Sputtered MnCu metallic coating on ferritic stainless steel for solid oxide fuel cell interconnects application

MnCu (Mn:Cu = 1:1, atomic ratio) metallic coatings have been deposited by magnetron sputtering on bare and on 100 h pre-oxidized SUS 430 steel for planar solid oxide fuel cells interconnects application. After oxidation at 800 °C in air, the MnCu coating directly deposited on the bare steel has been thermally converted to (Mn,Cu)₃O₄ spinel with Fe, containing discrete CuO on the outer surface. Nevertheless, the converted (Mn,Cu)₃O₄/CuO layer from the MnCu coating deposited on the pre-oxidized steelis almost free of Fe. A double-layer oxide structure with a main (Mn,Cu)₃O₄ spinel layer atop a Cr-rich oxide layer has been developed on the bare and pre-oxidized steel samples with MnCu coatings after thermal exposure. The outer layer mainly consisted of (Mn,Cu)₃O₄ spinel has not only significantly suppressed Cr outward migration to the scale surface, but also effectively reduced the area specific resistance (ASR) of the scale. The sputtered MnCu metallic coating is a very promising candidate for steel interconnect coating material.     

A promising NiCo2O4 protective coating for metallic interconnects of solid oxide fuel cells

The NiCo₂O₄ spinel coating is applied onto the surfaces of the SUS 430 ferritic stainless steel by the sol-gel process; and the coated alloy, together with the uncoated as a comparison, is cyclically oxidized in air at 800 °C for 200 h. The oxidation behavior and oxide scale microstructure as well as the electrical property are characterized. The results indicate that the oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 8.1 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹, while the electrical conductivity is considerably improved due to inhibited growth of resistive Cr₂O₃ and the formation of conductive spinel phases in the oxide scale.     

Metallic interconnects for solid oxide fuel cell: A review on protective coating and deposition techniques

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.     

Co/LaCrO3 composite coatings for AISI 430 stainless steel solid oxide fuel cell interconnects

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.     

Natural CuFe2O4 mineral for solid oxide fuel cells

Natural mineral, cuprospinel (CuFe₂O₄) originated from natural chalcopyrite ore (CuFeS₂), has been used for the first time in low temperature solid oxide fuel cells. Three different types of devices are fabricated to explore the optimum application of CuFe₂O₄ in fuel cells. Device with CuFe₂O₄ as a cathode catalyst exhibits a maximum power density of 180 mW/cm² with an open circuit voltage 1.07 V at 550 °C. And a power output of 587 mW/cm² is achieved from the device using a homogeneous mixture membrane of CuFe₂O₄, Li₂O-ZnO-Sm_0.2Ce_0.8O₂ and LiNi_0.8Co_0.15Al_0.05O₂. Electrochemical impedance spectrum analysis reveals different mechanisms for the devices. The results demonstrate that natural mineral, chalcopyrite, can provide a new implementation to utilize the natural resources for next-generation fuel cells being cost-effective and make great contributions to the environmentally friendly sustainable energy.     

Development of the spinel powder reduction technique for solid oxide fuel cell interconnect coating

Mn_0.9Y_0.1Co₂O₄ spinel coatings are developed for solid oxide fuel cell (SOFC) alloy interconnects by a novel powder reduction technique. Material properties, electrical performance and long-term stability of the coatings are explored. The coating is about 9 μm in thickness and adheres well to the alloy substrate without any cracking or delamination. The area specific resistance (ASR) remains almost unchanged and is less than 3 mΩ cm² even though the coated alloy undergoes oxidation at 800 °C for 1017 h and ten thermal cycles from 800 °C to room temperature. The coated alloy presents excellent electrical performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect.     

