Cu- and Ni-doped Mn1.5Co1.5O4 spinel coatings on metallic interconnects for solid oxide fuel cells

An interconnect in solid oxide fuel cells electrically connects unit cells and separates fuel from oxidant in the adjoining cells. Metallic interconnects are usually coated with conductive oxides to improve their surface stability and to mitigate chromium poisoning of a cathode. In this study, Mn_1.5Co_1.5O₄ (MCO) spinel oxides doped with Cu and Ni are synthesized and applied as protective coatings on a metallic interconnect (Crofer 22 APU). Doping of Cu and Ni into MCO improves sintering characteristics as well as electrical conductivity and thermal expansion match with the Crofer interconnect. The dense layers of Cu- and Ni-doped MCOs are fabricated on the interconnects by a slurry coating process and subsequent heat-treatment. The coated interconnects exhibit area-specific resistances as low as 13.9–17.6 mΩ cm² at 800 °C. The Cu-doped MCO coating acts as an effective barrier to evaporation and migration of Cr-containing species from the interconnect, thereby reducing Cr poisoning of a cathode.     

Comparison of oxidation behaviours among three Fe–Cr based alloys for solid oxide fuel cell interconnect

Abstract

Two metallic alloys, containing comparable amounts of Cr, underwent oxidation in hot air simulating (the solid oxide fuel cell cathode atmosphere) for various periods. The results demonstrated that the oxidation kinetics of Crofer22 APU and equivalent ZMG232 followed the parabolic rate law and oxidation rates increased with temperature. Typical oxidation rates of Crofer22 APU and ZMG232 upon annealing treatment are approximately 0·21 orders of magnitude lower than that of ZMG232. An oxide scale electron probe microanalyser, a scanning electron microscope and X-ray diffractometer were adopted to verify the applicability of Fe–Cr based alloys in the solid oxide fuel cell interconnect component. Two alloys contain comparable amount of Cr, Mn and Fe, and their surface oxides as analysed are indicated to be Cr₂O₃ and (Mn,Fe)Cr₂O₄ spinel compound. In summary, Crofer22 APU had the best oxidation resistance of any of the alloys of interest.     

Electrical contacts between cathodes and metallic interconnects in solid oxide fuel cells

In this work, simulated cathode/interconnect structures were used to investigate the effects of different contact materials on the contact resistance between a strontium doped lanthanum ferrite cathode and a Crofer22 APU interconnect. Among the materials studied, Pt, which has a prohibitive cost for the application, demonstrated the best performance as a contact paste. For the relatively cost-effective perovskites, the contact ASR was found to depend on their electrical conductivity, scale growth on the metallic interconnect, and interactions between the contact material and the metallic interconnect or particularly the scale grown on the interconnect. Manganites appeared to promote manganese-containing spinel interlayer formation that helped minimize the increase of contact ASR. Chromium from the interconnects reacted with strontium in the perovskites to form SrCrO₄. An improved performance was achieved by application of a thermally grown (Mn,Co)₃O₄ spinel protection layer on Crofer22 APU that dramatically minimized the contact resistance between the cathodes and interconnects.     

Life prediction of coated and uncoated metallic interconnect for solid oxide fuel cell applications

In this paper, we present an integrated experimental and modeling methodology in predicting the life of coated and uncoated metallic interconnect (IC) for solid oxide fuel cell (SOFC) applications. The ultimate goal is to provide cell designer and manufacture with a predictive methodology such that the life of the IC system can be managed and optimized through different coating thickness to meet the overall cell designed life. Crofer 22 APU is used as the example IC material system. The life of coated and uncoated Crofer 22 APU under isothermal cooling was predicted by comparing the predicted interfacial strength and the interfacial stresses induced by the cooling process from the operating temperature to room temperature, together with the measured oxide scale growth kinetics. It was found that the interfacial strength between the oxide scale and the Crofer 22 APU substrate decreases with the growth of the oxide scale, and that the interfacial strength for the oxide scale/spinel coating interface is much higher than that of the oxide scale/Crofer 22 APU substrate interface. As expected, the predicted life of the coated Crofer 22 APU is significantly longer than that of the uncoated Crofer 22 APU.     

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.     

