulated anode and cathode sides of the sample, led to significant surface iron oxide growth on both coated and uncoated 13Cr steel samples, yet with remarkable difference in corrosion morphology. The coated steel samples suffered from a localized corrosion attack with formation of iron oxide nodules and significant coating detachment, while uniform iron oxide growth was observed on the uncoated steel. The results of this work indicate that the perovskite conversion coating can be a promising approach to protect 13Cr steels in simple oxidizing environments, although further studies and improvements are required for long‐term protection in the more aggressive conditions.

Characterization of a new Fe-based alloy as metallic interconnect in SOFC anode environment

The oxidation resistance and electrical contact property of a low-Cr-containing alloy, named as FeCro, was examined as metallic interconnect at 750 °C in simulated solid oxide fuel cell (SOFC) anode environment. This alloy was proved to process excellent oxidation resistance at 750 °C in N2 + 4.85% H2 + 3% H2O (g) for up to 1000 h. The thermally grown oxide scale, around 1 μm thick, consisted of Mn doped (Mn, Cr)3O4 spinel with Cr2O3 sublayer. The oxidation kinetics obeyed the parabolic law with two rate constants, 5.23 × 10−14 g2 cm−4 s−1 (0–100 h) and 3.85 × 10−15 g2 cm−4 s−1 (100–900 h) respectively. After 581 h of exposure in N2 + 3.75% H2 + 25% H2O (g) at 750 °C, the oxidation behavior of FeCro alloy in contact with Ni mesh under an applied current of 500 mA cm−2 was quite different from that in air or anode gas containing 3% H2O (g). The internal oxidation was promoted in the presence of water vapor, and a Fe- or Cr-rich internal oxide scale with a network structure was formed. Owing to a poor supply of Cr, Fe-rich external oxide scale composed of Fe3O4 and FeO was formed on the surface of FeCro alloy in which metallic compound Ni3Fe was embedded. Although FeCro alloy suffered from severe corrosion with non-protective Fe-rich oxide scale at 750 °C in N2 + 3.75% H2 + 25% H2O (g), the area specific resistance of Ni mesh/FeCro alloy was considerably low to offer sufficient metallic conduction.

Oxidation behaviour and phase transformations of an interconnect material in simulated anode environment of intermediate temperature solid oxide fuel cells

The oxidation behaviour and the phase transformations associated with high temperature exposure of a commercial ferritic interconnect steel, Crofer 22 H, was studied in a simulated solid oxide fuel cell (SOFC) anode atmosphere at 700 °C. Special emphasis was placed on the formation of the intermetallic sigma phase. No sigma phase was detected in the bulk alloy after 500 h of exposure of bare specimens. However, specimens which were pre-coated with a layer of nickel showed formation of an interdiffusion zone after as little as 2 h of exposure and sigma phase formation occurred after 10 h. The presence of the nickel layer, which simulates the contact between ferritic steel interconnects and a nickel mesh in a SOFC results in the formation of an austenitic zone and accelerated formation of a σ-phase rich layer at the ferrite/austenite interface. The ferritic steel is transformed into austenite due to the inward diffusion of nickel, σ-phase started to nucleate at the transformed austenite grain boundaries. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface due to more pronounced diffusion of Fe, compared to Cr, in the direction of the Ni-layer. Different possible mechanisms for the nucleation and growth of σ-phase were identified. The experimental results led to the conclusion that sigma nucleates in the austenite and grows following an isothermal eutectoid-like decomposition. The kinetics of σ-phase formation and the depth of the interdiffusion zone were found to follow a traditional diffusion relationship. It was observed that as the Ni-concentration increases the sigma-phase re-dissolves and thus the zone which, contains sigma phase moves deeper into the ferritic steel with exposure time. Interdiffusion processes between the nickel layer and the ferritic steel result not only in accelerated formation of σ-phase but also in the formation of Cr-rich oxides within the nickel layer.