Studies on elements diffusion of Mn/Co coated ferritic stainless steel for solid oxide fuel cell interconnects application

The spinel structure of manganese cobalt oxide (Mn,Co)₃O₄ is one of the most promising coatings for solid oxide fuel cell (SOFC) stainless steel interconnects. The stoichiometric Mn_1.5Co_1.5O₄ composition has properties that are preferable to other Mn/Co ratios, for example a higher conductivity and a thermal expansion coefficient that matches the typical steel substrate. However, previous work showed the Mn/Co ratio changes during operation due to the diffusion of Mn from the substrate. The results presented here are on three coatings with different compositions (namely; pure Co, Mn₂₀Co₈₀, and Mn₄₀Co₆₀) with each coating composition deposited to a thickness of 800 nm, 1500 nm, and 3000 nm. The coatings were applied by DC magnetron sputtering and then machine cut into coupons for isothermal annealing at 800 °C in air using a batch-type furnace for 2, 10, 50, 250, and 1000 h. The morphology, chemical composition (including surface and cross sections of the layers) and structures of the oxides formed were analyzed by SEM, EDS and XRD. Analysis of the element diffusion (Mn, Co, Cr, Fe) shown here points to an optimized coating recipe of Mn₄₀Co₆₀.     

Investigation of MnCu0.5Co1.5O4 spinel coated SUS430 interconnect alloy for preventing chromium vaporization in intermediate temperature solid oxide fuel cell

The MnCu_0.5Co_1.5O₄ spinel coating is proposed as a protective coating for SUS430 alloy to improve its oxidation resistance and prevent chromium vaporization. The coated alloy is exposed to dual atmosphere (Air/H₂–3%H₂O) at 750 °C for 200 h, exhibiting a stable spinel structure on the air side, but reduced to MnO, Cu and Co on the fuel side. The coating layer could maintain integrated and dense with a thickness of 13–14 μm. The experiment results shown that the MnCu_0.5Co_1.5O₄ coating is an effective diffusion barrier that can inhibit oxidation and chromium vaporization of metallic interconnect. The relatively low amount of Cr deposition on LSM cathode on coated condition is considered associating with the stable electrochemical performance under current density of 400 mA cm^?2. The above results indicate that MnCu_0.5Co_1.5O₄ spinel is a promising coating for interconnect alloy of solid oxide fuel cell.     

Enhancing the long-term stability of Ag based seals for solid oxide fuel/electrolysis applications by simple interconnect aluminization

Ag-(0–8 mol%) CuO is used to successfully join aluminized ferritic stainless steel interconnect to the ceria-gadolinia (CGO) barrier layer of a solid oxide fuel/electrolysis cell by reactive air brazing at 1000 °C in air. The wetting of AgCuO on CGO is tailored by varying the CuO content. The effects of the CuO content on the joint microstructure are discussed. The long-term stability of brazed joints is evaluated by aging in oxidizing (air) and reducing (4% H₂–50% H₂ON₂) atmospheres at 800 °C for 250 h. An Ag-2mol% CuO braze results in the best joint stability during aging. Aluminization of the steel to create an alumina surface layer provides excellent protection of the steel both during the joining process and aging in the 2 atm. No degradation related to steel corrosion and outward diffusion of elements from the steel can be observed.     

Performance studies of solid oxide fuel cell cathodes in the presence of bare and cobalt coated E-brite alloy interconnects

The electrochemical performances of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) electrodes were studied by half-cell measurements in the absence of chromia-forming alloy, in the presence of bare and Co coated E-brite alloy interconnects, respectively. The surface and cross-section properties of the bare and Co coated E-brite alloys, and LSCF electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). The results showed a rapid degradation in LSCF performance when the bare E-brite alloy was used as interconnect. The growth of chromia scale on the E-brite alloy and the increase of Cr content throughout the LSCF electrode were observed. The uniform and dense Co coating process was developed to coat the E-brite alloy by using both electroless and electrodeposition methods. It was demonstrated that the Co layer effectively mitigates the Cr migration, leading to improved electrochemical stability of LSCF electrodes.     

Characteristic of (La0.8Sr0.2)0.98MnO3 coating on Crofer22APU used as metallic interconnects for solid oxide fuel cell