Cold-start icing characteristics of proton-exchange membrane fuel cells

Understanding the icing characteristics of proton-exchange membrane fuel cells (PEMFCs) is essential for optimizing their cold-start performance. This study examined the effects of start-up temperature, current density, and microporous layer (MPL) hydrophobicity on the cold-start performance and icing characteristics of PEMFCs. Further, the cold-start icing characteristics of PEMFCs were studied by testing the PEMFC output voltage, impedance, and temperature changes at different positions of the cathode gas diffusion layer. Observation of the MPL surface after cold-start failure allowed determination of the distribution of ice formation at the catalytic layer/MPL interface. At fuel cell temperatures below 0 °C, supercooled water in the cell was more likely to undergo concentrated instantaneous freezing at higher temperatures (−4 and −5 °C), whereas the cathode tended to freeze in sequence at lower temperatures (−8 °C). In addition, a more hydrophobic MPL resulted in two successive instantaneous icing phenomena in the fuel cell and improved the cold-start performance.     

Determination of interfacial adhesion strength between oxide scale and substrate for metallic SOFC interconnects

The interfacial adhesion strength between the oxide scale and the substrate is crucial to the reliability and durability of metallic interconnects in solid oxide fuel cell (SOFC) operating environments. It is necessary, therefore, to establish a methodology to quantify the interfacial adhesion strength between the oxide scale and the metallic interconnect substrate, and furthermore to design and optimize the interconnect material as well as the coating materials to meet the design life of an SOFC system. In this paper, we present an integrated experimental/analytical methodology for quantifying the interfacial adhesion strength between the oxide scale and a ferritic stainless steel interconnect. Stair-stepping indentation tests are used in conjunction with subsequent finite element analyses to predict the interfacial strength between the oxide scale and Crofer 22 APU substrate.     

Facile modified polyol synthesis of FeCo nanoparticles with oxyhydroxide surface layer as efficient oxygen evolution reaction electrocatalysts

The oxygen evolution reaction (OER) at anode requires high overpotential and is still challenging. The metallic core-oxyhydroxide layer structure is an efficient method to lower an overpotential. We synthesized Fe rich FeCo core-Co rich FeCo oxyhydroxide layer with a different particle size of 173 nm, 225 nm, and 387 nm (FeCo 173, 225, 387) through a difference in the reduction rate of Fe/Co precursors using facile modified polyol synthesis. To investigate the effect of conductivity, CoFe₂O₄ nanoparticles of 80–130 nm were synthesized. Among samples, FeCo 173 showed remarkable catalytic performance of 316 mV at a current density of 10 mA/cm² in 0.1 M KOH compared to RuO₂ (408 mV), FeCo 225 (323 mV), FeCo 387 (334 mV), CoFe₂O₄ (382 mV). Moreover, FeCo 173 showed good stability for 60,000 s while RuO₂ showed a gradual increase in overpotential to maintain 10 mA/cm² after 15,000 s in chronopotentiometry. The excellent performance was attributed to Fe-rich metallic core, a small amount of Fe doping into CoOOH, and the synergic effect between the active site of Co rich FeCoOOH and conductive Fe rich metallic core. Following this result, it shows that the use of such FeCo electrodes has advantages in the production of hydrogen via electrochemical water oxidation.     

MnCo1.9Fe0.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the interconnection plates can be made from stainless steel. This is a big economic advantage, but energy losses can be caused by undesirable reactions between the alloys and other SOFC components. The use of coatings on interconnect stainless steels can reduce this degradation. A MnCo_1.9Fe_0.1O₄ (MCF) spinel not only significantly decreases the contact resistance between a La_0.8Sr_0.2FeO₃ cathode and a stainless steel interconnect, but also acts as a diffusion barrier to prevent Cr outward migration through the coating. The level of improvement in electrical performance depends on the ferritic substrate composition. For Crofer22APU and F18TNb, with a Mn concentration of 0.4 and 0.12 wt%, respectively, the reduction in contact resistance is significant. In comparison, limited improvement is achieved by application of MCF on IT-11 and E-Brite containing no Mn. No influence of the minor additions of Si or Al is observed on contact resistance. The MCF protection layer bonds well to the stainless steel substrates under thermal cycling, but the thermal expansion difference is too large between the La_0.8Sr_0.2Co_0.75Fe_0.25O₃ contact layer used and Crofer22APU and IT-11.     