Electrical Resistance Measurements and Microstructural Characterization of the Anode/Interconnect Contact in Simulated Anode-Side SOFC Conditions

Metallic interconnects in solid oxide fuel cell (SOFC) stacks are often in direct contact with a nickel/yttria stabilized zirconia (Ni/YSZ) cermet anode. Interdiffusion between the two components may occur at the operating temperature of 700–850°C. The alteration of chemical composition can result in phase transformation of the steel and in formation of oxides with a poor electrical conductivity in the anode. In this study, the area specific resistance (ASR) of the steel Crofer 22 APU, in contact with a Ni/YSZ anode with and without a tape casted CeO2 barrier layer was measured in simulated SOFC anode conditions at 800°C. The microstructure in the contact area was characterized using scanning electron microscopy techniques. The ASR was low for the steel in direct contact with the Ni/YSZ anode. Nickel diffusion into the steel resulted in a fine grained zone, which was identified as ferrite. The zone is austenitic at the exposure temperature but transforms to ferrite during cooling. When a CeO2 nickel diffusion barrier layer was used The ASR was considerably higher. These results imply that nickel diffusion is not only detrimental: It leads to microstructural instability but also results in a low electrical resistance of the anode/interconnect contact.

Pre-treatment and oxidation behavior of sol–gel Co coating on 430 steel in 750 °C air with thermal cycling

Research and development efforts continue to improve oxidation resistance and electrical conductivity of solid oxide fuel cell (SOFC) interconnects. This research evaluates the performance of a ferritic stainless steel (AISI 430 — a candidate SOFC interconnect material) with and without a Co coating (via sol–gel dip coating technique) with various pre-treatments, during cyclic oxidation exposures up to 750°C in laboratory air. A pre-treatment exposure of Co coated samples to a 750°C reducing atmosphere (prior to oxidation exposures) led to the formation of an effective Co-based spinel oxide coating that lowers oxidation rates by more than 40 times, and substantially lowers area specific resistance (ASR) in comparison to uncoated specimens. The development of effective sol–gel Co coatings with reducing pre-treatments, and their evolution within simulated SOFC interconnect environments is presented and discussed.

Lean-burn NOx emission control via simulated stack of solid oxide fuel cells with Cu-added (LaSr)MnO3 cathodes

A stack of solid oxide fuel cells (SOFCs) can be used as an automotive catalytic converter for lean-burn NOx emission control. With a simulated SOFC stack, it is demonstrated that this converter can reduce NOx in the lean-burn engine exhaust to zero no matter how large the inlet NOx concentration is. An SOFC unit operating at 600°C was constructed with Ni–YSZ as the anode, YSZ as the electrolyte, and Cu-added La0.8Sr0.2MnO3–Ce0.9Gd0.1O1.95 as the cathode. The NOx conversion increases not only with decreasing NO concentration in the low NO concentration range but also with increasing NO concentration in the high NO concentration range. In addition, the NOx conversion increases with increasing O2 concentration and can increase with adding SO2. Simultaneous control of CO emission via oxidation can be performed in this converter.

Electrical stability of a novel sealing glass with (Mn,Co)-spinel coated Crofer22APU in a simulated SOFC dual environment

A novel alkaline-earth silicate (Sr–Ca–Y–B–Si–Zn) sealing glass was developed for solid oxide fuel cell (SOFC) applications. The glass was sandwiched between two metallic interconnect plates and tested for electrical stability in a dual environment at elevated temperatures of 800–850 °C. A ferritic stainless steel (Crofer22APU) was used as the metallic interconnect material in the as-received state and coated with (Mn,Co) 3O 4 spinel. The isothermal aging results showed stable electrical resistivity at 800–850 °C for ∼500–1000 h. The electrical resistivities at 800 or 850 °C of the spinel coated samples were lower than the as-received ones; however, they were still several orders of magnitude higher than typical SOFC functional parts. Interfacial microstructure was characterized and possible reactions are discussed.

Oxidation Behavior and Electrical Property of a Ni-Based Alloy in SOFC Anode Environment

The oxidation behavior and electrical property of a Ni–Mo–Cr alloy at in a simulated solid oxide fuel cell (SOFC) anode environment (humidified and diluted ) have been studied. The thermally grown oxide scale presents a multilayered structure, which is similar to that formed in air, with densely grown in between an underneath layer of the intermetallic compound and a less compact top layer consisting of with flakes embedded. The oxidation kinetics obey a two-stage parabolic law with an initially fast oxidation (0–170 h at a rate of ) followed by a slower steady oxidation (170–1000 h at a rate of ). The reducing atmosphere, even with a significantly lower oxygen partial pressure, has a negative effect on the high temperature oxidation resistance; however, it enhances the adherence of the scale to the substrate. The outward diffusion of Ti ions changes the composition, lattice parameters, and diffraction peak positions of the formed oxides, leading to Ti-doped and the spinel. The area specific resistance of the oxide scale preformed at for 1000 h is at , which is higher than that formed in air and lower than that of other studied Ni-based alloys.