This study reports the high temperature oxidation kinetics, area specific resistance (ASR), and interfacial microstructure of metallic interconnects coated by (La_0.8Sr_0.2)_0.98MnO₃ (LSM) in air atmosphere at 800 °C. An efficient LSM conductive layer was fabricated on metallic interconnects for solid oxide fuel cells (SOFCs) by using a wet spray coating method. The optimum conditions for slurries used in the wet spray coating were determined by the measurement of slurry viscosity and coated surface morphology. The surface roughnesses of the substrates were increased through sandblast treatment. The adhesive strength of the interface between the coated layer and the metal substrate increased with increased surface roughness of the metallic interconnects. The electrical conductivities of the coated substrates were measured by using a DC two-point and four-wire method under air atmosphere at 800 °C. Of note, the Crofer22APU treated at 1100 °C in N₂ with 10 vol.% H₂ showed long-term stability and a lower ASR value than other samples(heat-treated at 800 °C and 900 °C). After an 8000-h oxidation experiment the coated Crofer22APU substrate, the ASR showed a low value of 23 mΩ cm². The thickness of the coated conductive oxide layer was about 10-20 μm. These results show that a coated oxide layer prevents the formation and the growth of scale (Cr₂O₃ and (Mn, Cr, Fe)₃O₄ layer) and enhances the long-term stability and electrical performance of metallic interconnects for SOFCs.     

NiMn2O4 spinel as an alternative coating material for metallic interconnects of intermediate temperature solid oxide fuel cells

In an effort to improve the performance of SUS 430 alloy as a metallic interconnect material, a low cost and Cr-free spinel coating of NiMn₂O₄ is prepared on SUS 430 alloy substrate by the sol-gel method and evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. A oxide scale of 3-4 μm thick is formed during cyclic oxidation at 750 °C in air for 1000 h, consisting of an inner layer of doped Cr₂O₃ and an outer layer of doped NiMn₂O₄ and Mn₂O₃; and the growth of Cr₂O₃ and formation of MnCr₂O₄ are depressed. The oxidation kinetics obeys the parabolic law with a rate constant as low as 4.59 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹. The area specific resistance at temperatures between 600 and 800 °C is in the range of 6 and 17 mΩ cm². The above results indicate that NiMn₂O₄ is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells.     

Oxidation behavior of (Co,Mn)3O4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects

Ceramic coatings are being explored to extend the lifetime of stainless steel interconnects in planar Solid Oxide Fuel Cells (SOFCs). One promising coating is Co_1.5Mn_1.5O₄ spinel, which is deposited using various techniques, resulting in different coating thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation (0, 3, 10 and 100 h in 800 °C lab air) prior to coating. Samples were coated with 2 μm CoMn alloy using magnetron sputtering and were subsequently annealed in 800 °C air for 0, 10, 100 or 1650 h. Oxidation behaviors were evaluated as a function of these exposures, as well as in dual atmospheres and during area specific resistance (ASR) measurements in 800 °C lab air. Preoxidation was found to inhibit Fe and Cr transport from the stainless steel into the coating and preoxidized samples exhibited a substantially thinner surface layer after oxidation. After ASR testing for 1650 h in 800 °C air, the trend of the preoxidized sample values remained level while trend of the non-preoxidized sample values showed an increase. Observed oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects are presented and discussed.     

Promising alloys for intermediate-temperature solid oxide fuel cell interconnect application

The formation of a low Cr-volatility and electrically conductive oxide outer layer atop an inner chromia layer via thermal oxidation is highly desirable for preventing chromium evaporation from solid oxide fuel cell (SOFC) metallic interconnects at the SOFC operation temperatures. In this paper, a number of ferritic Fe–22Cr alloys with different levels of Mn and Ti as well as a Ni-based alloy Haynes 242 were cyclically oxidized in air at 800 °C for twenty 100-h cycles. No oxide scale spallation was observed during thermal cycling for any of these alloys. A mixed Mn₂O₃/TiO₂ surface layer and/or a (Mn, Cr)₃O₄ spinel outer layer atop a Cr₂O₃ inner layer was formed for the Fe–22Cr series alloys, while an NiO outer layer with a Cr₂O₃ inner layer was developed for Haynes 242 after cyclic oxidation. For the Fe–22Cr series alloys, the effects of Mn and Ti contents as well as alloy purity on the oxidation resistance and scale area specific resistance were evaluated. The performance of the ferritic alloys was compared with that of Haynes 242. The mismatch in thermal expansion coefficient between the different layers in the oxide scale was identified as a potential concern for these otherwise promising alloys.     