Ferritic stainless steel interconnects for protonic ceramic electrochemical cell stacks: Oxidation behavior and protective coatings

Protonic ceramic fuel or electrolysis cells (PCFC/PCEC) have shown promising performance at intermediate temperatures. However, these technologies have not yet been demonstrated in a stack, hence the oxidation behavior of the metallic interconnect under relevant operating environments is unknown. In this work, ferritic stainless steels 430 SS, 441 SS, and Crofer 22 APU were investigated for their use as interconnect materials in the PCFC/PCEC stack. The bare metal sheets were exposed to a humidified air environment in the temperature range from 450 °C to 650 °C, to simulate their application in a PCFC cathode or PCEC anode. Breakaway oxidation with rapid weight gain and Fe outward diffusion/oxidation was observed on all the selected stainless steel materials. A protective coating is deemed necessary to prevent the metallic interconnect from oxidizing.To mitigate the observed breakaway oxidation, state-of-the-art protective coatings, Y₂O₃, Ce_0.02Mn_1.49Co_1.49O₄, CuMn_1.8O₄ and Ce/Co, were applied to the stainless steel sheets and their oxidation resistance was investigated. Dual atmosphere testing further validated the effectiveness of the protective coatings in realistic PCFC/PCEC environments, with a hydrogen gradient across the interconnect. Several combinations of metal and coating material were found to be viable for use as the interconnect for PCFC/PCEC stacks.     

CoMo heterohierarchical foam-structured cathode for anion exchange membrane water electrolyzer

It is important to consider the synergy of heterostructures to improve the slow kinetics of water dissociation in the alkaline hydrogen evolution reaction (HER). Herein, we report a simple method to design a heterohierarchical CoMo catalyst. The CoMo catalyst was prepared by simple one-pot electrodeposition on carbon paper (CP). The CoMo/CP catalyst was optimized for the alkaline HER by controlling the electrodeposition bath conditions, potential, and time. The optimized catalyst shows the heterohierarchical structure containing the electrically conductive metallic Co in the bulk and Mo-incorporated Co containing Mo⁴⁺ at the surface. It exhibited a lower HER overpotential of 0.11 V at ?20 mA/cm² compared to those of the others owing to the synergetic effect of the between the Co and Mo incorporated Co. The results highlight the advantages of the simple method developed herein for the design of heterohierarchical catalysts.     

Evaluation of several low thermal expansion Fe–Co–Ni alloys as interconnect for reduced-temperature solid oxide fuel cell

Several low thermal expansion Fe–Co–Ni alloys including HRA 929C, Thermo-Span, EXP 4005 and Three-Phase were evaluated as interconnect for reduced-temperature solid oxide fuel cell (SOFC). The isothermal oxidation behaviors of the four alloys were determined at 800 °C in air corresponding to the SOFC cathode environment. The results indicate that the mass gains of HRA 929C and Thermo-Span increased continuously with oxidation time, and were higher than those of EXP 4005 and Three-Phase, both of which exhibited low oxidation rate after the first-week exposure due to the formation of a semi-continuous Al₂O₃ inner layer. Compared to the ferritic alloy Crofer 22 APU, these low thermal expansion alloys exhibited inferior oxidation resistance; however, the area specific resistance (ASR) of the oxide scales thermally grown on these alloys was lower than that for Crofer 22 APU, as a result of the formation of a highly-conductive, Cr-free surface spinel layer. The promises and problems of these low thermal expansion alloys were discussed with regard to SOFC interconnect application.     

3D multiphysics modeling aided APU development for vehicle applications: A thermo-structural investigation

Solid oxide fuel cells (SOFC) are suitable for on-board electricity generation as Auxiliary Power Unit (APU) to support the electric power supply in heavy-duty vehicles. In order to satisfy the requirements of a lightweight fuel cell stack for mobile applications, thin-walled components must be used for the stack structure. This necessity is associated with material, process and design difficulties that must be solved in order to achieve a successful utilization. In this work a novel lightweight SOFC stack design with metal-supported cell was studied both numerically and experimentally. The metallic components are made from the Intermediate Temperature Metal (ITM), a high performance, high chromium ferritic stainless steels alloy. The multiphysics modeling approach (fluid dynamics, heat transfer, structural mechanics) was utilized in this work to predict the temperature distribution and the thermo-structural behavior of the new developed design. Geometric details of the fuel cell stack components as well as appropriate nonlinear, temperature and time-dependent constitutive models were developed to describe the material behavior. Experimental data were used to determine the material model parameters and validated the simulation results. The three-dimensional stress and deformation distributions in the individual stack components were evaluated and their maximum values for elements at risk were identified. Thus, the developed model enables the investigation of sustainability and serviceability of the structural elements to ensure a reliable operation of the stack. The developed computational model can be used as a design tool for parametric studies and optimization analysis to investigate the effects of process boundary conditions, material properties as well as geometrical design parameters and their variation on the induced thermal stresses.     