Advanced PVD protective coatings for SOFC interconnects

In an effort to enable inexpensive ferritic steels as interconnects (bi-polar plates) in planar solid oxide fuel cell (SOFC) stacks, advanced large area filtered arc physical vapor deposition (LAFAD) of MCrAlYO ceramic protective coatings was investigated ( M = Ti , Co and/or Mn). This manuscript presents three unique LAFAD MCrAlYO coatings, which were applied to 430 ferritic steel specimens and subjected to various SOFC relevant exposures. The coatings’ microstructure, composition, area specific resistance (ASR) and Cr volatility were evaluated as a function of exposure to 800 ∘ C air for up to 1000 h. Significant improvement in simulated SOFC interconnect performance was observed with coated vs. uncoated specimens (e.g., increased thermal stability and decreased ASR and Cr volatility). Outward Mn transport (from steel substrate) was observed (to varying extents) in all coatings. This appeared to promote surface crystallization and lower ASR values with extended exposure time. Coatings containing Mn and/or Co as-deposited exhibited the lowest and most stable ASR values, and more-uniform surface crystallization compared with coatings without. Coatings without Mn and/or Co as-deposited retained an amorphous surface structure, apart from local regions of high Mn outward transport, which evolved surface crystallites concentrated at topological features on the substrate steel surface (e.g., roll marks). ASR values for these coatings decreased significantly with exposure time. The engineering of LAFAD MCrAlYO coating processes to enhance SOFC interconnect performance of ferritic steels is presented and discussed.

Simulated SOFC Interconnect Performance of Crofer 22 APU with and without Filtered Arc CrAlON Coatings

The solid oxide fuel cell (SOFC) interconnect performance of Crofer 22 APU [Crofer 22 APU High Temperature Alloy MSDS No. 8005 June, 2004 ThyssenKrupp VDM (2004)] with and without protective large area filtered arc coatings [V. I. Gorokhovsky, R. Bhattacharya, and D. G. Bhat, Surf. Coat. Technol., 140, 82 (2001)] from the Cr-Al-O-N elemental system has been investigated as a function of exposure to simulated SOFC cathode-gas phase conditions at 800°C. Area specific resistance (ASR) was measured using a four-point probe method with platinum paste as an electrical contact. The sample surface morphology and chemical composition were assessed using scanning electron microscope/energy-dispersive spectroscopy and Rutherford backscattering spectroscopy. Significantly improved surface stability and ASR were observed for coated vs uncoated samples. Changes in coating composition and/or architecture were also observed to influence the SOFC interconnect relevant performance.

Chemical interaction between Crofer 22 APU and mica-based gaskets under simulated SOFC conditions

Mica gaskets and also composite gaskets containing compressive mica interlayers are under consideration as sealing materials in solid oxide fuel cells (SOFC). To study potential interactions between the interconnect steel Crofer22APU and the mineral phases vermiculite (exfoliated) (K,Mg,Fe)3(Si,Al)4O10(OH)2) and talc (Mg3Si4O10(OH)2), corrosion experiments were conducted in simulated SOFC conditions. The opposite walls of the gaskets were simultaneously exposed to air and wet H2. A substantial increase in the thickness of the oxide layers formed by the interconnect steel is observed with specimen containing talc. The Cr2O3/(Cr,Mn)3O4 duplex layer normally formed on the Crofer is replaced by a thicker (factor of 5–10) layer of a complex microstructure that is assumed to contain Cr2O3, (Cr,Mn,Mg,Fe)3O4 and Fe2O3 phases. The modified microstructure is found in the entire air manifold, with an increased thickness up to a distance of 300 μm from the mica. It is proposed that magnesium is the critical component responsible for the accelerated oxide formation. A decomposition of talc is observed, which is discussed as the mechanism for the release of magnesium.