Perovskite materials for highly efficient catalytic CH4 fuel reforming in solid oxide fuel cell

SOFC (solid oxide fuel cell, SOFC) is recognized to be efficient green energy technology in the 21st century. However, when hydrocarbons are directly used as fuel, carbon deposition is easy to occur in Ni-based anode, thus losing electrochemical catalytic activity. Fuel pre-reforming is also called on-cell reforming of hydrocarbons, which has been a promising solution for alleviating the carbon deposition problem in cermet anodes to varying degrees. And the key factor is to find an efficient and stable fuel reforming catalyst. Perovskite oxides have stable structure, highly catalytic activity and adjustable thermal expansion coefficient for using on the cells, showing great potentials of application for fuel reforming. In this paper, we summarize the application of perovskite catalyst in CH₄ fuel reforming based on the research of our group and other scholars, and puts forward the corresponding views and perspective, especially in perovskite catalyst with Ni exsolution.     

Performance of stainless steel interconnects with (Mn,Co)3O4-Based coating for solid oxide electrolysis

Mixed transition-metal oxide coatings are commonly applied to stainless steel interconnects for solid oxide cell stacks. Such coatings reduce oxidation and Cr evaporation rates, leading to improved degradation rate and stack lifetime. Here, the ChromLok? MCO-based composition (Mn,Co)₃O₄ is applied to Crofer 22 APU stainless steel and evaluated specifically for application in solid oxide electrolyzer stacks operating around 800 °C and utilizing oxygen-ion-conducting solid oxide cells. The MCO coating is found to decrease the stainless steel oxidation rate by about one order of magnitude, and decrease the Cr evaporation rate by fourfold. The coating also dramatically lowers the rate of area-specific resistance increase for stainless steel coupons oxidized for 500 h with constant current applied, from 33 mΩ1cm² kh^?1 for an uncoated coupon to less than 4 mΩ1cm² kh^?1 for coated coupons. The coating is demonstrated on full-scale interconnects for single-cells, where the coating dramatically reduces degradation rate, and for a stack, which displays stable operation for 700 h.     

Cobalt and molybdenum co-doped Ca2Fe2O5 cathode for solid oxide fuel cell

Recently, Brownmillerite oxides Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) (M = transition metal such as Co, Mo), have been drawing attention as they possess mixed ionic and electronic conductivity. Fe site of parent Ca₂Fe₂O₅ (CFO) structure is partially substituted by Co and/or Mo as well as CoMo co-doping and tested as cathodes in SOFC. Physical characterizations such as X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and Brunauer–Emmett–Teller (BET) have been carried out to assess the phase formation, microstructure, presence of constituent elements, particle size, and surface area of the cathode, respectively. The Co doped CFO cathodes have better percolation, large surface area, and extended triple phase boundary. Further, the doped CFO cathodes exhibited chemical compatibility with other cell components during fabrication and cell testing as evident from SEM micrographs. The Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) oxides show a semiconductor behaviour having sufficient electrical conductivity values in the SOFCs operating temperature 600–800 °C range. The best electrical conductivity, 0.47 S/cm at 800 °C and the corresponding activation energy of 0.17 eV is exhibited by Ca₂Fe_1.8Co_0.2O₅ (CFCO), whereas Ca₂Fe_1.8Mo_0.2O₅ (CFMO) and Ca₂Fe_1.8Mo_0.1Co_0.1O₅ (CFMCO) cathode shows electrical conductivity 0.11 S/cm and 0.15 S/cm at 800 °C, respectively. CFMO performed better with SDC than YSZ electrolyte between 600 and 700 °C although the lowest area specific resistance (ASR) of 1.28 Ω cm² at 800 °C is observed for CFMO with YSZ electrolyte. Similarly, CFMCO provided low ASR at lower temperature with SDC than that with YSZ electrolyte but exhibited lowest ASR of 0.41 Ω cm² at 800 °C with YSZ. The CFCO cathode shows lower ASR with YSZ than that with SDC for all the temperature and provided lowest value of ASR 0.21 Ω cm² at 800 °C. CFCO cathode has been tested in 900 μm thick electrolyte (SDC/YSZ) supported solid oxide fuel cell (SOFC) CFCO-SDC/SDC/NiO-SDC and CFCO-YSZ/YSZ/NiO-YSZ provided maximum power densities of 171 and 506 mW/cm² (i-R corrected) at 800 °C, respectively.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.