Refueling-station costs for metal hydride storage tanks on board hydrogen fuel cell vehicles

Refueling costs account for much of the fuel cost for light-duty hydrogen fuel-cell electric vehicles. We estimate cost savings for hydrogen dispensing if metal hydride (MH) storage tanks are used on board instead of 700-bar tanks. We consider a low-temperature, low-enthalpy scenario and a high-temperature, high-enthalpy scenario to bracket the design space. The refueling costs are insensitive to most uncertainties. Uncertainties associated with the cooling duty, coolant pump pressure, heat exchanger (HX) fan, and HX operating time have little effect on cost. The largest sensitivities are to tank pressure and station labor. The cost of a full-service attendant, if the refueling interconnect were to prevent self-service, is the single largest cost uncertainty. MH scenarios achieve $0.71–$0.75/kg-H₂ savings by reducing compressor costs without incurring the cryogenics costs associated with cold-storage alternatives. Practical refueling station considerations are likely to affect the choice of the MH and tank design.     

A comparison of ignition characteristics of slurry fuels prepared using coal processing waste and finely divided coal

The scientific novelty of the research is that for the first time differences in the conditions and characteristics of the ignition and burning of droplets of slurries prepared on the basis of coals and waste from their enrichment have been established. The practical significance of the research results is that they illustrate the prospects of utilization of the numerous coal enrichment wastes by combustion in the composition of aqueous slurries with the generation of a rather large amount of energy and a relatively small negative environmental impact. The most significant characteristics were compared: the limiting (minimum) temperature; the ignition delay times; the maximum combustion temperature; the concentration of the main gas anthropogenic emissions. It has been found that fuel mixtures prepared from wet waste of coal flotation are characterized by higher inertia and ignition temperatures compared to slurries with high-quality coal dust. However, the established differences considering the availability and low cost of filter cakes illustrate the prospects of waste derived fuel combustion. The combustion heat of the investigated slurries based on coal and filter cake with addition of petroleum products differs by no more than 5–30%. The average difference between the duration of ignition for fuel droplets based on dust and filter cake of coking and low-caking coals is about 20%. At that the addition of waste turbine oil (10% wt.) into the filter cake reduces the duration of ignition by 12–25% and the ignition temperature – by 10–15 °C without a significant increase in anthropogenic gas emissions. The difference between the minimum ignition temperatures of coal and waste coal based slurries was from 10 °C to 80 °C. On environmental and economic indicators, coal waste is more attractive than coal.     

Synthesis of binary Co–Mg–O oxide system and study of its behavior in reduction/oxidation cycling

The nanoscale Co–Mg–O oxide sample (15 wt% in terms of Co₃O₄) was prepared from the corresponding xerogel synthesized by the modified sol-gel method. The hydrolysis of as-prepared Mg(OCH₃)₂ was carried out using the aqueous solution of Co(NO₃)₂ precursor. The CoO_x nanocrystallites of about 10–20 nm in size were shown to be uniformly distributed within the MgO matrix. The reduction of Co–Mg–O in H₂ flow was found to proceed in two separate stages within the temperature ranges of 200–350 °C and 350–600 °C. The prepared binary Co–Mg–O system was demonstrated to possess completely reproducible reduction behavior in the consecutive reduction/reoxidation cycles. The phase composition of the sample exposed to both the reducing and oxidative environment was followed by an in situ X-ray diffraction analysis performed at temperatures of 25, 300, 500 and 700 °C. The determined lattice parameters for MgO (a = 4.219 Å) and Co₃O₄ (a = 8.110 Å) were found to be slightly increased as compared with the values from Powder Diffraction File, so that the formation of joint non-stoichiometric (Mg_1-xCo_x)O and (Co_3-xMg_x)O₄ phases was suggested. The strong chemical interaction of cobalt oxide with MgO matrix was also evidenced by the data of a diffuse reflectance UV–vis spectroscopy.     

Steam reforming of acetic acid for hydrogenproduction: Catalytic performances of Ni and Co supported on Ce0·75Zr0·25O2 catalysts

The catalytic steam reforming of acetic acid over both Ni/ and Co/Ce_0·75Zr_0·25O₂ (CZO) catalysts in the temperature range of 450–650 °C and steam-to-carbon molar ratios of 3–9 was studied. It was found that the complete acetic acid conversion was achieved for all the conditions investigated. Nevertheless, the C–C bond cleavage conversion was attained less than the acetic acid conversion at a given condition due to carbon deposition on the catalyst. However, hydrogen yield was obtained in the same trend as C–C bond cleavage conversion as well. The results revealed that the CZO as an active support prefers to promote the ketonization reaction to the C-C bond cleavage reaction at a lower temperature, and vice versa at a higher temperature. The Ni/CZO catalyst exhibits higher C–C bond cleavage conversion than the Co/CZO catalyst particularly at 650 °C whereas the Co/CZO catalyst is more active for ketonization reaction at low temperatures. However, as an increase in reaction temperature, the Co/CZO catalyst promotes ketonization reaction more pronouncedly toward aldol-condensation reaction thus giving rise to the carbon deposition. The results deduced from the effect of space velocity on the activity and product distribution suggested that the steam reforming of acetic acid over Ni/CZO catalyst is dominated by decomposition of acetic acid, while that of Co/CZO catalyst by ketonization reaction.     

Detonation propagation characteristics of H2/O2/AR in multi-angle bifurcation tubes

The propagation characteristics of the detonation wave in the bifurcated tube with the angular variation range of 30°–90° are simulated with 25% AR as dilution gas for H₂/O₂ mixture fuel at chemical equivalence ratio using the solver DCRFoam built on the OpenFOAM platform. The diffraction and reflection phenomena of detonation waves passing through bifurcation tubes with different angles are studied and analyzed. The results show that the distance from regular reflection to Mach reflection increases with the increase of the bifurcation angle so that after one reflection, the detonation forms three reflection forms with the angle of the different bifurcation tubes. After the first reflection, the detonation waves are more likely to induce the formation of transverse waves in the low-angle bifurcation tube. The lowest collision pressure after the detonation collides with the upper wall to form a secondary reflection occurs in the bifurcation tube between 50° and 60°.     

Promising components of waste-derived slurry fuels

The paper presents the results of experimental studies of energy (calorific value, ignition delay times and threshold ignition temperatures, duration and temperature of combustion) and environmental (CO₂, NO_x and SO_x emission) characteristics of fuel slurries based on pulverized wood (sawdust), agricultural (straw), and household (cardboard) waste. Wastewater from a sewage treatment plant served as a liquid medium for fuels. Petrochemical waste and heavy oil were additives to slurries. The focus is on obtaining the maximum efficiency ratio of slurry fuel, calculated taking into account environmental, cost, energy and fire safety parameters. All slurry fuels were compared with typical coal-water slurry for all the parameters studied. A comparison was also made between slurries and traditional boiler fuels (coal, fuel oil). The relative efficiency indicator for waste-based mixtures was varied in the range of 0.93–10.92. The lowest ignition costs can be expected when burning a mixture based on straw, cardboard and oil additive (ignition temperature is about 330 °C). The volumes of potential energy generated with the active involvement of industrial waste instead of traditional coal and oil combustion are forecasted. It is predicted that with the widespread use of waste-derived slurries, about 43% of coal and oil can be saved.     

Seal glass compatibility with bare and (Mn,Co)3O4 coated Crofer 22 APU alloy in different atmospheres

To prevent gas mixing and leakage during solid oxide fuel/electrolyzer cell operation, the interconnect/seal glass interface should bond well and remain stable. A SrO-La₂O₃-Al₂O₃-SiO₂ (SABS-0) seal glass has been bonded to bare Crofer 22 APU alloy and (Mn,Co)₃O₄ coated Crofer 22 APU alloy. The stability of the interconnect/SABS-0 interface has been studied in air and H₂/H₂O atmospheres at 800 °C for 1000 h. The interconnect/seal glass interaction involves the oxidation of the bare and (Mn,Co)₃O₄ coated Crofer 22 APU alloy surfaces, inter-diffusion of elements, chemical reaction, and the devitrification of the SABS-0 glass. The study shows that the thermal treatment atmosphere greatly affects the interfacial stability of both bare Crofer 22 APU/SABS-0 and (Mn,Co)₃O₄ coated Crofer 22 APU/SABS-0 samples. The interfacial stability is better in the H₂/H₂O atmosphere for both samples. The instability of the (Mn,Co)₃O₄ coating under the thermal treatment conditions degrades the interfacial compatibility of the (Mn,Co)₃O₄ coated Crofer 22 APU/SABS-0 sample.